Fabrication of transparent polycrystalline silicon nitride ceramic

As hot pressing silicon nitride, sintering additives such as MgO, Al ₂O₃ and Y₂O₃ are generally used. The use of sintering additives changes the properties of hot pressed silicon nitride such as mechanical, thermal and optical. In the present paper, silicon nitride was hot pressed with various amount of AlN as a sintering additive. Mechanical properties such as density, hardness, Young's modulus, fracture toughness and flexual strength of hot pressed silicon nitride were measured with regards to the content of AlN. Also microstructure was observed using a SEM. The effect of α/β phase fraction on the mechanical and optical properties of silicon nitride was investigated as a function of the amount of AlN additive. Transparent polycrystalline silicon nitride consists of 75 vol.% a-phase Si₃N₄ and 25 vol.% β-phase Si ₃N₄. The elongation of β-Si₃N₄ was suppressed. The resulting polycrystalline silicon nitride showed superior properties such as about 64% transmittance at wavelength of 2.5 μm with 300 μm thickness.