Molecular dynamics study on friction of polycrystalline graphene

Abstract

Graphene, as a one-atom thick 2-dimensional material, is an ideal solid lubricant for small length scale devices such as NEMS/MEMS (nano/micro-electro-mechanical systems) and is often synthesized using chemical vapor deposition (CVD). While CVD-grown graphene consists of a large number of randomly-oriented grains, the effects of this polycrystalline structure on graphene friction still remain far from completely understood. In this study, we investigate the tribological properties of the multigrain structure against a pristine (defect-free single-crystal) graphene surface using molecular dynamics (MD) simulations. The MD simulations presented here test the friction of multiple such configurations created by a novel method mimicking the natural growth of grains in two dimensions. The simulation results reveal that most multigrain configurations exhibit persistent negligible friction without transiting to high friction states. However, there also exist several configurations having relatively large friction forces compared with those from perfectly-aligned single-crystal graphene interactions. A systematic analysis of the independent effects of the grain orientation and grain boundary explains these observations. In particular, it is found that there exists a critical range in the misalignment angle of grain (0.3–1.5°, to the pristine surface) where the shear stress (friction force per unit area) is two or three times larger than the perfectly-aligned grain. Thus, configurations which contain grains with a small misalignment angle in this critical range exhibit large friction forces, despite their substantially smaller grain area. Moreover, for those configurations where all grains have misalignment angles greater than the critical angle, grain boundaries act as the primary source of friction.