Low pressure arc spraying of reactive materials

Abstract

In spite of the introduction of low pressure plasma spraying, disadvantages remain in the case of powdered materials because of their large specific surface area. Thus tantalum, for example, is already embrittled before the spraying operation and the gases adsorbed on the very large surface area of the powder compared with that of a wire limit the purity of the coatings even if high purity gases are employed. Low pressure arc spraying, however, combines the advantages of wire spraying with those of spraying in low pressure chambers in that it yields homogeneous chemically pure coatings of low porosity. At the same time, the process is more economical than low pressure plasma spraying. Stable operation of conventional arc spraying heads at low pressure is not possible because the environmental pressure causes considerable enlargement of the arc between the electrodes being burned off. The result is that, among other things, the arc causes seizure of the contact nozzles and melts them off thereby rendering the spray heads unusable. In low pressure arc spraying the melting process takes place in an antechamber maintained at a pressure above atmospheric to ensure a safe burn-off along with appropriate droplet formation. The low pressure acts on the particles emerging from the nozzle and ensures intense degasification. In this way coatings are produced which are outstanding as regards density and adhesion to the substrate. The arc may burn between two or more current-carrying wires or between one centrally arranged wire and an annular backing electrode, located in the nozzle, or the surface of the substrate. By applying a voltage between an annular metallic electrode insert at the underside of the combustion chamber and the base metal, a transferred arc of selectable polarity can additionally be started and maintained as in low pressure plasma spraying. Coatings of titanium and tantalum prepared by low pressure arc spraying are presented. Optical, electron microscopy, metallographic and electrochemical analyses reveal the properties of the coatings.