The effect of Cu grain size on transition from EHL to BL regime (Stribeck curve)

► The effects of grain size of Cu on friction and wear under lubricant conditions were studied. ► Preliminary severe plastic deformation improves remarkably the friction and wear properties. ► A balance between geometrical and structure parameters is preserved in steady state. ► The parameter, K, incorporated in the Sommerfeld number, allows a prediction of the Stribeck curves. Recently friction and wear of copper (Cu) samples (the average grain size, d = 30 μm) rubbed against a hardened steel in steady friction were studied. The effects of load and sliding velocity were interpreted by the Stribeck curve and the transitions from elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL) region were analyzed. It was shown that the wear rate is proportional to a load both in the EHL and in the BL regions. The hardness of thin surface layers saturates during repeating sliding of Cu that is close to the hardness of nanocrystalline Cu produced by different severe plastic deformation (SPD) processes. The present investigation was initiated with three objectives similar to our previous works with the Cu samples: first, to study the friction and wear behavior of Cu surfaces with different grain sizes under different lubricant conditions; secondly, to examine the structural evolution of Cu subsurface layers under friction in different contact conditions and thirdly, to evaluate the effect of grain refinement by equal channel angular pressing (ECAP) on friction and wear. The Stribeck curves for the Cu-steel pairs with different virgin grain sizes of Cu ( d = 1 μm, 30 μm and 60 μm) are presented. All Stribeck curves have the similar shape and only shifted depending strongly on virgin hardness of the Cu samples. The transitions from one lubricant region to another depend on the virgin grain size. The lesser the grain size of Cu, severe the condition of transition to BL region. Grain refinement by ECAP ( d = 1 μm) shifts the Stribeck curve in the range of severe contact conditions (high loads and low sliding velocities). Preliminary severe plastic deformation of Cu samples leads to remarkable decreasing the wear loss in comparison to coarse grain size samples. The hardness of thin surface layers (∼1 μm) saturates during repeating sliding of Cu and this value is close to the hardness of nanocrystalline Cu produced by different SPD processes. Since plastic deformation and wear occur in ML and BL regions, the known lubrication numbers cannot be used for descriptio