Investigation of the running-in process in photoinduced superlubricity

Photoinduced superlubricity on TiO2 surfaces is a newfound phenomenon which draws researchers' attention. This study provides a new method to achieve superlubricity (COF < 0.01) with an external light field. However, photoinduced superlubricity can only be realized under specific conditions. Improper running-in conditions, such as speed, load, and pH value, will lead to superlubricity failure even after ultraviolet illumination on the TiO2 surface. In this paper, different running-in loads, speeds, or pH values were used in the experiment of photoinduced superlubricity, and the worn surfaces after running-in and testing in 70% v/v glycerol aqueous solution were investigated thoroughly. Results reveal that the morphology of worn scars differs under different running-in conditions. While the running-in speeds and loads are too low (< 0.03 m/s and < 2 N) or too large (> 0.1 m/s and > 9 N), the photoinduced superlubricity will fail because of wrong lubrication state. When the pH value of running-in solution is less than 4.5, photoinduced superlubricity is easier to achieve. In discuss, mixed lubrication is believed to be the key to success of photoinduced superlubricity, because the elastohydrodynamic effect, doublelayer effect and adsorption of glycerol molecules works at the same time. In addition, due to the formation of the SiO2 layer on the Si3N4 ball and better attraction to lubricant molecules with hydroxyl radicals on the TiO2 surface, running-in in solutions with low pH values contributes to the success of photoinduced superlubricity. In any event, the ultraviolet illumination can reduce the friction coefficient of the TiO2/Si3N4 tribological system and can realize photoinduced superlubricity under appropriate running-in conditions.