Fracture Toughness, Ionic Conductivity, and Low‐Temperature Phase Stability of Tetragonal Zirconia Codoped with Yttria and Niobium Oxide

Tetragonal zirconia (t‐ZrO 2 ) solid solutions were prepared with additions of up to 1.5 mol% of niobium oxide (Nb 2 O 5 ) into 3‐mol%‐yttria‐stabilized t‐ZrO 2 (3Y‐TZP). The influence of pentavalent cation doping on fracture toughness, ionic conductivity, and the tetragonal‐to‐monoclinic phase transformation in the temperature range of 120°‐210°C was investigated. Fracture toughness and ionic conductivity increased and decreased, respectively, as the Nb 2 O 5 content increased, which indicated that the annihilation of oxygen vacancies in 3Y‐TZP was responsible for the instability of the t‐ZrO 2 lattice. The activation enthalpy related to the conductivity was ~83 kJ/mol, regardless of the dopant content, which was consistent with that for the low‐temperature degradation of 3Y‐TZP doped with Nb 2 O 5 . Degradation under an applied electric field occurred only on the specimen surface that was in contact with the anode, which suggests that depletion of the oxygen vacancies led to the degradation. The identical activation enthalpies and the involvement of the vacancy migration in both processes fortified the belief that the low‐temperature degradation of yttria‐stabilized t‐ZrO 2 is attributed to oxygen vacancy diffusion.