Characterization and sliding behavior of HFCVD diamond coatings on WC–Co

The effect of three conditioning treatments (scratching, S; scratching and nitric acid etching, SE; scratching, Murakami etching and nitric acid etching, SM) on the properties and dry sliding wear behavior of diamond coatings on WC–Co substrates produced by hot filament CVD was investigated. In specimen S the critical strain energy Gc for coating spallation was found to be similar to the available strain energy due to the residual stresses alone, G0. In the other two specimens, where cobalt was removed from the substrate surface before coating, Gc was found to be 1.1 (specimen SE) and 1.8 (specimen SM) times G0. The dry sliding tests against an austenitic stainless steel counterface revealed, for all the coatings, a high run-in friction coefficient (about 0.6) followed by a tribological stage characterized by a very low friction coefficient (about 0.05). However, due to the high counterface ductility a transfer layer was soon formed, which increased friction coefficient and, thereby, the applied friction stress. In specimens S, this produced a rapid removal of the coating by buckling. In the other two specimens, only localized damage regions were detected, due to buckling for specimen SE and wedging for specimen SM. Additional sliding tests of specimen SM were carried out against an Al alloy counterface, a very soft material, and a granite, a very hard and brittle material, in order to highlight the influence of counterface on the dry sliding behavior of the coating. In the case of sliding against the Al alloy counterface, transfer phenomena started immediately after the run-in stage and a tribological stage with a low friction coefficient was not observed. In the case of sliding against granite, the friction coefficient continuously decreased during sliding tending to zero.