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High density, inductively coupled RF plasma source and its application in nitriding of stainless steel
Author
Luo, Weiyi
Supervisor
Xu, Shuyan
Abstract
The thesis presents an experimental and theoretical investigation of a low frequency, high density, inductively coupled plasma (ICP) source and its results of applications in plasma nitriding of AISI 304 stainless steel. The experimental apparatus and diagnostics, including the RF voltage and current probes, the magnetic probe, the single Langmuir probe and the optical emission spectroscopy technique, are described in detail.
The measurements of global electrical parameters for Ar and N2 discharges show that there are two discharge states in the low frequency ICP source: E-mode state with a faint light emission and H-mode state with a bright light emission. The E-mode discharge is characterised by a low plasma resistance and a high plasma reactance. On the other hand, the H-mode discharge is featured by a high plasma resistance and a low plasma reactance. In Ar and N2 gas discharges the E-H mode transition and then the inverse H-E mode transition have been experimentally studies by changing RF input power and/or planar coil current. It is shown that there is a "discharge hystersis" on this ICP source. The current transition from E- to H-mode has been measured against the filling gas pressures for Ar and N2 discharges. The experimental curve of Ar discharge has a similar form to the typical Paschen curve - electrical breakdown curve in neutral gases.Using a single Langmuir probe, the measurements of microscopic plasma parameters show clearly that the E-mode discharge has a low electron density, high electron temperature and high plasma potential, whereas the H-mode discharge has a high electron density, low electron temperature and low plasma potential. In particular, the plasma parameters during the process of E-H mode and inverse H-E mode transitions have also a "hysteresis" with the changing input power or coil current. It is found that the ICP source can produce a plasma of high density, up to 8x1012 cm-3, and of uniform distribution for H-mode Ar discharge.
The optical emission spectra from Ar, N2 and mixture (Ar/N2/H2) discharges have been analysed. The plasma-generated particles are mainly excited atomic and molecular species.
The distributions of time-varying magnetic fields inside the ICP chamber for vacuum, Ar and N2 gas discharges have been measured using a magnetic probe. By comparing the measured results with a theoretical analysis of a two-dimensional model of ICP source, it is found that a travelling-wave, not a standing wave in vacuum, is produced in the H-mode Ar plasma. The measured data show that the low-frequency RF fields penetrate much deeper into the plasma than usual industrial-frequency (13.56 MHz) fields.
The low frequency, high density ICP source was used to nitride AISI 304 stainless steel. A series of experiments has been conducted in a low pressure, low temperature, N2/H2/Ar plasma mixture. The thickness of the nitrided layers varies from 38 to 79 um for various processing time at - 400 V bias voltage. It increases faster during the first nitriding hour. The microstructure, phase and composition of the nitrided surface layers are characterised by means of scanning electron microscopy/energy dispersive x-ray diffraction (SEM/EDX) and x-ray diffraction (XRD). It is found that the nitrided layer has crystalline structure with various phases. The distribution of the nitrogen content has a step function: high in the nitrided layer and almost zero elsewhere. The content of Cr, however, remains constant over the entire substrate/nitrided layer. After nitriding, the micro hardness of the stainless steel samples is increased by a factor of about 6 and their free corrosion potential is improved by ~20%.
The measurements of global electrical parameters for Ar and N2 discharges show that there are two discharge states in the low frequency ICP source: E-mode state with a faint light emission and H-mode state with a bright light emission. The E-mode discharge is characterised by a low plasma resistance and a high plasma reactance. On the other hand, the H-mode discharge is featured by a high plasma resistance and a low plasma reactance. In Ar and N2 gas discharges the E-H mode transition and then the inverse H-E mode transition have been experimentally studies by changing RF input power and/or planar coil current. It is shown that there is a "discharge hystersis" on this ICP source. The current transition from E- to H-mode has been measured against the filling gas pressures for Ar and N2 discharges. The experimental curve of Ar discharge has a similar form to the typical Paschen curve - electrical breakdown curve in neutral gases.Using a single Langmuir probe, the measurements of microscopic plasma parameters show clearly that the E-mode discharge has a low electron density, high electron temperature and high plasma potential, whereas the H-mode discharge has a high electron density, low electron temperature and low plasma potential. In particular, the plasma parameters during the process of E-H mode and inverse H-E mode transitions have also a "hysteresis" with the changing input power or coil current. It is found that the ICP source can produce a plasma of high density, up to 8x1012 cm-3, and of uniform distribution for H-mode Ar discharge.
The optical emission spectra from Ar, N2 and mixture (Ar/N2/H2) discharges have been analysed. The plasma-generated particles are mainly excited atomic and molecular species.
The distributions of time-varying magnetic fields inside the ICP chamber for vacuum, Ar and N2 gas discharges have been measured using a magnetic probe. By comparing the measured results with a theoretical analysis of a two-dimensional model of ICP source, it is found that a travelling-wave, not a standing wave in vacuum, is produced in the H-mode Ar plasma. The measured data show that the low-frequency RF fields penetrate much deeper into the plasma than usual industrial-frequency (13.56 MHz) fields.
The low frequency, high density ICP source was used to nitride AISI 304 stainless steel. A series of experiments has been conducted in a low pressure, low temperature, N2/H2/Ar plasma mixture. The thickness of the nitrided layers varies from 38 to 79 um for various processing time at - 400 V bias voltage. It increases faster during the first nitriding hour. The microstructure, phase and composition of the nitrided surface layers are characterised by means of scanning electron microscopy/energy dispersive x-ray diffraction (SEM/EDX) and x-ray diffraction (XRD). It is found that the nitrided layer has crystalline structure with various phases. The distribution of the nitrogen content has a step function: high in the nitrided layer and almost zero elsewhere. The content of Cr, however, remains constant over the entire substrate/nitrided layer. After nitriding, the micro hardness of the stainless steel samples is increased by a factor of about 6 and their free corrosion potential is improved by ~20%.
Date Issued
1998
Call Number
TN752.C3 Luo
Date Submitted
1998