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 moiré is smooth and superlubric at incommensurate twists. Surprisingly, load provokes a first-order structural transformation, where the highest energy moiré nodes are removed—an unexpected Aubry-type transition for which we provide a Landau theory leading to a full 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. Both superlubricity and strong Aubry locking hold promises for studies and applications in micro and nano mechanics and devices.

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• Friction
• Grain boundaries
• Structural order parameter
• Tribology
• Solid-solid interfaces
• Molecular dynamics