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Introduction The widespread utilization of ceramic thermal-barrier coatings (TBCs) in both energy and propulsion systems has, to a large extent, been enabled by the development of advanced deposition technologies. The refractory nature of TBC materials such as yttria partially stabilized zirconia (YSZ) with melting points approaching or in excess of 3000 K requires ultrahigh temperature materials processing capabilities. Hence, thermal plasmas and electron beam sources have become primary and preferred methods of manufacturing. The former involves melt fabrication of powdered ceramics, while the latter is based on evaporation and vapor deposition from ceramic ingots. A remarkable attribute of this process development is the scale of implementation of such advanced materials. According to various industrial sources, some 1–1.5 million kilograms of YSZ was atmospheric plasma sprayed (APS) onto engine components in 2011 alone. Aero-engine components benefiting from APS TBCs include combustors, vanes, and turbine shrouds, while TBCs are plasma sprayed onto both rotating and stationary parts of large land-based power-generation engines. On the other hand, virtually all hot section rotating turbine blades of aircraft-engines contain TBCs deposited

via electron-beam physical vapor deposition (EBPVD) processes. These applications are expected to grow in the next two decades, especially as the importance of TBCs continues to grow due to ever increasing demands for fuel efficiency. A critical aspect of ceramic TBCs, in addition to the material, is the coating defect architecture facilitated by processing. Both APS and EBPVD TBCs are comprised of some 10–30% porosity, which reduces the already low thermal conductivity of YSZ by an additional 100–150%.1,2 EBPVD coatings display conductivities from 45 to 65% of bulk values, depending on process conditions, while as-deposited APS coatings can show properties as low as 20% of bulk values. This substantial reduction in thermal conductivity is attributed to the assortment of deposition-induced defects in these coatings, including pores of various sizes and morphologies, as well as a myriad array of interfaces of different character and length scales. Figure 1 shows exemplary scanning electron micrographs of these coatings identifying the nature and dimensions of these defects. Given the scale of reduction of thermal properties due to these defects, their manipulation and control via processing have been an important research endeavor. In recent years,

Sanjay Sampath, Center for Thermal Spray Research, Department of Materials Science and Engineering, Stony Brook University; [email protected] Uwe Schulz, German Aerospace Center, Institute of Materials Research, Germany; [email protected] Maria Ophelia Jarligo, Institute of Energy and Climate Research (IEK-1), Forschungszentrum Jülich GmbH, Germany; [email protected] Seiji Kuroda, National Institute for Materials Science, Japan; [email protected] DOI: 10.1557/mrs.2012.233

© 2012 Materials Research Society

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