Unexpected formation of ε iron nitride by gas nitriding of nanocrystalline α-Fe films

Polycrystalline iron thin films exhibiting different crystallite sizes, which were deposited on alpha-Al2O3 substrates by molecular beam epitaxy, were nitrided in a NH3/H-2 gas mixture. After different nitriding treatments the specimens were prepared and analyzed by focused ion beam microscopy and x-ray diffraction. It was found that formation of epsilon-Fe3N1+x occurs upon nitriding of specimens exhibiting a small crystallite size at values of the thermodynamic nitriding parameters for which pure gamma(')-Fe4N1-x is predicted to form according to bulk thermodynamics. This unexpected phenomenon is explained as a consequence of the nanocrystalline nature on the thermodynamics of the binary system iron-nitrogen. (C) 2007 American Institute of Physics.