Effects of Plasma Nitriding on Microstructure and Tribological Properties of CoCrMo Alloy Implant Materials

Medical forged CoCrMo alloy was treated by plasma nitriding process. The microstructures were characterized by 3D profiler, SEM and XRD. The tribological properties were investigated under lubrication of 25% bovine serum solution. Results show that plasma nitriding is a promising process to produce thick, hard, and more wear resistant layers on the surface of CoCrMo alloy. All nitrided samples showed an important increase in the surface hardness due to the formation of harder CrN and Cr2N phases with compact nano-crystalline structures. The typical hardness values of HV0.05 increased almost two times than untreated one. Under bovine serum lubrication, at low nitriding temperature the Coefficient of Friction (COF) of nitrided sample was lower than that of untreated sample, but at high nitriding temperature the COF was almost the same as the untreated one. Compared with the untreated sample, the nitrided samples showed lower wear rates and higher wear resistance under different nitriding temperatures. The adhesive wear is the main mechanism for untreated CoCrMo alloy and the wear mechanisms of nitrided ones are the fatigue wear and slight adhesive wear. It is concluded that the improvement of wear resistance is ascribed to the hard nitride formation of CrN and Cr2N phases at the nitrided surfaces.