Abstract: Porous silicon nitride ceramics (Si₃N₄) were fabricated by pressureless sintering using different particle size of silicon nitride powder. Lu₂O₃ was used as sintering additive. According to phase relationships in the ternary system Si₃N₄-SiO₂-Lu₂O₃, porous Si₃N₄ ceramics with different phase composition were designed through the change of the content of SiO₂ which was formed by the oxidation at 800°C in air. Porous Si₃N₄ with different phase compositions was obtained after sintering at 1800°C in N₂ atmosphere. A small content of SiO₂ favored the formation of secondary phase Lu₄Si₂O₇N₂, while large content of SiO₂ favored the formation of secondary phase Lu₂Si₂O₇ and Si₂N₂O. Porous Si3N4 ceramics with secondary phase Lu₄Si₂O₇N₂ had a flexural strength of 207 MPa, while that with secondary phase Si₂N₂O and Lu₂Si₂O₇ had lower flexural strength.

Key words: Porous silicon nitride ceramics, Secondary phase, SiO₂