Microstructural characterization of electron beam-physical vapor deposition thermal barrier coatings through high-resolu
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12/29/04
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Microstructural Characterization of Electron Beam–Physical Vapor Deposition Thermal Barrier Coatings through High-Resolution Computed Microtomography ANAND KULKARNI, HERBERT HERMAN, FRANCESCO DeCARLO, and RAMESH SUBRAMANIAN Thermal barrier coatings (TBCs), deposited using the electron beam–physical vapor deposition (EB-PVD) process, comprise a unique architecture of porosity capable of bridging the technological gap between insulation/life extension and prime reliance. The TBC microstructures consist of columnar structure, nucleated via vapor condensation, along with a high degree of intercolumnar porosity, thus providing enhanced stress relief on thermomechanical loading and also accommodating misfit stresses resulting from CTE mismatch. In this article, we report the characterization of these coatings using high-resolution synchrotron-based X-ray computed microtomography (XMT) at 1.3-m resolution. Experiments focused on quantitative characterization/visualization of imperfections in these coatings and on the relative changes in microstructural features upon isothermal annealing. The influence of time/temperature of exposure was investigated and the results were correlated with elastic modulus.
I. INTRODUCTION
YTTRIA-STABILIZED zirconia (YSZ) coating systems are widely used for the thermal, oxidation, and hot corrosion protection of high-temperature components in gas turbines and diesel engines.[1,2] These coatings allow increased operating temperature of the engine and therefore enhanced efficiency, increased durability, and extended life of metallic components subjected to high temperatures and high stresses.[3,4] Also achieved are reduced cooling requirements to metallic components at the high operating temperature. These YSZ topcoats are deposited using atmospheric plasma spray (APS) or electron beam–physical vapor deposition (EB-PVD) processes, each producing distinctively different microstructures.[5,6] The EB-PVD process offers the opportunity to generate coatings with columnar microstructures with intercolumnar spacing across the thickness of the coating, thus providing an inherently superior strain tolerance and thermal shock resistance. Significant lifetime enhancements are thus achieved.[7] Other advantages include increased erosion resistance and good surface finish, required for aerodynamic efficiency.[8] Moreover, this process has the advantage of minimizing the closure of cooling holes during deposition. One drawback, undesirable to applications as a thermal barrier coating (TBC) and resulting from the columnar structure, is high thermal conductivity as compared with APS coatings. This is a significant factor controlling coating behavior in-service. In EB-PVD, crystal nuclei are formed on favored sites upon vapor condensation, generated by heating the source ingot material with an elecANAND KULKARNI, Postdoctoral Associate, and HERBERT HERMAN, Distinguished Professor, are with the Department of Materials Science and Engineering, State University of New York, Stony Br
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