Iron nitrocarburising in flowing post-discharge: Evolution of the compound layer

In this work, the monitoring of mass increase during post-discharge nitrocarburising treatments by sample controlled thermogravimetry (SCTG) is presented. For the first time, a thermobalance is mounted on a post-discharge reactor to study the kinetics of the compound layer growth by following carbon and nitrogen uptakes. Pure iron specimens are successfully nitrocarburised in N2-H2-CH4 post-discharge. The treatments lead to a compound layer composed by a dual phase: a γ′-Fe4(N,C) sublayer beneath a ε-Fe2(N,C)1−x layer. The monitored mass gain exhibits an incubation time due to the nucleation step of only 100 s before a parabolic mass increase. Iron nitrocarbide layers synthesised at 853 K in post-discharge were investigated by optical microscopy, X-ray diffraction (XRD) analysis and Sputtered Neutral Mass Spectroscopy (SNMS). The carbon concentration in the substrate does increase after treatment despite it remains lower than necessary to avoid the growth of the γ′-phase.