Modern methods of material production, such as additive manufacturing, as well as material surface modification by means of high-energy thermal radiation, such as surface texturing or structuring, are excellently suited to meet the requirements of innovative material design, lightweight construction and surface functionalization. The applicability of surface hardening processes, such as thermochemical heat treatment to further improve surface properties in terms of hardness and wear behavior, is still largely unknown for the aforementioned new materials and surface conditions.
The technology of active screen plasma nitriding using an active screen made of carbon shows, especially for stainless steels, a more efficient nitrogen and carbon uptake as well as geometry-independent and contour-true treatment results. Therefore, this process is suitable for the application of complex shaped components or profiled surfaces. During the thermochemical heat treatment of austenitic stainless steels, so-called expanded austenite is formed as a result of the diffusion of nitrogen and/or carbon into the surface, leading to a significant increase in hardness and the formation of a wear-resistant surface layer.
On the example of selected austenitic stainless steels, the present study shows the potentials and the variety in the application of an active screen plasma nitriding/nitrocarburizing process (i) on substrates produced by selective laser beam melting and (ii) on electron beam modified surfaces. In both cases, the interaction of the substrate with thermal radiation to affect the substrate microstructure occurs prior to thermo-chemical surface treatment.
For additively manufactured specimens produced by selective laser beam melting it is shown that the surfaces can be homogeneously nitrided both in the finished state and in the as-built state. In addition, thicker layers of expanded austenite are produced compared to conventional plasma nitriding. For electron beam treatment, both the example of a uniformly remelted surface area and the profiling with a high aspect ratio are used to demonstrate the surface engineering possibilities offered by the application of active screen plasma nitriding in order to vary the nitrogen/carbon element depth profiles and corresponding properties. Material specific results are presented including X-ray diffraction analysis, electron probe microanalysis, scanning electron microscopy and complementary hardness measurements.