The correlation of the scattering coefficient and morphology‐based microstructure in yttria‐stabilized zirconia coatings

Abstract

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.