Effects of sulfur impurity on the scale adhesion behavior of a desulfurized Ni-based superalloy aluminized by chemical v
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I.
INTRODUCTION
STATE-of-the-art thermal barrier coatings (TBCs) consist of a strain-tolerant Y2O3–stabilized-ZrO2 (YSZ) layer prepared by electron beam physical vapor deposition and a metallic bond coat, which provides oxidation protection. The dominant failure mode observed in this particular material system is the progressive fracture along the interface region between the bond coat surface and its thermally-grown oxide scale upon exposure to high-temperature oxidation and thermal fatigue.[1,2,3] From this perspective, improving the scale adhesion behavior of bond coat materials is a critical part of developing more reliable TBCs. It has been well established that the oxidation resistance of Ni-based alloys and bond-coat materials in high-temperature environments is largely dictated by their ability to preferentially form an adherent Al2O3 scale on the metal surface. For example, a common practice, which has been used to extend the oxidation life of Ni-based alloys, is to enrich the alloy surface with a higher Al concentration (26 to 30 wt pct) relative to that in the alloys (5 to 6 wt pct for single-crystal alloys). This type of Al surface enrichment by gas phase pack or chemical vapor deposition (CVD)[4–7] results in the formation of an aluminide phase (i.e., b-NiAl with substrate alloying elements as major impurities) at the alloy surface. The resulting aluminide coating provides a sacrificial Al reservoir and, thus, a longer oxidation life. Eventually, this Al reservoir is depleted by the continued cycle of scale growth and spallation as well as by Al diffusion into the alloy structure. It is important to point out that, in the development of reliable TBCs, this sacrificial W.Y. LEE, formerly a Research Staff Member at Oak Ridge National Laboratory, is with the Department of Materials Science and Engineering, Stevens Institute of Technology, Hoboken, NJ 07030. I.G. WRIGHT, and B.A. PINT are Research Staff Members at the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6156. Y. ZHANG, Ph.D. Student, and P.K. LIAW, Professor, are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200. Manuscript submitted July 24, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
metallic coating concept is no longer applicable, since the failure of the TBCs is dictated by the ability of the coating’s initial scale to maintain a strong bond with its adjacent metal surface and YSZ layer upon oxidation and thermal fatigue. It has been a common industrial practice to incorporate Pt in the aluminide coating structure to improve scale adhesion,[4] although the mechanism by which Pt exerts its beneficial effect is not well understood. Another widely used coating system is vacuum plasma–sprayed NiCoCrAlY, which contains small amounts (less than a few weight percent) of reactive elements such as Y, Hf, Zr, etc.[8] In this coating system, the knowledge of improving scale adhesion through doping reactive elements is utilized to increase the cyclic oxidation performance of the coatin
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