Use of a solid carbon precursor for DC plasma nitrocarburizing of AISI 4140 steel

A new surface treatment concept of solid carbon source plasma nitrocarburizing (SCS-PNC) was proposed. Carbon fiber carbon composite (CFC) was used as a solid carbon precursor for conventional DC plasma nitriding process in a nitrogen-hydrogen plasma. The effect of plasma power on the phase composition of the compound layer on AISI 4140 steel was investigated. The results of the SCS-PNC process were compared to conventional DC plasma nitriding (PN) and plasma nitrocarburizing (PNC) using methane as carbon-bearing gas. The SCS-PNC process provides increasing carbon concentration within the compound layer with increasing plasma power density. In the compound layer a constant carbon content of 1.5 wt.-% C was obtained for high plasma power densities and the ε-carbonitride Fe2-3(C,N)x formation was significantly promoted reducing the porous seam zone. Moreover, precipitation of unwanted cementite Fe3C in the compound layer was effectively suppressed for SCS-PNC compared to treatment by PNC with methane addition.