Hydrothermal degradation of cubic zirconia

Abstract

Tetragonal ZrO 2 undergoes a tetragonal to monoclinic transformation with the presence of water or water vapor when annealed at relatively low temperatures (65–400 °C); such a transformation has been shown to be due to the annihilation of oxygen vacancies. Nevertheless, cubic ZrO 2 has always been presumed to be stable under similar conditions. However, after annealing at 250 °C under a vapor pressure of 2.6×10 −2 atm for 2 years, the 8 mol% Y 2O 3-doped cubic ZrO 2 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 ZrO 2. 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 ZrO 2.