Investigation of the ASPN process of low alloy steel by using Ni or Cr coated active screens

The mechanism of active screen plasma nitriding (ASPN) and the influence of the active screen's coating material on the surface properties of the nitrided layer was investigated on low alloyed steel samples, using chromium or nickel-coated active screens. ASPN experiments were performed at 510 degrees C, for 5 h in a 75% N-2 + 25% H-2 gas mixture using tempered 42CrMo4 type low alloy steel. The base material of the screens was unalloyed steel, but the screens were electroplated with chromium or nickel. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), atomic force microscopy (AFM), and secondary neutral mass spectrometry (SNMS) were used for the characterization of surface properties of the samples and the screen. SNMS results show that the coating material of the active screen (chromium or nickel) was deposited on the surface of the samples in the form of compounds, with layer thicknesses between 160 and 200 nm (chromium) and 290-420 nm (nickel). Chemical analysis by X-ray photoelectron spectroscopy (XPS), performed at the interface of the steel sample and the deposited compounds reveals that iron-nitride was only formed in the case of samples that were treated with a Cr-coated active screen. Here the decomposition and oxidation of the unstable chromium-nitride provided the nitrogen atoms and the chemical drive for the formation of iron-nitride. In the case of the Ni-coated active screen, only molecular nitrogen was observed in the treated steel samples. The results prove that - contrary to widespread models on ASPN nitriding mechanisms - physically adsorbed nitrogen plays little role in the nitriding process of iron.