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°C in N2. At temperatures below the melting point of the oxides, α-Si3N4 whiskers of high aspect ratio were coated with weak adhesion to the substrate. At temperatures above the melting point, β-Si3N4 whiskers with diameters of 0.02 to 0.07μm (low aspect ratio) were coated with strong adhesion to the substrate. When the Si3N4 substrates coated with the β-Si3N4 whiskers were used as membrane filters, it could separate particles as fine as 0.05μm in diameter.
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Kawai et al. (1999) studied this question.