Studies on plasma-nitrided iron by scanning electron microscopy, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy

The tribological and mechanical properties of several iron alloys are improved by nitriding processes. In this work the plasma process was employed because it offers various advantages as compared to other methods used in surface modifications. For example, it is a nonpollutant method and the nitriding times are reduced. Four samples were prepared by nitriding the iron substrates in a gas mixture of 80% of H2 and 20% of N2 under a pressure of 900 Pa, discharge frequency of 10 kHz and temperature of 500 °C, for 1, 2, 4, and 6 h. The samples were analyzed by optical microscopy, scanning electron microscopy (SEM), microhardness technique, glancing angle x-ray diffraction (GXRD), and x-ray photoelectron spectroscopy (XPS). SEM and GXRD results showed the presence of γ′-Fe4N and a small amount of ε-Fe3N phase. XPS was employed in order to obtain the chemical-state and quantitative informations on the plasma-nitrided iron surfaces. The surface N/Fe atomic ratios obtained by XPS were close to the ideal value of 0.25 for Fe4N. The nitrogen concentration close to the surface is lower than that found in the bulk for all samples. This effect is explained by the presence of H2 in the gas mixture, which not only cleans the surface but also removes nitrogen by sputtering.