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

摘要

Polycrystalline iron thin films exhibiting different crystallite sizes, which were deposited on α-Al_2O_3 substrates by molecular beam epitaxy, were nitrided in a NH_3/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 ε-Fe_3N_(1+x) occurs upon nitriding of specimens exhibiting a small crystallite size at values of the thermodynamic nitriding parameters for which pure γ'-Fe_4N_(1－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.