Characterization of plasma-nitrided iron by XRD, SEM and XPS

The plasma nitriding technique has been used to improve the tribological and mechanical properties of materials, especially iron-based alloys. In this work, the pulsed glow discharge (PGD) technique was used for nitriding pure iron. Three samples were nitrided in a gas mixture of 80 vol.% H2 and 20 vol.% N2 under a pressure of 400 Pa, discharge frequency of 9 kHz, and temperature of 580 °C. Samples A, B and C were nitrided for 30 min, 60 min and 90 min, respectively. The nitrided iron samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). SEM micrographs showed differences in the surface morphologies among the samples. XRD identified the γ′-Fe4N and ε-Fe2–3N phases in all samples. The Rietveld analysis was performed in order to investigate the concentration variations of the phases as a function of nitriding time. XPS was employed to obtain chemical-state and semi-quantitative information, and revealed a thin film of Fe2O3 covering a Fe3O4 layer formed on top of the Fe2–3N and Fe4N layers.