Topography rules the ultra-mild wear regime under boundary lubricated gross-slip fretting corrosion

Fretting corrosion of biomedical taper junctions has raised concerns about adverse local tissue reactions. A low release of ions and other reaction products into the body is desirable, and this can be achieved by, e.g., optimizing the topography. Retrieval analysis has revealed that circumferential machining marks with high ridges and low valleys reduce the release of wear products. Thus, we hypothesized that a rougher titanium surface decreases metal release under gross-slip fretting because of a change in wear mechanisms. Fretting tests (polished CoCr29Mo6 alloy pins (Ra ≈ 0.006 μm) against fluted Ti6Al4V cylinders (Ra ≈ 5.6 μm)) were carried out. The worn surfaces were analyzed using SEM and LRS (Laser-Raman Spectroscopy) while ICP-MS of the lubricant (BCS) rendered the wear loss. Results were compared with the wear loss of identically polished CoCr29Mo6 pins rubbing against fine-machined Ti6Al4V cylinders (Ra ≈ 0.245 μm) reported earlier. The fluted topography led to microploughing and microcutting on both CoCr29Mo6 and Ti6Al4V surfaces, triggering tribocorrosion and mechanical mixing of tribomaterial containing Cr-, Mo-, and Ti-oxides and denatured/cleaved proteins. While friction was controlled by mechanical-dominated microploughing and microcutting the steady-state wear was characterized by chemical-dominated tribocorrosion. The total wear loss of the fluted system was half of the fine machined system, albeit the Co-side lost more. Despite the rougher topography the tribomaterial still acted as boundary lubricant and protected the surfaces.