Plasma nitrocarburizing is applied to produce compound layers preferably consisting of ε-Fe_2-3(N,C) carbonitrides which are very favorable for low alloyed steels under tribological or/and corrosive loading. During conventional plasma nitrocarburizing, the plasma discharge is applied directly to the materials surface. In the last 20 years, a new technique was developed where the plasma seam is relocated to an active steel screen surrounding the components to be treated, called active screen plasma nitriding. For both technologies, the present study reports the possibilities and limitations of plasma nitrocarburizing using both gaseous and solid carbon precursors.
For the AISI 4140 (42CrMo4) steel, conventional plasma nitrocarburizing was carried out with the addition of CH₄ or CO₂ to the N₂-H₂ plasma and with variation of the process parameters N₂-H₂ ratio and carbon-bearing gas amount. In comparison, active screen nitrocarburizing with an active screen made of steel was also performed with the addition of CH₄ or CO₂ to the N₂-H₂ plasma. Furthermore, the study investigates the possibilities and limits of active screen plasma nitrocarburizing with an active screen made of carbon fiber-reinforced carbon. The related experiments were perfomed by varying the N₂-H₂ ratio, the plasma power applied to the active screen as well as by adding oxygen-containing carbon-bearing gases.
As a result of the narrow process window of conventional plasma nitrocarburizing, the compound layers produced using carbon-bearing gases show only marginal structural differences and, besides high volume fractions of $\gamma$'-Fe₄N phase, they exhibit only a low volume fraction of ε-Fe_2-3(N,C). In active screen technology using a carbon active screen, within the compound layers an enrichment of up to 2-3 wt.% carbon was reached, and the formation of $\gamma$'-Fe₄N nearly suppressed. By varying the plasma power at the carbon active screen the concentration of reactive species was changed and thus the phase composition and structure of the compound layer was specifically influenced.
The compound layers produced on AISI 4140 were investigated with respect to their structure, composition and corrosion behavior in media containing sodium chloride. In order to classify the technological possibilities, the compound layers generated with a solid carbon source were compared with compound layers generated by using gaseous precursors such as CH₄ and CO₂.