On the fundamental mechanisms of active screen plasma nitriding

Active screen (AS) is an advanced technology for plasma surface engineering, which offers some advantages over conventional direct current (DC) plasma treatments. Such surface defects and process instabilities as arcing, edge and hollow cathode effects can be minimised or completely eliminated by the AS technique, with consequent improvements in surface quality and material properties. However, the lack of information and thorough understanding of the process mechanisms generate scepticism in industrial practitioners. In this project, AISI 316 specimens were plasma carburised and plasma nitrided at low temperature in AS and DC furnaces, and the treated samples were comparatively analysed. Two diagnostic techniques were used to study the plasma: optical fibre assisted optical emission spectroscopy, and a planar electrostatic probe. Optimum windows of treatment conditions for AS plasma nitriding and AS plasma carburising of austenitic stainless steel were identified and some evidence was obtained on the working principles of AS furnaces. These include the sputtering of material from the cathodic mesh and its deposition on the worktable, the generation of additional active species, and the electrostatic confinement of the plasma within the operative volume of the furnace.

Study of active screen plasma processing conditions for carburising and nitriding austenitic stainless steel

The nonlinear propagation of dust-ion-acoustic (DIA) waves in a dusty plasma with bi-Maxwellian electrons, namely, lower and higher temperature electrons (composed of negatively charged stationary dust, inertial ions, and non-inertial two-temperature-electrons) is investigated by deriving the Gardner equation using the reductive perturbation technique. The basic features (amplitude, width, etc.) of the hump (positive potential) and dip (negative potential) shaped DIA solitons (Gardner solitons, i.e., GSs) are found to exist beyond the Korteweg-de Vries (K-dV) limit. These DIA-GSs are qualitatively different from the K-dV and modified K-dV solitons. It is also shown that depending on the parameter σ (where σ=Te1/Te2, Te1 and Te2 being the temperatures of two distinct electrons and Te1≪Te2), the DIA-GSs exhibit hump and dip shape solitary structures. The implications of our results in understanding the localized nonlinear electrostatic perturbations observed in double-plasma machines, rf discharge plasma, n...

Dust-ion-acoustic Gardner solitons in a dusty plasma with bi-Maxwellian electrons

Generation of large-volume plasma by making use of multi-needle plasma at low-pressure

A simple scheme for generating a uniform, steady-state, large-volume plasma is presented. The weakly magnetized plasma is created by direct ionization of the background gas by low-energy electrons generated from thermionic filaments. An annular arrangement of the filaments ensures a uniform plasma density in the radial direction as predicted by theory. Experiments have been performed to characterize the plasma generated in such a configuration. In order to explain the experimental observation, a bulk plasma theory based on plasma transport by means of cross-field diffusion is developed. As assumed in the theoretical model, the experimental measurements indicate a uniform plasma density along the axis. Both the theory and experiment indicate that the plasma density is a function of the square of the external magnetic field. The theory also predicts the plasma density to be proportional to the neutral density to the two-thirds power, in agreement with the experimental data. The experimental data agree well with theoretical prediction for a broad range of system parameters. >

A model for steady-state large-volume plasma generation

We describe a scheme in which the primary beam electrons are electrostatically confined in a low‐pressure dc discharge plasma which can substantially improve the density (300 or 400% in our case). The electrons are heated to 2 eV with a fraction thermalized to ∼10 eV by the extended presence of the primary beam electrons. Ion noise observed was not changed substantially from that without confinement. (It was slightly better than unconfined.) The plasma noise is strongly dependent on filament geometry. We also describe a second electrostatic confinement scheme in which both the primary electrons and the plasma are confined, resulting in a further density increase.

D. A. Rasmussen

Papers

An improved low‐pressure dc discharge