Friction and wear behaviors of nanocrystalline surface layer of pure copper

Surface mechanical attrition treatment (SMAT) was employed to fabricate a nanocrystalline surface layer on a pure copper plate. The grain size is about 10 nm in the top layer and increases with an increasing depth from the treated surface. The tribological behavior of the nanocrystalline surface layer was investigated under dry conditions. Experimental results show that the load-bearing ability is markedly enhanced with the nanocrystalline surface layer relative to the coarse-grained form. The friction coefficient of the nanocrystalline layer is lower than that of the coarse-grained copper when the applied load is below 20 N. With increase of the load, the difference in wear resistance between the SMAT and the conventional Cu decreases. When the load exceeds 40 N, for the SMAT Cu, there occurs a transition of wear regime from local damage to delamination of a mechanical mixed layer. There is an abrupt increase of the wear volume, which corresponds to the wearing away of the nanocrystalline layer. The enhanced wear properties of the nanocrystalline surface layer are correlated with the stability of the mechanical mixed layer and the high hardness of the nanocrystalline structure.