低温下非氧化物陶瓷的断裂机理、裂纹扩展与性能研究

发布时间：2019-06-13 11:54

【摘要】：随着人类科学技术的进步,低温技术在航空航天、热核聚变以及超导领域得到了广泛的应用。结构陶瓷具有硬度高、耐磨损、电绝缘等特点,可以与金属和高分子材料形成互补,在某些低温应用领域发挥独特的优势。目前关于非氧化物陶瓷低温断裂机理和性能的研究开展的还较少,因此本文从影响结构陶瓷力学性能的因素入手,研究了几种具有典型结构的非氧化物陶瓷低温环境下断裂模式、裂纹扩展、桥接、气孔与材料性能的关系,揭示了材料低温性能变化的规律和原因。研究发现氮化铝陶瓷低温下断裂韧性、抗弯强度和韦伯模数三个力学性能指标均有增加的趋势。出现这一变化的直接原因是低温环境下材料穿晶断裂比例增加,导致材料断裂表面能增加。同时,通过分析氮化硅陶瓷与氮化铝陶瓷低温断裂机理的区别,发现材料中晶界相的性质、体积分数和分布位置影响了陶瓷低温断裂模式的变化规律。当温度从293K降低到77K时,氮化硅陶瓷的稳态断裂韧性从5.19MPa·m1/2增加到6.51MPa·m1/2,增幅为25.4%。利用原位降温拉曼光谱测得某一桥接结构293K下桥接应力的最大值为0.7GPa,而在77K下桥接应力的最大值增加到1.0GPa,这说明桥接应力增加是低温下氮化硅陶瓷韧性改善的主要原因之一。通过对氮化硅陶瓷不同温度(293K、159K和77K)下产生的60条裂纹中4323个裂纹偏转和320个桥接结构进行统计分析,发现虽然低温下残余应力能够影响材料中沿晶断裂的比例,但沿晶断裂时裂纹偏转角度分布以及不同角度裂纹偏转形成桥接的概率由材料显微结构决定,不受温度变化影响。以Si3N4-Y2O3-Al2O3体系为研究对象,发现改变烧结助剂Y2O3和Al2O3质量分数或配比时,氮化硅陶瓷中晶界相体积、热膨胀系数和材料显微结构都会发生变化,进而对材料低温性能产生影响,其中显微结构的影响是主导因素。研究发现低温下碳化硅晶粒弹性模量增加,气孔诱发裂纹扩展阻力增大导致材料的断裂强度增加;低温下残余应力场的存在会降低无压烧结碳化硅陶瓷中气孔诱发裂纹有效尺寸,并导致韦伯模数增加。
[Abstract]:With the progress of human science and technology, low temperature technology has been widely used in aerospace, thermonuclear fusion and superconductivity. Structural ceramics have the characteristics of high hardness, wear resistance, electrical insulation and so on. They can complement each other with metal and polymer materials, and play a unique role in some low temperature applications. At present, there are few studies on the fracture mechanism and properties of non-oxide ceramics at low temperature. Therefore, starting from the factors affecting the mechanical properties of structural ceramics, the relationship between fracture modes, crack propagation, bridge, porosity and material properties of several kinds of non-oxide ceramics with typical structure at low temperature is studied in this paper, and the law and reason of the change of material properties at low temperature are revealed. It is found that the fracture toughness, bending strength and Weber modulus of aluminum nitride ceramics increase at low temperature. The direct reason for this change is the increase of transgranular fracture ratio at low temperature, which leads to the increase of fracture surface energy. At the same time, by analyzing the difference of low temperature fracture mechanism between silicon nitride ceramics and aluminum nitride ceramics, it is found that the properties of grain boundary phase, volume fraction and distribution position affect the variation of low temperature fracture mode of ceramics. When the temperature decreased from 293k to 77K, the steady-state fracture toughness of silicon nitride ceramics increased from 5.19MPa 路M1 / 2 to 6.51MPa 路M1 / 2, an increase of 25.4%. The maximum bridge stress of a bridge structure at 293k is 0.7 GPA, while the maximum bridge stress increases to 1.0 GPA at 77K, which indicates that the increase of bridge stress is one of the main reasons for the improvement of toughness of silicon nitride ceramics at low temperature. Through the statistical analysis of 4323 crack deflection and 320 bridge structures in 60 cracks produced at different temperatures (293K, 159K and 77K) of silicon nitride ceramics, it is found that although the residual stress can affect the proportion of intergranular fracture in the material at low temperature, the distribution of crack deflection angle and the probability of bridge formed by crack deflection at different angles are determined by the microstructure of the material and not affected by the temperature change. Taking Si3N4-Y2O3-Al2O3 system as the research object, it is found that when the mass fraction or ratio of sintering auxiliaries Y2O3 and Al2O3 is changed, the grain boundary phase volume, thermal expansion coefficient and microstructure of silicon nitride ceramics will change, and then the low temperature properties of the materials will be affected, and the influence of microstructure is the dominant factor. It is found that the increase of elastic modulus of silicon carbide grain and the increase of pore induced crack propagation resistance lead to the increase of fracture strength at low temperature, and the existence of residual stress field at low temperature will reduce the effective size of pore induced crack in pressuless sintered silicon carbide ceramics and lead to the increase of Weber modulus.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ174.1

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