Faculty Sponsor's Department:
ZrO2-based ceramics doped with TaO2.5 and YO1.5 are of great interest for next-generation thermal barrier coatings (TBCs) used in gas turbine blades. Highly doped tetragonal compositions have shown a potential for phase stability at elevated temperatures (~1500 °C) that is coupled with outstanding fracture toughness. The design of advanced TBCs, however, requires comprehensive thermodynamic information on the ZrO2-YO1.5-TaO2.5 system. Consequently, assessments of the ternary equilibria require an understanding of the constituent binary systems. While the ZrO2-YO1.5 and YO1.5-TaO2.5 binaries are well studied, data on ZrO2-TaO2.5 are incomplete. The aim of this project is to elucidate phase equilibria in the ZrO2-TaO2.5 system at temperatures relevant to TBC applications. Specimens were synthesized via reverse co-precipitation and heat treated at temperatures ranging from 1250 °C to 1500 °C for 100 h. X-ray diffraction was used for phase identification, in addition to determining any relevant transformation temperatures in the terminal oxide phases (i.e. ZrO2 and TaO2.5) and the intermediate Ta2Zr6O17 phase. Scanning electron microscopy was utilized to analyze specimen microstructure, with energy dispersive x-ray spectroscopy employed to determine the homogeneity ranges of ZrO2, TaO2.5 and Ta2Zr6O17. Results from these experiments are then used to refine the ZrO2-TaO2.5 binary phase diagram between 1250 °C to 1500 °C. This in turn, provides a more complete evaluation of the ZrO2-YO1.5-TaO2.5 system, which can promote the development of future TBC systems.