The Role of Chemical Composition for the Ductility and Microstructure of Thin NiAl Films
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The Role of Chemical Composition for the Ductility and Microstructure of Thin NiAl Films Patrick Wellner, Oliver Kraft and Eduard Arzt Max-Planck-Institut für Metallforschung, Stuttgart, Germany
ABSTRACT In this work, NiAl films deposited by magnetron co-sputtering onto silicon substrates with compositions between 45.0 and 51.5 at-% Al and a thickness of 1.2 µm were studied. The films exhibited grain sizes comparable to the film thickness. Furthermore, the films were found to have fiber textures with the strongest component being (110) for the very Ni-rich and (111) for the stoichiometric and Al-rich films. The residual stresses in the Ni-rich and stoichiometric films after annealing were found to be independent of composition, while the Al-rich films showed a significant decrease in film stress. This stress drop can be related to severe cracking in the Alrich films, which has not been observed in the stoichiometric and Ni-rich films, indicating that the fracture toughness depends on composition. Based on a simple estimate, the fracture toughness of these Al-rich films was determined to be in the range of 1 to 2 MPa m-1/2, which is somewhat smaller than commonly reported bulk values for polycrystalline NiAl.
INTRODUCTION NiAl films can be used as a protective coating for structural components against oxidation and corrosion at high temperatures, e.g. as a coating for turbine blades. The intermetallic phase ßNiAl exhibits many advantageous properties, such as high melting point, high thermal and electrical conductivity and excellent oxidation resistance [1]. However, the well known brittleness below the brittle-ductile transition of about 450 °C is a major drawback for the application of NiAl as a structural material. Although the brittleness of bulk NiAl has been thoroughly investigated, little is known about the ductility of NiAl thin films. With respect to the application as a coating material, however, knowledge of the ductility of NiAl films is extremely important in order to guarantee the integrity of NiAl coatings during temperature changes. In order to perform a general study on the ductility of NiAl films, different compositions of NiAl were deposited on Si substrates, and the influence of composition on film microstructure and ductility was investigated.
EXPERIMENTAL NiAl films with a composition in the range of 45.0 to 51.5 at-% Al were deposited by magnetron co-sputtering on Si substrates. The Si substrates were additionally coated with 50 nm thick layers of amorphous SiOx and amorphous SiNy as diffusion barrier between the Si and the NiAl film. The thickness of the films was 1.2 ± 0.1 µm for each composition. Prior to deposition, the bare substrates were heated at 120 °C for 2 hours in a pre-vacuum chamber (base pressure 1x10-7 mbar) and cleaned in the deposition chamber (base pressure 5x10-10 mbar) by ion bombardment using a Kauffmann ion source. The deposition was performed without additional L9.6.1
heating with a Ni power of 150 W and an Al power in the range of 138 to 178 W. After deposition, th
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