S-phase surface engineering for longer lasting metal-on-metal implants : a tribocorrosion evaluation

Abstract

Several publications have raised concerns with regards to premature failure of metal-on-metal hip joint replacement implants, especially those with large diameter heads. The failure of such implants was attributed to the combined effect of wear and corrosion which causes an adverse reaction by the human body. Different surface engineering techniques are used to mitigate the effect of tribocorrosion such as S-phase surface engineering techniques, which were shown to reduce significantly volumetric loss by the synergistic effect of tribocorrosion in stainless steels. The successful formation of the S-phase on the surface of Coo-Cr-Mo alloys has therefore raised interest in investigating the tribocorrosion performance of a biomedical grade F-1537 Co-Cr-Mo alloy. In this study treatment was carried out by the commercially established low temperature carburising process carried out by Bodycote, where such process was successful in crating S-Phase on both Co-Cr-Mo alloys and 316L stainless steels. Tribocorrosion testing was performed using a linear reciprocating ball-on-disc tribocorrosion tester, taking the innovative approach of having both the ball and the flat sample made of cobalt-chromium alloys. This approach was further investigated by comparing it with ceramic-on-metal tests and tribocorrosion testing of stainless steels. The electrolyte used in this study (Ringer’s solution) was modified to investigate the effect of proteins on tribocorrosion, resulting in the formation of a bio-tribolayer which improved the lubricating properties of the ball-on-disc contact. Results show an increase in hardness by a factor of 2.6 following low temperature carburising with values reaching 14.45 GPa for the Co-Cr-Mo disc. Whilst low temperature carburising reduced volumetric loss by half on Co-Cr-Mo discs following ceramic-on-metal testing; the corresponding results for metal-on-metal combinations show no improvement following treatment, although a milder form of abrasion wear was evident. Further investigations showed that such result is inherit to the metal-on-metal ball-on-flat nature of testing accompanied with a variation in response to treatment as the treated Co-Cr-Mo ball surface resulted in a shallower and softer S-phase when compared to the untreated disc.