The yttria-stabilized zirconia (YSZ) top coat provides a thermal resistance function in the thermal barrier coatings used in gas turbines and Diesel engines. Besides the thermal conductivity, the thermal radiative properties, especially their dependence on the coating microstructure, under the high temperature combustion temperature are critically needed to the design and operation of the thermal barrier coating (TBC) systems. In this study ceramic oxide films made of thermally sprayed YSZ powder are prepared with three average porosities 5.9%, 14.5%, and 23.3% at film thickness from 283 to 955 μm. These films are fabricated with the air plasma spray (APS) deposition on the aluminum surface. The porosity changes are accomplished by varying the spraying parameters or ceramic oxide powder particle size. The room-temperature, spectral directional-hemispherical 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, depending on the film’s optical thickness. The films are then mounted, sectioned, and polished for SEM imaging. Using the image processing tools developed in-house, the porosity and pore size distribution (PSD) are obtained for each film. A numerical algorithm is used to convert the two-dimensional PSD into a three-dimensional PSD assuming all pores are spheriod. The absorption and scattering coefficients can be computed directly by the Mie theory based on the electromagnetic wave scattering from a distribution of sphere sizes. The new approach provides a predictive model of radiative properties based on the pore size distribution and pore number density, which are dependent on the APS spraying parameters, powder size and morphology. Comparison of radiative properties obtain by direct Mie theory computation and those obtained by reduction from spectral measurement is made and discrepancy is discussed.

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