Sintering condition and microstructure of translucent silicon nitride ceramics

Translucent Si₃N₄ ceramics are expected to use for a special optical application in high temperature and corrosive environment owing to its high mechanical strength and thermal and chemical stability. To achieve the translucency of Si₃N₄ ceramics, fine-grained microstructure with an appropriate sintering aid is important to reduce light scattering. The Si₃N₄ ceramics were fabricated by spark-plasma sintering (SPS) with AlN-MgO additive. Fine α-Si₃N₄ powder with high purity was useful to obtain translucent Si₃N₄ sintered body. When the additive content was large, α-SiAlON formed, whereas β3-Si₃N₄ formed at a small additive content, revealing the high translucency of both phases in visible light region. The AlN/MgO ratio should be on the SiAlON forming line (AlN/MgO = 3/1) in Si₃N₄-AlN-MgO phase diagram. The short-time sintering at high temperature (1850 °C, 5 min) by SPS was effective to obtain the dense and fine-grained microstructure, and the transmittance of 22∼28% at the wavelength of 900 nm was achieved under the optimal conditions.