Infrared-optical properties and heat transfer coefficients of semitransparent thermal barrier coatings

Abstract

Thermal barrier coatings (TBC) which are used in aircraft and land-based gas turbines for thermal insulation of thermally highly loaded components are usually semitransparent in the infrared spectral region at higher temperatures. Thus, at turbine surface conditions the total heat transfer coefficient of TBCs increases above the heat transfer coefficient caused by solid heat conduction alone. The solid thermal conductivity of electron beam physical vapor deposited (EB-PVD) partially yttria stabilized zirconia (YSZ) coatings derived from laser flash measurements were correlated with the microstructure of the coatings, which was adjusted by defined heat treatments. To obtain the contribution of the radiative transfer on the total heat transfer coefficient, infrared-optical characterizations were carried out at ambient and elevated temperatures. A theoretical model was developed which can be used to describe the heat transfer through semitransparent, absorbing and scattering media. Finally, the total heat transfer, caused by solid thermal conduction, radiative transfer and an interaction of both is derived for the coatings prepared in this work. Additionally the measurement method BBC (black body boundary conditions) which is suitable to determine spectral transmittance and emittance at elevated temperatures is introduced.