Fig. 1 shows the microstructure of WC–Co–Cr powder. The particles were mostly spherical with some having elongated shape. Fig. 2a and b show the microstructures of the transverse section of the HVOF and DS coated samples in the as-sprayed condition prior to grinding. The DS coated sample resulted in slightly denser coating. Table 1 shows the percentage porosity and microhardness values of the coating for the two coating processes. The microhardness in the case of DS was found to be slightly higher. Very high kinetic energies involved in the HVOF and DS processes help in flattening the particle upon impact and provide improved adhesion to the substrate and lower porosity in the coating. Among the HVOF and DS processes, the latter is known to have higher particle velocities compared to the former. Further, the particle temperature is marginally higher (thus the lower strength material being more amenable to flattening upon impact) in the DS process as compared to the HVOF process. Hence, the DS coating is expected to give slightly higher microhardness value and also lower porosity. The Table 1 also shows the average surface roughness of the coating in the as-sprayed condition. The surface roughness was higher in the DS coating. The morphology of the ground surface is shown in Fig. 3a and b for HVOF and DS coatings, respectively. It shows the grinding induced surface damage in both the cases. Fig. 4a and b show the microstructures of the transverse sections of the coating in the ground condition for HVOF and DS samples, respectively. It can be noted that the microstructure of the HVOF coating developed numerous cracks beneath the surface up to a depth of 150–200um. However, such cracks were minimum in the case of DS samples (Fig. 4b).
Table1Coating characteristics of as-sprayed coating
Fig. 1. SEM micrograph of WC–10Co–4Cr powder.
Fig. 2. SEM micrographs of transverse section of WC–10Co–4Cr coating in as-sprayed condition: (a) HVOF coating; (b) DS coating
Fig. 3. SEM micrographs showing the surface morphology of WC–10Co–4Cr coating after grinding: (a) HVOF coating; (b) DS coating.
Fig. 4. SEM micrographs of transverse section of WC–10Co–4Cr coating in as-ground condition: (a) HVOF coating; (b) DS coating.
The residual stress measurement of the WC–Co–Cr coating was carried out using the X-ray diffraction technique. Table 2 shows residual stress values. In the as-sprayed condition, the coating was found to have compressive stresses which possibly occurred due to the differential thermal contraction between the coating and the substrate during the powder deposition process. In fact, the residual compressive stress in the detonation gun coating was found to be much higher than that for the HVOF coating. The residual stress measurements were also carried out on the ground coated samples. It was found that grinding in general resulted in higher residual compressive stresses as compared to their as-sprayed counterpart in the case of both HVOF and DS samples. Given the fact that coolant was used while grinding, the heat generation at the surface was minimal, the compressive residual stress was possibly as a result of the mechanical interaction of abrasive grains with the work piece leading to non-uniform plastic deformation which induced compressive residual stresses. The super imposed hydrostatic compressive stress present during the grinding process in the depth direction of the specimen below the grinding wheel is responsible for high compressive residual stress below the ground surface. The stress measurement in the direction normal to and along the grinding direction on the samples further revealed that the compressive stress normal to the grinding direction (across the lay) was higher than that along the lay. Such anisotropic trend in compressive residual stress has been earlier observed in bulk Si3N4 specimens after grinding. Even after grinding, the DS samples were found to have higher residual compressive stress than that of the HVOF coated samples as shown in Table 2.
Table 2 Residual stress for the WC–Co–Cr coating before and after grinding
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