| Abstract: |
A relatively new process, suspension plasma spraying (SPS), permits to obtain finely structured coatings with graded properties for energy or environmental applications. However, the plasma torches, even powered with direct current regulated sources, generate highly fluctuating plasma. It results in nonhomogeneous treatment of the suspension particles injected into the plasma jet and consequently, in lack of the reproducibility of coatings properties. Consequently, since many years, special efforts have been devoted to understand the origin of electric instabilities in the plasma jet. The previous works in our laboratory have shown that these instabilities mainly originate in the stick and slip motion of the arc inside the nozzle, known as the restrike mode, and the compressibility effects of plasma forming gases in the cathode cavity, so-called Helmholtz oscillations. The following work highlights a new approach to the instabilities of plasma jet. The arc jet working in a pulsed laminar regime, obtained by coupling Helmholtz and restrike modes in a mechanism of phase-locked loop, is presented. The properties of this pulsed arc plasma were studied by implementing Time-Resolved Optical Emission Spectroscopy which allowed analyzing the plasma chemical kinetics. The pulsed arc plasma can be an alternative method expected to control heat and momentum transfers by applying a synchronous material injection into the plasma jet. The synchronization of suspension injection with the plasma is possible by the use an innovative injection system based on drop-on-demand (DOD) technique. The suspension used in the experiment was composed of titanium dioxide (90% TiO2 rutile phase) powder and water. The treatment of the particles into the plasma jet was observed by time-resolved imaging system and Time-Resolved Optical Emission Spectroscopy. The emission of suspension into the plasma jet was regulated by adjusting the required injection time delay over one period of arc voltage oscillation (700 µs), i.e. ranging from 0 µs to 620 µs, where 0 µs corresponded to the lowest level voltage (~ 40 V). The results of the experiments showed that the interaction between the plasma and materials depended on the injection time delay.
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