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 alpha-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 (approximately 1300-degrees-C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (greater-than-or-similar-to 1500-degrees-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.