Investigations on the mechanism of superlubricity achieved with phosphoric acid solution by direct observation

摘要

In this work, the contact region between a Si_3N_4 ball and a SiO_2 plate with the lubrication of phosphoric acid solution is observed directly by an optical microscope combined with a Raman microscope to understand the superlubricity mechanism. It is found that the wear on the friction surfaces mainly occurs at the beginning of the test and nearly disappears after the friction coefficient reduces to 0.05. When the superlubricity appears (μ = 0.004), there is only a limited amount of solution available to the contact (forming starvation state), resulting in an "H" distribution surrounding the contact region. Moreover, it is observed that the hydrogen bond effect in the solution is enhanced with time going by, and finally a thin film with hydrogen bond network among H_3PO_4, H_2PO_4~-, and H_2O is formed on the friction surfaces, leading to the superlubricity. By employing this direct observation approach, the structure of the confined solution and the superlubricity mechanism of phosphoric acid solution are finally investigated and discussed.