A tribocorrosion appraisal of a dual layer PVD coated CoCrMo alloy tribopair

A dual layer PVD coating consisting of a CrN layer on top of a CoCrMo(C) S-phase underlayer (CrN/S) was deposited on low-carbon wrought CoCrMo alloy discs and CoCrMo Stellite® 21 spheres. The tribocorrosion response of the uncoated CoCrMo tribopairs and coated CrN/S tribopairs was studied under reciprocating sliding conditions in Ringer's and diluted foetal bovine serum (FBS) solutions under anodic potential and elastic contact for a sliding time of 2 h. Compared to the uncoated tribopair, CrN/S successfully mitigated both oxidation and mechanical material losses and resulted in a low dynamic coefficient of friction and smoother scar morphologies in both test solutions. For the uncoated tribopair, the presence of proteins acted as a barrier to charge transfer and lowered the friction during tribocorrosion testing contributing to a marked reduction in material loss, primarily by a reduction in oxidative losses, but resulted in a rougher scar morphology. Tribocorrosion tests of the coated tribopair were also carried out for a duration of 24 h and revealed minimal material losses in diluted FBS and no evidence of other damage. It, however, resulted in substantial thinning of the outer CrN layer, transverse microcracking and delamination when testing in Ringer's solution. This coating loss produced a source for coating debris which led to three-body micro-abrasion damage. FIB investigation revealed that the observed transverse microcracks were limited to the outer layer and hence have not provided electrolytic pathways to the substrate interface. The excellent tribocorrosion response of dual layer CrN/S on CoCrMo metal tribopairs in diluted FBS renders them good candidates for further investigation to mitigate tribocorrosion damage of biomedical CoCrMo alloys.