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.
The feasibility of a novel approach of plasma nitrocarburizing with active screen (AS) made of carbon-fiber reinforced carbon (CFC) has been demonstrated in a comparative study of two types of AS material, steel and CFC, under similar process conditions. Besides the excellent thermo-mechanical properties of the CFC material against the conventional steel, an uncontrollable sputter deposition of the steel screen material during the nitriding can be avoided by the use of the CFC material. Furthermore, a solid-carbon-source concept based on the generation of highly reactive precursor gases, in particular of the unsaturated hydrocarbons HCN and C2H2 directly in the process, avoiding external supply of gaseous hydrocarbons, has been realized in the plasma-enhanced thermochemical treatment. The high nitriding effect of the process atmosphere obtained with the CFC AS yielded a significant improvement of nitriding results for different treated steels.
The active screen plasma nitrocarburizing technology is an improvement of conventional plasma nitrocarburizing by providing a homogeneous temperature distribution within the workload and reducing soot formation. In this study, an industrial-scale active screen (AS) made of carbon-fibre-reinforced carbon serves as the cathode as well as the carbon source for the plasma-chemical processes taking place. The pulsed dc discharge was maintained at a few mbar of pressure while simultaneously being fed with a mixed gas flow of hydrogen and nitrogen ranging from 10 to 100 slh. Using in situ infrared laser absorption spectroscopy with lead salt tuneable diode lasers and external-cavity quantum cascade lasers, the temperatures and concentrations of HCN, NH3, CH4, C2H2, and CO have been monitored as a function of pressure and total gas flow. To simulate industrial treatment conditions the temperature of the sample workload in the centre of the reactor volume was kept at 773 K by varying the plasma power at the AS between 6 and 8.5 kW. The resulting spectroscopically measured temperatures in the plasma agreed well with this value. Concentrations of the various species ranged from 6 × 1013 to 1 × 1016 cm−3 with HCN being the most abundant species.
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