The present invention relates to a ceramic composite particle and a production method thereof, in particular, to a ceramic composite particle having a homogeneous microstructure of fine ceramic particle phases and a binder phase which is formed by a reaction between a ceramic particle and a sintering aid and a method for producing such a ceramic composite particle. The ceramic composite particle has good moldability and is very suitable for producing a densified sintered body having a fine and homogenous crystal structure.
Ceramic sintered bodies of silicon nitride, etc., have been expected to be satisfactory for structural applications under severe conditions due to their high mechanical strength, high heat resistance, high thermal shock resistance and high abrasion resistance. Generally, a densified sintered body in which the fine ceramic particle phase of silicon nitride, etc., is uniformly dispersed may be considered to be obtainable from a starting ceramic powder finely pulverized. However, a ceramic powder having a particle size nano-meter order has an excessively large specific surface area and is bulky to result in a poor moldability.
For example, when a fine ceramic powder having an average particle size of nano meter order is injection molded under a low pressure, a large amount of binder for wetting the surface of individual ceramic particles is required to prepare a compound having an optimum viscosity. In the case of a slip casting method, a large amount of solvent is required to prepare a slurry having a suitable viscosity. For these reasons, a preform (green body) obtained from the fine ceramic powder having an average particle size of nanometer order has only a small density and is difficult to be densified. Therefore, to obtain a densified sintered body, the preform must be sintered while directly compressing the preform as in a hot press method, etc. However, such a direct compressing method limits the choice of the shapes of the sintered body.
Since the ceramic powder of silicon nitride, etc., is a difficult-to sinter material, the ceramic powder is sintered usually in the presence of a sintering aid such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, etc., to densify the sintered body. However, it is difficult to uniformly disperse the components by merely mixing the sintering aid with the ceramic powder. The nonuniform powder mixture will result in segregation of the components in the sintered body, in particular, the segregation of the sintering aid makes the microstructure of the sintered body nonuniform.
To improve the physical properties of the ceramic sintered body, it has been proposed to obtain a composite sintered body by sintering a preform formed from a composite powder prepared by mixing a plurality of different ceramic components and heat-treating the resultant mixture. For example, to improve the properties of the silicon nitride ceramic body, various ceramic components have been used. Of these ceramic components, silicon carbide has been preferably used because of its ability of exhibiting a good oxidation resistance, a high-temperature strength and a high mechanical strength. However, a sintered body obtained from a mere mixture of silicon nitride and silicon carbide hardly has a microstructure in which the components are uniformly dispersed, and includes segregation of the components. Therefore, various methods have been proposed to produce a sintered body having a microstructure in which a silicon carbide particle is finely dispersed in the silicon nitride phase.
Japanese Patent Laid-Open No. 2-160669 discloses a silicon nitride-silicon carbide composite sintered body having a microstructure in which silicon carbide having an average particle size of 1 .mu.m or less is dispersed in a grain boundary of silicon nitride and fine silicon carbide particles having a particle size from several nano- meters to several hundred nanometers are dispersed in silicon nitride particles. The composite sintered body is produced by a liquid phase sintering of an amorphous silicon nitride-silicon carbide composite powder or a silicon nitride-silicon carbide powder mixture, each being capable of forming fine silicon carbide having an average particle size of 0.5 .mu.m or less during the liquid phase sintering, at 1500-2300.degree. C. in the presence of a sintering aid which forms a liquid phase during the liquid phase sintering process. However, the starting powders are bulky and have a poor moldability due to their small particle sizes. Also, it is required for obtaining a densified sintered body to directly compress a preform during the sintering process as in the hot press method, etc. This limits the choice of the shapes of the sintered body.
Japanese Patent Laid-Open No. 3-261611 discloses a method of producing a silicon nitride composite powder for producing a silicon nitride-silicon carbide composite sintered body. The method is characterized by simultaneous carbonizing and nitriding reactions of a metallic Si powder. The simultaneous carbonizing and nitriding reactions are caused by heating a powder mixture of the metallic Si powder and a carbonaceous powder at 1400.degree. C. or below in an inert gas atmosphere containing nitrogen. It is taught that the composite powder produced by the disclosed method has a nano composite structure having a disperse phase of nanometer order. However, since the composite powder is very fine in its particle size, the preform of the composite powder has a low density. In addition, since the composite powder contains no sintering aid, the composite powder is mixed with a sintering aid prior to the sintering process. Therefore, the microstructure of the resultant sintered body is not satisfactorily homogeneous due to the nonuniform dispersion in the starting powder mixture of the sintering aid and the composite powder. Further, since the metallic Si powder is nitrided in the absence of a sintering aid, the silicon nitride formed in the composite powder is likely to be in the form of the .beta.-phase, which deteriorates the sinterability of the composite powder.