Effect of Aluminum Content on Mechanical Properties and Thermal Conductivities of Sintered Reaction-Bonded Silicon Nitride

摘要

A systematic study was conducted of the effect of Al impurities on the mechanical and thermal properties of sintered reaction-bonded silicon nitrides (SRBSNs). 2 mol.% Y_2O_3 and 5 mol.% MgSiN_2 were added as sintering aids to high-purity raw Si powder, followed by respective additions of 0, 0.01, 0.1, 0.2, and 0.4 mass% of Al relative to Si. The prepared powder mixes were press formed and nitrided in a 0.1 MPa nitrogen atmosphere at 1400℃ for 8 h. Postsintering of the nitrides was conducted under a pressurized nitrogen atmosphere of 0.9 MPa at 1900℃ for 6 h. The SRBSN specimens exhibited hardly any difference in their four-point bending strength, which ranged from 800 to 850 MPa. However, their fracture toughness clearly decreased with increasing additive Al content. This is thought to be due to a reduction in the proportion of coarse prismatic grains improving the toughness with increasing additive Al content. A significant reduction in the thermal conductivity was also observed with increasing additive Al content. This is conjectured to be due to the formation of SiAlON through a substituted solid solution of aluminum (Al) and oxygen (O) in β-Si_3N_4 crystals. An inductively coupled plasma and hot gas extraction method were employed to directly analyze Al and O in β-Si_3N_4 particles. Approximately half of the additive Al was confirmed to be in solid solution in β-Si_3N_4 particles along with the solid solution of O in a chemical formula that nearly corresponds to β-Si_(6-z)Al_zO_zN_(8-z). The chemical analysis results give a Z value range of 0.0006-0.024.