Fabrication and characterization of austenitic stainless steel : nitrogen films

Abstract

In the present work, the Fe-N based film systems fabricated by reactive magnetron sputtering were investigated, with particular emphasis on the stainless steelnitrogen (SS-N) system. For the purpose of understanding the phase evolution and various structural and compositional features of the films as a function of nitrogen doping and deposition parameters, a series of SS-N films were prepared under various deposition conditions. Structural characterization was conducted by a variety of experimental and analytical techniques, including grazing angle incidence X-ray diffraction, secondary electron imaging, energy dispersive spectroscopy, high resolution field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The mechanical properties of the films were evaluated by nanoindentation and microscratch tests. In addition, the room temperature magnetization behavior of the films was measured by the vibrating sample magnetometry technique. These efforts have facilitated a better understanding of the phase evolution and structural and properties characteristics of the resultant films. It was found that although sputtering of the austenitic stainless steel target with face centred cubic structure in pure argon atmosphere resulted in the formation of a ferrite film with body centred cubic structure, doping of the film with nitrogen by sputtering in argon and nitrogen gas mixtures led to a series of structural changes in the film. With increasing amount of nitrogen doping, the crystalline structure of the films changed from bcc ferrite (α), to a mixture of bcc ferrite and expanded fcc austenite (γ N) supersaturated with nitrogen, and to the MN (metal nitride) type nitride phase. Subtle differences in phase evolution depended on % N 2 , power density and the substrate temperature. This MN type phase, where M represents all the metallic elements in the stainless steel, i.e. M=Fe+Cr+Ni+Mo, has a face-centered cubic structure and equiatomic (M/N=l) stoichiometry. Further studies were conducted to investigate the influence of various deposition parameters such as nitrogen gas composition in the sputtering gas mixture, vii