High-temperature tribology of silicon nitride lubricated with cesium-based inorganic films

The high temperature sliding friction and wear of silicon nitride was investigated under unlubricated and solid lubricated conditions. Experiments were performed in laboratory air (18.7% ± 10.0% R.H.), mostly at 600°C, using a ball-on-disk configuration. Two cesium-based inorganic films were studied; a sodium silicate bonded cesium oxythiotungstate (Cs₂WOS ₃) coating and a cesium silicate chemical reaction film of the form Cs₂O·xSiO₂. The selection of these materials was based on previous studies that showed excellent high-temperature performance on sub-scale and full-scale ceramic bearing components. Wear surfaces were characterized by surface profilometry and optical and scanning electron microscopy (SEM). Surface chemistry of selected posttested samples was analyzed using Auger electron spectroscopy (AES) with depth profiling, Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy.; Although both of the cesium-based films studied were effective in reducing wear at 600°C, the best tribological performance was obtained with thin chemical reaction films annealed in a sulfur-rich oxidizing environment. Friction coefficients as low as 0.04 and wear factors in the range of 4 × 10 −9 to 1 × 10−8 mm3/N·m were obtained at 600°C with this system. These are comparable to boundary liquid lubricating films at much lower temperatures. The data provide conclusive evidence that neither tungsten nor molybdenum is necessary for low friction at 600°C. The results also suggest that sulfur and cesium play important roles in the formation of a lubricious film. The results show that the lubricating film initially consists of Cs₂SO₄ deposits that give a μ of 0.10 at 600°C. These deposits subsequently dissolve SiO ₂/Si₃N₄ asperities during sliding via a hot-corrosion mechanism to produce very smooth surfaces. This dissolution process leads to the formation of a lubricious cesium silicate film with μ ∼ 0.04. The tenacity and endurance of the film are thought to be enhanced by the inward diffusion of oxygen and cesium. Diffusion of magnesium out from the silicon nitride substrate (i.e., sintering aid) was also observed and resulted in higher friction.