Transformation of Tetragonal Zirconia Polycrystals by Annealing under Humidity Controlled Conditions

Changes in the crystalline phase and microstructure on the surface resulting from low-temperature annealing of tetragonal zirconia polycrystals doped 2, 3 and 4 mol%Y₂0₃ were investigated under humidity controlled conditions at 0—7. 35 kPa of water vapor pressure and 100—600°C for 1—50 hr. The tetragonal phase Zr0₂ transformed to the monoclinic structure by annealing at low temperature conditions such as 100—500°C. The phase transformation caused microcracks on the surface and resulted in significant loss of fracture strength. The critical temperatures of the phase transformation of Zr0₂-2 mol%Y₂0₃, Zr0₂-3 mol%Y₂0₃ and Zr0₂-4 mol%Y₂O₃ were 500, 400 and 350°C, respectively. The phase transformation proceeded rapidly on the surface but quite slowly inside the body. The rate of the phase transformation on the surface was first order with respect to the concentration of the tetragonal Zr0₂ and increased with increasing water vapor pressure by Langmuir type relation. The appearent activation energy of the phase transformation was about 85 kJ/mol.