As mentioned earlier, grinding is normally adopted for finishing of carbide coatings. Therefore, influence of grinding on the tribological properties needs to be understood. In the literature, this aspect has not been addressed so far. However, effect of grinding on monolithic ceramic material has been reported by Srinivasan et al. Thus, the present study represents one of the first to report on the effect of grinding on the tribological behaviour of thermally sprayed coatings.
The improvement in erosion resistance as a result of grinding in the case of HVOF coating and DS coating is quite significant as shown in Fig. 1. Fig. 2 shows the SEM micrographs of the surface morphology of the eroded surface of coatings deposited by HVOF process. Fig. 2a and b are the eroded surface of ‘as-coated’ and ‘as-ground’ samples, respectively, tested at normal impact. Fig. 2c and d are the eroded surface morphology of ‘as-coated’ and ‘as-ground’ specimens tested at 30◦ impact angle.
Fig. 1. Steady-state volume erosion rate.
It is clearly noticed in Fig. 2a that in ‘as-coated’ condition tested at normal impact, the eroded surface consists of numerous craters formed by erodent particle impact. The craters that are larger in diameter consisted of characteristics of particle pull out induced voids and gross spalling of the coating. In some regions the pull out areas are surrounded by lip suggesting strain localisation which is a common feature in ductile erosion mode. Such ductile erosion is possibly on the Co–Cr metallic binder phase. Otherwise, the erosion involved material removal by spalling of coating dominated by fracture of carbide particles and their pull out from the binder phase. Thus, the erosion is predominantly brittle in nature. The grinding has changed the erosion morphology significantly. As shown in Fig. 2b, the ‘as-ground’ specimens tested at normal impact contained craters which are smaller in size as compared to that in ‘as-coated’ specimens. Thus, the grinding has resulted in reduction in erosion rate at normal impact. At oblique impact also, similar trend is observed. For example, the ‘as-coated’ specimens tested at 30◦ impact angle consisted of deeper grooves along the particle impact direction as a result of ploughing of erodent particles on the coating. In the ground specimen tested at 30◦, the width and depth of the grooves were much smaller as compared to the grooves in ‘as-coated’ condition suggesting lower penetration in the as-ground specimens. Fig. 8c and d also indicate particle pull out along the grooves. The damage due to particle pull out was much larger in the ‘as-coated’ (Fig.2c) condition than in the ‘as-ground’ condition.
Fig. 2. SEM micrographs showing the eroded surface morphology of WC–10Co–4Cr coating by HVOF process. (a) and (b) As-sprayed and as-ground condition, respectively, at normal impact; (c) and (d) as-sprayed and as-ground condition, respectively, at 30◦ impact. The erodent particle flow direction is indicated by the arrow mark.
As mentioned earlier, surface grinding on HVOF and DS coatings resulted in compressive residual stress. In tungsten carbide based material, it is known that the erosion rate is governed by brittle erosion mechanisms dominated by particle fracture and fracture involving crack nucleation and propagation at the particle-matrix interface. In brittle materials, the compressive residual stress is known to mitigate impact wear damage as reported by Srinivasan et al. in the case of monolithic Si3N4 as it is known to impede crack initiation and propagation. Therefore, the fracture of carbide grains and fracture at the particle-matrix interface has been significantly reduced by the higher compressive residual stress in the ‘as-ground’ specimens leading to lower volume wear rate. When comparing the compressive residual stress in ‘as-ground’ HVOF coating with that of DS coating, DS coating resulted in higher compressive stress. However, the erosion rate (Fig. 1) at normal impact in as-ground specimens is found to be nearly same for these two coatings. This could be attributed to the following. The volume wear rates shown in Fig. 1 were estimated assuming a constant density value for both HVOF and DS coatings. However, the actual density values will differ based on the porosity and microcracks in the coating. Given the fact that HVOF specimens in ‘as-ground’ condition contained higher density of cracks as compared to DS coatings, the actual density of ‘as-ground’ HVOF coating is expected to be relatively lower as compared to the density of ‘as-ground’ DS coating. Therefore, the relative volume wear rate for ‘as-ground’ HVOF specimens would be higher even under normal impact as compared to ‘as-ground’ DS coating if the actual density of the coating is taken into account.
Compared to HVOF coating, DS coating has shown more improvement in erosion resistance at 30◦ impact angle after grinding as shown in Fig. 1. The severity of fracture is less intense at oblique impact, and also higher compressive residual stresses impede crack nucleation and propagation. It is to be noted that in case of HVOF coating, grinding also induced cracks that are parallel to the ground surface. These cracks will reduce the erosion resistance at oblique impact as these cracks cannot offer resistance to shear force present during impacting of particles at oblique angle resulting in higher erosion.
To conclude, this study shows that finishing operation influences the coating characteristics and its performance, and the grinding improves the erosion resistance of the WC–Co–Cr coated by HVOF and DS processes. Improvement in erosion resistance may possibly be due to an increase in compressive residual stresses.
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