Blown Arc Plasma Source for the Elaboration of Finely Structured Coatings

Abstract

New emerging technologies using blown arc plasma sources allow elaborating finely structured coatings which find many applications fields. Among them, Suspension Plasma Spraying consists in injecting within a dc plasma jet a liquid jet containing submicron solid particles which are plasma-sprayed to form coatings. This new technology has renewed the interest devoted to the understanding and the development of blown arc plasma sources. This paper focuses on a non-exhaustive description of standard (subsonic) dc plasma torches involving blown arcs related to the Suspension Plasma Spraying (SPS). The emphasis is put on important features for SPS, i.e. on the influence of operating parameters on plasma properties, the understanding of the origin of electric arc instabilities and their influence on suspension injection, and at last, the coating characteristics. parameters and their easiness of installation. Plasma forming gases can be either axially injected within the torch or with an azimuthal velocity component and are heated up by Joule effect in the arc region. Due to plasma confinement within the anode-nozzle, the resulting plasma jet presents a strong axial convective velocity which stabilizes the arc and blows it downstream. This stabilization effect mainly operates on the cathode jet at the cathode tip and on the arc column, but the dynamics of the arc root at the anode wall depends on (im)balances of hydrodynamic and magnetic forces exerting on current lines. Moreover, strong arc current densities at the anode wall (> 10