EFFECT OF COMPOSITION ON PHASE EVOLUTION AND MICROSTRUCTURE OF REACTION BONDED MULLITE (RBM)

Compositions of Al2O3+Si, SiO2+Al and Al+Si systems were prepared to study the effect of reaction bonding process on the mullite formation. The composition of each system was adopted according to mullite stoichiometery and sintered in 700-1600°C range. Results showed that the formation of reaction bonded mullite starting from Al2O3+Si mixtures, proceeded in two partially overlapping steps, the oxidation of Si to SiO2, and the reaction of SiO2 and Al2O3 to form mullite. In this system, up to 1400°C, conversion of Si to SiO2 was taken place and cristobalite formed, but mullite formation was not observed. Mullite phase started to form at 1450°C. Results indicated that complete reaction was not occurred up to 1600°C and 2 hours soaking time. XRD patterns of samples in Al+SiO2 system showed that the reaction through sequences:(a) reduction of SiO2 by Al, (b) formation of ?-Al2O3 and SiO2-derived Si oxidation, and (c) mullite formation. X-ray diffraction patterns of heat-treated Al+Si system showed that reaction between Al and oxygen at 900°C was occurred with the reaction product being ?-Al2O3. Oxidation of Si and formation of mullite were not detected in this system. SEM micrographs showed that both Al2O3+Si and SiO2+Al systems have similar microstructures, which consisted of ?-Al2O3, mullite and free Si. The microstructures of the samples in Al+Si system consisted of ?-Al2O3, free Al and Si with intermetallic Al-Si compound.