Reaction-bonded mullite/zirconia composites

The feasibility of fabricating dense, low-shrinkage, mullite/ZrO2 composites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined. Compacts pressed from an attrition-milled powder mixture of Al, Al2O3 and zircon were heated in air according to a two-step heating cycle. The phase evolution and microstructural development during reaction bonding were traced by X-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy on samples extracted from various points along the heating cycle. It is seen that, as in conventional RBAO, Al oxidizes to γ-Al2O3 which then transforms to α-Al2O3 between 1,100 and 1,200 C. The zircon dissociation commences at ∼ 1,400 C and is practically complete by 1,500 C. Mullite enriched in Al2O3 forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite, t- and m-ZrO2, and residual α-Al2O3. The flexure strength of the composite is superior to that of pure mullite, and ∼ 80% of the strength is retained up to 1,200 C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure