REACTION-BONDED MULLITE ZIRCONIA COMPOSITES

The feasibility of fabricating dense, low-shrinkage mullite/ZrO2 compo sites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined, Compacts pressed from an att rition-milled powder mixture of Al, Al2O3 and zircon were heated in ai r according to a two-step heating cycle, The phase evolution and micro structural development during reaction bonding were traced by X-ray di ffraction, nuclear magnetic resonance, and scanning electron microscop y on samples extracted from various points along the heating cycle, It is seen that, as in conventional RBAO, Al oxidizes to gamma-Al2O3 whi ch then transforms to alpha-Al2O3 between 1100 degrees and 1200 degree s C. The zircon dissociation commences at similar to 1400 degrees C an d is practically complete by 1500 degrees C. Mullite enriched in Al2O3 forms initially, but 3:2 stoichiometry is attained in the final produ ct which consists of mullite, t- and m-ZrO2, and residual alpha-Al2O3. The flexure strength of the composite is superior to that of pure mul lite, and similar to 80% of the strength is retained up to 1200 degree s C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure.