Nonmonotonic interfacial friction with normal force in two-dimensional crystals

Classic Amontons' law states that friction force increases monotonically with the increase of normal force. Here a nonmonotonic variation of interfacial friction with normal force is revealed in two-dimensional van der Waals bilayers by extensive first-principles calculations. The results show that the interlayer sliding energy barriers and shear strengths of MoS2, hexagonal boron nitride, and graphene bilayers turn to decrease with the relative movement of top and bottom monolayers from the lowest to the highest energy stacking state under normal pressures beyond 1.8, 14.7, and 7.6 GPa, respectively. Meanwhile, the interlayer binding energy differences between the highest and the lowest energy stacking state are nonmonotonic as the normal pressure increases. This anomalous frictional behavior in two-dimensional crystals is effectively interpreted by the opposite contributions of interlayer van der Waals and Coulomb interactions.