Phase structure and thermal evolution in coating films and powders obtained by sol-gel process .2. ZrO2-2.5 mole % Y2O3

Powders and coatings of zirconia doped with 2.5 mole % yttria have been produced via the sol-gel route. The phase structure and subsequent thermal evolution in heating and cooling cycles have been investigated using mainly perturbed angular correlations spectroscopy. Thermal analyses and XRD as a function of temperature have also been performed to obtain complementary information. Upon heating, the amorphous gels crystallized into the tetragonal structure and showed the same hyperfine pattern and thermal behavior as observed in tetragonal zirconia obtained by the ceramic route: the two configurations of vacancies around zirconium ions denoted as t(1) and t(2) forms and their mutual t(1) --> t(2) transformation. While the powder sample exhibited an incipient thermal instability above 1000 degrees C and underwent completely the t(2) form to m-ZrO2 transition during subsequent, gradual cooling below 500 degrees C, the coating retained the tetragonal phase within the whole temperature range investigated. Hyperfine results suggest that the tetragonal phase stabilization is favored by the highly defective nature of the tl form and consequently hardened by the availability of oxygen. The PAC derived activation energy for the fast diffusion of the oxygen vacancies inherent to the t(2) form was determined as 0.54 +/- 0.14 eV.