UC Irvine

This thesis focuses on understanding the role of moiré patterns formed between graphene and metal substrate in governing the interfacial frictional behavior. The frictional system consists of graphene supported on a bulk copper substrate, which is investigated by atomistic simulations. The thesis first discusses moiré patterns appearing between graphene and copper, whose morphology depends on crystallographic orientation and relative positions. Depending on system temperature and the contacting nature, two extreme friction phenomena, namely superlubricity and supersticky, have been found, both of which are mediated by moiré patterns. The superlubricity effect can be achieved by tuning the rotation angle of moiré patterns to high value. However, such a condition only happens in metastable states with high potential energy, which induces the graphene layer spontaneously incline to lower energy state with small rotation angles, causing the disappearance of superlubricity. It is found that introducing grain boundaries in graphene can stabilize the high-angle moiré patterns and therefore retain superlubricity. On the other hand, supersticky effect was found to occur at ultra-low temperatures, at which stable moiré pattern persists and raises the resistance to sliding. In the end, the thesis provides details on the developed mathematical models, which describe the rotation and translation process of hexagonal moiré patterns, simplifying the generation of moiré patterns due to graphene rotation and lateral sliding movement.