Plasma Transferred Arc Coating

The plasma transferred arc (PTA) coating process was developed in the 1960s for the rebuilding and repair of worn parts and the hard facing of parts exposed to extreme abrasion and corrosion. Regular steel parts with an appropriate PTA coating can exhibit superior corrosion and wear-resistant behavior even compared to specialty alloys. The process is significantly different from other coating processes, including plasma spraying, as the surface of substrate is locally molten in the process creating a strong metallurgical bond between the molten coating material and the substrate if they are metallurgical compatibles. The substrate, in most of the cases, serves as the anode for the transferred arc and only exceptionally as cathode to significantly limit the heat flux to the substrate and clean its surface. The powder to be deposited is introduced into the arc plasma through two or more orifices located on a ring surrounding the exit of the plasma torch nozzle. A further annular slot surrounding the powder injection ring provides the shield gas flow, necessary to avoid reaction of the molten metal with the environmental air. Once the molten metal cools and freezes, it creates a metal matrix composite coating, the mechanical and metallurgical properties of which can differ from those of the substrate or the coating material. Compared to other thermal spray processes, PTA has the major advantage that alloy formation can take place in the metal pool during the process, allowing for a wider range of combinations of metal compounds or metal matrix composites. This chapter is devoted to PTA used for coatings and hard facing, describing their basic design features, typical performance characteristics, and applications.