Observations and mechanisms of fracture in polycrystalline alumina

Abstract

The fracture mechanics of polycrystalline alumina have been investigated using the double cantilever beam cleavage method. The fracture resistance of alumina was measured at temperatures up to 1500°C. The fracture resistance of alumina increased with increasing grain size for all temperatures studied. Electron fractography studies of the fractured surface indicate that the large grain alumina contains more transgranular fracture than the small grain material. This preponderance of transgranular fracture is shown to accompany a higher fracture resistance in the material. It is thus concluded that one mechanism of variation of strength in these materials is through variation in the relative amounts of transgranular and intergranular fracture. The lower softening point of SiO 2 causes alumina with this material as a binder to lose its strength at lower temperatures than does MgO bonded material. The mechanism of fracture in these polycrystalline ceramics leads to a suggested method for improved mechanical strength.