Micro-asperity tribocorrosion of CoCrMo, Ti6Al4V, and 316 stainless steel in air and physiological solution: Small scale reciprocal sliding of a single diamond tip

Abstract

Fretting and corrosion of metallic medical devices are significant degradation mechanisms that impact performance. Multi-asperity contact surfaces undergo a progressive engagement-damage-redistribution process distributed over the nominal contact surface, implying that relatively short-lived fretting contact engagements of individual asperities are a central element to fretting corrosion processes in modular taper junctions of total hip replacements. In this work, the contact mechanics of single asperity hard-metal surfaces under small scale reciprocal sliding (SSRS) between two contacting surfaces in air and solution (i.e., tribocorrosion), typically less than 100 μm cyclic motion are presented and discussed for Ti–6Al–4V, CoCrMo, and 316L SS alloys. Characteristic scaling factors, based on asperity radius, hardness, and modulus, for characteristic stress and a dimensionless asperity radius are presented based on the interaction between elastic and plastic contact mechanics. The contact and tribological interactions of a 17 μm radius spherical diamond asperity was used to explore damage effects on these alloys in both air and phosphate buffered saline. The volume abraded, depth of penetration, extent of plastic deformation and oxide debris formation were captured as functions of cycles, load, and solution presence/absence. Damage was analyzed and quantified using digital optical microscopy, atomic force microscopy and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The results were analyzed with ANOVA statistics (α < 0.05). The extent of damage follows CoCrMo < Ti–6Al–4V < 316L SS. The wear characteristics varied significantly between alloys with more plastic deformation associated with CoCr and more shearing particle and ribbon formation present for Ti–6Al–4V and 316L SS. Solution immersion did not significantly increase volume loss for any alloy but did increase the amount of released and embedded oxide debris for 316L SS. Titanium also exhibited oxide debris generation and embedding. Importantly, significant (<5 μm) penetration depths were obtained after 100 cycles, indicating that single asperities, continuously engaged with metal under very low loads, rapidly generate extensive debris and plastic deformation.