Preparation and Properties of Zirconia-Toughened Mullite Ceramics.

Compacts formed from high-purity fused submicrometer mullite powders, both pure and with the additions of 5 to 25 vol% ZrO2 of particle size 1, 2, or 4 μm (average), were sintered in air at 1610°C for 6 h to form dense ceramic bodies. Mechanical properties (flexural strength, Kic, and Young's modulus) were measured both before and after thermal shock. The strength after thermal shock increased considerably in systems containing additions of 10 to 20 vol% ZrO2; Kic also increased, but not so markedly. The low fraction of tetragonal ZrO2 (0.1 to 0.2) in the as-fired samples and the increase in Young's modulus after thermal shock for some strengthened samples supported the view that the interaction of residual internal stresses induced by quenching and the accompanying martensitic transformation of ZrO2 result in potential nucleation sites for microcracking. Thus, microcracking may be the predominant energy-absorbing mechanism responsible for strengthening and toughening the quenched ceramic bodies.