Materials based on the non-transformable tetragonal phase in the ZrO2-(Y/Yb)O1.5-TaO2.5 systems are of significant interest for next-generation thermal barrier coatings (TBCs) owing to their thermodynamic stability at high temperature, resistance to deleterious phase transformations on thermal cycling, low thermal conductivity, and potentially adequate toughness. The current study investigates fundamental issues pertaining to the implementation of these compositions for TBC applications and associated concerns potentially impacting coating durability. Topics explored include: (i) material processability by electron beam physical vapor deposition (EB-PVD), (ii) TBC microstructure evolution, (iii) relationships between toughness and composition, and (iv) toughening mechanisms, including ferroelastic domain switching.
Key challenges identified in the fabrication of ZrO2-(Y/Yb)O1.5-TaO2.5 TBCs by EB-PVD are compositional variability throughout the coating thickness and the development of suboptimal columnar microstructures. The former prevents deposition of single-phase materials and potentially compromises the phase stability of the structure and the latter degrades the strain tolerance of the coating. Despite these complications, compositions in the two-phase t-ZrO2 + c-ZrO2 and three-phase t-ZrO2 + c-ZrO2 + M’-YTaO4 phase fields can be deposited by EB-PVD such that the columnar microstructure is adequate and the stability of the tetragonal phase is retained for at least 400 hours at 1250°C and 1500°C.
The toughness of the investigated multi-phase compositions, which is expected to influence the durability of thermal barrier coatings based on these materials, is demonstrated to be significantly dependent on composition and microstructure. The influence of composition arises from the relative fractions of the phases present at equilibrium and the differences between their intrinsic toughness. Minimizing the amount of secondary phases (c-ZrO2 and M’-YTaO4), particularly c-ZrO2, is a path to optimize the toughness of the tetragonal-phase material. The role of microstructure is related to the fracture pathway and prevalence of toughening mechanisms such as crack deflection, grain bridging, grain pullout, and microcracking, which had not been previously reported. It is shown for the first time that domains suggestive of ferroelastic switching are contained within non-transformable tetragonal grains. The domains form on growth and/or densification of precursor-derived powder compacts as a result of short-range ordering between Y3+ and Ta5+ cations, as predicted by first-principle calculations. The ordering patterns in the zirconia solid solution are reminiscent of those predicted and observed for the various forms of the yttrium tantalate. However, direct evidence of ferroelastic switching within the process zone of a crack remains elusive. The results suggest that additional understanding of the relationship between toughness and microstructure is required for the implementation of materials based on the non-transformable tetragonal phase in thermal barrier coatings.