How does the volume expansion of zirconia particles help close a crack?

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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 context of advanced structural ceramics, the process known as transformation toughening utilizes the phase change of zirconia to inhibit crack growth.

When fine tetragonal zirconia particles are dispersed within a ceramic matrix, they remain stable at room temperature due to the surrounding material's constraint. However, as a crack advances, the associated stress field triggers these particles to transform into their monoclinic form. This phase change is accompanied by a 3 percent volume expansion.

Behind the crack front, this cumulative increase in volume creates a process zone where the expanding particles exert a compressive force on the crack. This force effectively acts to close the crack and provides significant resistance to further propagation, a mechanism that gives transformation-toughened zirconia (TTZ) its enhanced fracture toughness.1