Ratcheting wear of a cobalt-chromium alloy during reciprocated self-mated dry sliding

Cobalt-chromium alloys find usage in environments where reliable wear and friction properties are required. However, the sliding wear particles generated presents significant health risks in nuclear and medical applications. Thus, there is great motivation to develop cobalt-free alternatives. These alloys are known to undergo several physical changes at the interface during dry sliding, sensitive to the loading conditions and environment. Due to these micro-structural alterations, the wear behaviour of the alloy is modified, which linear Archard-like wear models do not capture. To better understand the wear performance a cobalt-chromium alloy in-situ, and to aid their replacement, a mechanistic model of wear would be desirable. To understand the essential physical phenomena required in the modelling of cobalt-chrome systems, a systematic experimental study was performed for a hot-isostatically pressed cobalt-chromium hard-facing alloy. To date, no such in-depth self-mated tribological study has been conducted for this alloy under these processing conditions. Tests were done under combinations of sliding speed (0.02–0.5 m/s) and normal load (40–1000N). Platelet wear and subsurface cracking was seen in all tests, with considerable work-hardening in the subsurface, as well as evidence of plastic deformation at the wear surface. These results suggest the platelet wear observed is more likely a consequence of a plastic ratcheting mechanism, known as ‘ratchetting wear’ and not delamination wear. Unique to this study, the cross-sectional nano-indentation study showed the stiffness of material at and below wear interface to drop significantly. The changes in material properties and a plastically-driven wear mechanism have implications for the development of a mechanistic wear model.