Friction and Wear of Silicon Nitride at 150°C to 800°C

Abstract

The friction coefficient of hot-pressed Si₃N₄ on itself was measured in the temperature range of 150 to 800°C. Sliding occurred in dry argon, dry and humid air and the sliding velocity was varied from 0.1 to 10 cm/s. In dry argon and dry air, the friction coefficient is f = 0.8 ± 0.05 over the whole temperature range and wear occurs by fracture as it does at room temperature. In humid air, the friction coefficient is a complex function of sliding velocity, load, and temperature. Values of as low as f = 0.2 are obtained in a narrow velocity range at temperatures between 150 and 650°C. They are associated with the formation of a soft, amorphous, surface layer of silicon dioxide. At 800° C, the friction coefficient is f = 0.8 in all atmospheres. At every temperature and load, there is a velocity above which the friction coefficient increases rapidly and wear by microfracture is predominant. We speculate that the observed behavior is due to a balance between tribochemical reaction rate and the rate of mechanical wear.