Studies of the Yttrium Tantalate – Zirconium Oxide Quasi-Binary

Abstract

For thermal barrier coating (TBC) applications, the high temperature tetragonal phase of ZrO2 has been the widely used material for years due to its many favorable properties, including low thermal conductivity and high thermal expansion coefficient. It is known that the addition of Y2O3 stabilizes the t-phase to room temperature, enabling its use in TBCs. However, as engine operating temperatures push to higher temperatures, challenges with phase stability and heat transport become apparent, leading to a search for new TBC materials. One promising candidate is the YO1.5-TaO2.5-ZrO2 system. This dissertation explores the synthesis, structural characterization, and thermal and optical properties of compositions along the YTaO4-ZrO2 quasi-binary. Using a variety of characterization techniques, the phase diagram along the YTaO4-ZrO2 quasi-binary is developed. The behavior of the crystal structures and the phase transformations within the YTaO4 solid solution region is explored as a function of ZrO2 content. The extent of the solubility limits for the YTaO4 and ZrO2 phases is established as a function of temperature, revealing a two-phase regime in the intermediate region of the YTaO4-ZrO2 quasi-binary. The implications of this intermediate region include an extensive stable tetragonal phase field, small grain size with limited grain growth, and low thermal conductivity that is independent of temperature. The two tetragonal phases display structural similarities, which when combined with the microstructural analysis of compositions in this intermediate region, suggests the presence of a higher temperature phase above 1600°C. Next, the heat transfer behavior is investigated for compositions along the YTaO4-ZrO2 quasi-binary using thermal and optical techniques. The experimental results indicate compositions in the intermediate region are better thermal insulators than the current yttria-stabilized zirconia TBC coatings. Additionally, the thermochemical compatibility between the quasi-binary compositions and alumina is studied since alumina is the most commonly formed TGO in TBCs.