Hydrodynamic effect on the superlubricity of phosphoric acid between ceramic and sapphire

In this work, a super-low friction coefficient of 0.003 was found between a silicon nitride ball and a sapphire plate lubricated by phosphoric acid solution. The wear mainly occurred in the running-in period and disappeared after superlubricity was achieved. The friction coefficient was effectively reduced from 0.3 to 0.003 at a constant speed of 0.076 m/s, accompanied by a 12-nm-thickness film. The lubrication regime was indicated to change from boundary lubrication in the running-in period to elastohydrodynamic lubrication in the superlubricity period, which is also supported by the results of the friction coefficient versus sliding speed. In addition, the experimental results showed good agreement with theoretical calculations based on the elastohydrodynamic lubrication theory, suggesting a significant hydrodynamic effect of phosphoric acid on superlubricity.