Nitrocarburizing of stainless steel using low-pressure plasma processes is an established technology. Circumventing substrate bias is possible when using active screens while screens made from carbon avoid gaseous carbon containing precursors. Nevertheless, segregation of carbon and nitrogen is always observed with the carbon containing layer situated near the interface to the substrate and the nitrogen containing layer near the surface. However, this implies a much higher diffusivity of carbon than for nitrogen. For very long process times, the nitrogen containing layer to be transversed by late incorporated carbon is much thicker than at the beginning of the process.

The current investigation does present information on the relative diffusivities of carbon and nitrogen as function of temperature and process time. At 420 °C, for a treatment time of 5 hours, a nearly complete transport of carbon through the growing nitrogen containing phase is still observed. But for 7 hours, the relative carbon content decreases, implying that a finite carbon diffusivity is present, reducing the growth of the carbon containing layer at long times.

Furthermore, depth-resolved data on the phase formation from in-situ XRD investigations during sputter etching reveal a complex, layered structure with the austenite grains differentiating into nitrogen- and carbon-expanded austenite. The ferrite grains of the duplex steel show a strongly disordered structure with faint reflections of disordered ferrite or martensite within the nitrogen-containing layer. At the same time, increasing the treatment temperature will result in the formation of CrN precipitates by the decay of the expanded phases, starting at the surface and growing deeper towards the original substrate. At 460 °C for 5 hours and 420 °C for 7 hours, CrN together with a martensitic Fe-Ni phase is observed. The obtained results give a more detailed insight into the dynamics of nitrocarburizing, allowing further development and easing adaptation in industrial environments.

Financing by Deutsche Forschungsgemeinschaft (DFG, project number 557879687) is gratefully acknowledged by D.M. and A.D.

Dr. Stephan Mändl