Spectroscopy of Low Temperature Plasma
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The third item leads to the chemical vapor deposition of Fe 16 N 2 thin films on the template substrate [ 7 ]. This solution nitriding method induced the nitrogen atoms even into high chromium content steels including austenitic stainless steels [ 9 ].
The nitrogen works to stabilize the austenitic phase even with less nickel content. These nickel-free HNS have coarse grains, resulting in embrittlement, difficulty in welding, and insufficient stability in working. In parallel with research on HNS, ion- and radical nitriding processes were developed with the use of the direct current DC -plasma and DC-pulse plasma technologies [ 11 ].
Hence, the stainless steels and Fe-Cr alloys were hardened by fine precipitation of CrN; however, the chromium content in the matrix was reduced by CrN-precipitation reaction to lower the original corrosion resistance [ 13 ]. In addition, this high temperature plasma nitriding was mainly governed by the nitrogen diffusion process; the nitrogen solute content exponentially decreases from the maximum nitrogen solid solubility of 0. Most of engineers and companies related to plasma nitriding believe that chemical reaction of chromium with nitrogen should drive the nitrided layer formation and hardening.
British research group [ 16 ] first found the nitrogen super-saturated lattices in the austenitic stainless steels by low temperature plasma nitriding. In addition, various new engineering is expected to start from this nitrogen super-saturated Fe N [ 17 ].
First, Radio-frequency RF -DC plasma nitriding system is introduced with comments on the essential difference from other plasma nitriding processes such as DC- and DC-pulse plasmas. Quantitative plasma diagnosis equipment is stated to describe the nitrogen-hydrogen plasmas. In particular, the effect of hydrogen content in the mixture gas on the nitriding process is analyzed to determine the optimum condition. A hollow cathode device is proposed to intensify the ion and electron densities. The nitrogen super-saturation is described by SEM-EDX; the elastic distortion is directly calculated by the lattice strain.
The phase transformation, the plastic straining as well as the microstructure refinement are analyzed by EBSD. The nitrogen diffusion path is mainly estimated by the grain boundary diffusion process. These processes are mutually related to form a synergetic loop to drive this low temperature inner nitriding. When this loop is sustained during nitriding, the nitriding front-end advances homogeneously into the depth of stainless steel matrix.
Once this loop is shut down at any point, the inner nitriding localizes by itself only to form a heterogeneous microstructure. High-density RF-DC plasma nitriding system is introduced together with comments on the quantitative plasma diagnosis of nitrogen-hydrogen plasmas and on the hollow cathode device to intensify the ion and electron densities. In the following, a detail of RF-DC plasma generation as well as a hollow cathode device is stated together with plasma diagnosis equipment in the present system. The nitriding parameters as well as controlling procedure are specified on the panels.
All through the nitriding process, the measured pressure, temperature as well as gas pressure is automatically controlled by the process computer. Through the telecommunication, time history of RF- and DC-voltages and currents are also monitored on the panel to recognize the temporal status of RF-DC plasmas.
Research:Optical Emission Spectroscopy
RF-DC plasma nitriding system. The thermocouple is inserted into this cathode plate to monitor the T H. In the vacuum chamber, the specimen is placed inside a hollow cathode setup on the cathode plate, which is electrically connected with DC generator. In the standard plasma diagnosis, two methods are often employed to quantitatively describe the nitrogen-hydrogen mixture plasma state; that is, emissive light optical spectroscopy EOS and Langmuir probe LP.
The LP was also utilized in the diagnosis to describe the effect of [H] on the generated plasmas. Through the direct measurement of I-V curves at the probe tip, the electron resistivity as well as the ion and electron densities are analyzed to describe the plasma state. In particular, the electron resistivity is proportional to the enhancement of plasma chemical reaction.
Variation of the measured resistivity in the plasmas with increasing the hydrogen content in the mixture gas. The hollow cathode device is utilized to intensify the ion density in the nitrogen-hydrogen plasmas. The LP is employed to directly measure the ion density in the hollow. In each position, the tip was fixed at the center of hollow. This is common to the hollow device effect where the ionization is enhanced at the vicinity of outlet in the hollow [ 30 ].
EDX device and software were utilized to make element mapping over a specified depth for nitrogen, chromium, iron, and carbon. The inverse pole figure IPF was determined for each constituent grain to describe the change in microstructure through the nitriding. In addition, the kernel average misorientation KAM and the phase mapping were also measured to explain the plastic straining and phase transformation processes, respectively. Essential processes in this low temperature plasma nitriding are described by chemical analyses. High-density RF-DC plasma systems [ 36 , 37 , 38 , 39 , 40 ] provides a new way to further analyze this low temperature plasma nitriding by experiments.
The pre-sputtering only with the use of nitrogen gas was first performed for 1. The nitrided layer thickness reaches to be Formation of uniform nitrogen super-saturated layer reveals that inner nitriding advances homogeneously into the depth of matrix.
This strain slightly increases to be 5. This elastic distortion in the nitrogen super-saturated lattices just corresponds to the previous report in [ 41 ]. The grains housing these elastically distorted lattices are forced to deform plastically to compensate for strain incompatibility between the nitrogen unsaturated and the super-saturated lattices in grains.
EDX as well as the micro-Vickers testing are utilized to investigate the nitrogen content and hardness depth profiles. A nitriding front-end is defined by the position in depth where the measured hardness coincides with the substrate hardness; the nitrided layer thickness E after nitriding for In the high temperature nitriding, the nitrogen content exponentially decays from the maximum nitrogen solubility limit of 0.
EBSD provides a tool to describe the interrelation among the phase transformation, the plastic straining, and the microstructure evolution. After [ 42 ], the measured cross-sectional KAM profile can be identified as an equivalent plastic strain distribution.
That is, uniform phase transformation and plastic straining change themselves across this critical depth by their localization to grains. The neighboring lattices to elastically distorted ones by phase transformation are forced to make plastic distortion.
Phase transformation and plastic straining in the above reflects the microstructure change by the nitrogen super-saturation. Each grain with a specified crystallographic orientation is represented by a different color. The plastically strained grains are partially decomposed into several or tens of subgrains with different crystallographic orientations. LTN of austenitic stainless steels is essentially different from the conventional plasma nitriding at higher temperatures. No nitrides are formed in the matrix so that no change in the original chromium content proves less change in the original corrosion resistance of stainless steels.
Owing to fine grain size in the homogeneously nitrided layer, higher strength is expected to this high-nitrogen stainless steel surface. In addition, the fine-grained two-phase structure has a role to improve the trade-off-balancing between strength and fracture toughness and to increase the fatigue life [ 2 ].
How to extend this homogeneously nitrided layer toward the nitriding front-end must be an engineering issue to be discussed further. Inner nitriding mechanism in this low temperature plasma nitriding of austenitic stainless steels is discussed with importance on the difference between the homogeneous and heterogeneous nitriding processes.
Nitrogen solute, penetrating from the surface under high-nitrogen flux, occupies with an octahedral vacancy sites in the fcc-structured lattice as suggested by [ 43 ].
Spectroscopy of low temperature plasma
Original grain is distorted and decomposed into fine subgrains by this plastic straining. First published: 28 January About this book Written by a distinguished plasma scientist and experienced author, this up-to-date work comprehensively covers current methods and new developments and techniques, including non-equilibrium atomic and molecular plasma states, as well as such new applications as gas lasers. Containing numerous appendices with reference data indispensable for plasma spectroscopy, such as statistical weights and partition sums and diatomic molecules. For plasmaphysicists, spectroscopists, materials scientists and physical chemists.
Appendix H is only available online.
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Author Bios V. Ochkin , after receiving his PhD degree from the P. Lebedev Physical Institute, now serves as deputy director of this institution as well as head of the Laser Technology Section of the Russian Academy of Engineering Sciences. The main fields of his scientific interests are physical and chemical kinetics of gas discharge active medias, frequency tunable gas lasers, dispersive resonators and Waveguide lasers as well as various kinds of plasma.
Professor Ochkin has authored more than publications and several books. Free Access. Tools Get online access For authors.