Phase Equilibria and Toughness of Zirconia-Based Thermal Barrier Coatings
Thermal barrier coating (TBC) systems are critical to the performance of gas turbine engines in the aviation and power generation industries. As engine operating temperatures are increased to improve efficiency, ceramic topcoats that depend on a metastable tetragonal phase, namely t’-8YSZ (ZrO2 + 8 ± 0.5 mol% YO1.5), rapidly degrade. Alternative TBC compositions that exhibit combinations of properties superior to 8YSZ are needed to meet the efficiency targets of next generation gas turbines. In this work, two approaches are pursued based on either a single-phase, non-transformable tetragonal structure or multiphase compositions that offer the possibility of adequate toughness and resistance to molten silicates.
Previous laboratory investigations of the CeO2-TiO2-ZrO2 (CeTiSZ) system revealed a relatively large, non-transformable tetragonal field with exceptional tetragonality is stable at 1350°C. In the present work, challenges with transitioning the earlier laboratory results to industrial scale coatings are elucidated. Two important effects not previously encountered in the deposition of current TBCs are identified. Understanding the driving force for segregation during deposition as well as the potential reducibility of the cations upon heating is critical to maintaining stable phase equilibria in thermally sprayed coatings. The reduction of cerium (IV) upon heating places an inherent limit on the temperature capabilities of CeTiSZ coatings.
Contrary to the CeTiSZ system, the ZrO2-YO1.5-TaO2.5 (ZYTO) system does not experience reduction of the cations at high temperatures. A stable, non-transformable tetragonal phase exists at temperatures up to 1500°C, albeit in a much narrower composition range. Although the ZrO2-rich region of the ternary had been studied previously, there was a paucity of information on the phase equilibria in the rest of the diagram. Systematic investigation of the entire YO1.5-TaO2.5 binary phase diagram elucidates the equilibrium phases and serves as a solid foundation for the investigation of the ZYTO ternary. For the first time, isothermal sections of the complete ZYTO ternary are experimentally determined at 1250°C and 1500°C. Two-phase fields in the ZYTO ternary suggest that multiphase compositions with improved combinations of properties may be possible. Specifically, a large two-phase field in which fluorite and YTaO4 (YT) are stable offers the potential to increase the toughness of fluorite, which, with sufficient rare earth content, can mitigate damage caused by molten silicates. Additionally incorporating YT as a second phase has the potential to improve the CMAS resistance of phase stable tetragonal compositions.
Similar to the phase equilibria, there was a dearth of information on the toughness values and associated toughening mechanisms of compositions in the ZrO2-lean portion of the ZYTO phase diagram. Micro-indentation and micro-3-point bend experiments indicate that YT contributes to the toughness of multiphase samples. It is shown for the first time that samples containing YT exhibit microstructural features clearly consistent with domain wall motion, the fundamental mechanism underpinning ferroelastic toughening.