Formation of Fe-Based Amorphous Coating Films

by Thermal Spraying Technique

Masahiro Komaki1, Tsunehiro Mimura1, Yuji Kusumoto1,Ryurou Kurahasi1, Masahisa Kouzaki2 and Tohru Yamasaki3

1Nakayama Steel Works, Ltd., R&D Division, Osaka 551-8551, Japan2Nakayama Steel Works, Ltd., Osaka 551-8551, Japan3University of Hyogo, Himeji 671-2280, Japan

Some amorphous Fe-Cr-P-C coating films having high hardness and high corrosion resistance have been produced by a newly developedthermal spraying technique. In order to control the temperatures of the powder particles in the flame spray and the substrate, a newly developedcylindrical nozzle, with external cooling nitrogen gas, was mounted to the front end of the thermal spraying gun. Fe70Cr10P13C7 films withvarious external cooling gas velocities between 20m/s and 40m/s exhibited entire amorphous structure without oxides and/or unmeltedparticles. Corrosion-resistance of the films was observed in immersion tests using various corrosive liquids. An amorphous film was formed onthe surface of the shaft sleeve of the slurry pump by using the cylindrical nozzle. This shaft sleeve was installed in the slurry pump of chemicalfertilizer maker’s production line and the life test was done under the real operation condition for two months.[doi:10.2320/matertrans.MAW201022]

(Received April 27, 2010; Accepted June 1, 2010; Published August 25, 2010)

Keywords: iron-chromium-phosphorus-carbon, amorphous coating film, thermal spraying method, rapid cooling, hardness, corrosion

resistance

1. Introduction

Thermal spraying is a superior technique for producingamorphous alloy coating films with large area on variousindustrial materials.1–4) Especially, Fe-Cr based amorphousalloys have high hardness and high corrosion resistance.5–8)

However, the Fe-Cr based amorphous alloys having highmelting temperatures of about 1000�C have not yet beenwell produced by previous thermal spraying methods. Theimportant problems in these methods may be due to thecontamination of oxide and the density of pores in thethermal sprayed films.1,2,9–13) Recently, there are someresearch papers that were able to inhibit the oxide formationby the shield nozzle installed at the front end of theatmospheric plasma spray gun or the high velocity oxygenfuel gun.14,15)

In the present study, formation of some amorphous Fe-Cr-P-C coating films having high hardness and high corrosionresistance have been demonstrated by a newly developedthermal spraying technique. In order to control the temper-atures of the powder particles in the flame spray and thesubstrate, a newly developed cylindrical nozzle with externalcooling nitrogen gas was mounted at the front end of thethermal spraying gun. Cooling rates of the sprayed sampleson SUS316L substrates were estimated to attain about106�C/s by measuring the temperature gradient of thespraying flame.

2. Experimental Procedures

The thermal spraying equipment used in this study wascomposed of a gas flame spraying gun and a newly developedcylindrical nozzle. The thermal spraying material was gas-atomized powder (grain size: 38–63 mm) of Fe70Cr10P13C7(at%) with a melting temperature of 997�C. A SUS316L(100mm� 50mm� 3:2mm) substrate, shot blasted by

alumina powder, was used. In order to keep a reductionatmosphere in the flame, an acetylene-rich gas fuel mixturewas used. Nitrogen gas was introduced into the cylindricalnozzle to prevent the formation of oxides in the coating film.The thermal spray gun was mounted on a robot arm, scannedat the rate of 300mm/s in feed rate and 10mm in sprayingpitch, and an alloy layer of up to 500 mm in film thickness wasproduced. Structural analysis of the coating films was doneby X-ray diffraction (Cu K� radiation with a graphitemonochromator, 40 kV-200mA) and transmission electronmicroscope (TEM:HF2000-200 kV). The amorphous alloycoating films, separated from the substrate, were immersed invarious kinds of corrosive liquid for the maximum period of4-weeks. For comparison, standard specimens of Hastelloy Cand commercially produced pure titanium were used. Anamorphous film was formed on the surface of the shaft sleeveof the slurry pump by using the cylindrical nozzle. This shaftsleeve was installed in the slurry pump of chemical fertilizermaker’s production line and the life test was done under thereal operation condition for two months.

Figure 1 shows a schematic illustration of cylindrical-nozzle type thermal spray gun. In order to control thetemperatures of the powder particles in the sprayed flame andthe substrate, and to prevent the formation of oxides in thecoating film, a newly developed cylindrical nozzle withexternal cooling nitrogen gas was mounted at the front end ofthe thermal spraying gun.

Figure 2 shows the cooling-gas flow rate, over thedistance from the cylindrical nozzle head to the substrate,measured by a pitot tube. The cooling-gas flow rate can becontrolled up to 60m/s by adjusting the gas pressure in thecylindrical nozzle. To keep the optimal temperature and flowstates of the spraying flame, the flow rate of the cooling gaswas set to the nearly equal to that of the spraying flame.In this study, optimal flow rate of the sprayed flame was30�40m/s.16)

Materials Transactions, Vol. 51, No. 9 (2010) pp. 1581 to 1585#2010 The Japan Institute of Metals

http://dx.doi.org/10.2320/matertrans.MAW201022

Figure 3 shows the thermal gradients of the sprayingflame, over the distance from the cylindrical nozzle head tothe substrate, measured by a thermocouple. In this diagram,the 0mm gradient curve indicates the thermal gradient at theflame center, while the 5mm and 10mm gradient curvesindicate the thermal gradient at the positions outlying fromthe center of flame. Although the center of flame hits thesubstrate at a temperature of approximately 1000�C, theflame temperature decreased sharply to about 300 to 500�Cfor the 10mm and the 5mm respectively positions.

Figure 4 shows the compositions of the thermal sprayingflame, over the distance from the cylindrical nozzle head to20mm and 70mm, measured by the orsat apparatus. In thisexperiment, nitrogen gas or compression air were used as acooling gas. In order to keep a reduction atmosphere in thespraying flame, an acetylene-rich gas fuel mixture was used.When nitrogen gas was used, oxygen was contained in thespraying frame, 0.2% and 0.8% at positions of 20mm and70mm away from the cylindrical nozzle head, respectively.On the other hand, when compression air was used, oxygenwas contained 8% and 14% in the spraying frame at samepositions.

3. Results

3.1 Structure of the coating filmsFigure 5 shows the optical micrographs in cross-section of

the thermal spray coating films with and without the newlydeveloped cylindrical nozzle. When the thermal spray gunwithout the cylindrical nozzle was used, many inclusions ofoxides have been observed in the coating film as shown in

C2H2

O2

Flame

Molten range Quenching range

Spraying film

Cooling gas N2

Cooling gas N2Spray powder material (Carrier gas: N2)

Cylindrical nozzle head

Internal cooling gas

Cylindrical nozzle (twofold cylinder)

Front end of thermal spray-gun nozzle

Substrate

Fig. 1 Schematic illustration of cylindrical-nozzle type thermal spraying gun.

Coo

ling-

gas

flow

rat

e, v

/m/s

Distance from the cylindrical nozzle head, l/mm

0.60MPa

0.35MPa

0.25MPa

0.15MPa

0 20 40 60 80 100

70

60

50

40

30

20

10

0

Position of the substrate

Fig. 2 Cooling-gas flow rate.

Spra

yed

flam

e te

mpe

ratu

re, T

/ °C

Distance from the cylindrical nozzle head, l/mm

0mm

5mm

10mm

0

200

400

600

800

1000

1200

0 20 40 60 80 100

1400

5mm

10mm

Cylindrical nozzle

Cross section of the flame0mm position

Position of the substrate

Fig. 3 Flame temperature gradient.

Com

posi

tion

of th

e sp

rayi

ng f

lam

e (v

ol.%

)

Distance from the cylindrical nozzle head, l/mm

0

5

10

15

20

0

5

10

15

20

0

5

10

15

20

CO

20mm 70mm 20mm 70mm

O2

CO2

Cooling Gas:N2 Cooling Gas:Air

CO

O2

CO2

Fig. 4 Compornent of the spraying flame.

1582 M. Komaki et al.

Fig. 5(a). On the other hand, shown in Fig. 5(b), when thethermal spray gun with the cylindrical nozzle was used, high-quality amorphous sprayed film without the inclusion ofoxides can be produced.

Figure 6 shows the X-ray diffraction patterns of thethermal spray coating films with and without the cylindricalnozzle. When the thermal spray gun without the cylindricalnozzle was used, crystalline peaks from many kinds of oxideswere observed. On the other hand, when the thermal spraygun with the cylindrical nozzle was used, X-ray diffractionpeaks are well-broadened, indicating structure of the film isthe amorphous.

Figure 7 shows a TEM image and a selected areadiffraction pattern (SADP) of the sprayed coating filmfabricated by using the cylindrical nozzle. It consists ofamorphous phase in general. The SADP with halo ringsalso supports that the coating film has an amorphousstructure. Compositions of the coating film obtained byEDX analysis are as follows; Fe: bal, Cr: 11.6 at%, P:13.7 at%, indicating a similar composition to that of the gas-atomized powder.

3.2 Corrosion testsThe amorphous alloy coating films separated from the

substrates were immersed in various kinds of corrosiveliquid. For comparison, standard specimens of Hastelloy Cand commercially produced pure titanium were used. Table 1shows the results of the corrosion tests. In the corrosionstandard for chemical plant materials, a corrosion rate in theweight variation of between 0�0:5 g/m2 day is consideredgood.17) Corrosion of the amorphous sprayed film scarcelyprogressed after an initial increase arising from the gener-ation of an oxide layer. On the other hand, both Hastelloy Cand titanium slightly corroded. Accordingly, the corrosiontests have clarified that the sprayed amorphous film exhibiteda high corrosion resistance, equal to that of Hastelloy C andcommercially produced pure titanium, under most condi-tions. However, the corrosion resistance test in hydrochloricacid did not show a favorable result for the Fe70Cr10P13C7film. The corrosion liquids containing strong oxidizers suchas nitric acid and sulfuric acid etc. usually exhibit the highformability of the passive films on the sample surface. Onthe contrast, the hydrochloric acid is the nonoxidizing acid,

(a) (b)

Substrate (SUS316L)

Amorphous

Substrate (SUS316L)

Amorphous

Oxide

Fig. 5 Optical micrographs in the cross section of the coating films: (a) Without the cylindrical nozzle; (b) With the cylindrical nozzle.

20° 40° 60° 80° 100°2θ

With the cylindrical nozzle

Without the cylindrical nozzle

Inte

nsity

Fig. 6 X-ray diffraction patterns of the spray coating films.

Diffracted pattern

Site being diffractedand EDX analyzed

Fig. 7 TEM image of the spray coating film with the cylindrical nozzle.

Table 1 Results of the corrosion tests.

Corrosion rate in the weight variation (g/m2 day)

CorrosiveTemperature

Immersion AmorphousStandard materials

liquids period sprayed

filmHastelloy

CTitanium

Hydrochloric

acid, 35%6 h �7890 �4:56 �312

Nitric acid,

10%+0.065 0 �0:004

Sulfuric acid,

5%+0.049 0 �0:013

Sulfuric acid,

70%�0:011 �0:013 �0:117

Caustic soda,

48%

Room

temperature4 weeks

+0.024 �0:008 �0:004

Sodium

hypochlorite,

12%

+0.079 0 0

Salt water,

3%+0.154 0 0

Phosphoric

acid, 5%+0.100 — —

Formation of Fe-Based Amorphous Coating Films by Thermal Spraying Technique 1583

indicating the formation speed of the passive film may beslower than that of the other liquids.

3.3 Life test of the sprayed coated slurry pump shaftsleeve

An amorphous film was formed on the surface of the shaftsleeve of the slurry pump by using the cylindrical nozzle.This shaft sleeve was installed in the slurry pump of chemicalfertilizer maker’s production line and the life test was doneunder the real operation condition for two months.

Figure 8 shows the diagrammatic illustration of the slurrypump. The shaft sleeve of the slurry pump was worn easilybecause of the griding effects of slurry that sandwichedbetween the driving shaft and the seal. Therefore, thedevelopment of the material of high hardness and highcorrosion resistance has been demanded.

Table 2 shows the life test conditions for the shaft sleeveof the slurry pump. Surface of the SUS304 shaft sleeve wasshot blasted by alumina powder, and then NiCr undercoatlayer with about 50 mm thickness was formed. The Fe-Cr-P-Camorphous film of 300 mm thickness was coated on the NiCrlayer. Finally the surface of the coated shaft sleeve wasground with a diamond grindstone and finished up. As aresult, the thickness of the remained Fe-Cr-P-C coating filmwas about 150 mm.

Figure 9 shows the wear track of the shaft sleeve afterlife test for two months. There is no wear track in theamorphous thermal spraying shaft sleeve though wear trackof 4 mm is admitted in conventional SCS23 product (Fe: bal,

Cr: 20mass%, Ni: 30mass%, Mo: 3mass%, Cu: 3mass%).The amorphous thermal spraying shaft sleeve indicatedhigh corrosion resistance and high abrasion resistance(HV700�900).

4. Discussion

Figure 10 shows a schematic diagram explaining thetemperature distribution of the flame just prior to the inpacton the substrate. When the cylindrical nozzle is not used, thehigh temperature area at about 1000�C is spread over theentire flame. In contrast, when the cylindrical nozzle is used,the high temperature area is well converged to the center ofthe flame as expected from the results of Fig. 3.

Slurry inhalation

Slurry exhalation

Seal

Pump drive shaft

Impeller

LinerCover plate

Fe-Cr-P-C amorphouscoated shaft sleeve

Fig. 8 Diagrammatic illustration of the slurry pump.

Table 2 Test conditions and results of the varification test by using the

amorphous coated and uncoated shaft sleeve of the slurry pump.

Item Conventional product Material under test

Substrate: SUS304

Material SCS23Under coat: NiCr 50mmTop coat: Fe70Cr10P13C7

150mm

Test circumstanceSlurry pump of chemical fertilizer maker’s

production line

Required

performance

High hardness and high corrosion resistanc in

slurry of pH2

Depth of wear track

after two months4 mm 0mm

Wear track withslurry and seal

SCS23 material

Depth of wear track : 1.5 µ m

0 0

4

Fe70Cr10P13C7 coated materialµ m

µ m

µ m

Fig. 9 Wear track of the shaft sleeve after two months.

Lower temperature area

(a) (b)

High temperature areaabout 1000°C

Fig. 10 Schematic diagram explaining the temperature distribution of the

flame just prior inpact on the the substrate: (a) Without the cylindrical

nozzle; (b) With the cylindrical nozzle.

1584 M. Komaki et al.

Figure 11 shows the changes of the substrate temperatureas a function of the thermal spraying time with and withoutthe cylindrical nozzle. The thermal spray gun was mountedon a robot arm, scanned at the rate of 300mm/s in feed rateand 10mm in spraying pitch. Temperature of the substrate atthe surface center point was measured by radiation ther-mometer. When the thermal spray gun without the cylindricalnozzle was used, a hot area expands over all the flame, thuscausing a rapid increase of substrate temperature beyond thevitrification point (493�C) and, accordingly, crystallization ofthe coating occured easily. In contrast, when the thermalspray gun with the cylindrical nozzle was used, the substratetemperature saturated gradually under the vitrification tem-perature (400�C), indicating the high formability of theamorphous film. Therefore, the high scanning speed of300mm/s is very effective for rapid cooling of the coatingfilms, preventing the crystallization of the former coatedamorphous layer.

5. Summary

Some amorphous Fe-Cr-P-C coating films having highhardness and high corrosion resistance have been producedby a newly developed thermal spraying technique. In order tocontrol the temperatures of the powder particles in the flamespray and the substrate, a newly developed cylindrical nozzlewith external cooling nitrogen gas was mounted to the frontend of the thermal spray gun. When the thermal spray gunwith the cylindrical nozzle was used, the substrate temper-

ature increased gradually because of the lower temperaturearea surrounding the high temperature area of about 1000�C.Cooling rates of the sprayed samples on SUS316L substrateswere estimated to attain about 106�C/s by measuring thetemperature gradient of the spraying flame. Fe70Cr10P13C7films with various external cooling gas velocities between20m/s and 40m/s exhibited an entirely amorphous structurewithout oxides and/or unmelted particles. Corrosion testinghas clarified that the sprayed amorphous films exhibited ahigh corrosion resistance equal to that of Hastelloy C andcommercially produced pure titanium. An amorphous filmwas formed on the surface of the shaft sleeve of the slurrypump by using the cylindrical nozzle. This shaft sleeve wasinstalled in the slurry pump of chemical fertilizer maker’sproduction line and the life test was done under the realoperation condition for two months. The amorphous thermalspraying shaft sleeve indicated high corrosion resistance andhigh abrasion resistance.

REFERENCES

1) A. Kobayashi, S. Yano, H. Kimura and A. Inoue: Surf. Coat. Technol.

202 (2008) 2513.

2) Z. Zhou, L. Wang, F. C. Wang, H. F. Zhang, Y. B. Liu and S. H. Xu:

Surf. Coat. Technol. 204 (2009) 563.

3) A. H. Dent, A. J. Horlock, D. G. McCartney and S. J. Harris: Surf. Coat.

Technol. 139 (2001) 244.

4) S. Yoon, H. J. Kim and C. Lee: Surf. Coat. Technol. 200 (2006) 6022.

5) M. Naka, K. Hashimoto and T. Masumoto: J. Japan Inst. Metals 38

(1974) 835.

6) K. Hashimoto: J. Japan Inst. Metals 15 (1976) 203.

7) K. Hashimoto: J. Japan Inst. Metals 18 (1979) 362.

8) K. Asami, H. Kimura and A. Inoue: J. Jpn. Soc. Powder Powder Metall.

54 (2007) 795.

9) K. Kishitake, H. Era and F. Otsubo: J. JTSS 5 (1996) 476.

10) F. Otsubo, H. Era and K. Kishitake: Quarterly J. Japan Weld. Soc. 19

(2001) 54.

11) M. Fukumoto, C. Yokoi, M. Yamada, T. Yasui, M. Sugiyama, M.

Ohara and T. Igarashi: Quarterly J. Japan Weld. Soc. 25 (2007) 323.

12) M. Fukumoto, Y. Okuwa, M. Yamada, T. Yasui, Y. Motoe, K.

Nakashima and T. Igarashi: Quarterly J. JapanWeld. Soc. 26 (2008) 74.

13) N. Nagao, M. Komaki, R. Kurahashi and Y. Hrihara: J. Japan Inst.

Metals 71 (2007) 742.

14) T. Fukushima and S. Kuroda: Quarterly J. Japan Weld. Soc. 20 (2002)

439.

15) N. Sakakibara, Y. Manabe, Y. Hiromoto and Y. Kobayashi: Quarterly

J. Japan Weld. Soc. 24 (2006) 181.

16) A. Hasui: Shinpan-youshakougaku, (Sanpoushuppan, Tokyo, 1996)

p. 28.

17) S. Hatano: Kagakusouti-zairyoutaisyokuhyou, (Kagakukougyosya,

Tokyo, 1984) p. 194.

0

200

400

600

800

0 60 120 180

Thermal spraying time, t/s

With the cylindrical nozzle

Without the cylindrical nozzle

Vitrification temperature(493°C)

Subs

trat

e te

mpe

ratu

re (

°C)

Fig. 11 Changes of the substrate temperature as a function of the thermal

spraying time when the thermal spray gun scanned at the rate of 300mm/s

in feed rate and 10mm in spraying pitch.

Formation of Fe-Based Amorphous Coating Films by Thermal Spraying Technique 1585