Comparison of different models for the determination of the absorption and scattering coefficients of thermal barrier coatings

The thermal radiative properties of thermal barrier coatings (TBCs) are becoming more important as the inlet temperatures of advanced gas-turbine engines are continuously being pushed higher in order to improve efficiency. To determine the absorption and scattering coefficients of TBCs, four-flux, two-flux and Kubelka-Munk models were introduced and used to characterize the thermal radiative properties of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings. The results show that the absorption coefficient of YSZ is extremely low for wavelengths <6 μm and the scattering coefficient is high and decreases with increasing wavelength. The obvious deviation of absorption and scattering coefficients obtained by the Kubelka-Munk model from those values calculated by four-flux and two-flux models indicates that surface reflection is an important parameter which cannot be neglected when characterizing the radiative property of the coating. The excellent agreement of predicted reflectance and transmittance spectra by the two-flux and four-flux models for coating thicknesses >200 μm suggests that when the coating thickness is larger than around twice the average scattering distance, the collimated flux can be simply treated as a diffuse flux inside the coating, and thus the two-flux model can be used to determine the absorption and scattering coefficients as a simplification of the four-flux model.