Depth-related microstructure of rf plasma nitrocarburized austenitic stainless steel

The depth dependence of elemental composition, phase distribution, and cross-sectional morphology of rf plasma nitrocarburized 304 austenitic stainless steel were investigated using glow discharge optical spectroscopy (GDOS), grazing incidence X-ray diffraction (GIXRD), and optical microscopy, respectively. A step-wise mechanical polishing method was used to remove successive sublayers of the treated surface. It was found that the properties of the nitrocarburized layer depend critically on the plasma gas composition, which controls the supersaturation of nitrogen and carbon through the compound layer depth. Iron nitride phases and/or nitrogen-expanded austenite (γN) were detected in the nitrocarburized layer prepared at high plasma nitrogen (N2) content. In the compound layer processed at high plasma carbon (C2H2) content, besides the carbon-expanded austenite phase (γC), carbide phases were found predominantly in the top-layer, in which the carbon concentration has a maximum value of ∼2 wt.%. The lattice expansion of the expanded austenite phases changes with sampling depth, depending on local variations in nitrogen and carbon content. The applied rf plasma processing power influences significantly nitrogen and carbon distribution in the treated sublayers.