Effect of carbon and CO gas atmosphere on alpha-SedlON generationin the surface of a 6-Si,N, ceranucs was investiguted. Siliconnitride ceramics having the composition, Si,N. 92-Y2O3, 5-Al2O3 3(mass), was sintered at 1750 deg C for 2h. in four sagyer conditionsunder a N2 pressure of 0. l-100 MPa. The four conditions were in acarbon sagger with carbon powder-bed, in a carbon sagger withoutpowder-bed, in a BN sagur without powder-bed and in a BN sagger withSi,N.AsN mixed powdenbed. The 100 beta-Si3N4 ceramics, which weresintered in the BN sagyer with Si,N.SN mixed powder-bed, were heatedunder the N2 pressure of 0. l -l 00 M Pa in the same four sagger. Thecrystal phases of the ceramics were influenced by the N, pressure andthe sagger conditions. A SiC thin layer was formed in the surface ofthe beta-Si3N4. ceramics when the 100 beta-Si3N4. ceramics wereheated at 1 750 deg C for Zh. in a carbon powder under a N, pressureof 0.1 MPa. In the inner part of the ceramics under the SiC layer,alpha-SiAION and melilite (Si3N4. .Y2O3) were generated. These wereobserved to a depth of 60-70 mu m from the surface after peeling theSiC layer. The alpha-SiAION and melilite are thought to be generatedby the following mechanism, The SiC layer excludes the liquid phaseincluding Y and Al, which are in the grain boundary ofthe beta-Si3N4,from the layer.Then, the liquid phase concentrates between the SiClayer and the beta-Si3N4. bulk, and Y and Al in the liquid phasereact with beta-Si3N4, and alpha-SiAlON and melilite are formed.
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