Subsurface microstructural alterations during sliding wear of biomedical metals. Modelling and experimental results

Underneath contact surfaces under sliding wear distinct microstructural changes have been found in biomedical materials. These are attributed to the nature of the contact stress (and strain) field. The experimental investigation of subsurface areas of retrieved metal-on-metal hip joints and laboratory specimens revealed that the worn surfaces of these fcc CoCrMo alloys in general consist of a nano-crystalline (nc) layer with a thickness of up to 200 nm. This layer is chemically modified by mechanical mixing, which introduces the interfacial medium (e.g. proteins) into the subsurface volume. Below 200 nm one often finds a nc-layer of the same grain size but having only the chemical composition of the base material. Thus, the interface between the mechanically mixed nc-layer and the underlying one cannot be distinguished by grain size, but by chemical composition. In comparison to shear fatigue tests, these materials show a distinct tendency for ratchetting and they depict the same lattice defects as in a siding wear stress field. This contribution shows that computer simulation and experimental validation can support each other in order to understand the complicated interaction of wear mechanisms under sliding wear qualitatively. But there is no general description available and any aspect requires its own simulation method.