Mechanics Of Electric Field Induced Failure In Ferroelectric Ceramics

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...