EROSION RESISTANCE OF CR C -25% (NI -20CR) COATING AT ...E-mail address: [email protected] ABSTRACT Erosion studies were carried out using an air-jet erosion test rig at a velocity

EROSION RESISTANCE OF CR3C2 -25% (NI-20CR) COATING AT DIFFERENT TEMPERATURES

Journal of Emerging Trends in Engineering, Science and Technology, ISSN 2394-5354, Vol 3, No. 2, 2015Open Access Peer Reviewed Refereed International Journal, ijoetest.wix.com/joetest

42

EROSION RESISTANCE OF CR3C2 -25% (NI-20CR) COATING ATDIFFERENT TEMPERATURES

Rakesh Bhatia a*, Hazoor Singh a, Buta Singh Sidhub

aYadavindra College of Engineering, Punjabi University, G.K.Campus, Talwandi Sabo, Bathinda,Punjab 151302, Indiab Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurtala Punjab, India

*Corresponding Author: Tel:+91-99880-96160E-mail address: [email protected]

ABSTRACT

Erosion studies were carried out using an air-jet erosion test rig at a velocity of 35 m s−1 and impingement angles of30° and 90° on uncoated and High Velocity Oxy Fuel (HVOF) coated 75% Cr3 C2 -25% (Ni-20Cr) T-91 boiler tubesteel at different temperatures. Coatings were characterized before and after the erosion test using Scanning ElectronMicroscopy (SEM), Energy Dispersive X-ray Analysis (EDAX) and X-Ray Diffraction (XRD) techniques. T-91boiler tube steel has shown ductile behaviour, whereas coated samples exhibited brittle behaviour. However, 75%Cr3 C2 -25% (Ni-20Cr) coating gave the least thickness loss at room temperature and 30

0 impingement angle amongthe temperatures under study.

Keywords: Porosity, High Velocity Oxy Fuel (HVOF), Coating, Erosion.

1. Introduction

Erosion is one of the great unsolved problems in engineering. Researchers agree that several factorscontribute to the erosion of material components, subjected to solid particle impingement. Erosion is the removal ofmaterial from a surface by impact, cutting, or frictional wear action of solid particles entrained in a high velocityfluid medium. Solid particles transported in gas or liquid flows cause severe damage on industrial components andlead to expensive repair and part replacement [1]. The high-temperature erosion behavior of materials is believed todepend on complex interactions between the materials, several temperature-dependent material properties, and theoperative erosion mechanism(s) that, in turn, is related to particle shape, velocity, and impingement angles. Erosionresistance must generally be determined by laboratory techniques since the complexity of most eroding systemsrenders testing under service conditions difficult [2].

Gat and Tabakoff [3] have classified the temperature-sensitive material properties into two groups: Type Iproperties are those which decrease the erosion resistance with increasing temperature and include mechanicalstrength, modulus of elasticity, surface hardness, and fatigue resistance. Type II properties tend to increase erosionresistance with increasing temperature and comprise ductility, recrystallization temperature, which determines Solidparticle erosion (SPE).

The metal wastage caused by impacts of solid particles on surfaces that are simultaneously oxidized atelevated temperature is a problem in several types of energy systems, ranging from heat exchanger tubes influidized-bed combustors to steam turbine nozzles and blades. It can reduce the efficiency and life of the plant,increase maintenance costs and cause safety hazards. High temperature erosion–corrosion of heat exchanger tubesand other structural materials in coal-fired boilers is recognized as the main cause of downtime at power-generatingplants, which could account for 50–75% of their total arrest time [4]. Fly ash erosion-corrosion (E-C) of superheater,reheater and economizer tubing in the back pass of boiler is a serious problem in biomass-fired as well as in othertypes of boilers [5].

Thermal sprayed coatings are currently used to reduce erosion–corrosion on a large number of componentsin various industries, including the energy conversion and utilization system such as coal gasifiers, combustionboilers, steam turbines, gas turbines, and ID fan ducts and blades [6-7]. Many coatings are now undergoing testingin aggressive environments of boilers. Cr3C2–NiCr thermal spray coatings are extensively used to mitigate hightemperature erosive wear in fluidised bed combustors and power generation/transport turbines. The useful operating



43

temperature range of these coatings starts at the upper limit of the WC based cermet system (450–500°C) to amaximum of 850–900°C [7-9].

Research has been carried out regarding mechanisms and relative rates of erosion of materials to mitigatethe effect of impact damage. Laboratory scale trials have focused on materials, tested under ambient temperaturesand with relatively low particle impact velocities.

2. Experimental Details

2.1 Development of coatings

T-91 boiler tube steel was selected as substrate material. The substrate material selected for this study have beenprovided by Guru Nanak Dev Thermal Power Plant , Bathinda, Punjab (India) and are used for fabricating boilertubes. Strips were cut from the boiler tube using milling machine. The specimens with dimensions of approximately22mm×15mm×3 mmwere prepared from these strips using surface grinder and slitting wheels. The specimens werepolished with SiC papers down to180 grit, and subsequently were grit-blasted with alumina powders (Grit 45) beforedevelopment of the coatings by the HVOF process.Commercially available75%Cr3C2 -25% (Ni-20Cr) coatingpowders was used in the study. Detail of the coating powders are given in Table 1. Fig. 1 shows the micrographs of75%Cr3C2 -25% (Ni-20Cr) coating powder having spherical and irregular shaped particles along with the EDAXanalysis of the powder showing composition in % wt confirming the powder blend.

Table 1 Details of coating powders

Coating Powder Type Size

75%Cr3C2 -25% (Ni-20Cr)FST K – 856.23

Flame Spray Technologies (The Netherlands)

Agglomerated Sintered -45 + 15 μm

Fig. 1 SEM and EDAX analysis of 75%Cr3C2 -25% (Ni-20Cr) coating powder.



44

The coatings were developed at M/S Metallizing Equipment Co. Pvt. Ltd. (Jodhpur, India) by using commercialHVOF thermal spray systems. The Hipojet-2100 HVOF system was used for powder spraying. Liquefied petroleumgas (LPG) was used as a fuel. All of the process parameters were kept constant throughout the coating process. Thespecimens were cooled with compressed air jets during and after spraying. The spray parameters used for theHipojet-2100 system were an oxygen flow rate of 250 LPM, a fuel (LPG) flow rate 60 LPM, an airflow rate of 900LPM, a spray distance of about 200 mm, a fuel pressure of 8 kg/cm2, an oxygen pressure of 9 kg/cm2, and airpressure 5 kg/cm2 and powder feed rate of 28 g/min. The coated samples were cloth wheel-polished and thensubjected to XRD, and SEM/EDAX analysis to characterize the surface and cross-sectional morphology of thecoatings. The XRD analysis was carried out with a Diffraction patterns were obtained by Bruker AXS D-8 AdvanceDiffractometer (Germany) with CuKα radiation. A scanning electron microscope (JSM-6610, Jeol, New York) withan EDAX attachment (Oxford, UK) was used for SEM/EDAX analysis. The porosity measurements were made withan image analyzer with Dewinter Material Plus 1.01 software based on ASTM B276.The image was obtainedthrough the attached PMP3 inverted metallurgical microscope with stereographic imaging. To identify the cross-sectional details, the samples were cut across the cross section, mounted in transoptic powder, and subjected tomirror polishing. The coating thickness was measured with a scanning electron microscope. The microhardness ofcoatings was measured by the Omnitech Microhardness tester.

2.2 High Temperature Erosion TestsThe solid particle high temperature erosion tests were performed on air jet erosion test rig TR-471-M10 Air JetErosion Tester (Ducom Instruments Private Limited, Bangalore, India) capable of conducting tests at roomtemperature as well as high temperature, available with IIT Roorkee (India), as per ASTMG76 . The rig consisted ofan air compressor, erodent feeding system, mixing chamber, furnace unit, specimen holder, nozzle, erodentcollection chamber, pneumatic control box and electrical control box. The test method utilizes a repeated impacterosion approach involving a small nozzle delivering a stream of gas containing abrasive particles which impacts thesurface of a test specimen.Dry compressed air was mixed with the erodent particles, which were fed at a constant rate from hopper through

erodent feeding system in the mixing chamber and then accelerated by passing the mixture through a convergingnozzle made of inconel material of 4 mm diameter. These accelerated particles impacted the specimen kept in thefurnace unit consisting of specimen heater and air heater. The specimen could be held at angles of 300 and 900 withrespect to the impacting particles using an adjustable sample holder. The discharge rate of the particles could becontrolled by varying the frequency of motor speed in the erodent feeding system. The erodent feeding systemconsists of a hopper which allows erodent to fall under gravity through throat on a wheel which is rotated by a motorthrough timer belt. Motor speed determines the extent of discharge. Higher the speed of motor greater is thedischarge and vice versa. The impact velocities of the particles could be varied by varying the pressure of thecompressed air. The details about conditions of erosion testing are given in Table 2.

The uncoated as well as the coated specimens were polished down to 1m alumina wheel cloth polishing toobtain similar condition on all the samples before being subjected to erosion run. The samples were cleaned inacetone, dried, weighed to an accuracy of 1×10-5 g using an electronic balance, eroded in the test rig for 3 hours andthen weighed again to determine weight loss. In the present study standard alumina 50 micron ( supplied with Rigby Ducom Instruments Private Limited, Bangalore, India) was used as erodent . In general, Erosion resistance ismeasured using weight loss technique by measuring the weights before and after the test. But at high temperature,weight change measurements leads to flawed results due to oxidation of samples. In order to overcome thelimitations of the weight change technique, a different technique was used for the present investigation. Erosionresistance was measured in terms of thickness loss after the erosion testing.



45

Table 2 Erosion Test conditions

Erodent material Alumina (Irregular shape)

Erodent Specifications 50 micron Al2O3

Particle velocity (m/s) 35m/s

Erodent feed rate (g/min) 2 gm/min

Impact angle (°) 30, 90

Test temperature Sample at Room temperature & Air Temperature 900ºC

Sample Temperature 200ºC & Air Temperature 900ºC



Nozzle diameter (mm) 4

Test time (Hrs) 3 Hours

All the specimens subjected to erosive wear were analyzed for the characterization of erosion products. The

specimens were analyzed using surface SEM and EDAX.

3. Results and Discussions

The thickness of the coatings as measured from SEM micrographs taken along the cross section of themounted sampleshas been reported to be 350 μm and is in similar range as reported by Bhatia et al. [7], Wang et al.[10], Singh et al. [11], Sidhu et al. [12], Roy et al. [13], Espallargas et al. [14].The SEM micrographs taken from thecross-section of the coatings has been shown in Fig.2.The porosity of the coatings is a property which has asignificant role to play when high temperature erosion resistant coatings are to be developed. The porosity of thecoatings has been found to be less than 1.5% , similar results for porosity of the coating has been have been reportedby Bhatia et al. , Sidhu et al., Singh et al., et al., Murthy et al. and Fedrizzi et al. in their studies[7, 12,15-18].Surface roughness of coating has been evaluated by Surface roughness tester Mitutoyo Model SJ 201, the value ofsurface roughness has been reported as 5.272 μm which is in accordance with results reported by Bhatia et al.Espallargas et al., Fedrizzi et al., Roy et al. and Yin et al. [7, 14, 18-20].



46

Fig. 2 SEM micrograph showing cross-section of as coated 75% Cr3 C2 -25% (Ni-20Cr) sample

From Fig. 3 showing the SEM morphology and EDAX of the surface of the as sprayed coatings it can be observedthat the microstructures of the Cr3C2-NiCr coatings have almost uniformly distributed irregularly shaped finegrained microstructure. It can be inferred that the coating has uniform and dense microstructures and exhibitslayered morphologies due to the deposition and resolidification of molten or semimolten droplets. Surface SEM ofcoatings also indicated presence of melted, partially melted and unmelted particles, which are identified in thecoating by their size and surface morphology.

Figure 3 EDAX analysis of the surface of 75% Cr3 C2 -25% (Ni-20Cr) coating

Resin

Coating

Substrate

Ck 5.47%Ok 3.13%Crk 49.77%Fek 0.23%Nik 29.88%Nbk 3.96%Mok 5.67%

Ck 11.42%Ok 7.37%Crk 50.76%Fek 0.2%Nik 20.85%Nbk 4.53%Mok 2.88%

Ck 1.45% Ok 0.98%Crk 79.70% Fek 0.16%Nik 7.59% Nbk 3.19%Mok 4.44%

Voids Molten Particles



47

XRD was performed to determine phase changes/formations which takes place during HVOF spray process. For thisthe coating powder and HVOF as sprayed specimens were subjected to XRD analysis obtained by Bruker AXS D-8Advance Diffractometer (Germany) with CuK radiation. The specimens were scanned in 2θ range of 10 to 1100 andthe intensities were recorded at a chart speed of 1 cm/min and with Goniometer speed 10/min. The diffractometerinterfaced with software provides ‘d’ values directly on the diffraction pattern. X-ray diffractograms as shown inFig. 4 indicate the patterns received for 75%Cr3C2 -25% (Ni-20Cr) powders along with as coated samples. TheXRD diffraction pattern of powders indicated presence of Ni and Cr3C2 phases. The as coated samples prepared byHVOF spraying technique indicated Cr26C6, Cr7C3 and NiCr phases along with Ni, Cr and Cr3C2 phases. This isexpected in case of thermal spray coating deposition, which involves rapid cooling of the molten particles onto therelatively cooler substrate material in the as-sprayed coating. In general, little change in the coating composition isobserved for HVOF spray coatings as oxidation andphase transformation that occurs due to higher flame velocityand lower flame temperature of the HVOF process, which would limit the decomposition process.

Figure 4 XRD analysis of 75% Cr3 C2 -25% (Ni-20Cr) powder and as coated sampleIn many high temperature aggressive environments, coatings may have to face the erosion-corrosion attacksimultaneously. The softer coatings may be more at risk to this erosion –corrosion attacks at elevated temperatures.Microhardness of the coatings was measured by 300-gram load provided to the needle for penetration. Themicrohardness of the substrate has been in the range of 246-275 Hv and has been in range of 774-950 Hv for coatingas shown in Fig 5. The measured values of microhardnessof HVOF coating under study are found to be in goodagreementwith the findings reported by Bhatia et al. , Murthy et al., Roy et al., Yin et al., Wirojanupatump et al.Mathews et al, Zorawski et al., Scrivani et al.,Wang and Lee, Wang and Shui, Wang and Lee, Wang et. al.,Hawthorne et al., Uusitalo et al. and Sidhu et al. [7, 17, 19-31]. The higher microhardness of the coating than thesubstrate may be attributed to carbide content present in the coating.

Cr Ni Cr3C2NiCr Cr26 C6Cr7 C3

75% Cr3 C2 -25% (Ni-20Cr) powder

75% Cr3 C2 -25% (Ni-20Cr) sample

Inte

nsit

y (A

rbitr

ary

Uni

ts)



48

Fig. 5 Microhardness profile of the Coatings and substrate

3.1. Erosion mechanism and surface morphology

The mechanism by which material is removed from a coating under erosive conditions may be either ductile orbrittle. The ductile erosion occurs by cutting and deformation mechanism, whereas brittle erosion occurs by crackingand chipping mechanism of the fractured and loosened pieces whose size is determined by the grain size of thecoating [32].

From Fig. 6 it can be noticed that the solid particle erosion of the substrate is maximum at 30° impact angles, whichwas behaving in what is normally termed a ductile manner. Hence while considering the effect of impact angle it canbe inferred that at all the temperatures the thickness loss of uncoated T-91 substrate at 900 impact angle is lower thanthat at 300. However, while considering the effect of temperature, the thickness loss of substrate increases withincrease in temperature. The thickness loss of uncoated substrate at 30 0impingement angle is almost double at 600degree Celsius when compared with loss at room temperature, whereas at 900 impingement angle the thickness lossof the uncoated substrate at 600 degree Celsius is almost 2.5 times than at room temperature.For the 75% Cr3 C2 -25% (Ni-20Cr) coating it was observed that the thickness loss of the coating was lower at 300impact angle andincreases with increase in temperature. The coatings indicated somewhat brittle behavior of erosion.The thicknessloss even at room temperature was greater than that of uncoated substrate at 600 degree Celsius. But at the sametime the thickness loss with increase in temperature increases marginally. The specimens of coatings possessedsimilar chipped morphology, indicating the same brittle erosion mechanism. The coating eroded at high impactangle, showed a surface morphology with number of crater sites, originating from individual impacting erodentparticles. The Brittle behavior was not characterized by typical fracture or chipping mechanism but rather by intenselocalized plastic flow producing lips around the crater periphery as suggested by Hearley et al [33]. As evident fromthe cross-sectional micrograph shown in Fig 2 small splat and grain size, low porosity and absence of cracks mighthave been favorable factors for hot erosion resistance. It was also proved elsewhere that small NiCr particle withuniform distribution might provide improved impact resistance of coatings [34]. Levy [35] has observed that it iseasier for the erodent particles to knock off pieces of exposed surface and get the greater removal rateif porosity ofthe coating is greater.Further Vicenzi et al [36] reported in their study against high temperature erosion that reduced

Substrate Coating

Distance in µm

Micr

ohar

dnes

s (H v

)



49

amount of energy is required for the removal of parts of the surface if porosity is higher as the porosity acts toreduce the mechanical strength as well as modulus of elasticity.

Figure 6 Effect of sample temperature on the thickness loss.

Figure 7 shows the SEM micrographs of the surface morphologies of uncoated T-91 boiler tube steel eroded atdifferent sample temperatures with different impingement angles. In fig 7 (a) surface morphology of uncoated T-91sample at room temperature with 300 impingement angle shows formation of pits whereas at same temperature thePloughing action is visible with 900 impingement angle in fig 7(b). Fig 7 (c) and (d) shows surface morphology ofuncoated substrate at 2000 Celsius sample temperature where plastic flow of material is visible at 30° and pitformation takes place at 900 impingement angles respectively. Uncoated T-91 at 4000 Celsius sample temperatureand 300as well as 900impingement angles have shown plastic flow of material along with start of lip and craterformation as shown in fig 7 (e) and (f) respectively.

Thick

ness

loss

(μm

)

0

20

40

60

80

100

120

Unco

ated

T-9

1

Cr3

C2-(

Ni-2

0Cr)

Unco

ated

T-9

1

Cr3

C2-(

Ni-2

0Cr)

Unco

ated

T-9

1

Cr3

C2-(N

i-20C

r)

Unco

ated

T-9

1

Cr3

C2-(

Ni-2

0Cr)

RoomTemperature

200 Degree 400 Degree 600 Degree

30 Degrees

90 Degrees

Sample Temperature

Craters on the surface

(b)

Plouging action

(a)



50

Figure 7 SEM micrographs showing surface morphology of the uncoated eroded surfaces of T-91 boiler tube steel(a) At room temperature with 30°impingement angle (b) At room temperature with 90°impingement angle (c) At2000 C with 30°impingement angle (d) At 2000 C with 90°impingement angle.

In Fig 7 (g) and (h) respectively it can be visualized that at 6000 Celsius sample temperature and 300 impingementangle Ploughing seems to be the reason for erosion while at 900 impingement angle plastic flow and deformation byformation of lips and craters seems to be the erosion mechanism. Figure 8 shows the SEM micrographs of thesurface morphologies of 75% Cr3 C2 -25% (Ni-20Cr) coated T-91 boiler tube steel samples eroded at differentsample temperatures with different impingement angles.

(c)

Plastic Flow

(d)

Pits formed on the surface

Plastic Flow

(e) (f)



51

Figure 7 SEM micrographs showing surface morphology of the uncoated eroded surfaces of T-91 boiler tube steel(e) At 4000 C sample temperature with 30°impingement angle (f) At 4000 C sample temperature with90°impingement angle (g) At 6000 C sample temperature with 30°impingement angle (h) At 6000 C sampletemperature with 90°impingement angle.

Fig 8 shows the scanning electron micrographs of 75% Cr3 C2 -25% (Ni-20Cr) coated sample eroded at differenttemperature and different impact angles.. Fig. 8 (a) and (b) show the SEM micrographs at room temperature, wherefig 8 (a) Surface morphology at 30°impingement angle shows extensive plastic flow whereas fig 8(b) shows highlyplastically deformed surface of coating at 90° with lips and craters which may be the cause of brittle erosionmechanism in this case. Fig 8 (c) and (d) shows the SEM micrographs of the 75% Cr3 C2 -25% (Ni-20Cr) coatedsample at 2000 Celsius sample temperature with 300 and 900 impingement angle respectively. With 300

impingement angle the formation of pits can be visualized in fig 8(c), whereas formation of pits and holes in thecoating surface are visible in fig 8(d) clearly showing the mechanism of erosion.

(g)

Ploughing action

(h)

Plastic Flow and deformation

(b)

CratersLips

(a)

Plastic FlowLips



52

Figure 8 SEM micrographs showing surface morphology of the 75% Cr3 C2 -25% (Ni-20Cr) coated sample surfaces(a) At room temperature with 30° impingement angle (b) At room temperature with90° impingement angle (c) At2000 C sample temperaturewith 30° impingement angle (d) At 2000 C sample temperature with90° impingementangle (e) At 4000 C sample temperature with 30° impingement angle (f) At 4000 C sample temperature with90°impingement angle

Lips

(c)

Erodent Particles

(d)

Localized Pit Formation

(f)

Lips Craters(b)

(e)

Craters Lips Alumina Particle



53

Fig. 8 (e) shows the micrograph of the sample at 4000 Celsius sample temperature and with 300 impingement angle,microcutting scars (MC) along with raised lips and craters are visible in this micrograph. Fig 8 (f) shows surfacemicrograph of the sample at 4000 Celsius sample temperature and with 900 impingement angle, eroded coatingshows the craters (C) and lips (L) formed by the impact of erodent particles indicating intenseplastic deformation ofthe surfaces The coating eroded at high impact angle, showed a surface morphology with number of crater sites,originating from individual impacting erodent particles.

Figure 8 SEM micrographs showing surface morphology of the 75% Cr3 C2 -25% (Ni-20Cr) coated sample surfaces(e) At 6000 C sample temperature with 30° impingement angle (f) At 6000 C sample temperature with90°impingement angle

Figure 8 (g) and (h) shows SEM micrographs of 75% Cr3 C2 -25% (Ni-20Cr) coated sample surfaces at 6000 C

sample temperature with 30° and 900 impingement angles. The pit formation can be visualized in both themicrographs whereas in fig 8 (h) formation of lip can also be visualized. The results have indicated that themaximum thickness loss has been reported at this temperature ant both the impingement angles. The morphologiesof the eroded surface presented in Fig. 8(except for sample at 2000 Celsius and 900 impingement angle [fig 8 (d)])show no evidence of macro or microcracking in the coatings.The solid particle erosion rate of the uncoated substrate steel indicated maximum erosion took place for the samplestested at 300 impact angles, whereas thickness loss was lesser when the samples were tested at 900 impact angle.Such behavior is typical of a ductile material as proposed by Wang and Shui, Sidhu et.al., Wang et.al., Murthy et. al.and Mishra et.al. [26, 32, 37-39]. The material subjected to erosion initially undergoes plastic deformation and islater removed by subsequent impacts of the erodent on the surface. The material deformations in these cases werein a ductile manner so that the material is plastically deformed at impact sites with shearing at the edges of theimpact crater. This resembles the ductile degradation mechanisms referred to as ploughing or type I cutting byHutchings and Winter [40]. The morphology examination of the eroded surface also showed that the ploughingbecame more evident with the decrease of jet angle of erosive particles. Therefore, examination of surfaces oferoded coating suggests that the erosion of HVOF sprayed Cr3C2-NiCr coating occurs through microcutting andploughing of soft binder matrix by hard abrasives. With progress of erosion, the harder carbide particles are exposedand then gouged out by further impact of abrasive particles, which leads to further removal of the matrix by cutting.

(h)

Pits Lips

(g)

Ploughing action



54

It has been observed in general that the coated T-91 boiler tube steel sample showed higher thickness loss thanthat of uncoated T-91 steel. By contrast, the coatings exhibited greater thickness loss at a steep impact angle (900)than at a shallow impact angle (300), the coatings were deemed “brittle” materials. The coating at 300 impact anglehas shown decrease in thickness loss when the sample temperature was increased from room temperature to 2000C,with continuous increase in the thickness loss values afterwards at 4000C and 6000C. The initial decrease inthickness loss between room temperature and 2000C might be due to an increase in the fracture toughness of theCr3C2- NiCr coating. The increase in thickness loss between 400

0C and 6000C was due to a decrease in flexuralstrength of the Cr3C2- NiCr coating as suggested by Wang and Luer [41].Erosion in coatings containing less than 80 vol.% carbide, is controlled by the metallic binder (NiCr) but still thecoatings behaves in a brittle manner because erosion in this case is controlled by the “skeletal network” of carbides,where removal of individual carbide grains can be seen by impacting particles as suggested by Wang and Luer [41].The Brittle behaviour shown by the coatings under study was not characterized by typical fracture or chippingmechanism but rather by intense localized plastic flow producing lips around the crater periphery as suggested byHearley et. al., [42].

4. Conclusions

1. The overall ductile behavior has been revealed by SEM micrographs for uncoated T-91 boiler tube steel asthickness loss at 30o impingement angle is greater than that at 90o impingement angle.2. Results showed that the erosion rate of the uncoated boiler tube steel increases with the increase of the testtemperature at both 30o and 90o impingement angles.3. 75% Cr3C2 -25% (Ni-20Cr) coated sample shows the brittle mechanism of erosion. No evidence of macro ormicro-cracking was observed in the coatings after erosion.4. The coatings at all the temperatures have shown higher erosion rate than the uncoated T-91 boiler tube steelregardless of the impact angle.4. The results suggest that the erosion performance of the HVOF sprayed Cr3C2-NiCr coating was controlled by thecohesion between splats in the coating. The spalling-off of lamellae from the interlamellar interface was mainlyresponsible for the erosion of HVOF Cr3C2-NiCr coatings at relatively high erosion angles. Thus the erosionresistance could be further enhanced by improving splat cohesion.

REFERENCES

[1]. S.C. Agarwal, M. A. H. Howes, “Erosion-Corrosion of Materials in High Temperature Environments: Impingement Angle Effects inAlloys 310 and 6B under Simulated Coal Gasification Atmosphere”, J. Materials for Energy Systems 7 (1986) 370-381.

[2] Kevin J. Stein, Brian S. Schorr, Arnold R. Marder, “Erosion of thermal spray MCr–Cr C cermet coatings” Wear 224 (1999) 153–159[3] N. Gat, W. Tabakoff, "Effects of Temperature on the Behavior of Metals Under Erosion by Particulate Matter," Journal of Testing and

Evaluation, 8 (1980) 177-86.[4] V. Higuera, J. Hidalgo, BelzunceVarela , A. Carriles, S. Menéndez, Poveda Martınez, “High temperature erosion wear of flame and

plasma-sprayed nickel–chromium coatings under simulated coal-fired boiler atmospheres” Wear 247 (2001) 214–222.[5] Bu-Qian Wang, “Erosion-corrosion of coatings by biomass-fired boiler fly ash” Wear 188 (1995) 40-48.[6] Bu Qian Wang, “Hot erosion behavior of two new iron-based coatingssprayed by HVCC process” Wear 255 (2003) 102–109[7] R. Bhatia, H. Singh, B.S.Sidhu, “Hot Corrosion Studies of HVOF-Sprayed Coating on T-91 Boiler Tube Steel at Different Operating

Temperatures” Journal of Materials Engineering and Performance, 23, 493-505.[8] S. Matthews , B. James , M. Hyland, “Erosion of oxide scales formed on Cr3C2–NiCr thermal spray coatings” Corrosion Science 50

(2008) 3087–3094[9] S. Matthews, B. James , M. Hyland “High temperature erosion of Cr3C2-NiCr thermal spray coatings -The role of phase microstructure”

Surface & Coatings Technology 203 (2009) 1144–1153[10] J. Wang, B. Sun, Q. Guo, M. Nishio, H. Ogawa, “Wear resistance of Cr3C2-NiCr Detonation Spray Coating”, Journal of Thermal Spray

Technology, 11(2), 261-265, (2002)[11] H.Singh, B.S. Sidhu, S. Prakash,” Comparative characteristic and erosion behavior of NiCr coatings deposited by various HVOF spray

processes”, Journal of Material Engineering and Performance, 15(6), 699-704, (2006)[12] T. S. Sidhu, S. Prakash , R. D. Agrawal, “Characterisation and Hot Corrosion resistance of Cr3C2-NiCr coating on Ni Base superalloys in

an aggressive environment”, Journal of Thermal Spray Technology, 15(4), 811-816, (2006)



55

[13] Manish Roy, Andreas Pauschitz, Reinhard Polak, Friedrich Franek,” Comparative evaluation of ambient temperature friction behaviour ofthermal sprayed Cr3C2–25(Ni20Cr) coatings with conventional and nano-crystalline grains” , Tribology International, 39, 29-38, (2006)

[14] N. Espallargas, J. Berget, J. M. Guilemany, A. V. Benedetti, P. H. Suegama, “Cr3C2–NiCr and WC–Ni thermal spray coatings asalternatives to hard chromium for erosion–corrosion resistance” , Surface and Coatings Technology, 202, 1405-1417, (2008)

[15] Hazoor Singh, Buta Singh, S. Prakash, “Mechanical and Microstructural Properties of HVOF sprayed WC-Co and Cr3C2 –NiCr coatingson the boiler tube steels using LPG as the fuel gas”, Journal of Material Processing Technology, 171 (2006) 77-82.

[16] M. Factor, I. Roman, “Microhardness as a simple means of estimating relative wear resistance of carbide thermal spray coatings: Part 1.Characterization of Cemented Carbide Coatings”, Journal of Thermal Spray Technology, 11 (4) (2002) 468-481.

[17] J. K. N. Murthy, S. Bysakh , K. Gopinath , B. Venkataraman, “Microstructure dependent erosion in Cr3C2–20(NiCr) coating deposited by adetonation gun”, Surface and Coatings Technology, 204, (2007) 3975-3985.

[18] L. Fedrizzi, L. Valentinelli, S. Rossi, S. Segna , “Tribo Corrosion behavior of HVOF Cermet coatings”, Corrosion Science, 49 (2007)2781-2799.

[19] Manish Roy, A. Pauschitz, J. Wernisch, F. Franek, “The influence of temperature on the wear of Cr3C2–25(Ni20Cr) coating—comparisonbetween nanocrystalline grains and conventional grains”, Wear, 257 (2004) 799-811,

[20] B. Yin , G. Liu, H. Zhou, J. Chen, F. Yan, “Sliding wear behavior of HVOF sprayed Cr3C2-NiCr/CeO2 Composite coatings at elevatedtemperature upto 8000C”,Tribology Letter, 37, (2010) 463-475.

[21] S. Wirojanupatump, P. H. Shipway and D.G. Mc Cartney , “The influence of HVOF powder feedstock characteristics on the abrasive wearbehaviour of CrxCy–NiCr coatings”, Wear, 249 (2001) 829–837.

[22] S. Matthews , M. Hyland, B. James, “Long Term Carbide Development in High Velocity Oxy Fuel/High Velocity Air Fuel Cr3C2–NiCrcoatings heat treated at 9000 C” , Journal of thermal Spray Technology,13(4) (2004) 526-536,

[23] Wojciech Żórawski, Stefan Kozerski, “Scuffing resistance of plasma and HVOF sprayed WC12Co and Cr3C2-25(Ni20Cr) coatings”,Surface and Coatings Technology, 202 (2008) 4453-4457.

[24] A. Scrivani, S. Ianelli, A. Rossi, R. Groppetti, F. Casadei, G. Rizzi, “A contribution to the surface analysis and characterisation of HVOFcoatings for petrochemical application”, Wear, 250 (2001) 107-113.

[25] B.Q. Wang, S.W. Lee, “Elevated temperature erosion of several thermal-sprayed coatings under the simulated erosion conditions of in-bedtubes in a fluidized bed combustor”, Wear, 203–204, (1997) 580–587.

[26] B.Q.Wang, Z.R. Shui, “The hot erosion behavior of HVOF chromium carbide-metal cermet coatings sprayed with different powders”,Wear, 253 (2002) 550–5577.

[27] B. Wang, S.W. Lee, “Erosion–corrosion behaviour of HVOF NiAl–Al2O3 intermetallic-ceramic coating”, Wear, 239 (2000) 83–90,[28] J. Wang, B. Sun, Q. Guo, M. Nishio, H.Ogawa, “Wear resistance of Cr3C2-NiCr Detonation Spray Coating”, Journal of Thermal Spray

Technology, 11(2) (2002) 261-265.[29] H. M. Hawthorne, B. Arsenault, J. P. Immarigeon, J. G. Legoux, V. R. Parameswaran, “Comparison of slurry and dry erosion behavior of

some HVOF thermal sprayed coatings”, Wear, Vol. 225–229 (1999) 825–834.[30] M. A. Uusitalo, P. M. J. Vuoristo, T. A. Mantyla, “Elevated temperature erosion–corrosion of thermal sprayed coatings in chlorine

containing environments”, Wear, 252 (2002) 586–594,[31] H. S. Sidhu, B. S. Sidhu, S. Prakash, “Wear Characteristics of Cr3C2-NiCr and WC-Co coatings deposited by LPG fueled HVOF,

Tribology International, 43 (2010) 887-890.[32] Hazoor Singh Sidhu, Buta Singh Sidhu, S. Prakash, “Solid particle erosion of HVOF sprayed NiCr and Stellite-6 coatings”Surface &

Coatings Technology, 202 (2007) 232–238.[33] J.A. Hearley , J.A. Little, A. J. Sturgeon, “The erosion behaviour of NiAl Intermetallic Coatings produced by high velocity oxy-fuel

thermal spraying, Wear 233-235 (1999) 328-333.[34] Bu Qian Wang, Zheng Rong Shui “The hot erosion behavior of HVOF chromium carbide-metal cermetcoatings sprayed with different powders” , Wear, 253 (2002) 550–557.[35] A.V. Levy, “The erosion–corrosion behaviour of protective coatings”, Surf. Coat. Technol. 36 (1988) 387–406.[36] J. Vicenzi, D.L. Villanova, M.D. Lima, A.S. Takimi, C.M. Marques, C.P. Bergmann, “HVOF-coatings against high temperature erosion

(300 °C) by coal fly ash in thermoelectric power plant”, Mater. Des. 27 (3) (2006) 236–242.[37] Wang, B. Q. and Luer, K., (1994), “The Relative Erosion-Corrosion Resistance of Commercial Thermal Sprayed Coatings in a Simulated

Circulating Fluidized Bed Combustor Environment,” Proc. of the 7th National Spray Conf., 20-24th June, Boston, Massachusetts, pp. 115-20.

[38] Murthy. J.K.N., Rao. D.S. and Venkataraman. B., (2001), “Effect of Grinding on the Erosion Behavoir of a WC-Co-Cr Coating Depositedby HVOF and Detonation Gun Spray Processes”, Wear, Vol. 249, pp. 592-600.

[39] Mishra. S.B., Prakash. S., and Chandra. K., (2006), “Studies on erosion behavior of plasma sprayed coatings on a Ni-based superalloy,”Wear, Vol. 260, pp. 422–432.

[40] I.M. Hutchings, R.E. Winter, Wear 27 (1974) 121.[41] Wang, B. Q. and Luer, K., (1994), “The Relative Erosion-Corrosion Resistance of Commercial Thermal Sprayed Coatings in a Simulated

Circulating Fluidized Bed Combustor Environment,” Proc. of the 7th National Spray Conf., 20-24th June, Boston, Massachusetts, pp. 115-20.

[42] Hearley, J.A., Little, J.A. and Sturgeon, A.J., (1999), “The erosion behaviour of NiAl intermetallic coatings produced by high velocity oxy-fuel thermal spraying,” Wear, Vol. 233–235, pp. 328–333.

Documents

EROSION RESISTANCE OF CR C -25% (NI -20CR) COATING AT ...E-mail address: [email protected] ABSTRACT Erosion studies were carried out using an air-jet erosion test rig at a velocity