Tribochemical characterization of the lubrication film at the Si3N4Si3N4 interface sliding in aqueous solutions

A chemical characterization of the thin film at the interface between two silicon nitrides sliding in water was performed to analyze the relationship between the tribological properties (sliding with a low coefficient of friction μ = 0.01), the chemical form of lubricating layers and their practical application. The observed low sliding friction, tribo-induced reactions and subsequent film formation are discussed in terms of reactions between silicon nitride and aqueous solutions. Sliding experiments were carried out in pure water, 1% hydrogen peroxide and NaCl solutions, and the worn surface was quantitatively analyzed by EPMA and SEM. In a 1% hydrogen peroxide solution, the coefficient of friction decreased at an earlier stage of the sliding test than in pure water. This result was attributed to an oxidizing agent which accelerated the surface oxidation reaction. But in a 3% sodium chloride solution, the coefficient of friction remained high: μ = 0.8. Analysis of the wear debris of the silicon nitrides in the 3% sodium chloride solution showed that the debris had a composition equal to Na2O · nSiO2, indicating that the reaction of Na+ with this surfaces suppressed the formation of a lubricating film. We propose that the lubricating surface film is composed of a SiOH bond, which is insoluble in water and is the most probable chemical form.