CHANGES OF MICROSTRUCTURE AND HIGH-TEMPER ATURE STRENGTH OF CERAMIC COMPOSITE IN THE MULLITE-ZRO2-AL2O3 SYSTEM USING AN IN-SITU REACTION BETWEEN SYNTHETIC ZIRCON AND AL2O3

Ceramic composite was synthesized using an in-situ reaction between sy nthetic zircon and Al2O3. High-temperature strength did not decrease u p to 1500 degrees C for the sample having only mullite for matrix. Whi le, the strength decreased above 1300 degrees C for others. With incre ase of the Al2O3 precipitated in the mullite matrix phase, the mullite grains became isotropic and small, which may occur the creep deformat ion. On the contrary, the ceramic composite used in the in-situ reacti on revealed excellent high-temperature strength. It was found that ZrO 2 and Al2O3 particles dispersed in the grains of mullite matrix, contr ibuted to increase the room temperature strength, and to improve effec tively the high-temperature strength. Remarkable difference of the mic rostructure was not observed in the composite obtained from the natura l zircon instead of the synthetic zircon at same composition. However, high-temperature strength decreased markedly due to the increase of i mpurity content in the natural zircon. High-temperature strength of th e ceramic composite obtained from synthetic mullite, ZrO2 and Al2O3 po wders by the mixing method, decreased significantly at 1300 degrees C. This behavior can be explained by the shape and size of the mullite i n matrix, or the quantities of ZrO2 and Al2O3 precipitated at the grai n boundary.