Radiative properties of thermal barrier coatings at high temperatures

Surface radiation represents an important mechanism for heat loss at high temperatures. Thermal control may require improved heat dissipation of highly emitting surfaces in order to keep the maximum temperature below a certain critical value in high-temperature turbine systems. Emissivity allows determining the surface temperature based on thermal spectra measurement or thermal imaging of the turbine blades. In this study, the emissivities of different coating samples including the metal substrate have been measured over a wavelength range 0.4-1.08 mu m in the temperature range 400-1150 degrees C and high values of emissivities are observed. The data are also compared with the theoretical values of emissivity. The comparison between the theory and experiment are, however, poor because the experimental data are obtained at high temperatures, while the theoretical values are calculated using the values of refraction and absorption indices at room temperature in the Fresnel reflection formula. The optical constants of the samples are computed by the Lorentz elastically bound electron theory of insulator and the Drude free-electron theory of metals.