A new approach towards performing plasma nitriding of CrCoMo medical grade alloys using HIPIMS discharge

CoCrMo alloy, although an acceptable choice for metal-on-metal prosthetic implants, requires enhancement of its mechanical and tribological properties. In this work, HIPIMS discharge was utilised to perform a low-pressure plasma nitriding (HLPN) process. XRD studies revealed the formation of a compound layer consisting of a desirable mixture of Co2-3N and Co4N phases and a (111):(200) [43:57] texture. The process led to a higher rate of nitrogen diffusion, (case depth of 2.5µm in 4 hours) than that observed for a conventional dc plasma nitriding process (DCPN) (2.1µm in 18 hours). The HLPN treatment resulted in a higher hardness of 23 GPa as compared to 20 GPa achieved for DCPN and substantially higher than that measured for the bare alloy (7.9GPa). Consequently, a significant improvement in dry sliding wear resistance (KC=1.18×10-15m3N-1m-1) and lower friction coefficient (µ=0.6) was achieved as compared to the untreated specimens ((KC=6.0×10-14m3N-1m-1) and µ=0.8). The defined H/E and H3/E2 values of 0.078 and 0.135 respectively indicated that the HLPN process enhances toughness. Potentiodynamic polarisation tests in Hank’s solution showed a significant improvement in corrosion resistance; ECorr value of -218mV was considerably nobler compared to the untreated alloy (ECorr=-775 mV) whereas corrosion current densities of ICorr= 5×10-5 mAcm-2 were found to be two orders of magnitude lower as compared to the untreated CoCrMo alloy with ICorr= 2 analysis revealed that the plasma nitrided layers present a reliable barrier against Me-ion release from CoCrMo alloy. Enhancement of mechanical and corrosion properties and favorable Me-ion release performance combined with the significant reduction in process time makes HLPN a powerful technique for surface treatment of medical grade CoCrMo alloys.