Toughening of Nontransformable t ′‐YSZ by Addition of Titania

Substitution of TiO 2 for ZrO 2 into single‐phase tetragonal ( t ′) 7YSZ has been shown to induce a twofold increase in toughness. This enhancement has been achieved by increasing the tetragonality of the unit cell, upon substituting Ti 4+ for the larger Zr 4+ cation. The observed behavior is consistent with ferroelastic toughening, but direct evidence of the mechanism is lacking. Adding TiO 2 to 7YSZ has the additional benefit that it diminishes the transformability of the depleted tetragonal form: specifically, no monoclinic phase was observed even after equilibration at 1600°C, followed by low‐temperature annealing. The combination of properties bodes well for potential application as a thermal barrier coating in gas turbine engines.