MODEL EXPERIMENTS CONCERNING ABNORMAL GRAIN-GROWTH IN SILICON-NITRIDE

Model experiments were designed to study abnormal grain growth in Si3N 4-based ceramics. Experiments relating inhomogeneous crystalline secon dary-phase distribution to exaggerated grain growth conclusively showe d that abnormal grain growth is not governed by secondary-phase distri bution, because a rapid homogenization of locally formed liquid occurs via capillary forces. Further investigations were focussed on intrins ic properties of the alpha-Si3N4-starting powders. The influence of: ( i) beta-Si3N4-grain morphology; (ii) beta-Si3N4-nuclei density, and (i ii) beta-Si3N4-grain-size distribution of the powder blends on microst ructural development were analyzed. The results revealed that a large basal plane of beta-Si3N4 seeds energetically and kinetically favours grain growth. However, this effect is only partly responsible for abno rmal grain growth. The formation of elongated Si3N4 grains, such as in in situ reinforced Si3N4 materials, strongly depend on the amount and grain-size distribution of beta-Si3N4 nuclei present in the alpha-Si3 N4-starting powder.