【作者】 朱廷；

【导师】 杨卫；

【作者基本信息】 清华大学 ， 固体力学， 1999， 博士

【摘要】 铁电陶瓷作为新一代功能陶瓷的代表，其可靠性问题日益受到人们的关注。 铁电陶瓷在电场或力电耦合载荷作用下的典型失效模式为电致断裂和电致疲 劳。本文尝试采用断裂力学、畴变细观力学和介电物理学相结合的方法，从实验 和理论角度对铁电陶瓷的电致失效力学问题进行了研究。论文从电场驱动的裂尖 微结构演化导致电致失效这一基本思路出发，获得了裂尖集中电场诱发90°铁电 畴变的实验图像，提出了电致畴变断裂的理论模型，阐明了铁电陶瓷电致断裂， 疲劳裂纹扩展和裂纹偏折的失效机理。在铁电陶瓷电致失效研究这一新领域所完 成的主要工作有： 一、首次报道了裂尖集中电场诱发的90°铁电畴变的实验现象。利用不同取向铁 电畴的腐蚀速率不同的原理，获得了裂尖前方晶粒内电场诱发的90°a-c畴 结构的SEM照片。这一观测结果为电场驱动裂尖微结构演化的模型提供了 清晰的物理过程图像，并为进一步的理论分析提供了直接的实验依据。 二、提出了基于细观物理机制的小范围畴变模型。区别于传统的介电失效，揭示 了由裂尖集中电场引起畴变，畴变导致非协调应力，由应力驱动断裂的电致 失效机理。在此基础上，进行了电致断裂、断裂韧性的各向异性和断裂韧性 关于电场的非对称变化的理论分析。解决了基于线性压电本构的电致断裂分 析与实验观测不一致的矛盾。 三、观测了循环电场低于矫顽电场加载时，电致疲劳裂纹扩展的规律：首次阐明 了电致疲劳裂纹扩展的机理。不同于Cao和Evans的实验报道（矫顽电场以 下，裂纹不发生扩展），通过长焦距显微镜实时观测了循环电场的幅值低于 矫顽电场时，铁电陶瓷中疲劳裂纹扩展的规律。在实验基础上，理论分析了 铁电畴在交变电场作用下循环翻转而产生循环应力，以及由循环应力驱动裂 纹以起裂、扩展、止裂、再起裂的模式循环向前扩展的过程。定量给出了电 致疲劳裂纹扩展量与电场翻转次数的理论预测值，并与实验观测结果吻合较 好。 四、求解了铁电陶瓷中偏折裂纹的问题，讨论了电场对裂纹偏折的影响。基于线 性压电本构，将偏折裂纹等效为连续分布的位错和电偶极子，采用推广的 Stroh方法，建立了以位错密度和电偶极子密度为未知量的奇异积分方程 组。数值计算结果表明，1．在垂直拉应力和垂直正电场作用下，裂纹不发 生偏折，沿直线扩展；2．在混合型机械载荷作用下，垂直正电场将增加裂 纹偏折角度，3．沿水平侧向施加的正电场和水平拉应力促进裂纹偏折，而 沿水平侧向施加的负电场和水平压应力抑制裂纹偏折。更多还原

【Abstract】 Reliability concerns of ferroelectric ceramics, as one of the most important functional ceramics, call for a better understanding of their failure mechanisms. Under an electric field or a combined electric and mechanical loading, the ferroelectric ceramics are susceptible to electric fracture and electric fatigue. By the methods of fracture mechanics, mesoscopic mechanics and dielectric physics, the problem of electric field induced failure in ferroelectric ceramics is experimentally and theoretically studied in this dissertation. We approach the problem based on the perception that electric field induced microstructure evolution at the crack tip will lead to fracture and fatigue. It follows that a micrograph of intensified electric field induced 90° domain switching at the crack tip is obtained, a theoretical model of domain switching induced fracture is proposed, and the failure mechanisms of electric fracture, electric fatigue and crack kinking are revealed. The major works include: 1. Experimental observation on the electric field induced 9O° domain switching at the crack tip is first reported. The difference in etch rates of domains with distinct orientations gives rise to topographical contrast which can be used to identify the domain configuration. A SEM micrograph of 90° a-c domain configuration in the grain ahead of a crack tip is obtained. The result reveals a clear physical picture of electric field induced microstructure evolution, and lends a direct experimental support to the theoretical analysis. 2. A mechanism based small scale domain switching model is proposed in the dissertation. In contrast to the conventional perception of dielectric failure, the failure mechanism is revealed as follows: the intensified electric field in the vicinity of a crack tip drives domains to switch, the switched domains induce incompatible stress under the constraint of unswitched materials, and consequently the induced stress leads to fracture. The model is capable of explaining the experimental phenomena, such as electric fracture, fracture toughness anisotropy, asymmetric variation of fracture toughness of poled ferroelectrics under positive and negative electric fields. The inconsistency between the experimental observation and the electric fracture analysis based on the liner piezoelectric constitutive equations is resolved. 3. Electric field induced fatigue crack growth in ferroelectric ceramics is observed and the mechanism of fatigue crack growth is revealed. In contrast to the experimental report by Cao and Evans, we found that the crack would extend under a cyclic electric field below the coercive field by the in-situ observation through a long focal length optical microscope. The mechanism of fatigue crack growth is understood as follows: under an alternating electric field, the field concentrated around the crack causes the ferroelectrics to undergo cyclic local domain switching, which generates a cyclic driving stress field near the crack tip. Driven by the switching induced stress field, the crack extends in a repeated process of initiation, growth, arrest and re-initiation. The prediction on the crack growth versus electric field reversals agrees well with experimental measurements. 4. Crack kinking in ferroelectric ceramics is explored within the framework of the linear piezoelectric constitutive relation, with an emphasis on the effect of electric field. The kink of a crack is mode更多还原