Improved Tribocorrosion resistance of a CoCrMo implant material by carburising

Tribocorrosion damage is a cause for the premature failure of hip implants made of cobalt-based alloys. Low-temperature carburising can be a plausible solution towards mitigating the tribocorrosion damage of articulating components. This diffusion treatment introduces a supersaturated carbon solid solution, termed S-phase, which hardens the CoCrMo alloy without detriment to the corrosion resistance. This work investigates and compares the tribocorrosion behaviour of untreated and carburised ASTM F1537 CoCrMo alloys tested in Ringer’s solution using a reciprocating sliding configuration against a polycrystalline alumina counterface under different electrochemical conditions. The research shows that whereas the carburised alloy suffered a slightly higher wear loss under a cathodic potential, it was able to reduce the material losses considerably when tested under both open circuit and anodic potential conditions. Under anodic conditions material losses by corrosion due to wear dominated. The better tribocorrosion resistance of the carburised layer was attributed to the better qualities of the passive film for the carburised sample coupled with an increased load support.peer-reviewe

Tribocorrosion response of duplex layered CoCrMoC/CrN and CrN/CoCrMoC coatings on implant grade 316LVM stainless steel

The objective of this work was to improve the tribocorrosion performance of biomedical grade 316LVM stainless steel via the application of duplex layered coatings. Two different PVD coatings were deposited via magnetron sputtering: (1) an underlying CoCrMo carbon S-phase layer followed by an outermost CrN layer designated as CrN/S and; (2) an underlying CrN layer followed by an outermost CoCrMo carbon S-phase designated as S/CrN. Tribocorrosion of the coated and uncoated 316LVM was studied against an alumina ball which was made to slide in a reciprocation motion in Ringer's solution. The contributions of wear, corrosion and their synergy were elucidated by performing tests under open circuit, cathodic and anodic potential conditions. The resultant wear scars were analysed via SEM, EDS, optical microscopy and contact profilometry. Both coatings displayed superior tribocorrosion response to AISI 316LVM with CrN/S exhibiting the best performance. CrN/S displayed lowest coefficient of friction under all electrochemical conditions (~0.10–0.15) and a material loss under anodic condition with a 93% decrease when compared to 316LVM. The CrN/S scars exhibited a smooth morphology under all test conditions in contrast with the other two test materials which displayed shearing marks oriented along the sliding direction under OCP and anodic conditions. By considering the resultant scar morphologies, a hypothesis is being proposed to explain the tribocorrosion degradation mechanisms encountered.peer-reviewe

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.peer-reviewe