Tungsten carbide phase transformation during the plasma spray processD. Tu, S. Chang, C. Chao, and C. Lin

Citation: Journal of Vacuum Science & Technology A 3, 2479 (1985); doi: 10.1116/1.572862 View online: http://dx.doi.org/10.1116/1.572862 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/3/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in A Numerical Study of the Mechanical Behavior of Silicon Carbide due to PressureInduced PhaseTransformations During Nanoindentation AIP Conf. Proc. 908, 1167 (2007); 10.1063/1.2740968 Hydrogen Retention In PlasmaSprayed Tungsten AIP Conf. Proc. 837, 12 (2006); 10.1063/1.2213055 Laser processing of polycrystalline diamond, tungsten carbide, and a related composite material J. Laser Appl. 18, 117 (2006); 10.2351/1.2164472 Atomistic processes during nanoindentation of amorphous silicon carbide Appl. Phys. Lett. 86, 021915 (2005); 10.1063/1.1849843 Phase transformation of tungsten films deposited by diode and inductively coupled plasma magnetron sputtering J. Vac. Sci. Technol. A 22, 281 (2004); 10.1116/1.1642651

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Tungsten carbide phase transformation during the plasma spray process D. Tu, S. Chang, C. Chao, and C. Lin Chung-San Institute a/Science and Technology, P.o. Box 8678, Kangshan, Taiwan, Republic a/China

(Received 8 April 1985; accepted 8 June 1985)

Tungsten carbide coatings applied by the plasma spray process have been widely used in wear applications. In the W-C-Co ternary system, tungsten carbide can either be present as WC or W2C. Frequently WC transforms into W2C during the plasma spray process. In this study, tungsten carbide/17% cobalt coatings were applied by both the air plasma and the vacuum plasma spray processes using different power levels and plasma gases. The W 2C phase was found by x-ray diffraction techniques in the air plasma sprayed coatings. Decarburizing of the WC in the presence of oxygen took place in the plasma. In this study air plasma sprayed coating hardness and microstructure are superior to those of the vacuum plasma sprayed ones. However, the vacuum plasma coatings were found to be more wear and impact resistant than the air plasma coatings. This performance difference may be attributed to the presence of hard and brittle W 2C phases in the air plasma sprayed tungsten carbide coatings.

I. INTRODUCTION

Tungsten carbide coatings are the most widely used wear resistance coatings in industry. Tungsten carbide powders, mixed or alloyed with a binding agent, such as cobalt, can be readily applied to a workpiece by various thermal spray pro­cesses such as detonation gun, flame spray and plasma spray. I During the spray process, tungsten carbide powders are exposed to very high temperatures and different atmo­spheres. Decarburization may take place during this thermal exposure.

Two different tungsten carbide phases (WC and W 2 C) are usually found in commercially available powders and coat­ings. WC contains 6.13 wt. % carbon and W 2 C contains 3.16 wt. % carbon.2 The microhardness (kg/mm2) ofWC is 2400 while that of W 2 C is about 3000.3 Although W 2 C is harder than WC, in most applications W 2 C is not a desirable phase due to its brittleness.

In recent years, vacuum or reduced pressure plasma spraying has gained more attention because of its capability of producing high performance coatings from oxygen-sensi­tive materials.4

•5 The purpose of this study is to compare the

difference in the WC to W 2 C transformation in air versus vacuum plasma spray processing. II. EXPERIMENTAL PROCEDURE

Fine grade (minus 325 mesh) tungsten carbide/17% co-balt composite powders were used for both air and vacuum plasma spraying. The spray parameters are listed in Table I. Argon primary gas (45-60 l/min) and helium secondary gas (36--150 l/min) were employed. Power levels varied from 35 to 42.3 kW. Coatings were applied on grit blasted mild steel substrates.

Coating samples were sectioned and polished with dia­mond paste and alumina powder for metallographic exami­nation. Extreme care was taken to polish vacuum plasma sprayed samples to prevent coating pull out. Vickers micro­hardness was measured using a 300 g load. Ten randomly selected points of the cross section of each sample were test­ed and the readings averaged.

·Published without author correction.

Slightly polished coated samples and powder were ana­lyzed by x-ray diffraction for phase identification. A copper tube with a nickel foil filter were used to provide the x-ray source. A voltage of 40 k V and a current of20 rnA were used. Scanning range was 25° to 90° 2e. The scanning rate and the time constant were 4° per min and 1 s, respectively.

Selected wear/impact test samples (IOvs 9, IOvs 7,3 vs4, and 5 vs 6) were evaluated on wear/impact test equipment schematically shown in Fig. 1. Wear test samples (7 X 13 mm) ofSAE 4340 steel were hardened to RC 35 and coated with tungsten carbide. A fixed load of 50 kg (0.55 kgf/mm2) was applied to the sample pair continuously while the rub­bing surfaces were moved against each other at the frequen­cy of 300 cycles per min. The displacement was 10 mm. The sample pair also experienced impact load every 2 s. The load was equivalent to 70 kg falling from a height of 12 mm. Assuming the impact contact time is 0.005 s, the total impact load is 104 N (112 kgf/mm2). Samples were examined at 20 h intervals up to 80 h for weight changes and cracks. After 80 h of testing, equal to 1.44 X 105 impacts, the samples were sec­tioned for crack measurement.

FIG. \. Schematical drawing of wear/impact equipment.

2479 J. Vac. Sci. Technol. A 3 (6), Nov/Dec 1985 0734-2101/85/062479-04$01.00 © 1985 American Vacuum SOCiety 2479 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 79.170.128.150 On: Thu, 08 May 2014 05:43:01

~ < I» P en l2. -I ID () :::J' :::I TABLE I. Plasma spray parameters and coating characteristics. ~ 1> < ~ Items Spray parameters

So> Z P Primary Secondary Spray !T' Sample Energy gas (Ar) gas (He) distance Spray z No. (kW) (l/min) (l/min) (mm) process 0 < ...... 0 ID () .... 40 60 3 (H2) 350 Vacuum CD CD U1

2 35 SO 36 350 Vacuum

3 38 45 54 350 Vacuum

4 41 45 75 350 Vacuum

5 40 SO 100 350 Vacuum

6 40 SO 120 350 Vacuum

7 39 SO SO 350 Vacuum

8 42 SO SO 350 Vacuum

9 38 47.5 ISO 130 Air

10 38 SO SO 130 Air

Metallography

Apparent Tungsten porosity Precipitate carbide (vol. %) (vol. %) shape

5 60 Globular

4 65 Globular

4 65 Globular

6 60 Globular

5 60 Globular

5 55 Globular

6 60 Globular

5 60 Globular

Crystalline 60 precipitate

Crystalline 50 precipitate

Micro-hardness

HV(300g)

890

930

970

970

900

870

760

970

1140

1100

X-ray diffraction phase identification

Major Minor

WC-Co-W WC unknown

WC WC-Co-W unknown

WC WC-Co-W unknown

WC WC-Co-W unknown

WC-Co-W WC

unknown

WC-Co-W WC unknown

WC WC-Co-W unknown

WC WC-Co-W unknown

WC-Co-W WC+W2C unknown

WC+W2C WC-Co-W unknown

I\) .... CD 0

-I t:: Cb ... III ,... -I t:: :::I co !i ID :::I () I» .. c-o: ID '0 :::J' I» /II ID

I\) .... CD o

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2481 Tu et al. : Tungsten carbide phase

(a) 50 )Jm

FIG. 2. Micrographs of No.2 vacuum plasma sprayed tungsten carbide coating.

III. RESULTS AND DISCUSSION Coated sample microstructural characteristics, micro­

hardness and x-ray phase identification are listed on Table I along with the spray parameters. Numbers 9 and 10 air plas­ma sprayed tungsten carbide coating quality is far better than those of the vacuum plasma sprayed samples. Apparent porosity of the air plasma sprayed samples is around 1 % compared with 4%-6% in the vacuum plasma ones. Coating pull out during the sample preparation may contribute to part of the porosity found in the vacuum plasma samples. The volume percentage of the tungsten carbide precipitates varies from 50% to 65%. No correlation can be seen between the percentage of precipitates and the spray param­eters.

The shapes of the tungsten carbide in the air and vacuum plasma sprayed samples were extremely different. Typical photomicrographs are shown in Figs. 2 and 3. The vacuum

(a) 50 )lm

FIG. 3. Micrographs of No.9 air plasma sprayed tungsten carbide coating.

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

2481

(b)

20 )Jm

plasma sprayed tungsten carbide precipitates are globular and less distinct from the matrix, while the air plasma sprayed tungsten carbide precipitates are very distinct from the matrix and tend to agglomerate together.

The average hardness of the vacuum plasma sprayed sam­ples is about 200 points softer than those of the air plasma sprayed ones. This study was designed to evaluate the WC to W 2 C transformation rather than to optimize the vacuum plasma parameters. Further detailed evaluations are needed to determine the actual mechanisms which cause the metal­lography and hardness differences between the air and the vacuum plasma sprayed tungsten carbide coatings.

Plasma spray powder x-ray diffraction analysis showed that WC is the major phase with minor phase peaks with lattice spacing of 1.26, 1.42, 2.05, and 2.09 A. No matching data could be found in the JCPDS file. No W2 C or cobalt phase were found in the plasma powder. For all the vacuum

(b)

20 J.lm

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2482 Tu et al. : Tungsten carbide phase

TABLE II. Wear/impact test results.

Results' Total weight loss (mg)

Sample Sample pair No.

10 b 10 vs 9 9 b

10 b 10 vs 7 7 350

3 300 3 V5 4 4 270

5 290 5 vs 6 6 310

• Results are measured after 80 h of testing. b Negligible weight loss.

(a)

(b)

Cracks

severe severe

severe none

minor none

minor none

50)Jm

50}Jm

FIG. 4. Micrographs of No. 10 air plasma sprayed tungsten carbide coating after 80 wear/impact test.

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

2482

plasma sprayed samples, the diffraction peaks were almost identical to that of the plasma powder. This observation in­dicates no we to w 2 e transformation during the vacuum plasma process.

Numbers 9 and 10 air plasma sprayed samples showed a considerable amount (estimated to be 20% by volume) of W 2 e present at the expense of we phase.

The remarkable difference in the we to w 2 e transforma­tion between the air and the vacuum plasma sprayed pro­cesses can be mainly attributed to differences in the spray atmospheres. In vacuum plasma spraying, the powder may experience higher temperatures and longer dwell times be­cause of the longer flame and spray distance. But due to the absence of oxygen, decarburization can only take place by vaporizing carbon into the environment. Brewer et al. have found that at 2500 K we is only slightly volatile. 3 In the air plasma spray, oxygen can act as a very effective catalyst by completing the chemical equation 2e + O2 = 2eO. This equation is thermodynamically more favorable than the oxi­dation of cobalt or tungsten at high temperatures. 6

The wear/impact test results are listed in Table II. It is interesting to note that the air plasma sprayed coatings ex­hibited higher hardness but suffered much more severe cracking problems than the softer vacuum plasma sprayed coatings. The brittle W 2 e phase formed during the air plas­ma spray process is believed to have a detrimetal effect on the fatigue crack resistance. Figure 4 shows the partially pene­trated crack and completely penetrated crack of sample No. 10 after 80 h of testing. The crack lay perpendicular to the rubbing direction and propagated to the substrate. After the crack completely penetrated the coating, it started to propa­gate along the coating/substrate interface until it merged with another crack causing the coating to spall off.

IV. CONCLUSION

The we to w 2 e phase transformation takes place during air plasma spraying while the we phase remains unchanged during the vacuum plasma spray process. The oxygen in the air plasma environment promotes the decarburization. In the wear/impact tests, the harder air plasma sprayed tung­sten carbide coatings suffer more severe cracking problems than the softer vacuum plasma coated counter parts. The brittle W 2 e phase in the air plasma sprayed coating may be responsible for the poor fatigue cracking resistance.

IA. R. Nicoll, "Protective Coatings and Their Processing-Thermal Spray." CEI course on high temperature materials and coatings, 1984, Finland.

2M. Hanson, Constitution of Binary Alloys (McGraw-Hili, New York, 1958), p. 391.

'P. Schwarzkoph, Refractory Hard Metals (MacMillan, New York, 1953), p.170.

'S. Shankar, D. E. Koenig, and L. E. Dardi, J. Met. 33(10), 13 (1981). sH. Gruner, Thin Solid Films 118, 409 (1984). 6L. S. Darken and R. W. Gurry, Physical Chemistry of Metals (McGraw­Hill, New York, 1953).

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