Plasma nitrocarburizing based on active screen technology using a carbon-fiber reinforced carbon (CFC) active screen was applied in an industrial-scale unit for thermochemical surface treatment of austenitic stainless steel. This concept is based on the use of a solid-carbon-source for the generation of highly reactive gases directly in the active screen plasma. Plasma nitrocarburizing of AISI 316L stainless steel was performed with 50% H₂ and 50% N₂ at a pressure of 3 mbar without the use of any additional carbon bearing gas.
For these specific experiments, a plasma discharged CFC active screen, paired with a substrate holder at floating potential was used. Thus, no charged particles are in the vicinity of the sample surface, and sputtering of the surface is avoided. Only neutral atoms, molecules and radicals are contributing to the insertion of carbon and nitrogen into the stainless steel surface. For a temperature of 460 °C and duration of 300 minutes, a near-surface layer of 7 µm containing up to 25 at.% nitrogen and an underlying layer containing up to 10 at.% carbon were detected. This result is in agreement with conventional nitrocarburizing treatments indicating that ions are not required for the thermochemical modification of this steel.
At the same time, a rather homogeneous surface coverage with a carbon-containing phase was observed. A combination of scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS) results indicate that about 25 – 50 nm of carbon layer (possibly with some minor metal contamination) is found on the surface of the nitrocarburized stainless steel. A more detailed investigation of the SIMS results shows that inside the original steel surface, a significant segregation of Cr near the steel surface within a depth interval of 50 nm is found, followed by a strong segregation of Mn (again about 50 nm thick).
When looking at the time evolution of this effect, starting with an effective nitriding time of 0 (only heating to 460 °C and cooling back to room temperature), 60 and 180 minutes, a progressive formation of the carbon-rich surface structure and the segregation is observed in parallel. It is hypothesized that this surface segregation of specific alloying elements is supporting the agglomeration of the carbon-rich phase – which in turn increases the uptake of carbon and nitrogen from the gas phase – acting similarly to a catalyst.