Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat-isolative topcoat. Several recent research activities are concentrating on developing improved bondcoat or topcoat materials; for the topcoat especially, those with reduced thermal conductivity are investigated. Using advanced topcoat materials, the ceramic coating can be further divided into layers with different functions. One example is the double-layer system in which conventional yttria-stabilized zirconia (YSZ) is used as bottom and new materials such as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. In addition, new functions are introduced within the TBCs. These can be sensorial properties that can be used for an improved temperature control or even for monitoring remaining lifetime. Further increased application temperatures will also lead to efforts for a further improvement of the reflectivity of the coatings to reduce the radiative heat transfer through the TBC.

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Yttria partially stabilized zirconia (YSZ) coatings are widely used for thermal barrier coatings (TBCs) to increase operating temperature of gas turbines. In the wavelength range where most of the radiation by walls and combustion gas is emitted within the gas turbine YSZ is semitransparent leading to increasing radiation heat flows into the components at increasing service temperatures. The objective of this work is to optimize the diffuse reflectance of plasma‐sprayed TBCs by improving the coating microstructure such that the reflectance of radiation is increased. As a result, a more efficient thermal screening of the underlying metallic substrate is achieved. In this work, air plasma‐sprayed and suspension plasma‐sprayed (SPS) coatings of 7% YSZ using powder of different grain size distributions and different spray parameters were deposited. The reflectance and transmittance has been investigated in the wavelength range from 0.3 to 2.5 μm. The SPS‐coatings showed the highest reflectance up to 94% at 1.5 μm wavelength. In addition, the scattering and absorption coefficients of the sprayed TBCs calculated with the Kubelka–Munk two flux model showed strong correlation with the measured porosity. By improving the microstructure, we were able to reduce thermal conductivity while increasing scattering of radiation, resulting in lower heat flow and lower temperature at the metallic substrate. These results are strengthened by numerical calculations.

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