Spectroscopic investigations of plasma nitrocarburizing processes using an active screen made of carbon in a model reactor

Abstract

The active screen plasma nitrocarburizing technology significantly reduces the risk of cementite precipitation in the compound layer of nitrocarburized materials and of soot production compared to conventional plasma nitrocarburizing. In a laboratory-scaled plasma nitriding monitoring reactor, PLANIMOR, using an active screen made of carbon, low-pressure pulsed dc N 2 -H 2 plasmas have been studied by infrared laser absorption spectroscopy (IRLAS) techniques. Applying IRLAS, using tunable diode lasers (TDL) and a quantum cascade laser (QCL) as radiation sources, the evolution of the concentrations of eight stable molecular reaction products, C 2 H 2 , C 2 H 4 , CH 4 , HCN, NH 3 , CO, C 2 H 6 , and C 2 N 2 , and of the CH 3 radical, have been monitored. By using the line ratio method, the rotational temperatures of HCN and CO could be determined in a range of 300-400 K and 300-500 K, respectively. Analysing the profile of the CH 3 Q(3-3) absorption line, the gas temperature of this radical, i.e. the temperature in the vicinity of the plasma zone, has been found to range between 400-800 K. The concentrations of the detected molecular reaction products were found to be in the range of 10 12 -10 16 molecules cm −3 with HCN and NH 3 as the most abundant reaction products. Additionally, the respective conversion efficiencies to the product molecules (R C ≈5×10 12 -2×10 16 molecules J −1 ) have been determined for different mixing ratios of N 2 :H 2 in the feed gas and plasma power values. Taking into account the concentrations of all carbon-containing species, a maximum of the carbon combustion of the screen material of up to 96 mg h −1 has been found for a N 2 -H 2 ratio of 1:1 and the highest plasma power of the screen of P screen =106 W.

Influence of the Active Screen Plasma Power during Afterglow Nitrocarburizing on the Surface Modification of AISI 316L

“…As already observed by Puth et al [19] in a model reactor the four species with the highest concentrations (i.e,. HCN, NH 3 , C 2 H 2 , and CH 4 ) produced at the AS showed a dependency on the screen power.…”

“…As already reported by Puth et al [19] for model reactor conditions, the production rate of reactive species formed at a solid carbon source depends on both screen power and feed gas composition. The reported results are consistent with the spectroscopic results that were obtained for an industrial-scale reactor in the present study.…”

“…In previous studies conducted at a similar setup, CH 4 , C 2 H 2 , HCN, and NH 3 were found to be the predominant species in an ASPNC plasma [19,29]. Accordingly, the column-averaged absolute concentrations of the four species were monitored using two spectral windows of an infrared external-cavity quantum cascade laser (EC-QCL) at approximately 1356.3 and 1387.8 cm −1 .…”

“…By use of high-resolution infrared laser absorption spectroscopy (IRLAS), such as quantum cascade laser absorption spectroscopy (QCLAS), quantitative analysis of the gas composition can be conducted [16][17][18]. Previous studies by IRLAS have shown that the use of an AS made of CFC in an ASPNC reactor together with N 2 and H 2 as precursor gases results in a highly reactive process gas containing different saturated (CH 4 , C 2 H 6 ,) and unsaturated hydrocarbons (C 2 H 2 , C 2 H 4 ), radicals (CH 3 ), ammonia (NH 3 ), cyanides (HCN, C 2 N 2 ), and carbon monoxide (CO) [19]. By the variation of the process parameters (e.g., flow rate, N 2 -to-H 2 ratio in the precursor gas, and pressure), the concentrations of reactive species can be changed [20].…”

“…Using IRLAS measurements in a model reactor for ASPNC processes, Puth et al [19] showed that the production rate of reactive species can be controlled not only by the composition of the precursor gas but, in particular, also by the screen power. By monitoring the absolute concentrations of reactive species, they showed the dependence of the concentrations of reactive species on the screen power applied to a perforated CFC plate.…”