Revealing the maximum microhardness and thickness of hardened layers for copper with various grain sizes

Surface strengthened Cu samples with ultra-fine grain (UFG), fine grain (FG), and coarse grain (CG) matrixes were prepared by cold-rolling, controlled annealing, and then treated by surface spinning strengthening (3S) to reveal the effect of grain size on the feature of the hardened layer. The results indicate that the maximum microhardness of the hardened layer was slightly affected by grain size. The thickness of the hardened layer is significantly influenced by grain size. When decreasing the grain size of matrix from CG to UFG, the thickness of the hardened layer displays an increasing then deceasing trend, and approaches in the highest thickness at FG.