Dry sliding wear mechanisms of HIPIMS plasma nitrided CoCrMo alloy for medical implant applications

Unlike the state-of-the-art plasma nitriding technologies where a low ionisation glow discharge is used, nitriding of CoCrMo alloy was carried out in an industrial size vacuum coater, utilising novel high ionisation, high power impulse magnetron sputtering plasma discharge mixed with unbalanced magnetron sputtering discharge. Three different nitriding voltages were used for the nitriding process. To generate more ions, two pairs of Cr and Nb targets were also operated at low power in N2:H2 gas atmosphere. Dry sliding pin on disc tests were performed at room temperature to measure the coefficient of friction and to estimate the wear coefficient. In the case of samples nitrided at lower nitriding bias voltages such as -700 V and -900 V, the oxidative mild wear mechanism was identified as the dominant wear mechanism. For higher voltage of -1100 V, despite the formation of an oxide based tribolayer severe abrasion was identified as the predominant wear mechanism due to the operation of three body contact wear involving high hardness wear particles. Among the three nitrided samples, the sample nitrided at -900 V was identified as the best with the lowest wear coefficient of Kc = 1.11×10-15 m3N-1m-1 and Knoop microhardness of 2230 HK0.010.