【作者】 李红波；

【导师】 韩杰才；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2011， 博士

【摘要】 六方氮化硼（hexagonal boron nitride,h-BN）是一种优良的高性能结构陶瓷。现代科学技术的发展对陶瓷的低成本、高性能更提出高要求。本文以价格相对低廉的B4C为B源,以Si和Ti为还原剂,在高压氮气环境中采用燃烧合成-热等静压方法,低成本制备出性能优良的h-BN基复合陶瓷。由于h-BN陶瓷的低弹性模量、热膨胀系数和高的热导率,该体系的结构陶瓷具有优良的耐腐蚀性以及抗热震性能。首先对B4C-Si-N2体系进行了热力学和动力学分析,研究了该体系的反应机理,分析了毛坯致密度、反应氮气压力和稀释剂对反应过程和产物性能的影响。研究表明,随反应氮气压力增加,致密化作用增强,产物相对密度提高,力学性能大幅提高,但120MPa氮气压力下有Si₃N₄相生成;随毛坯相对密度的提高,产物致密度先升高后降低,在毛坯致密度52.3%有最大值,其原因是过高的毛坯致密度提高了氮气的渗透阻力,导致反应不完全;稀释剂SiC、BN的加入降低了反应温度、反应速度和烧结性能。同时加入的稀释剂没有反应原位合成生成的BN和SiC结合强度高,因此产物的性能降低。通过优化工艺参数,在毛坯致密度52.3%,100MPa反应氮气压力条件下,制备了抗弯强度83.3MPa,断裂韧性1.96MPa·m^1/2的h-BN-SiC复合陶瓷。然后对B4C-Ti-N2体系进行热力学分析,实验研究了Ti含量对反应和产物组织性能的影响。随着原料中B4C/Ti从1/1变化到1/4,产物的抗弯强度和断裂韧性分别从42MPa和0.7MPa·m^1/2提高到67MPa和1.1MPa·m^1/2,产物力学性能提高的原因为产物中Ti（C,N）相氮含量增加。而且由于氮气渗透阻力小,产物表面Ti（C,N）相的N含量高于产物内部Ti（C,N）相的N含量。上述两个反应体系的实验结果表明,B4C-Si-N2体系的产物致密度较高,性能较B4C-Ti-N2体系好,其原因主要由于Si的熔点（1410oC）小于Ti（1668oC）的熔点,液相烧结较为明显。通过引入液相烧结作用明显、综合力学性能优良的Al-TiB₂-N2体系,结合B4C-Si-N2体系,通过燃烧合成制备了h-BN-AlN基复合陶瓷。产物致密度和综合力学性能有较大提高,但反应过程中TiB₂在高温高压氮气下部分分解氮化,造成产物组织结构不均匀。因而用热稳定性较好的TiN取代TiB₂,在100MPa高压氮气下通过燃烧合成制备了组织均匀h-BN-AlN基复合陶瓷。实验研究结果表明,AlN-TiN体积含量对h-BN-AlN陶瓷复合材料力学性能有关键影响:当AlN-TiN体积含量低于50%,AlN相含量少,没有形成连续结构,材料的强度以h-BN相的强度为主导,断裂方式主要为沿晶断裂。而AlN-TiN体积含量高于50%时,AlN相已经形成了连续结构,产物的抗弯强度和断裂韧性显著提高,在AlN-TiN体积含量70%时,达到274MPa和5.1MPa·m^1/2。产物的断裂主要是AlN穿晶断裂为主。通过对AlN-TiN体积含量为30%、50%和70%的h-BN-AlN基复合陶瓷的热震试验表明,h-BN-AlN基复合陶瓷的抗热震性能优异。在h-BN相含量高于50%时,h-BN为基体相,h-BN的高热导率、低热膨胀系数和弹性模量使材料的抗热震性能优异。AlN-TiN含量30vol%和50vol%在1200oC残余强度分别为原始强度的67%和32.5%,热震温差（?T）分别达1000oC和700oC。主要原因是低强度片层状的h-BN在产物中相当于微裂纹,起到缓解热应力作用,加上均匀分布的TiN颗粒弥散强化作用,材料的热震温差（（?）T）在AlN-TiN含量70vol%仍高达800oC。最后对以B₂O₃为B源,Mg和Al为还原剂,在高压N₂中燃烧合成非导电h-BN-MgO和h-BN-Al₂O₃进行了初步研究。研究表明B₂O₃-Mg/Al在高压氮气中剧烈反应生成h-BN-MgO和h-BN-Al₂O₃复合陶瓷,反应基本过程是Mg/Al和B₂O₃发生置换反应生成MgO/Al₂O₃,置换出的B和氮气反应生成BN。但产物中分别有少量硼酸镁（3MgO·B₂O₃）和硼酸铝（9Al₂O₃·2B₂O₃）生成,这是由于反应温度和速率很高,Mg、Al会气化挥发,生成的MgO和Al₂O₃分别和剩余的B₂O₃结合而生成3MgO·B₂O₃和9Al₂O₃·2B₂O₃。由于Mg、Al会的挥发,造成产物致密度不高。更多还原

【Abstract】 Hexagonal boron nitride （h-BN） is a high-performance structural ceramic with excellent properties. The development of technology raises requirements for advanced ceramics with increasing properties and low cost. In this paper, h-BN-based ceramic composites were prepared by combustion synthesis and instantaneously hot isostatic pressing （SHS-HIP） ignited under high nitrogen pressure from relative cheap raw materials of B₄C and Si or Ti, in which B₄C served as B source and Si or Ti acted as reducing agents. Based on the low elastic modulus, coefficient of thermal expansion and high thermal conductivity, h-BN-based ceramic composites have excellent corrosion resistance and thermal shock resistance.Firstly, thermodynamic and kinetic parameters of the B₄C-Si-N₂ system were calculated theoretically, and the combustion synthetic mechanism was analyzed. Process parameters such as compacts porosity, nitrogen pressure and diluent content have significant effect on the microstructure and properties of the products. The research results show that higher relative density and mechanical properties of the products were obtained with the increasing of nitrogen pressure. However, Si3N4 phase formed under higher nitrogen pressure over 120MPa; The relative density of the products has a maximum value at compacts relative density of 52.3%. h-BN-SiC ceramic composites with bending strength and fracture toughness were 83.3MPa and 1.96MPa·m1/2, respectively, were prepared under 100MPa nitrogen pressure from the compacts with relative density of 52.3%.Thermodynamic calculations of the B₄C-Ti-N₂ system were analyzed. The effect of Ti content in reactant on reaction, structure and mechanical properties of the products were studied by experiments. With the B₄C/Ti ratio changed from 1/1 to 1/4, bending strength and fracture toughness of the h-BN-TiCN ceramics increased from 42MPa and 0.7MPa·m1/2 to 67MPa and 1.1 MPa·m1/2, respectively. The reason for that is the increasing Ti（C,N） phase content in products. The N content in Ti（C,N） phase on the product surface is higher than that of interior product because of lower nitrogen penetration resistance during combustion reaction. In comparison with the experimental results of these two systems, B₄C-Si-N₂ system shows more intensive liquid-phase sintering, higher relative density and mechanical properties because of lower melting point of Si （1410oC） than that of Ti （1668oC）. Relative density and mechanical properties of h-BN-AlN-based ceramic composites improved notably through introducing Al-TiB₂ combustion system into B4C-Si system because of good liquid-phase sintering of Al during combustion process. However, the thermal decomposition of TiB₂ phase under high nitrogen pressure and temperature during the combustion synthesis of B4C-Si-Al-TiB₂-N2 system leads to the non-homogeneous structure in resulting products. In order to overcome this drawback, we obtain homogeneous h-BN-AlN-based products by replacement of TiB₂ with TiN because of higher stability of TiN under high nitrogen pressure and temperature in experiments. Volume content of AlN-TiN has a critical effect on the mechanical properties of h-BN-AlN-based composites. When volume content of AlN-TiN was lower than 50%, h-BN was the main phase and determined the mechanical properties. Fracture characteristics show intergranular type. When volume content of AlN-TiN exceeded 50%, AlN was the main phase and dominated the mechanical properties and fracture characteristics exhibit transgranular type. Bending strength and fracture toughness improve to 274MPa and 5.1MPa·m1/2, respectively.Thermal shock resistance test on products with AlN-TiN content of 30, 50 and 70vol% were carried out. The results show that h-BN-AlN-based ceramic has excellent thermal shock resistance. h-BN was the matrix when h-BN phase was higher than 50vol%, and the excellent thermal shock resistance resulted from the low elastic modulus, coefficient of thermal expansion and high thermal conductivity of h-BN phase. The residual strength was 67% and 32.5% of original strength, and thermal shock temperature difference （（?）T） reached 1000oC and 700oC when AlN-TiN content was 30vol% and 50vol%, respectively. Microcracks bring by weak h-BN phase and dispersion strengthening of TiN grains make the thermal shock temperature difference （（?）T） reached 800oC when AlN-TiN content was 70vol%.h-BN-MgO and h-BN-Al₂O₃ composites were prepared by combustion synthesis under high nitrogen pressure from powder compacts of B₂O₃, acting as B source, and Mg and Al, acting as reducing agent, respectively. The reaction mechanisms of these two combustion systems were replacement reaction of thermite. 3MgO·B₂O₃ and 9Al₂O₃·2B₂O₃ phase were observed because of high reaction temperature and velocity, as well volatilization of Mg and Al. So the relative density was not high.更多还原