Solid lubrication of silicon nitride with cesium-based compounds: Part II - Surface analysis

Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscop y (XPS), and Auger electron spectroscopy (AES) were used to characterize th e wear surfaces of selected samples from Part I of the authors study. Resul ts are presented for films generated an silicon nitride (Si3N4) originally coated with cesium oxytrithiotungstate (Cs2WOS3), cesium sulfate (Cs2SO4), and a hydrated cesium silicate (Cs2O . 3SiO(2). nH(2)O), all applied in a s odium 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 a nd S were not detected on samples that originally contained these elements, suggesting that decomposition had taken place. To simulate the reactions t hat might occur in a tribo-contact, mixtures of Si3N4, and Cs2WOS3 powders were heated in air to 700 degrees C and analyzed using XPS and Bremsstrahlu ng-excited AES. It was found that Cs2WOS3 accelerates the formation of SiO2 on Si3N4 under static conditions. These results support our hypothesis tha t high temperature chemical reactions between the cesium-containing compoun ds and the Si3N4 surface form a lubricious cesium silicate film. A mechanis m is proposed based on the glass-modifying tendency of alkali metals and th e hot-corrosion of Si3N4.