Multilayered ruthenium-modified bond coats for thermal barrier coatings
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AL barrier coatings (TBCs) are an enabling materials technology for advanced gas turbine engines as they expand the operating temperature capability of metallic components[1,2] with attendant benefits to the engine fuel efficiency and environmental impact. A typical TBC system (Figure 1) comprises multiple layers with distinct but complementary functionalities.[3] The Ni-based superalloy component (often an internally cooled turbine blade or vane) is the load-bearing member of the system. Thermal protection is provided by the ‘‘top coat,’’ a 100 to 300 mm layer of 7 wt pct yttria-stabilized zirconia (7YSZ) applied either by electron-beam physical vapor deposition (EB-PVD, as in Figure 1) or atmospheric plasma spray (APS).[2] The coating microstructure is tailored to be ‘‘strain tolerant’’ by promoting columnar grains with open boundaries (EBPVD) or microcracking of splats (APS).[3] Incorporation of porosity during deposition reduces further the inherently low thermal conductivity of the 7YSZ thermal barrier.[4,5,6] Because 7YSZ is an oxygen conductor and contains extensive porosity, oxidation protection must be built in by other means, namely, through a thin, dense, and continuous ‘‘thermally grown’’ aluminum a-Al2O3 oxide layer (TGO in Figure 1).[7] Zirconia generally bonds well to alumina at high temperature, and thus, the TGO also provides a strong foundation for the thermal barrier.[8] Because superalloy compositions are not typically optimized for oxidation resistance, their surfaces are chemically modified to produce a sound, durable TGO.[3,9] This modified metallic layer is known as the ‘‘bond coat’’ (BC) and is generally based on one or more of the intermediate Ni-Al intermetallic phases, most notably b-NiAl (B2) or g9-Ni3Al (L12), with other elements in solid solution.[10–15] The ability of B. TRYON, Research Fellow, Q. FENG, Senior Research Fellow, and T.M. POLLOCK, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] R.G. WELLMAN, Senior Research Fellow, and J.R. NICHOLLS, Professor, are with Cranfield University, Cranfield, Bedford, MK 43 OAL, United Kingdom. K.S. MURPHY, Senior Project Engineer, is with Howmet Research Corporation, Whitehall, MI 49461. J. YANG, Research Specialist, and C.G. LEVI, Professor, are with the Materials Department, University of California, Santa Barbara, CA 93106. Manuscript submitted March 9, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
the BC to support the TGO is sensitive to initial processing procedures and subsequent in-service thermal cycling, because its chemistry constantly changes due to oxidation and interdiffusion with the substrate.[15–22] Of particular interest to this study are the single-phase (B2) Pt-modified NiAl-based BCs that have been shown to outperform conventional (B2) NiAl for high-temperature TBC systems.[23,24] These BCs are fabricated by first plating the Ni-base superalloy with a thin layer of Pt (;5 to 7 mm thick), followed by annealing at high temperatu
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