Strengthening mechanisms of graphene coated copper under nanoindentation

We conduct molecular dynamics simulations to investigate the strengthening mechanisms of monolayer graphene coated copper during nanoindentation. It was found that the fracture of the graphene coating includes three stages: (i) initiation of decagon defects, (ii) growth of decagon defects, and (iii) formation of pearl necklace like structure. After the graphene breaks, the intact graphene coating around the indentation still plays a role in strengthening the copper substrate. The shear modulus mismatch and the lattice mismatch at the interface result in the image and misfit stresses, which act as a barrier to impede dislocations to slip out of the copper surface, thereinto, the image stress is the dominant. In addition, it is interesting to observe pristmatic partial dislocation loop and twinning planes during the indentation