Fabricationand Properties Of SiC-AlN Multiphase Ceramics Prepared By Gelcasting

Silicon carbide (SiC) ceramic is an advanced structural material with excellent properties such as high bending strength at high temperature, low density, high thermal conductivity, resistance to extreme temperature and abrasion, low thermal expansion, and it is widely used in many industries. However, the covalent bonding between Si and C and low diffusion coefficient makes the sintering of SiC difficult without the application of sintering additives. Aluminium nitride (AlN) is similar with SiC in crystal lattice, density and so on.They can react and form an extensive solid solution at temperature between 1800℃and 2100℃to improve the sintering behaviour of SiC and obtain SiC-AlN multiphase ceramics. The driving forces for densification are volume and surface diffusion which are enhanced by the addition of AlN and Y₂O₃. Fracture toughness and bending strength increase greatly through the solid solution and the formation of Y-Si-Al-O-N liquid phase.An aqueous gelcasting process for the preparation of dense SiC-AlN composites has been described in this work. The content of AlN and Y₂O₃ is 7%～20wt% and 3wt%. The paper mainly contains following several experimental procedures: (1) Reports on SiC-AlN slurrys rheological properties were investigated. (2) The influence of monomer content and solid content on green body has been described; the procedure of binder burnout was confirmed by TG/DTA analysis. (3) The relationship of microstructure, properties and AlN content, sintering temperature and solid content was discussed. The results are followed: because AlN content is minority, the influence of AlN that react with water on gelcasting minimize. The hydrolysate of AlN, which can seem to be sintered additives, was even beneficial to densify SiC-AlN ceramics. The best fabricating condition of SiC-AlN suspensions with solid loading of 40vol% and low viscosity was content of dispersant being 0.2wt%, PH being 13.01～13.10, content of monomer being 5wt% and balling time being 10h. When AlN content increases too much, more and more AlN is hydrolytic, the slurry viscosity raises rapidly and is unstability.As increasing monomer content and solid content, the strength of green body improved correspondingly that can be machining using standard machining equipment. However, it also leads to pack unhomogeneously and large cracks on the surface of green body. When the monomer content and solid content is 5wt% and 40vol%, we prepare the green body of bending strength being 14.2MPa, contractiveness being 3.4%, relative density 42.2%. The surface of green body is sprayed a proper amount of polyethylene glycol(PEG) to prevent the inhibiton of oxygen which occurred in gel casting during the formation of polymer networks. Through the TG/DTA analysis of green body, We establish the procedure of binder burnout: Binder burnout was operated in muffle furnace in air at 600℃for 2h, at a heating rate of 1.5℃/min, then cool at room temperature in air. When enhancing AlN content, the amount of SiC-AlN solid solution increases, the grain size is fine, fracture mode transites from transcrystalline fracture to intergranular fracture, cracks expan producing bypass and deflection,which improve the strength and fracture toughness of multiphase ceramics. When the introducing content of AlN reaches to 12wt%, the bulk density, flexural strength, fracture toughness and elastic modulus sintered at 2000℃are 2.94g/cm³,329.5MPa,5.49MPa·m^1/2,299.6GPa, respectively. However, if AlN content is too much, sintered sample shows coarse-grained microstructure that make the behavior of SiC-AlN composite deteriorate. The higher sintering temperature, the better the mechanical properties of SiC-AlN multiphase ceramics. When the introducing content of AlN reaches to 10wt%, the bulk density, flexural strength, fracture toughness and elastic modulus sintered at 2050℃are 2.95g/cm³,365.8MPa,5.31 MPa·m^1/2, 308.9GPa. Enhancing solid content, the pressureless sintering SiC-AIN multiphase ceramic possesses superior sintering properties and mechanical properties and then deteriorates.As sintered temperature and AlN content are both constant, the amount of solid solution varies little in different solid content SiC-AlN multiphase ceramics. In a proper range of AlN content and sintered temperature, solid solution increase with elevating AlN content and sintered temperature. After SiC-AlN ceramics experience 1000℃thermal shock, the residual bending strength fall dramatically and the microstructure of fracture surface change remarkably. Oxidation film emerges on the ceramics surface and is more thick when enhancing AlN content of SiC-AlN ceramics.