Fundamental issues regarding the implementation of gadolinium zirconate in thermal barrier systems.

摘要

Gadolinium zirconate is a promising material to replace 7-wt% yttria stabilized zirconia (7YSZ) as the insulating oxide layer in thermal barrier coatings (TBC's). This project investigated fundamental issues regarding the implementation of Gd2Zr2O7 for TBC applications. The issues examined were (i) the phase equilibria in the A1O1.5 -GdO1.5-ZrO2 system, (ii) the diffusional interactions between electron beam physical vapor deposition (EB-PVD) Gd 2Zr2O7 coatings and sapphire substrates, (iii) the growth and use of 7YSZ/Gd2Zr2O7 bi-layer coatings either as diffusion barriers or as protective layers, and (iv) strategies to toughen Gd-doped zirconias through Ti4+ additions.;Two types of materials were used in these investigations, compacts of liquid-precursor derived powders, and EB-PVD coatings on alumina substrates. Phase equilibria studies were performed on powder compacts heat treated for up to 3 weeks at 1200°C. Interdiffusion studies were performed on EB-PVD single (Gd2Zr2O7) and bi-layer (7YSZ/Gd 2Zr2O7) coatings heat-treated at 1200°C for up to 192 hours. Another group of bi-layer coatings was exposed to siliceous melts at 1300°C for up to 4 hours. Samples for the toughness studies were sinter-forged, and then tested using micro-indentation techniques. Characterization was carried out by XRD, TEM, SEM, EDS, FIB etching, and Raman microprobe.;Gd2Zr2O7 and all ZrO2 compositions with >32%GdO1.5 were found to react with AlO1.5 to form a perovskite GdAlO3 interphase, compromising the interface and Al2O3 layer. The diffusion path inferred was in agreement with the proposed equilibria. The evidence suggests that the fluorite-pyrochlore ordering may be second order, but the issue remains to be fully elucidated.;Gd2Zr2O7 can deposit epitaxially upon 7YSZ via EB-PVD. 7YSZ was found to be an effective diffusion barrier against Gd3+ migration, with ∼20 mum estimated as a minimum thickness. A Gd2Zr2O7 layer can prevent infiltration by molten siliceous deposits, but the protection offered is sensitive to microstructural defects.;The toughness investigation found that addition of smaller cations (Ti 4+) can lead to increased toughness in Gd-doped zirconias and not fully justified by transformation toughening. Ferroelastic toughening was suggested, but confirmation has remained elusive. Rapid partitioning into equilibrium phases was a significant issue for implementation of t'-based materials in the GdO1.5-TiO2-ZrO2 system.