Abstract
In most small and medium enterprises (SMEs) that conduct plasma spraying, a one-cathode, one-anode plasma generator (OCOAPG) is used due to its cost-effectiveness. To achieve high deposition rates, the highest possible fraction of the injected powder has to be melted and accelerated toward the substrate. A sufficiently long and reproducible residence time in plasma is needed that is adequate in relation to the amount and size of the particles This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. In OCOAPG, an arc is operated between a cathode and a central tubular anode, which causes different instabilities in the effluent plasma jet. Due to the instable interaction between the plasma jet and the carrier gas jet continuously incorporating the powder, fluctuations occur resulting in reduced coating quality. Coating systems with a higher number of electrodes (and hence using several arcs) show higher stability and therefore can provide higher coating quality. However, due to their complexity and cost intensity, the investment hurdle for SMEs usually turns out to be too high. Recently, research steps to improve the plasma spraying process with OCOAPG have been undertaken by using a controllable current source to create a uniform interaction between the particle and the gas jet. As the movement of the anodic arc attachment can be actively controlled by current pulses, the plasma jet can be lengthened and shortened at periodic intervals. Furthermore, by pulsing the particle delivery synchronously to the power modulation, an improved particle penetration and consistent residence times can be achieved. First diagnostic results, including example coatings, are presented and discussed within the paper.
