Effect of Sintering Atmosphere on the a/p-Transformation of Si_3N_4

摘要

Effect of carbon and CO gas atmosphere on a-SiAlON generation in the surface of a β-Si_3N_4 ceramics was investigated. Silicon nitride ceramics having the composition, Si_3N_4 92-Y_2O_3 5-Al_2O_3 3 (mass%), was sintered at 1750℃ for 2h. in four sagger conditions under a N_2 pressure of 0.1-100 MPa. The four conditions were in a carbon sagger with carbon powder-bed, in a carbon sagger without powder-bed, in a BN sagger without powder-bed and in a BN sagger with Si_3N_4/BN mixed powder-bed. The 100% β-Si_3N_4 ceramics, which were sintered in the BN sagger with Si_3N_4/BN mixed powder-bed, were heated under the N_2 pressure of 0.1-100 MPa in the same four sagger. The crystal phases of the ceramics were influenced by the N_2 pressure and the sagger conditions. A SiC thin layer was formed in the surface of the β-Si_3N_4 ceramics when the 100% β-Si_3N_4 ceramics were heated at 1750℃ for 2h. in a carbon powder under a N_2 pressure of 0.1 MPa. In the inner part of the ceramics under the SiC lay er, α-SiAlON and melilite (Si_3N_4 ·Y_2O_3) were generated. These were observed to a depth of 60-70μm from the surface after peeling the SiC layer. The a-SiAlON and melilite are thought to be generated by the following mechanism,: The SiC layer excludes the liquid phase including Y and Al, which are in the grAlN boundary of the β-Si_3N_4, from the layer. Then, the liquid phase concentrates between the SiC layer and the β-Si_3N_4 bulk, and Y and Al in the liquid phase react with β-Si_3N_4, and α-SiA10N and melilite are formed.