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Why can tiny tetragonal zirconia particles remain stable at room temperature?

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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 the field of advanced structural ceramics, tiny tetragonal zirconia particles remain stable at room temperature primarily due to their extremely small size—typically less than 1 micrometre—and their intimate contact with a surrounding matrix, such as cubic zirconia or alumina.1

While pure zirconia naturally undergoes a 3 percent volume expansion when transforming from a tetragonal to a monoclinic structure upon cooling below 1,150°C, this constraint from the surrounding matrix prevents the transformation from occurring spontaneously at room temperature. This stability is essential for transformation toughening, a mechanism where the stress field ahead of an advancing crack triggers the tetragonal particles to transform into the monoclinic form. The resulting volume expansion exerts a closing force on the crack, significantly increasing the material's fracture toughness.1