S-phase against S-phase tribopairs for biomedical applications

S-phase, or expanded austenite, is a corrosion resistant diffused hardened layer which can be created in austenitic stainless steels and cobalt-chromium(Co-Cr) alloys. It is a precipitate-free metastable supersaturated solid solution of nitrogen or carbon or both. S-phase layers formed on biomedical grade austenitic stainless steels have demonstrated significantly enhanced in-vitro wear and corrosion properties. To date, all these tribo-corrosion studies on S-phase treated alloys were conducted using an alumina or tungsten carbide ball as the counterface material. Testing S-phase against S-phase is both scientifically interesting and technologically important in view of their potential applications for the articulating surfaces of metal-on-metal joint prostheses. In this work, biomedical grade 316LVM and High-N stainless steel discs together with AISI 316 balls were low temperature plasma surface alloyed with: nitrogen (nitriding); carbon (carburising); and both carbon and nitrogen (carbonitriding). The S-phase layers created by these treatments were in-vitro corrosion-wear tested in Ringer's solution using an S-phase engineered ball reciprocating against an S-phase engineered disc. In addition, self-mated tribopairs of untreated stainless steel and Co-Cr alloy were used as benchmarks. The results demonstrate that the self-mated S-phase tribopairs can produce a marked decrease in material loss when compared to self-mated untreated stainless steel tribopairs. This is partially because of significantly increased surface hardness and thus an enhanced mechanical support for the surface oxide film. The combined wear loss of the S-phase stainless steel tribopairs was close to that of the benchmark Co-Cr tribopair. It can be concluded from this work that S-phase surface engineering can effectively combat scuffing or seizure of biomedical austenitic stainless steels when self mated. Therefore S-phase hardened austenitic stainless steels could compete against more expensive alloys such as Co-Cr alloys in metal-on-metal wear applications in the biomedical industry.