Hydrothermal degradation of cubic zirconia

Tetragonal ZrO2 undergoes a tetragonal to monoclinic transformation with the presence of water or water vapor when annealed at relatively low temperatures (65-400 deg. C); such a transformation has been shown to be due to the annihilation of oxygen vacancies. Nevertheless, cubic ZrO2 has always been presumed to be stable under similar conditions. However, after annealing at 250 deg. C under a vapor pressure of 2.6x10-2 atm for 2 years, the 8 mol% Y2O3-doped cubic ZrO2 ceramic samples cracked. Even though the sample phase was still predominantly cubic, subsequent X-ray diffraction studies indicated that a kind of phase transformation actually occurred, while TEM investigations and electron diffractions demonstrated the precipitation of monoclinic ZrO2. After annealing, the electrical conductivity of the sample was found to increase by about 30%, suggesting proton conduction. This is the first observation of the hydrothermal degradation in cubic ZrO2