The role of kyanite in the improvement in the crystallization and densification of the high strength mullite matrix: Phase evolution and sintering behaviour

Fine particles of kyanite were used as reinforcement in mullite matrix composite. Mix of metakaolin and meta-alumina (calcined bauxite) received different fraction of fine kyanite particles: 37.5, 42.5 and 50 mass%. The objectives were to improve the crystallization of kaolin-based mullite matrix reducing the amorphous content and the grain growth. Packing process and densification were enhanced, leading to the action of kyanite at low temperature (<1250 °C) as phase separator and nucleation sites for the transformation of metakaolin to acicular mullite. At high temperature (>1350 °C), the kyanite particles are decomposed to mullite still without glassy phase and the metastable alumina presented in the mix reacts with amorphous silica to produce additional mullite. The sintering behaviour, the thermal expansion behaviour of ~0.5 % between 200 and 1000 °C and the coefficient of the thermal expansion of 5.92, 5.15 and 6.92 × 10−6/°C for KY3, KY4 and KY5, respectively, between 200 and 1000 °C, and their decrease at higher temperature demonstrated the optimum refractoriness of the mullite matrix composites developed.