Abstract: The Si3N4 ceramics were prepared by hot-pressing at 1800℃using different amounts of Y2O3-Al2O3 as sintering additives and α-Si3N4 as raw powder.The effects of Y2O3-Al2O3 content on phase composition , densification , microstructure and mechanical properties were studied .The results show that dense Si3N4 ceramics could be hot-pressed by the addition of 6wt;Y2O3-Al2O3 at 1800 ℃.The densification of Si3N4 ceramics was not affected by the further increase of Y2O3-Al2O3 content, whereas the amounts of α-Si3N4 and β-Si3N4 phases in the ceramics were influenced by the Y2O3-Al2O3 content. The higher Y2O3-Al2O3 content could promote the α-βphase transformation .Regardless of the Y2O3-Al2O3 content , the Si3N4 ceramics had bimodal microstructures composed of fine equiaxedgrains and large elongated grains.When the Y2O3-Al2O3 content increased to 10wt;, the Si3N4 ceramics with higher content of elongated grains show more bimodal microstructure .Based on the present study , it was found that the Si3N4 ceramics with high hardness and high strength could be prepared by the addition of low Y2O3-Al2O3 content(6wt;), and the Si3N4 ceramics with high toughness and high strength could be obtained by the addition of high Y2O3-Al2O3 content(10wt;).

Key words: The Si3N4 ceramics were prepared by hot-pressing at 1800℃using different amounts of Y2O3-Al2O3 as sintering additives and α-Si3N4 as raw powder.The effects of Y2O3-Al2O3 content on phase composition , densification , microstructure and mechanical properties were studied .The results show that dense Si3N4 ceramics could be hot-pressed by the addition of 6wt;Y2O3-Al2O3 at 1800 ℃.The densification of Si3N4 ceramics was not affected by the further increase of Y2O3-Al2O3 content, whereas the amounts of α-Si3N4 and β-Si3N4 phases in the ceramics were influenced by the Y2O3-Al2O3 content. The higher Y2O3-Al2O3 content could promote the α-βphase transformation .Regardless of the Y2O3-Al2O3 content , the Si3N4 ceramics had bimodal microstructures composed of fine equiaxedgrains and large elongated grains.When the Y2O3-Al2O3 content increased to 10wt;, the Si3N4 ceramics with higher content of elongated grains show more bimodal microstructure .Based on the present study , it was found that the Si3N4 ceramics with high hardness and high strength could be prepared by the addition of low Y2O3-Al2O3 content(6wt;), and the Si3N4 ceramics with high toughness and high strength could be obtained by the addition of high Y2O3-Al2O3 content(10wt;).