Abstract:Medical forged CoCrMo alloy was treated by DC plasma nitriding process to alter its surface characteristics and improve mechanical and tribological properties. The phase compositions, microstructures, surface microhardness, and wettability were characterized by using X-ray diffraction, scanning electron microscope, micro-hardness tester, and optical dynamic/static contact angle meter. Friction and wear characteristics were investigated on ball-on-disc tribotester under dry sliding conditions. The experimental analyses confirmed that plasma nitriding process is promising in producing thicker, harder, highly hydrophilic and more wear resistance layers on CoCrMo alloy for biomedical applications. At lower nitriding temperatures and shorter time, the nitrided layer structure was composed of σ-CoCr and CrN phases. At higher nitriding temperatures and longer time, σ-CoCr and CrN as well as Cr2N phases appeared to occur in nitrided specimens. With increasing nitriding temperatures and time, case depth thickness, surface roughness, microhardness and wear resistance increased while water contact angle decreased and became highly hydrophilic. Compared with untreated CoCrMo alloy, all nitrided specimens showed an important reduction in wear rate and wear scar width, and the nitrided specimen at 800 ℃-8h exhibited the lowest wear rate and better dry-sliding wear resistance. The wear mechanisim for untreated specimen was mainly adhesive wear while the nitrided specimens changed the wear mechanisim to fatigue wear, abrasive wear and slight adhesive wear because of breakage of nitrided layers. From the experimental results shown in this paper, it is concluded that the improvements of mechanical and tribological properties are attributed to thicker case depth and hard σ-CoCr, CrN and Cr2N phases formed in forged CoCrMo alloy. Moreover, the wettability of nitrided specimen was mainly determined by its surface roughness.