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

Abstract

Thermal barrier coatings (TBCs) using yttria‐stabilized zirconium dioxide (YSZ) are widely used in gas turbines to protect metal components against the high combustion product temperature. Increasing combustion temperature and pressure, radiative heat transfer becomes an essential portion of the overall heat transfer in TBCs. This necessitates a greater understanding of the thermal radiative properties of YSZ films, especially in the near‐infrared wavelength range. The commonly used Kubelka–Munk (KM) method in the radiative property reduction from the measured transmittance and reflectance spectra of YSZ films can incur inaccurate results when the coating optical thickness is not sufficiently large. The discrete ordinates method with the asymmetric spherical ring angular quadrature can solve the radiative transfer equation with good accuracy in optically thin media. Considering the solution accuracy and computational efficiency, a hybrid approach of combining the KM and discrete ordinate methods is used to invert radiative properties. The absorption and scattering coefficients of air plasma sprayed YSZ films are determined over the wavelength range from 1 to 2.6 μm at room temperature. Over this near‐infrared wavelength range, the scattering coefficient decreases with the increasing wavelength, and the absorption coefficient is very small overall.

The correlation of the scattering coefficient and morphology‐based microstructure in yttria‐stabilized zirconia coatings

“…The detailed reduction method is given in the previous work by the authors. 25 The results are shown in Figure 9. In the same batches, the scattering coefficient decreases with increasing wavelength.…”

“…The absorption and scattering coefficients at room temperature are reduced over the wavelength range of 1.4–2.4 µm using the measured transmittance and reflectance data. The detailed reduction method is given in the previous work by the authors 25 . The results are shown in Figure 9.…”

“…For each coating, 20 SEM images of the coating's cross section are taken, and the porosity is determined, per ASTM E2109, 35 by using the image processing software tools developed in-house. 25 The average coating thickness is also determined by the SEM images, and the standard deviation of thickness is calculated. More details of YSZ powders and porous YSZ coatings can be found in Table 1.…”

“…Comparing the model scattering coefficient with that reduced from spectral measurement of coating film reflectance and transmittance, the agreement is only qualitatively as reported by the authors. Also noted there are substantial differences in the scattering coefficients reduced from the experimental data by the discrete ordinate method and two‐flux approximation, respectively, even in the small wavelength range ( λ < 3 µm) where the optical thickness of the coating film is very large 25 . The microstructure information based on a spheroid geometry not only ignores many pore details but also does not represent the actual coating microstructure morphology.…”

“…Various pore types are identified in the insert image. (B) YSZ topcoat after 50 h of laser rig thermal cycling, high‐temperature gradient test 25 …”