ACCELERATED REACTION BONDING OF MULLITE

A suitable way to process mullite ceramics and mullite-matrix composit es with low dimensional changes (''near-net-shape processing'') is rea ction bonding, using silicon metal and corundum (alpha-Al2O3) as start ing materials, because sintering-induced shrinkage is compensated by v olume expansion caused by the silicon-oxidation-induced volume expansi on, This work describes a new reaction-bonded mullite (RBM) processing route that proceeds at much lower temperatures (less than or equal to 1350 degrees C) than in ''normal'' RBM systems (less than or equal to 1550 degrees C). Accelerated silicon oxidation and mullite formation are effected by adding low amounts of yttria (Y2O3) or ceria (CeO2) to the green powder mixtures, which causes the formation of transient, m etastable, and low-viscosity Y-AI-Si-O and Ce-Al-Si-O partial melts. A fter long-term heat treatments at less than or equal to 1350 degrees C , however, the intermediate liquids recrystallize and leave behind RBM ceramics that are homogeneous, substantially glass-free, and relative ly dense. These two RBM materials consist of mullite, alpha-Al2O3, and Y2Si2O7 or CeO2, respectively.