Ni 3 N compound layers produced by gaseous nitriding of nickel substrates; layer growth, macrostresses and intrinsic ela
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Franziska Lienert Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany
Shun Li Shang and Zi-Kui Liu Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802
Eric Jan Mittemeijer Max Planck Institute for Intelligent Systems, D-70569 Stuttgart, Germany; and Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany (Received 18 September 2011; accepted 24 January 2012)
Ni3N was prepared by gaseous nitriding of nickel substrates using gas mixtures of high nitrogen activities, composed of NH3 and H2 at 1 atm and at temperatures between 175 °C and 550 °C. At least above 300 °C closed Ni3N layers developed, which possess distinct compressive macrostrain parallel to the surface. The observed hkl-anisotropy of the macrostrain could be ascribed to the elastic anisotropy as indicated by the single-crystal elastic constants of Ni3N obtained from firstprinciples calculations performed in this work. The macrostress originates from the thermal misfit between layer and substrate, developing upon cooling. The extent of macrostress is reduced by partial misfit accommodation by plastic deformation as well as by porosity.
I. INTRODUCTION
Among the nitrides of iron (Fe), cobalt (Co) and nickel (Ni), the iron nitrides have attracted by far the largest scientific interest due to a myriad of already realized and potential, future applications, e.g., as hard1 and ferromagnetic2 materials. Relatively little information is available on nickel and cobalt nitrides. In any case, as already apparent from the earliest preparative studies by Juza,3,4 the nickel and cobalt nitrides are less stable than the iron nitrides, and consequently more difficult to synthesize. This is indicated by the relatively narrow parameter windows for temperature and gas-phase composition pertaining to the gas treatments for preparation of pure nickel and cobalt nitride (powder) specimens3,4; in particular application of very high NH3 contents in the atmosphere is necessary. In recent years, research on nickel nitrides has been devoted to the development of various “chemical” preparation methods, especially for Ni3N.5–10 The production of thin nickel-nitride layers by deposition techniques, in particular by reactive sputtering and investigation of these layers has been considered as well.11–15 Notwithstanding this intense research activity, lack of fundamental knowledge on the intrinsic properties of the Ni-N system and the associated nitrides is evident. In particular, phase-diagram a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.40 J. Mater. Res., Vol. 27, No. 11, Jun 14, 2012
information is scarce16; a calculation of phase diagram (CALPHAD) assessment thereof has only been possible on the basis of estimation of many important thermodynamic quantities.17 The available knowledge on the Ni-N system can be summarized as follows: the existence of the following Ni-N phases has been shown16: Ni4N-I and Ni4N-II
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