R-curve behaviour of PZT ceramics near the morphotropic phase boundary
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The mechanical failure of PZT ceramics was characterized by measuring R-curves for compositions near and at the morphotropic phase boundary (MPB) where tetragonal and rhombohedral phases coexist in equal quantities. The R-curve behaviours (an increasing fracture toughness with crack extension) were identified by indentation-fracture testing and they were analysed to determine the key parameters. The fracture toughness of the PZT ceramics consisted of three different terms, representing particular microstructural processes in front of advancing cracks, that is, intrinsic cleavage, 90° domain switching and microcracking. Their relative contributions to an overall crack-extension resistance varied with the length of the advancing crack and, more importantly, with the compositions of the PZT. In the compositional range where the tetragonal phase was dominant, the R-curves were determined by domain switching and microcracking. However, the compositional dependency of the fracture toughness was due to the microcracking mechanism. On the other hand, in regions rich in rhombohedral phases, the R-curves were essentially determined by domain switching in the crack-tip area. The R-curves characterized by the domain-switching mechanism were insensitive to the compositional variation near the MPB. Our results also demonstrated that R-curve analysis could be used to probe further into the microstructural responses of materials in front of advancing cracks and to quantify them particularly in systems like PZT where several different toughening processes compete with each other.
KeywordsFracture Toughness Tetragonal Phase Crack Extension Morphotropic Phase Boundary Compositional Range
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