Sintering and microstructural investigations on combustion processed mullite

Abstract

Various grades of mullite have been prepared by the combustion route using different fuels and silica sources of the redox mixture. Sintering behaviour and microstructure of combustion processed mullite have been investigated. Pure mullite sintered at 1650 °C showed 3% shrinkage and was porous. The microstructure shows the presence of equiaxed grains. Hot isostatic pressing (100 MPa, 1500 °C, 30 min) followed by sintering at 1700 °C yielded >95% theoretical density. Mullite with 5 wt% of YO or MgO sintered at 1650 °C achieved high density (≈95% theoretical density) with a shrinkage of 18 and 22% respectively. The microstructure showed the presence of anisotropic elongated mullite grains and corundum particles. The enhanced densification of mullite with additives is attributed to the formation of liquid phase which facilitates diffusion.