Study On The Preparation Of Microcrystalline SiC And The Properities Of Reaction Bonded SiC Ceramic

Silicon Carbide(SiC) is significantly applied in high-temperature structural materials and is one of the most promising materials, as it has excellently properties such as high-melting-point, high strength, high hardness and wear-resistant. Reaction Bonded Silicon Carbide (RBSC) could be applied as sealing element, heat exchanger devices and nozzle materials. There are highly impurity content, broad particle size distrbution, lead to various molding and sintering methods of conventional RBSC, its performances are unsatisfactory. In this paper, nanometer SiC power is used as main raw material due to it’s chemical composition was uniformly, narrow particle size distrbution, excellent purty and chemical activity.Nanometer were made by shape-system, in situ coagulation and sol-gel methods,and it’s impurities in nanometer were estiminated by chemical method. The green body of RBSC was made by powder after mechanical milling and ultrasonic dispersion method, and its microstrture was uniformly. The effort of molding method such as pressure forming,pressure casting, gel-casting/pressure forming on microstructure and mechanical properties of RBSC. The effort of lubricant types and residuals silicon on the properties of materials are analysed. RBSC is prepared with low-temperature sintering process. The main viewpoints are showed as followed:(1)The nanometer SiC power with particle size of300nm,51nm and54nm were respectively made with comparing the three methods. The content of oxyten content was3-5%in nanometer SiC. Sol-Gel method was the best method to synthetize nanometer SiC, the impurties were the lowest in nanometer SiC which was made with sol-gel method. The best methods to estiminate the impurties of SiO2and C were found.(2) The best dipersion condition was confrimed by suspending liquid light absorption method, the dispersion principle was showed with molecular structure.(3) The density of RBSC with different forming methods could be controlled by calculated the carbon density(pc) of biscuits. The residual silicon and "silicon line" could be estiminated with suitablely pc, it was significantly applied to control the phase composition of RBSC.(4) Compared to RBSC which was made by micrometer raw materials,the fixture strength, microhardness and fracture toughness of RBSC ceramics which made by super-fine particle size SiC and carbon power with gel-casting/pressure forming method were increased15.89%,7.5%, and3.9%respectively,up to824±19MPa,28.1GPa, and4.26MPa-m1/2respectively. The grain growth was restrained with nanometer raw materials and low-temperature sintering process.(5) Lubricant which adding2vol%PEG could improve the density of green body and strength of sintered body. The quantity amount controlled the size and distribution of free silicon. The linear relationship between material strength and the content of free silicon: σb=-7.51OVSi+956.9. Free silicon was smaller size and more uniformly distributed, the strength was higher.(6) The grain size suffers the sintering temperature and holding time: the average size of SiC grain is400-500nm of sintering materials in1480℃heating1h. They grow up9～10times.Its grain grow1-2μm higher than1480℃.The main fracture RBSC material way was transcrystalline fracture. There was a small amount intercrystalline fracture.