Fabrication of mullite composites by cyclic infiltration and reaction sintering

Mullite composites were fabricated by infiltrating porous preforms containing alumina and silica powder with a liquid mullite precursor and then, by reaction sintering of the infiltrated preforms. Cyclic infiltration to saturation could introduce 15–25 wt.% of mullite precursor into preforms with 45–55% open porosity. The increase in the green density accounted for the significant reduction in sintering shrinkage of the preforms. The addition of 20 vol.% of 3 mol% yttria–zirconia in the preforms lowered the sintering temperature and increased the sinterability of the mullite composites. The reactive formation of mullite from alumina and silica started at 1300°C and became more prominent with higher sintering temperatures. The presence of mullite powder, infiltrated mullite and 3 mol% yttria–zirconia could enhance the reactive formation of mullite. SEM observations revealed that all the mullite grains were equiaxed. The reactively formed mullite and the infiltrated mullite grains were smaller than the mullite grains derived from the mullite powder in the preforms.