The effect of the structural parameters on the friction and wear properties of some FCC metals

Friction and wear are very complex thermo-mechano-chemical processes of contact interaction. For a better understanding of the mechanisms of plastic deformation affecting friction and wear, four face-centered cubic (FCC) metals (Ag, Cu, Ni, and Al) were studied using a pin-on-disk rig. The topmost surface layer presents nanocrystalline (NC) and ultrafine grain (UFG) structures with the transition to a sublayer with a lamellar structure parallel to the direction of friction. The thickness of the topmost layers is mainly determined by stacking fault energy (SFE), hydrostatic pressure, and shear stress. The thermo-mechanical control of plastic flow during the steady friction state was considered. The effective shear stress as the activation barrier, the strain rate sensitivity, and the contact temperature were calculated. The values of the strain rate sensitivity, m, indicate that NC and UFG surface layers for Ag, Cu, and Ni, exhibit low capacities for strain hardening. The friction of FCC metals in lubricated conditions is characterized by a switch from dislocation-mediated plasticity to grain boundary (GB) plasticity in the topmost surface layers. The similarity between friction with low and high coefficients of friction during dry and lubricated conditions was considered. The wear resistance of studied materials can be compared to strength improvements and easy shearing of the topmost surface layer under dry and lubricated conditions. Dislocation glide and grain growth play a key role in the hardening/softening of surface layers both in dry and lubricated conditions.