Phase stability of thermal barrier oxides based on t'-zirconia with trivalent oxide additions

Zirconia stabilized with 7+/-1 wt.% addition of yttria (7YSZ) is widely used for thermal barrier coatings (TBC's) on actively cooled gas turbine components, selected partly because of its superior durability under thermal cyclic conditions. As deposited, 7YSZ occurs as a metastable single-phase tetragonal solid solution (t') that is thermodynamically stable against the deleterious transformation to monoclinic upon cooling. However, at high temperatures t' is driven to decompose diffusionally into an equilibrium mixture of high-Y cubic and low-Y tetragonal; the latter becomes transformable to monoclinic compromising the mechanical integrity of the system.; This dissertation explores the effects of trivalent stabilizers, including Y, Sc and selected rare-earth oxides (REO's), on the phase stability of the resulting solid solutions in zirconia. The REO additions are of interest because they can potentially enhance the insulation efficiency on the coating allowing higher operating temperatures. However, understanding of their effects on phase stability and potentially on cyclic durability at the projected use temperature in next generation engines (1200-1400°C) is insufficient to guide the design of coatings with the desirable combination of lower thermal conductivity and acceptable durability. Sc was also investigated because of previous reports on the higher phase stability of materials doped with Sc, and Y served as the baseline.; The experimental approach is based on powders synthesized by reverse co-precipitation of precursor solutions, usually compacted and then subjected to a variety of heat treatments, following their evolution by means of X-ray diffractometry, dilatometry, transmission electron microscopy and Raman spectroscopy. The use of powders facilitated the synthesis of a wider range of compositions that would not have been possible by coating deposition approaches, and because the synthesis occurs at low temperature, it also enabled the starting materials to be metastable single-phase solid solutions.; Salient findings of technological relevance include: (i) a clear relationship between t' stability and the solute cation size for singly-doped compositions, with an optimum for Yb; (ii) a trend of increasing phase stability with increasing the total content of stabilizers within the t'-forming range; (iii) the comparative effects of co-doping 7YSZ with different cations, wherein the smaller cations (Yb, Sc) lead to more stable solutions than the larger ones (Gd, Sin, Nd, La). Of more fundamental interest are the observations that (iv) the decomposition path of t' is different depending on whether the annealing is performed below or above the T0 F/t curve, (v) the compositions of the separated phases evolve over time toward their equilibrium values, wherein the rate of evolution of the depleted t' phase and its interplay with the position of the T0 t/m curve determine the tolerance of the solid solution to partitioning before monoclinic formation ensues, (vi) the differences in behavior between the transformations of the high temperature cubic phase, highlighted by the comparison of YSZ with YbSZ.; A conceptual framework was developed to explain the experimentally observed variations on phase stability based on thermodynamic and kinetic considerations. The findings highlight the value of reliable information on equilibrium phase diagrams, free energy descriptions and T0F/t and T0t/m at the temperatures of interest in understanding these problems. There is also a paucity of information on the relevant diffusion parameters as well as understanding of the mechanisms and their connection to the dopant characteristics. The results of this work provide improved understanding of the problem of phase stability in TBCs and an expanded knowledge base to guide the design of TBCs with optimal phase stability.