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 degrees C to elucidate phase constitution by X-ray diffraction. The composition ranges of nontransformable 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 ZrO2NdTaO4 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 nontransformable 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.

Automated summary

This study focuses on the effects of rare-earth dopants on the phase equilibria and phase stability of tetragonal zirconia in ZrO2-RETaO4 systems. The composition range of non-transformable t-ZrO2 in ZrO2-YbTaO4, ZrO2-DyTaO4, and ZrO2-GdTaO4 is 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 ZrO2NdTaO4 and ZrO2-LaTaO4. These findings are valuable for the development of appropriate thermal barrier coatings.