This research investigates the frictional properties of extended planar graphitic contacts with polycrystalline surfaces using molecular dynamics simulations. The simulations reveal that kinetic friction is primarily determined by shear-induced buckling and unbuckling of corrugated grain boundary dislocations, resulting in a non-monotonic relationship between friction and normal load/temperature. A phenomenological two-state model effectively captures these effects, enabling the characterization of tribological properties in large-scale polycrystalline layered interfaces without extensive atomistic simulations. The observed negative differential friction coefficients at high loads can mitigate the typical linear scaling of grain-boundary friction with surface area, potentially restoring superlubricity at larger length scales.

Xiang Gao, Wengen Ouyang, Michael Urbakh, Oded Hod