【作者】 袁彪；

【导师】 贾德昌；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2011， 硕士

【摘要】 碳化硅陶瓷因具有高温强度高、比重轻、高热导率、抗氧化、耐化学腐蚀、耐磨损性好、热膨胀系数小等优良特性,在国民经济领域获得广泛的应用。但碳化硅的共价键合很强,晶界能和表面能之比过高,扩散系数低,很难制备出致密的烧结体。由于AlN和SiC材料在原子尺寸、晶体结构上非常相似,能在1800～2100℃的温度范围内形成固溶体,有效地改善材料的力学性能和微观组织结构。本文以SiC、AlN和Y₂O₃为原料,成分以SiC为主体,AlN含量在7wt%～20wt%,添加剂Y₂O₃含量为3wt%,采用凝胶注模成型的方法制备SiC-AlN复相陶瓷。系统地研究了浆料制备工艺条件对流变学性能的影响规律;单体含量、固相体积分数对坯体性能的影响规律及坯体排胶工艺;进而研究了成分配比、烧结温度、固相含量对SiC-AlN复相陶瓷的显微组织结构和性能的影响规律。结果表明:由于AlN成分较少,AlN的水解对凝胶注模成型的影响在可接受范围之内,水解产物在烧结过程中可作为烧结助剂促进致密化。通过对SiC-AlN浆料流变学性能研究得到最佳工艺参数:PH值13.00～13.10,分散剂含量0.2wt%,单体AM含量5wt%,混料时间10h,可制备出流动性较好,固相体积分数40vol%的SiC-AlN浆料。另外,AlN含量过多,水解程度增大,浆料稳定性恶化。单体含量和固相体积分数越高,坯体强度也越高,可对其进行机加工。但同时也造成坯体成分不均匀,排胶时易出现变形开裂。单体含量5wt%,固相体积分数40vol%,所制备的坯体抗弯强度14.2MPa,收缩率3.4%,致密度42.2%,可满足凝胶注模成型坯体的要求。坯体表面喷洒质量分数20wt%的聚乙二醇（PEG）溶液,可大大减轻表面起皮现象。通过对坯体TG/DTA热分析,最终制定坯体排胶制度:1.5℃/min升至600℃,保温2h,最后随炉冷却至室温。随着AlN含量的增加,SiC-AlN固溶体增加,晶粒细化,断裂方式由穿晶断裂过渡至沿晶断裂,裂纹扩展产生绕道与偏转效应,材料的力学性能大为改善。AlN含量为12wt%时,复相陶瓷密度、抗弯强度、断裂韧性和弹性模量分别为2.94g/cm³,329.5MPa,5.49MPa·m^1/2,299.6GPa。AlN进一步增加,复相陶瓷由于失重率较高,坯体成分不均而导致力学性能下降。而烧结温度的提高,也导致SiC-AlN复相陶瓷力学性能也大幅提高。当烧结温度为2050℃时,AlN含量10wt%,其密度、抗弯强度,断裂韧性,弹性模量分别达到2.95g/cm³,365.8MPa,5.31 MPa·m^1/2,308.9GPa。随着固相含量的增加,SiC-AlN复相陶瓷的力学性能相应提高。烧结温度不变,AlN含量一定,不同固相含量的复相陶瓷中的固溶体含量在中基本不变。在一定范围内,AlN含量和烧结温度越高,生成的固溶体越多。经过高达1000℃的热震后,SiC-AlN复相陶瓷残余抗弯强度大幅下降,热震前后断口形貌显著改变,表面有氧化膜的形成,且AlN含量越多,表面氧化膜越致密。更多还原

【Abstract】 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.更多还原