Solid Lubrication of Silicon Nitride with Cesium-Based Compounds: Part II — Surface Analysis

Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) were used to characterize the wear surfaces of selected samples from Part 1 of the authors study. Results are presented for films generated on silicon nitride (Si3N4) originally coated with cesium oxytrithiotungstate (Cs2WOS3), cesium sulfate (Cs2SO4), and a hydrated cesium silicate (Cs2O·3SiO2·nH2O), all applied in a sodium silicate binder (Na2SiO3). Results show the presence of mostly Si, O, and Cs within the wear tracks of post-tested specimens. In some cases, W and S were not detected on samples that originally contained these elements, suggesting that decomposition had taken place. To simulate the reactions that might occur in a tribo-contact, mixtures of Si3N4 and Cs2WOS3 powders were heated in air to 700°C and analyzed using XPS and Bremsstrahlung-excited AES. It was found that Cs2WOS3 accelerates the formation ofSiO2 on Si3N4 under static conditions. These results support our hypothesis that high temperature chemical reactions between the cesium-containing compounds and the Si3N4 surface form a lubricious cesium silicate film. A mechanism is proposed based on the glass-modifying tendency of alkali metals and the hot-corrosion of Si3N4