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Kinetics and Thermodynamics of Molten Silicate Attack on Thermal and Environmental Barrier Coatings


Enhancing the efficiency of gas turbine engines requires higher operation temperatures and the materials capable of surviving the increasingly challenging environment. Ceramic barrier coatings, with carefully engineered microstructures, protect the structural components within the hottest sections of the engine. However, these coatings are susceptible to damage mechanisms arises from the ingestion of siliceous debris, which can melt and deposit on the coating’s surface. Thermomechanical strains develop that are either mitigated or exacerbated by the thermochemical interactions between the coating and the melt. This work investigates these thermochemical interactions and their pertinent kinetics and thermodynamics.

Dense compacts or single crystalline pieces of barrier coating oxides were placed into a semi-infinite 1D diffusion couple geometry with one of two synthetic silicate melts at 1200–1400 °C. Concentration profiles within the melt were obtained and fit to partial differential equations quantitatively describing the coating dissolution rate into the melt and diffusivities therein. Cation diffusivities were most affected by the melt composition, whereas the ratio of rare-earth (RE3+) oxides to ZrO2 or HfO2 most strongly affected the initial detachment rate of barrier oxides into the melt. Ultimately, the dissolution kinetics were sufficiently slow to delay melt saturation and the nucleation of reprecipitated or reaction phases that limit coating degradation. This delay was worse for barrier oxides with low concentration of RE3+ elements. Finite element models—using the gathered kinetic data but applied to small length scales relevant for real coatings—suggest this delay will be controlled primarily by the initial interface detachment rate in practice.

After the initial dissolution transient period, the crystallization of reprecipitated and reaction phases was investigated qualitatively using electron microscopy and chemical analysis techniques. The presence of only a small amount of RE3+ oxide in the dissolving material (e.g., 7%) kinetically hindered the crystallization of reaction products—even those based on Zr4+ or Hf 4+—favoring instead reprecipitated phases, deviating from the expected thermodynamic response predicted by CALPHAD databases. Conversely, those barrier oxides free of RE3+ (e.g., HfO2) more readily crystallized reaction products such as (Zr,Hf)SiO4, or Ca2HfSi4O12; those containing a substantial amount of RE3+ (e.g., Gd2Zr2O7) rapidly crystallized a RE-apatite, nominally Ca2RE8(SiO4)6O2.

Finally, the thermodynamics of Y-Al-Fe-garnet formation, i.e., the solid-solubility limits of substitutional cations Ca2+, Mg2+, Fe2+, and Si4+, their crystallographic site preference, and the competition between garnet and other phases was investigated. Long duration heat treatments afforded equilibrated samples, for which the phase assemblage was analyzed using X-ray diffraction, electron microscopy, and standardized chemical analysis techniques. A key factor in the stability of garnet was the Fe:Al ratio of the system. Indeed, increasing the Fe:Al ratio of the as-synthesized powder significantly increased the Ca2+ and Si4+ solubility and the quantity of garnet present, with a concomitant decrease to the quantity of other important reaction phases such as apatite.

This dissertation advances the understanding of thermochemical interactions between protective barrier coatings and molten silicates, which is critical to design robust coatings. The quantitative kinetic data and thermodynamic information enables computational approaches to coating design.

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