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
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ratio decreased.