OPTICAL AND RADIATIVE PROPERTIES OF YSZ PLASMA-SPRAYED COATINGS

This paper is devoted to the study of the optical and radiative properties of YSZ plasma-sprayed coatings which are widely used as thermal barrier coatings (TBCs). The effects of microstructure and oxygen under-stoichiometry have been investigated by performing directional-hemispherical reflectance and transmittance measurements over the YSZ translucent spectral region of 0.25-8 µm. The effect of under-stoichiometry has been isolated by annealing samples in air at 500°C during various times. Using the Gouesbet-Maheu model, the radiative properties (absorption and scattering coefficients) of the coatings have been evaluated by numerical inversion of their optical spectra. The differences in the scattering coefficients are discussed in terms of pore size and volume fraction, while the absorption level is correlated to the under-stoichiometry of YSZ. A prediction of the radiative properties of the coatings based on their microstructural description is attempted by applying the Mie theory in an absorbing host medium (YSZ) including monosize spherical pores. INTRODUCTION YSZ plasma-sprayed coatings are widely used as thermal barrier coatings (TBCs) to protect metal parts from high temperature combustion gases in turbine engines [1]. To ensure high performance to TBCs, an extensive effort has been carried out to correlate the spraying parameters to the resulting microstructure and macrostrutural properties such as the thermal conductivity or the thermo-mechanical resistance. Hovewer, only a few studies [1,2,3] have investigated the radiative properties of plasma-sprayed YSZ, although radiation becomes the major heat transfer mechanism at high temperature and the coating material (YSZ) is nearly translucent over the 0.5-5 µm wavelength range where thermal radiation is mostly concentrated. Therefore, a better knowledge of the radiative properties of YSZ plasma-sprayed coatings is necessary to predict and optimize their insulating ability. A very difficult issue in such a study lies in the large variety of microstructural defects of plasma-sprayed coatings and some possible chemical modifications of the material during plasma spraying such as oxygen reduction for a metal oxide in a reducing atmosphere [4,5]. Indeed, plasma-sprayed coatings are built-up on a prepared substrate by successive impinging, flattening and solidification of particles which were previously accelerated and heated in a plasma jet. The coatings exhibit a lamellar structure including a complex void structure with interlamellar pores, globular pores and intralamellar cracks. The size of the voids ranges from sub-micron to more than 10 µm with various shapes (spherical shape for globular pores, flat shape for interlamellar pores and microcracks). To our knowledge, no model is currently available to predict the radiative properties of such highly heterogeneous materials. To circumvent this difficulty, some simple phenomenological approaches are presented in this paper that aim to characterize the radiative properties of plasma-sprayed coatings and then to predict them from some simple microstructural descriptions. Our study is limited to the investigation of the optical and radiative properties of YSZ plasma-sprayed coatings at room temperature over the 0.5-8 µm wavelength range where the microstructural effects are predominant. The effects of under-stoichiometry are also addressed by performing annealing in air at 500°C during 5 minutes to 24 hours and following the resulting variation of the optical and radiative properties. This paper is organized as follows. In Section 1, our different experimental equipments are described. Section 2 is devoted to the results of the coating characterization that lead to classify our samples according to their main microstructural features such as the pore size, the pore shape and the lamellar