Preparation Of SiC Ceramic Toughened With Nano-TiN/SiCw

Based on the present research of SiC ceramic and nanocomposite ceramic, the composition of SiC nanocomposite ceramics toughened with nano-TiN and SiC whiskers was designed. The dispersion behavior of nano-SiC, SiC whiskers and nano-TiN in aqueous medium was studied, and the dispersion condition to keep them stable was found out. Sintering aids YAG was added by sol-gel method and composite slurry was prepared. Then the SiC nanocomposite powders were prepared by spray granulation. After two-step molding and sintering pressureless with liquid phase, we prepared SiC nanocomposite ceramics, analyzed sintering behavior, mechanical properties, phase composition and microstructure, and studied the strengthening and toughening mechanisms of nano-TiN and SiC whiskers. The main tasks and conclusions were as follows:(1) The dispersion behavior of nano-SiC, SiC whiskers and nano-TiN in aqueous mediumWith the increasing of pH value, the surface potential of nano-SiC, SiCw and TiN changed from positive to negative. The good dispersibility and stability of nano-SiC were obtained when pH value was from 3 to 7. During the whole range of pH value, the dispersion stability of nano-SiC whiskers was poor. TMAH can improve the dispersion properties of TiN, but didn't work on nano-SiC and SiCw. PEG 2000 had a certain effect on nano-SiCw, but didn't work on nano-SiC. (NaPC₃)₆ can make nano-SiC, SiCw and TiN achieve good dispersion. The best result was obtained when the content of (NaPO₃)6 was 0.4mg/ml. Under this condition, absolute value of Zeta potential of nano-SiCw was more then 55mv and the relative sedimentation height of suspension remained over 92% after 24h for three types of powder.(2) Preparation of SiC composite slurry by sol-gel processing and characteristics of SiC powdersSintering aid YAG was prepared by sol-gel processing and then composite slurry was prepared. SiC composite slurry displayed typical shear thinning. With the increase of solid content, viscosity and shear stress increased, but the dispersion performance improved. The 20wt% solid content slurry was more suitable for spray drying with low viscosity/shear stress and good dispersion. Through Spay drying, the apparent density of granulation powders increased by 65%-120% and the angle of repose decreased to 27-32⁰, which showed significant improvement in flowability. With two-step molding, the relative density of green bodies was between 55% and 59%, which was an ideal density.(3) Singtering, structure and properties of SiC nanocomposite ceramicsWith the increasing of sintering temperature, the relative density, bending strength, hardness and fracture toughness increased first and then decreased. The optimum sintering and mechanical properties was gained by two stage sintering including 1900℃for 15min and subsequent 1750℃for 45min. The effects of reinforcements on ceramics were different; the relative densities were more than 94% when sintered at 1750℃expert the sample with 10% TiN. The highest bending strength was gained when adding 5%℃TiN; the highest hardness and fracture toughness was gained when adding 5% TiN and 5% SiCw. In conclusion, SiC nanocomposite ceramics adding 5% TiN and 5% SiCw had best performance, with 94.3% relative density, while the bending strength, Vickers hardness and fracture toughness were 484.2MPa,18.95GPa and 7.84MPa-m^1/2 respectively.During sintering process, TiN can react with matrix to produce TiC and A1N. Grain sizes of ceramics were between 200-400nm under sintering at 1750℃. Nanoparticles can restrict the growth of grains, and produced grain boundary strengthening, crack deflection and bridging.