GRAIN-GROWTH STUDIES OF SILICON-NITRIDE DISPERSED IN AN OXYNITRIDE GLASS

Isothermal growth of beta-Si3N4 crystals dispersed in an oxynitride glass (Y-Si-Al-O-N) was studied by electron microscopy after heat treatment at temperatures between 1550-degrees and 1640-degrees-C for 1 to 18 h. The beta-crystals exhibited growth striations introduced by intermediate coolings and these striations were used for developing a sophisticated technique for analysis of growth. It was determined that alpha/beta-transformation and Ostwald ripening can be treated as different kinetic stages of grain growth, while beta-nucleation was found to be insignificant. The mean diameter of the needlelike beta-grains was almost constant during phase transformation, indicating negligible growth of the beta-prism plane; growth was mainly one-dimensional with the maximum mean length and aspect ratio just at the end of the phase transformation. The growth rate of the beta-basal plane was independent of diameter, indicating interface-controlled growth. During Ostwald ripening, the length distribution broadened and the aspect ratio of smaller grains decreased. Dissolution of small grains caused an increase in the mean diameter, while the mean aspect ratio decreased.