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

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.