Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part II.: Impact of the carbon and water contaminants on a low-pressure N 2 –H 2 discharge

The density evolution of H C N , N H 3 , H 2 O and C O molecules over time was monitored by laser absorption spectroscopy in a low pressure DC pulsed discharge in N 2 –H 2 gas mixtures with addition of CH 4 or O 2 . The discharge was maintained in an industrial-scale, active screen (AS) plasma nitrocarburizing (ASPNC) reactor with a steel AS. The measured species densities were analysed using a simplified kinetic model that includes three characteristic times for chemical processes in the ASPNC reactor. The shortest time (1–4 min) was associated with the gas residence time in the reactor, the middle one (about 20 min) was assigned to surface reactions on the AS and workload, whereas the largest one (about 3–5 h) was assigned to surface reactions on the cold reactor walls. The work highlights the importance of monitoring the gas composition during plasma nitrocarburizing processes in order to maintain defined treatment conditions and compensate for continuously changing chemical kinetics at the internal reactor surfaces.