Structural superlubricity — friction coefficients below 0.01 — requires incommensurate contact between two crystalline surfaces. The lattices don't mesh, so the lateral forces cancel. The problem is that this cancellation is fragile: defects, contamination, and edges destroy it. Practical superlubricity under engineering conditions (high pressure, humidity, repeated cycling) has remained elusive.
Unless one surface is amorphous.
Amorphous/crystalline interfaces achieve friction coefficients around 0.008 over 100,000 cycles at 12.7 GPa and 40% humidity. The disorder of the amorphous surface is what makes the superlubricity persistent. In crystalline/crystalline contacts, the incommensurability is precise but delicate — any defect creates a local commensuration that pins. In amorphous/crystalline contacts, there is no crystalline order to locally match. The disorder prevents the defects that normally destroy superlubricity.
This is the opposite of the expected relationship between order and performance. Precision (crystalline/crystalline) gives better peak performance but fragile durability. Imprecision (amorphous/crystalline) gives slightly worse peak numbers but survives real-world conditions. The meta-interface — neither fully ordered nor fully disordered — is permanently incommensurate because it cannot accidentally become commensurate.
The engineering implication: stop trying to make surfaces more perfect. Make one of them deliberately imperfect and the friction problem solves itself for 100,000 cycles under conditions that would destroy any crystalline contact in the first hundred.