  The effect of graphene flakes on the strength of Ni-graphene composites is
investigated using molecular dynamics simulation. Rather than introducing
flakes as flat structures into the Ni matrix, as it is common in available
studies, we introduce them into the heated liquid Ni and let the structures
equilibrate in a 14-ns molecular-dynamics run; these structures are then
quenched to obtain the composites. By varying the interaction of flake edge
atoms with the Ni matrix, two different flake morphologies -- wrinkled vs flat
-- are obtained. The mechanical properties, and in particular the composite
hardness, are investigated by a simulated nanoindentation test. The flake
morphology affects the plastic activity and the hardness of the composites.
Wrinkled flakes show a higher potential in absorbing dislocations than flat
flakes, resulting in a considerably reduced hardness of the composite. No
effect of the changed interaction between graphene edge atoms and the Ni matrix
on the dislocation activity and hardness could be observed. Furthermore, we
demonstrate that a high graphene content in the plastic zone leads to an
increased dislocation absorption and weakens the composite.
