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

Compositions of Al۲O۳+Si, SiO۲+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 ۷۰۰-۱۶۰۰°C range. Results showed that the formation of reaction bonded mullite starting from Al۲O۳+Si mixtures, proceeded in two partially overlapping steps, the oxidation of Si to SiO۲, and the reaction of SiO۲ and Al۲O۳ to form mullite. In this system, up to ۱۴۰۰°C, conversion of Si to SiO۲ was taken place and cristobalite formed, but mullite formation was not observed. Mullite phase started to form at ۱۴۵۰°C. Results indicated that complete reaction was not occurred up to ۱۶۰۰°C and ۲ hours soaking time. XRD patterns of samples in Al+ SiO۲ system showed that the reaction through sequences: (a) reduction of SiO۲ by Al, (b) formation of a- Al۲O۳ and SiO۲-rderived Si oxidation, and (c) mullite formation. X-ray diffraction patterns of heat-treated Al+Si system showed that reaction between Al and oxygen at ۹۰۰°C was occurred with the reaction product being a- Al۲O۳ Oxidation of Si and formation of mullite were not detected in this system. SEM micrographs showed that both Al۲O۳+Si and SiO۲+Al systems have similar microstructures, which consisted of a- Al۲O۳, mullite and free Si. The microstructures of the samples in Al+Si system consisted of a- Al۲O۳ free Al and Si with intermetallic Al-Si compound.

کلیدواژه ها:

MULLITE ، REACTION BONDING OROCESS ، ALUMINA ، SILICON ، SILICA