Phase equilibria and non-transformable tetragonal zirconia in ZrO2-RETaO4 systems and their stabilization mechanism: Experiments and calculations

The non-transformable tetragonal zirconia (t-ZrO2) in ZrO2-RETaO4 systems offers significant promise in the development of next generation thermal barrier coatings. This work focuses on the effects of rare-earth dopants on phase equilibria and phase stability of tetragonal zirconia in ZrO2-RETaO4 systems. Precursor-derived oxides were heat treated at 1500 °C to elucidate phase constitution by X-ray diffraction. The composition ranges of non-transformable t-ZrO2 in ZrO2-YbTaO4, ZrO2-DyTaO4 and ZrO2-GdTaO4 are measured to be about 30–41, 22–29 and 16–19 mol% RE0.5Ta0.5O2, respectively. However, non-transformable t-ZrO2 cannot be found in ZrO2-NdTaO4 and ZrO2–LaTaO4. Formation abilities of t-ZrO2 in ZrO2-RETaO4 systems are determined to be inversely proportional to the ionic radius of RE3+. Based on our calculations, stabilization mechanisms of non-transformable t-ZrO2 were considered to be related to degree of strain energy release and high-temperature decomposed reactions of the doped t-ZrO2. Firstly, RE-Ta pair dopants could relieve the strain energy of the strained tetragonal zirconia structure, which would prevent the displacive transformation of t-ZrO2→m-ZrO2. Secondly, the solid solubilities of RE-Ta dopants should be large enough to ensure the doped t-ZrO2 phase cannot decompose before inhibiting t-ZrO2→m-ZrO2. For large ionic radius of Nd3+ and La3+ dopants, the decomposed reactions of doped t-ZrO2 can take place even at very low dopant concentration, which explains the observations that t-ZrO2 cannot be detected in their whole compositions. The emerging results are useful to the development of appropriate thermal barrier coatings.