Microstructural evolution at the worn surface: a comparison of metals and ceramics

In order to predict the ability of a material to support the load at contacting asperities, the flow stress of the surface must be known. In addition, the manner in which the flow stress changes as a result of interaction with the environment and/or the counterface also needs to be understood. However, there is little quantitative data on the work hardening behaviour at worn surfaces. Both linear hardening and the achievement of a saturation flow stress have been predicted. In this paper, detailed transmission electron microscopy is used to quantitatively determine worn surface microstructure for a wide range of materials tested under similar conditions. Materials investigated included single phase fcc metals (γ-Fe and Al alloys), two phase metals (e.g. Al-12%Si), metal matrix composites (e.g. Al-4%Cu-δ-Al2O3), multi-component systems (e.g. tool steels with added TiC) and oxide ceramics (alumina and zirconia). The minimum subgrain size is determined for the deformed metals where no mechanical mixing occurs and is related to the work hardening behaviour. The wear rate is considered as a function of the total depth of deformation. The manner in which mechanical mixing alters the surface mechanical properties is considered. Finally, a comparison is made with the worn surfaces of two oxide ceramics.