Coating of fine-grained Si3N4 whiskers on a porous Si3N4 substrate by gas phase processing for membrane filter application

Silicon nitride (Si3N4) whiskers were coated on a porous Si3N4 substrate, which was coated with SiO2-Al2O3-Y2O3 oxides, by gas phase processing using amorphous Si3N4 and TiO2 powders as source materials. Coating was carried out in the interval 1350-1550 degrees C in N-2. At temperatures below the melting point of the oxides, alpha-Si3N4 whiskers of high aspect ratio were coated with weak adhesion to the substrate. At temperatures above the melting point, beta-Si3N4 whiskers with diameters of 0.02 to 0.07 mu m (low aspect ratio) were coated with strong adhesion to the substrate. When the Si3N4 substrates coated with the beta-Si3N4 whiskers were used as membrane filters, it could separate particles as fine as 0.05 mu m in diameter.