Analysis of the Effect of Bending, Fatigue, Erosion …doras.dcu.ie/17302/1/hussain_al_fadhli_RR.pdfAnalysis of the Effect of Bending, Fatigue, Erosion-Corrosion and Tensile Stresses

Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion and Tensile Stresses on

HVOF Coating o f Metallic Surfaces

by

Hussain A1 Fadhli B.Sc M.Sc

A thesis submitted in fulfillment o f the requirement for thedegree o f

Doctor of Philosophy

Supervisor:

Dr. Joseph Stokes

Dublin City University School o f Mechanical and Manufacturing Engineering

March 2006

DECLARATION

1 herby certify that this material, vvliioh I now submit for assessment cm the

programme o f study leading to the award of Doctor o f Philosophy, is entirely my

own work and has not been taken from the work o f others save and to the extent

that such work has been cited and acknowledged within the text o f my work.

1.1). Number: 53147278

I

ACKNOWLEDGMENTS

First o f all I thank ALLA H , w ho bestow ed m e w ith the strength to com plete this

work.

I shall always rem ain indebted to m y supervisor Dr. Joseph Stokes, who helped me

throughout m y thesis by his invaluable inspiration, encouragem ent and continuous

guidance. He w as alw ays there to help me in the difficult tim es during this work. I

am also extrem ely thankful to P rofessor B ekir Sami Y ilbas for his valuable

suggestions and guidance during the startup plan and the execution o f this work. I

am deeply grateful to P rofessor Saleem H ashm i (H ead o f School) and Dr. M alika

A rdhaoui for their help, support and enhancem ent o f this work.

I am grateful for the support o f the M echanical Services Shops D epartm ent in the

Saudi Oil C om pany (Saudi A ram co) represented by Mr. A laaeldeen M ustafa, Mr.

Sulaim an A1 Rasheed, M r. A bdullah A l-Jam eel, Mr. M oham m ed A l-Sultan, Mr.

A bdulrahm an A l-D khail and Mr. N aser A l-N aim i (H ead o f Departm ent). I am also

grateful for the support o f M artial Testing and M etallographic Group/CSD ,

Corrosion Testing U nit/R & D C at Saudi A ram co and M echanical Engineering

D epartm ent at K ing Fahd U niversity o f Petroleum and M inerals for supporting this

research work. This research w ould not have occurred w ithout utilizing their

laboratory facilities, their support is acknowledged.

F inally , I w ould like to thank m y parents and m y w ife w ho prayed and provided

m e the m oral support that I needed the m ost throughout m y study.

II

ABSTRACT

Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic Surfaces

By

H u ssa in A l-F ad h li BSc, M Sc

The aim o f the present study w as to evaluate the bending, fatigue, tensile stresses

and erosion-coiTosion characteristics o f HVO F therm ally sprayed Inconel-625

pow der coatings w hen deposited on three d ifferent m etallic surfaces: (a) plain

stainless steel (SS), (b) spot-w elded stainless steel (SW -SS), and (c) a com posite

surface o f stainless steel and carbon steel w elded together (C-SS-CS) under three

conditions: (i) surface as-coated, (ii) surface as-coated and subjected to aqueous

static corrosion for tw o weeks, and (iii) surface as-coated and subjected to aqueous

static corrosion for four weeks.

The predictions o f the residual stresses generated during the deposition o f the

coating over the d ifferent m etallic surfaces were also perform ed using analytical

and num erical m odel studies. V alidation o f the m odels w as earned out by

com paring the results to experim ental data. The surface m orphology and the

elem ental com position o f the coatings before and after each test w ere exam ined

using SEM and EDS techniques.

The results indicate that the presence o f weld or having a com posite substrate

form ed by two different m aterials does affect the residual stresses at the interface,

as it lowers bending, tensile, fatigue strength and enhancing corrosion effect, as

observed by the analytical and experim ental analyses carried out in this research.

M icroscopic obseivation o f the fractured surfaces showed that the cracks were

form ed at the interface betw een the coating and substrate m aterial as well as w ithin

the coating. Presence o f non-m elted particles in the coating enhanced crack

propagation and erosion rates. The presence o f oxides, nam ely AI2O 3 at the

interface betw een the coating and the substrate m aterial left over from the surface

preparation process also had an effect. H ence careful control is required w hen

applying w elds beneath surface coatings.

Ill

TABLE OF CONTENTS

Page

Declaration i

Acknowledgements ii

Abstract iii

Table o f Contents iv

List o f Figures ix

List o f Tables xiv

CHAPTER 1 INTRODUCTION 1

CHAPTER 2 LITERATURE REVIEW 5

2.1 Introduction 5

2.2 Surface Science 7

2.2.1 Wear 7

2.2.2 Corrosion 10

2.2.3 Biosion-Corrosion 1 1

2.2.4 Fatigue 12

2.3 Thermal Spray Coating Technique 15

2.3.1 Initiation o f Thermal Spray Coating 15

2.3.2 Thermal Spray Coating Types and Characteristics

16

IV

2.3.3 Comparison between Thermal Spray Processes

21

2.4 Feedstock Materials 23

2.4.1 Types o f Thermal Spray Materials 23

2.4.2 Methods o f Powder Production 24

2.4.3 Methods o f Powder Characterization 26

2.5 The IIVOF Process 27

2.5.1 Introduction 27

2.5.2 IIVOF Coating Structure and Properties 28

2.5.3 Parameters Affecting the Coating Quality 30

2.5.4 Advantages and Disadvantages o f the HVOF 32Coating

2.5.5 Application o f the IIVOF Coating 34

2.6 Review o f Previous Studies with Focus on IIVOFThermally Sprayed lnconel-625 Coatings Testing 36

2.6.1 Bending Test 36

2.6.2 Fatigue lest 36

2.6.3 Erosion-Corrosion Test 38

2.6.4 Tensile Bonding Test 39

2.6.5 Residual Stress 41

2.7 Spray Process Modelling 44

CHAPTER 3 EXPERIMENTAL EQUIPMENT AND 45PROCEDURES

3.1 Introduction 45

3.2 IIVOF Thermal Spraying System 46

3.2.1 DJ9H Gun 47

V

3.3 Powder Material 49

3.4 Experimental Matrix 50

3.5 W orkpiece 52

3.5.1 Substrate Sample 52

3.5.2 W elding Procedure 53

3.5.3 Surface Preparation 54

3.5.4 Spraying 55

3.6 Coating Characterization Techniques 59

3.6.1 Metallographie Preparation 59

3.6.2 Microscopic Analysis Techniques 62

3.7 Mechanical Tests Preparation 65

3.7.1 Static Corrosion Test 65

3.7.2 Bending (Flexural) Test 65

3.7.3 Fatigue Test Preparation 67

3.7.4 Jet Impingement (Erosion-Corrosion) Test 69

3.7.5 Tensile Testing 71

3.8 Measurements o f Residual Stress 73

3.8.1 Clyne’s Analytical Method 73

3.8.2 Modelling Using ANSYS Finite Element 77Analysis

CHAPTER 4 RESULTS AND DISCUSSION 82

4.1 Introduction 82

4.2 Coating and Substrate Characterisation 83

VI

4.3 C orrosion B ehaviour o f Sprayed Inconel-625 92A pplied over D ifferent M etallic Surfaces P rior to Testing

4.4 B ending B ehaviour o f H igh V elocity O xy-Fuel 97(HV OF) Therm ally Sprayed Inconel-625 Coatingson D ifferent M etallic Surfaces

4.4.1 E ffect o f Substrate V ariation on C oating Per- 97form ance Due to Bending Testing

4.4.2 D eposit C haracterisation Post B ending Test- 100ing

4.5 Fatigue B ehaviour o f H igh V elocity O xy-Fuel 104(HVOF) T herm ally Sprayed Inconel-625 C oatingson D ifferent M etallic Surfaces

4.5.1 E ffect o f Substrate V ariation on C oating 104Perform ance D ue to Fatigue Testing

4.5.2 D eposit C haracterization Post Testing 107

4.6 Erosion-C orrosion B ehaviour o f H igh V elocity 111Oxy-Fuel (HV OF) Therm ally Sprayed Inconel-625 Coatings on D ifferent M etallic Surfaces

4.6.1 D eposit C haracterisation (post erosion) 111

4.6.2 D ifferent Substrate Types (com posite and w elded)

112

4.6.3 C oating over Spot-W elded Stainless Steel (SW -SS)

113

4.6.4 Coating over Tw o D issim ilar M etals

(C-CS-SS)

114

4.6.5 W eight Loss 114

4.6.6 Influence o f Tim e 115

4.6.7 Slurry E ffect 116

4.7 Tensile B ehaviour o f H igh V elocity O xy-Fuel 118 (HV OF) Therm ally Sprayed Inconel-625 C oatings on D ifferent M etallic Surfaces

VII

4.7.1 Effect o f Substrate Variation on Coating 118Performance Due to Tensile Testing

4.7.2 Deposit Characterization Post Testing 120

4.8 Modelling o f Residual Stresses Variation with 126Substrate Type

4.8.1 Validating the Model 128

4.8.2 Comparison o f Each Model Based on 131Substrate Type

C H A PT E R 5 CO N CLU SIO N S AND R ECO M M EN D A TIO N S 139

5.1 Conclusions 139

5.2 Recommendations for Future Work 145

PU BLICA TIO N S ARISIN G FROM TH IS W O R K 146

REFERENCES 148

LIST OF FIGURES

Figure Description PageNo. No.

1 M ajor categories o f w ear................................................................................ 8

2 Industrial pump impeller subjected to impact (erosion-corrosion) 10w ear......................................................................................................................

3 Pitting and cracking at the junction between web and flange o f a cast 11316 type stainless steel impeller subject to flow o f seaw ate r..............

4 Photograph o f pump shaft failure due to cyclic fatigue......................... 14

5 Timeline development o f the therm al spray industry............................. 16

6 Schematic diagram o f the flame spray process....................................... 17

7 Schematic diagram o f the electric arc spray.............................................. 18

8 Schematic diagram o f the detonation gun spray system ........................ 19

9 Schematic diagram o f the plasm a spray system ...................................... 20

10 Schematic diagram o f the HVOF spray system ........................................ 21

11 D ifferent produced powder morphologies, as a result o f using dif- 25ferent powder production methods

12 Schematic diagram o f the HVOF spray gun used in this research....... 28

13 Schematic diagram o f the HVOF coating structure showing deposit 30features

14 Schematic diagram o f the HVOF spray process variables and 32parameters

15 Schematic o f quenching stresses in the individual lam ellae................ 42

16 Schematic o f cooling stresses....................................................................... 43

17 H igh velocity oxygen fuel system ............................................................... 46

IX

18 Schematic diagram o f DJ9H H V O F spraying gun................................. 48

19 Scanning Electron M icroscope view o f the manufactured Inconel- 49625 powder.

20 Photograph o f the substrate types used in the te st.................................... 52

21 Schematic diagram o f the GTAW welding process................................. 53

22 Photograph o f grit blasting cham ber............................................................. 55

23 Photograph o f the first test samples holder, for spraying angles o f 45 56to 90°...................................................................................................................

24 Schematic diagram showing the effect o f spraying angle 57

25 Photograph o f the modified test samples holder, spraying angle o f 5890°........................................................................................................................

26 SEM image showing the coating thickness.............................................. 58

27 Photograph o f ESEM (Philips X L-30)....................................................... 63

28 Schematic diagram o f the 3-point bending test....................................... 6 6

29 Photograph o f the three types o f samples used in the bending te s t.... 67

30 Photograph o f the INSTRON 8501 fatigue/tensile testing m achine... 6 8

31 Geometry o f the spot welded fatigue samples used in each test.......... 69

32 Schematic representation o f the erosion-corrosion testing rig .............. 70

33 Photograph o f the three forms o f samples types used in the jet- 71im pingem ent test.............................................................................................

34 Schematic description o f the generation o f curvature in a bi-m aterial 76plate as a result o f misfit strain......................................................................

35 C lyne’s m ethod used to determine distributed stress in thermal spray 78coatings..............................................................................................................

36 Sample deflection caused by m om ent.......................................................... 79

37 Stress distribution in the coating during deposition process................ 80

3 8 Schematic o f residual stresses developed in welding jo in t.................. 81


X

39 Typical cross section o f the coated sam ple............................................... 84

40 M agnified view o f the area depicted as A in Fig. 39 .............................. 84

41 (a) EDS spectrum o f the particle identified as C in Figure 61, 8 6

indicating high Ni, Cr, and Mo peaks, (b) EDS spectrum o f the dark inclusions as D in Figure 61, indicating presence o f C^Cb. (c) spectrum o f interface between coating and substrate as b in Figure60, indicating presence o f AI2O3 and Si2 0 3 .............................................

42 Fi X-Ray line scan analysis o f Inconel-625 coating................................ 87

43 X-ray-maps o f Inconel-625 coating over stainless steel surface.......... 89

44 Line scan analysis o f stainless steel and weld substarte.......................... 90

45 'Cross section o f sample before test. (A) Inconel-625 coating over 91stainless steel and sweld., (B) Stainless steel substrate, (C) Weld., (D)The heat affected zone......................................................................................

46 Photograph o f composite stainless steel and carbon steel (C-SS-CS) 92after three point bending test and subjected to corrosion ambient prior to te st........................................................................................................

47 (a) Coating cross-section before the corrosion test........................................ 94

47 (b) Coating cross-section after the corrosion test............................................ 95

48 SEM showing close view o f the heat affected zone (H AZ).................... 96

49 Load and displacement characteristics o f the workpieces during the 983 point bending tests for stainless steel workpiece (S S ) ......................

50 Load and displacement characteristics o f the workpieces during the 993 point bending tests for spot welded stainless steel workpiece (SW-SS)

51 Load versus displacement characteristics o f the workpieces during 100the 3 point bending tests ................................................................................

52(a) Close view o f coating cross-section.............................................................. 101

52(b) Coating cross-section after the 3-point bending te st................................. 102

53(a) Crack sites in the coating................................................................................. 103


XI

53(b) Close view o f crack sites n the surface post bending.............................. 103

54 S-N plot for axial fatigue testing o f a plain stainless steel (SS) 105surface coated surface coated with Inconel-625 ....................................

55 S-N plot for axial fatigue testing o f a spot-welded stainless steel 105(SW -SS) surface coated with Inconel-625................................................

56 S-N plot for axial fatigue testing o f a composite surface (C-SS-CS) 106coated w ith Inconel-625................................................................................

57 Coating life cycle over plain and w elded surfaces at the minimum 107alternating stress (148 M Pa).......................................................................

58 Sample post the corrosion-fatigue test: (A ) Stainless steel substrate. 109(B) Inconel-625 coating. (C) Interface between coating and substrate............................................................................................................

59 Cross section o f Inconel-625 coating post the fatigue test when 109subjected to four weeks corrosion..............................................................

60. Coating surface after je t impingement testing............................................ I l l

61 M agnified view o f the area depicted as E in Fig. 6 0 ............................ 112

62 Cross section o f the Inconel-625 coating applied over various met- 113allic substrate...................................................................................................

63 V ariation o f weight loss with tim e................................................................ 116

64 Effect o f adding silica sands to the testing fluid....................................... 117

65 Stess strrain curve for plain stainless steel workpiece (SS).................. 119

6 6 Stess strrain curve for spot-welded stainless steel workpiece (SW- 119SS)

67 Stess strrain curve for composite stainless and carbon steel work- 121piece (C-SS-CS)

6 8 Cross section o f tested coating...................................................................... 122

69 Photograpah o f plain stainless steel (SS) after tensile test................... 122

70 Coating after corrosion-tensile test................................................................ 123


X II

7 1 Cross section o f tested coating after tensile test.......................................... 124

72 Coating surface after tensile testing................................................................. 125

73 Deflection o f plain stainless steel (SS) sam ple due to therm al load... 126

74 Deflection o f plain stainless steel (SS) sam ple due to m om entum 127load

75 Overall stress distribution across ihe sam ple............................................... 128

76 Finite elem ent and analytical stress analysis o f Inconel-625 deposit 129over plain stainless steel substrate (SS)

77 Finite elem ent, experim ental and analytical stress analysis o f a 0.2 130mm thick deposit

78 Variation o f residual stresses based on substrate type............................ 132

79 Variation o f residual stresses at different regions in (SW -SS ).............. 13

80 V ariation o f residual stresses at different regions in (C -SS-C S).......... 136

81 Residual stresses across the SW -SS sam ple.................................................. 137

82 Residual stresses across the C-SS-CS sam ple............................................... 138


XIII

No.

1

2

3

4

5

6

7

8

9

10

11

12

13

LIST OF TABLES

Description PageN o .

ASTM: Wear test methods grouped by form of wear................ 9

Characteristic of thermal spray processes and coatings.............. 22

Coating porosity in various Diamond Jet HVOF coatings.......... 29

Benefits o f using HVOF coatings................................................. 33

Chemical Composition of Diamalloy 1005 Powder.................... 49

Shows the test matrix used in this research................................... 51

Specification of the rod welding used in the experiment tests.... 54

Process parameters of HVOF thermal spray.................................. 56

Material properties of the coating and the different metallic 7 7

substrates used in the modeling.......................................................

Elemental composition of coating (%wt)...................................... 96

Chemical composition of locations B and C as shown in Fig. 11058

Mass loss results from jet impingement testing during (500 hrs) 115timing

Quenching and cooling stresses variation between coating and 136substrate

XIV

CHAPTER 1: INTRODUCTION

1INTRODUCTION

In m any industrial applications, including oil and gas production and in the

petrochem ical fields, the behaviour o f m aterial surfaces is o f m ajor im portance.

The appropriate selection o f bu lk m aterials and the coating o f equipm ent

com ponents is an im portant consideration for the econom ic success o f these

industries. M oreover, the need to m inim ise cost and enhance the reliability o f

ro tating and stationary fluid m achinery equipm ent that is subjected to highly

erosive and corrosive environm ents is m andatory. In m any cases, surfaces are

exposed to m ultiple deterioration m echanism s, such as w ear and corrosion, at the

sam e time, thus the probability o f m aterial failure is increased. It is estim ated that

the cost o f unw anted w ear and corrosion in the U SA alone is in excess o f U S$ 3

b illion per year. This alone provides significant im petus for research in the area o f

protective coatings [1, 2 ].

T herm al spraying is one o f the m ost useful techniques used to produce coatings to

p ro tec t com ponents from the effects o f w ear and corrosion. Today, increasing use

is being m ade o f therm al spray deposits in technologies developed in industrial

fields such as oil and gas, the m arine, m edicine, the petroleum , steel, textiles and

prin ting industries [3], Therm al spray technology has developed rapidly over the

past tw enty years, in line especially w ith progress in aircraft engines and the

chem ical industries [4], N ew m aterials and surfacing contact has led to the

developm ent o f engineering coatings that provide protection from wear, corrosion,

and elevated tem peratures, and allow for reliable perform ance in increasingly

hostile environm ents [5, 6 ],

________________________________________________________________________lAnalysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

CHAPTER 1: INTRODUCTION

The high velocity oxygen fuel (HVOF) therm al spraying technique has been

w idely adapted in m any industries due to its high flexibility and cost effectiveness

[7], This techn ique’s w ide applications include com ponent repair involving either

rebuilding w orn out parts or enhancing the surface properties by using coatings

m aterial w ith better characteristics. The superior characteristics o f H VO F coatings,

w hich include high bonding strength and low er porosity and oxidation, have

increased the efficacy o f this process w hen com pared to o ther coating techniques

[8 , 9, 10]. H ow ever, despite huge progress in the developm ent o f this technology,

practical applications in the repair field still present m any research challenges, and

problem s such as detected coating failures w hich occur in day to day operation still

require solutions. The coating m aterial used in this investigation, Inconel-625

pow der, m anufactured by Sulzer M etco (D iam alloy 1005), has various applications

in several industries due to its superior corrosion resistance [10]. The application

o f this coating m aterial is very com m on; how ever, the application o f this m aterial

over w elded and dissim ilar m aterials has not yet been w idely researched [11 , 12 ,

The report initially describes the coating characterisation o f the Inconel-625

deposit. Follow ing this, the m ateria l’s resistance to several m echanical tests -

including bending, fatigue, erosion-corrosion, and tensile testing, w here the coating

m aterial is applied over plain, w elded and dissim ilar substrate m aterials - w ill be

investigated. The substrate m aterials w hich w ere selected are SS 316 and C arbon

Steel 4140, and the w elding m aterial chosen is SS 309.

The rem ainder o f the report is divided into three chapters. C hapter Two gives a

sum m ary o f re levant literature concerning HVO F spraying processing and design,

equipm ent and theory, and applications focusing on the Inconel-625 m aterial. The

literature w ill also present previous research on coating bending, fatigue, erosion

corrosion, and tensile tests. V arious research sources w ere considered, including

conference proceedings, w ith m ore focus on the annual therm al spray conference

____________________________________________________________________________________ 2Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metal lie SurfacesAl-Fadbli, Hussain Y

CHAPTER 1 : INTRODUCTION

proceedings as it is dedicated to therm al spray coating processes. R elevant

jou rnals supported by industry data w ere also considered. It was noted that the

testing o f H VO F sprayed coating m aterials was increasing and that m ost o f the

re la ted research w as relatively new ; this assisted in selecting the latest, m ost up-to-

date research in this regard [14, 15].

The experim ental procedure is described in C hapter Three. This chapter is divided

into six m ain sections, w hich include full descriptions o f the spray system used,

pow der m aterial m orphology, substrate surface preparation, coating

characterisation techniques, m echanical tests sam ple preparation as w ell as the

spray process m odelling technique. The m echanical tests sam ple preparation,

w hich is the core o f this work, is divided into four parts, w here each part is

concerned w ith the experim ental procedure and set-up for the specific tests,

including bending, fatigue, erosion-corrosion, and tensile tests. M odifications

added to each experim ental set-up are explained separately in each part and the

A ST M standard used is addressed. Spray process finite elem ent m odelling

(residual stress developm ent during the cooling process o f the coating) is presented

in the final section o f C hapter Three. This section describes the finite elem ent

analysis technique and procedures used in the research.

The experim ental results and discussion are presented in C hapter Four. The results

show a detailed characterisation o f the Inconel-625 pow der including Scanning

E lectron M icrocopy (SEM ) and Energy D ispersive Spectroscopy (EDS) analysis.

M echanical properties such as porosity, m icrohardness o f the tested coating that

had been studied in the previous research [16] are also discussed. The m echanical

test results are show n separately in four parts to indicate the effect o f each

individual test. The m echanical tests results as a w hole are correlated w ith the

m icrostructural analysis and discussed to conclude the chapter.

F inally, C hapter F ive presents the overall conclusions draw n from the results and

provides recom m endations for future w ork in this area. Such future research m ight

____________________________________________________________________________________ 3Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

C H A P T E R I : IN T R O D U C T IO N

include expanding the current study to include other powders such as WC-Co as

this is on one o f the most widely used powders, especially in the areas o f high

temperature application and the aerospace industries [17]. Eight different articles

were published out o f this work: two journal papers, three conference papers and

three posters, a list o f which may be consulted in the “Publications Arising from

This Work” section at the end o f this thesis.

___________________________________________________________________________ 4Analysis uT ihc ItfTeci ol'Bending, Fatigue, iirosion-Conosion. and Tensile Stresses on IIVOK Coaling ofMetnllic SurlhccsAl-J-ndhli, Hussain Y

CHAPTER 2: LITERATURE SURVEY

2LITERATURE SURVEY

2.1 IN T R O D U C T IO N

Surface engineering is defined as a m ultidisciplinary activity w hich aim s to tailor

the properties o f the surface o f an engineering com ponent so that its function and

serviceability can be im proved [18]. Surface treatm ent can be used in m any

d ifferen t w ays to im prove the com ponen t’s m echanical p roperties such as

hardness, strength and toughness, w hich results in im provem ents in the

com ponen t’s fatigue, w ear and corrosion resistance [19]. S im ilarly, environm ental

p roperties such as resistance to high tem perature oxidation, aqueous corrosion and

solid ification can be im proved by selecting a surface treatm ent technique such as

the therm al spray process [2 0 ].

A lthough for m ost applications the surface is no t m ade totally independent from its

bu lk m aterial, the dem ands on surface and bu lk properties are often quite different.

For exam ple, in the case o f a sea-w ater pum p shaft exposed to high s lu n y fluid

flow , the bu lk o f the m aterial m ust have sufficient bending and fatigue strength at

the surface to provide an acceptably safe service life under repetitive cyclic loading

on the shaft. Conversely, the surface o f the m aterial m ust also possess sufficient

resistance to oxidation and erosion-corrosion under the conditions o f service fluid

im pingem ent and corrosion attack to achieve that same com ponent life.

In m any instances, it is either m ore econom ical or absolutely necessary to select a

p roper coating m aterial w ith the required properties to enhance the com ponent

surface [21, 22], H ow ever, in som e practical cases the surface o f the bu lk m aterial

_______________________________________________________________________________________________________5Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

C H A P T E R 2: L IT E R A T U L E S U R V E Y

under the same working condition may not be uniform. In other words, either it

has som e wekl spots random ty distributed over the surface or it is joined to another

material with different properties. Hence, applying a coating over these different

surfaces requires further investigation in order to achieve optimum results and

minimise costs and potential safety hazards resulting from catastrophic equipment

failures.

The literature survey o f this report focuses on several points, including surface

science, thermal spray coating technique, feedstock materials, and a detailed

literature on the HVOl7 process theory and application. This will lead to a section

describing the HVOl7 thermally sprayed Inconel-625 coating, focusing on (he

testing and characterisation o f this material. Finally, a background o f the literature

in the area o f the spray process modeling will be addressed.

________________________________________________________________________<jAnalysis of the UfTect of liending, Fatigue. Rrosioit-CoiTfision, and Tensile Stresses an HVOl-' Con! my of Metallic Surfaces At-frulhli. Hussain Y


2.2 S U R F A C E S C IE N C E

The first step in the process o f understanding therm al spray coating technology is

to understand the surface to w hich the coating is applied. The concept o f defining

surfaces as the exterior phase o f an object has now been expanded to being defined

as the interface betw een solid objects and their surroundings [23]. The reason for

expanding this concept is that coatings are applied no t only to enhance surfaces

and restore w orn out parts, bu t also to pro tect these surfaces from their in-service

harsh environm ents [24], The com ponents’ interfaces encounter physical,

chem ical, electrical and other factors during perform ance. These forces can

degrade the com ponents and cause them to fail [25], Surface coatings, such as

therm al spray coatings, can help deal w ith these circum stances and reduce their

effect. B efore one can decide on the coating processes to be used to provide the

best solutions, one needs to further explain in detail the m echanism s that cause

these m aterials to fail. Three o f the m ost significant problem s that need to be

addressed are; corrosion, fatigue and erosion-corrosion.

2.2 .1 W ear

W ear can be defined as the physical rem oval o f a m aterial from a solid surface by

another solid m aterial [26], The in teraction is characterised by load and relative

m otion w here both can produce several kinds o f wear, including the m odes o f

sliding wear, im pact w ear and rolling contact w ear or any com bination o f these

types. A sum m ary o f the characteristics o f the d ifferent w ear types is shown in

F igure 1.

M ultibody im pact w ear can be further broken dow n into erosive w ear and

cavitation w ear (or cavitation erosion) [26, 27], Erosive w ear involves the cum ula

tive effects o f m any particles striking a surface.

---------------------------------------------------------------------------------------------------------------------------------------- 7Analysis ofthe Effect ofBending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadhli, Hussain Y


These particles can be solids, liquid droplets, collapsing bubbles, or solid-liquid

m ixtures called slurries [28]. Slurry is defined as a m ixture o f solid particles in

liquid, w ith a consistency as to be capable o f being pum ped like a liquid [29]. One

m ajor part o f this research is dealing w ith the m ultibody im pact w ear resulted from

je t im pingem ent fluid (slurry) tests, the effect o f w hich causes the dam age shown

in Figure 2. H ence, the follow ing section w ill discuss the theory o f erosion-

corrosion separately. A list o f A STM w ear test m ethods, organised by type o f

w ear and surface dam age, is given in Table 1.

W ear

Slidin g wear Impac w ear R olling co ntact w ear

A brasive (cutting) w ear 2 -bodyM ulti body (3 -body)

A d h esive w ear

Fretting W ear

Fatigue w ear

P olish ing w ear (chein -m ech. abrasion)

C orrosive W ear

2-body im pact w ear

M ultibody im pact w ear

— Erosion

Solids

Liquids

Gases

Slurries

Electric sparks

— Cavitation

Pure rolling contact

R olling slid in g contact

Shdmg !--------- 1>

Impact

• V * \ "\ / i \ a

R ollingoFigure 1 M ajor categories o f wear, [30].

_________________ _____________________________________ ___________________________ 8Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Table 1 A STM : W ear test m ethods grouped by form o f wear, [30],

Form of wear Designation Title Means of wear measurement

A brasive wear, 2 -body

G 56Test M ethod for A brasiveness o f Ink-Im pregnated Fabric P rinter R ibbon

Surface profiling or other m ethod

G 132Test M ethod for P in A brasion Testing

M ass loss

G 119G uide for D eterm ining Synergism betw een W ear and Corrosion

M ass loss and corrosion- related m easurem ents

Erosive w ear, cavitation fluid

G 32Test M ethod for Cavitation Erosion U sing V ibratory Apparatus

M ass loss

Erosive wear, liquid droplets

G 73Practice for L iquid Im pingem ent E rosion T esting

M ass loss

Erosive w ear, slurry G 75

Test M ethod for D eterm ination o f Slurry A brasiv ity (M iller N um ber) and Slurry A brasion Resistance R esponse o f M aterials (SA R N um ber)

M ass loss

Erosive w ear, solid particles

G 76Test M ethod for Conducting Erosion Tests by Solid Particle Im pingem ent U sing Gas Jets

M ass loss

Fretting w ear D 4170Test M ethod for Fretting W ear Protection o f Lubricating Greases

M ass loss ratio

Sliding w ear D 2266

Test M ethod for W ear Preventative C haracteristics o f L ubricating G rease (Four-Ball M ethod)

W ear scar d iam eter

Surface dam age, galling

G 98Test M ethod for Galling R esistance o f M aterials

Visual inspection, critical load for galling

Surface dam age, scoring

D 2782

Test M ethod for Extrem e-Pressure Properties o f L ubricating Fluids

"OK" value o f m axim um m ass (weight)

for load ju s t below critical scoring condition

_____________________________________________________________________________________ 9Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

CH A PIER 2: LITERATURE SURVEY

Figure 2 Industrial pum p im peller subjected to im pact (érosion-corrosion) wear,

[31].

2 . 2 . 2 C o rro sio n

C orrosion is the resu lt o f the undesirable chem ical in teraction o f a surface w ith its

environm ent [32], There are several factors that contribute to corrosion failure,

including condensation o f the containing w ater-soluble ionic species such as

chlorides and sulfites, the degree o f exposure to w ind and sun, the shape o f the

structure, the presence o f crevices, jo in ts, im properly applied w elds, vibrations,

and design stresses (Figure 3), [33]. In general, corrosion m ight be classified into

several basic categories: uniform , pitting, intergranular, crevice, and galvanic [34].

V arious types o f testing techniques can be used to investigate corrosion failures or

____________________________________________________________________ 10Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


to evaluate the resistance to corrosion o f m etals and alloys. These include

accelerated tests, electrochem ical tests and sim ulated-service tests [35, 36, 37].

Figure 3 P itting and cracking at the junction betw een w eb and flange o f a cast 316

type stainless steel im peller as a result o f flow o f sea water, [33].

2 .2 .3 E ro s io n -C o rro s io n

Erosion-corrosion is a form o f m aterial degradation th a t involves electrochem ical

corrosion and m echanical w ear processes resulting from fluid m otion [38]. For

m ost m aterials, there are critical velocities beyond w hich protective film s are

sw ept aw ay and th is accelerates corrosion. The critical velocity differs greatly from

one m aterial to another and m ay be as low as 0.6 to 0.9 m /s (2 to 3 ft/s) for soft

m etal alloys to m ore than 27 m /s (90 ft/s) for som e hard m etals such as titanium

[39]. D uring the erosion-corrosion m echanism , the total m aterial loss is m ainly



com posed o f the sum o f the separate pure electrochem ical corrosion and the

surface w ear process [40]. M oreover, the deterioration m echanism is dependent on

a w ide range o f variables that include the properties o f the erodent particles (for

exam ple kinetic energy, flux, hardness, shape and size), the targeted m aterial (for

exam ple hardness and strain hardening coefficient), and the environm ent (salinity,

tem perature, and pH ), as w ell as the com plex hydrodynam ic effect such as im pact

velocity and im pact angle [41]. In practical applications the erosion-corrosion

phenom ena can dam age surfaces in a variety o f ways. O ne o f the m ost com m on in

the oil and gas industries occurs in sub sea pum p internals, w here im peller w ear

rings and valve stem s handle rapidly flow ing or im pinging liquid stream s (Figure

2). This kind o f dam age is accentuated w hen the flow ing slurries contain highly

corrosive (for exam ple sea w ater) m ixtures and especially w hen contam inated w ith

solid particles [42], O bviously, this can lead to substantial operating breakdow n

m aintenance, and increased production costs in a w ide variety o f offshore oil and

gas industries.

R ecent studies o f erosion-corrosion have been focused on m etallic m aterials,

ranging from low -grade cast iron and carbon steel to the h igher grade o f austenitic

and duplex stainless steels and high grade n ickel alloys [42], As these m aterials

suffer from aggressive erosion-corrosion conditions, there is always a strong

incentive for alternative surface engineering options to be developed, one o f w hich

is therm al spray coatings [30, 42],

2 .2 .4 F atigue

Fatigue can be defined as the failure that occurs in structures w hen subjected to

dynam ic and fluctuating stresses having a m axim um stress value less than the

ultim ate tensile strength o f the m aterial [43], It is estim ated that 90% o f all

m etallic failures are a resu lt o f fatigue [44], The process o f fatigue usually starts

w ith the initiation o f cracks that propagated at a certain tim e and becom e long

enough to cause failure (Figure 4), [45]. H ow ever, th is m echanism o f failure is

__________________________________________________________________________________________ 12Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

C H A P T E R 2: L IT E R A T U R E S U R V E Y

catastrophic and insidious, and can occur suddenly without warning. In practice,

except for a few relatively brittle materials, ihe prediction o f the fatigue life o f a

material is complicated because fatigue life is very sensitive to small changes in

loading conditions, local stresses, and the local characteristics o f the material |46],

As it is difficult to account for these minor changes in either the dynamic stress-

prediction techniques or the fatigue-failure criteria, this results in a targe degree o f

uncertainty, inherent in analytical predictions o f fatigue life. Therefore, the

designer must rely on experience with similar parts and eventually on qualification

testing o f prototypes or production parts.

In general, factors that influence fatigue life include [47, 48]:

• Type o f loading (uniaxial, bending, torsional)

• Shape o f loading curve

• Frequency o f load cycling

■ Magnitude o f stresses

• Part size

• Fabrication method and surface roughness

• Operating temperature

• Operating atmosphere.

___________________________________________________________________________ 13A n alysis o f th e I; flee t o f B en d in g , l:a tigue . K rosinn-C orrosion . an d T en sile S tresses on H V O F C o a lin g o l'M cla lM t S urfacesA l-F:idiili. H ussain Y


Figure 4 Photograph of pump shaft failure due to cyclic fatigue, [45],

_____ _______________________ ______ _ _________________________________ 14Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


2.3 T H E R M A L S P R A Y C O A T IN G T E C H N IQ U E

2.3.1 In itia tio n o f T h erm al S p ray C oating

T herm al spraying w as first discovered and used at the state o f last century and

research in this field has progressed ever since (Figure 5). The recognised

beginning o f therm al spraying is believed to have been in 1911, w ith a flam e spray

process (m etallizing) developed by Schoop and G uenther [49], In the 1950s, the

b road application o f new refractory m aterials (m ostly re la ted to the aerospace

industry) com m enced and this saw the b irth o f the detonation gun deposition

spraying process (invented by R. M. Poorm an, H. P. Sargent, and H. Lam prey and

patented in 1955) [49]. The 1970s w itnessed the developm ent o f vacuum plasm a

spraying (VPS) and the atm ospheric p lasm a spraying (APS) [50], F inally, during

the 1980’s, w ith the developm ent o f H igh V elocity O xy-Fuel (HV OF), therm al

spraying becam e w idely recognised as an industrial technology [50].

------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 1 5Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


P00D

%>gQ .'3©-

UJ

3U0

250

M

sMa

T 200V♦*©

TJc a

High-energy p lu riu

Water-jet i tripping

appl icat ions

150

Low-energy p lu o u

Dttooation gun

Axime form* Mctco

100

50

Ballud'ibook

Sdioopinvent*procew

1

Proem/ motion

doted-loopcontrol

1900 \ m

Newmaterials/OEM

a p p lications

2000

Figure 5 T im eline developm ent o f the therm al spray industry, [49].

2 .3 .2 T h erm al S p ray C o a tin g T ypes and C harac teris tics

The therm al spray technique can be grouped into a num ber o f d ifferent processes:

flame spraying, arc spraying, p lasm a spraying, detonation gun spraying, and

H V O F spraying. The next section w ill b riefly cover each o f these processes, but

w ill focus chiefly on HVO F as this was the process used in this research.

___________________________________________________________________________________16Analysis oftlie Effect ofBending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


(1) F lam e S p ray P rocess

The flam e spray process uses a com bustible gas as a heat source to m elt the

coating m aterial (Figure 6). The coating m aterial can either be pow der or w ire

form on deposition. The heat source is a m ix o f fuel gas-oxygen flame. D ifferent

fuel gases m ay be used including acetylene (C2H 2) and propane (C3H 8) w here

acetylene produces the h ighest com bustion tem perature. The Jet gas speeds are

typically low, below 100 m /s (330 ft/s) and particle velocities around 80 m /s (260

ft/s), due to the process having relatively low pressure and low flow rate [50]. The

com bustion tem perature is usually above 2600°C (4700°F) [50]. H igh levels o f

porosity and oxidation are considered a m ajor disadvantage in the flam e spray

p rocess [51].

F igure 6 Schem atic diagram o f the flam e spray process, [51],

(2) A rc S p ray P rocess

In the arc spray process, a p a ir o f electrically conductive w ires (w hich w ill atom ise

the final deposit) are m elted by m eans o f an electric arc (Figure 7). The m olten

_____________________________________________________________________________________ 17Analysis of the Effect of Bending, Fatigue, Erosion-Coirosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


m aterial is atom ized by com pressed air and propelled tow ards the substrate

surface. E lectric arc spray coatings are norm ally denser and stronger than their

equivalent com bustion spray coatings. Low running costs, high spray rates and

efficiency m ake it a good tool for spraying large areas and at h igh production rates.

The disadvantages o f the electric arc spray process are that only electrically

conductive w ires can be sprayed and i f substrate preheating is required, a separate

heating source is needed. The m ain applications o f the arc spray process are anti

corrosion coatings o f zinc and alum inum and m achine elem ent w ork on large

com ponents [49, 50].

(3) D e to n a tio n G un P rocess

The detonation gun basically consists o f a long, w ater cooled barrel w ith inlet

valves for the gases and pow der (Figure 8). O xygen and fuel (acetylene m ost

com m only) are fed into the barrel along w ith a charge o f pow der. A spark is used

to ignite the gas m ixture and the resulting detonation heats and accelerates the

pow der to supersonic speeds dow n the barrel. A pulse o f nitrogen is used to purge

---------------------------------------------------------------------------------------------------------------------------------------- 18Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic Sui facesAl-Fadhli, Hussain Y


the barrel after each detonation. This process is repeated m any tim es a second.

The operation frequency is typically four to eight cycles per second, giving a

relatively low spray rate o f 0.3 to 0.9 kg/hr. The high kinetic energy o f the hot

pow der partic les on im pact w ith the substrate results in a bu ild up o f a very dense

and strong coating [52],

F igure 8 Schem atic diagram o f the detonation gun spray system , [52],

(4) P la sm a S p ray P rocess

In the p lasm a spray process, m aterial in the form o f pow der is in jected into a very

h igh tem perature p lasm a flam e, w here it is rapidly heated and accelerated to a high

velocity (Figure 9). P lasm a is initiated by a high current discharge, produced from

electric current, w hich causes localised ionization for the gas (argon, nitrogen,

hydrogen, or helium ). The resistance heating from the arc causes the gas to be

ionized and reach extrem e tem peratures, form ing a p lasm a [50], The tem perature

o f the gas can be as high as 15,000-20,000°C, depending on the gas used, and the

operating energy often reaches 720 M J (682,800 BTU) [51]. Therefore, plasm a

spraying has the advantage o f spraying very h igh m elting po in t m aterials such as

__________________________________________________________________________________ 19Analysis ofthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


refractory m etals like tungsten and ceram ics like zirconia, unlike com bustion

processes. Plasm a spray coatings p robably account for the w idest range o f therm al

spray coatings and applications, w hich therefore m akes this process the m ost

versatile . The disadvantages o f the p lasm a spray process are relative h igh cost and

the com plexity o f the process [49, 50],

(5) T he H ig h V e lo c ity O xy-F ue l (H V O F)

The H igh V elocity O xy-Fuel (HV OF) therm al spray process is basically the same

as the pow der flam e spray process except tha t this process has been developed to

p roduce extrem ely high spray velocities. This is achieved by using a nozzle

arrangem ent to acquire the velocities together w ith high flow rates o f gases (Figure

10). This process is relatively new com pared to other therm al spray m ethods [52],

A detailed explanation o f this process w ill be introduced in Section 2.5 as this is

the process that was used in the current work.

_______________________________________________________________________________________________________20Analysis of the Effect of Bending, Fatigue, Erosion-Con o si on, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli. Hussain Y


Figure 10 Schem atic diagram o f the HVO F spray system [52],

2.3 .3 C o m p ariso n b e tw een T herm al S p ray P rocesses

The d ifferent therm al spray processes described earlier have different

characteristics. Each o f these processes can be distinguished based on their coating

quality and the requirem ents o f the applications. In other w ords, it is difficult to

perm anently p refer any one o f these process over another. For exam ple, the only

process that could be used for h igh m elting tem peratures is p lasm a spraying.

H ow ever, this process is still considered com plicated and relatively expensive, and

has coating properties that differ from those found w hen using H V O F [52],

The je t param eters; (i) tem perature, (ii) velocity, (iii) gas flow , (iv) gas type, (v)

pow er, the partic le feed param eters; (i) partic le size (ii) tem perature, (iii) velocity,

(iv) feed rate, and deposit characteristics param eters; (i) density, (ii) bond strength,

and (iii) oxides, have a great influence on the coating properties (Table 2).

___________________________________________________________________________________21Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


R esearch is on-going to optim ise the coating end product quality o f each o f these

processes using the above m entioned param eters [49,52], In this study, the H V O F

process w as selected to deposit Inconel-625 because it produces low porosity, high

bonding strength, w ith high efficiency and sim ple process often used in repair

industry [51,53],

Table 2 C haracteristic o f therm al spray processes and coatings [49].

IN i? l i -v d r« i iv ÎMonalionA ttr ib u t i Klamc spray uAiruti gim Wire arc Air plasma Vacuum plasma

le t

Jel ifm perauue. K 351») 5500 55 « ) >25,(X)0 15.000 12.0(K)Ji-t vdnciticK, m/:■

(HA)5» KKl

( ! « ) 3ÍXII500-1200

(IMXMOCIO)>1000 0 .3300) 5IV-100 (160-300) 3ÍHMOOO

i lo o a 3.KI0I21» í) 0 U i7 0 0 H M l

G;is How, sl.m !(10-’ M Jill) 11 IVI NVA 500-3000 100-200 150-250G as lypcs O , ucciykne C H ^ C W ^ O ’ O .. acetylene Air, N „ Ar A r . f e . lU N - , A i.H c .J1 .Power input, kW ' 20 150 ,100’ N/Á 2 5 ■10-200 ■40-«TI

P a rtic le feed

Piriticle Icriipcr.ilikfciiri.w). "C n - )

2ÍQClOt5UO) 3300 (.6000) N/A > 3800(>6900) >3800 OfiyOO.1 >3K00 (x59(XV)

P a ll id i velocities, m/s (li/si

M M Ofl 1160 300) 200-1000(700-3300)

N/A 50-100(160-300)

200-800(700-2600)

200-600 (700-20001

M ateiial feed nue. g/rain

30-S0 15-50 N/A 150 2000 50-150 25-150

D ep o s it/c o a tin g

Densily range i'-i) » 5-90 >95 >95 80-95 90-95 90 -99Homi strength. M Pa

(ksi:)7- IS (1-3) 68(10 ) 82 (12 ) 10 4 0 (1 .5 -6 ) <68 « 1 0 } >6S (>10)

O sitlcs H ifh M oderate Small M oderate to hi^li M oderate tu coarse Noneto dispersed

____________________________________________________________________________________ 22Analysis ofthe Effect of Bending, Fatigue, Evosion-Covrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadlili, Hussain Y


2 .4 F E E D S T O C K M A T E R IA L S

2.4 .1 T ypes o f T herm al S p ray M ateria ls

Therm al spray m aterial types are characterised into the follow ing forms: w ires,

rods, and pow ders. W ires are m ade from either m etals or alloys and can be used

only in the arc spraying or flam e spraying processes. The w ires are produced using

form ing technique processes such as w ire drawing.

R ods are m ainly used in the flam e spraying process. The m ain disadvantage o f

using rods is the short length o f the sintered rod, w hich gets used up in a very short

tim e; therefore, to continue the process o f deposition, a new rod m ust be added to

the torch. This interruption in the process leads to a change in the coating

m icrostructure.

Pow ders are the m ost com m on m aterial used in the field o f therm al spraying. This

is m ainly due to the reliability o f their m anufacturing route and the variety o f their

chem ical com positions.

It is also im portant to recognise that com position is not the only im portant variable.

Pow der partic le shape, size distribution, hom ogeneity and m icrostructure all

depend on the m anufacturing process and these processes influence the spraying

process and coating quality [49,50], The H V O F process uses pow der as its coating

m aterial, hence this form o f m aterial, w ill be discussed further in the follow ing

section.

__________________________________________________________________________________ 23Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli. Hussain Y


2.4 .2 M eth o d s o f P o w d er P ro d u c tio n

T herm al spraying pow ders can be m anufactured according to different m ethods,

the m ethod chosen depending m ainly on the m aterial and the process to be used

during spraying. Paw low ski [50] describes in detail the d ifferent pow der types and

production m ethods, including the production o f m etallic and ceram ic pow ders.

The m ost popular pow ders that are currently in use are [51]:

• M etals (e.g. M olybdenum ) and alloys (se lf fluxing alloys such Inconel-625)

• O xide ceram ics (e.g. AI2O 3)

• C erm ets (G raphite cladded w ith N i or W C agglom erated w ith 12 w t% Co)

• Carbides (C ^C a)

The m ethod o f pow der production can be sum m arised into four groups (F igure 11):

• The atom ization m ethod

• Fusing or sintering

• Spray drying (A gglom eration)

• C ladding

The atom ization m ethod is m ainly applied to the m anufacture o f m etals and alloy

pow ders. The purity o f the atom ized pow der depends on the atm osphere inside the

m elting cham ber, the atom ization m edium , and the cooling m edium in the

atom ization tower. Pow ders produced by this process usually have a sm all internal

porosity and, due to their spherical shape, and provide excellent flow ability [51].

Fusing or sintering w as the first m ethod used to produce oxide, carbide and cerm et

pow ders [50], In this m ethod, the m aterial com pound is m elted, cast into a m ould

and crushed into pow der form. The pow der particles produced by this m ethod are

relatively dense w ith angular and blocky m orphologies (Figure 11) [49, 51],

The spray drying or agglom eration technique is the m ost versatile technique as it

perm its the agglom eration o f any kind o f m aterial. The flow ability o f spherical

_______________________________________________________________________________________________________________ 2 4Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


pow der partic les form ed using this m ethod can be better than that form ed by other

m ethods [50], This m ethod consists o f introducing a slurry containing finely

d ispersed m aterial to be agglom erated together w ith an organic b inder and w ater,

atom ized and dried in a drying cham ber and collected using a cyclone for therm al

spray use [51].

C ladding pow ders (com posite pow der) consists o f coating a m aterial w ith porous

or dense layer o f another m aterial. This process is applied m ainly w hen m aterials

need to be protected from a flam e, to enhance adhesion or to enhance flow ability

[49, 51]. The pow der Inconel-625 under investigation w hich is m ainly m ade up o f

N ickel and C hrom ium was form ed using the atom ization pow der production route.

(c) Spray diiecl

(b) Fused and Crushed(n) Gas-ntonuzed

Figure 11 D ifferen t produced pow der m orphologies, as a result o f using d ifferent

pow der production m ethods [49, 51].



2.4 .3 M eth o d s o f P o w d er C harac te risa tio n

It is m andatory to characterise the pow ders prior to applying therm al spray coating

as the pow der size distribution, in com bination w ith shape and particle surface

condition, w ill determ ine the flow behaviour o f the particles w ith in the spray gun

and the m elting in the torch. C onsequently, the end product quality o f the coating

w ill be affected. For instance, using sm all size partic les in a h igh m elting

tem perature system such as the p lasm a spray process results in pow der evaporation

w hich reduces the decom position efficiency o f the coating [51]. Conversely, using

large particle size in a m oderate flam e tem perature system such as H V O F results in

having non-m elted particles in the coating [52],

There are several m ethods o f pow der characterisation as w ell as the techniques

used, including [49, 51]:

• Particle size (M echanical sieve analysis, X -ray absorption)

• C hem ical com position (X-ray florescence, X RF spectroscopy)

• Shape o f the grain ( SEM or O ptical m icroscopy O M )

• D ensity and flow ability (A STM B 329-76)

__________________________________________________________________________________ 26Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


2.5 T H E H V O F P R O C E S S

2.5.1 In tro d u c tio n

During the last two decades, the high-velocity oxy-fuel (HVOF) process

has proved a viable technological alternative to the many conventional thermal

spray processes such as plasma [52], In principle, the spray process is similar to

the detonation gun process with the exception that HVOF operates using a

continuous, steady state process. Fuel (either kerosene, acetylene, propylene and

hydrogen) and oxygen are fed into the chamber, combustion produces a hot high

pressure flame which is forced down a nozzle, increasing its velocity (Figure 12).

The coating material, which is a powder, is fed axially into the high energy gas

stream where the expanding gas forces the particles through a nozzle at supersonic

velocity. Gas velocities have been measured in the range from 1,500 to 2,000 m/s

(4,900 to 6,500 ft/s), on the order o f up to five times the speed o f sound [54], The

Ideal Stochiometric combustion equation for propane is:

C^HS + 5 0 2 — ► 3C O 2 + 4 H 2 0 A H = - 2 2 2 0 K J / m ol C 2 H S Equation 1

The compressed air pinches and accelerates the flame and acts as a coolant for the

HVOF gun. HVOF coatings can be very dense, strong and show low residual

tensile stress or in some cases compressive stress, which enables thicker coatings

o f certain materials to be applied than that possible with the other processes [55],

The very high kinetic energy o f particles striking the substrate surface does not

require the particles to be fully molten to form high quality HVOF coatings. This

is certainly an advantage for the carbide type coatings and this is where this

process really excels. HVOF coatings are used in applications requiring the

highest density and strength not found in most other thermal spray processes. N ew

applications, previously not suitable for thermal spray coatings are becoming

viable, such as gas turbines blades, biomedical applications, curvature surfaces

(ball valves), to mention a few [56, 57].

_______________________________________________________________________________________________________________2 7Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Figure 12 Schem atic diagram o f the axial feed type H VO F spray gun used in this

research.

2 .5 .2 H V O F C o atin g S truc tu re and P roperties

It is obvious that the m echanical behaviour o f industrial m aterials/coatings depends

strongly on their m icrostructure. Several coating features determ ine the H V O F

deposit properties, including pores, splats (lam ellar), unm elted or resolidified

partic les, grains, oxide inclusions, phases, cracks, and bond interface (Figure 13).

Porosity and oxidation are tw o o f the m ost influencing param eters on H V O F

coating characterisation in w hich all rem aining param eters can be deeply related to

either one or bo th o f them [58].

_______________________________________________________________________________________________________________2 8Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


(1) P o ro sity

T he H V O F spray deposit usually contains som e level o f porosity, typically

betw een 0 and 5%. This porosity results from the h igh num ber o f unm elted or

reso lid ified partic les that becom e trapped in the coating creating poor cohesion that

leads to prem ature coating cracking, delam ination, or spalling [59], M oreover,

porosity in H V O F deposits low ers the coating hardness and contributes to poor

surface finishes, hence decreasing w ear resistance [60, 61]. Table 3 shows porosity

results for various D iam ond Jet H V O F coatings.

T able 3 C oating porosity in various D iam ond Jet H V O F coatings [62].

C oating m aterial P orosity (%)

N ickel/C hrom ium M olybdenum Base Superalloy (Inconel-625)

1

Tungsten C arbide-C obalt <0.5

C obalt B ase A lloy 1.5

C hrom ium C arbide/ N ickel C hrom ium 1

T ool Steel <1

A l20 3- 1 3 T i0 2 1.2

(2) O x ides

O xide inclusions in H VO F deposits are usually detected as dark, elongated phases

that appeared as strings in the coating cross section, parallel to the substrate.

O xides are produced w hen a heated particle interacts w ith the atm osphere or w hen

the coating surface is heated during deposition [63], HVO F coating oxides can be

m inim ized using a num ber o f m ethods: reducing the average tem perature o f the

partic le by low ering the heating capacity o f the spray je t, reducing the dw ell tim e

_____________________________________________________________________________________29Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


o f the particle by low ering the spray distance or increasing velocity, or by using the

p roper pow der particle size (30p.m ± 5 fim) as large partic les have a low er surface

area to volum e ratio [69].

O xidizedparticle

Grains

Bonding Interlace Roughenedsubstrate

Cracks

Unmeltedparticle

Figure 13 Schem atic diagram o f the HV O F coating structure show ing deposit

features [49].

______________________________________________________________________________ -_______________________________ _ 3 0Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


2.5 .3 P aram ete rs A ffec tin g th e H V O F C oating Q uality

There are m any param eters that have a direct effect on the H V O F coatings’

characterisation. These could be either in the form o f surface preparation

param eters such as grit type, b lasting conditions, grit feed rate and sam ple surface

roughness, or operational param eters like pow der feed rate, stand o ff distance, gun

pressure, w ork piece transfer speed, w ork piece surface tem perature, and

equipm ent from different m anufacturers thus producing differen t coatings structure

[65].

P revious research indicates tha t both spray distance and pow der feed rate

variations, w ith in the lim it o f m anufacturer specifications, show the highest

influence on the coating characterisation [16]. In the p lasm a spray process alone

there are m ore than one hundred param eters that all have a d irect effect on the end

p roduct quality o f the coating [16, 52, 66], F igure 14 illustrates the m ajor process

param eters that affect HVO F coating quality.

In the H V O F coating process, w here the com bustion process takes place inside the

gun at very h igh gas flow rates, a supersonic flam e speed o f up to approxim ately

2000 m /s can be achieved, leading to very high partic le speeds o f up to

approxim ately 800 m /s [67], A lthough this high kinetic energy leads to a very high

degree o f partic le deform ation producing better coating bonding and higher coating

density, the num ber o f unm elted or sem i-m olten partic les is increased during

spraying. This is m ainly due to the fact that higher partic le velocities result in

shorter dw ell tim e, so there is less tim e for particle heating. H ence, enhancing one

o f the process param eters can affect other properties o f the deposit [49, 68],

_______________________________________________________________________________________________________________ 31Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Thermo I

s p r o y s o u r c e

* Powder s i z e & shape

* Powder t h e r m a l p r o p e r t i e s i C a r r i e r gas f l o w* I n j e c t i o n ge o me t r y

u

'i- Powaer* Type o f t h e r m a l e n e r g y

* Gas f l o w* Gas c o m p o s i t i o n

* T e m p e r a t u r e /

i f Cool i n g /

- « P a r t i c l e i m p a c t e n e r g y -¥■ P a r t i c l e i m p a c t a n g l e

* S u b s t r a t e t y p e

S u b s t r a t e t e m p e r a t u r e

c * J e t e x i t v e l o c i t y & t e m p e r a t u r e

* P a r t i c l e v e l o c i t y & t e m p e r a t u r e -x P a r t i c l e t r a j e c t o r y

Figure 14 Schem atic diagram o f the H V O F spray process variables and param eters

[49].

2 .5 .4 A d v an tag es and D isad v an tag es o f the H V O F C o atin g

(1) A d v an tag es

H V O F coatings, like other therm al spray coating processes, have several overall

advantages: a w ide range o f m aterials can be deposited as a coating, low

processing cost, w ide range o f coating thickness, w ide range o f applications, and

m in im al therm al degradation to substrate. However, the H V O F process offers a

num ber o f advantages over, and alternative to, com peting processes including

higher bond strength, low er porosity, low er flam e tem perature and low er capital



cost. The HVO F therm al spray process is a uniquely d istinguished process due to

its h igh partic le velocity produced via the supersonic velocity o f the je t gun.

C onsequently , several advantages are m aintained due to this high partic le velocity,

including [49, 62]:

1. M uch shorter exposure (to tem perature) time in flight due to high particle

velocities

2. Short particle exposure tim e in am bient air once the je t and particles leave

the gun, w hich results in low er surface oxidation o f particles

3. U niform and efficient particle heating due to the high turbulence

experienced by the particles

4. L ow er ultim ate particle tem peratures com pared to other processes

Table 4 sum m arises the reasons the H VO F process produces such high quality

coatings.

Table 4, B enefits o f using H VO F coatings [62],

Coating benefit Main reasons for this benefit

H igher density (Low er porosity) H igher im pact energy

Im proved corrosion barrier Less porosity

H igher hardness ratings B etter bonding, less degradation

Im proved w ear resistance Harder, tougher coating

H igher bond and cohesive strength Im proved particle bonding

L ow er oxide content Less in flight exposure tim e to air

Few er unm elted particle content U niform particle heating

G reater chem istry and phase retention R educed tim e at h igher tem perature

T hicker coatings Less residual stress

Sm oother as sprayed surface H igher im pact energies

________________________________________________________________________________________________________________3 3Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on IIVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


(2) D isad v an tag es

H V O F coatings also have som e disadvantages, and it is im portant to understand

these so that design or repair engineers can avoid p lacing H V O F deposits in

situations w here the required perform ance is not achievable [62, 69], These

disadvantages include:

1. F lam e tem perature is relatively low, in the order o f 2,900°C (5,250°F),

m aking it difficult to spray ceram ics and refractory m etals. Since dw ell

tim e (the am ount o f tim e the pow der is in the flam e) is short, hea t transfer to

large pow der partic les m ay no t be sufficient. It requires finer pow der

partic le size and tighter tolerance on partic le size distribution than other

processes.

2. T he am ount o f heat content in the HV O F system is very high, so over

heating o f the substrate is quite likely. Therefore, extra cooling o f the

substrate is necessary, and cooling w ith liquid C O 2 is now a standard w ith

the new H V O F process [62, 69].

3. M asking (C overing o ff o f certain parts o f a com ponent to deposit only on

specified areas) o f the part is still a great problem as only m echanical

m asking is effective. I t is very difficult and tim e consum ing to design an

effective m ask for a com plex com ponent w ith areas w hich do no t require

deposition.

2.5 .5 A p p lica tio n s o f th e H V O F C oating

H V O F coatings have num erous applications that can be used in several industries

including oil and gas, aerospace, autom otive, chem ical, electronic, energy,

______________________________________________________________________________________________________________ _ 3 4Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAI-Fadhli, Hussain Y


shipbuilding, m edicine, printing, paper, and decorative. A b rie f list o f these

applications include coatings used for [49, 51, 70]:

1. D im ensional restoration and repair

2. C orrosion resistance

3. W ear prevention

4. O xidation resistance

5. Therm al insulation and control

6. A brasive activity

7. A bradable seals

8. B iom edical com patibility

9. Lubricity and low friction surface.

It is im portant to note that Inconel-625 pow der is used extensively in the (oil/gas)

repair industry due to its excellent corrosion resistance. This m aterial has both

C hrom ium and N ickel that enhance the property o f corrosion resistance and can be

easily m ade using gas atom ised process as indicated previously. A lso, it has

outstanding characteristics in term s o f low er porosity and reduced oxide content

[71]. H ow ever, as the application o f this m aterial is influenced by the substrate

type and behaviour, it needs to be thoroughly exam ined and evaluated in

aggressive environm ents and over d ifferent m etallic substrates.

The specific application considered in this w ork is the repair o f sea w ater pum p

shafts for dim ensional restoration, w ear and corrosion pro tection using H VO F

Inconel-625 therm ally sprayed coating.



2.6 R E V IE W O F P R E V IO U S S T U D IE S W IT H F O C U S O N H V O F T H E R M A L L Y S P R A Y E D IN C O N E L -6 2 5 C O A T IN G S T E S T IN G

The follow ing section gives a b rie f review o f the previous testing investigations

carried out in the field o f HVOF therm ally sprayed Inconel-625 coatings including

bending (flexural), fatigue, erosion-corrosion, tensile as w ell as residual stresses

determ ination.

2.6 .1 B en d in g T est

T he w ork that has been carried out on the three-point bending test o f HV O F

therm ally sprayed N i-based coatings is very lim ited [72-74], Y ilbas et al. [72]

investigated the effect o f laser treatm ent on HVO F sprayed Inconel-625 coatings

w hen subjected to a three-point bending test. The results indicated that the elastic

lim it o f the coating reduced after the laser treatm ent. A lso, the origin o f crack sites

w as in the coating/substrate interface. D uring the four-point bending test, the

stress p rofile o f coating show ed sim ilar behaviour for the sam e pow der sprayed

using tw o differen t coating system s, nam ely HVO F and P lasm a [73]. It was also

observed tha t the four-point bending stress profile for the coated sam ple was

sim ilar to the tensile test profile. The four-point bending test w as u tilised by

H jornhede et al. [74] to com pare the adhesion strength o f H V O F, arc spray and

laser cladding applied over (F elC r0 .5M o) base m aterial. C oating delam ination

occurred due to the initial form ation o f transverse cracks in the coating post

bending.

2 .6 .2 F a tig u e T est

One im portant application o f H V O F coatings is their use in dynam ic com ponents

in oil and gas industrial equipm ent [75], These coatings are w idely used to restore

w orn or undersized high strength steel parts and com ponents that w ere once

__________________________________________________________________________________ 36Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


previously repaired using the conventional arc w elding technique, hence the

m icrostructure o f the substrate has changed [76], In service, these com ponents are

generally subjected to severe cyclic loading in an aggressive environm ent [77],

C onsequently, investigations into corrosion and fatigue properties o f HVOF

coating over p la in or w elded surfaces are now o f the u tm ost im portance [78],

C onsiderable research studies have been carried out to investigate various affecting

param eters on the fatigue properties o f HVO F coated surfaces [79]. H ow ever, the

study o f HV O F coating over w elded surfaces has to date been very lim ited [80-97].

The evaluation o f the fatigue strength o f a m aterial is strongly affected by substrate

type and sprayed coating [80]. H ow ever, the fatigue strength o f a coated specim en

was no t as greatly affected as that o f a non-coated specim en w hen both w ere

evaluated under the sam e testing conditions in highly corrosive environm ents [81,

82]. Previous w ork indicates that N ickel based alloy coatings such as

N iC rM oA lFe and Colom onoy 88 show n good resistance to fatigue w hen tested in

various corrosive m edium s [16]; however, varying the spraying system param eters,

adjusted these resulting fatigue properties [83, 84],

The H V O F coating process produces significantly less p it form ations w hen

com pared to o ther techniques such as A tm ospheric Plasm a Spraying (A PS) and

H igh F requency Pulse D etonation (HFPD), hence the process produces coatings

w ith high fatigue strengths [85, 86], Proper control o f the coating thickness

significantly enhances the coating fatigue strength properties and it is possible to

com bat interfacial delam ination by selecting an appropriate coating thickness [87].

This m ay be related to the variation o f residual stresses w ith the coating thickness

[88], The fatigue behaviour o f thinner coatings (200 pm ) has been found to be

better than th icker ones (450 pm), as the possibility o f crack initiation w ithin

th icker coatings is higher, as well as the fact that com pressive residual stresses are

h igher in th icker coatings [89, 62].

_____________________________________________________________________ ______________ 37Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


A lthough surface preparation enhances the coating-substrate bond, the fatigue

strength o f substrate m ay decrease due to b lasting [90]. D uring the grit b lasting

process, partic les that are retained in the substrate act as stress concentrators and

enhance the nucleation o f fatigue cracks and give a rise to a decrease in the fatigue

strength o f the b lasted m aterial [91-94], V arious studies have indicated that there

is a relationship betw een com pressive residual stress on both the coating bonding

and its fatigue strength [95-96], In general, fatigue failure can m ostly be attributed

to m icro and m acro-cracking o f either the coating m aterial or the coating substrate

interface, w hich is also a resu lt o f the attenuation o f com pressive residual stresses

that arise during the quenching process during deposition [97],

2 .6 .3 E ro s io n -C o rro s io n T est

O ne o f the m ost p rom inent concerns in the oil and gas production industry is the

behaviour o f m aterials in an aggressive corrosive environm ent w ith the presence o f

suspended sand partic les w hich contribute to corrosion, erosion and overall w ear o f

the surface. Surface m etal loss o f internal com ponents in ro tating and stationary

equipm ent can be attributed to pure electrochem ical corrosion, pure m echanical

erosion and the synergistic interaction o f the tw o [98], This effect plays a m ajor

role in equipm ent reliability and m aintenance costs, especially i f the equipm ent

operates in off-shore industries w here the internal com ponents are exposed to

corrosive environm ents. The erosion-corrosion, w ear and therm al resistance o f

m etallic alloys used in a w ide range o f such equipm ent have been show n to be

im proved in an effective and econom ical w ay by m eans o f H igh V elocity Oxy-

Fuel (HV OF) coatings [99, 100],

The rap id developm ent o f the HV O F coating process has increased the dem and

for various types o f coating m aterials including Inconel-625. This m aterial has

outstanding properties w hich m ake it superior w hen applied as a H VO F coating in

order to p reven t corrosion [101-103]. H ow ever, the coating m aterial can

significantly deteriorate w hen subjected to the im pact o f dry or solid erodent

_____________________________________________________________________________________38Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


particles w here the erodent partic le size and the degree o f fluid im pingem ent are

the principal affecting param eters [104, 105], O ther param eters, such as the

influence o f tim e, solid loading, im pingem ent angle, tem perature and particle size

and velocity, as w ell as the erosion-corrosion m echanism , m ust also be taken into

consideration [106, 107].

W hen com paring the influence o f dry and slurry erosion on HVOF coatings, it was

show n that erosion rates in dry particles im pact w ere about three orders o f

m agnitude higher than those in slurry system s [108], This difference probably

reflects the real erodent target im pact velocities w hich are m itigated in the slurry

test by the w ater m edium [109]. V arious studies on the erosion-corrosion o f

HV O F sprayed N i-based coatings indicated that m aterial loss rates, considered to

be low during solid-free im pingem ent, increased significantly w hen solid partic les

w ere added [110]. However, m ost o f these tests w ere carried out on single

substrate type and only a lim ited num ber o f tests considered the com bination o f

H V O F erosion-corrosion o f coatings applied over different substrates [111].

M oreover, the testing o f coatings applied over w elded surfaces has no t yet been

fully understood. In order to select appropriate coating m aterials for fluid

m achinery, bo th coating erosion-corrosion behaviour and the substrate m aterial

response needs to b e clarified.

2 .6 .4 T en sile B o n d in g T est

B ond strength is considered to be one o f the m ost influential properties o f HV O F

coatings. This is because it represents a m easure o f the HVO F coating’s ability to

adhere to the substrate. Practical experience as w ell as theoretical analysis

perform ed on d ifferent H VO F system s and m aterials have show n that there are

several factors w hich influence coating-substrate bonding. These factors include

h ea t treatm ent and surface preparation prior to deposition, the properties o f the

pow ders used for spraying, the contact tem perature betw een the particles o f the

coating and substrate, and the physical properties o f the specim en m aterial and

_______________________________________________________________________________________________________________ 3 9Analysis ofthe Effect ofBending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadhli, Hussain Y


coatings. R eports indicate that the fuel/oxygen ratio and spray distance have the

m ost significant effect on coating-substrate bonding [112, 113]. W hen optim ising

these param eters for the deposition o f N iC rB Si therm ally sprayed pow der using the

H V O F process, a bond strength o f 41-52 M Pa w as achieved [114].

Further investigations indicated that the tensile bonding strength o f N i-C r coatings

decreases w hen the coatings w ere subjected to very h igh tem peratures, such as

using coatings in bo iler com ponents. For instance, w hen 80N i-20Cr was tested at

750°C using the adhesion test as per A STM C 633, the tensile bond strength

decreased from 68 M Pa to 56 M Pa [115]. M oreover, the bonding strength o f

N iC rB Si increased as the substrate surface roughness increased, w here m axim um

bonding was achieved w hen a surface roughness o f R a 5.8(xm w as used [116]. It

was also show n that adding W C -C o to N iC rB Si during spraying changes the

adhesion m echanism from m echanical interlocking only to chem ical bonding and

m echanical interlocking [117, 118], Li et. al. [119] found that w hen a bond coat

o f about 50(j.m thickness w as introduced betw een the coating (N iCrBSi) and the

substrate (m ild steel), the adhesive strength o f the coating increased from 40 M Pa

to 56 M Pa. M oreover, w hen a W C -C o interlayer bond coat w as applied to

N iC rB Si-40w t% (W C -C o) com posite coating, the bond strength then increased to

65 M Pa.

The tensile bonding strength o f HVO F sprayed N i-based coatings can be

significantly affected by the state o f the deposited particles, w here significant

m elting o f the sprayed particle did not contribute to an increase o f bonding strength

[120]. The deposition o f partially m elted large particles presented a dense

m icrostm cture and yielded strengths o f m ore than 76 M Pa [121]. H ow ever, W ang

et al. [122] states tha t spray particle tem perature, velocity and m om entum do not

correlate directly w ith bond strength. As coating corrosion resistance affects

bonding, HV O F sprayed N i-based coatings show ed better corrosion resistance than

stainless steel coatings w hen both w ere tested under highly corrosive environm ents

[123],

_______________________________________________________________________________________________________________4 0Analysis of the Effect of Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


It is im portant to note that the tensile test for coating to substrate adhesion is often

used as a determ ination o f the po in t o f failure o f coatings. H ow ever, for m ost

coating m aterials, including Inconel-625 pow ders, several factors can affect the

m ethodology o f the tensile test and the resultant behaviour o f the coating. Firstly,

in practical applications, the coating does not experience a pure tensile stress.

There are in fact several stresses, including shear, bending, tensile and

com pressive, all acting on the deposit. Secondly, during the A STM tensile test, the

coating/adhesive interface or in terior coating m aterial often fails, instead o f the

coating/substrate interface [124],

2 .6 .5 R esid u a l S tress

There are m ain ly tw o m echanism s o f residual stress developm ent in therm ally

sprayed coatings, quenching stress and cooling stress.

(1) Q uenching Stress

A ccording to Paw low ski [50], as m any as 5 to 15 lam ellae ex ist in a single pass o f

spray. A s the lam ellae solidify they contract, bu t are constrained by each other,

and by the substrate, thus generating high tensile stresses in the individual lam ellae

as shown in F igure 15. Tensile quenching stress is unavoidable and m ay be

estim ated b y the follow ing [62]:

ery = a c ( T m - T s ) E c E q u a t i o n 2

W here o q is the quenching stress (Pa), E c is the elastic m odulus o f the coating (Pa),

a c is the coefficient o f therm al expansion o f the coating (/°C), T m is the m elting

tem perature o f individual lam ella (°C), and Ts is the tem perature o f the substrate

(°C).

_________________________________________________________________ 41Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Figure 15 Schem atic o f quenching stresses in the individual lam ellae, [62],

(2) C ooling Stress

W hen deposition is eased or interrupted, cooling stresses generate, m ainly due to

the therm al expansion co-efficient m ism atch betw een the substrate and the coating

m aterial. I f the coating contracts to a greater extent than the substrate (ac > a s), a

tensile stress is generated in the coating [62]. This m ay lead to adhesion loss and

cracking o f the coating. I f the coefficients are equal, then no cooling stress will

develop. I f the coating contracts by a sm aller am ount than the substrate (a c < a s),

the resulting cooling stress w ill be com pressive as show n in F igure 16 [62]. The

cooling stress can be estim ated using the follow ing equation [62, 64, 124]:



<T -Ec{Tf - T R\ a c - a , )

1 + 2 E XE T

Equation 3

W here a c is the cooling stress (Pa), E c is the Y oung’s m odulus o f the coating (Pa),

E s is the Y oung’s m odulus o f the substrate (Pa), a c is the coefficient o f therm al

expansion o f the coating (/°C), a s is the coefficient o f therm al expansion o f the

substrate (/°C), Tc is the tem perature o f the coating (°C), T s is the tem perature o f

the substrate (°C), Tf is the deposition tem perature (°C), and TR is the room

tem perature (°C). H ow ever, this equation does not take into account; conduction

and the tem perature gradient across the sample.

Subs+ra+e c o n t r a c ts t o a g re a te r e x te n t than s u b s t r a te c r e a t in g com press ive c o o l in g

L ame I 1 a

\s t r e s s

___ ~— — C— - ■■ ■■ &! f

J

-

S u b s t ra te

E n l arge v ie w

Figure 16 Schem atic o f cooling stresses, [62],

_____________________________________________ ______________________________ _ 43Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


2.7 S P R A Y P R O C E S S M O D E L L IN G

C onsiderable research studies have been carried out on the developm ent o f m odels

capable o f predicting w hat happens at various stages during and after the therm al

spraying process. Previous research earned out in the M PRC (M aterials

P rocessing R esearch Centre) included sim ulation o f heat transfer though a

therm ally sprayed sam ple, analysis o f residual stresses due to deflection loading

using the A NSY S Finite E lem ent A nalysis (FEA) Package [62], O ther w ork has

considered the sim ulation o f m ulti pow der/gas flow through a m ulti pow der HVO F

(H igh V elocity O xy-Fuel) feed device using the FLO TR A N CFD (Com putational

F lu id D ynam ics) [69].

S im ulation results obtained from the FEA dem onstrate the capability for residual

stresses prediction in therm ally sprayed layer com posite and stresses that arises

during the production processes o f coated w ork pieces such as engine liners [125].

G hafouri et al. [126] use FE A for the prediction o f residual stresses at the

coating/substrate interface and upon variation in coating thickness. R esults

show ed that high stresses w ere present at the coating/substrate interface and

stresses increased w ith an increase in coating thickness and tem perature.

FE A can be used effectively to explain the behaviour o f coating during the

deposition process based on the variation o f som e influencing factors including

coating thickness, coating m odulus. M oreover, residual stresses have been shown

to be influenced by the deposit fracture toughness [127].

It is im portant to note that all previous w orks involved the sim ulation o f HVO F

coating applied on one substrate type. H ow ever, the present w ork includes the

sim ulation and m odelling o f HVOF coating applied on different substrate m aterial

types. This w ork also includes the analysis o f residual stresses during the

deposition process for therm ally sprayed sam ple applied over d ifferent substrate

m aterials.

_______________________________________________________________________________________________________________ 4 4Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metal lie SurfacesAl-Fadhli, Hussain Y

CHAPTER 3: EXPERIMENTAL EQUIPMENTA N D P R O C E D U R E S

3EXPERIMENTAL EQUIPMENT AND PROCEDURES

3.1 IN T R O D U C T IO N

The present study investigates the bending, fatigue, erosion-corrosion, and tensile

characteristics o f HVOF therm ally sprayed Inconel-625 coating m aterial w hich in

som e cases w ere subjected to aqueous static corrosion. Investigations o f each test

w ere carried out w hen the coating m aterial w as applied over three d ifferent

m etallic surfaces: (a) p lain stainless steel (SS), (b) spot-w elded stainless steel (SW -

SS), and (c) a com posite surface o f stainless steel and carbon steel w elded together

(C-SS-CS).

In this chapter, the HVOF therm al spray facility used to deposit the Inconel-625

coating is described follow ed by a description o f the pow der m orphology, size and

characteristics used in this research. Inform ation is provided on the fabrication o f

the w orkpiece fabrication, surface preparation, and w elding procedure undertaken.

This is follow ed by a full description o f the coating process and characterisation

techniques used to analyse the coated specim en. The experim ental procedures o f

each m echanical test is described according to their associated standard. The

chapter concludes w ith a description o f the residual stresses calculation procedure

and the finite elem ent m ethod used in this work.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 4 5Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

CHAPTER 3: EXPERIMENTAL EQUIPMENTAND PROCEDURES

3.2 H V O F T H E R M A L S P R A Y IN G S Y S T E M

A Sulzer M etco M anual H ybrid D iam ond Jet (using propylene fuel gas) w ith gas

supply unit, D J9H - H and held gun body, 9M P-D J pow der feed rate control, and

D JFW flow m eter w ere used to produce the Inconel-625 coating (Figure 17). The

air, fuel and oxygen w ere m aintained at pressures 7.2, 6.2 and 10.3 Bar,

respectively. The w ork specim en w as m aintained at 475°C (by the flam e o f the

gun) and rotated at 250 rev/m in using a D ean Sm ith B-5 lathe m achine. The

com bustion process took place w ith in the gun. This has a prom inent influence on

the coating integrity, hence m ore details about the D J9H gun are discussed in the

nex t section.

F igure 17 H igh velocity oxygen fuel system.

____________________________________________________________________________ 46Analysis of the Effect of Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

C H A P T E R 3: EXPERIMENTAL EQUIPMENTAND PROCEDURES

3.2.1 D J9H G un

The Sulzer Metco HVOF spraying process uses oxygen and fuel gas to produce a

high velocity gas stream in the gun nozzle. This gas stream, when ignited exits the

gun, as a luminous white, supersonic flame containing diamond-shape shock

waves hence the name “Diamond Jet”, (Figure 18). The combination o f high fuel

gas and oxygen flow rates and high pressure leads to the generation o f this

supersonic flame. The gun can accommodate different fuel gases including

acetylene (C2H2), ethylene (C2H4), hydrogen (H2), propylene (C iH 6), and propane

(C3H 8). Each gas provides different combustion temperatures. The latter was used

in this research as this is the fuel used in repair applications and has the ideal

combustion for the powder material used Inconel-625 [49], The powder coating

material is fed into the high energy gas stream where the expansion gas, forces the

particles through a series o f nozzles at supersonic velocities. The high kinetic

energy o f the powder produces well bonded coatings with high bond strength and

low porosity [52], It should be noted that HVOF systems from different

manufactures produce quite different coating properties. Harvey et al. [128]

described different HVOF systems and the important differences between them.

Significant details making them quite different from each other are; powders feed

position in the spraying gun, gas flow rates and choice o f fuel gas. In some

systems the powder is fed into the combustion zone, in other systems it is fed into

the exhaust barrel. Feeding powder into the combustion chamber as used in this

research, m aximises the heat transfer between the flame and the powder particles.

This increase in the heat transfer, in parallel with spray distance and powder feed

rate, plays a major roll in controlling coating oxidation. Hybrid cooled spraying

gun specifications are also available, where instead o f air (as used in this research),

cold water can be used to keep the gun cool around the outside o f the nozzles to

prevent deformation o f the internal section o f the gun (Figure 18).

_______________________________________________________________________________________________________________ 4 7Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-FadhIi, Hussain Y

C H A P T E R 3: E X P E R IM E N T A L E Q U IP M E N TAND PROCEDURES

Victor i n l o t i f h y b r id syslem is t o t>e used

Figure 18 Schematic diagram o f DJ9H H VO F spraying gun, [49].



3.3 P O W D E R M A T E R IA L

The pow der under investigation w as Inconel-625 pow der (or know n as Sulzer

M etco D iam alloy 1005). Its com m ercial com position is p resented in Table 7.

F igure 19 show the m orphology o f the atom ized powder. These particles had

spherical shape and a size range o f 40 ± 11 ^m . The Inconel-625 pow der is

typically used as a coating in corrosive and erosive applications like seawater

environm ents [16, 129],

T able 5 C hem ical Com position o f D iam alloy 1005 Pow der [16].

Powder Material Chemical Composition %

N i Cr M o Fe Co

Inconel-625 66.5 21.5 8.5 3 0.5

F igure 19 Scanning Electron M icroscope view o f the m anufactured Inconel-625

powder.

_______________________________________________________________________________________________________49Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


3.4 E X P E R IM E N T A L M A T R IX

To evaluate the effect o f each substrate type and service m edium , nine sets o f test

specim en w ere proposed. These sets o f specim ens selected represented a

com bination o f three substrate types: (i) P lain stainless steel (SS), (ii) Spot-w elded

stainless steel (SW -SS), (iii) C om posite surface o f Stainless Steel and C arbon Steel

w elded together (C-SS-CS); in three environm ental service m edium s: (i) as coated,

(ii) as coated and subjected to aqueous static corrosion for tw o w eeks, (iii) as

coated and subjected to aqueous static corrosion for four w eeks. T w enty eight

specified specim ens o f each set type w ere m anufactured, w here seven test sam ples

w ere dedicated for each o f the four m ain m echanical tests (Bending, Fatigue,

E rosion-C orrosion, and Tensile), w ith the proviso that som e o f the specim ens w ere

kept as back up in the case o f the need to repeat tests. Som e o f the testing

standards required less than seven sam ples to analyse and obtain reliable data. For

exam ple, three sam ples o f each case for the erosion-corrosion test and four sam ples

for the tensile test w ere adequate for reliable analysis. C oating characterisation

was then perform ed post m echanical testing on each test sam ple, (total o f 252

sam ples w ere fabricated o f w hich 189 w ere analysed) (Table 6).



Table 6, Show s the test m atrix used in this research.

Po w d er S ubstra te Serv ice M ediumN u m b er o f Sam ples

B end ing Fatigue E-C * T ensile

As coated 4 7 6 4

Plain stainless steel (SS)

As coated and subjected to aqueous static corrosion for two weeks

4 7 6 4

As coated and subjected to aqueous static corrosion for four weeks

4 7 6 4

As-coated 4 7 6 4

Inconel-625(Diamalloy

Spot-welded stainless steel (SW-SS)


4 7 6 4

1005) As coated and subjected to aqueous static corrosion for four weeks

4 7 6 4

Composite surface o f Stainless Steel and Carbon Steel welded together (C-SS- CS)

As coated 4 7 6 4


4 7 6 4

As coated and subjected to aqueous static corrosion for four weeks

4 7 6 4

* E -C = E rosion -con osion test

_______________________________________________________________________________________________________ 51Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


3.5 W O R K P IE C E

3.5.1 S u b stra te Sam ples

A ISI 4140 carbon steel and A ISI 316 stainless steel (150x40x4 m m 3) substrates

w ere used. These can be considered the m ost com m on m aterials used in the oil

and gas industries and are norm ally subjected to highly corrosive and erosive

environm ents. Three different sam ple types w ere used: (a) p lain stainless steel

(SS), (b) spot w elded stainless steel (SW -SS), and (c) com posite surface o f

stainless steel and carbon steel w elded together (C-SS-CS), (Figure 20).

F igure 20 Photograph o f the substrate types u sed in the test.

_______________________________________________________________________________________________________________ 5 2Analysis ofthe Effect ofBending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadhli, Hussain Y


3.5 .2 W eld in g P ro ced u re

Sam ples fabricated w ere degreased in benzene and w ashed w ith de-ionized w ater

p rior to w elding process. Sam ples were spot-w elded using Gas T ungsten Arc

W eld ing (G TA W ) m anual process. The G TA W process consisted o f a pow er

source, m anual gas tungsten arc torch and filling w ire as show n in Figure 21. The

stainless steel 309L electrode used w as 2.4 m m in diam eter and can be applied on

any austenitic stainless steel. D uring the w elding process the arc w as p laced at a

d istance o f 2 to 3 m m from the w ork piece. The w eld spot was 3 m m in depth and

5 m m in w idth. Pure argon was used as shielding gas at flow rate o f 71/m in. The

specifications o f the rod w elding process used in this research are show n in Table

7.

F igure 21 Schem atic diagram o f the G TA W w elding process.

_______________________________________________________________________________________________________ 53Analysis of the Effect of Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Table 7, Specification o f the rod w elding used in the experim ent tests.

B A SE M ETA L FIL L E R M ETA L H EA T TR EA TM EN TSubstra teM aterialS p ecifica tion

T h ick n essR ange

A W S N o (C lass)

Size M axD epositT h ick n ess

P re -H eatT em p.

In ter-p ass P W H T T em p.

C arb o n Steel to A usten itic S ta in less Steel

1 .6 - 1 2 m m SS 309L 2.4 m m 1 2 m m 100°C I 7 7 OQ N one

3.5 .3 S u rface P rep a ra tio n

The sam ples w ere subsequently cleaned, p reheated to 450°C in a furnace for three

hours to takeou t any contam ination in the substrate and then grit b lasted w ith

AI2O 3 particles 20 m esh (83 |im ), m anufactured by K. C. A brasive Com pany w ith

the follow ing chem ical com position AI2O3 (96-97% ), Ti02(2.75%), Si02 (0 .61%),

Fe203 (0.54% ), [16]. B lasting was conducted at a pressure o f 550 kPa, (80 Psi) and

at a distance o f 38 cm , (15 inches) norm al to the surface o f the specim en, (Figure

22). The surface roughness o f the substrate ranged from 5 to 6 (Ra, jim).



3.5 .4 S p ray ing

The spraying parameters used lo produce the HVOF coatings used in this work are

given in Table 8 . Firstly, special samples holder were fabricated to allow the

spraying o f 24 samples all at once, (Figure 23). Later, it appeared that this holder

was unable to maintain a spraying angle o f 90° on each sample as the assembly

rotated which is recommended to avoid splats overlapping that result in increased

coating porosity and thereby decreasing coating bonding (Figure 24), [49], Hence,

another holder was fabricated to maintain 90° spraying angle for each samples

(Figure 25 (a)) which also resembles the real life application o f repairing shafts

(Figure 25 (b)). The coating thickness was Fixed at 400 ± 10 |im , whatever the

working conditions, (Figure 26).

________________________________________________________________________ 55Analysis o f lh e litlect o f Bending, Fatigue, lirosion-l orrosion. imd Tensile Stresses on I IVOF C oaling o f M tiallic SurfacesAl-Fadhli, Hussain Y


Table 8 Process parameters o f HVOF thermal spray.

Spray l’ara meters

OxygenPressure

(kPa)

Fuelpressure

(kPa)

Air pressure (kPa)

Powder feed rate (m3/h)

Spray rate (kg/h)

Spraydistance

(m)

Vitine 1034 620 724 0.81 6.35 0.28

____________________________________________________________________________________ _ 56A nul) sis o f 'ih e ElTect o f B cnd iiig , Fatigue, E rosion -C orro sion , and T cn sile S tresses on H V O F C orn ing o f M eta llic S iirlhccsA l-Fîitlhli, l ÎLisstnn V


Figure 24 Schematic diagram showing the effect o f spraying angle.

------------------------------------------------------------------------------------------------------------------------------- 57A n alysis o f the E ffec t o f B en d in g , F aligue , E rosion -C orro sion . ant) T e n sile S tresses o n IIV O F C o a lin g o f M eta llic S urfacesA l-I:adtili- H ussain Y


! *Sample

Spray at 90“ ’

Figure 25 Photograph o f the modified test samples holder, for spraying

angles o f 90 (a) and shaft requiring repair (b).

Figure 26 SEM image showing the coating thickness.

___________________________ ________ :_________ 58Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f M etallic SurfacesAl-Fadhli, Hussain Y


3.6 C O A T IN G C H A R A C T E R IS A T IO N T E C H N IQ U E S

C oating characterization is a critical step in the assessm ent o f therm al spray

coating quality. It is im portant to note that the preparation o f the test specim ens

prior to characterisation has a prom inent influence o f the coating m icroscopy.

D ifferent grinding or polishing techniques can result in a com pletely d ifferent

coating m icrostructures [130], The follow ing sections w ill explain the

m etallographic preparation process used, and the m icroscopic analysis techniques

such as; O ptical, SEM and EDS used to characterise the deposit.

3.6.1 M eta llo g rap h ic P rep a ra tio n

M etallography is the technique taking im age for topographical or m icrostructural

features on prepared surfaces o f m aterials [131]. The properties and perform ance

o f coating m aterials m ay be controlled by studying the m icrostructures using

m etallography. M etallographic specim en preparation is an im portant tool for the

characterisation o f therm ally sprayed coatings in term s o f revealing the

coating/substrate interface, coating layer m orphology, and location. The

m etallographical process can be divided into four different stage; sectioning,

m ounting, grinding and polishing.

(1) Sectioning

Sectioning is a necessary step before the m ounting to reduce the size o f test

sam ples or to explore its hidden cross-section. The test sam ples were sectioned

using an ISO M ET low speed diam ond cutting saw m anufactured by BEUH LER.

A n O il-base lubricant w as used as the cooling m edium . A low speed cutting saw

w as used to m inim ise stresses and distortion o f the sectioned surface, w hich can

often be introduced by cutting processes.

_______________________________________________________________________________________________________________ 5 9Analysis of the Effect of Bending, Fatigue, El os io 11-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


(2) Mounting

There are two basic techniques o f mounting; cold and hot. Cold mounting requires

very simple equipment consisting o f a cylindrical ring which serves as a mould and

a flat piece o f glass which serves as the base o f the mould. The sample is placed on

the glass within the cylinder and the mixture poured in and allowed to set Cold

mounting takes about 40 minutes to complete. However, hot mounting uses both

heat and pressure to thermoset resin. This method, in some cases, is not preferred

particularly i f the sample has a loose layer on its surface. However, in the thermal

spray deposits case the coating has adequate adherence to its substrate, hence it is

not effected by the hot mounting. In hot-mounting the sample is surrounded by an

organic polymeric powder (thermoset resin) which melts under the influence o f

heat (up to 200°C). Pressure is also applied by a piston, ensuring a high quality

mould free o f porosity and with intimate contact between the sample and the

polymer. Therefore, in the current research, the hot mounting process SIMPLIMET

2000 made by BUEHLER was used to mount different types o f coatings and

powder samples.

(3) Grinding

The prepared mount (sample contained in a thermoset mould) was then subjected

to a series o f grinding steps, with the aim o f producing a surface which would

reveal its micro structure characteristics. The BUEHLER MOTOPOL unit was

used to grind samples, automatically. Automation o f grinding and polishing stage

was essential as it eliminates operational error such as applied load and rotation per

minute [64], The grinding was divided into two stages; plane grinding and fine

grinding.

(a) Planar Grinding

_______________________________________________________________________________________________________________ 6 0Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

C H A P FER 3: EXPERIMENTAL EQUIPMENTAND PROCEDURES

The purpose o f using plane grinding was to remove the damage due to the

sectioning stages and to produce a flat plane for subsequent grinding. A P60 grit

(very coarse) Silicon Carbide paper was initially used to remove all damage

without creating new defects in the coating. This paper lasted a duration o f around

60sec, after which the abrading particles were washed away. Planar grinding was

performed in a contact pressure o f 100 to 150 kPa and using water as a lubricant

for 60sec.

(b) Intermediate Grinding

The intermediate grinding step was concerned with the removing the damage

caused by the planar grinding. Silicon carbide paper was used to fine grind the

samples, but this procedure went from a coarse P200 grit up to fine paper P I200

grit. Each abrasive size can be used for five minutes in turn, starting from P200 grit

and finishing with P I200 grit. In the current work, only the P240 and P600 grit

papers were used to fine grind the different coating samples. A s this found to

adequately reveal the characteristics received from this research. Each paper was

used for duration o f 4 minutes. Intermediate grinding was performed at a contact

pressure o f 100 to 150 kPa and using water as a lubricant.

(4) Polishing

The objective o f the polishing stage was to remove the abrasion-damage layer that

may appear as scratches or as slip/twin/shear damage beneath the surface caused

by the intermediate grinding stage. Optical microscopy requires that a specimen

must be both flat and highly reflective [132], There were two steps involved in the

polishing procedure; diamond polishing and fine oxide particle polishing. Diamond

abrasives are very effective during the polishing stage and considered as an

adequate tool to prepare a coating for general microscopic examination. The most

com mon diamond particle sizes are 6 , 3 and 1 micron. Normally each abrasive size

can be used for 5 minutes in turn, starting from 6 micron finishing by 1 micron

[64], In the current research, Aluminium oxide polish with 0.05 micron o f particle

__________________________________________________________________________________ 61Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadlili, Hussain Y

CH A P T E R 3: EXPERIMENTAL EQUIPMENTAND PROCEDURES

size w as used for a duration o f 3 m inutes to prepare the coated specim ens.

Polish ing w as perform ed at a contact pressure o f 100 to 150 kP a and using w ater as

a lubricant.

3 .6 .2 M ic ro sco p ic A nalysis T echn iques

(1) Scanning E lectron M icroscope (SEM )

Scanning electron m icroscopy (SEM ) is a m ethod for h igh-resolution im aging o f

surfaces. The SEM uses electrons to form im age, m uch as a light m icroscope uses

v isib le light to reflect an image. The advantages o f SEM over optical m icroscopy

include greater m agnification (up to 300,000X ) and m uch greater depth o f field

producing h igh im age resolution [133]. A n incident electron beam is raster-

scanned across the sam ple's surface, and the resulting electrons em itted from the

sam ple are collected to form an im age o f the surface trough a cathode tube. The

im age is typically obtained using secondary electrons to get the best resolution o f

fine surface topographical features. A lternatively, im aging w ith backscattered

electrons gives contrasts based on atom ic num ber to resolve m icroscopic

com position variations, as w ell as, topographical inform ation [133], Q ualitative

and quantitative chem ical analysis inform ation can also be obtained using an

energy dispersive x-ray spectrom eter, a devise attached to the SEM , w ill be

explained in the nex t section. The scanning electron m icroscope used in the current

study, w as a Philips X L-30 SEM equipped w ith Energy D ispersive Spectrom eter

(ED S), M odel DX-4 b y ED A X , (Figure 27). Polished/as sprayed sam ples were

p laced in the SEM vacuum cham ber, and analysed to reveal coating characteristics

such as; voids, non-m elted particles, coating thickness, and oxides inclusions.



Figure 27 Photograph of the ESEM (Philips XL-30),

(2) Energy Dispersive X-Ray Spectroscopy (EDS)

Energy dispersive x-ray spectroscopy (EDS) is defined as an analytical method

used in the determination of elemental chemical composition, and normally

performed in conjunction with a scamiing electron microscope (SEM) [134], The

technique utilises x-rays that are emitted from the sample during the bombardment

by the electron beam to characterise the elemental composition of the analysed

material. Features as small as about l(j.m can be analysed [134]. When the test

sample is bombarded by the electron beam, electrons are ejected from the atoms

along the sample's surface. A resulting electron vacancy is filled by an electron

from a higher shell, and an x-ray is emitted from the sample to balance the energy

difference between the two electrons. The EDS x-ray detector measures the

number of emitted x-rays versus their energy. The energy of the x-ray is

characteristic of the element from which the x-ray was emitted.

____________________________________________________________________________ 63Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


A spectrum of the energy versus relative counts of the detected x-rays can be

obtained and evaluated for qualitative and quantitative determinations of the

elements present in the sampled volume [135], It is important to note that EDS

analysis of light elements such as oxygen is at best semi-quantitative and the

relative accuracy depend on the amount present in the sample. For examples, the

errors in the analysis for concentrations below 5 % can be as high as ± 50%

[136,137],

In the current research a scanning electron microscope with an EDS X-ray

instrument, provided by EDAX company was used to investigate elemental

analysis of the HVOF thermally sprayed Inconel-625 coatings. The analysis was

used to measure the amount of element contained in the steel samples such as;

Oxygen, Aluminium, Silicon, Nickel, Chromium, and Molybdenum.

_______________________________________________________________________________________________________ 64Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


3.7 MECHANICAL TESTS PREPARATION

The mechanical tests carried out in this research include: bending, fatigue, jet

impingement (erosion-corrosion), and tensile tests. These tests were prepared

according to the international standards; ASTM D 790, ASTM E-739, ASTM G

73, and ASTM E8 for bending, fatigue, je t impingement, and tensile tests

respectively. 70% of the tested samples were subjected to static corrosion test prior

to subjecting these samples to each of the mentioned mechanical tests. The

following sections will explain the preparation of each test in detail.

3.7.1 Static Corrosion Test

The static corrosion test were carried out in an aqueous environment simulating the

sea water in the gulf region. The electrolytic solution used in the immersion tank

was 0.1N H2SO4 + 0.05 NaCl. The specimens prior to testing were degreased in

benzene and washed with de-ionized water. The immersion tank facilitated the

circulation of the electrolytic solution and it had capacity of accommodating ten

flat samples at a time. Consequently, specimens were hung into the immersion

tank and checked with the conditions on a regular base. The specimens were left

in the immersion tank for two and four weeks duration.

3.7.2 Bending (Flexural) Test

A bending test method was used to determine the flexural properties of the HVOF

coated specimens and how these properties varied with specimen substrate type

and atmospheric conditions. The bending test was carried out using the 3-point

bending test according to the ASTM D 790 international standard [138]. Test

samples of rectangular cross-section was placed on two supports and loaded by

means of a loading nose midway between the supports, (Figure 28). The

specimens were allowed to deflect up to maximum displacement of 20 mm for all

------------------------------------------------------------------------------------------------------------------------------------------------------------- 65Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


types of coated samples. The test was carried out using an INSTRON 300

instrument equipped with a deflection measuring device where the error in the load

measuring system was ± 1 % of the maximum applied load. The loading nose had

a cylindrical surface with a radii of 5mm (0.197 in), to avoid excessive indentation,

or failure due to stress concentration directly under the loading nose. The test was

carried out at room temperature 23 ± 2°C and 50 +/- 5% relative humidity. To

ensure reliability a total of five samples for each test condition were tested. Figure

29 shows the shape and conditions of the sample after a typical 3-point bending

test.

F

▼

Figure 28 Schematic diagram of the 3-point bending test.

_______________________________________________________________________________________________________________ 6 6Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


As c oated

Subjected to two weeks coiiusion

Subjected to four weeks corrosion

Figure 29 Photograph of the three types of samples used in the bending test.

3.7.3 Fatigue Test Preparation

An INSTRON 8501 material testing system was used to obtain the fatigue data as

it had a capacity o f + 100 kN, (Figure 30). The displacement and percentage strain

was set to ± 75 mm, ± 25% respectively. The fatigue tests were conducted using

the Wave Maker FLAPS Software initially ramped to a mean load level and then a

sinusoidal loading with a frequency o f 20 Hz at a stress ratio of R

(R=cimin/amax)= 0.1 of the specified value of am. The maximum cyclic stress

ranged was set to approximately 70% to 90% of the tensile strength of the substrate

material. All tests were carried out in air at room temperature 23 ± 2 °C and

conducted according to the Statistical Analysis of linearized Stress-Life (S-N)

Fatigue Data found in ASTM E-739-91 [139]. A total of 42 samples were

employed for evaluating the corrosion-fatigue properties o f the coated specimens,

(21 plain stainless steel substrate and 21 spot welded stainless steel substrate,

____________________________________________________________________ _______________________________ 67Analysis ofthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


which was within the range of specimens required in S-N testing for reliability data

according to the ASTM E-739 standard (12-24) [139]. Three samples were tested

at each stress level which ranged from (150 to 450 MPa), where each sample

represent the test condition (as coated, as coated and subjected to corrosion for two

weeks period and as coated and subjected to corrosion for four weeks period). The

test samples geometry employed is shown in Figure 31.

Figure 30 Photograph of the INSTRON 8501 fatigue/tensile testing machine.

Computer conùol

YLoading ramps

__ ____________________________________________________________________________________________________________ 6 8Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Figure 31 Geometry of the spot welded fatigue/tensile samples used in each test.

3.7.4 Jet Impingement (Erosion-Corrosion)

The erosion corrosion experiment was carried out using a je t impingement rig

(Figure 32). This consisted of a flow loop through which natural seawater (pH 8.3)

or sand slurry was circulated. The system was designed to test three samples at the

same time. The je t angle was set at 90° to the sample surface and the fluid

impinged at a velocity of 40 m/s, at a water slurry temperature of 50°C and a

pressure of 14 Bar and 17 Bar after heating to simulate the erosion-corrosion effect

on the coating at extreme summer temperatures. The specimens were rinsed in

water, dried and weighed with a precision balance which had an accuracy of ± 0.1

_______________________ ________________________________________________ 69Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


mg. Special Teflon holders were fabricated to ensure the prevention of any

eventual leakage. The methodology of the test was to consider two conditions: (i)

different substrate types tested every 20 hours, up to 100 hours, (ii) different

substrate types tested in 500 hour runs. Figure 33 shows the three forms of sample

used in the jet-impingement test.

The test was performed according to ASTM G73 “Practice for Liquid

Impingement Erosion Testing” [140]. To improve the chance of reliability, a total

of six samples for each test condition were tested.

Figure 32 Schematic representation of the erosion-corrosion testing rig.

_______________________________________________________________________________________________________ 7 0Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAJ-Fadh 1 i, Hussain Y


Figure 33 Photograph of the three forms of samples types used in the

jet-impingement test.

3.7.5 Tensile Testing

The aim of conducting the tensile test was to evaluate the tensile strength of the

different substrate materials coated with HVOF thermally sprayed Inconel-625

powder and subjected to different corrosive environments. The tests were

conducted using the INSTRON 8501 hydraulic testing machine. Statics tests were

conducted to acquire stress strain plots for all test specimens, considering the

variations produced by the processes shown in Table 6. The tests were performed

under load controlled at a displacement rate of 0.08 mm/sec, largely as per ASTM

E 8, "Standard Method of Tension Testing of Metallic Materials" [141], The test

specimen employed for determining the stress strain behaviour of the coated

materials were of rectangular geometry and not dog-bone shape. The reason for

that was because the spot welded samples were assured to fail in the mid-section

region of the sample due to the deterioration in the material properties because of

________________________________________________________________________________________________________71Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metal lie SurfacesAl-Fadhli, Hussain Y


(lie heal affected zone resulting from the weld. A nother reason w as to ensure the

coating w ould fail before the substrate, m aking it unnecessary to reduce ihe cross

section area by dog-boning to force the failure to occu r in ihe specim en m id

section, To effectively evaluate Ihe coating perform ance, both stainless stee! and

carbon steel uncoated m aterials were also tested. The grips were placed 25 mm

from each end o f ihe sam ple in size o f (150x40x4 m m 3) and the test was carried

out at room tem perature.

______________________________________________________________________ 72Analysis ol'lhe lifted of Bunding, Fatigue, Lirosion-Corrosion, and Tensile Slresscs on HVOF Coating o f Metallic SurlaccsAl-l adhli. Hussain Y


3.8 MEASUREMENT OF RESIDUAL STRESS

Residual stresses in HVOF coatings has been recognised as one of their most

important characteristics. Coating deformations such as spalling or cracking can

result from residual stress variations between the coating and substrate [142],

Moreover, several coating performance indicators including bonding strength,

thermal resistance, erosion-corrosion, fatigue, and bending characteristics are

strongly influenced by the nature of thermal stresses [143], There are several

methods used to measure residual stresses including hole drilling method, X-ray

diffraction method, and deflection method (Almen) [143-147]. X-ray diffraction

and the Hole drilling methods were found to be the most used methods in recent

time [148], Similarly, Clyne derived a simpler analytical method to determine the

residual stresses variations across the coating and substrate [149], In this study, the

prediction of residual stresses of coatings applied over different metallic substrates

were determined using two methods; (i) Clyne’s analytical method, and (ii)

Modelling using ANSYS Finite Element Analysis.

3.8.1 Clyne’s Analytical Method

Residual stress determination using Clyne’s analytical method is a quick and

reliable method compared with other methods such as the X-ray diffraction and

hole drilling method [150], In this method, a simple coating system (just two

layers), can be considered. It is useful to consider the situation of a two layered

system in terms of misfit strains, that is, relative differences between the stress free

dimensions of the two layers [151]. Tsui and Clyne [152] used an analytical

method, which considered a pair of plates bonded together with a misfit strain

As in the x-direction as shown in Figure 34. The stress distribution through the

coating thickness, stress at the coating-substrate interface, as well as at the bottom

of the substrate can be analytically predicted using Clyne’s method [153]. The

stress distribution was derived by Clyne [153], using the following equations:

[Stress at the top of the deposit]

__________________________________________________ ___________________________________________________ _ 73Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


= - A e - K (ll - S )

[Stress at the BoLtom o f the deposit!

ö-s |,.=ü = - A e E„ H E.¡} E ',, + H E i j

- E , k 5

[Stress at the Top o f the Substrate]

a.' E . li E Ì

\>-H = A£l l 3

J i Ej +HE] )- E ( k {H + 5)

[Stress at the Bottom o f the Substrate]

o-,Lp = A*x hE '„ + i M t J

- E , KÔ

W here A e , E r and E \ , k (C urvature) are given as,

A e = {as ~ a r )A T

E ' = - * * -' O ' O

*(1 - 0

K =6E, ,Es (h + H)kHA£

E l h* + 4 E jE , ft*H + 6 E,t Es i? H 2 + 4 Etl EJi l i 1 + E;H*

Equation 4

Equation 5

Equation 6

Equation 7

Equation 8

Equation 9

Equation 10

Equation 1 I

______________________________________________________________________ 74Analysis of the lifTect o f Bending. r:aiiy,uc, Erosi on-Corrosion. and Tensile Stresses on HVOF Coating of Metallic SurfacesAM'ndhii, Hussain Y


Where

a c - co-efficient of thermal expansions of the coating (K)"1

a s = co-efficient of thermal expansions of the substrate (K)"1

vc = poison’s ratio of the coating

vs = poison’s ratio of the substrate

AT = temperature difference between the substrate and coating (K)

5 = overall deflection of the beam (m)

k = the curvature of the beam (given as 1/R, R is bending radius of thesample, m"1)

Stokes [62] found this method to be very effective in measuring residual stress in

WC-Co deposits; hence the method was used in the present study. In this method a

sample of substrate size (40mm wide x 150 mm long x 4 mm thick) and coating

thickness of 0.4mm was considered. The coating material was applied over three

different metallic surfaces: (a) plain stainless steel (SS), (b) spot-welded stainless

steel (SW-SS), and (c) a composite surface of stainless steel and carbon steel

welded together (C-SS-CS). Following deposition, the distributed stresses on the

coating and the different metallic substrates were deduced by measuring the

resulting deflection of the samples and using Equations 4 to 7.

Equation 2 was used to calculate the quenching stress in the deposit [62], and the

stress generated in the coating due to thermal expansion mismatch between the

coating and substrate can be estimated using Equation 3. Residual stress due to

deposition can be estimated by adding both quenching and cooling stresses. It is

important to note that these equations were used to estimate the overall residual

stresses in the coating during the deposition and cooling processes and it was not

possible to use these to determine the residual stress at specific points like

equations (4-7). This is because determining the residual stress at specific points is

dependent of the coating and substrate thickness.

_______________________________________________________________________________________________________ 7 5Analysis o f the Effect o f Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli. Hussain Y


Figure 34 Schem atic description o f the generation o f curvature in a bi-m aterial

plate as a result o f m isfit strain, adapted from [151 j.

_______________________________________________________________________ _ 76Analysis o f the Effect o f (lending. Fatigue, Erosion-Conosion, and Tensile Stresses on I lVOt; Coaling o f Metallic SurfacesAl-Fadhli, Hussain V


Table 9 Material properties of the coating and the different metallic substrates usedin the modeling. [16, 49],

Material(Bulk)

E (N/mm^) a (K"l) *k (W/m.K) V Bulk Yield (MPa)

C.S (AISI) 4140

205 x 103 13.7x 10-6 42.7 x 10-3 0.3 675

SS 316 193 x 103 15.9 x 10 6 16.3 x 10-3 0.3 290

SS 309 200X 103 15 X 10-6 15.9 X10-3 0.3 280

Inconel-625 207 x 103 12.8 xlO -6 9.8 xlO -3 0.31 483

*k heie represent conductivity required in the ANSYS model.

3.8.2 M odeling U sing A N SY S Finite E lem ent A nalysis

Two of the samples involved welding, this resulted in inhomogeneity of the

substrate, Clyne’s method can not be used in this situation as it only deals with

plain substrates. Hence, prediction the residual stress of the coating over different

metallic substrates require another method of modelling. Equation 2 and 3 were

used to calculate the variation of residual stresses due to substrate type change.

These results were then compared to the model results.

During the deposition of Inconel-625 coating over metallic substrate, stresses due

to the quenching of the lamella and cooling of the coating generated a moment at

the end of the sample, causing a deflection, (Figure 35). As the simulation of both

quenching and cooling in one system is quite difficult, the method of simulation

used in this research relies on the deflection of the sample post-spraying, as

proposed by Stokes [62],

____________________________________________________________________________ 77Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

CHAPTER 3: EXPERIMEN TAL EQUIPMENTAND PROCEDURES

Figure 35 C lyne’s m ethod used to determ ine distributed stress in therm al spray

coatings, adapted from [62].

Tw o approxim ate m ethods were approached to dem onstrate the num erical

form ulation o f this system. The first m ethod was to apply a known displacem ent

(deflection) to the sam ple while both ends were locked, (Figure 36). T his would

generate a known deflection that was found from the curvature in equation 11.

This deflection would cause an internal stresses in the deposit and the substrate

(m om ent m odel).

______________________________________________________________________ 78Analysis ofthe Effect o f Bending, Fatigue, Riosion-Cormsion, ami Tensile Stresses on 11VOF Coating o f Metallic SurfacesAUFndhli, ¡Hussain Y


The second method (Thermal model) was to apply a series of temperature loads (as

produced by the convective thermal gradient across the sample) to the coated

sample until the desired deflection was achieved (that found using equation 11).

To correctly model the residual stresses in deposit, Stokes [62] found that if one

used the stress at the top of the coating and the bottom of the substrate from the

moment model and the stress at the bottom of the coating and the top of the

substrate (interface) from the thermal model this would provide results similar to

that found experimentally, Figure 37 [62], This would then give a predictive

model of residual stresses distributed across the deposit.

______________________________________________________________________ 79Analysis o f the Effect o f Bending, Fatigue, Erosion-Con osion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-FadVili, Hussain Y


Figure 37 Stress distribution in the coating during deposition process.

This work considered the developm ent o f residual stresses during the spraying

process. The presence o f a weld in the base material generated a residual stresses

as show n in Figure 38. However, the effect o f this residual stresses was reduced

during the surface preparation (shot peening) o f the sam ple prior to spraying. Heat

treatm ent in the oven for three hours at 450°C before and after spraying also

reduced the effect o f residual stresses generation in welded substrates [154],

Therefore, any stresses arising in the sam ples were purely due to therm al spraying.

_________________________________________________________ 80Analysis ol thc I-fleet of Iiending. Fatigue. Frosioii-Cortosion. and Tensile Stresses on ttVOF Coating of Metallic SurfacesAl-Fadtili, Hussain Y


Figure 38 Schem atic o f residual stresses developed in w elding jo in t [154],

________________________________________________________________________ 81Amitysis ol'the HITect o f Bending. I: aligne, firosinn-Conosion, and Tensile Stresses on I IVOt: Coaling ofMetallic '¡uriaccsAl-l-'a<lhli, Hussain Y

CHAPTER 4: RESULTS A N D DISCUSSION

4RESULTS AND DISCUSSION

4.1 IN TR O D U C TIO N

The present study experim entally analysed the m echanical properties o f the

Inconel-625 coated m aterial, through coating characterisation and m echanical

testing. The m aterial’s resistance to several m echanical tests - including bending,

fatigue, erosion-corrosion, and tensile testing, w here the coating m aterial are

applied over plain, w elded and com posite substrate m aterials was investigated to

determ ine the effect o f possible failures o f this m aterial when subjected to sever

corrosive and erosive environm ents com bined with d ifferent m echnical loadings

during operation. Furtherm ore, residual stresses evaluation using m odelling and

analytical m ethods w ere used to predict the influence o f residual stress w hen this

coating was applied over different m etallic substrates.

________________________________________________________________________ 82Analysis o f the I: fleet o f Bending, I'alignc, Erosion-('orrosion, and Tensile Stresses on IIVOP Coaling of Metallic SurfacesAl-l-'ndhli. Hussain Y

CHAPTER 4: RESULTS AND DISCUSSION

4.2 C O A TIN G A N D SU B STR A T E C H A R A C TE R ISA TIO N

Detailed characterisation for the Inconel-625 coating material applied over

different metallic substrates was performed prior to mechanical testing to compare

the characteristics of the present coating with manufacturer specifications and

ensure that the coating under testing was produced at its best quality.

Figure 39 shows a typical cross section of a coated sample where the Inconel-625

deposit (A) is approximately 400|j,m in thickness deposited onto the substrate (B).

The coating porosity (measured microscopically) ranged from 1% to 3% with the

noticeable presence of oxide formation around the resolidified splats. The lamella

structure can be observed in the coating and some randomly distributed cavitations

at the interface between the coating and the substrate are also evident. Figure 40

shows a magnified view of the area depicted as (A) in Figure 39, illustrating that

such a deposit contains large semi-molten powders (C), which presumably were

formed due to one or all of the following: (i) the non-uniformity of the powder

manufactured - in some cases several particles are attached together (as shown in

Figure 19), and (ii) the high cooling rate of splats during inflight time. Moreover,

the melting of such joint particles is more difficult than that of small spherical

particles when one considers the energy and particle thermal exposure time (dwell

time) during the HVOF process.

______________________________________________________________________ _ 83Analysis o f the Effect o f Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Tiff

I b b e

B

&«

'4 iw m

Aco.V Spot Hagn

Ï1 5.0 W / 4 .0 2 5 5xDel W D Exp

BSE foa 2 2loo \m

0 .0 Toit Sample 1 : Inco 6 2 5 Costino

Figure 39 Typical cross section of the coated sample

I

• v V

o IN * *♦ >>

it

/ » \ ’• : >* : . 4 •

4 . . *_ iLy -M. •' "S '■» '■ *“ iD • *V

VSvV r

" - il

.V S po t Ma^n 16 .0 kV 5 u 2 0 0 0k

Det W D

BSE 9 .9

■i£ > \ •,i’'

- \ - < TI----------------1 10 pm0.2 Ton Sample No: 3

Figure 40 Magnified view of the area depicted as A in Fig. 39.

_______________________________________________________________________________________________________ 84Analysis of the Effect of Bending, Faligue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


The EDS analysis was conducted on three selected points across the deposit to

determine its approximate local chemical composition (wt%) and the variation of

oxygen presence in the coating along with thickness. Figure 41 (a) shows the EDS

spectrum of the particle identified as (C) in Figure 40 and indicates the presence of

large peaks of Ni, Cr, and Mo, which are the three main elements of the Inconel-

625 powder. However, Figure 41 (b) shows the EDS spectrum of the particle

identified as (D) in Figure 40. The EDS analysis showed that the oxide formed in

the coating contained mainly the Cr203 phase. This is mainly due to the high

temperature of the splats and presence of air molecules captured in voids between

the non-melted or semi-molten particles and the next bonded splats. It was also

observed from the EDS analysis that other types of oxides were present at the

interface between the coating and the substrate, namely and SÍO2 ( from the AI2O3,

TÍO2, SÍO2, Fe203 grit blast) as shown by Figure 41 (c). This is because some grit

blasting material remained after the surface preparation process. Coating porosity,

microhardness and bond strength, and variation of thickness were as follows:

porosity ranged from 2-3%, microhardness ranged from 350 to 450 (Vicker), and

tensile bonding measured using ASTM C 633 ranged from 700 to 850 Bar,

comparable with earlier observations [16].

________________________________________________________________________ _ 85Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Wt<X A t* 5620 470118S2 1777129 2238

Fe 619 544Mo 9.24 5.10Mn 126 055Si 1 i00 1.75

Mu

«

Element WtW At%Ct 3005 15.77 cr0 2234 2887Al 219 138C 1934 4638

o Ft 358 109Cr 1.14 037Mo 10.55 500Hi 10B1 3.14M»

III lit Ml «Il S II III Ml III IIIft)_______________________________________ (çl

Figure 41 (a) EDS spectrum of the particle identified as C in Figure 40, indicating high Ni, Cr, and Mo peaks, (b) EDS spectrum of the dark inclusions as D in Figure 40, indicating presence of Q 2O3. (c) EDS spectrum of interface between coating and substrate as b in Figure 39, indicating presence of SiC>2.

Figure 42 shows the X-ray linescan of the coatings cross section at 2000X

magnification. The linescan represents 128 consecutive analysis points on an

arbitrary line A_________ B across the coating. At each o f the 128 analysis points,

the concentrations o f the six determined elements (Ni, Cr, Fe, Mo, Nb, and O)

were measured. The data was then superimposed onto the original image as a

colour-coded scatter plot for each element. The plots show variations in elemental

concentrations as the line traversed from bright to gray to dark areas across the

image. It was noted that the coating had a reasonably uniform composition along

the line with no major variations in coating composition.

_______________________________________________________________________________ 86Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

Element Wt%Si 2331 1409O 35 74 37 92Al 219 138C 31.47 44.47Fe 3.58 109Cr 1.14 037Mn 083 0 18Hi 174 050

CHAPTRR a - r e s u it s a n d d is c u s s io n

Sample 2A:Line Scan Analysis of lnconel-625 Coating over StainlessSteel-Weld Metal

00 20 40 60 80 100 120

128 Data P o in ts o v er 2.35mm D istance

Figure 42 X-Ray line scan analysis of lnconel-625 coating.

X-ray maps showing the distribution of O, Mo, Cr, Fe and Ni across the width of the

coating, can be seen in Figure 43. The backscattered electron (BSE) image of the

coating is shown in the first box on top left (a) of Figure 43. In this image a sharp

interface can be observed between the coating on the left and the Fe-substrate on the

right. The intensity o f each colour reflects the concentration of the element

represented by that colour. For example, high concentrations was represented by

bright colours, whereas the absence of that particular element was represented by

some electronic noise in a black background. Thus, Ni, Cr, Mo, the main

constituents of the coating, show up as bright colours in their respective X-ray maps.

In contrast, the low intensity of oxygen (yellow) in the oxygen map suggest that only

a minor presence of oxygen existed in the coating (Figure 43 (b)). Also, as expected

the Fe (red colour) was low in the coating but it can be readily seen in the Fe-

substrate to the right of Figure 43 (e).

________________________________________________________________________ 87Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


làSJf ri.

Labi I----1 lOOum (a)153x kV:15 T i l t : 0

O K 153x

- h 1 OOum kV: 15 T i l t : 0

kV: 15 T i l t kV : 15 T i l t

Figure 43 X-ray-maps of lnconel-625 coating over stainless steel surface, (a) sample cross section, (b) Oxygen (O) . (c) Molybdenum (Mo), (d) Chromium (Cr). (e) Iron (Fe). (f) Nickel (Ni).

_________________________________________________________ 88Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Figure 44 shows another X-ray linescan of the coating cross section at 2000X

magnification. This linescan also represents 128 consecutive analysis points on an

arbitrary line A_________ B across the composite substrate (stainless steel and

weld). At each of the 128 analysis points (3-5|im electron spot) on the line

concentrations of seven elements (Fe, Cr, Ni, Mo, Si, Mn and C) were measured. It

was observed that the substrate had a change in composition as the line

A__________ B shifted from stainless steel to the heat affected zone (HAZ) and

then to the weld. This variation was due to the fact that both materials are stainless

steel but of different types 316, and 309. Figure 45 shows a typical cross-section

of the Inconel-625 coating over plain and welded stainless substrate pre testing.

The figure shows the uniformity of the coating distribution over both surfaces. The

heat effected zone between the weld and the stainless steel (Zone D in Figure 45) is

protected by the coating layer to minimize corrosion effect.

________________________________________________________________________ 89Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


o>®3

20

Sample 2A: Line Scan Analysis of lnconel-625 coated Carbon Steel/W eld Metal Boundary

«ft. •

\ ■ A A / VW , - • **- ** - • - ** ÉL ■ ■ - ■ — - Ù ^ r ■ .......... " . . ■ " ? “

Stainless Steel ‘ •*, Weld Metal

• * ™ •^ t » * •

I

40 100 120128 Data Points over 2.35mm Distance r 500 m

C K----- SiK

MoL----- CrK

MnK----- FeH----- NiK

Figure 44 Line scan analysis o f stainless steel and weld substarte.

50 n20

_______________________________________________________________________ _ 90Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


. J

A n

Coating

C

Weld Metal

D

— - V* >- , • ■

BStainless Steel

• s

■ A cc .V Spot Magn l l 5.0 kV 4.0 50x

Del WD Exp 1---------------------------1 500 putBSE 11.1 1485 0.2 Torr #2A: CS-Weld-SS: 1% Sand

III---------------------------------------------------------------=----------------------------------------------------------------- 1

Figure 45 Cross section of sample before test: (A) Inconel-625 coating over stainless steel and weld., (B) Stainless Steel substrate, (C) Weld., (D) The heat affected zone.

_________________________________ _________________________________ 91Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


4.3 CORROSION BEHAVIOR OF SPRAYED INCONEL-625 APPLIED OYER DIFFERENT METALLIC SURFACE PRIOR TO TESTING

All samples which were subjected to corrosive environment for two or four weeks

duration showed a good resistance to static corrosion and no coating failure was

observed prior to bending, fatigue, erosion-corrosion and tensile tests. Figure 46

shows a composite sample stainless steel and carbon steel welded sheets together

(C-SS-CS) after three point bending test. It should be noted that the sample was

subjected to static corrosion in ambient aqueous solution for four weeks duration.

It can be observed from the figure that the sample portion of a carbon steel

substrate is more susceptible to corrosion as compared to other portion of the

sample that has only stainless steel substrate.

Figure 46 Photograph of composite stainless steel and carbon steel (C-SS-CS) after three point bending test and subjected to corrosion in ambient aqueous solution

prior to test.

_____________________________ __________________________________________________________________________92Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadlili, Hussain Y


Figure 47 shows SEM micrograph of coating cross-section before and after the

static coiTOsion tests. In general, the coating presented a lamellar structure, which

corresponds to melting of splats before impinging onto the workpiece surface.

Moreover, in some regions, partially melted particles corresponding to the

microstructure of the stacking of the particles in elliptical or near-spherical shapes

are observed. The oxygen content in the splats is found to be low. This can be seen

from Table 10, in which the elemental analysis obtained from EDS are given. This

indicates that even for well melted particles, oxide formed during in flight was

mainly distributed over the surface. It should be noted that the oxidation of the

powder in a flame occurs around the particle surface. The oxygen content in the

particles increases with time due to oxygen diffusion into the powder. Moreover,

after the particles flies off the temperature zone of the flame, oxidation of the

particles becomes slower. Also, if the powder particle size becomes large, the

oxygen content increases. Small pores unevenly distributed in the coating cross

section are observed. No microcrack was observed in the coating cross-section.

Moreover, no corrosion products at the coating surface or at interface between

coating and base substrate material is observed extensively. However, coating

material is corroded locally. This may be because of the inter-corroded pores and

splats boundaries, the electrolyte could reach the substrate and it then corrodes the

material. It appears that the deterioration of the coating begin along the splat

boundaries and corrodes the material nearby.

The cavitations in the interface between the coating and base material accelerate

the corrosion of the coating as well as base material, i.e. as a results of the inter

corroded porosity (crevice corrosion), the electrolyte may penetrate the coating and

attack the substrate material, even tough the coating is resistant to the corrosive

environment. Consequently, the substrate material can be corroded (crevice

corrosion) beneath the coating lowering the mechanical properties at the interface.

__________________________________________________ _______________________ 93Analysis o f the Effect o f Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y

f HAFTER 4: RESULTS AND DISCUSSION

Figure 47 (a) C oating cross-seclion before the corrosion test.

____________________ _________________________________________________________________ 94Aimlysis o f the Effect o f Bending, Kaliguu, Erosion-Corrosion, and Tensile Stresses on 11 VOI1 Coating ofMcinllic SurfacesAl-Sradlffl Mussain Y


Figure 47 (b) Coating cross-section after the corrosion test.

It was found that the HAZ corroded easily once the corrosive solution penetrated

into the coating (Figure 48). This indicates that Inconel-625 coating prevent

corrosion of these different metallic substrates and minimizes the effect of

corrosion on joint of two composite materials.

_______________________________________________________________________________________________________ 9 5Analysis ofthe Effect of Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadlili, Hussain Y


*

Figure 48 SEM showing close view of the heat affected zone (HAZ).

Table 10 Elemental composition of coating (%wt).

0 Al Cr Fe Ni Mo 0

At surface 5.8 1.2 22.1 6.7 58.8 5.3 5.8

At coating cross-section

2.3 1.4 22.5 3.2 64.2 6.3 2.3

_______________________________________________________________________________________________________ 96Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


4.4 BENDING BEHAVIOUR OF HVOF THERMALLY SPRAYED INCONEL-625 COATINGS ON DIFFERENT METALLIC SURFACES

4.4.1 Effect of Substrate Variation on Coating Performance Due to Bending Testing

(1) Plain Stainless Steel Sample (SS)

Figure 49 shows the stress strain curve obtained from the three-point bending tests

for plain stainless steel (SS) substrates tested under four conditions; i) surface un

coated (SS-NC-AR), ii) surface as coated (SS-AR), iii) surface as coated and

subjected to aqueous static corrosion test for two weeks (SS-2WK), iv) surface as

coated and subjected to aqueous static corrosion test for four weeks (SS-4WK). It

should be noted that the three-point bending tests were carried out at constant

strain rate and the bending tests were terminated when the coating failed. The

elastic behaviour was almost the same for all workpieces, provided that the elastic

limit for the un-coated was less than that corresponding to coated workpieces.

However, as the bending load increased, all workpieces behave similarly. When

the stress strain characteristics for all workpieces were compared, the stress of

work pieces with coating was higher than that corresponding to the un-coated

workpiece for a known strain, even if where the coating was subjected to abrasive

corrosive medium for four weeks. This indicate that Inconel-625 powder coating

enhance the corrosion resistance of stainless steel workpiece, and increase the

workpiece resistance to bending. However, as the period of exposure to corrosive

environment increased, coating resistance to corrosion decreased.

------------------------------------------------------------------------------------------------------------------------------------------------------------- 97Analysis o fthe Effect ofBending, Fatigue, Eiosion-Conosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadlili, Hussain Y


SS-NC-AR SS-AR SS-2WK SS-4WK

Strain (%)

Figure 49 Stress versus strain characteristics of the workpieces during the 3 point

bending tests for stainless steel workpiece (SS).

(2) Spot welded Stainless Steel Sample (SW-SS)

It should be noted here that the corrosion resistance of the spot welded stainless

steel (SW-SS) coated workpiece significantly decrease compared to the plain

stainless steel (SS) ones. The SS ones peaked at 470 —> 550 MPa where as the

spot welded ones peaked from 525 —> 625 MPa. No major variation between the

two weeks and the four weeks exposed samples (Figure 50). Obviously, the length

o f time did not have an effect, it was the corrosion media that reduce the coating

capabilities. This is may be due to the presence of weld where the variation in

material composition and presence of HAZ decreases corrosion resistance.

____________________________________________________________________________________ 98Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coaling o f Metallic SurfacesAi-Fadhli, Hussain Y


SW-SS-AR — SW-SS-2WK SW-SS-4WK

70U -_ __ _ " — ■- - - • — ——-.... - ___

600-

500 "

400

L, I

00 1 2 3 4 5 6

Strain (%)


bending tests for spot welded stainless steel workpiece (SW-SS).

(3) Composite Carbon Steel and Stainless Steel Sample (C-SS-CS)

Figure 51 shows the stress strain curve characteristics obtained from the three-

point bending tests for the composite stainless steel and carbon steel workpiece (C-

SS-CS). It was found that the coating enhanced the workpiece resistance to

bending by small factor o f approximately 20 MPa. However, when the workpiece

was subjected to static corrosion, the coated surface exhibited less resistance to

bending (reducing by up to lOOMPa) , even less than the un-coated surface. The

presence of carbon steel in the substrate obviously was effected by the corrosive

media, but this media had more of an effect over time on the carbon steel

compared to plain steel and spot welded steel.

________________________________________________________________________ 99Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


However, comparing the composite stainless steel subjected to corrosion for four

weeks to the same treatment applied to the plain stainless steel sample both were

able to sustain the same stress level ~ 480MPa. However, the deterioration was

much higher for the composite substrate (~ 120MPa)compared to plain (~ 30MPa)

or spot welded samples (~ lOOMPa).


bending tests.

4.4.2 Deposit Characterisation Post Bending Testing

Deposit characterisation post corrosion and bending test showed that the brittleness

of the coating triggered crack formation in the coating. Since the coating size was

about 400 |im, it is possible that the maximum of both normal and shear stresses

took place at base metal-coating interface. However, the crack initiation at the

interface relieved the stress levels in this region which caused spalling or buckling

of the coating. A tensile shear deformation occurs in the coating due to direction

of the bending. The crack initiated at the coating surface and propagated through

the coating thickness Figure 52 (a).

_________________________________________________________________________________100Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


When the crack surface was examined closely plastic deformation and

delamination around the coating edge was observed due to the action of shear

stress. Moreover, the internal stresses possibly created local stress concentrations,

especially at defect locations around the coating base material interface. Such

defects have a marked effect on the failure mechanism of the coating and the stress

concentration at these defects are often factors higher than the mean internal

stresses [155], As soon as the local internal stress for crack propagation is reached,

the entire coating fails in this region, which can be observed from Figure 52 (b).

Figure 52 (a) Close view of coating cross-section

________________________________________________________________________ 101Analysis of the Effect o f Bending, Fatigue, Eros ion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Figure 52 (b), Coating cross-section after the 3-point bending test.

Figure 53 shows SEM micrographs of the coating post the three-point bending

tests. The multi cracking o f the surface can be observed. The crack extends

almost straight along the coating surface. This indicates that the coating does not

conform to the plastic deformation occurring in the substrate material as it is a

brittle material. The small crack spacing indicates that shear lag separation in the

coating took place. In this case, crack regions in the coating dissipate energy

through shear deformation. This was particularly true for the workpieces subjected

to the corrosive environments.

-------------------------------------------------------------------------------------------------------------------------- 102Analysis o fthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Figure 53 (b) Close view of crack sites on the surface post bending.

---------------------------------------------------------------------------------------------------------------------------------------- 103Analysis ofthe Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadlili, TIussain Y


4.5 FATIGUE BEHAVIOUR OF HVOF THERMALLY SPRAYED INCONEL-625 COATINGS ON DIFFERENT METALLIC SURFACES

4.5.1 Effect of Substrate Variation on Coating Performance Due to Fatigue Testing

The S-N plot for the axial fatigue tests of the coating over plain, spot- welded and

composite surfaces are presented in Figures 54, 55 and 56, respectively. All of the

figures illustrate the results obtained at the three different conditions, un-coated,

as-coated, as-coated and subjected to two weeks of corrosion, and as-coated and

subjected to four weeks of corrosion. Coating the sample did enhance the fatigue

behavior of the samples. It was observed in all cases that the fatigue strength

decreased as the period of coating exposure to corrosive environment increased

and more reduction in fatigue strength took place at samples exposed to the four

week corrosion period. In fact after 4 weeks the material behave similar to that of

an uncoated samples in all cases. This may be associated with low bonding

strength of the coating at high corrosive environments. The spot welded samples

behave well compared to the plain samples in terms of fatigues strength. However,

for the composite stainless steel and carbon steel surface C-SS-CS, the fatigue

strength decreased significantly, by more 60%, when the coating was subjected to

the corrosive medium at both periods; two weeks and four weeks. Again the

presence of carbon steel in the substrate of the composite caused the composite

surface to become more susceptible to corrosion than the plain and spot-welded

stainless steel surfaces (Figure 56).

_______________________________________________________________________________________________________ 104Analysis of the Effect of Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


&cs

500

450

400

350 •

300

250 -

200

150

100 -

50 0

♦ O B

« <3

* CM

♦ A o m♦ A o ■

o ■

11 ■ 1 1

<

1 T 1 1

►A

1 H-----1---- 1---- — i---- r —i— i— 1

■ (a) As-CoatedO (b) A s-Coated-2 weeks in NaCl ▲ (c) As-Coated-4 w eeks in NaCl ♦ (d) Non-Coated

0 100 200 300 400

Number o f cycles to failure (N) for SS

I---1---1---1--- 1---1—

500 600Thousands

Figure 54 S-N plot for axial fatigue testing of a plain stainless steel (SS) surface

coated with Inconel-625.

500

450

400

350

£ 300s- 2 5 0

I 200150

10050

0

♦AOB

«* O I

♦A♦A♦A

OO *

♦ A

100

■ (a) As-CoatedO (b) As-Coated-2 weeks in NaCl A(c) As-Coated-4 weeks in NaCl ♦ (d) Non-Coated

O

200 300 400

Number o f cycles to failure (N) for SW-SS

500 600Thousands

Figure 55 S-N plot for axial fatigue testing of a spot-welded stainless steel

(SW-SS) surface coated with Inconel-625.

---------------------------------------------------------------------------------------------------------------------------------105Analysis ofthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadlili, Hussain Y


Figure 56 S-N plot for axial fatigue testing of a composite surface (C-SS-CS)

coated with Inconel-625.

Figure 57 shows the comparison of coating life cycle over plain and welded

surfaces at minimum alternating stresses of 148MPa applied for different durations

of corrosive environments. Coating the samples did increase the fatigue strength

of each sample. It was noted that both plain and welded surfaces behaved with a

similar trend in terms of corrosion fatigue test, when coated by Inconel-625

powder. However, the carbon steel in the composite sample, cased the life cycle to

be decreased significantly when the sample was subjected to its corrosive medium.

However, after 4 weeks all the samples were reduced to a fatigue strength similar

to their uncoated counter parts.

_______________________________________________________________________________________________________1 0 6Analysis o fthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


600000

500000

400000

300000£Ù& 200000 '

100000 •

N on-C oated As- C o a te d 2 w eek sS a m p le Condition

-ss - sw-ss c-ss-cs

4 w eek s0

Figure 57 Coating life cycle over plain and welded surfaces at the minimum

alternating stress (148 MPa).

4.5.2 Deposit Characterization Post Testing

The Inconel-625 coating surface appeared to have high resistant to corrosion, due

to the presence of well stacked partially deformed particles in its microstructure.

In this study, pit formation at the surface was not observed. However, the presence

of unmelted and alumina particles, in the coating interface possibly caused

localised corrosion which took effect after a long period of exposure. As a result

of interconnected porosity, corrosion media may have penetrated the coating and

attacked the substrate, even though the coating itself was resistant to the particular

corrosive environment as shown in Figure 58 and 59.

It should be noted that most of the steel samples were highly susceptible to service

corrosion particularly in a chloride containing aqueous media [16]. As a result, the

substrates corroded beneath the coatings causing the coating to blister and spall.

____________________________________________________________________________________ 107Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y

C UAPTER 4: RESULTS AND DISCUSSION

Figure 58 shows the cross-section of the coating after the corrosion-fatigue test. It

was observed that cracks were formed at both the interface between the coating

and substrate, and in the coating itself. Coating crack size was measured as high as

49(.im and the interface gap up to 38|im. Although this amount of porosity was

small 1 to 3% (using microscopical technique) when comparing to other coating

systems, the presence of voids and un-melted particles in the coating would

significantly effect its performance when subjected to a highly corrosive

environment as shown in Figure 59. The EDS spectrum analysis was measured at

both locations of the points B and C (Figure 58), to determine the approximate

local chemical composition of both areas. It was observed that the location of

point B shows the normal composition of Inconel-625 as a Nickel based alloy.

However, the presence of a large Aluminum peak is clearly shown in the location

of point C (TablelO). Presence of Aluminum in the coating substrate interface

resulted from using it as the grit blasting material in the surface preparation

process. It is tire opinion of the author that these aluminum particles contributed to

the rising of the stress concentration at the substrate surface, resulting in the

initiation of fatigue cracks which led to specimen fracture and introduce localised

residual stresses.

_________. __________________________________ ________________________ __________________________________ 1 0 8Analysis of the Effect of Bending, Fatigue, Erosion-CoiTosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Figure 58 Sample post the corrosion-fatigue test: (A) Stainless Steel substrate. (B)

Inconel-625 coating. (C) Interface between coating and Substrate

Voids

U n-m elted panicles

Figure 59 Cross section of Inconel-625 coating post the fatigue test when

subjected to four weeks corrosion.

________________________________________________________________________________ 109Analysis ofthe Effect of Bending, Fatigue, Erosion-Conosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadhli, Hussain Y


Table 11 Chemical composition of locations B and C as shown in Figure 58,

Location B Location C

Element W t% At % Element W t% At %

0 12.03 33.80 O 34.29 37.42

Mo 10.42 4.88 C 27.86 40.50

Cr 18.77 16.22 Al 30.52 19.75

Fe 2.68 2.16 Si 0.25 0.15

Ni 56.10 42.94 Mo 0.29 0.05

Cr 0.97 0.33

Fe 4.12 1.29

Ni 1.49 0.44

____________________________________________________ _________________________________ 110Analysis ofthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofMetallic SurfacesAl-Fadhli, Hussain Y


4.6 E R O SIO N -C O R R O SIO N B E H A V IO U R OF H IG H V EL O C ITY O X Y -FU E L (H V O F) T H ER M A LLY SPRA Y ED IN C O N EL-625 C O A TIN G S O N D IFFER EN T M ETA L LIC SU RFA CES

4.6.1 D eposit C haracterization (post erosion)

The surface microstructure of the Inconel-625 after 500 hours of slurry jet

impingement is shown in Figure 60. It indicates that the metal removal was almost

uniform over the tested surface due to the proper setting of the jet impingement

nozzle stand-off distance and nozzle size, which controls the je t flow over the

whole targeted area. Figure 61 shows the micrograph of the eroded surface at

higher magnification. It can be seen that the semi-molten particles were more

susceptible to metal removal from the coating surface after the tests. This situation

was also observed by Kunioshi et al. [156] for Ni-based alloys where coating

damage was more concentrated around unmelted particles.

Figure 60 Coating surface after je t impingement testing.

______________________ ___________________ _______________________________ 111Analysis ofthe Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

CHAPTER 4 RESU1 TS AND DISCUSSION

Figure 61 Magnified view of the area depicted as E in Figure 60.

4.6.2 D ifferent Substrate Types (C om posite and W elded)

Figure 62 shows a cross section of the Inconel-625 coating applied over various

metallic substrates. This reflects the actual use of such a coating in real

applications in the oil/gas industries, where the parts are composed of different

substrate materials after repair, that is repair through welding, filling, and so on.

Thus the substrate has the form of a metal composite. Welding always results in

the formation of residual stresses, which are the function of stiffness and the

thermal expansion coefficient between the deposit and the substrate material [157].

Consequently, coating over welded substrate raised the stress levels in the deposits

causing microcracks or the separation of deposits from the substrate surface [12].

To avoid such an occurrence, the substrates (carbon steel and stainless steel) were

preheated prior to welding as preheating improves weldability by reducing the

cooling rate and the level of thermal stresses, as well as the shrinkage and possible

cracking of the joint [158]. In addition, post-heat treatment and shot peening,

__________________________________________________________________________________________ 112Analysis o f the Effect o f Bending, Fatigue, Eros ion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


which is another method of stress relief, eliminated the remaining residual stresses

in the welded workpiece. Therefore, the welded substrates acted as a uniform

composition material during the deposition in the same way as the sample with no

welding.

Figure 62 Cross section of the Inconel-625 coating applied over various metallic

substrates.

4.6.3 C oating over Spot-W elded Stainless Steel (SW -SS)

Approximately 70% of the samples tested in the present study involved coatings

which were applied over welded surfaces, including composite samples

(C-CS-SS). It was noted during the coating characterisation of the SW-SS sample

that there was no significant change in coating properties, particularly

microstmctural properties, compared to that applied on the plain stainless steel

substrate. This may be due to the fact that the HVOF coating, unlike the welding

process, mainly relies on the mechanical interlocking of splats to the substrate

surface, regardless of the substrate material type. After the je t impingement tests,

_______________________________________________________________________________________________________ 113Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metal lie SurfacesAl-Fadhli, Hussain Y


coating over spot-welded stainless steel also showed similar behaviour to that of

the coating over plain stainless steel surface. This may be because of the heat

treatment of the welded surface prior to coating, following which residual stress

levels in the workpiece surface region were minimized.

4.6.4 Coating over Two Dissimilar Metals (C-CS-SS)

Figure 62 illustrates the coating over stainless steel-weld-carbon steel. In terms of

coating microstructure, the substrate material and the coating were not affected by

the impingement tests. This suggested that the HVOF coating structure was not

affected by the inhomogeneity of the substrate material. However, the results of

the je t impingement tests showed that greater mass loss occurred over the C-CS-SS

region compared to that experienced over the SW-SS and the plain SS coated

regions. The mass loss was associated with the vulnerability of carbon steel to

corrosion.

4.6.5 Weight Loss

The erosion test results are presented in Table 12, which shows the weight loss and

erosion rate per substrate type. Erosion rate results were obtained twice for each

case, before and after cleaning. It was observed that in some cases, when the 1%

silica sand erosive media were used, the final weight before cleaning resulted in a

weight gain rather than weight loss. This was due mainly to the presence of silica

sand impinged inside the voids of the eroded surface. The presence of voids in the

coating also increased coating loss, particularly when the impinged fluid contained

1% silica sand. The percent weight loss (erosion rate) was higher for carbon steel

than for stainless steel.

_______________________________________________________________________________________________________ 114Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Table 12, Mass loss results from jet impingement testing during (500 hrs) timing.

Substratetype

Seawatercondition

Initialweight(gm)

Final weight (before cleaning)

(gm)

Final weight (after cleaning)

(gm)

Weig ht loss (gm)

Erosion rate (mg/g)

Average erosion

rate (mg/g)SS Pure 13.24 12.93 12.92 0.31 2.36E-04

Pure 13.35 13.04 13.03 0.32 2.41E-04 2.30E-04

Pure 13.86 13.57 13.56 0.29 2.14E-04

1% Sand 13.21 13.21 12.76 0.45 2.43E-04

1% Sand 12.95 12.44 12.44 0.51 2.63E-04 2.5 IE-04

1% Sand 13.21 12.79 12.78 0.43 2.47E-04

SW-SS Pure 13.65 13.43 13.42 0.23 5.05E-04

Pure 13.12 12.78 12.77 0.34 5.05E-04 5.30E-04

Pure 12.99 12.99 12.67 0.32 5.78E-04

1% Sand 13.52 13.02 13.01 0.51 3.41E-04

1 % Sand 13.25 12.74 12.73 0.52 3.95E-04 3.54E-04

1% Sand 14.00 13.48 13.46 0.53 3.27E-04

c-ss-cs Pure 13.55 12.88 12.87 0.68 3.78E-04

Pure 14.06 13.36 13.35 0.71 3.93E-04 3.85E-04

Pure 13.86 13.07 13.06 0.80 3.83E-04

1 % Sand 13.23 12.24 12.23 0.99 7.53E-04

1% Sand 13.56 12.32 12.31 1.25 9.24E-04 8.68E-04

1% Sand 14.02 12.73 12.72 1.29 9.26E-04

4.6.6 Influence o f Tim e

Figure 63 illustrates the variation of weight loss for the different sample conditions

with respect to time. In all cases, as the experimental time increased, the weight

loss of the coated surface increased. The weight loss of both plain SS and SW-SS

exhibited similar behaviour, thus the presence of a weld spot in the substrate had

no significant effect on the weight loss of the coating. However, the composite

substrate (C-CS-SS) exhibited a much higher degree of weight loss and this

significantly increased after 60 hours of testing compared to the other tested

conditions. This is mainly because the impinged fluid attacking the substrate

material from the sides during testing, that is leaking through the holders, resulted

in corrosion that took place in the base material in the region below the coating.

_____________________________________________________________________________________ 115Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metal lie SurfacesAl-Fadhli, Hussain Y


0.12

0.1

'S 0.08(/>(/>o—11 0.06£O)‘35 0.04

0.02

20 40 60 80

Tim e (hours)

100

■sw-ss-c-ss-cs

120

Figure 63 Variation of weight loss with time.

4.6.7 Slurry E ffect

Figure 64 shows the variation of weight loss due to fluid condition (pure sea water/

pure sea water containing 1% silica sand) for the three tested substrate types. In all

cases, the weight loss increased by approximately 50% when the fluid contained

silica sand. Again, both plain SS and SW-SS behaved similarly, presenting no

effect of adding weld spots to the substrate with respect to coating resistance.

However, the weight loss increased significantly for the C-CS-SS substrate. Both

the stainless steel substrate materials (type 306) and weld (type 309) showed

excellent resistance to corrosion. However in the case of the 4140 carbon steel

substrate, the degree of corrosion was very high, hence the weight loss in the case

of the C-CS-SS substrate was not due to weakness in the coating properties, but

rather to the corroded area in the carbon steel portion of the tested sample.

________________________________________________________________________________________________________ 1 1 6Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


1.0E-03

9.0E-04

B.OE-04

_ 7.0E-04 o>|> 6.0E-04

1 5.0E-04 cec 4.0E-04O2 3.0E-04Ul

2.0E-04

1 .OE-04

O.OE+OO

SS

B Pure se a w ater

□ Pure s e a w ater containing 1% silica sand

sw-ss Test Sample

c-ss-cs

Figure 64 Effect of adding silica sands to the testing fluid.

____________________________________________________________________________________ 117Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


4.7 TE N SIL E B EH A V IO U R O F H V O F TH ER M A LLY SPRA Y ED IN C O N EL-625 C O A TIN G S O N D IFFER EN T M ETA LLIC SU RFA CES

4.7.1 E ffect o f Substrate V ariation on C oating Perform ance D ue to T ensile Testing

Figure 65 shows the tensile strength behaviour of the plain stainless steel

workpiece. It was noted that the coated workpiece exhibited higher tensile strength

than the un-coated workpiece, even when the coated workpiece was subjected to a

severe corrosive environment. This was similar for the cases of spot-welded

stainless steel (SW-SS) and composite stainless steel (C-SS-CS) as shown in

Figure 66 and Figure 67, respectively. This indicates that the Inconel-625 coating

enhanced the strength behaviour of the stainless steel workpiece (as found in the

bending test). It was also found that the corrosion effect on the coated workpiece

for both periods two weeks and four weeks was minimal, and they showed a

similar behaviour to that of the coated plain stainless steel (SS) workpiece as

received. The effect of corrosion became more effective for the SW-SS workpiece,

especially when the sample was subjected to aqueous static corrosion for four

weeks (Figure 66). This again might be due to the presence of weld in the

workpiece which made the workpiece more susceptible to corrosion than when it

had no weld.

_______________________________________________________________________________________________________118Analysis o f the Effect o f Bending, Fatigue, Erosion-Con osion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Figure 65 Stress strain curve for plain stainless steel workpiece (SS).

S W - S S - N C S W - S S - A R S W - S S - 2 W K — S W - S S - I W K

T e n s i l e S t r a i n

Figure 66 Stress strain curve for spot-welded stainless steel workpiece (SW-SS).

_____________________________________________________________________________________ 119Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


More influence of the corrosion was observed for the (C-SS-CS) workpiece

(Figure 67). This might be due to the presence of both the weld and the carbon

steel material as a part of the substrate workpiece material.

It is important to note that the yielding limit for the tested samples was changed

based on the substrate type and the period of exposure to corrosive media. For

instant, the yielding for plain stainless steel sample tested after four weeks of

exposure to corrosive media dropped from 600MPa to 420MPa when changing the

sample type to spot welded stainless steel SW-SS. Approximately, the same value

was obtained when substrate type was changed to composite stainless steel and

carbon steel C-SS-CS. It was observed that the presence of weld or composite

material enhance the strength of the material. This may be related to the materials

properties which modifies after welding. It is important to mention that even if the

material strength was enhanced, the corrosion effect behave similarly like all other

cases reported from other tests i.e., the coating resistance to the tensile test

decreased when the exposure to corrosive environment increased.

4.7.2 D eposit C haracterization Post Testing

Figure 68 shows a typical cross section of the Inconel-625 coating over the

stainless substrate before corrosion and tensile testing, where the uniformity of

coating distribution over substrate surface is observed. Figure 69 shows the sample

after tensile test, subjected to aqueous static corrosion for four weeks. It was

observed that the coating exhibited a transverse failure through out the sample.

This is could be related to the brittleness of the coating. The coating cross-section

after the tensile test is shown in Figure 70. It was noted that cracks propegated

around the non-melted particles. This is may be due to the availability of

surounding voids created during spraying. These voids will also aid in the cracks

propagation through aqueous solution towards the coating substrate interface. This

is clear from the close view of coating cracks subjected to four weeks aqueous

corrosion shown in Figure 71. Figure 72 shows how cracks propagated towards the

________________________________________________________________________ 120Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


weekest points, while some of the cracks intiated and then stopped when they met

the tough region.

. C - S S C S N C — C - S S C S - A R C - S S - C S - 2 W K C - S S - C S ^ t W K

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Tensile Strain

Figure 67 Stress strain curve for composite stainless and carbon steel workpiece

(C-SS-CS).

_________________________________________________________ _ 121Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Figure 68 Cross section of tested coating.

Figure 69 Photograph of plain stainless steel (SS) after tensile test.

_______________________________________________________________________________________________________________1 22Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating ofM etallic SurfacesAl-Fadhli, Hussain Y


Figure 70 Coating after corrosion-tensile test.

______________________________________________________________________ 123Analysis ofthe Effect ofBending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadlili, Hussain Y


<cc.V Spot Magn Det WD Exp I---------------------1 :Æ0kV5f l lOOx BSE 14.1 782 0.2 Torr Sample 1■SBBaESMAtl- I t iMi .• J*M - 1

Figure 71 Cross section of tested coating after tensile test.

_______________________________________________________________________________________________________1 2 4Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y

C H A P T E R 4: R E S U L T S A N D D IS C U S S IO N

Figure 72 Coating surface after tensile testing.

--------------------------------------------------------------------------------------------------------------------------------- 125Analysis o f the Effect o f Bending, Fatigue, Erosion-Cortosion, and Tensile Stresses on HVOF Coating of Metal lie SurfacesAl-Fadhli, Hussain Y

CHAPTER 4: RESULTS A N D DISCUSSION

4.8 MODELLING OF RESIDUAL STRESSES VARIATION WITH SUBSTRATE TYPE

As mentioned in chapter 3, the residual stresses modelling was carried out in two

ways; one by subjecting the sample to various temperatures until the resulting

deflection equal that found using Clyne’s method (Thermal Model). In this case

the deflection value obtained at the centre of the sample in the Y-direction, was

0.037 mm (Figure 73). The procedure to calculate the deflection using Clyne’s

method is shown in [62]. Once the deflection was achieved then the Von Mises

stress distribution across the sample was outputted. According to Stokes [62], the

important stress results were at the interface of the model for the thermal load

analysis.

NODAL S O L U T I O N

S T E P = 1 SUB =1 T I M E = 1 SEQV DHX = . 0 3 7 8 2 5 SMN = 5 . 5 5 3 SMX = 3 4 5 2

Figure 73 Deflection of plain stainless steel (SS) sample due to thermal load.

________________________________________________________________________________ 126Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAI-Fadhli, Hussain Y


The second method involved applying a deflection to the sample in order to

generate a moment (Figure 74). Again, according to Stokes [62], the stress

distribution at the top of the deposit and at the bottom of the substrate were of

interest in determining the stress distribution of the sample. The results were found

by carrying out a path plot across the various locations (Top of Coating, Interface

and Bottom of Substrate) to get the exact value for stress at each of these locations.

The results of each model were then combined to produce an overall stress

distribution across the sample (Figure 75).

NODAL SOLUTION

STEP=1 SUB =1 TIME=1 SEQVDHX = . 0 3 6 6 9 5 SMN 03 3 85 6SMX = 1 9 9 2

"7Œ— IS E P 1 2 2 0 0 5

1 3 : 1 9 : 4 6

(JLVG)

8 = 0.037 mm

MPa. 0 3 3 0 5 6 4 - 3 2 . 7 8 3 8 8 5 . 5 3 4 1 3 2 0 1 7 7 1

2 2 1 . 4 0 9 6 6 4 . 1 5 9 11D7 1 5 5 0 1 9 9 2

Figure 74 Deflection o f plain stainless steel (SS) sample due to moment load.

______________________________________________________________________ 127Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


Figure 75 Overall stress distribution across the sample.

4.8.1 Validating the Model

Clyne’s method was applied using the same deflection used in the FEA. For

instant, the deflection value calculated using Clyne’s method for this case was

found to be 0.037 mm. It is important to note that this value (0.037 mm) was

obtained based on the physical properties of the tested materials and the applied

temperature. The same value is then used in the FEA to determine stresses. Stokes

[62] used this method and found it to be very effective to determine the residual

stresses in WC-Co deposit.

Figure 76 shows a comparison between the combined modelling and the analytical

results obtained using Clyne’s method. It was observed that the profile of the

stress distribution along the coating and substrate was similar with a maximum

________________________________________________________________________________128Analysis of the Effect o f Bendingi Fatigue, Erosion-Con-osion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


variation of approximately 20% at the top o f the deposit. As the model and dyne’s

method are based on numerical and analytical techniques, therefore these results

should be validated using experimental data.

SS-Clyne's SS-Modelling

CO

240

200 ♦

160 '

« 120 1

00cn

Coating SubMratc

2 3

Through Thickness (mm)

Figure 76 Finite element and analytical stress analysis of Inconel-625 deposit over

plain stainless steel substrate (SS).

Stokes [62] validated the proposed model and Clyne’s analytical method

experimentally (Figure 77). The results of the research showed that the variation

found between stresses obtained by Clyne’s and the experimental results were

negligible. Hence, it becomes appropriate to use Clyne’s method in this case to

validate the models proposed in this research. The maximum difference between

FEA and Clyne's was found to be 23%, 40%, and 37% at the top of the coating, the

interface and the bottom of the substrate respectively (Figure 77). However,

Clyne’s method does not take into account conduction and thermal difference

through the sample, hence this would contribute to the error, where as the model

did.

_________________________________________________________________ 129Analysis of the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadbli, Hussain Y


Figure 77 Finite element, experimental and analytical stress analysis o f a 0.2mm

thick deposit [62],

It is important to note that Clyne’s method can be only applied where deposit exist

over single material substrates. Therefore this method was implemented to predict

the stress distribution in the stainless steel substrates without a weld or joined to

other steel material coated with Inconel-625.

For the plain stainless steel substrate (SS), the stress at the top of the deposit was

found to be 205 MPa using Clyne’s and 165 MPa using FEA. So the model predict

an error of 24% which was similar to that found by Stokes [62] for WC-Co

deposit. In both methods, the stress decrease across the coating by 4.8%

approximately. This small change in stress was associated to the small deposit

thickness used (0.4 mm). Stokes [62] found that the stresses changed significantly

with increased coating thickness. The model stress at the interface ranged from

150 MPa on the deposit interface side to 87 MPa at the substrate side. Clyne’s

________________________________________________________________________________ 130Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadbli, Hussain Y


distribution at this interface indicated a change from 190 MPa to 92 MPa, so while

the model did not match these values, it did predict the same profile.

The change in the stress is due to the difference in the physical properties between

coating and substrate (misfit strain) as indicated previously by Clyne’s [152], The

stress at the bottom of the substrate was found to decrease by more than 75%

compared to the stress at the top of the substrate. This large decrease in the stress

was mainly associated with the large substrate thickness ( 4 mm) used compared to

coating (0.4 mm).

4.8.2 Comparison of Each Model Based on Substrate Type

(1) Measurements at the Centre of the Model

Figure 78 shows the distribution o f stresses taken from models based on substrate

type, plain stainless steel (SS), spot welded stainless steel (SW-SS), and composite

stainless steel and carbon steel (C-SS-CS), where the measurements were taken at

the centre of the sample. As described in the previous section, Clyne’s equations

(Equation 4-7) could not be applied to this section when the various coated

samples were compared. It was found that the stresses at the top of the deposit

were equal to 165 MPa in all cases. This is mainly because the material used for

deposition is the same in all cases (Inconel-625) and the substrate type had no

influence on stress on the top of the coating. However, a noticeable change was

observed at the coating/substrate interface for each substrate type. Using equations

2 and 3 for quenching and cooling, this result can be validated. The stress that

affects the deposit mainly occur during spraying, therefore the quenching stress

contributes to the stress at this stage according to equation 2. Therefore, substrate

type has no influence on the quenching stress (Table 11). Therefore this suggests

the results of all three substrates types having the same stress at the top of their

respective coatings. For SS, the stress was found to be ranged from 88 to

120MPa, however, for the SW-SS the stress value increased by 17%. The stress

__________________________________________________________________________________131Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadlili, Hussain Y

CHAPTER 4: RESUT TS AND DISCUSSION

value was increased by 25% when the substrate is change to be (C-SS-CS). This is

due to the variation of substrate material properties. Again, looking at equations 2

and 3, the effect of stress at the interface is mainly due to cooling stress and misfit

strain (equation 8).

When the substrate type changes more, both stiffness and coefficient of thermal

expansion varies between the samples, thus resulting in a change in cooling stress

(Table 11). All stresses were found to be quite similar in the range of 78-80 MPa

at the bottom of the substrate. As the substrate was quite thick the effect of

applying a coating to such a substrate had little effect on stress generated at the

bottom of the substrate.

ss sw-ss c-ss-cs

£C/D

185

165

145

125

105

85

65

45

25

5

Toaling

0.5

Substraic

» * I ‘ ‘ I “* ' I '

1.5 2 2.5 3 3.5


4.5

Figure 78 Variation of residual stresses based on substrate type.

(2) Comparison of Stresses at Different Regions in the SW-SS Substrate

______________________________________________________________________ 132Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y


Figure 79 show s the stress variation in the spot welded stainless steel (SW -SS)

sam ple. It was found that, at the top o f the deposit, the stress at the welded region

increased by 27% com pared to the non-w elded region. This variation is due to the

fact that the weld at the centre was subjected to m ore stress than the stainless steel

at the edges. The stress at the interface show s the highest effect. It can be seen that

the w elded region (I) ranged from 145 M Pa (on coating side) to 100 M Pa (on

substrate side), w hereas the un-welded region (II) only experiences a change o f 10

M Pa. Hence, while the deflection experienced in region (II) w as lower than that in

region (I), there was also another contribution to this stress change. This can be

validated again by cooling stress and m isfit strain equations. The change in

stiffness and coefficient o f therm al expansion across these tw o regions would have

increased the stress in the welded zone (Table 11).

____________________________________________________________________________ 133Analysis o f the l-ffcct of Bending, l;atigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coaling ot'Mclnllic SurfacesAl-Fiulhli, Hussain Y


W ild

H < I'.>!•»! i

SS

Region II

SW-SS (Region I) SW-SS (Region II)

185 -165 «145 -

nT 125 -,CL

105 -( n111 85 •

65 •45 -

25 -5 -I

H ating Subsume

2 3T h ro u g h t T h ic k n es s (mm)

Figure 79 Variation of residual stresses at different regions in (SW-SS).

(3) Comparison of Stresses at Different Regions in the C-SS-CS Substrate

Figure 80 shows the stress variation in the composite stainless steel and carbon

steel C-SS-CS sample. It was found that, at the top of the deposit, the stress at the

welded region (III) was 28% higher than that of the stress at the carbon steel

Region (I) and the plain stainless steel Region (II). This variation was again as a

result of the weld at the centre been subjected to more stresses than the carbon steel

(CS) and the stainless steel (SS) at the edges. Moreover, the stress ranges at the

interface increased dramatically in the welded region by 28% compared to stainless

steel region and by 31% compared to carbon steel region due to stresses generated

_____________________ __ __________________________________________ 134Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


by presence o f weld. Again this is due to the stiffness and coefficient o f therm al

expansion effects on the misfit strain and cooling m echanism s involved in the

stress developm ent in the deposit (Table 9).

Using equations 2 and 3, Fig. 81 show s how overall residual stress changes across

die SS 316 to SS 309 (w eld) back to SS 316 sam ple. The weld increases the stress

in the deposit by 12 M Pa across the w elded region. C om paring that to the stress

for the C arbon-Steel weld-Siainless Steel substrate, Figure 82 show s that the stress

increases from stainless steel to weld by 12 M Pa, but from the weld to the Carbon

Steel by a further 88M Pa. Therefore, w elding two different steels together w ith a

weld o f another steel has a dram atic effect on residual stress generation during and

post deposition. A lthough these values are low, it m ust be understood that they are

approxim ate, therefore the realistic effect m ay not be appreciated in these results.

____________________________________________________________________________ 135Analysis o f ihc Effect o f Howling, I'nligue, Erosion-Corrosion, ami Tensile Stresses on I I VOfr C'oaiing o f Metallic SurfacesAl-I;adhli. Hussain Y


Figure 80 Variation of residual stresses at different regions in (C-SS-CS).

Table 13 Quenching and cooling stresses variation between coating and substrate.

Stress (MPa) SS SW-SS C-CS-SS

*Quenching 265 265 265

**Cooling -163 -153 -64

Overall Residual Stress 98 112 199

*oq=Ec x (Tc-Ts) x a c ,l i e x AT (ac - as)]

1 + 2 -

Ectc

Esls

________________________________________________________________________________ 136Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


115 00

110.00

raCL2

105 00

CO

100.00

95.00

f • • 1 I11

WeldSS 316 SS 309 !

1 \SS3I6

5 10 15 20 25 30 35 40 45

Through thickness (mm)

Figure 81 Residual stresses across the SW-SS sample.

______________________________________________________________________ 137Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


250 00

200 00

"S'Cg 150 00

83) 100 00

(0 S S Weld cs50 00

0.00 i l »10 15 20 25 30 35 40 45


Figure 82 Residual stresses across the C-SS-CS sample.

--------------------------------------------------------------------------------------------------------------------------------------- 138Analysis oilhe Elìcci o f Bending, Fatigue, Emsion-C'orrosion, and Tensile Stresses on IIVOE Coating o f Metallic SurfacesAl-l-adhli, Hussain Y

CHAPTER 5: CONCLUSIONS ANDRECOMMENI ) ATIONS

5CONCLUSIONS AND

RECOMMENDATIONS

5.1 C O N C LU SIO N S

In the present study, coating charactaristics and mechanical properties of HVOF

Inconel-625 coating onto stainless and carbon steel weld joints were examined.

The material’s resistance to several mechanical tests - including bending, fatigue,

erosion-corrosion, and tensile testing, where the coating material was applied over

plain, welded and composite substrate materials has been investigated to determine

the effect of possible failures of the substrate material when subjected to sever

corrosive and erosive environments combined with different mechnical loadings.

Furthermore, residual stresses evaluation, using modelling and analytical methods,

were used to predict the influence of residual stress when the coating was applied

over different metallic substrates. The conclusions resulting from invistigations

are summarised as follow:

• The EDS analysis on page 85 showed that the oxide formed in the coating

contained mainly the Cr203 phase. This is mainly due to the high

temperature of the deposited particles and presence of air molecules

captured in voids between the non-melted or semi-molten particles and the

next bonded splats. It was also observed from the EDS analysis that other

types of oxides were present at the interface between the coating and the

substrate, namely AI2O3 and Si02 . This is because some grit blasting

material remained after the surface preparation process where both A I 2 O 3

and S i0 2 grits were used in workpiece surface preparation prior to coating.

___________________________ _______________________________________________________139Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

C H A P T E R 5: C O N C L U S IO N S A N DR E C O M M E N D A T IO N S

• The oxygen content in the deposited particles was found to be low and

oxide formed in flight was mainly distributed over the surface. The

deterioration of the coating occurs along the deposited particles boundaries

and corrodes the material nearby as shown by the SEM analysis.

• The cavitations in the interface between the coating and base material seem

to have accelerated the corrosion of the coating as well as base material, i.e.

as a results of the inter-corroded porosity, the electrolyte could then

penetrate the coating and attack the substrate material, even tough the

coating was resistant to the corrosive environment. Consequently, the

substrate material could be corroded beneath the coating lowering the

mechanical properties at the interface.

• Heat Affected Zone (HAZ) corroded easily once the corrosive solution

penetrated the coating. This indicates that Inconel-625 coating protected

these different metallic substrates against corrosion and minimised the

effect of corrosion on two materials attached together.

• Inconel-625 powder coating enhanced the corrosion resistance of stainless

steel workpiece, and increased the workpiece resistance to bending.

However, as the period of exposure to corrosive environment increased

from two weeks to four weeks, coating resistance to corrosion decreased.

• Using bending tests analysis, the corrosion resistance of the spot welded

stainless steel (SW-SS) coated workpiece significantly decreased compared

to the plain stainless steel (SS) ones. The SS ones peaked at 470 —> 550

MPa where as the spot welded ones peaked from 525 —» 625 MPa. No

major variation between the two week and the four week exposed samples

was noted. Also, the length of time, within the tests performed, did not

_________________________________________________________________ 140Analysis o f the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


have an effect, it was the corrosion media that reduced the coating

capabilities.

• In the bending test analysis for the composite carbon steel and stainless

steel (C-CS-SS), the presence of carbon steel in the substrate obviously was

effected by the corrosive media, but this media had more of an effect over

time on the carbon steel compared to plain steel and spot welded steel.

Comparing the composite stainless steel subjected to corrosion for four

weeks to the same treatment applied to the plain stainless steel sample both

were able to sustain the same stress level ~ 480MPa. However, the

deterioration was much higher for the composite substrate (~

120MPa)compared to plain (~ 30MPa) or spot welded samples (~

lOOMPa).

• Deposit characterisation post bending test showed that the brittleness of the

coating triggered crack formation in the coating layer. Since the coating

thickness was about 400 (im, it was possible that the maximum of both

normal and shear stresses took place at base metal-coating interface.

However, the crack initiation at the interface relieved the stress levels in

this region which avoided spalling or buckling of the coating.

• In the bending test analysis, plastic deformation and delamination around

the coating edge was observed due to the action of shear stress. Moreover,

the internal stresses possibly created local stress concentrations, especially

at defect locations around the coating base material interface and as soon as

the local internal stress for crack propagation was reached, the entire

coating failed in this region.

• For the axial fatigue test, both plain and welded stainless steel surfaces

behaved similarly for the three different corrosion durations.

_________________________________________________________________ 141Analysis of the Effect of Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


• It was observed that the coating enhanced the fatigue resistance of different

metallic substrates. There was a reduction of 300 MPa in the fatigue

strength of the coating when subjected to four weeks aqueous corrosion

medium in both plain and welded surfaces. The fatigue strength reduced to

that of their uncoated counter parts after 4 weeks exposure.

• Microscopic observations of the fracture surfaces of the coated test

specimen showed that the fatigue cracks was formed at both the interface

between coating and substrate and in the coating itself. Availability of non

melted particles and voids in the coating, and the presence of Si02 particles

in the coating substrate interface contributed to crack initiation in the

coating and the coating-substrate interface, respectively.

• Erosion corrosion tests were conducted and SEM investigations of the

specimen surfaces showed that the material removal was concentrated more

around the unmelted or semi-molten particles rather than any other

locations in the coating.

• The coating was found to be highly sensitive to the presence of sand

particles in the impinging fluid. As the period of coating exposed to the

flow of slurry fluid increased, the weight loss increased significantly. This

increment was dependent on the type of substrate material.

• It was found from the tensile tests that Inconel-625 coating enhanced the

strength behaviour of the stainless steel workpiece (as found in the bending

test). It was also found that the corrosion effect on the coated workpiece

for both periods of two weeks and four weeks was minimal, and they

showed a similar behaviour to that of the coated plain stainless steel (SS)

workpiece as received. This showed that that the Inconel-625 was a good

alloy choice for use in industry, especially when corrosion resistance is

required.

________________________________________________________________________________ 142Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAI-Fadhli, Hussain Y


• Inconel-625 coating enhanced the yielding strength of all samples tested

regardless of its substrate type and the period of exposure to corrosive

environment.

• Presence of weld in the workpiece made the workpiece more susceptible to

corrosion than when it had no weld. More influence of the corrosion was

observed for the (C-SS-CS) workpiece. This may be due to the presence of

both the weld and the carbon steel material as a part of the substrate

workpiece material. Therefore, even if the presence of the weld might not

result into the coating failure, it could be highly recomended to avoid it

unless there is no choice except having it. In this case, regular inspection of

the coating for evidance of corrosion attack would be highly recommended.

This was because when the corrosive solution penetrated the coating the

weld and the carbon steel area would corrode very fast resulting into a

catastrophic failure. This could be de due to electrocell formation between

the disimilar metals, leading to electrochemical corrosion.

• For the plain stainless steel substrate (SS), the stress at the top of the

deposit was found to be 205 MPa using Clyne’s and 165 MPa using FEA.

An error between the model and analytical of 24% can be seen. In both

methods, the stress decreased across the coating by approximately 4.8%.

This small change in stress was associated with small deposit thickness

used (0.4 mm).

• The FEA stress model indicate a stress at the interface ranged of 150 MPa

on the deposit interface side and 87 MPa at both of the substrate. Clyne’s

distribution on the other hand, indicated a change from 190 MPa to 92

MPa, so while the model did not match these values, they did predict the

same profile. The change in the stress was due to the difference in the

physical properties between coating and substrate material (misfit strain).

________________________________________________________________________________ 143Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


• The stress at the bottom of the substrate was found to be below the stress at

the top by more than 75%. This large difference in the stress was mainly

associated with the large substrate thickness ( 4 mm) used as compared to

coating (0.4 mm).

• The numerical study, analytical and experm intal methods showed that the

stress at the interface had the highest effect on coating failure. The

difference in stiffness and coefficient of thermal expansion between the

substrate and the weld and between these and the deposit increased the

stress above the welded zone in the deposit. This increase in residual stress

makes the sample more susceptible to bending, fatigue, erosion-corrosion,

and tensile stresses, therefore it should be avoided in repair.

________________________________________________________________________________ 144Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y


5.2 R EC O M M EN D A TIO N S F O R FU T U R E W O R K

The results docum ented in the p resen t research are significant, how ever further

recom m endations are as follows:

• Testing C urved Surfaces:

A ll tests w ere perform ed on p la in or round surface such as plates and shafts;

how ever, assessm ents o f coating perform ance on spherical surfaces could help

to im prove the practical application o f the coating onto valves and spherical

joints.

• C om parison and Coating o f o ther M aterials:

The current study was very com prehensive for the lnconel-625 m aterial as a

N ickel based alloy; how ever, in the industry m any different types o f pow ders

are used. Therefore, it is h ighly recom m ended to com pare the available tests

to o ther types o f pow ders such as those that contain carbides.

• Experim ental T est for R esidual Stresses:

The prediction o f residual stresses in this w ork w as done using m odeling and

analytical m ethods and the results w ere validated using experim ental w ork

w hich was conducted on o ther type o f pow der W C-Co. Therefore, it is

recom m ended to extend the current w ork to include experim ental test for the

residual stresses, w hich is generated w hen depositing lnconel-625 pow ders.

__________________________________________________________________________________________ 145Analysis of the Effect ofBending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating of Metallic SurfacesAl-Fadhli, Hussain Y

PUBLICATIONS ARISING FROMTHIS WORK

PUBLICATIONS ARISING FROM THIS W ORK

Journals

1. Al-Fadhli, Hussain Y, J. Stokes, M .S.J. H ashm i, and B.S Y ilbas “HVOF C oating o f W elded Surfaces: C orrosion-Fatigue B ehaviour o f Stainless Steel C oated w ith Inconel-625 A lloy” , Surface & Coatings Technology, (2005), A rticle in Print, (A vailable Online).

2. Al-Fadhli, Hussain Y, J. Stokes, M .S.J. H ashm i, and B.S Y ilbas “The E rosion-C orrosion B ehaviour O f Inconel-625 C oating T herm ally Sprayed U sing H V O F Process A pplied O ver D ifferent M etallic Surfaces” , Surface & C oatings Technology, (2005), A rticle in Print, (A vailable Online).

Conference Papers

3. Al-Fadhli, Hussain Y., “Influence o f H VO F setting param eters on N ickel- B ased A lloy Pow der (D iam alloy 1005) C oating Properties” , Proceeding from the 1st international surface engineering congress and the 13th IFH TSE Congress, (2002), pp. 549-554.

4. Al-Fadhli, Hussain Y., B.S. Y ilbas, and M .S.J. H ashm i, “H V O F-C oating o f M etallic Surfaces: Coating R esponse to C orrosion and B ending” Proceeding o f the In ternational Therm al Spray C onference, Basel, Switzerland, M ay 2-4, 2005 - pp. 695-699.

5 . Al-Fadhli, Hussain Y., J. Stokes, M .S.J. Hashm i, B.S. Y ilbas, I. Taie and S. M ehta “M odelling o f H V O F C oating D eposit A pplied over D ifferent M etallic Surfaces and T ested in H ighly Corrosive Environm ents” . 11th M iddle East C orrosion C onference February 26 - M arch 1, 2006, A c c e p t e d f o r P u b l i c a t i o n

_____________________________________________________________________________________ 146Analysis o f the Effect o f Bending, Fatigue, Erosion-Corrosion, and Tensile Stresses on HVOF Coating o f Metallic SurfacesAl-Fadhli, Hussain Y

PUBLICATIONS ARISING FROMTHIS WORK

Poster Papers

6. Al-Fadhli, Hussain Y., J, Stokes, M.S.J. Hashm i, and B,S. Y ilbas “Erosion- C orrosion C haracteristics o f Inconel-625 Coating Sprayed by the IIV OF Process and Applied over D ifferent M etallic Surfaces” . WOM 2005, San Diego, IJSA.

7. Al-Fadhli, H ussain Y., J. Stokes, M .S.J. H ashm i, and B.S Y ilbas “Tensile Strength Behavior o f Plain and W elded Stam less Steel Coated w ith Inconel- 625, Therm ally Sprayed Using HVOF C oating” , AM PT 2005, W isla, Poland.

8. Al-Fadhli, Hussain Y., J. Stokes, M.S.J. Hashm i, and B.S. Y ilbas "Post Test A nalysis o f Inconel-625 (HV OF) C oating U sing SEM and EDS after Exposure to Erosion-Corrosion T est” , M &M 2005, Hawaii, USA.

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