Session: 02-03 Thermal and Environmental Barrier Coatings

Paper Number: 80853

80853 - The Pore Size Dependence of the Radiative Scattering Coefficient in Yttria-Stabilized Zirconia Films 

The yttria-stabilized zirconia (YSZ) top coat provides thermal resistance function in the thermal barrier coating used in gas turbines and Diesel engines.  Besides the thermal conductivity, the thermal radiative properties under the high temperature combustion temperature are critically important to for the design and operation of the TBC systems.  At the present time, there is scarce of information of how the radiative properties, particularly the scattering coefficient, are influenced by the coating microstructure, which can be manipulated by the thermal spray process.  In this study YSZ films are prepared with three porosities and at various film thickness: 8%, 15%, and 23%.  The film thicknesses are from around 200 to 700 mm.  These films are fabricated with the air plasma spray (APS) deposition on the aluminum surface.  The porosity changes are accomplished by varying the spraying parameters and ceramic oxide powder size.  The room-temperature, spectral directional-hemispherical transmittance and reflectance data are measured over the wavelength range from 1.35 to 14 μm.  The film transmittance and reflectance measurements are conducted on a Fourier-Transform Infrared spectrometer with an integrating sphere equipped with InGaAs and liquid-nitrogen cooled MCT detectors.  The radiative properties of absorption and scattering coefficients are reduced from the spectral transmittance and reflectance data by using the discrete ordinate method or the Kubelka-Munk four-flux method, depending on the film's optical thickness.  The films are then mounted, sectioned, and polished for SEM imaging.  Using the image processing tools developed in-house, the porosity and pore-size distribution (PSD) curve are obtained for each film.  A numerical algorithm is used to convert the two-dimensional PSD into a three-dimensional PSD, while assuming all pores are spherical.  With the same spherical pore assumption, the absorption and scattering coefficients can be computed directly by the Mie theory based on the electromagnetic wave scattering from a sphere.  The new approach provides a direct computation of radiative properties based on the pore size distribution and pore number density, which are dependent on the APS spraying parameters and powder size.  Comparison of radiative properties obtain by direct Mie theory computation and those obtained by reduction from spectral measurement is made and the discrepancy is discussed.  The direct computation of the thermal radiative properties is important for the TBC design.

Presenting Author: Pei-Feng Hsu Florida Institute of Technology

Presenting Author Biography: Dr. Hsu is a professor of mechanical engineering in Florida Institute of Technology. He is a fellow of ASME and an associate fellow of AIAA. His main research areas are thermal/environmental barrier coating testing and analysis, thermal radiative heat transfer in high temperature environment, radiative property measurement and modeling.

Authors:

Yao Wang Florida Institute of Technology
Pei-Feng Hsu Florida Institute of Technology
Mary Helen Mccay Florida Institute of Technology