Microstructural Characterization of Yttria-Doped Zirconia Coatings with Electron Microprobe Wavelength Dispersive Compos
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Microstructural Characterization of Yttria-Doped Zirconia Coatings with Electron Microprobe Wavelength Dispersive Compositional Mapping Ryna B. Marinenko, Jennifer R. Verkouteren, and David S. Bright, National Institute of Standards and Technology, NIST, Surface and Microanalysis Science Division, Chemical Science and Technology Laboratory Gaithersburg, MD 20899 ABSTRACT The use of digital electron microprobe x-ray compositional mapping with wavelength dispersive spectrometers to understand the microstructure of yttria stabilized zirconia thermal barrier coatings is described. Data from quantification of element x-ray maps can be utilized to infer what phase or phases are present. Analysis of a plasma-sprayed coating prepared from a fused and crushed feedstock is compared to an annealed specimen of the same material. INTRODUCTION For more than a decade, yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been prepared from feedstocks composed of 0.06-0.08 mass fraction Y2O3 stabilized zirconia [1]. The resulting coating is a tetragonal zirconia phase, also called non-transformable or t’ zirconia that has the mechanical and thermal stability needed for a good TBC. In service, TBCs are exposed to high temperatures, often above 1100°C, causing phase transformations that result in increased residual stress and eventual spalling. To improve the performance of these TBCs characterization of the microstructure before and after exposure to service conditions is needed. Wavelength dispersive compositional mapping with the electron probe microanalyzer (EPMA) was used to study YSZ TBCs. Phases present in TBCs may be only a few to several micrometers in one or more dimensions; EPMA provides the resolution needed to recognize compositional variations on the micrometer scale. For the YSZ specimens studied here, different phases not visible with an optical microscope or in SEM secondary electron (SE) or backscatter (BS) images became obvious in EPMA digital x-ray element maps. Results of analyses of plasma sprayed coatings prepared from a fused and crushed feedstock are described. Data from quantification of the element maps are used to infer which phases are present. EXPERIMENTAL Two specimens of a plasma-sprayed YSZ-TBC were studied. The coating was 4-5 mm thick, purposely prepared for bulk analysis [2] and considerably thicker than industrial coatings that are usually about 300 µm thick. The TBC was prepared from a fused and crushed YSZ feedstock powder with a nominal 0.08 mass fraction of Y2O3 (0.087 mole fraction YO1.5 or 0.063 Y mass fraction). The stoichiometric phase is (Zr1-xYx)O2-x/2. One specimen was “as sprayed” and the other was annealed for 1h at 1400°C. Polished cross-sections about 2 cm long and 5 mm wide were coated with 10 nm of carbon. X-ray maps of the cross-sections were acquired with digital beam control in 64 pixel x 64 pixel arrays, a 2-s acquisition time per pixel, at an excitation potential of 25 kV, and a beam current of 50 nA. All maps were 50 µm x 50 µm. From a Monte Carlo calculation
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