Reaction‐Bonded Mullite/Zirconia Composites

The feasibility of fabricating dense, low‐shrinkage, mullite/ ZrO 2 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, A1 2 O 3 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, AI oxidizes to γ‐Al 2 O 3 which then transforms to α‐AI 2 O 3 between 1100° and 1200°C. The zircon dissociation commences at ∼1400°C and is practically complete by 1500°C. Mullite enriched in Al 2 O 3 forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite, t ‐ and m‐ZrO 2 , and residual α‐AI 2 O 3 . The flexure strength of the composite is superior to that of pure mullite, and ∼80% of the strength is retained up to 1200°C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure.