Research on fracture mechanism and mechanical properties of polycrystalline graphene by nanoindentation: A molecular dynamics study

Randomness of grain boundaries makes it difficult to reach a broad consensus about mechanical properties of polycrystalline graphene (PG). In the present paper, based on principle of Voronoi diagram, the models of PG with different grain sizes were established, and the fracture mechanism and mechanical properties were investigated by molecular dynamics (MD). The results showed that the crack initiation point of PG always located at the multiple junction of grain boundaries, and the crack propagation and fracture mode of PG was mainly dependent on not only the relative size but also the relative location of the indenter and grain boundaries. Additionally, the effects of grain size, indentation speed, temperature and indenter diameter on the mechanical properties were studied, which showed some interesting and different phenomena from the tensile case, e.g., the grain size seems no regular effect on mechanical properties. Furthermore, the ultimate indentation force, indentation depth and fracture showed an increase trend with the increase of indenter diameter and indentation speed, while they decreased with the increase of temperature. But when it came to the elastic modulus, it showed a decreasing trend with the increase of indenter diameter and indentation speed, while it first increased and then decreased with the increase of temperature.