Co–Cr–Mo alloys: Improved wear resistance through low-temperature gas-phase nitro-carburization

UNS R31538, a Co–Cr–Mo alloy for biomedical applications, was surface-engineered by LTNC (low-temperature nitro-carburization), leading to co-infusion of concentrated interstitially dissolved carbon and nitrogen. LTNC was performed by encapsulating the alloy together with solid urea in a silica ampoule, evacuated and backfilled with Ar. Upon heat treatment, pyrolysis of the urea generates molecular species that provide carbon and nitrogen to diffuse into the alloy and, at the same time, “activate” the surface for carbon and nitrogen infusion by removing the alloy's native surface oxide and forming a micrometer-scale nitride layer. The treatment generates an outer zone rich in nitrogen and an inner zone rich in carbon. While the outer zone contains nitride precipitates, the inner zone consists of a uniform solution of interstitially dissolved carbon with atom fractions up to 0.035, orders of magnitude higher than the equilibrium solubility limit of carbon in UNS R31538. The treatment increases the surface hardness of UNS R31538 to 16 GPa, twice as high as that of the as-received state. Moreover, it improves the wear resistance (wear-loss volume) and the accompanying acoustic emissions (“squeaking”) by one order of magnitude. Accordingly, surface engineering by LTNC has potential to improve the performance of load-bearing Co–Cr–Mo components, e.g. in medical implants.