Elastic recovery induced strengthening effect in copper/ multilayer-graphene interface regions revealed by instrumental nanoindentation

To distinguish the strengthening effect of multilayer graphene (MLG) on the copper matrix from the intrinsic strengthening effect of copper grain boundary (GB), the nanoindentation behavior in the Cu/MLG interface boundary (IB) region is investigated and further compared with those in the copper GB region and pure copper grain interior (GI) regions. The indentation displacement recovery ratio, elastic work ratio, and indentation hardness in the Cu/MLG IB regions are significantly enhanced as compared with those in the copper GB region and copper GI regions. The strengthening effect in the Cu/MLG IB regions can be attributed to the elastic recovery behavior during the unloading period induced by the MLG, which is further confirmed in both the experimental and simulated indentation topography evolutions. In addition, gradient strengthening effect is revealed from the gradual increase in the indentation displacement recovery ratio, elastic work ratio, and indentation hardness with gradual decreasing distance from the MLG in the Cu/MLG IB regions. The findings in this paper can be utilized to precisely enhance the mechanical performance of copper matrix composite materials by tailoring the interfacial microstructure and property.