TRIBOLOGICAL CHARACTERISTICS OF SI3N4, CERAMIC IN HIGH-TEMPERATURE AND HIGH-PRESSURE WATER

The effects of sliding speed and dissolved oxygen on the tribological behavior of Si3N4 sliding on itself in water were investigated at room temperature and at 120 degrees C saturated vapor pressure. The friction coefficients and specific wear rates at 120 degrees C mere much larger than those at room temperature and had a minimum at about 0.4 m/s, whereas the specific wear rate of the disk increased with increasing the sliding speed. The wear rate at lower sliding speeds in water at 120 degrees C is considered to be primarily controlled by the increase of the contact stress on the asperities which are formed by the dissolution of grain boundaries of the Si3N4 ceramic and the subsequent dissolution of the silica layer of the reaction product. However, the wear rate at higher sliding speeds is governed by the direct oxidation and microfracture of the Si3N4 substrate. The tribochemical reaction to produce NH, mainly occurred at all sliding conditions in water at room temperature and 120 degrees C, and the reaction to produce H, gas appeared slightly only at the sliding speeds above 0.4 m/s at 120 degrees C. The tribological behavior was independent of dissolved oxygen concentration for all sliding conditions in water at room temperature and 120 degrees C.