Fabrication of Porous Si₃N₄-Lu₂Si₂O₇ Composite Ceramics

To improve the corrosion resistance of porous Si₃N₄ used in the high temperature environments, which contain water vapor and volatile species, porous Si₃N₄-Lu₂Si₂O₇ composite ceramics were fabricated by a process of oxidation bonding and pressureless sintering in flowing N₂ atmosphere at the temperatures lower than 1550°C. The pores in ceramics were formed by removing the pore-forming agent (phenolic resin). SiO₂ derived by the oxidation of Si₃N₄ at 1250°C in air reacted with Lu₂O₃ at various sintering temperatures, leading to the formation of Lu₂Si₂O₇ with excellent oxidation and corrosion resistance. The phase composition, microstructure, and mechanical properties of the porous ceramics depended on the sintering temperatures. Si₃N₄-Lu₂Si₂O₇ composite ceramics with the porosity of 52% and the pore size of 0.2 μm were obtained by oxidation bonding at 1250°C for 1 h and pressureless sintering at 1550°C for 2 h using 4 wt pct Lu₂O₃ additive.