Tribology of Mullite Ceramics at Elevated Temperatures

Abstract

Friction and wear tests from room temperature to 1000°C were carried out for two kinds of mullite ceramics, and compared with those of alumina ceramics. Samples prepared from the natural raw materials (M-1) and the sol-gel method (M-2) were used. The former contained considerable amount of impurities and they formed glassy phase in the grain boundaries whereas the latter was highly pure and dense ceramics. No significant change in either coefficient of friction or wear loss was observed against temperature in the M-1 material. In the M-2 material, the coefficient of friction was almost constant up to 600°C but slightly increased above that temperature. The specific wear loss, defined as the wear volume divided by the load and the wear distance, slightly increased from room temperature to 600°C and then significantly decreased above 800°C. A film-like region formed by very fine grains, which was similar to the one previously observed in the alumina wear surface, was observed in the surface worn at 1000°C. The observed microstructure suggests that the surface film may be derived due to the plastic deformation of grains by dynamic recrystallization during sliding under the shear stress field at high temperatures. A certain correlation between the wear loss and the fracture toughness KIC was found although no correlation between the wear loss and hardness was found.