Lubricated Fretting Wear of Gradient Ultrafine-Grained Copper Induced by Burnishing

The surface of pure copper was repeatedly burnished by rollers using large loads,introducing the plastic deformation with different strain levels into the surface layer. The microstructure and grain distributions of a plastic deformation surface layer with the thickness of 150 μm at different depths were analyzed, and the microhardness was also measured. The results show that,a gradient ultrafine-grained (GUG) structure was induced on the surface layer of copper. The average grain size increased from 662 nm in the topmost surface to 94 μm in the matrix with the increasing depth. The hardness of the topmost surface increased by 45% than that of the matrix and gradually decreased with the increase of depth. The fretting wear properties of GUG Cu and the coarse-grained (CG) Cu were investigated at the SRV IV tribo-meter at room temperature. The results indicate that the friction coefficients of GUG Cu first decreased and then increased, which were consistent with that of CG Cu under large loads. The wear mechanism of GUG Cu was mainly abrasive wear but the main wear forms of CG Cu were abrasive wear and fatigue spalling accompanied by oxidation wear in some regions. The fretting wears resistance of GUG Cu increased by more than 10 times than that of CG Cu.