Plasma nitriding of Ti6Al4V alloy and AISI M2 steel substrates using D.C. glow discharges under a triode configuration

Plasma nitriding of different substrates such as Ti6Al4V and AISI M2 steel was achieved by means of D.C. glow discharges assisted by thermionic emission (triode configuration). The higher ionisation levels achieved using a D.C. triode configuration reduced the treatment time and nitriding temperature required to obtain hard and reasonably deep nitrided cases in the Ti6Al4V alloy. For this alloy, surface hardness values of 560–600 HV0.05 and nitrided layer depths of 30–40 μm were achieved at 700 °C. Up to this nitriding temperature, hardening seemed to be mainly accomplished by incorporation of nitrogen in the α-Ti phase, as the formation of a titanium nitride layer on the surface could not be detected by SEM. For the AISI M2 steel, hard (1379–1524 HV0.05) nitrided layers of 100–150 μm were obtained at 480–500 °C for 240 min, depending on total pressure, gas composition and substrate bias. For the highest nitrogen concentration and lowest bias voltage, no compound layer was formed. The triode plasma nitriding process allowed a significant reduction in processing time for both M2 steel and Ti6Al4V alloy in comparison to conventional D.C. diode plasma nitriding.