The importance of characterizing the submicrometrical region of the layer formed on nitrided austenitic stainless steel

Full text: The application of plasma nitriding on austenitic stainless steels can produce a nitrided layer of some micrometers, improving hardness, wear and corrosion resistances, optimizing their use as biomaterials or for industrial applications. Depending on the parameters used in the process, the nitrided layer is formed by the nitrogen diffusion layer called expanded austenite γN, which spreads into inner region. On nitrided layer surface, if besides γN, some iron and chromium nitrides are formed, this region is known as composite layer. There is no consensus about the composite layer formation, mainly because of the characterization limitation, which may mask the presence of such nitrides. The study about some samples, nitrided at 673 K, for 4 h, in 80% H2- 20% N2 gas mixture, at different pressures: 4; 6 and 10 Torr, can illustrate this important question. A recent paper correlated some magnetic properties of their nitrided layer. The results pointed to the formation of a submicrometer layer occurring on the outermost surface of them, which indicated the occurrence of nitrides. In order to do a more accurate characterization, the surface of the non-cross-sectional samples was analyzed by x-ray diffraction (XRD) using a synchrotron source (XRD2-LNLS) with a 7,5 keV beam energy in a grazing-incidence geometry, for different superficial depths, on the top of the layer.The results can identify reliably the presence of the non-stoichiometric Fe_2-3 N for all the samples and Fe₄N for 10 Torr sample and show that these nitrides, decrease rapidly with depth. This contributes to better elucidate this important question in the nitriding process, showing that a carefully characterization, particularly from the top of the nitrided layer, as shallow as possible, is mandatory to understand this system and the use of a synchrotron source, for performing the XRD is essential. (author)