Effect of In-process Cryogenic Cooling in the Burnishing Process on the Solid Particle Erosion Behavior of HVOF Cermet Coating

The paper presents the role of in-process cryogenic cooling in the burnishing process carried out on high velocity oxy-fuel (HVOF) thermal spray coating (WC-10Co-4Cr). The work on the same coating on dry burnishing with a novel grinding wheel-shaped tool was published earlier. The average normal burnishing forces, measured with the help of dynamometer, were more than thrice the tangential forces during the cryogenic burnishing with the same tool. The burnishing process not only modified the surface but also the cross-section as the micro-hardness in the cross-section was also more in the cryogenic burnished specimen. The decrease in the surface porosity and the roughness of the HVOF thermal spray coating (WC-10Co-4Cr) after cryogenic burnishing was not as much as reported after dry burnishing. The compressive residual stresses, as determined through XRD, were more on the cryogenic burnished surface as compared to those on HVOF as-sprayed surface, but lesser than those reported for dry burnished. The cryogenic burnished specimens performed much better in both oblique and normal impact air-jet solid particle erosion (SPE) testing, than HVOF as-sprayed coating. In spite of having higher porosity, as compared to that reported for dry burnished specimens, the cryogenic burnished specimens performed better against SPE due to higher compressive residual stresses.