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 per formed at the same substrate temperature (550 degrees C) and bias volt age (500 V), using the following gas mixtures: 8% N-2 in Ar, 8% N-2 in Ne, N-2 + H-2 in equal proportions, and N-2 only. By careful control of the discharge parameters, most of the bombardment energy was transp orted to the substrates by 500 eV ions in all cases; these were mainly Ar+ ions in the N-2 + Ar run and N-2(+) ions in the other runs, notab ly by the action of Penning ionization in the N-2 + Ne run. We found t hat the surface hardness was not significantly influenced by the type of ion delivering the bombardment energy, although we suggest that ion s would need sufficient mass to cause (for example) sputtering, if the substrate is susceptible to contamination during nitriding. Furthermo re, the results showed that there was a strong dependence on the avail ability of nitrogen thermal neutral molecules during processing; we su ggest that there may be a critical value of nitrogen gas density or ra nge of values for effective plasma nitriding. so that a process may be inadequately supplied (resulting in reduced case hardness) or over-su pplied (increasing the significance of white layer formation).