Surface Processes in Tribocorrosion of Highly Alloyed Metallic Materials

Highly alloyed corrosion-resistant metallic materials are used in a range of applications, including biomedical, oil and gas, and aerospace applications. As one particular example, cobalt-chromium (CoCr) alloys are commonly employed in tribological applications because of their mechanical properties (high hardness) as well as good wear resistance. Additionally, thanks to their excellent corrosion resistance, CoCr alloys have widely been used as lynchpin materials for components exposed to aggressive environments, such as joint implants, valves in chemical plants, gripper latch arms in nuclear industries. The corrosion resistance of CoCr alloys, which has been extensively investigated, has been shown to be due to the formation of a protective Cr oxide surface film, which also prevents the release of metal ions. The composition of the oxide film, which depends on the environment, strongly affects the degree of protection. In spite of the success of CoCr alloys, early failures of components made of these materials have been observed in some applications due to degradation phenomena resulting from the coupling of corrosion and wear (also called tribocorrosion). As one particular example, in the case of metal-on-metal (MoM) bearings made of CoCr alloys, which currently constitute ~35% of over 200,000 primary total hip replacement procedures performed annually in the U.S., the degradation has been shown to be mainly induced by tribocorrosion phenomena. In spite to the relevance of the topic, remarkably little is known about the tribocorrosion behavior of CoCr alloys. It has previously been determined that CoCrMo hip bearing surfaces form tribofilms comprising the original chemical constituents of the base alloy and the remnants of denatured proteins and in addition can undergo microstructural changes and chemical reactions with the joint environment during articulation that produce a mechanically mixed zone of nanocrystalline metal and organic constituents, referred to as a biotribolayer. This layer appears to be critical to reducing wear and corrosion. Despite these results, significant ambiguity still remains concerning the correlation between microstructural modifications of the surface and subsurface metals, surface chemical changes (i.e., composition and thickness of the oxide layer), and biocompatibility.
The proposed research project aims to develop a fundamental understanding of dependence of biocompatibility on tribocorrosion-induced variations in surface chemistry and materials structure. This will be achieved through the investigation of the dynamic surface chemical and metallurgical processes occurring in the near-surface region of CoCr as a function of tribological and electrochemical conditions. The correlation of tribocorrosion measurements, advanced surface chemical characterization and metallurgical analysis of the surface/subsurface regions with the resulting protein adsorption behaviour will provide a full picture of the mechanism(s) underpinning the degradation of CoCr in a range of applications, including as materials for joint implants.