Dislocation interactions during nanoindentation of nickel-graphene nanocomposites

The processes occurring during nanoindentation of graphene-reinforced Ni substrates are studied using molecular dynamics simulation. A systematic study is presented by positioning the graphene flake at various depths below the Ni surface and also terminating it at various lateral distances from the indent point. We find that the dislocations generated by the indentation in the vicinity of the indenter never penetrate the graphene flake. However, they can bend around the flake and thus reach positions below it. If the indenter touches the flake, dislocations are nucleated in the Ni layer beyond the flake. In this case, interface cracks between Ni and graphene are created. The composite material has a smaller hardness than pure Ni. This is caused by an attractive interaction of the graphene flake with the Ni dislocations; when the dislocations arrive at the Ni-graphene interface they induce height depressions of the flake that alleviate the pressure in the top Ni layer and weaken the force on the indenter.