What phase change occurs when pure zirconia cools below 1,150 °C?

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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.

When pure zirconia (zirconium dioxide, ${ZrO}_{2}$ ) cools below approximately 1,150 °C, it undergoes a phase transformation from a tetragonal crystal structure to a monoclinic crystal structure.

This structural change is significant in the field of materials science because the transition from the tetragonal phase to the monoclinic phase is accompanied by a volume expansion. In the context of advanced structural ceramics, this expansion can induce internal stresses that may lead to cracking or failure in pure zirconia components. To mitigate this, stabilizers are often added to the material to retain the high-temperature cubic or tetragonal phases at room temperature, a process that enhances the material's toughness through transformation toughening.1