Wear of CoCrMo alloys used in metal-on-metal hip joints: A tribocorrosion appraisal

A good biocompatibility, excellent mechanical properties and high corrosion resistance characterize CoCrMo alloys. Therefore they are widely used for artificial joints in biomedical implants. However, the degradation of the implants during service life leads to the release into the body of toxic ions and wear particles. This continuous degradation is of concern for long-term stability of the implants. Published literature has highlighted the relevance of lubrication as well as metallurgical and contact mechanical factors on the degradation of CoCrMo implant alloys. Recent experimental investigations have proposed tribocorrosion, i.e., the interplay of mechanical wear and corrosion by the body fluids, as one of the crucial degradation mechanism of implants. Tribocorrosion is sub-discipline of tribology and corrosion that recently made significant progresses in mechanistic understanding and modelling. The present work aims at evaluating published results on the degradation of CoCrMo alloys using existing tribocorrosion concepts. Results show that wear accelerated corrosion due to mechanical removal of the passive film during sliding is a major contribution to the overall degradation. Further, a transition from low (10−6N/mm3m) to high (10−4N/mm3m) wear coefficients was found at a threshold electrode potential close to 0.2 VSHE These findings clearly show that electrochemical phenomena play a key role on the tribological behaviour of biomedical CoCrMo alloy implants. ► The electrode potential is a key factor determining wear of CoCrMo alloys. ► Wear accelerates corrosion of CoCrMo alloys in and outside the contact zone. ► Tribocorrosion models were compared to simulator data published on CoCrMo degradation. ► Wear accelerated corrosion accounts for a large fraction of overall wear.