Current state‐of‐the‐art environmental barrier coatings (EBCs) for Si‐based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3·2SiO2) or mullite + BSAS ((1−x)BaO·xSrO·Al2O3·2SiO2, 0 ≤x≤ 1) layer, and a BSAS top coat. Areas of concern for long‐term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt‐infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low‐melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water‐vapor‐containing atmospheres. Both reactions can be EBC life‐limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.

Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directionalhemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 lm. Spectra were collected over a wide range of coating thickness from 60 to almost 900 lm. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surfacetemperature pyrometer measurements and on (2) applying midinfrared reflectance to monitor TBC delamination are discussed.

High reflectance thermal barrier coatings consisting of 7% Yittria‐Stabilized Zirconia (7YSZ) and Al2O3 were deposited by co‐evaporation using electron beam physical vapor deposition (EB‐PVD). Multilayer 7YSZ and Al2O3 coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 μm wavelength. The variable 7YSZ and Al2O3 multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 μm. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m‐K for the 20‐layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations.

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