Deformation and fracture of zirconia ceramics stabilized by CeO2. II. Crack resistance

Abstract

Investigations of crack resistance of four types ofzirconia-base ceramics stabilized by 9% CeO2 are described. In three types of ceramics cracks develop against the background of the formation of a wide zone of transformation transitions, whose size decreases with decrease in the grain size of the materials. In the ceramics with the finest grain composition (1.14 lain) such a zone is absent. The measured results are highly divergent as a function of the method used for estimating the length of the cracks, namely, visual examination and determining the compliance of the specimens. It is assumed that the divergence can be attributable to the presence of a zone of transformation transitions at the tip of a crack and along it.In [1] we presented the results of testing ceria-stabilized zirconia ceramics for strength and deformability. The present work is devoted to the investigation of the crack resistance of such ceramics.We tested specimens 4.8 x 2.8 x 25 mm in size with an initial notch of depth 2.4 mm and width 0.25 mm for crack resistance. The specimens were subjected to three-point bending with the distance between the supports being 20 ram. In order to make observations of the initiation and development of cracks and the formation of a zone of transformation transitions easier, one of the side faces of the specimens was polished.We used the traditional technique of constructing Rcurves, although we were aware of its deficiency due to inelastic deformation of the ceramics. The technique was chosen because we were not convinced that the concept of J-integral was applicable to transformation-strengthened materials. The only energy parameter used was the specific energy of fracture.Just as in experiments on determining strength and deformability we recorded the dependence between the applied force P and the deflection 8 of a specimen (the P-5 diagram) and the acoustograms of force-total AE count (the P-N diagrams). The specimens fractured in a few loadingunloading cycles. After each cycle the length of the crack was where c o is the compliance of the specimen without a notch; Y is the K-calibration [3]; cz is the relative (with respect to the specimen's height h) depth of the stress concentrator.The P-5 diagrams for Ce-TZP-II ceramics are presented in Fig. 1, which also shows calculated (from the P -5 diagrams) dependences between the crack length lcr and the stress intensity coefficient K~, which is found from the relation of linear fracture mechanicswhere a is the nominal stress attained in the given load cycle. Despite the stepwise shape of the R-curves obtained, especially those for the visual data, we can say that the latter are quite different from the data determined from the compliance. Judging by the visual measurements, the values of K m tend to increase with the growth of a crack, while the results on compliance show a tendency to decrease. We can say that the Ce-TZP-II-Ce-TZP-IV materials virtually do not differ within the scattering range, whereas the Ce-TZP-I ceramics 0034-3102/95/3603-078512.50