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The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal AI,O, and Y,O, sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both AI,O, and Y,O, sols could simultaneously be adsorbed on the S i c particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-Sic) with 2 wt % A1,0, and 1 wt% Y,O, were prepared by freeze granulation/ pressing and sintering at 1880°C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y,O, and A1,0, added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.

Pressureless sintering of silicon nitride requires addition of sintering agents. The main part of this study was done in order to homogenize the distribution of sintering agents, in this case Y z 0 3 , in a silicon nitride matrix. Colloidal 10-nm Y203 particles were electrostatically adsorbed on SiJN4 particle surfaces. The adsorption was studied by X-ray fluorescence analysis and electrophoretic measurements. Addition of Y2O3 sol to a Si3N4 suspension decreased the viscosity of the suspension. The slip casting properties of Si3N4 suspensions with added Y203 sol were examined, and the homogeneity of Y 2 0 3 in the green compacts was compared with conventionally prepared samples. An improved microstructural homogeneity was obtained when Yz03 sol particles were adsorbed on the Si3N4 particle surfaces. [

Organic silanes and titanates were added to a system of polyethylene and silicon nitride intended for injection molding and to a model system of paraffin oil and silicon nitride. The effect of the additions on viscosity was studied. Results from the model system agree comparatively well with the results from the system intended for injection molding. The model system can be used for screening tests. A glycidoxytrimethoxysilane reduces the viscosity of the polyethylene-silicon nitride system. In combination with tetrabutyl titanate, the effect is more pronounced. [