Suspension and Air Plasma‐Sprayed Ceramic Thermal Barrier Coatings with High Infrared Reflectance

Abstract

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.

Thermal Conductivity Analysis and Lifetime Testing of Suspension Plasma-Sprayed Thermal Barrier Coatings

“…The amount and size of the SPS porosity allows for the reduction in coating thermal conductivity through decreased phonon transport and decreased IR radiation transmittance. As IR radiation is an important contributor to the total heat flux in a combustion environment; the ability to reduce the radiative contribution has an advantage for increased engine performance [19].…”

Thermal radiative properties of zirconium oxide films in the near‐infrared wavelengths

“…where μ is the direction cosine, and q is the radiative heat flux. It can be shown that when the medium optical thickness in a given problem is large, O (10), the second term is much smaller than the first term on the right hand side of Equation (2). Therefore, the angular variation of intensity is effectively nonexistence.…”

“…However, the embedded diffusion approximation in the KM four flux method indicates that the method is only accurate in the optically thick media. Stuke et al 10 . compared the formulations of the two‐flux approximation and the KM approximation.…”

“…The same equation was also derived by using the Milne-Eddington approximation or the moment method. 10 Eldridge and Spucker 9 used the KM four-flux method, which was based on the KM theory, 26 to reduce scattering and absorption coefficients from spectral transmittance and reflectance of YSZ films of various thicknesses. The original KM theory yields simple expressions of the diffuse reflectance and transmittance for the computation, but its domain of validity is restricted.…”

Laser Cladding of Embedded Sensors for Thermal Barrier Coating Applications

“…On the other hand, this cannot be considered equivalent to transparency in general. In studies of the optical properties of plasma-sprayed YSZ in the near infrared (NIR) and long wave infrared (LWIR) range, which are also important for radiative heat transfer in turbine application, it has been shown that, depending on the fraction and morphology of pores, the reflectivity of the TBCs reaches values close to 1 [20,23,24]. Accordingly, laser radiation is not able to dissipate in the depth of the porous substrate but is reflected back to the powder stream.…”