Model Experiments Concerning Abnormal Grain Growth in Silicon Nitride

摘要

Model experiments were designed to study abnormal grain growth in Si_3N_4-based ceramics. Experiments relating inhomogeneous crystalline secondary-phase distribution to exaggerated grain growth conclusively showed that abnormal grain growth is not governed by secondary-phase distribution, because a rapid homogenization of locally formed liquid occurs via capillary forces. Further investigations were focussed on intrinsic properties of the α-Si_3N_4-starting powders. The influence of: (ⅰ) β-Si_3N_4-grain morphology; (ⅱ) β-Si_3N_4-nudei density, and (ⅲ) β-Si_3N_4. grain-size distribution of the powder blends on microstructural development were analyzed. The results revealed that a large basal plane of β-Si_3N_4 seeds energetically and kinetically favours grain growth. However, this effect is only partly responsible for abnormal grain growth. The formation of elongated Si_3N_4 grains, such as in in situ reinforced Si_3N_4 materials, strongly depends on the amount and grain-size distribution of β-Si_3N_4 nuclei present in the α-Si_3N_4-starting powder.