Microstructure and Tribological Properties of Plasma Nitrided Biomedical CoCrMo Alloys

Abstract

Abstract Medical forged CoCrMo alloy was treated by DC plasma nitriding process to alter its surface characteristics and to improve mechanical and tribological properties. The phase composition, microstructures, surface microhardness, and wettability were characterized by 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 analysis confirms that the 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 for shorter time, the nitrided layer structure is composed of σ-CoCr and CrN phases. At higher nitriding temperatures and for longer time, σ-CoCr and CrN as well as Cr2N phases appear in nitrided specimens. With the increasing of nitriding temperatures and time, case depth thickness, surface roughness, microhardness and wear resistance increase while water contact angle decreases and becomes highly hydrophilic. Compared with untreated CoCrMo alloy, all nitrided specimens show an important reduction in wear rate and wear scar width, and the nitrided specimen at 800 °C/8 h exhibits the lowest wear rate and the best dry-sliding wear resistance.