The growth of a smooth tribofilm seems necessary to achieve superlow friction with a-C:H coatings. First, removal of oxide layers, either by etching or by sliding, leads to tribofilm build-up. Then, sliding surfaces evolve, by hydrogen release and sp{sup}2 bonds formation, and through tribochemical reactions with environment. Finally, viscoplasticity of a-C:H films is well correlated with superlubricity. We can suspect that mechanical and rheological properties of the interfacial material are critical in preserving superlow friction: it has to stay in the contact area without scratching the flat, and to smooth itself to reduce asperities interactions.
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