Beschreibung
Despite the polycrystalline nature, piezoelectricity can be observed in ferroelectric ceramics after the electrical poling, which aligns domains along the direction of external electric field. Due to piezoelectricity, ferroelectric ceramics have been used for a variety of devices such as actuators, sensors, and transducers. In order to ensure long term reliability, it is required to understand crack growth-related fracture behavior under mechanical load. To this end, in the present work, the influence of crystal structure as well as second phase inclusion on fracture behavior of ferroelectric ceramics has been investigated. Ferroelasticity plays an important role during fracture of ferroelectric ceramics, such as PZT. Prior to examining the fracture behavior, the macroscopic stress-strain behavior as a function of PZ/PT ratio and temperature were investigated to provide an insight into ferroelasticity of PZT in the thermal operating range of many actuators. In conjunction with an X-ray diffraction (XRD) analysis, temperature- and composition-dependent ferroelastic responses are discussed in terms of the change in lattice structure. It was found that the ferroelastic characteristics, such as the coercive stress and remanent strain, were closely correlated to the lattice distortion. With decreasing lattice distortion, found either by modifying the PZ/PT ratio or the temperature, a decrease of coercive stress was observed. At elevated temperature, a decrease in remanent strain due to the reduced lattice distortion was found. This was, however, not the case for compositional changes in lattice distortion. The abnormal increase in was observed in rhombohedral compositions, which is likely due to either a change of lattice parameters or stress induced R?T phase transition. Soft PZT ceramics were found to display the R-curve behavior regardless of changes in composition or temperature. MPB compositions of PZT showed a larger ferroelastic toughening, which is attributed to the enhanced domain switchability corresponding to a lower coercive stress and possibly stress induced R?T phase transformation. In comparison to tetragonal PZTs, rhombohedral compositions were found to display a larger ferroelastic toughening. For pure tetragonal PZTs there was no difference in R-curve behavior even though the lattice distortion changed significantly with PbTiO3 content. Taking into account the lattice distortion of each phase composition, a predominant factor influencing ferroelastic toughening is coercive stress, which is related to the number of switchable domains and the size of process zone. Soft PZT ceramics exhibited a decrease in maximum toughness as well as shielding toughness with increasing temperature. Due to the decrease of lattice distortion, a decrease in the coercive stress and remanent strain at elevated temperature was observed. The analysis of the backswitching strain demonstrated that domain backswitching is stress- and temperature-dependent, showing an increase with increasing temperature. Domain backswitching reduces the effective remanent strain in the crack wake, which is responsible for the reduced ferroelastic toughening at higher temperatures. Using large PMN-PT single crystals, produced by means of the SSCG technique, fracture resistance-crack extension behavior was characterized. Both single crystal and polycrystalline PMN-PT were found to show the R-curve behavior, which is primarily due to ferroelastic toughening. In terms of overall toughening effect, a single crystal orientated along a certain polar axis was comparable to the polycrystal materials. R-curve behavior of single crystals was found to be anisotropic, depending on their crystallographic orientation. PZTTZ composites showed the reduction of grain size of PZT matrix, signifying that that ferroelectricferroelastic response should decrease. Despite this, it was found that the insertion of second phase TZ particles enhanced the fracture toughn
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