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.
In a distributed antenna array reactor, microwave H2-CH4-CO2 plasmas with admixture of N2 used for the low-temperature deposition of nanocrystalline diamond (NCD) films are studied by in situ infrared laser absorption spectroscopy (LAS) and optical emission spectroscopy techniques. The experiments are carried out in order to analyze the dependence of temperatures and species densities as a function of the admixture of nitrogen. The evolution of the concentrations of the methyl radical (CH3), and of five stable molecules (NH3, HCN, CH4, C2H2, and CO), are monitored in the plasma processes by LAS using tunable lead salt diode lasers and external-cavity quantum cascade lasers (EC-QCL) as radiation sources. OES is performed simultaneously to obtain complementary information about (i) the degree of dissociation of H2 precursor gas, (ii) the gas temperature and therefore (iii) the density of atomic hydrogen, a key species in the chemistry of NCD deposition plasmas. The species temperatures are not significantly affected by the nitrogen addition. The concentrations of the various species are in the range between 1011 to 1015 molecules cm−3. HCN and CO are the major products in the plasma besides atomic hydrogen. The analysis of the nitrogen and carbon mass balances of the measured species shows that in addition to NH3 and HCN other nitrogen containing species are produced in the plasma which were not probed. It is shown that the formation of HCN consumes C atoms that can be provided from hydrocarbon species and from the deposition of carbon-containing films on the reactor walls, which results in a decrease of the measured densities of hydrocarbon species.
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.
The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process. Laser absorption spectroscopy was used to determine the resulting process gas composition at different levels of oxygen admixture using O2 and CO2, respectively, as well as the long-term trends of the concentration of major reaction products over the duration of a material treatment of ARMCO® iron. The short-term studies of the resulting process gas composition, as a function of oxygen addition to the process feed gases N2 and H2, showed that a stepwise increase in oxygen addition led to the formation of oxygen-containing species, such as CO, CO2, and H2O, and to a significant decrease in the concentrations of hydrocarbons and HCN. Despite increased oxygen concentration within the process gas, no oxygen enrichment was observed in the compound layer of ARMCO® iron; however, the diffusion depth of nitrogen and carbon increased significantly. Increasing the local nitrogen concentration changed the stoichiometry of the ε-Fe3(N,C)1+x phase in the compound layer and opens up additional degrees of freedom for improved process control.
Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.
The work is devoted to the development of laser absorption spectroscopy (LAS) of plasma-assisted processes for application under industrial conditions. The interpretation of the LAS measurements was revised by taking into consideration the temperature gradient along the absorption path, which is unavoidable in a reactor for thermochemical treatment. The revision is based on the measurement of HCN, NH3, H2O and CO molecular lines in an industrial-scale, active screen plasma nitrocarburizing (ASPNC) reactor with a steel active screen (AS). It shows that an effective temperature determined from Doppler broadening could be assigned to each measured spectral line. The effective temperature does not only reflect the temperature gradients along the line-of-sight but also the line strength dependence on temperature for the specific spectroscopic transition. Lower limit estimates of the molecular densities are proposed based on the determined effective temperatures under the assumption of a Boltzmann distribution of the population density over the molecular levels at any local volume of the reactor. For a more accurate interpretation of LAS data of plasma-assisted processes, the spatial distribution of the temperature along line-of-sight has to be known and needs to be taken into account to obtain the molecular densities.
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