Microstructural aspects of wear in soft tribological alloys

Soft tribological alloys, based on the Al–Sn and Cu–Pb systems, have been used for decades in journal bearings, but detailed studies on the interaction between wear and microstructures in such materials are rare. This work analyses a series of laboratory tests and case studies to elucidate the relationship between wear and the underlying microstructural processes in these alloys. SEM-results of sections through the tribolayer reveal two important processes. While the tribological compatibility between the materials prevents the formation of a transfer layer, auto-transfer of seized bearing material onto the bearing surface is common under severe loading conditions. This results in mechanical mixing of the alloy components, producing metastable and driven phases in the tribolayer. A simple analysis based on the free energy of mixing provides qualitative understanding of these effects. The second feature is formed by localised zones of severe plastic deformation showing pronounced vorticity. These are distinguished from other documented vortices formed during wear by the fact that their axis is parallel to the sliding direction and by the large strain level and may be explained by cyclic plasticity, induced by hydrodynamic pressure oscillations due to asperities on the rotating shaft.