Molecular mechanics of polycrystalline graphene with enhanced fracture toughness

Although polycrystalline graphene generated by chemical vapor deposition features defects at grain boundaries, experimental results show that the strength of polycrystalline graphene is comparable to that of the pristine graphene. This is in contrast to the widespread knowledge that defects typically weaken a material's strength. Here, we examine why polycrystalline graphene has high strength and high fracture toughness, by combining an innovative algorithm with classical molecular dynamics simulation to systematically build well-stitched (99.8% heptagon and pentagon defects without void) polycrystalline graphene models with regular and irregular grain boundaries, and use these models to systematically examine the fracture toughness of polycrystalline graphene composed of grains of different characteristic length. Our study reveals that polycrystalline graphene under fracture releases up to 50% more energy than the pristine graphene. Per mechanism, we find that grain boundaries increase the critical energy release rate to fracture by reducing stress concentration and making branches near the crack tip. We conclude that these effects are likely governed by the out-of-plane deformation of polycrystalline graphene.