STATIC FATIGUE AND FRACTURE-TOUGHNESS OF MULLITE CERAMICS AT 1200-DEGREES-C

The fatigue behavior for mullite ceramics containing 71, 72 and 74 wt% Al2O3 (71A, 72A and 74A) was measured at 1200-degrees-C by the static fatigue technique. The fracture toughness was evaluated from the slow crack growth magnitude (fatigue fracture surface) combined with the ap plied stress. The crack growth parameter (N) of 71A, 72A and 74A was 1 7, 23-24 and 26, and the fracture toughness (K(IC)) was 4.3, 2.4 and 1 .7 MPa.m1/2, respectively. These tendencies correspond to a decrease o f the glass phase in the grain boundary. The fracture modes of the gra ins in the fatigue fracture surface for 71A was intergranular fracture in equiaxial crystals and transgranular fracture broken in the rod-li ke crystals, for 72A intergranular fracture in equiaxial crystals and transgranular fracture broken in slightly remained rod-like crystals, and for 74A only intergranular fracture. The fracture mode in the cata strophic fracture surface was transgranular for 71A, 72A and 74A. It w as considered that the fracture toughness of mullite at 1200-degrees-C increased with a decrease of Al2O3 content because the crack tip beca me blunter by increasing the glass phase.