Motion behavior of graphene sheets and friction characteristics between the interfaces of silicon-based materials

Abstract

The <i>ab initio</i> molecular dynamics method is used to simulate the friction process of the graphene sheet as lubricant added to the silicon-based material interface under the action of compression and shear, and the influence of water molecules and oxidation of graphene surface on the movement behavior of graphene sheet are studied. In a dry environment, the pristine graphene (PG) sheet will slip only when the pressure is high. Owing to the presence of surface functional groups, a strong force is formed between the graphene oxide (GO) sheet and the substrate. The direction of the hydroxyl groups on the surface of the upper slab is consistent as the upper slab moves at a constant speed, resulting in the fact that the force between the GO sheet and the upper slab is greater and the GO sheet slides forward with the upper slab. Owing to the formation of mechanical interlock between the GO sheet and the lower slab surface, the GO sheet no longer slips when the pressure is high. In a humid environment, the interface structure affects the overall distribution and movement state of water molecules. The water molecules between the PG sheet and the upper slab are adsorbed only on the surface of the upper slab and always remain in a “flat” state, and their motion behavior is consistent with the upper slab’s. Comparing with a dry environment, the PG sheet starts to slip only when the pressure is high. Since the hydroxyl orientation angle on the surface of the upper slab is greater than the hydroxyl orientation angle on the surface of the GO sheet, the water molecules gradually change from the "flat" state to the slightly “upright” state as the pressure increases. The change of the orientation of water molecules makes the bonding strength between water molecules and the GO sheet gradually decrease, leading to a relative slip between them. The change in the movement behavior of the graphene sheet causes the shear plane to change. There is a positive correlation between the velocity fluctuation mean square error of the graphene sheet and the friction coefficient as the oxidation rate of graphene sheet increases under different coverages of water molecules, indicating that the motion behavior of the lubricant affects the interface friction characteristics.