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Why can anisotropic grains cause microcracks in polycrystalline ceramics?

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advanced structural ceramics Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.

In polycrystalline ceramics, grains are considered anisotropic when their physical or mechanical properties vary depending on the crystallographic direction. This anisotropy can lead to the formation of microcracks due to the mismatch of properties between adjacent grains.12

Specifically, when a material is not isometric, it may exhibit different thermal expansion coefficients along different crystallographic axes. As the ceramic cools during fabrication, these varying expansion rates cause the grains to contract at different magnitudes in different directions. This differential contraction generates internal stresses at the grain boundaries. When these localized stresses exceed the material's strength, tiny microcracks form.32

While these microcracks may seem detrimental, they are sometimes utilized as a toughening mechanism in advanced structural ceramics. By opening up around the path of a main crack, these microcracks can absorb energy and exert a closing force on the main crack tip, thereby increasing the material's overall fracture toughness.2