Gel-Casting Prepared Porous Si₃N₄ Ceramics with Different Contents of Y₂O₃ and Al₂O₃ Additives

Porous Si₃N₄ ceramics were prepared via a controlled gel-casting process followed by gas pressure sintering with 8 wt.% Y₂O₃ and Al₂O₃ additives. The gel-casting parameters including slurry pH value, AM: MBAM ratio, degassing procedure, drying condition and decarbonize process were optimized, and the effects of different Y₂O₃: Al₂O₃ ratios on the microstructure and properties of the Si₃N₄ ceramics were investigated. With an optimized gel-casting process, a high solid loading of 46.5 vol.% was achieved, and as the Y₂O₃ content decreased from 6 to 1 wt.%, the porosity of Si₃N₄ ceramics declined from 36.3 to 27.5% while the β-Si₃N₄ phase content reduced from 38.9 to 15.0%. The bending strength and elastic modulus of the ceramics varied with different Y₂O₃: Al₂O₃ ratios and were influenced by the porosity as well as the tailored interlocking structure of elongated β-Si₃N₄ grains. The Si₃N₄ ceramics also exhibited low and stable dielectric constants.