Fracture mechanisms of a coarse-grained, transparent MgAl2O4 at elevated temperatures

The fracture of a transparent, large-grain-sized MgAl2O4 spinel has been studied through temperatures of 1400°C. Fracture toughness values, falling between about 1.3 and 2.3 MPa.m12;1/2, behaved sigmoidally with temperature, with a lower shelf transition appearing near 800°C. Rising R-curves, displaying a run-arrest character, were found at both the lowest and the highest temperatures, while only minimal values of dKR/dΔa were produced at intermediate temperatures of 800°C. This fracture character is ascribed largely to finite concentrations of a residual LiF pressing aid identified on the fracture surface, while additional influences associated with the lower shelf region at the highest temperatures may include the increased fraction of intergranular crack path and the onset of plasticity. This cubic monolithic ceramic displayed strong nonlinear fracture behavior in both temperature regimes. In both cases the fracture character is linked directly to an active toughening mechanism in the wake region, which depends upon crack face bridging.