| The influence of nano-Al2O3 on the sintering and the properties of the Si3N4 ceramics was researched in this paper .The samples with different amount of nano-Al2O3 were obtained by using pressureless sintering at 1600 , 1650 , 1700 in the nitrogen atmosphere. The microstructure and the composition of the ceramics were determined by the means of X-ray, SEM, micro-hardness meter etc.It is show that the Sisty ceramics can be densified at 1650C to % percent of the theory density through the addition of nano-Al2O3 (the value could be 90 percent by other technique).The crystalline growth of the cylindrical -Si3N4 and the ratio of its longitude to its diameter are increased with the addition of nano-Al2O3. A uniform microstructure and an fined crystal as well as more Sialon phases can be obtained in the Si3N4 ceramics through the addition of that. It can be ruled out that an increase in the properties of Si3N4 ceramics can be achieved through the introduction of nano-Al2O3.As to the influence of the content of nano-Al2O3, it is found that the largest bending strength of the ceramics being 720.06 MPa among the samples sintered at 1700 C appears at 8% nano-Al2O3 content while at 1650 C the largest toughness being 8.61Mpa m172 does at 10% nano-Al2O3 content.But the content of nano- Al2O3 should be controlled between 6 and 12 percent. When above 12 percent the nano-Al2O3 is excess and there are a lot of free corundum grains on the grain boundary, therefore the property of the ceramics increases no longer as the content of nano-Al2O3 continuosely increased. While it is under 6 percent, there is a lot of Si3Al267N4O4 phase after being sintered, which is the infancy product of the Sialon ceramics, and therefore the best properties of the ceramics can be reached hardly.
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