Low-temperature degradation in yttria-stabilized tetragonal zirconia polycrystal doped with small amounts of alumina: Effect of grain-boundary energy

Low temperature degradation (LTD) behavior was examined in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZP) doped with 0–1.1 mol% Al2O3 sintered at 1500 °C. 0.12 mol% Al2O3-doped Y-TZP exhibited monolithic microstructure without dispersion of Al2O3 grain, while second-phase Al2O3 particles were observed in the Y-TZP sintered bodies doped with 0.54–1.1 mol% Al2O3. In the Y-TZP doped with 0.12 mol% Al2O3, neither amorphous nor second phase was present along the grain boundaries, but Y3+ and Al3+ ions segregated in the vicinity of the grain boundaries. The tetragonal-to-monoclinic (T → M) phase-transformation rate during the accelerated LTD test significantly decreased at 0.12 mol% Al2O3, but the phase-transformation rate slightly increased with the increasing doping amount of Al2O3. The suppressed T → M phase-transformation in Al2O3-doped Y-TZP is explained in terms of decrease in grain-boundary free energy of tetragonal phase owing to Al3+ ions’ grain-boundary segregation.