Multiple Electrodes and Cascaded Nozzles: A Review of the Evolution of Modern Plasma Spray Torches

Conventional one-cathode/anode plasma spray guns are susceptible to aging. One reason is the large power density, especially at the arc roots on the cathode tip and the anode wall. Anode wear results in a thinner boundary layer and a reduced arc root motion, which increases the local thermal load. This also results in a voltage drop, and thus a reduction in power level when the power source is operated in a constant current mode. In addition to electrode wear, the instantaneous arc morphology and the time-dependent voltage waveform are strongly correlated to each other, especially when hydrogen or nitrogen is used as secondary plasma gas. Such arc dynamics are a major phenomenon that broadens the distribution of particle in-flight characteristics. The inevitable wear of electrodes and the inherent power fluctuations were the starting point for the development of new concepts for modern plasma spray guns. Multi-electrode plasma torches were developed to improve operational stability and lifetime. They became popular due to their good stability and high-power plasma jet, even when operated with inert gases only. In this context, cascaded torch nozzles were introduced, which effectively limit the axial movements of the anodic arc attachment. Such a design includes a stack of neutrodes in front of the anode, which are electrically insulated from each other. Since the arc is more stable, the power demand is virtually constant and the treatment of the feedstock particles is more uniform than with the conventional non-cascaded torches. In this review, the mechanisms leading to electrode wear and arc fluctuations in single-cathode/anode plasma guns are explained. Some concepts of multi-electrode torches and cascaded torch nozzles are presented. Examples of experimental results obtained by diagnostic methods are also given.