Photoinduced superlubricity on TiO₂ surfaces

Superlubricity control is of great interest in both industry and scientific research, and several methods have been proposed to achieve this goal. In this work, ultraviolet (UV) light was introduced into titanium dioxide (TiO₂) and silicon nitride (Si₃N₄) tribosystems to accomplish photoinduced superlubricity. The friction coefficients (COFs) between Si₃N₄ balls and TiO₂ plates in the mixtures of sulfuric acid (H₂SO₄) solution and glycerol solution were obviously reduced, and the system entered the superlubricity region (COF < 0.01) after UV illumination at a speed of 56 mm/s. However, the COF was much larger without UV treatment than that with UV treatment. The formation of silica (SiO₂) layers on the surfaces of Si₃N₄ balls and the elastohydrodynamic effects were determined to be fundamental to the low friction in this experiment, and the enhancement of the combination between the TiO₂ surface and the hydroxy group of glycerol by UV illumination was the key to the photoinduced superlubricity in this system. These findings showed one method for achieving superlubricity by introducing a light field that could be further applied to special working conditions.