Arc voltage fluctuations: Comparison between two plasma torch types

In plasma spray process, the arc root lifetime at the anode wall is limited and new arc roots are continuously created according to restrike, takeover or mixed modes. It results in voltage fluctuations at frequencies in the few thousands hertz range. Such phenomena play a key role in the spray process through the fluctuations of particle trajectories and correspondingly their velocity and temperature time-variations at impact. They have been intensively studied for Ar–H2 plasmas produced by Sulzer-Metco PTF4 and Praxair plasma torch. However, the arc movement depends on several parameters such as the plasma spray parameters, the plasma gas mixture composition, the electrode geometry and the plasma forming gas injector design. In this paper, two commercial plasma torches, PTF4 and 3MB, running both with argon–hydrogen gas mixtures, have been studied. This research work shows the geometry effect and the cold boundary layer thickness on the arc voltage fluctuations, as well as the particle thermal treatment and the resulting coating properties.