Strains, stresses and microcracks in pure zirconia polycrystals
Résumé
In pure dense zirconia-based materials, cooling induces a martensitic phase transition associated with a large variation of the ZrO2 molecular volume and an anisotropic thermal expansion, both leading to very high level of internal stresses and the formation of large multiscale microcrack networks. We investigate in this work the relationships between the main crystal planes or directions and the orientation of the microcracks using specimens that have undergone a single thermal cooling from the liquid state to room temperature. Two distinct crack orientations have been identified. The first one is observed on the {001}m planes of monoclinic crystals, and the second one on the {110}c planes of the parent cubic phase. We propose to explain these observations through the calculation of stresses and strains at the crystal scale induced by phase transitions all along the cooling process. The experimental observations of crack orientations are in good agreement with the strain model based on a phenomenological approach of the martensitic transformation. Accordingly, the strain accommodation mechanisms that occur during this transition appear to be a critical factor in the formation of crack or twin boundary between individual monoclinic crystals.
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