Study On Reaction-bonded Silicon Carbide Composites

Reaction-bonded silicon carbide (RBSC) has both advantages of silicon carbide ceramics, such as thermal resistance, wear resistance, oxidation resistance, high hardness and high thermal conductivity, and advantages of preparing techniques, such as simplified process, short sintering systems and net-shape sintering. All the advantages made RBSC one of the widely used structural ceramics. But the free silicon contained in RBSC restrains the working temperature of RBSC and reduces its mechanical properties. Consequently, reaction-bonded B₄C/SiC composites by gel-casting were prepared in this subject to increase the properties of RBSC and widen its application area.Reaction-bonded silicon carbide composites by gel-casting were prepared in this subject. The effect of gel-casting parameters on properties of C/SiC slurry and green bodies, the effect of particle grading of silicon carbide with different particle size on properties of RBSC green bodies and sintered bodies, and the effect of B4C, as the reinforcement phase, on properties of B₄C/SiC slurry, green bodies and sintered bodies were investigated in this subject.The traditional modeling method which is now commonly used in industry is slip casting, of which the main shortages are the asymmetry in density and cracks after drying. In this experiment, the gel-casting method was used to form the green bodies of RBSC and reaction-bonded B₄C/SiC composite. It is a net-shape modeling technology by the polymerization of organic monomer in the slurry of C/SiC or C/B₄C/SiC. The formed green bodies by gel-casting have many outstanding properties, such as uniformity in density, high density, high flexural strength and machinability. Based on the research of gel-casting parameters, C/SiC slurry with 60vol% solid phase content, which met the requirements for modeling in gel-casting, was obtained. The green bodies by gel-casting avoided all the problems that exist in other modeling methods.To assure the reaction of silicon and carbon during the filtering of silicon, the proper density of green bodies, which has enough porosity to complete the filtration of silicon and guarantee sufficient mechanical properties of green bodies and sintered bodies, is required. Consequently, the particle grading of silicon carbide powder was investigated and the green bodies with desirable densities were obtained, and the effects of particle grading of raw materials on densities of green body and sintered body were also investigated. Based on the research on particle gradings of raw materials, it is confirmed that when the mass ratio of silicon carbide with 3μm particle size and that with 20μm is 3:1, the formed green bodies have uniform structure and good machinability. The density of green body guarantees the complete reaction between silicon and carbide during the reaction bonding process, which assures the high density of sintered body and the excellent mechanical properties for industrial applications.Due to the brittleness of reaction-bonded silicon carbide, B4C was added as an additive to increase the toughness of SiC-matrix materials. Reaction-bonded B₄C/SiC was fabricated by gel-casting using silicon carbide, boron carbide and carbon black as raw materials in this subject. The effects of content and particle size of B4C on the viscosity of C/B₄C/SiC slurry and the effect of TMAH content on the surface of green bodies were investigated. The optimal gel-casting parameters for B₄C/SiC were confirmed. Based on the toughing effect of B4C, the mechanical properties of reaction-bonded silicon carbide composites, such as hardness and fracture toughness, are improved and the application areas are widened.Through series of experiments, C/SiC and C/B₄C/SiC slurries, with low viscosity and satisfying requirements of gel-casting, were successfully prepared; the hardness and fracture toughness of reaction-bonded silicon carbide composites were increased to a certain extent, which widened the application area of reaction-bonded silicon carbide composites.