Spectroscopic Investigations of Plasma Nitriding and Nitrocarburizing Processes Using an Active Screen: A Comparative Plasma Chemical Study of Two Reactor Types

Low-pressure pulsed DC N-2-H-2 plasmas with admixtures of CH4 were investigated in a comparative study in two different types of reactors, (i) a cylindrical industrial scale active screen plasma nitriding (ASPN) reactor and (ii) a laboratory scale plasma nitriding monitoring reactor (PLANIMOR) with a linear electrode configuration. Applying infrared laser absorption spectroscopy (IRLAS) the evolution of the molecular concentrations of four stable molecules, CH4, HCN, NH3, and C2H2 has been monitored. The degree of dissociation of the carbon containing precursor, CH4, varied between 80 and 95%. The concentrations of the molecular reaction products have been found in a range of 10(11) to 10(16) molecules cm(-3). By analyzing the development of the molecular concentrations at changes of the screen plasma power, the gas mixture and the admixture of CH4, a similar behavior of the monitored species depending on the varied parameters has been found in both reactors with NH3 and HCN as the main products of plasma conversion. The fragmentation efficiency of methane has been determined, decreasing from 2.4 to 0.8x10(15) molecules J(-1) with the increasing power of the screen plasmas in both reactors. With the help of optical emission spectroscopy (OES) the rotational temperature of the N-2(+) ions in the plasma could be determined, being in a range of 650 ... 920 K where the rotational temperature of the screen plasma in the ASPN reactor is approximately 100 K higher than that in PLANIMOR. Also with power the ionic component of nitrogen molecules, i. e. the intensity of the N-2(+)-(0-0) band of the first negative system, which has been used as the reference parameter for comparing both reactors, increases strongly in relation to the intensity of the neutral component, represented by the N-2-(0-0) band of the second positive system. In addition, steel samples have been treated in both the ASPN reactor and PLANIMOR leading to comparable nitriding results. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim