Experimental study on radiative properties of air plasma sprayed thermal barrier coatings

In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are investigated by measuring the spectral normal-hemispherical reflectance and transmittance of samples with various thickness and porosities in a broad spectral band ranging from 250 nm to 15 μm. The spectral absorption and scattering coefficients are obtained by a four flux model based on the experimental data. The results show that the porous zirconia ceramics exhibits high scattering coefficient (>15,000 m−1) and low absorption coefficient (<500 m−1) in the spectral range from 0.5 to 5.5 μm. Moreover, the scattering coefficient increases with increasing porosity, while the spectral absorption coefficient is not quite sensitive to the change of porosity. In order to examine the effects of microstructure of porous zirconia coatings, a theoretical model of Mie scattering is established, and a fairly good agreement with the experimental data is achieved. The results indicate that the scattering coefficient heavily depends on the microstructures of coatings, i.e., pore size, porosity and pore distribution, and this will in turn considerably affect the thermal radiation heat transfer inside the coating.