Emittance of Y2O3 stabilised ZrO2 thermal barrier coatings prepared by electron-beam physical-vapour deposition

Electron-beam physical-vapour deposited (EB-PVD) thermal barrier coatings are used in aircraft and land-based gas turbines for thermal insulation of thermally highly loaded components like turbine blades. Conventional duplex thermal barrier coatings (TBCs) consist of two different layers. A partially yttria stabilised zirconia top coat serves as thermal barrier while a NiCrAIY or CoCrAIY bond coat below provides adhesion and oxidation protection of the metallic substrate. The ceramic top coat shows a columnar microstructure which offers some advantages in comparison to plasma-sprayed TBCs. Besides having a low thermal conductivity, TBCs have to emit thermal radiation efficiently in order to reduce the heat load on the coated parts. In this study the emittance of TBCs is measured as a function of coating thickness. This is necessary for a detailed infrared analysis as PYSZ (partially yttria stabilised zirconia) is known to be semitransparent in the near infrared region The directional-spectral emittance in the wavelength range from 2 μm to 18 μm is measured in an angular range from 0° to 70° with respect to the surface normal and for a temperature range from 400 K to 1100 K by a direct radiometric method. Furthermore, the directional-spectral emittance was measured via the integrating sphere method normal to the surface at ambient temperature in the wavelength range from 0,25 μm to 18 μm. The dependence of the emittance on wavelength, emission angle, temperature, and layer thickness is discussed.