Finely dispersed Si3N4-SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si3N4--SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24-80 m(2)/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were alpha-Si3N4, beta-Si3N4, beta-SiC, and X-ray amorphous Si3N4. Dense materials were prepared both by hot pressing at 1800 degrees C under a load of 30 MPa and gas-pressure sintering at 1600-1900 degrees C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were liner grained, and densified more rapidly than materials sintered from commercial powders.