The influence of the N2–H2 mixture composition on the spectroscopic and temporal behaviour of glow discharge characteristics in pulse-supplied nitriding processes

The purpose of the research was to develop experimental basis for monitoring and real-time control of the plasma nitriding process performed at DC-pulsed substrate biasing. For this purpose correlation between I–U characteristics of pulsed glow discharge (f=2 kHz, Δ=95%) and the temporal evolution of active elements concentration in the generated plasma had to be determined. Experiments were carried out in an industrial arc-vacuum device adapted to execution of continuous ‘duplex’ surface treatment processes. Investigated processes of plasma nitriding were executed at constant pressure (3 mbar) of N2–H2 atmosphere within a wide range of N2 content (from 5 to 100%). Current and voltage of the discharge were automatically adjusted by the control system in such a way that the substrate temperature was kept constant at chosen value of 520 °C, enabling, depending on the atmosphere composition, creation of nitrided layers of different structures: from single-phase layer, containing diffusion zone α-Fe(N) only, to multi-phase one containing both diffusion zone α-Fe(N) and compound zone ε+γ′, in the substrates made of hot working steel 1.2367. Optical emission spectra of generated plasma as well as temporal courses of selected emitters intensities (N2(C), N2+(B), Hα) were recorded in the vicinity of the substrate. On the basis of the obtained results, the correlation between power supply parameters, gas composition and temporal evolution of the relative concentrations of the selected active species was determined. It was shown that the kinetics of the investigated emitters is strongly controlled by electrons. Three stable optical signals, suitable for monitoring of the nitriding process were extracted from the plasma spectrum.