Why do transformation-toughened ceramics lose toughness at high temperatures?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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-toughened ceramics, such as transformation-toughened zirconia (TTZ), rely on a stress-induced martensitic phase transformation to inhibit crack propagation. In these materials, embedded tetragonal particles transform into a monoclinic form ahead of an advancing crack, creating a volume expansion that exerts a closing force on the crack tip.1

These ceramics lose their toughness at high temperatures because the driving force required for this phase transformation decreases as the temperature rises. Eventually, this driving force disappears entirely, preventing the transformation from occurring and causing the material to lose its toughening mechanism.1