Microstructure and mechanical properties of mullite/zirconia composites prepared from alumina and zircon under various firing conditions

Zirconia-dispersed mullite composites were prepared by reaction sintering of alumina and zircon powders under various firing conditions. Mullite formed by firing at 1635 degrees C apparently contained > 60 mol% Al2O3 and incorporated a small amount of zirconia. This transient mullite composition changed to the normal composition by treatments such as long firing time, annealing, two-step firing and/or seeding. Mullite/zirconia composites at room temperature showed good:strength and fracture toughness, but these properties decreased significantly at hi,oh temperature because glassy phase in the grain boundaries. Mechanical strength at high temperature could be improved by those firing treatments which reduced the amount of glassy phase, especially by two-step firing and seeding methods.