Radiative properties of zirconia coatings on inconel substrates

L. González-Fernández1,2, L. del Campo3, R. Pérez-Sáez1,4 and M. Tello1,4

1Departamento de Física de la Materia Condensada, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Leioa, Bizkaia, Spain
2Industria de Turbopropulsores, S.A, Zamudio, Bizkaia, Spain
3Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Bizkaia, Spain
4Instituto de Síntesis y Estudio de Materiales, Universidad del País Vasco, Bilbao, Spain

Keywords: thermal barrier coatings, FT-IR
property: spectral emittance, radiative properties
material: yttria stabilized zirconia

In this paper, the radiative properties of atmospheric plasma sprayed Yttria Stabilized Zirconia (YSZ) films layered on Inconel 718 substrates are studied. Ceramic films, as YSZ films, are very extensively used as thermal barrier coatings (TBCs) in medium and high temperature applications, and the radiative behavior of the TBCs is needed when performing calculation of radiative heat transfer. A Fourier transform infrared (FT-IR) spectrometer has been used to obtain the radiative properties of the samples between 2.5 and 22 `mu`m, and for temperatures ranging from ambient to 750 ºC. An integrating sphere has been coupled to the FT-IR spectrometer so as to measure the diffuse reflectance at ambient temperature, which can be used to indirectly calculate the spectral emittance. For higher temperatures, the spectral emittance is obtained by the direct radiometric method; the samples are heated in a controlled environment in an attempt to prevent oxidation, and the emittance is measured for several temperatures between 300 and 750 ºC by comparison of the radiance of the sample and that of a blackbody radiator. Samples with film thicknesses ranging from 10 to 300 `mu`m have been prepared to study the dependence of the radiative properties with film thickness. Additionally, the influence of the surface roughness has also been considered. Finally, the emittance has been measured during in situ oxidation of the samples at constant temperature, and the evolution of the radiative behavior has been studied.

The emittance of the coated samples is compared to the emittance of the substrate. The coated samples show a higher emittance in nearly all the studied spectral range. The maximum in the emittance spectra is found at the Christiansen point (`lambda` = 12.8 `mu`m), where it takes a value of 1. For wavelengths smaller than the Christiansen point, the YSZ film is semitransparent and a net contribution of the substrate is found. Moreover, for wavelengths higher than the Christiansen point, the YSZ film is opaque and the radiative behavior is influenced by surface roughness. Concerning the oxidation, the emittance in the semitransparent zone increases as oxidation advances due to the increase of the emittance of the Inconel substrate.

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