Phototribology: Control of Friction by Light.

In dry sliding, the coefficient of friction depends on the material pair and contact conditions. If the material and operating conditions remain unchanged, the coefficient of friction is constant. Obviously, we can tune friction by surface treatments, but it is a nonreversible process. Here, we report active control of friction forces on TiO2 thin films under UV light. It is reversible and stable and can be tuned/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show a lower atomic orbital overlapping on the surface, which leads to a friction reduction of up to 60%. We suggest that photocontrol of friction is due to the modification of atomic orbital interactions from both surfaces at the sliding interface.

Towards superlubricity in nanostructured surfaces: the role of van der Waals forces.

Hydrogenated amorphous carbon (a-C:H) thin films have a unique combination of properties that are fundamental in mechanical and electromechanical devices aimed at energy efficiency issues. The literature brings a wealth of information about the ultra-low friction (superlubricity) mechanism in a-C:H thin films. However, there is persistent controversy concerning the physicochemical mechanisms of contact mechanics at the atomic/molecular level and the role of electrical interactions at the sliding interface is still a matter of debate. We find that the hydrogenation of the outermost nanostructured surface atomic layers of a-C:H thin films is proportional to the surface potential and also to the friction forces arising at the sliding interface. A higher hydrogen-to-carbon ratio reduces the surface potential, directly affecting frictional forces by a less effective long-term interaction. The structural ultra-low friction (superlubricity) is attributed to a lower polarizability at the outermost nanostructured layer of a-C:H thin films due to a higher hydrogen density, which renders weaker van der Waals forces, in particular London dispersion forces. More hydrogenated nanodomains at the surface of a-C:H thin films are proposed to be used to tailor superlubricity.