Role of multigrain structure on friction of graphene layers

Graphene is a one-atom thick two-dimensional material and has a huge potential as a solid lubricant for industrial application. Graphene is often synthesized by chemical vapor deposition (CVD) because of its simplicity and scalability. However, CVD-grown graphene sheets have the multigrain structure, which may have detrimental effects on the superior properties of single-crystal graphene. In particular, the effects of this polycrystalline structure on graphene friction remain far from completely understood. In this study, we investigate the friction between multigrain graphene layers using molecular dynamics (MD) simulations. The MD simulations examine key factors such as grain size and orientation, and morphology of grain boundaries with several different multigrain configurations. The simulation results reveal that multigrain graphene layers exhibit low, but not completely negligible, friction. The observed frictional behaviors are analyzed in terms of the interactions between grains and grain boundaries (GB). In most models, the contribution of the grain-to-grain interaction to the total friction force is negligible, but some grain pairs exhibit relatively large friction if they configure the commensurate interface due to small mismatch in orientation. The interactions involving grain boundaries often exhibit large friction with the largest at the direct GB-to-GB contacts. An analysis shows that the increase in friction near the grain boundary region is directly related to the decrease in interlayer distance, which is caused by the warping of layers in the vicinity of grain boundaries.