Solid-Gas Reaction during Sintering of Si_3N_4 Ceramics (Part 2) -Beta to (#alpha#)′ Transformation of Si_3N_4 during Heat Treatment in Carbon Powder Bed-

摘要

Monolithic β-Si_3N_4 ceramics with additives of 5 mass% Y_2O_3 and 5 mass% Al_2O_3 were prepared by gas-pressure sintering at 2023 K at a N_2 pressure of 1 MPa with 50 mass%Si_3N_4/50mass%BN powder bed. When β-Si_3N_4 was heat-treated at 2023, 2073 and 2123 K at the N_2 pressures of 0.1 and 1 MPa with a carbon powder bed, a surface layer containing mainly SiC was generated. Inside the SiC layer, #alpha#-SiAION was generated.Moreover, melilite (Y_2Si_3O_3N_4) was observed when the N_2 pressure was 0.1 MPa. The reasons for the generations of #alpha#-SiAION and melilite were thought to be as follows. During the formation of the SiC layer, the liquid phase wiht Y-and Al-elements was excluded from the original surface layer. The liquid phase with Y- and Al-elements concentrated between the SiC layer and the β-Si_3N_4 bulk. Finally, Y and Al in the liquid phase reacted with β-Si_3N_4, and α-SiALON and melilite were formed.