Severe plastic deformation of four FCC metals during friction under lubricated conditions

The effects of stacking fault energy (SFE) on the sliding friction and wear of FCC metals (Ag, Cu, Ni, and Al) against Type 1040 steel were studied using a pin-on-disk rig. Friction results were presented as Stribeck curves. The transition from elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL) was studied. Friction and wear coefficients and the temperature near the contact were measured during the tests. It was found that friction in the BL region is characterized by formation of a nanocrystalline structure with a steady-state hardness (stress) for all studied metals. It was shown that the wear coefficient correlates well with the steady-state hardness/stress (σs) and SFE. It was found that SFE and melting temperature play import roles in the wear process. However, the grain size or contact temperature do not directly indicate a decreasing or increasing wear rate. Friction and wear of the investigated FCC metals is mainly determined by deformation hardening and dynamic recovery. The annihilation of dislocations occurs much more easily under Ni friction (high SFE) than the other metals. Annealing of Ni and Cu at relatively low temperature (~0.14 Tm) suggests that a recovery process occurred during frictional contact. The difference in behavior of Al compared with the other FCC metals is explained by its low melting temperature.