Microstructures and mechanical properties of mullite-zirconia composites made from inorganic sols and salts

This paper reports a flexural strength (four-point bending, RT) of 632 MPa achieved for a solgel prepared mullite-zirconia composite (MZYB), 30 wt% ZrO2 with 3 mol% Y2O3, sintered in air to 99.0% of theoretical density at 1550 degrees C for 4 h. The retained volume fraction of tetragonal ZRO2 on a ground surface was 94%. The presence of t-ZrO2 particles crystallized from the intergranular phase was believed to increase the strength by a stress-induced t → m transformation in the stress field of propagating cracks. For MZYB (1600 degrees C/4 h) with a t-ZrO2 fraction of 52% a fracture toughness (KIC) of 4.1 MPa1/2 was achieved, which presumably can be explained by microcrack nucleation at transformed m-ZrO2 particles and by a larger amount of transformable t-ZrO2. The microstructure and the mechanical properties were closely related to the size of the starting sol particles