The evolution of subsurface micro-structure and tribo-chemical processes in cocrmo-ti6al4v fretting-corrosion contacts: What lies at and below the surface?

Titanium and its alloys are attractive biomaterials because of their desirable corrosion, mechanical, biocompatibility and osseointegration properties. Ti6Al4V alloy in particular remains a prominent biomaterial used in Total Hip Arthroplasty (THA). Recently, researchers have shown interest in understanding the degradation mechanisms and the subsurface implications of fretting-corrosion at the modular taper interface in THA. The purpose of this study was to utilise advanced microscopy and spectroscopy to characterise in-vitro fretting-corrosion induced subsurface refinement and microstructural changes in Ti6Al4V alloy. In-vitro fretting-corrosion tests were carried at four displacement: ±10, ±25, ±50 and ± 150 μm for a CoCrMo – Ti6Al4V ball-on-flat material couple. Subsequently, high resolution micrographs of the alloy microstructure were obtained using the Transmission Electron Microscope (TEM) together with Energy Dispersed X-Ray spectroscopy (EDX). The degree of subsurface microstructural changes was observed to be linked to the slip regime and magnitude of energy dissipated at the interface. Strain-induced orientation were observed at the stick regime. The mixed and gross-slip regimes were both characterised with mechanical mixing and formation of nano-crystalline structures. Specific to the mixed fretting regime, fluid ingression and material entrapment at the interface led to further refinement of nano-crystalline structures which resulted in the formation of an amorphous Ti6Al4V structure. The interwoven relationship between energy dissipation, contact condition and mechanisms of clinical failure in Ti6Al4V alloy are discussed.