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transformation toughening
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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.
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transformation toughening

mechanics
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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.
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 method to enhance the mechanical properties of ceramics, particularly their toughness, by inducing a phase transformation. This process is most notably observed in zirconia (ZrO₂). When zirconia transforms from a tetragonal to a monoclinic form, it undergoes a significant volume expansion.

In transformation toughening, fine tetragonal zirconia particles are dispersed within a matrix. Ahead of a propagating crack, the stress field triggers these particles to transform into the monoclinic phase. This transformation causes a volume increase that exerts a closing force on the crack, resisting its further propagation. Ceramics treated this way, like transformation-toughened zirconia (TTZ), are sometimes called "ceramic steel" due to their strain behavior resembling steel. This mechanism significantly increases the energy required for crack propagation, making the ceramic much tougher than conventional brittle ceramics.1

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3 Britannica Sources
  1. advanced structural ceramics
  2. work hardening
  3. heat-treating

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