The influence of process gas characteristics on the properties of plasma nitrided steel

This study attempts to elucidate some of the effects of adding argon, neon and hydrogen to low pressure thermionically supported discharges used for plasma nitriding AISI M2 steel substrates. Four runs were performed at the same substrate temperature (550 °C) and bias voltage (500 V), using the following gas mixtures: 8% N₂ in Ar, 8% N₂ in Ne, N₂ + H₂ in equal proportions, and N₂ only. By careful control of the discharge parameters, most of the bombardment energy was transported to the substrates by 500 eV ions in all cases; these were mainly Ar⁺ ions in the N₂ + Ar run and N₂⁺ ions in the other runs, notably by the action of Penning ionization in the N₂ + Ne run. We found that the surface hardness was not significantly influenced by the type of ion delivering the bombardment energy, although we suggest that ions would need sufficient mass to cause (for example) sputtering, if the substrate is susceptible to contamination during nitriding. Furthermore, the results showed that there was a strong dependence on the availability of nitrogen thermal neutral molecules during processing; we suggest that there may be a critical value of nitrogen gas density or range of values for effective plasma nitriding, so that a process may be inadequately supplied (resulting in reduced case hardness) or over-supplied (increasing the significance of white layer formation).