Observations on the effect of calcium segregation on the fracture behaviour of polycrystalline alumina

The influence of calcium segregation to the grain boundaries of polycrystalline alumina on room temperature fracture behaviour has been investigated. In a commercial high-density single-phase alumina containing less than 5 ppm calcium by weight, thermal treatments were employed to achieve equilibrium segregation from 0.6 to 1.6 at % calcium without detectable changes in grain size (18m) or porosity distribution. Room temperature SENB test results revealed an inverse dependence of K _IC on calcium segregation levels in the range examined. Fractures were primarily intergranular in all specimens. Qualitatively, the relationship between K _IC and calcium segregation would be predicted from a consideration of the effect of such an ion on the interatomic spacing at the boundary. However, quantitative agreement with the model is poor, the measured effect being much greater than predicted. A relatively high K _IC value was achieved in a fine grained (2m) hot-pressed alumina containing very low levels of segregated impurities. This material exhibited substantial amounts of cleavage fracture. The higher fracture toughness of this alumina is discussed in terms of both increased intergranular and transgranular fracture stresses promoted by the relatively clean grain boundaries and small grain size, respectively.