An investigation of sliding wear behaviour of WC–Co coating

An investigation of sliding wear behaviour of WC–Co coating

Tribology International 44 (2011) 1711–1719 Contents lists available at ScienceDirect Tribology International journal homepage: www.elsevier.com/loc...

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Tribology International 44 (2011) 1711–1719

Contents lists available at ScienceDirect

Tribology International journal homepage: www.elsevier.com/locate/triboint

An investigation of sliding wear behaviour of WC–Co coating V. Rajinikanth, K. Venkateswarlu n CSIR National Metallurgical Laboratory, Jamshedpur 831007, India

a r t i c l e i n f o

abstract

Article history: Received 10 August 2010 Received in revised form 17 June 2011 Accepted 20 June 2011 Available online 5 July 2011

Dry sliding wear tests on specimens of mild steel (MS) and WC coated mild steel (MSC) specimens were performed against a hardened EN32 steel (EN32) and a WC coated EN32 steel (EN32C) discs. Four different combinations of specimen and counter surface were tested under dry sliding conditions. Results suggest that wear mechanisms differ depending on the combination of materials under sliding contact. Expectedly the MS specimen suffered high wear loss, but the MSC specimen showed interesting results. When slid against EN32, MSC specimens showed negative wear results whereas positive wear results occurred against EN32C. Steady wear rate was attained after a critical sliding distance. & 2011 Elsevier Ltd. All rights reserved.

Keywords: WC–Co coating Thermal spray HVOF Sliding wear

1. Introduction There has been a constant effort to develop super hard coatings with excellent mechanical properties to combat wear loss. WC–Co cermet is best known for its superior wear resistance-strength combination even at moderately elevated temperature up to 400 1C [1]. The cermets are generally used in dry sliding wear conditions because of their relatively high hardness and wear resistance. Wear behaviour of WC–Co cermets under various experimental conditions have been studied [2–5]. These studies suggest that the high wear resistance of this material is a function of carbide to binder ratio, carbide grain size and bulk hardness of the material [4,5]. Efforts were also made to study the wear resistance of thermally sprayed WC–Co coatings, i.e., combination of WC (hard constituent) and soft-ductile Co (binder). Thermal spraying is widely accepted as a low cost spray processing and used for many industrial applications [6]. Amongst the various techniques available for the fabrication of WC–Co coatings, high velocity oxy-fuel (HVOF), an advanced thermal spray technique, has gained much attention. HVOF thermal spraying has outperformed other methods for deposition of WC–Co powder [7–9] because the WC powder particles under the influence of higher velocities and lower temperature have less probability of getting decomposed during spraying [10]. In addition to the low degree of decomposition of WC powder during spraying, the low porosity levels of WC coatings favour high wear resistance. The low degree n Corresponding author. Presently working with Materials Science Division, CSIR National Aerospace Laboratories, Bangalore 560017, India. Tel.: þ91 80 25086244; fax: þ91 80 25270098. E-mail address: [email protected] (K. Venkateswarlu).

0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.06.021

of decomposition in HVOF thermal spray may be attributed to the small residence time of particles in the flame and low temperatures. However, the WC–Co powder morphology along with a type of HVOF thermal spraying system and other spraying parameters affect the coating microstructure which, in turn, affect the wear behaviour of the coatings. Yang et al. have studied the correlation between carbide grain size on the sliding and abrasive wear behaviour of WC-12%Co coatings at room temperature [11]. Usmani et al. have closely examined the influence of carbide grain size and on the sliding and abrasive wear behaviour of HVOF thermally sprayed WC-17%Co coatings [12]. These investigations showed that the carbide grain size is an important parameter in influencing the wear performance of WC–Co coatings. Sliding wear behaviour of WC–Co coatings at elevated temperatures up to 400 1C was investigated by Yang et al. [1]. In addition, they also showed that the specific wear rate of the coating increases with increase in carbide grain size at a given temperature, but decreases with increase in temperature for a given carbide grain size. Though significant efforts were made on investigating dry sliding friction and wear behaviour of WC–Co coatings, most of them are limited to the studies involving sintered alumina (Al2O3) as the mating material. In the present investigation, an attempt has been made to study the dry sliding friction and wear behaviour of MS and MSC specimens on a pin-on-disc machine. The sliding wear tests were carried out in two stages. Firstly, against a perfectly flat standard EN32 disc and against an EN32C disc. The mechanism of material loss and wear particle formation are investigated extensively and related to the coating microstructure, spraying conditions and powder characteristics. It is found that adhesion, the most prominent wear mechanism, in sliding wear can substantially be controlled by careful selection of the sliding materials.

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2. Experimental procedure 2.1. WC–CO coating The WC–Co powder used for HVOF thermal spraying was obtained from Metal Powder Company, Mumbai and has a composition of WC-12 wt% Co. The purity of the powder was 99.9%. The grain size of the WC particles was 1.570.5 mm. Before proceeding for the spraying process, 1 wt% urea solution in water was added to the WC powder for globulisation [13]. This enhances the flowability of powder during spraying and also maintains the homogeneity of the composition within the globules. Then the powder was dried at 120 1C for 1 h to remove the volatile materials and water. For the present investigation, one EN32 disc and MS substrates were coated with WC-12 wt% Co, using HVOF spraying technique. Prior to spraying, the substrates were degreased to remove all surface contaminants from the interstices and surface pores [14]. The coating surfaces of the mild steel substrates were polished using 120 grit emery paper. The specimens were mounted on the circumference of a horizontally rotating turntable maintained at an effective horizontal transverse rate. The gun system (HIPOJET4500, MEC, India) delivering a vertical transverse movement, was fixed on the transverse unit by a suitable mechanism. A mixture of oxygen, fuel gas (liquid petroleum gas) and a carrier gas (argon) along with WC powder was introduced into the combustion chamber. When the gas mixture was ignited, the controlled hot gas jet accelerated the powder downstream along the nozzle to impact the substrate. The velocity and temperature of the particles that come out from the nozzle was measured by employing a laser jet particle velocity analyser. Multiple detonations occurring within a short time at such high velocity helped in obtaining a coating of sufficient thickness. The temperature attained within the detonation gun was  1200 1C, as measured by an optical pyrometer. The substrates were maintained at a temperature of  200 1C in order to reduce thermal expansion during the coating process. The coating thickness was measured to be  200 mm using Quanix 8500, a German make thickness measurement tester. 2.2. Dry sliding wear Dry sliding wear tests were performed using a pin-on-disc machine (Model TR 20, Ducom, Bangalore, India) in conformity with ASTM G 99-05 standard. The following four combinations were selected for dry sliding friction and wear tests: (1) MS specimen against EN32 counter surface, (2) MS specimen against EN32C, (3) MSC specimen against EN32 counter surface, and (4) MSC specimen against EN32C counter surface. The specimen size for sliding wear test was 8 mm in diameter and 40 mm in length (inclusive of the WC coating, in case of the coated specimen). The counter surface (EN32 disc & EN32C disc) had a diameter of 165 mm and thickness of 7 mm (inclusive of the coating thickness, 200 mm). A sliding speed of 200 rpm was maintained for all the sliding tests. Prior to the tests, the specimens were made perfectly flat by moving the specimen mating surfaces on a polished steel disc at a very low load (0.1 kg) for sufficient period so that the whole specimen surface makes perfect contact with the counter surface. The wear loss in microns mentioned in this work was directly proportional to the height loss of the specimen. A LVDT, which was fixed to the sliding wear test machine, measures the height loss as the specimen was loaded by a cantilever system. The co-efficient of friction and frictional force were also continuously monitored and recorded separately during each sliding test. The variation in temperature of the specimen surface during the sliding wear tests was measured by inserting a thermocouple into the specimen,

approximately 5 mm above from the contacting surface of the specimen. The specimens were cleaned with and were weighed before and after each test to determine the weight loss. Each test was conducted continuously (without any interruption) up to a sliding distance of 3200 m at two different loads (1 and 2 kg). Standard metallographic techniques were used for microstructural studies on the WC–Co coated specimens. The polished and etched samples were examined in optical microscope and SEM (JEOL, JSM840A, Japan). The worn surfaces and debris after dry sliding wear tests were also collected and examined using SEM. The specimens were gold sputtered in case of wear debris prior to SEM examination. For elemental analysis, EPMA (JXA-8600 M, JEOL, Japan) was used and line profile quantitative elemental analysis was carried out by EPMA on abraded samples.

3. Results and discussion 3.1. Characterisation of starting materials and WC coating The SEM image of the as-received WC-12 wt% Co powder is shown in Fig. 1a. The average particle size of WC particles is  2 mm; these are fine particles cladded with cobalt. WC particles are globular in nature and the tendency to agglomerate with each other is quite evident. The energy dispersive spectroscopy (EDS) analysis of the WC–Co powder gave  12% cobalt (Fig. 1b). The optical image of MS substrate before deposition of WC–Co coating is shown in Fig. 2a along with the back scattered image of the WC–Co coating is in Fig. 2b. The presence of C, W, Co, Cr is clearly seen from the EDS pattern as shown in Fig. 2c. The microstructural detail shows the uniform distribution of WC particles within cobalt matrix. Excellent bonding between the WC particles and the cobalt matrix is also observed in the coating (Fig. 2b). This image of WC coating revealed that the average particles size is  1.570.5 mm indicatings that no substantial change has occurred in the particle shape or morphology of WC–Co after the coating. Further, SEM studies revealed dark (grey shades) and bright regions that contain dissolved tungsten and carbon apart from cobalt. This is in agreement with the microstructural studies by Shipway et al. [15]. The appearance of grey shades represent cobalt-rich binder matrix while the highly bright regions indicate strong presence of tungsten due to substantially high dissolution on the WC particles associated with decarburization from these regions [16]. The XRD spectrum of the deposited WC-12 wt% Co coating showed WC and Co (major peaks) along with (minor peaks) W2C (Fig. 3). Similar observation was also made by Yang et al. [11]. Presence of W2C in the coating can be attributed to a low degree of decomposition of WC during thermal spray. Degree of decomposition can further be reduced by employing low flame temperature and high particle velocity. This is in good agreement with the results of previous studies [12,17,18]. With regard to the carbide size, Yang et al. have found that decreasing the size of carbide particles in the starting powder led to an increase in the degree of decomposition of WC [11]. On the contrary, the amount of decomposition observed in the present investigation was relatively low in comparison to other studies [12,17–19]. It is also observed that WC and b-cobalt was highly retained in the coating (Fig. 2b). Such higher retention levels were reported for coatings deposited by HVOF process at very high gas velocities [20]. Prior to the deposition, the hardness of standard EN32 disc was  800 HV (62RC), while the mean bulk hardness of WC coated steel disc was 995 HV. MS specimen exhibited an average hardness of 180 HV; upon deposition of WC coating a surface hardness of  995 HV was obtained. Yang et al. reported that when hardness measurements were made using loads as high as 9.8 or 49 N, the indentations were big enough to include

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Fig. 1. (a) SEM image of the as-received WC-12 wt% Co powder and (b) EDS analysis of the WC–Co powder.

Fig. 2. (a) Optical image of MS substrate before deposition of WC–Co coating, (b) back scattered image of the WC–Co coating and (c) EDS pattern WC–Co coating along with substrate.

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Fig. 4. Dry sliding wear response of MS specimen on EN32 for different applied loads.

Fig. 3. XRD spectrum of the deposited WC-12 wt% Co coating.

pores and several splats, thus higher porosity and weaker splat cohesion in finer coating resulted in low hardness value [11]. In contrast, a fairly high bulk hardness value of 995 HV is observed for the WC–Co coating at 2 kg load. This is indicative of low porosity and increased splat cohesion of the coating obtained by HVOF spraying process. The coating principally consisted of WC particles within soft ductile cobalt matrix along with a minor presence of W2C. In this regard, some discernible differences were observed in comparison to other studies [17–19,21] that reported a still higher fraction of W2C phase. Delving into earlier studies, a matrix may have varying compositions depending upon the fractional replacement of ductile metallic cobalt phase by nanocrystalline CoxWyC or/and amorphous Co–W–C phase [17–19,21]. The earlier studies reasoned that increase in hardness is dependent on the increasing content of W2C in the deposited coatings, because W2C phase (HV¼29.4 GPa) is harder than WC phase (HV ¼23.5 GPa) [22], nanocrystalline CoxWyC or amorphous Co–W–C phase is harder than the b-cobalt phase and that the presence of either or both these phases results in a strong metallic cohesion between the WC particles and cobalt matrix. The hardness of the coatings increased substantially with an increase in W2C phase content, and this increases further when the Co6W6C phase substitutes metallic cobalt as the matrix phase [21]. The cobalt as binder phase must have melted and to form the unmelted carbide particles must have passed along with the flame depositing themselves during HVOF process thus resulting in high strength and good adhesion between the matrix and particles. 3.2. Sliding wear test The dry sliding wear response of MS specimen on EN32 for different applied loads (1 and 2 kg), for a sliding distance of 3200 m is shown in Fig. 4. The specimen suffered a wear loss of 180 and 220 mm for 1 and 2 kg load, respectively. Mild steel is (hardness 180 HV) relatively soft in comparison to counter EN32 disc (hardness 820 HV) and hence exhibited appreciable adhesion and resulted in substantial material removal. Since adhesion is more under higher loads, the wear loss for 2 kg was greater than 1 kg load. The abrupt increase in initial wear loss was due to the sudden increase in contact area and increased incidence of

Fig. 5. Wear tracks on MS specimens against EN32 disc (a) 1 kg, (b) 2 kg and (c) 2 kg, high magnification.

adhesion. After an initial sliding distance of 200 m, the contact stress reduced due to increased contact area and resulted in a moderate increase in wear loss. The wear tracks on MS specimens after a sliding distance of 3200 m is shown in Fig. 5. The wear tracks formed under 2 kg applied load is deeper than at 1 kg load. The groove width was  23.6 and 11 mm for 2 and 1 kg applied loads, respectively. The prime cause for material removal under high applied load is attributed to high adhesion between the opposing surfaces, which is possibly due to high ratio of adhesion force to contact force. The presence of strong adhesion force can be explained by the electron transfer between the contacting surfaces. Numerous free electrons are present in metals and, on contact; these electrons may be exchanged between the two solids to establish almost instantaneous bonding [23]. An image of the wear debris collected after a sliding distance of 3200 m, for 2 kg load is shown in Fig. 6. The average size of the debris particles is  3 mm. The

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Fig. 7. (a) and (b) wear track formed when MS specimen slid against EN32C at different magnifications.

Fig. 6. Wear debris collected after a sliding distance of 3200 m, for 2 kg load.

formation of wear particles (debris) is due to the action of adhesion between asperities and considerable plastic deformation of asperities caused during sliding motion. Material in the softer asperities is deformed in a series of shear bands to accommodate the relative movement, thus offering resistance to sliding along contact line of the asperities. When the shear bands reach a critical limit, a crack is initiated which later extends across the asperities causing an eventual detachment of the deformed asperities. Asperities with large slope angle tend to lose material to asperities with small slope angle [24]. The combined action of strong adhesion, abrasion and severe plastic deformation of the asperities contributes to high and continuously increasing material loss in MS specimen. The wear loss was higher in the case of MS specimen slid against EN32C disc with a bulk hardness value  995 Hv under different applied loads. The maximum wear loss corresponding to 1 and 2 kg load was 540 and  660 mm, respectively. Fig. 4 shows that the initial increase in wear loss was reduced after sliding distances of approximately, 150 and 200 m under applied loads of 1 and 2 kg, respectively. Beyond this the wear loss curve follows a moderately increasing path. The high wear loss in the initial stages was possibly due to high localised contact pressure at the real contact point between the opposing asperities; with increasing sliding distance the real contact area between the surfaces increases after having slid a certain distance. Fig. 7 shows the wear track formed on the specimen surface at different applied loads. The fine plugging out of materials and subsequent ploughing led to the formation of grooves. The mechanism of groove formation involved ploughing of soft MS surface by the hard WC particles. The groove width estimated as  2 mm, and is similar to the size of carbide particles in the coating. The amount of debris formed was more in case of higher applied loads. The wear debris was collected and examined under SEM (Fig. 8). The average size of debris particles is  3 mm. Iron along with minor

Fig. 8. SEM image of wear debris when MS slid against EN32C at 2 kg load.

presence of tungsten and cobalt were found through EDS examination of debris. The sliding wear behaviour of MSC specimen slid against standard EN32 disc and EN32C disc is shown in Fig. 9. Initially a negative trend of wear loss is clearly observed in case of MSC specimen slid against the standard EN32 disc. The negative wear is suggestive of the fact that the specimen initially gained weight, causing an increase in the sample height which resulted in the specimen being raised from the counter surface. It was observed that having a slid a critical sliding distance, associated with a net weight gain, a steady increase in wear loss has occurred. The critical sliding distance at which the wear rate acquires a steady value is observed to be a function of the applied load. It is also observed that the weight gain is even more with the application of a higher load (2 kg). The critical distance to reach the steady state value with 2 kg is more than at 1 kg applied load. Increase in weight gain or sample height at higher applied loads was due to greater adhesion. During the dry sliding, it was observed that the EN32 disc suffered wear due to relatively harder MSC specimen

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Fig. 9. Sliding wear performance of various coated samples.

being slid against it. The hard WC particles present on the surface of MSC specimen caused the wear of EN32 disc, and the wear loss increased further with increase in the applied load. An increase in applied load caused greater penetration of WC particles into the opposing surface. The wear out of EN32 disc started almost instantaneously when the specimen began sliding against it. Initially the wear particles were detached from the steel surface due to adhesion and then the worn out particles were subsequently removed by asperities contact to form true wear particles. Some of these wear particles were lost as debris from the system, with a few entrapped between the opposing surfaces in contact because the worn out particles are largely confined within the groove formed as a result of negative wear on the steel disc. The entrapped debris particles owing to its fine size produced negligible damage on the surfaces, possibly due to the fragmentation of wear particle under sliding motion. The debris obtained after fragmentation were very fine particles, and these fine particles were inefficient in wearing the surfaces as a third-body abrasive. The specific surface area of these fine debris particles was large enough to cause the reattachment of the particles to one another and to the specimen surface, readily forming the transfer film. The transfer film was formed by the mechanical alloying between the materials under sliding motion, resulting in the formation of particles consisting of lamella of the two materials. The formation of the transfer film has a dramatic effect on the wear rate [25]. The thin transfer film formed on the surface of MSC specimen preventes further wear. Due to the protruding hard asperities on the surface of MSC substrate held the abraded debris in the valleys between the asperities (locking action), the presence of transfer film was maintained on the surface of specimen. The locking action by the hard WC particles was more at higher applied loads because the debris entrapped experiences greater degree of compaction on the specimen surface. Due to this, the wearing of steel disc and the material gained by the specimen surface is high at higher applied loads. Hence, the width of the transfer film is expected to be more at higher applied loads but the stability of the transfer film decreases with increase in its width beyond a critical value. Therefore counter removal of transfer particles as wear debris from the transfer film occurred at higher applied loads that maintain both the thickness of the film below the critical value and the stability. Thus at equilibrium, the rate of transfer of wear particles to the specimen is balanced to the wear rate of the disc. The wear debris was formed entirely from the transfer layer on the specimen while the specimen

suffered almost negligible wear. The transfer of material from the steel disc is evident from EDS analysis, which confirmed the presence of iron in the debris collected. The wear behaviour was principally controlled by the combined action of material transfer and the counter formation and removal of mechanically mixed layer (MML). The sliding wear of MSC specimen slid against EN32C disc is shown in Fig. 9. When the MSC specimen was brought into sliding contact with the EN32C disc, the soft cobalt matrix suffered severe deformation. The compressive stress imposed by the hard asperities of WC particles on the specimen surface under sliding motion caused the deformation and subsequent extrusion of the cobalt matrix. The matrix underwent severe deformation, thereby causing a reduction in the matrix support, earlier imparted to the WC particles. This causes micro-cracking and pull-out of WC particles that leads to the formation of wear debris. Cobalt being soft and ductile was easily deformed and extruded, thus the debris so obtained was rich in cobalt content. The SEM image shows the bright and dark grey regions corresponding to tungsten carbide and cobalt binder phase (Fig. 10). The originally formed debris particles were not lost completely from the system and a part of them got entrapped between the opposing surfaces under sliding motion. The wear debris was reduced to even finer particles. The very fine particles in the debris reattached to form a transfer film on the surface of disc coming under sliding contact. In addition, since the debris was cobalt rich, the particles adhered fairly with minimum porosity. The dense film had good cohesion of particles due to fair performance of cobalt as a binder and thus protected the specimen against rapid wear. Consequently, the coating showed a very low wear rate. Since the pulled out fine carbide particles provided less damage to the WC coated counter surface and also the debris of finer carbides were less effective as third-body abrasives, the sliding wear rate decreased to a steady value. The steady wear loss in the mild wear regime is possibly due to the counter formation and removal of transfer film which maintained the thickness of the film. It is evident from Fig. 9 that wear loss increased with increase in contact pressure, but in the later stages a steady behaviour with continuous mild increase is observed. Yang et al. [11] have reported that higher the contact pressure the shorter is the sliding distance to enter the mild wear regime. In contrast, an opposite trend was observed in the present study that with increase in applied load the sliding distance was longer to acquire a steady wear condition. The afore stated wear mechanism lead to the formation of grooves (Fig. 11), which involved ploughing of the specimen surface by hard transfer particles. These transfer particles grew harder due to severe work hardening. The work hardened transfer particles under the application of normal and tangential stresses caused fair deformation of ductile cobalt matrix, but W2C and WC phases experienced poor deformation. Thus the ductile cobalt matrix was forced to protrude above the specimen surface, which led to the

Fig. 10. (a) SEM and (b) BEI-SEM image of wear debris when MSC sample slid against EN32c, 1 kg load.

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Fig. 11. Wear track of MSC when slid against EN32C at 1 kg load.

Fig. 13. (A) EPMA analysis on worn surface and (B) EDS analysis on mopped region.

Fig. 12. Co-efficient of friction versus sliding distance results on various coated samples at 1 and 2 kg loads.

removal of matrix in few places. After the removal of soft matrix, micro-cracking and detachment of hard WC and W2C phases occurred due to a reduced matrix support. The wear occurred principally due to the loss of some carbide grains and crack formation along a few carbide grain boundaries. Typical microcracks that are formed in the grooves is shown in Fig. 11. The carbide loss occurred due to the plugging out of carbide grains because of weakened bonding to the binder phase. The coefficients of friction during sliding wear when the MSC coated samples slid against EN32 and EN32C discs are shown in Fig. 12. It is evident that the coefficient of friction initially increases rapidly, decreases nearly to 500 m sliding distance and increases slowly and it is almost maintained the coefficient of friction values at similar levels. MSC samples tested against EN32C showed higher coefficient of frictional values and resulted in less wear, whereas MSC samples slid against EN32 disc at 2 kg load, exhibited lower coefficient of friction values. This is due to adhesion between the sample and the disc playing a significant role and also possibly due to (i) the interface heating increases

with increase in applied load, (ii) higher temperature making the matrix material softer thus increasing the flowability of matrix material increases, (iii) greater degree of MML formation and finally greater extent of surface smoothening at higher applied load and (iv) greater degree of surface smoothening of the counter surface due to greater degree of material transfer from the counter surface to the specimen. The EDS map (Fig. 13A) shows that the tribo film surface is rich in Fe (transfreed from steel) and low in Co that are uniformly mixed. The presence of finer WC particles embedded in the film provides the cutting edges on the surface for the increased wear of steel disc with increase in sliding distance. Fig. 13(B) shows the EDS map of worn surface indicating the fine WC particles are embedded in Fe-rich matrix. The corresponding EDS analysis of the mapped region indicated that the surface predominantly rich in both Fe and W, constitutes 85% of total concentration, which proves that the steel is getting adhered to the coating. The softness of cobalt is due to disintegration during the adhesive wear process enabling the formation of free WC particles which again get embedded in the Fe rich matrix. 3.3. Effect of temperature Fig. 14a and b shows the temperature attained in the specimen as a function of sliding distance. Initially the temperature increased rapidly up to  500 m (critical distance) of sliding

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porosity) WC coating may be used for protection against wear under dry sliding conditions.

Acknowledgements The authors would like to thank Dr S. Srikanth, Director, NML, Jamshedpur, for his kind permission to publish this paper. Thanks to Mr S.C. Modi, Managing Director, M/S Metallising Company, Jodhpur for his critical comments. We are thankful to Mr. M.K. Gunjan and Mr. B. Mahato for extending SEM and XRD studies, respectively. Critical review and corrections made by Dr.V.R. Ranganath, Scientist of CSIR-NAL, Bangalore is sincerely acknowledged. Fig. 14. (a) and (b) Temperature attained in the specimen as a function of sliding distance.

followed by a moderate trend. The critical distance increased with increase in applied load. Fig. 14 also shows that the maximum temperature recorded increased with increase in applied load, possibly due to greater dissipation of frictional power. The variation in temperature recorded could be reasoned by the following mechanism. The contact between specimen and counter surface at an applied load is limited to contacts between the asperities of opposing surfaces. The frictional heat generated in dry sliding between the surfaces conducts away through opposing asperities in contact. Since the true contact area between opposing asperities is smaller than the apparent contact area, the frictional energy and the resulting heat becomes highly concentrated at these contacts. The frictional power to sustain sliding is dissipated in the form of heat over the small contact areas of the asperities causing a rise in temperature of the sliding surfaces. When the load is increased greater dissipation of frictional power occurs in order to sustain the sliding, which in turn results in rise of temperature. It is believed that the temperature of the sliding surface is still slightly higher in comparison to the measurement point and the released heat can have an important influence on friction and wear levels. Nevertheless, structural changes of cermets do not take place at such temperatures [26].

4. Conclusions In addition to the spraying parameters, the carbide size and the degree of decomposition of WC strongly influence the microstructural properties of the coating. Dry sliding wear test performed on MS specimen showed high wear loss when slid against EN32 disc and EN32C disc. Under the same sliding conditions, when slid against EN32 disc the MSC specimen gained weight, signifying the transfer of worn particles due to severe cutting of the counter surface. The formation of transfer film and the removal of particles from the transfer layers as debris counter balanced each other to maintain constancy in the thickness of film. Hence, the thickness of the transfer film remained approximately constant at a particular load but increased with increase in applied load. The strength and stability of the transfer film to sustain sliding conditions depends on the ductility of the binder phase, cobalt. Cobalt owing to its appreciable ductility promoted the formation of dense transfer film, which showed good adherence under dry sliding conditions. It may be concluded that the equilibrium rate of film transfer to the disc may be due to the loss of wear particles as debris during sliding motion. Thus, HVOF sprayed dense (low

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