Speed dependence of liquid superlubricity stability with H3PO4 solution

Abstract

In the present study, the speed dependence of liquid superlubricity stability with phosphoric acid (H₃PO₄) solution was studied. Achieving superlubricity with H₃PO₄ solution is the result of the formation of a hydrated water layer with low shear strength combined with the hydrodynamic effect. The experimental results show that the superlubricity stability of a Si₃N₄/glass pair with H₃PO₄ solution strongly depends on speed and stable superlubricity is achievable when speed ranges from 0.075 m s⁻¹ to 0.209 m s⁻¹ under given loading conditions. Superlubricity failure under low speed is mainly attributed to the weak hydrodynamic effect. However, when the speed is too high, the hydrated water layer can hardly form due to the insufficient running-in process and serious substrate damage. Based on these results, a method was proposed to promote the achievable speed of stable superlubricity to m s⁻¹ level, namely performing the pre-running-in process under low or medium speed and then running at high speed. The results may help understand the liquid superlubricity failure mechanism and open a door to the wide application of liquid superlubricity in industrial areas.