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Why is the phase transformation in toughened zirconia described as martensitic?

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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.

The phase transformation in zirconia is described as martensitic because it shares fundamental characteristics with the transformation that occurs in rapidly quenched steel to form a phase known as martensite.1

In both materials, the term refers to a specific type of solid-state phase change that occurs without diffusion. In the case of zirconia, the transformation involves a structural shift from a tetragonal form to a monoclinic form, which is accompanied by a significant volume expansion. This process is triggered by the stress field ahead of an advancing crack, and the resulting volume increase exerts a closing force that resists crack propagation, thereby toughening the ceramic. Similarly, in metallurgy, the formation of martensite in steel occurs during rapid cooling (quenching), resulting in a hard, strong structure that is essential to the material's mechanical properties.12