ROLE OF O2-, OH- AND ANION VACANCIES IN THE DEGRADATION OF Y-TZP IN MOIST ENVIRONMENTS

X-Ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) hav e been used to study the changes in surface chemistry and crystallogra phy of yttria-tetragonal zirconia polycrystal during exposure to eithe r a moist air stream (200-degrees-C) or an autoclave environment (120 or 170-degrees-C). XPS measurements indicate that there is a significa nt increase in the O2- and %Zr3+ levels during the transformation of t he surface tetragonal phase to the monoclinic phase. No significant le vels of zirconium hydroxide species were detected during exposure to m oist air or autoclave treatment. Yttrium hydroxide was detected only a fter severe autoclave treatment at 170-degrees-C. A model is proposed whereby destabilisation of the tetragonal phase is caused by annealing of anion vacancies by O2-.