Influence of a crack on the electrical impedance of polycrystalline ceramics

The influence of a crack on the electrical impedance of single- and two-phase microstructures was investigated on zirconia ceramics experimentally by impedance spectroscopy and theoretically by extending the brick layer model. Accordingly a crack in a polycrystalline ceramic in general doubles the number of electrical time constants in the impedance spectra compared to the initial state. For single-phase materials this was experimentally proved on yttria stabilized specimens. However, in the case of realistic two-phase microstructures the spectra of the flawed materials are mostly characterized by two time constants as in the initial state. This also follows from the model and was found for ceria stabilized zirconia. Changes due to a crack always occur in the low-frequency range of the spectra. By means of the extended model and numerical field computations the qualitative and quantitative effects of a crack can clearly be reproduced. Consequently, the influence of a crack of any size on the electrical impedance can be predicted.