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 beta-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 beta-Si3N4 grains. The Si3N4 ceramics also exhibited low and stable dielectric constants.