Phase Stability and Physical Properties of Cubic and Tetragonal ZrO 2 in the System ZrO 2 –Y 2 O 3 –Ta 2 O 5

The cubic ( c ‐ZrO 2 ) and tetragonal zirconia ( t ‐ZrO 2 ) phase stability regions in the system ZrO 2 –Y 2 O 3 –Ta 2 O 5 were delineated. The c ‐ZrO 2 solid solutions are formed with the fluorite structure. The t ‐ZrO 2 solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m 1/2 ), and their instability/transformability to monoclinic zirconia ( m ‐ZrO 2 ) increases with increasing tetragonality. On the other hand, the t ‐ZrO 2 solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m 1/2 ), and their transformability is not related to the tetragonality.