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 sinter ing of alumina and zircon powders under various firing conditions. Mul lite formed by firing at 1635 degrees C apparently contained > 60 mol% Al2O3 and incorporated a small amount of zirconia. This transient mul lite composition changed to the normal composition by treatments such as long firing time, annealing, two-step firing and/or seeding. Mullit e/zirconia composites at room temperature showed good:strength and fra cture 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 treatme nts which reduced the amount of glassy phase, especially by two-step f iring and seeding methods.