Surface Engineering of Co─Cr Alloys by Low-Temperature Nitrocarburization

We report first experiments of surface engineering the Co─Cr alloy (UNS R31538) by low-temperature gas-phase nitro-carburization. Our approach is based on encapsulating the alloy with a solid reagent, which, on pyrolysis, activates the alloy surface by stripping the passivating surface layer and providing interstitial solute (carbon, nitrogen) to diffuse into the alloy. The processing temperature is chosen low enough to kinetically suppress the precipitation of carbides or nitrides by immobilizing metal atom diffusion, but still high enough to enable sufficient mobility of the interstitial solute. In this way, we succeeded in generating a uniform, precipitation-free “case” with atom fractions of interstitial nitrogen (0.075) and carbon (0.035) that are much higher than the corresponding equilibrium solubility limits. This doubles the surface hardness of the alloy and significantly improves the wear resistance in both air and simulated body environment.