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Glow Discharge
Published in Alexander Fridman, Lawrence A. Kennedy, Plasma Physics and Engineering, 2021
Alexander Fridman, Lawrence A. Kennedy
The negative glow and the Faraday dark space complete the cathode layer and provide a transition to the positive column. As was mentioned in Section 7.1.3, the negative glow region is a zone of intensive ionization and radiation (see Figure 7.4). Most electrons in the negative glow have moderate energies. However, quite a few electrons in this area are very energetic even though the electric field is relatively low. These energetic electrons are formed in the vicinity of the cathode and cross the cathode layer with only a few inelastic collisions. They provide a non-local ionization effect and lead to electron densities in a negative glow even exceeding those in a positive column (Figure 7.4). Details on the non-local ionization effect, formation and propagation of energetic electrons across the cathode into the negative glow and the Faraday dark space can be in particular found in publications of P. Gill, C.E. Webb (1977), J.P. Boeuf, E. Marode (1982) and S.Ya. Bronin, V.M. Kolobov (1983). The effect of intensive “non-local” ionization in a negative glow can be applied to form an effective electron source, the so-called hollow cathode discharge. Imaging a glow discharge with a cathode arranged as two parallel plates with the anode on the side. If the distance between the cathodes gradually decreases, at some point the current grows 100–1000 times without a change of voltage. This effect takes place when two negative glow regions overlap, accumulating energetic electrons from both cathodes. Strong photoemission from cathodes in this geometry also contributes to the intensification of ionization in the hollow cathode, see B.I. Moskalev (1969). Effective accumulation of the high negative glow current can be reached if the cathode is arranged as a hollow cylinder and an anode lies further along the axis. Pressure is chosen in such a way to have the cathode layer thickness comparable with the internal diameter of the hollow cylinder. The most traditional configuration of the system is the Lidsky hollow cathode, which is shown in Figure 7.10. The Lidsky hollow cathode is a narrow capillary-like nozzle, which operates with axially flowing gas. The hollow cathode is usually operated with the anode located about 1 cm downstream of the capillary nozzle. It can provide high electron currents with densities exceeding those corresponding to Child’s law (see Section 6.1.5, Eq. (6.14)). The Lidsky hollow cathode is hard to initiate and maintain in the steady state. For this reason, different modifications of the hollow cathode with external heating were developed to raise the cathode to incandescence and provide long-time steady operation (R.L. Poeschel, J.R. Beattle, P.A. Robinson, J.W. Ward, 1979; A.T. Forrester, 1988).
Improvement of wear resistance for carburised steel by Ti depositing and plasma nitriding
Published in Surface Engineering, 2018
Y. Yang, J. H. Guo, M. F. Yan, Y. D. Zhu, Y. X. Zhang, Y. X. Wang
In summary, the wear resistance of carburised M50NiL steel was further improved by depositing the Ti film followed by plasma nitriding. The microstructure evolution, nanohardness, load bearing capacity, friction and wear properties of the surface coating with different nitriding temperature were investigated. The TiN0.3, TiN and TiC are the main phase for the coating because of diffusion of nitrogen and back-diffusion of carbon. Moreover, a loose and porous a′C(N) layer forms on the surface of the specimens 800 and 850 due to sputtering effect of the steel hollow cathode.The specimen 750 demonstrates higher nanohardness (13.769 ± 0.738 GPa) than the carburised layer and good load bearing capacity (18 N). However, the nanohardness and load bearing capacity of the specimen 800 and 850 are substantially underestimated because of the loose and porous a′C(N) layer on the surface.The friction coefficient and wear rate of the specimen 750 decrease 23.1 and 72.2%, respectively, compared with carburised specimen. The specimen 750 shows slight adhesive wear while the carburised layer exhibited severe abrasive wear due to the hardness difference between the layer and the WC ball. The specimen 850 shows the best worn morphologies but the maximum wear weight loss because the loose and porous a′C(N) layer is easily peeled and counted in the weight loss.The sputtering effect of steel hollow cathode equipment causes a loose and porous a′C(N) layer formation on the surface as a impurity and thus prevent further diffusion of nitrogen into the Ti coating. This issue should be solved by adopting hollow cathode equipment made of refractory materials, such as Mo and W.