Microstructure and Thermo-Mechanical Behavior of NiAl Coatings
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Microstructure and Thermo-Mechanical Behavior of NiAl Coatings G. Dehm1, J. Riethmüller1, P. Wellner1, O. Kraft2, H. Clemens3, E. Arzt1 1
Max-Planck-Institut für Metallforschung, 70569 Stuttgart, Germany Institut für Materialforschung II, Forschungszentrum Karlsruhe and Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe (TH), 76344 Karlsruhe, Germany 3 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, 8700 Leoben, Austria 2
ABSTRACT In this study the thermo-mechanical behavior of a commercial Pt containing NiAl coating deposited on a Ni-base superalloy is compared with a Ni-rich NiAl coating sputter-deposited on a Si substrate. Both types of coatings possess high tensile room temperature stresses after thermal straining. The Pt-NiAl coating shows negligible plasticity as a result of solid solution and dispersion strengthening. In contrast, for the NiAl coatings on Si noticeable plasticity can be obtained if the film thickness exceeds the sub-micrometer range.
INTRODUCTION NiAl is applied as a protective high temperature coating for turbine blades used in engines [1]. In order to improve the oxidation resistance commercial NiAl coatings contain several at% Pt. Pt substitutes the Ni atoms of the ordered β-NiAl phase and suppresses the formation of ternary oxides which are less stable than a dense Al2O3 scale [2-4]. While the mechanical behavior of bulk NiAl has been extensively studied over the last decades little information is known about the mechanical behaviour of NiAl-based coatings. In this paper we report on thermal straining results of a commercial Pt containing NiAl coating deposited on a Ni-base superalloy. Since the microstructure of commercial coatings strongly depends on the processing route and chemical composition of the Ni-base superalloy additional straining experiments were performed on a model system. The model system consists of NiAl coatings sputter deposited onto a single crystal Si substrate with an amorphous diffusion barrier between the coating and the substrate to prevent interdiffusion. The deformation behavior of the coatings is characterized on the basis of stress-temperature cycles and correlated with microstructural and dimensional effects.
NiAl DEPOSITION AND STRESS MEASUREMENT Industrial NiAl coatings containing approximately 7 at% Pt were grown on (100) oriented Ni-base superalloy single crystals in a two step process. Initially, a several µm thick Pt layer was electro-deposited on the substrate and annealed at elevated temperatures in order to stimulate NiPt interdiffusion. Subsequently, Al was deposited by chemical vapour deposition. The Al diffuses into the Ni-Pt layer and forms a Pt alloyed NiAl coating. The commercial Ni-base superalloy substrate contained additions of Cr, Mo, W, Co, Ta, and Re.
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For microstructural and mechanical comparison with the industrial Pt-NiAl coatings pure NiAl films were grown as a model system by magnetron-sputtering at nominally room temperature. (100) oriented single-c
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