Correlation between strengthening and damage of Cu refined by different SPD processing and friction in different lubricant regions

Abstract

The aim of this work is to evaluate the strengthening and damage of surface layers of copper (Cu) after friction in different lubricant regions and to compare them with stress–strain properties of Cu refined by different methods of severe plastic deformation (SPD). Friction and wear tests were performed using a ring-on block rig. The structure and damage of Cu samples rubbed in Elastic Hydrodynamic Lubrication (EHL) and Boundary Lubrication (BL) were studied. It was shown that the formation of the laminar structure in surface layers under friction in the EHL region is accompanied by a generation of pores and their coalescence. The calculated value of the strain rate during friction in the EHL region was compared with the Coble creep and grain boundary sliding models. It was found out that the deformation hardening, nanocrystalline grain size and level of plastic deformation of thin surface layers under friction in the BL region are close to the parameters of refined structure of Cu obtained by different methods of SPD. Friction saturation in the EHL and BL regions is considered to be a result of the dynamic recovery based on thermally activated processes of accumulation and annihilations of dislocations. The relation between the dislocation accumulation and dynamic recovery determines the transition from one contact region to another. Constant values of the hardness, grain size and geometrical parameters of contact at the steady friction state in different lubricant regions indicate any connection between the structural and geometrical parameters of contact.