Development of Intermixed Zones of Alumina/Zirconia in Thermal Barrier Coating Systems
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INTRODUCTION
THE durability of thermal barrier coating (TBC) systems depends on the characteristics of the interfacial regions associated with the thermally grown oxide (TGO), which forms on the metallic bond coat during fabrication and service. In some cases, failures occur at the TGO/bond coat interface, in others at the TGO/ TBC interface, and in some cases mixed failures are observed. The durability of the interfaces is determined by the interplay of a number of factors that include the microstructure of the as-deposited TBC, the composition and morphology of the TGO, the composition and strength of the bond coat, and the test conditions. This article describes the development of the interfacial structures at the TGO/TBC interface for bond coats for which the TGO consists primarily of alumina. Particular emphasis is placed on the conditions under which an intermixed layer of alumina and zirconia can develop. Stiger et al.[1,2] first identified the intermixed zone on a platinum-modified aluminide bond coat on a Rene´ N5 substrate coated with standard 7 wt pct yttria-stabilized M.J. STIGER, R.W. JACKSON, and S.J. LANEY, Graduate Students, N.M. YANAR, Research Assistant Professor, and F.S. PETTIT and G.H. MEIER, Professors, are with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA. Contact e-mail: rwesleyjackson@ gmail.com A.S. GANDHI, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Chennai 600036, India. C.G. LEVI, Professor, is with the Materials Department, University of California–Santa Barbara, Santa Barbara, CA 93106-5050, USA. Manuscript submitted February 14, 2006. Article published online May 10, 2007. 848—VOLUME 38A, APRIL 2007
zirconia (7YSZ) (Figure 1). This coating, which was a commercially processed system with a grit-blasted bond coat and the TBC deposited by electron beam physical vapor deposition (EBPVD), was found to have a thin TGO (100-nm thick) in the as-processed condition. Cross-section transmission electron microscopy (XTEM) with selected area diffraction (SAD) indicated the TGO in the as-processed condition was a-Al2O3. The TGO, after an exposure of 10 hours at 1200 C in air, developed a columnar inner zone and an intermixed outer zone (Figure 1(a)), which contained precipitates. The entire TGO was determined by SAD to be a-Al2O3 and the precipitates were found by scanning TEM to consist of Zr, Y, and O. Figure 1(b) presents an XTEM micrograph of the intermixed zone showing that, for these exposure conditions, it consists of a matrix of aAl2O3 that contains particles of the oxides of Zr and Y. It was proposed that the alumina was growing partially outward into the YSZ during exposure and spherodizing entrapped YSZ particles. Subsequently, Brickey and Lee[3] studied a commercially fabricated TBC presumably with a similar processing history to that studied by Stiger et al. It was found by TEM and SAD that the TGO on the asprocessed coating was a-Al2O3 but that it al
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