Propylene flow, microstructure and performance of WC–12Co coatings using a gas-fuel HVOF spray process

Journal of Materials Processing Technology 213 (2013) 1653–1660

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Propylene flow, microstructure and performance of WC–12Co coatings using a gas-fuel HVOF spray process Qun Wang a,b,∗ , Jing Xiang b , Genyu Chen a , Yingliang Cheng b , Xinqi Zhao b , Shiqi Zhang c a b c

The State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, PR China College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, PR China Wuxi Fulaida Petroleum Machinery Co. Ltd., Wuxi 214192, PR China

a r t i c l e

i n f o

Article history: Received 2 January 2013 Received in revised form 16 April 2013 Accepted 17 April 2013 Available online 25 April 2013 Keywords: HVOF WC–Co Microstructure Abrasive wear Sliding wear

a b s t r a c t Five WC–12Co coatings were deposited by a high velocity oxy-fuel (HVOF) system using constant oxygen flow and varying propylene flow. The phase composition, microstructure, as well as abrasive and sliding wear performance of the as-sprayed coatings were investigated. The degree of tungsten carbide (WC) decarburization in the as-sprayed coatings increases while the coating porosity decreases with the increase of the propylene flow. The coating hardness, fracture toughness, resistance to abrasive and sliding wear increases with the increase of the propylene flow, reaches maximum and then decreases. At the low flow of the propylene, relatively loose coating microstructure is formed, which leads to fracturing and pulling off the WC particles during abrasive and sliding wear process. Herewith, at the high flow of the propylene, the high degree of the WC decarburization and high brittleness of the coating leads to micro-cutting during abrasive wear as well as to cracking and delamination of the coating in the sliding wear process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Tungsten carbide (WC)-based powders are widely used in highvelocity oxygen fuel (HVOF) spraying to produce dense coatings with high hardness and excellent wear resistance (Marple et al., 2001). Fang et al. (2009) and Wang et al. (2009) reported that HVOF sprayed WC-based coating exhibited better anti-wear performance as compared with hard chrome plating. Besides initial powder characteristics, the coating process parameters such as feed rate spray distance, type of fuel gas, fuel gas pressure and flow, oxygen gas pressure and flow are important for the performance of the assprayed WC-based coating. Some researchers have explored the effect of fuel flow on the performance of HVOF-sprayed WC/Co(Cr) coatings. Qiao et al. (2003) found that the fuel flow strongly influenced both the phase composition and the performance of the WC/Co coatings deposited by the propane-fueled DJ27000 spray equipment. In contrast, Wang et al. (2012a) found that the fuel flow exhibited little effect on the phase composition but strongly influenced the performance of WC–12Co coatings deposited using the kerosene-fueled JP-8000 spray system. Although the HVOF

∗ Corresponding author at: College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, PR China. Tel.: +86 013787113453; fax: +86 731 88823554. E-mail addresses: [email protected], [email protected] (Q. Wang). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.04.007

flame temperature is significantly lower than that used in the APS process, decarburization still cannot be completely avoided during the deposition process. The influence of the degree of decarburization on the properties of the WC/Co coating, including hardness, porosity, fracture toughness and wear resistance, has not been adequately identified. Therefore, five WC–12Co coatings with different degree of the WC decarburization were deposited using the Jet Kote® III HVOF spray system (Deloro Stellite) with a constant oxygen flow and various propylene flows. The microstructure and performance of the as-sprayed coatings under different wear conditions were investigated to identify the influence of the degree of WC decarburization on the properties of the WC/Co coatings. 2. Experiment 2.1. Materials The WC–12Co powder was sprayed onto a low-carbon steel substrate. The powder was spray-dried and agglomerated and then sintered into sprayable particles in the range of 10–45 ␮m. 2.2. Preparation of the HVOF coatings Prior to the spraying process, rectangular (200 mm × 57 mm × 5 mm) low-carbon steel samples were

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Table 1 The process parameters of five WC–12Co coatings. Coating code

Propylene flow (L/min)

Oxygen flow (L/min)

Feed rate (g/min)

Spray distance (mm)

C1 C2 C3 C4 C5

96 106 116 126 136

1020 1020 1020 1020 1020

55 55 55 55 55

175 175 175 175 175

degreased and grit blasted with 60-mesh Al2 O3 . An approximately 0.3 mm thick WC–12Co coating was deposited onto the substrates using the Jet Kote® III HVOF system equipped with mass flow meters. The spray parameters for five coatings are given in Table 1. Based on operating experience and the spray parameters provided by the manufacturer of the spray equipment, the other spray parameters were set as follows: the spray gun velocity in the horizontal direction was 1500 mm/s, and the step in the vertical direction was 2.5 mm; the powder carrier gas was argon with a flow of 57 L/min; the spraying angle was 90◦ ; and the temperature of the substrate during spraying was maintained below 150 ◦ C via compressed-air cooling. 2.3. Characterization The equation proposed by Evans and Wilshaw (1976) was used to calculate the fracture toughness of the as-sprayed WC–12Co coatings. Other characterizations of the powder and coatings were the same as in the previous work (Wang et al., 2012a). 2.4. Abrasion wear tests The coated specimens with a dimension of 57 mm × 25 mm × ∼5.3 mm were tested using a dry sand rubber wheel abrasion technique. A steel wheel covered with vulcanized rubber with a Shore hardness of 72 was turned against the test specimen under a load of 100 N. The rotation speed of the rubber wheel was 240 rpm. The flow of 40–70 mesh quartz sand was approximately 88 g/min. The coating mass loss was measured every 1000 revolutions and the total duration of the test was 3500 revolutions. The first 500 revolutions were to accommodate the system and were not taken into account in the wear measurements. All the samples before and after the test were cleaned ultrasonically for 5 min in acetone and then dried by a stream of hot air. The weight loss of the samples was measured using an electric balance with an accuracy of 0.1 mg and the wear data reported here is the average of three samples for each case.

3. Results 3.1. Phase composition The phase composition of powder and coatings were investigated using XRD. Fig. 1 shows the XRD analysis results for the WC–12Co powder and the corresponding coatings. The phase composition of all coatings is composed primarily of WC with various contents of W2 C, metal W and amorphous Co binder. When the flow of propylene was increased, the WC decomposition gradually increased and the C5 coating exhibited the most severe decarburization, with the most intense W peak in the XRD pattern among the patterns of all the as-sprayed coatings. In addition, the Co peak disappeared and some amorphous peaks appeared in the XRD patterns of all the as-sprayed coatings because of the rapid cooling rate of the molten and semi-molten sprayed particles after their impact with the substrate during the spraying process.

3.2. Microstructure of WC–12Co coatings The typical cross-sectional SEM images of the C1, C2 and C5 coatings are shown in Fig. 2. All the coatings exhibit good adhesion with the substrate according to their low-magnification cross-sectional images, whereas the amount of laminar-structured areas increases with the increase of propylene flow according to the highmagnification BSE images of the as-sprayed coatings. These white laminar-structured areas may form during the dissolution of WC into the Co binder, which has also been proposed by Qiao et al. (2003) and Wang et al. (2012b). In addition, it may also cause the decrease of the grain size of WC.

2700 2400 2100 Internsity/a.u.

2.5. Sliding wear tests The coatings were ground and polished to obtain an average surface roughness (Ra) of 0.08 ␮m. Before each test, the coating and ball were ultrasonically cleaned in acetone to remove any contaminants and grease, and then dried by the hot air. The dry sliding wear test was performed using a reciprocating ball-on-block UMT-3MT tribometer (Center for Tribology, Inc., Campbell, United States) at room temperature. A Si4 N3 ball (diameter of 9.525 mm) was used as the counter body. All tests were performed under a load of 50 N, a sliding speed of 0.075 m/s and a total time of 1800 s. The sliding wear data reported here is the average of three samples for each case.

1

1:WC 2:W2C 3:W 4:Co

1 1

23 2

C5 coating

1800

2

1500

C4 coating

1200

C3 coating

900

C2 coating

600

1

11

11

1

C1 coating

300 4 0 30

40

4 50

Powder 60

70

80

Fig. 1. XRD patterns of the WC–12Co powder and as-sprayed coatings.

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Fig. 2. The cross-sectional microstructures of C1 (a and b), C2 (c and d) and C5 (e and f) coatings.

3.3. Properties of WC–12Co coatings

The main peak height ratio of W2 C to WC phases increases while porosity decreases with the flow of propylene increase. Both the hardness and fracture toughness first increase with the flow of propylene and then decrease. Among five different coatings, the C2 coating exhibits the highest hardness and fracture toughness.

The hardness, porosity and fracture toughness of all the WC–12Co coatings, as well as the main peak height ratio of W2 C to WC phase, are listed in Table 2.

Table 2 Hardness, porosity, fracture toughness and the main peak height ratio of W2 C to WC phases of WC–12Co coatings. Coating

Hardness

C1 C2 C3 C4 C5

1012 1125 1102 1102 1080

HV0.3 ± ± ± ± ±

Porosity (%)

Fracture toughness MPa m(1/2)

Ratio of W2 C to WC

HV3 123 103 59 71 79

917 1003 979 1006 971

± ± ± ± ±

68 48 53 51 64

1.2 1.1 0.9 0.7 0.6

± ± ± ± ±

0.3 0.3 0.3 0.2 0.2

3.8 4.7 4.5 4.2 3.6

± ± ± ± ±

0.5 0.6 0.6 1.4 0.9

0.21 0.27 0.34 0.37 0.43

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2.8 HV0.3

1200

Wear loss

HV0.3

-5

1000

Wear Loss(×10 g/m)

2.4

2.0

1.6

800

1.2 600 C1

C2

C3

C4

C5

Coating Fig. 3. Schematic of deposition process of the WC–12Co particles.

4. Discussion 4.1. Decarburization mechanism of the HVOF spray WC–Co coating Numerous authors have reported that WC grains experience different degrees of decarburization during the HVOF process because of the oxidization reaction between carbon and oxygen. Vinayo et al. (1985) and Liao et al. (2000) proposed that the decarburization of WC resulted from the direct oxidation of WC, whereas Verdon et al. (1988) argued that the indirect oxidation of WC, which occurred when molten liquid cobalt was used as a media, was the primary model of decarburization. Kear et al. (2001) and Fincke et al. (1994) also supported the indirect decarburization mechanism of the WC. However, Chivavibul et al. (2007) and Guilemany et al. (1999) proposed that the WC decarburization in both direct and indirect models during the HVOF spraying process resulted in generation of W2 C, W and amorphous Co–W–C phases and this mechanism can also explain the decarburization process of the five as-sprayed WC–12Co coatings in this work. 4.2. Deposition process of the HVOF-sprayed WC–Co coatings According to the coating cross-sectional morphology (Fig. 2), the deposition process for WC–12Co particles with various sizes during HVOF spraying can be illustrated as in Fig. 3. The size distribution of the spray particles is 10–45 ␮m, so the spray particles were divided into fine, medium and coarse particles (Fig. 3). Different sized particles experienced different degrees of melting in the spray flame. The binder in the spray particles melts from the outside region to the inner region, so only the surface regions of the coarse particles were fully melted, and the core regions remained in a partially molten or in a soft solid state during the spray process. Fine particles, however, were overheated and fully molten. The degree of melting of medium-sized particles was in between that of the fine- and coarse-sized particles. The WC particles experienced the greatest degree of decarburization in the fully molten region (marked as “A”, “D” and “E” in Fig. 3) of the sprayed particles and generated the corresponding splats, marked as “a”, “d” and “e” in the coating, respectively, when they were deposited onto the substrate. A large number of nonWC phases, such as W2 C, W and amorphous Co, appeared in these splats (marked as “a”, “d” and “e”) because of the intensive decarburization of WC and the high cooling rate of the sprayed particles. With respect to the medium and inner parts of the coarse particles

Fig. 4. The hardness and wear loss of the HVOF sprayed WC–12Co coatings.

and the inner part of the medium size spray particles that are not fully overheated, many more WC particles would deposit on the substrate with a partly molten binder and result in less decomposition of the WC. In addition, the atomic mass of Co is less than that of W. Consequently, the back-scattering images of the binder taken in areas with no or few decarburization regions deposited by the medium and inner parts of coarse- or medium-sized particles are darker than those taken in areas with numerous decarburization regions (Figs. 2 and 3). 4.3. Abrasive wear mechanism of the HVOF-sprayed WC–Co coatings 4.3.1. Relationship among the decarburization degree, the hardness and the wear resistance of WC–12Co coatings The changes in the hardness and wear loss for all five coatings are shown in Fig. 4. The hardness of the coatings first increases and then decreases as the propylene flow is increased, whereas the wear loss shows almost the reverse tendency. The C2 coating, which was deposited with a propylene flow of 106 SCFH and with a constant oxygen flow of 1020 SCFH, exhibited the highest hardness and lowest abrasive wear loss. Fully molten sprayed particles in the plume increase the decarburization level of WC; however, they improve the density of the coating because of their strong ability to spread out. Yao et al. (1998) and Jacobs et al. (1999) reported that the decarburization of WC detrimentally affected the abrasive wear resistance of the coating as a result of the increased brittleness and the decreased content of hard particles; Schwetzke and Kreye (1999) and Picas et al. (2011) however, held the reverse opinion that a more compact structure and a higher hard particles content (W2 C), in contrast to the WC phase that results from the fully molten sprayed particles, are beneficial to the abrasive wear resistance for HVOF WC/Co coatings. The hardness and the wear resistance of the WC–12Co coatings first increases and then decreases with the increase of the propylene flow and the increase of the degree of decarburization according to the results in Table 2 and Fig. 2. The C2 coating, which had the main peak height ratio of W2 C to WC phase of 0.27, exhibited the highest hardness and the highest wear resistance. Both the greater and lesser decarburization degree of WC in the other coatings showed decreased wear resistance. Moderate decarburization in HVOF-sprayed WC–12Co coatings (the main peak height ratio of W2 C to WC phase is approximately 0.27 for the Jet Kote® HVOF spray system) produces beneficial effects on the abrasive

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Fig. 5. SEM micrographs of the worn surfaces of C1 (a), C2 (b) and C5 (c) and the BSE image of (c) (d).

resistance of the coating as a result of its highest hardness and fracture toughness. 4.3.2. Abrasive wear mechanism of HVOF-sprayed WC–Co coatings Typical abrasive worn surfaces of the C1, C2 and C5 coatings are shown in Fig. 5. Some differences were noted in the micrographs of the three worn coatings: the C1 coating exhibited fewer grooves but more pits and cracked WC particles, whereas the C5 coating exhibited more deep grooves and pits compared to the C2 coating. The C2 coating showed the smoothest worn surfaces without apparent cracked WC particles and deep grooves. These micrographs are consistent with the wear-loss results (Fig. 4). The percentage of the high-hardness WC grains decreased and that of other phases, such as the W2 C, W and amorphous Co phases, increased as the degree of decarburization increased. The indention morphologies in different regions with different degrees of decarburization are shown in Fig. 6 (the load was 0.98 N). The WC–12Co coating deposited using the HVOF process is a typical inhomogeneous material with a hardness that varies between regions (Fig. 6). The tested values of hardness (HV0.1) were 1565.3, 1226.2 and 926.5 in the regions marked “1”, “2” and “3,” respectively. The tested hardness values in the other regions which having the similar microstructure morphologies to those regions examined in Fig. 6 of the C5 coating exhibited similar variation pattern. Ludema and Kenneth (1996) proposed that the rate of material loss by abrasion strongly depends on the ratio () of the hardness of the abrasive (Ha ) and that of the wearing material (Hm ), that is: =

Ha Hm

(1-1)

If  < 0.8, the wear rate is low; If 0.8 <  < 1.25, the wear rate is moderate, which indicates a transition region;

If  > 1.25, the wear rate is high. Joseph and Albany (1992) reported that the hardness of SiO2 is approximately HV850.0; therefore, the ratios () between the abrasive hardness (Ha ) and that of the wearing material (Hm ) in the regions marked by “1”, “2” and “3” are 0.54 (<0.8), 0.69 (<0.8) and 0.92 (0.8–1.25) respectively. The wear rates in regions “1” and “2” are low, whereas the wear rate in region “3” belongs to the transition region, which exhibits a higher wear rate when compared to that in regions “1” and “2”. These results strongly agree with the micrograph of the worn surfaces in Fig. 5, i.e., grooves usually appear in regions where only a small number of hard WC particles remain because of the intensive decarburization of the WC phase. In contrast, the grooves can barely be observed in the regions that experienced a slight or moderate degree of decarburization and where a large number of WC particles remained. This difference can be attributed to the widespread distribution of high-hardness WC particles on the surface of the WC–12Co coating effectively hindering the penetration of the SiO2 abrasives. With respect to different WC–12Co coatings, the ratios of regions in each coating with different degrees of decarburization differ; for example, the ratio of “Region 3” in the C5 coating is significantly greater than that in the C1 or C2 coating, which resulted in lower hardness and poorer abrasive wear resistance. Jia and Fischer (1997) proposed an abrasive wear mechanism of the WC–Co coating as follows: (i) extrusion of the binder phase and its removal by plastic deformation and fatigue; (ii) undermining of the particles and subsequent particle pull-out; (iii) microcutting; (iv) carbide grain fracture; and (v) delamination of the coating. This wear mechanism can also be used to generally explain the wear process of the as-sprayed WC–Co coatings in this work. Herewith, the coatings, which exhibited different degrees of decarburization, also exhibited a different main wear failure mechanism. Based on the results in Fig. 5, more and more grooves appeared in the worn surfaces of the as-sprayed coatings as the degree of decarburization increased, especially for the C5 coating, whose

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Fig. 6. The indention micrographs in the cross-section of a C5 coating: (a) low-magnification SEM image; (b) BSE image of the region marked “1” in (a); (c) BSE image of the region marked “2” in (a); and (d) BSE image of the region marked “3” in (a).

dominant wear mechanism was “microcutting” accompanied, in part, by other wear mechanisms listed above. Low propylene flow during the deposition of the C1 coating resulted in lower flame temperature and lower flame velocity which correspondingly resulted in fewer molten particles and less-compact coating, respectively. Although a large amount of high-hardness WC particles can effectively protect the coating from being cut by the outside abrasives, the Co binder among the WC grains is inhomogeneous in the C1 coating, which made the WC grains susceptible to cracking and pulling out in the region with less binder (circled by the ellipse in Fig. 5a). The wear mechanism that includes extrusion of the binder phase and removal by plastic deformation and fatigue, carbide grain fracture and undermining of the particles and subsequent particle pulling-out is the dominant wear failure process for the C1 coating. The C2 coating applied using a reasonable propylene flow exhibited the smoothest worn surface with fewer cracked WC grains and fewer grooves compared with the C1 and C5 coatings. The wear mechanisms of the C2 coatings are similar, in part, to those of the C1 and C5 coatings.

The C2 coating had a similar worn surface as the C1 coating except for its less dark regions. The typical morphology and the EDS results for the dark region on the worn surface of the C1 coating are shown in Fig. 8. The dark region on the worn surfaces of the C1 coatings contains Si and N elements, which indicates that some debris in the dark region may come from the Si3 N4 counter body. The entrapped debris would result in further damage to both surfaces as thirdbody abrasives. Both Yang et al. (2003) and Zhang et al. (2009) found similar phenomena and they proposed that the sliding wear behavior of the WC–Co coatings generally occurs by Co-binder extrusion, carbide cracking, carbide pullout and material removal. In addition, different amount of the grooves were found on the sliding surfaces of all the as-sprayed coatings (Fig. 7b, d and f), which may be caused by the cutting with harder Si4 N3 counterpart material. The C5 coating, however, showed some apparent pits on its worn surface and exhibited much different worn surface morphology compared to the other coatings. As observed from the XRD results of the coatings (Fig. 1), intensive decarburisation took place in the C5 coating during the spraying process and the substantial amount of the brittle binder was generated, which facilitated the generation of cracks

4.4. Sliding wear mechanism of the HVOF-sprayed WC–12Co coatings The width of the sliding trace and the friction coefficient of all the WC–12Co coatings are shown in Table 3. The worn surface of C2 coating exhibited the most narrow sliding trace width and the highest friction coefficient among all the coatings, while the C5 coating showed the reverse results. The typical sliding worn surfaces of the C1, C2 and C5 coatings are shown in Fig. 7.

Table 3 The width of the sliding trace and the friction coefficient of WC–12Co coatings. Coating

Width of the sliding trace (mm)

C1 C2 C3 C4 C5

1.16 1.12 1.17 1.21 1.28

± ± ± ± ±

0.05 0.03 0.04 0.06 0.07

Friction coefficient 0.53 0.61 0.56 0.49 0.43

± ± ± ± ±

0.15 0.17 0.17 0.16 0.11

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Fig. 7. Low and high magnification images of the sliding worn trace morphology of the C1 (a and b), C2 (c and d), and C5 (e and f) coatings.

due to the poor bonding between the splats under the repeated attacks of the Si4 N3 ball. When the number and size of the microcracks increased and formed the nets, a large number of pits would generate on the sliding worn surface of the C5 coating (Fig. 7f) as a result of coating spallation/delamination. The low hardness and high brittleness of the C5 coating resulted in its poorest sliding wear resistance among all the as-sprayed coatings. The C2 coating showed a moderate decarburization, but the highest hardness and toughness, which contributed to its highest sliding wear resistance. Although its wear resistance was the poorest, the C5 coating exhibited the lowest friction efficient among all the coatings (Table 3), which was attributed to its the high brittleness and low hard WC phase content due to the intensive WC decarburisation. The rapid

spallation of coating material and the lack of sufficiently hard WC particles in the C5 coating ineffectively hindered the movement of the counterpart ball, which was beneficial to decrease its friction coefficient during the sliding wear test. As for C2 coating, a large number of remaining hard WC particles existed in the relatively dense coating and effectively hindered the movement of the Si4 N3 ball on the coating surface, which resulted in its higher friction coefficient. It is interesting to notice that high friction coefficient for the HVOF sprayed WC-based coating may not always mean high sliding wear rate, similarly, the low friction coefficient may also not always mean low one. The sliding wear behavior of the HVOF sprayed WCbased coating was greatly influenced by its hardness, toughness, phase composition and microstructure.

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Fig. 8. The morphology of entrapped particle debris (a) and the particle EDS analysis results (b) on the sliding worn surface of the C1 coating.

5. Conclusions (1) The degree of decarburization of WC in HVOF sprayed WC–12Co coatings gradually increased while the porosity decreased with the propylene flow increase. The hardness and the fracture of the coatings first increased and then decreased with the increase of the propylene flow. Moderate decarburization in the HVOF-sprayed WC–12Co coatings revealed beneficial effects on its abrasive and sliding resistance as a result of a compact microstructure, high hardness and high fracture toughness. (2) The feedstock with different sizes of particles and the different regions in the same sprayed particles experienced a different degree of melting, which led to different degree of decarburization. These variations were responsible for the different microstructure of the corresponding splats in the as-sprayed coatings. (3) The WC–12Co coatings with different degree of decarburization exhibited different dominant abrasive and sliding wear mechanisms. Acknowledgments This research was supported by the Project of Training Targets of Young Key Teachers of Common College & University in Hunan Province and the National Natural Science Foundation of China (Grant No. 51175165). References Chivavibul, P., Watanabe, M., Kuroda, S., Shinoda, K., 2007. Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC–Co coatings. Surface and Coatings Technology 202 (3), 509–521. Evans, A.G., Wilshaw, T.R., 1976. Quasi-static solid particle damage in brittle solids—I. Observations analysis and implications. Acta Materialia 24 (10), 939–956. Fang, W., Cho, T.Y., Yoon, J.H., Song, K.O., Hur, S.K., Youn, S.J., Chun, H.G., 2009. Processing optimization, surface properties and wear behavior of HVOF spraying WC–CrC–Ni coating. Journal of Materials Processing Technology 209 (7), 3561–3567. Fincke, J.R., Swank, W.D., Haggard, D.C.,1994. Proceedings of the 7th National Thermal Spray Conference. ASM Int., Boston, p. 325. Guilemany, J.M., De Paco, J.M., Nutting, J., Miguel, J.R., 1999. Characterization of the W2 C phase formed during the HVOF spraying of a WC–12Co powder. Metallurgical and Materials Transactions A 30 (8), 1913–1921.

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