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

TLDR

Transient viscous sintering of α‑alumina core/amorphous silica coating particles at ~1300 °C followed by conversion at ~1500 °C yields dense mullite and mullite‑based composites with zirconia, silicon carbide particles or whiskers. The process produced higher densification than recent studies, attributed to silica coating flow, delayed mullite formation, and a higher percolation threshold for network development.

Abstract

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.