Plasma systems having improved thermal spraying

Abstract

Apparatus for delivering heated powder to the plasma gun of the thermal spray plasma system includes a hollow heater tube coupled between a powder feeder and the plasma gun and a power supply coupled between opposite ends of the heater tube. The power supply heats the walls of the heater tube to a temperature below the melting point of the powder particles, to prevent melting and adhesion of the particles thereto. The length of the heater tube is selected to provide an adequate dwell-time of the powder particles therein for a given powder flow rate, so that even relatively large powder particles are heated to a temperature relatively close to that of the heater tube walls upon delivery to the plasma gun. The efficiency of the heater tube is improved by a water cooled reflector tube concentrically disposed about the outside of the heater tube, with oxidation of the tube surfaces being prevented by a flow of inert gas along the space between the heater and reflector tubes. The heated powder delivery apparatus can be used by itself and without the plasma gun to effect thermal spraying in instances where the powder is of the type that can be heated close to the melting point without adhering or clogging. One or more ring-shaped electrodes and associated DC power supplies can be disposed within the plasma stream to provide additional powder particle heating and control within the plasma stream itself.