Friction and Wear Reduction in Copper with a Gradient Nano-grained Surface Layer.

A gradient nano-grained (GNG) surface layer is fabricated on a commercial-purity Cu sample, in which a significant reduction in the coefficient of friction and the wear loss is obtained compared to the coarse-grained and the nano-grained counterparts. A novel mild ploughing mechanism without subsurface damage has been identified in the GNG sample, giving rise to a much reduced wear rate. Sliding induced surface deformation brings about the unique inhomogeneous substructure in the GNG Cu: the topmost layer persists with nanograins without being oxidized, underneath which deformation is well accommodated by grain coarsening adjacent to the dynamic recrystallization layer. Both subsurface structural evolution and stress field model confirm that sliding-induced strain localization is suppressed, which is responsible for the superior friction and wear behaviors of the GNG Cu.