Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.