Superlubric polycrystalline graphene interfaces
X Gao and WG Ouyang and M Urbakh and O Hod, NATURE COMMUNICATIONS, 12, 5694 (2021).
The effects of corrugated grain boundaries on the frictional properties of extended planar graphitic contacts incorporating a polycrystalline surface are investigated via molecular dynamics simulations. The kinetic friction is found to be dominated by shear induced buckling and unbuckling of corrugated grain boundary dislocations, leading to a nonmonotonic behavior of the friction with normal load and temperature. The underlying mechanism involves two effects, where an increase of dislocation buckling probability competes with a decrease of the dissipated energy per buckling event. These effects are well captured by a phenomenological two-state model, that allows for characterizing the tribological properties of any large-scale polycrystalline layered interface, while circumventing the need for demanding atomistic simulations. The resulting negative differential friction coefficients obtained in the high-load regime can reduce the expected linear scaling of grain-boundary friction with surface area and restore structural superlubricity at increasing length-scales. Achieving ultra-low friction at macroscopic scales is highly desirable. In this work molecular dynamics simulations of graphitic contacts incorporating corrugated grain boundaries reveal an unusual non-monotonic variation of friction with normal load and temperature due to dynamic buckling effects.
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