In this study, with a view to improve the mechanical properties, the influence of the change in the micro-structure, crystal phase and mechanical properties were evaluated, by changing the amount of one of the sintering additives, Al
2O
3 and the HIP sintering conditions, from the viewpoint of the micro-structure consisting of α-Si
3N
4, β-Si
3N
4 and glassy grain boundary phase.
The experimental results showed that 1) the combination of the sintering additives makes a various sorts of crystals and glassy phase formed which promotes the densification, 2) the addition of Al
2O
3 was effective in the densification, further, 3) as for the HIP sintering condition, N
2 gas pressure higher than 60MPa greatly influences the transformation from α-Si
3N
4 to β-Si
3N
4 with densification.
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