The optical and SEM microstructures and the room temperature mechanical properties of Si
3N
4-Er
2O
3-CaO-6 mol%, Al
2O
3 compact sintered under 0.1 MPa of nitrogen gas at 1973-2073 K for 3.6 ks were mainly investigated as a function of Er
2O
3 (2, 4, 8 mol%) and GaO (0-10 mol%) contents.
The results obtained were as follows; (1) The additions of Er
2O
3 and CaO to Si
3N
4-Al
2O
3 compact increased the relative density. (2) The a-β phase transformation and the grain growth of Si
3N
4 phase were restricted with increasing CaO content. (3) The crystalline grain boundary phases in Si
3N
4-8 mol%Er
20
3-CaO-A1
2O
3 compact were found to be mainly Er
4Si
2O
7N
2 and Er
2Si
3O
3N
4 at small and large CaO contents, respectively. (4) The transverse-rupture strength showed a maximum generally at about 8 mol%Er
2O
3 and 2 mol%CaO. The hardness increased with increasing CaO content. The result on the strength was discussed in relation to the relative density, grain size, kinds of grain boundary phases, fracture toughness, fracture source, etc.
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