PTWA internal coating system

The PTWA (Plasma Transferred Wire Arc) coating system developed by Flame-Spray Industries is a system for coating bores with a diameter of 65 to 350 mm. The feedstock is provided in wire form. The figure below provides an illustration of the system’s functionality. The nozzle unit is comprised of a thorium-doped tungsten cathode, an air-cooled pilot nozzle made of copper and an electrically conductive, wire feedstock which is fed vertically into the pilot nozzle. The plasma gas, a mixture of argon and hydrogen, is fed in through boreholes made tangentially to the circumference of the cathode holder (not shown in the figure). The placement of the boreholes results in a twisted gas flow along the cathode which escapes the nozzle at a high speed. The process is started by a high-voltage discharge which ionises and dissociates the plasma gas between the pilot nozzle and the cathode. The plasma that is then created flows through the nozzle tip at a high speed and expands along the longitudinal axis of the nozzle. In the process, the plasma is transported to the wire feedstock that is continuously being fed vertically into the nozzle, thus closing the electrical circuit. Melting and atomising of the wire are influenced by two phenomena. The wire is resistance heated though a high level of current strength, typically 65 - 90 amperes. The collision of the plasma against the pre-heated wire ensures that it melts and sputters. Layers manufactured with this method are characterised by a porosity of less than 2%. The application force deposition rate of this system is approx. 4 kg/h.

Schematic of the PTWA internal coating process