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1331 |
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Tuesday, June 03, 2008, Room 08 2:40 PM Plasma Spraying and Plasma Transferred Arc 3 New trends and modified plasma processes for the application of ceramic-based coatings |
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New approach to the problem of cathode wear in DC arc plasma torches |
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Tetyana Kavka* / Institute of Plasma Physics AS CR, v.v.i., Czech Republic Johannes Arnold / Institute of Technical Thermodynamics, DLR, Germany Asif Ansar Syed/ Institute of Technical Thermodynamics, DLR, Germany Guenter Roth/ Institute of Technical Thermodynamics, DLR, Germany Thomas Kupke/ Institute of Technical Thermodynamics, DLR, Germany Gottfried Schneider/ Institute of Technical Thermodynamics, DLR, Germany |
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High heat load on the surface of electrodes in DC arc plasma torches results in strong wear of electrodes. This takes place during the start up and shutting down procedure as well as during long-term running of the torch and results in arc voltage reduction. Such changes in voltage lead to a modification of the plasma jet properties and effect adversely reproducibility of deposits, which are undesirable for industrial applications. The present work is focused on studying the effect of cathode wear. Examinations of the influence of wear conditions on generated plasma and coating qualities are considered. In the process several cathodes are tested with different operation time. Moreover, the present work offers a new approach to compensate cathode wear in DC arc gas stabilized plasma torches by using moveable anode. With this design of torch, the distance between the cathode tip and anode can be varied. Voltage fluctuation analysis show the possibility of a reasonable arc modification by approaching the nozzle to the cathode caused by a change of the flow field inside the arc chamber and the arc dynamics. Smaller cathode tip-nozzle distances result in higher arc voltage with reduced fluctuations accompanied by a positive effect on the plasma jet properties. Thus, the negative influence of cathode wear on generated plasma properties can be suppressed and the lifetime of the cathode can be extended by simple approaching the nozzle. Effect of this arc modification on particles in the jet and results of coating tests made with the new torch will be presented as well. |