FATIGUE BEHAVIOR AND FRACTURE-TOUGHNESS OF MULLITE CERAMICS CONTAINING ZRO2 AT 1200-DEGREES-C

The static and dynamic fatigues of 72 and 74 mass% Al2O3 mullite ceram ics containing 10 vol% ZrO2 (72A-10Z, 74A-10Z) were measured at 1200 d egrees C, and the fracture toughness was evaluated from the size of a slow crack growth (SCG) region. The crack growth parameters N obtained by the static fatigue technique were 15 for 72A-10Z and 37 for 14A-10 Z. On the other hand, the N values obtained by the dynamic fatigue tec hnique were 23 for 72A-10Z and 30 for 74A-10Z, Fracture toughness (K-I C), obtained by the controlled surface flaw (CSF) method after the sta tic fatigue measurement, was 2.5 MPa.m(1/2) for 72A-10Z or 1.6 MPa.m(1 /2) for 74A-10Z, and was negligibly affected by the addition of ZrO2. The fracture toughness of 72A-10Z obtained after the dynamic fatigue m easurement varied considerably with stressing rate. The fracture tough ness obtained through static and dynamic fatigue measurements showed t he rising R-curve behavior. Enhanced fracture toughness and R-curve be havior in 12A-10Z were attributed to the fracture of rodlike crystals and/or the viscoelastic flaw of the glassy phase in the grain boundary .