Mechanical Properties of Mullite

The mechanical properties of alkoxy‐derived, high‐purity, translucent, theoretically dense mullite (3AI 2 O 3 .2SiO 2 ) were investigated over the temperature range room temperature to 1500°C. Large agglomerates were found to contribute to the formation of porosity nests which act as strength‐controlling flaws at room temperature as well as at high temperatures. Despite the slow crack growth above 1300°C, a slight increase in fracture stress and a large increase in K Ic were observed up to 1500°C. These increases are explained by the dominance of energy dissipation through plastic relaxation in the plastic zone over grain‐boundary sliding due to the presence of the glassy phase.