Determination of Scattering and Absorption Coefficients for Plasma‐Sprayed Yttria‐Stabilized Zirconia Thermal Barrier Coatings

Abstract

Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directionalhemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 lm. Spectra were collected over a wide range of coating thickness from 60 to almost 900 lm. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surfacetemperature pyrometer measurements and on (2) applying midinfrared reflectance to monitor TBC delamination are discussed.

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

“…(2) gave ∼0.010 µm −1 (514 and 632.8 nm) for APS-YSZ and ∼0.012 µm −1 (514 and 632.8 nm) for EBPVD-YSZ, which are of a similar magnitude to those previously determined for dense YSZ (0.05-0.19 µm −1 ) [18] and [43]. In particular, Eldridge et al [44] and [45] have determined experimentally the scattering and absorption coefficients of freestanding 8 wt.% APS-YSZ (11% porosity) using normalincidence directional-hemispherical reflectance and transmittance spectra over the range of 250 nm up to 15 µm (data for the range 0.8-15 µm can be found in Refs. [44] and [45]; the range between 250 and 1000 nm is provided by Eldridge and presented in Fig.…”

“…In particular, Eldridge et al [44] and [45] have determined experimentally the scattering and absorption coefficients of freestanding 8 wt.% APS-YSZ (11% porosity) using normalincidence directional-hemispherical reflectance and transmittance spectra over the range of 250 nm up to 15 µm (data for the range 0.8-15 µm can be found in Refs. [44] and [45]; the range between 250 and 1000 nm is provided by Eldridge and presented in Fig. 10).…”

Development of Porous YSZ Coatings with Modified Thermal and Optical Properties by Plasma Spray Physical Vapor Deposition

“…These confirm the diffuse (random) scattering nature of the PS-PVD coating despite vertically-alligned feather-like internal structures. Detailed procedure of this analysis can be found elesewhere (19) . Prior to deposition experiment, the pressure dependence on the loading effect of ZrO 2 powder injection to the hybrid plasma flame was simulated.…”

“…Measurement was carried out at room temperature under a constant nitrogen gas flow to minimize the effect of moisture (the optical properties of YSZ are less significantly dependent on temperature (18) ). The modified four flux model, proposed by Eldridge and Spuckler (19) , were employed to extract quantitative values of the scattering and absorption properties of the porous coatings. This analysis considers the collimated and diffused radiations in both incoming and out-going directions, assuming the isotropic scattering within the coating and zero emission from the specimen.…”

“…It is seen that the porosity of the PS-PVD reaches roughly 3 times higher than that attained by APS and EB-PVD and thermal conductivity is less than half of that generally attained by APS. (19) . Although the optical measurement was carried out under nitrogen gas flow environment, significant amount of the OH and CO absorption peaks are observed.…”