Modelling and experimental evidence of the cathode erosion in a plasma spray torchThe lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan-Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values.

]]>convectionerosionevaporationfield reversalionization layerplasma spray torchtungsten cathodeMaterials / Surfaces, Thermal Plasma Technologyf7116dbe-455b-451d-b7d7-8a8a48d829932022-07-07T11:44:18+02:002022-07-07T14:58:25+02:00enINP, Universidade da MadeiraModelling and experimental evidence of the cathode erosion - Figure 5The values of the temperature T_c (a), the current density j (b), and the electron temperature T_e (c) along the cathode surface (s denotes the distance measured from the center of the cathode) are computed in the framework of a model and its non-equlibrium boundary layer for electric currents of 400, 600, and 800 A. This model solves the equations of current continuity and heat transfer of the cathode applying boundary conditions for the cathode surface in contact with plasma, which are defined in terms of temperature on the cathode surface and the voltage drop in the boundary layer.

]]>2022-06-24T09:33:49+02:002022-07-07T14:51:34+02:00text/csvcsv11438https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/744abeecModelling and experimental evidence of the cathode erosion - Figure 6Number densities of neutral and charged species in the ionization layer of argon plasma at atmospheric pressure are computed for condtions corresponding to electric currents of 400, 600, and 800 A. It is shown that the computed number densities of argon atoms and ions approach their equilibrium values at a distance of about 100 µm. This distance is considered as the thickness of the ionization layer.

]]>2022-06-24T09:58:23+02:002022-07-07T14:51:34+02:00text/csvcsv16001203https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/b87a6f76Modelling and experimental evidence of the cathode erosion - Figure 7The ionization length for an argon plasma at atmospheric pressure is computed as a function of the electron temperature T_e for values of the temperature of heavy particles T_h of 1000, 2000, 3000, and 4000 K.

]]>2022-06-24T10:03:56+02:002022-07-07T14:51:34+02:00text/csvcsv2488https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/4f1d0240Modelling and experimental evidence of the cathode erosion - Figure 8Number densities of tungsten atoms n_wa are computed without (curves 1-3) and with account for ionization (curves 4-6). The number density of tungsten ions n_wi along the distance x is given by curves 7-9. The conditions for the computation correspond to the operating conditions of the plasma spray torch with electric currents of 400, 600, and 800 A.

]]>2022-06-24T10:10:27+02:002022-07-07T14:51:34+02:00text/csvcsv16349529https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/1bbd1d76Modelling and experimental evidence of the cathode erosion - Figure 9The density of net fluxes J_wa of tungsten atoms are obtained with input data corresponding to the operating conditions of the plasma spray torch (Table 1) applying two different boundary conditions (given by Eq. (15) and by Eq. (20)). It is shown that the computation with boundary condition given by Eq. (20) delivers physically unrealustic results since the net flux exceeds the limiting value J_vap.

]]>2022-06-24T10:17:30+02:002022-07-07T14:51:34+02:00text/csvcsv12048396https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/c5c88291Modelling and experimental evidence of the cathode erosion - Figure 10Parameters of the microarc plasma in atmospheric pressure argon are computed at a current density of 10^8 A/m^2: a) the temperatures of electrons and heavy particles (T_e, T_h); b) the number densities of electrons, argon atoms and ions; c) the modulus of the electric field.
It is shown that the plasma approaches thermal equilibrium at a distance of about 30µm (a), a deviation from quasi-neutrality occurs in the vicinity of the cathode (x<1µm) (b), a reversal of the electric field occurs in the vicinity of the anode (c).

]]>2022-06-24T10:25:08+02:002022-07-07T14:51:34+02:00text/csvcsv656365https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/2a41e536Modelling and experimental evidence of the cathode erosion - Figure 11Number densities of tungsten atoms and ions at a current density of 10^8 A/m^2 are computed by means of the microarc model. Similarly to the results obtained with the model of the ionization layer (Fig. 8), tungsten atoms are ionized within a distance of a few µm.

]]>2022-06-24T10:38:09+02:002022-07-07T14:51:34+02:00text/csvcsv288522https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/84dc4e02Modelling and experimental evidence of the cathode erosion - Figure 12The effect of the convection on the electric field (a), the diffusive velocity (b) and density of particle flux (c) at a current density of 10^8 A/m^2 is demonstrated. Average values of the convective velocity of 100 and 200 m/s are used to show the effect. The field reversal occurs closer to the anode (a) in the presence of convection. The reversal of the diffusive velocity is shifted with respect to the field reversal (b). The net flux of tungstem ions to the anode is fortified by the convection (c).

]]>2022-06-24T10:50:16+02:002022-07-07T14:51:34+02:00text/csvcsv917313https://www.inptdat.de/dataset/modelling-and-experimental-evidence-cathode-erosion-plasma-spray-torch/resource/cacdd770DKANhttps://www.inptdat.de