Spectroscopic studies of conventional and active screen N₂–H₂plasma nitriding processes with admixtures of CH₄or CO₂

Abstract

Low-pressure pulsed dc H 2 -N 2 plasmas with admixtures of CH 4 or CO 2 used for conventional (CPN) and active screen plasma nitriding (ASPN) were studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were performed in two modes: (i) the plasma at an internal model probe, driven by a bias voltage operated only, representing a CPN approach, and (ii) the screen plasma operated only, which corresponds to an ASPN experiment. Combining in situ tunable diode laser absorption spectroscopy with ex situ Fourier transform infrared spectroscopy the evolution of the concentrations of the methyl radical and of eight stable molecules, C 2 H 2 , CH 4 , C 2 H 4 , CO, CO 2 , NH 3 , HCN and H 2 O was monitored. The degree of dissociation of the carbon-containing precursor molecules varied between 40% and 98%. The methyl radical concentration was found to be in the range 10 11 -10 12 molecules cm −3 . By analysing the development of molecular concentrations with changes in gas mixtures and plasma power values, it was found that (i) C 2 H 2 , HCN and NH 3 were the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH 3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (R F (CH 4 ) ≈ (0.5-4) × 10 16 molecules J −1 , R F (CO 2 ) ≈ (0.5-3.4) × 10 16 molecules J −1 ) and the respective conversion efficiencies to the product molecules (R C (product) ≈10 13 -10 15 molecules J −1 ) were determined for different gas mixtures and plasma power values. With the help of OES the rotational temperature of the screen plasma could be determined, which increased with power from 600 to 850 K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the N + 2 -(0-0)-band, increased strongly in relation to the intensity of the neutral component, represented by the N 2 -(0-2)-band.

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

“…In the screen plasma of both reactors the bands of the N + 2 ion (FNS) and of the N 2 molecule (SPS) are the most intense emission features. In [15] and [17] it was already discussed that the ratio of the heads of these bands is the most sensitive emission feature depending on the plasma parameter. So it is a suitable reference parameter for the comparison of the screen plasma in both reactors.…”

“…1 the experimental arrangement of the ASPN plasma reactor used for nitriding and nitrocarburizing experiments with the TDLAS and the OES diagnostics systems is shown. Only a brief description is given here, further details can be found in [15]. The reactor consists of a cylindrical chamber with an effective volume of about 1 m 3 .…”

“…In addition, based on optical emission spectroscopy (OES), the intensity of the excited molecular nitrogen ions in relation to the excited neutral nitrogen molecules was analysed and the rotational temperature could be determined [15]. This requires advanced non-intrusive in situ plasma diagnostic techniques, which are able to provide quantitative information, in particular about absolute ground state densities of stable and transient molecular species.…”

“…The evolution of the concentration of the methyl radical and of eight stable molecules could be monitored. In addition, based on optical emission spectroscopy (OES), the intensity of the excited molecular nitrogen ions in relation to the excited neutral nitrogen molecules was analysed and the rotational temperature could be determined [15]. To analyze the temporal behavior of general processes and to evidence the working principle of ASPN electrostatic Langmuir probes have been combined with OES in another recent diagnostic study using the same reactor [16].…”

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

“…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].…”