Growth Stresses in Thermally Grown Oxides on Nickel-Based Single-Crystal Alloys
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STRUCTURAL materials that operate at high temperatures typically require protective surface layers to inhibit surface degradation in aggressive oxidizing environments. These protective layers may form either by intrinsic oxidation processes, or by application of protective coatings that form scales that are resistant to degradation. In these multilayered systems, differences in the properties of the layers relative to the properties of the substrate result in the development of stresses during layer growth and/or during thermal cycling, due to thermal expansion mismatch with the substrate. These stresses drive a number of different degradation modes, including buckling, delamination, and rumpling, Figure 1.[1–6] Under thermo-mechanical cycling conditions, cracking followed by oxidation of crack faces can further degrade the material.[7–18] The design of coatings and substrates that are resistant to these modes of LUKE H. RETTBERG, Graduate Student, MING Y. HE, Research Scientist, and TRESA M. POLLOCK, Materials Department Chair, are with the Materials Department, University of California Santa Barbara, Santa Barbara. Contact e-mail: rettberg@ umail.ucsb.edu BRITTA LAUX, Development Engineer, formerly with the Materials Department, University of California Santa Barbara, is now with Siemens AG, Berlin, Germany. DAVID HOVIS, Metallurgist, formerly with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, is now with Eaton Corporation, Euclid, OH. ARTHUR H. HEUER, Professor, is with the Department of Materials Science and Engineering, Case Western Reserve University. Manuscript submitted April 13, 2015. Article published online December 28, 2015 1132—VOLUME 47A, MARCH 2016
failure requires a detailed understanding of the mechanism of scale formation, as well as information on the stresses that develop during scale formation and thermal cycling. Al2O3 scale formation via selective oxidation of Al in multicomponent metallic and intermetallic systems is desirable for protection during operation in high-temperature oxidizing environments. The focus of this paper is on Al2O3-forming Ni-based single crystals that operate in a variety of high-temperature environments, including aircraft engines, energy generation systems, nuclear reactors, and chemical processing systems. These systems are typically employed in service with an intermetallic coating to improve oxidation resistance. For higher temperature environments, an additional ceramic thermal barrier coating is often employed.[1,2] During steady-state oxidation at high temperature, new a-Al2O3 forms at the surface of the intermetallic (if coated) or superalloy (if uncoated) by inward diffusion of oxygen and at transverse grain boundaries due to an additional outward counter-flux of Al from the substrate.[19,20] The Al2O3 formed on the grain boundaries is accommodated by lateral straining and creep of the neighboring oxide grains,[4,6,21] with oxide growth stresses that persist during continued growth.[22,23] The magnitude of
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