Superlubric-Locked Transition of Twist Grain Boundaries in 3D Crystals

Abstract: Properties of twist grain boundaries (TGB), long known structurally but not tribologically, are simulated under sliding and load, with Au(111) our test case. The load-free TGB moir\'e is smooth and superlubric at incommensurate twists. Strikingly, load provokes a first-order structural transformation, where the highest energy moir\'e nodes are removed -- an Aubry-type transition for which we provide a Landau theory and a twist-load phase diagram. Upon frictional sliding, the transformation causes a superlubric-locked transition, with a huge friction jump, and irreversible plastic flow. The predicted phenomena are robust, also recovered in a Lennard-Jones lattice TGB, and not exclusive to gold or to metals.