Thermal barrier coatings (TBCs) using yttria‐stabilized zirconium dioxide (YSZ) are widely used in gas turbines to protect metal components against the high combustion product temperature. Increasing combustion temperature and pressure, radiative heat transfer becomes an essential portion of the overall heat transfer in TBCs. This necessitates a greater understanding of the thermal radiative properties of YSZ films, especially in the near‐infrared wavelength range. The commonly used Kubelka–Munk (KM) method in the radiative property reduction from the measured transmittance and reflectance spectra of YSZ films can incur inaccurate results when the coating optical thickness is not sufficiently large. The discrete ordinates method with the asymmetric spherical ring angular quadrature can solve the radiative transfer equation with good accuracy in optically thin media. Considering the solution accuracy and computational efficiency, a hybrid approach of combining the KM and discrete ordinate methods is used to invert radiative properties. The absorption and scattering coefficients of air plasma sprayed YSZ films are determined over the wavelength range from 1 to 2.6 μm at room temperature. Over this near‐infrared wavelength range, the scattering coefficient decreases with the increasing wavelength, and the absorption coefficient is very small overall.
Air plasma sprayed (APS) thermal barrier coatings (TBCs) are a widely used technology in the gas turbine industry to thermally insulate and protect underlying metallic superalloy components. These TBCs are designed to have intrinsically low thermal conductivity while also being structurally compliant to withstand cyclic thermal excursions in a turbine environment. This study examines yttria-stabilized zirconia (YSZ) TBCs of varying architecture: porous and dense vertically cracked (DVC), which were deposited onto bond-coated superalloys and tested in a novel CO 2 laser rig. Additionally, multilayered TBCs: a two-layered YSZ (dense + porous) and a multi-material YSZ/GZO TBC were evaluated using the same laser rig. Cyclic exposure under simulative thermal gradients was carried out using the laser rig to evaluate the microstructural change of these different TBCs over time. During the test, real-time calculations of the normalized thermal conductivity of the TBCs were also evaluated to elucidate information about the nature of the microstructural change in relation to the starting microstructure and composition. It was determined that porous TBCs undergo steady increases in conductivity, whereas DVC and YSZ/GZO systems experience an initial increase followed by a monotonic decrease in conductivity.Microstructural studies confirmed the difference in coating evolution due to the cycling.
The microstructure dependence of thermal radiative properties is critically needed to the design and operation of the thermal barrier coating systems. In this study, different yttria-stabilized zirconia coating layers are fabricated by air plasma spray with three average porosities 5.9%, 14.5%, and 23.3% at coating thickness from 283 to 955 µm. The room-temperature, spectral directionalhemispherical transmittance, and reflectance are measured over the wavelength range from 1.35 to 2.5 µm. The radiative properties of absorption and scattering coefficients are reduced by using a hybrid method of the discrete ordinate method and the Kubelka-Munk four-flux method. Using the image processing tools developed in-house, the porosity and pore size distribution (PSD) are obtained from SEM images for each coating. A numerical algorithm is used to convert the two-dimensional PSD into a three-dimensional PSD assuming that all pores are spheroid. The scattering coefficient is directly computed by the Mie theory based on the PSD. The new approach provides a predictive model of radiative properties based on the PSD, which is extracted from the coating cross-section images. Comparison of radiative properties obtained by the direct Mie theory computation and those obtained by the reduction from spectral measurement is made and discrepancy is discussed.
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