Experimental study on the scattering and absorption coefficients of thermal barrier coatings at elevated temperatures

Abstract

The study on the scattering and absorption coefficients of thermal barrier coatings (TBCs) is becoming more and more important as the radiation heat transport through TBCs makes a greater contribution due to the continuous increase in turbine inlet temperature. The spectral reflectance and transmittance of the atmospheric plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs are measured by the two-substrate method at elevated temperatures from 850 to 1150 °C with the wavelength varying from 1.4 to 2.4 μm. The reflectance decreases with increasing wavelength and temperature, while the transmittance has a reverse rule. The high reflectance and transmittance of the 8YSZ TBCs demonstrate that the 8YSZ TBCs have high scattering and low absorption coefficients. The scattering and absorption coefficients of the 8YSZ TBCs are determined by the four-flux model. The scattering coefficient decreases approximately from 820 to 460 cm−1 as the wavelength increases with less temperature dependence. The absorption coefficient is extremely low (<0.1 cm−1), and it increases as the temperature increases with a little wavelength dependence. The transmittance of the 8YSZ TBCs cannot be simplified as a quasi-Beer’s law or an exponential term of the thickness, because the scattering coefficient is too large and the condition that the thickness of 8YSZ TBCs is much larger than the effective attenuation length is not met. Due to the low absorption coefficient, the 8YSZ TBCs can be treated as the pure scattering coatings without the effect of absorption. The zero-absorption two-flux model is a simple and accurate model to predict the scattering coefficient of the 8YSZ TBCs approximately as the pure scattering material.