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How does transformation toughening make zirconia less brittle?

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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.

Transformation toughening is a mechanism that significantly reduces the inherent brittleness of zirconia by utilizing a specific phase transformation to counteract crack propagation.1

Pure zirconia naturally undergoes a 3 percent volume expansion when cooling below 1,150° C, transitioning from a tetragonal to a monoclinic crystal structure. In structural applications, engineers stabilize this tetragonal form at room temperature by dispersing fine particles (less than 1 micrometre) within a matrix of cubic zirconia or alumina.1

When a crack begins to advance through the material, the stress field at the crack tip triggers the embedded tetragonal particles to transform into the monoclinic form. This transformation results in a cumulative volume increase in the wake of the crack. This expansion exerts a closing force on the crack, effectively bridging it and providing a resistance to further propagation that increases as the crack grows. Because this strain response to stress resembles that of steel, transformation-toughened zirconia is often referred to as ceramic steel.1