In situ toughening of mullite: Effect of yttria and zirconia additions

Mullite powder with a nearly stoichiometric composition (3Al(2)O(3). 2SiO(2)) Was doped with Y2O3 (0.5-3.0 wt%) and hot pressed at 1500 degreesC to obtain samples with nearly full density. The hot pressed material was subsequently annealed at 1750 degreesC to produce a self-reinforced microstructure consisting of elongated grains and an yttrium-aluminum-silicate glass at the grain boundaries. TEM observations revealed a non-uniform distribution of the glassy phase at the grain boundaries. with the glass concentrated at three-grain junctions and at some grain boundaries (two-grain junctions). The indentation fracture toughness of the self-reinforced mullite was similar to2 times the value for the hot pressed material with a fine-grained microstructure. The addition of ZrO2 particles (similar to 20 vol%) for enhancing the strength and fracture toughness of self-reinforced mullite was investigated. With the combination Of Y2O3 additive and ZrO2 particles, the fracture toughness was approximately the same as that for the self-reinforced mullite with only the Y2O3 additive. The enhancement of the strength and fracture toughness of self-reinforced mullite is discussed.