Improvement of Plasma Spray Torch Stability by Controlling Pressure and Voltage Dynamic Coupling

Abstract

The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.