Relation between the Arc Root Fluctuations, the Cold Boundary Layer Thickness and the Particle Thermal Treatment

In plasma spraying, the arc root fluctuations, modifying the length and density of the plasma jet, have an important influence on particle thermal treatment. These voltage fluctuations are strongly linked to the properties of the cold boundary layer, surrounding the arc column, depending on the plasma spray parameters (composition and plasma forming gas flow rate, current, etc.) and the plasma torch design (anode-nozzle internal diameter and shape, etc.). In order to determine the influence of these different spray parameters on the cold boundary layer properties and voltage fluctuations, experiments were performed with two different plasma torches from Sulzer Metco. The first one is a PTF4 torch with a cylindrical anode-nozzle, working with Ar-H₂ plasma gas mixture and the second one is a 3MB torch with both a conical and a cylindrical anode-nozzle, working with a N₂-H₂ plasma gas mixture. Moreover, the arc voltage fluctuation influence on particle thermal treatment was observed through the measurements of temperature and velocity of particles, using an yttria partially stabilized zirconia powder with a size distribution between 5 to 25 µm.