The Effect of Grain Size on Stribeck Curve and Microstructure of Copper Under Friction in the Steady Friction State

In the present work, the effect of grain size on the friction and wear behavior of a copper (Cu) samples under different lubricant conditions was studied. The structural evolution of Cu subsurface layers under friction in different lubricant conditions was considered. All friction tests were conducted under laboratory conditions using a block-on-ring rig. The effects of sliding velocity and load on the friction coefficient and wear rate of Cu with different grain size (1, 30, and 60 μm) were analyzed. The Cu samples with the average grain size of 1 μm were obtained due to severe plastic deformation (SPD) by equal channel angular pressing (ECAP). The Stribeck curves for Cu samples with different virgin grain sizes were considered. Elasto-hydrodynamic lubrication (EHL) and boundary lubrication (BL) regions were mainly studied in the present work. Similar Stribeck curves were found out for Cu samples with different virgin grain size. A load of the transition from the EHL to BL region was increased with a decrease of the grain size. While the friction coefficients were similar in the EHL and BL regions for the samples with different grain sizes, the wear rate was increased remarkably with an increase the virgin grain size. Flow localization during friction in the BL region led to formation of the vortex structure in subsurface layers. Based on the dependence of the microhardness upon the depth, the degree of hardening (H) was evaluated. A correlation between the coefficient of wear and the deformation hardening of Cu samples with different virgin grain sizes was revealed. In order to take into account the effect of the grain size and to predict the Stribeck curve, a parameter, K, as the ratio between hardness of tested and annealed samples, was incorporated into the lubricant number. The theoretical values of the Stribeck curve calculated for preliminary deformed Cu samples (d = 1 μm) and annealed samples (d = 60 μm) were well coincided with the experimental results.