Plasma spraying is a thermal coating process based on a superheated gas – known as a plasma. The gases argon, nitrogen, hydrogen, or helium flow between a tungsten cathode and water-cooled copper anode. Between the two electrodes an electric arc is initiated by a high-frequency discharge and then sustained with DC power. The arc ionizes the gas, creating a high-pressure gas plasma. This results in an increase in gas temperature, which can exceed 30,000 °C and expands the volume of the gas, and increases its pressure and velocity as it exits the nozzle.
Usually, powder is introduced into the gas stream – either just outside the torch or in the diverging exit region of the nozzle. The powder velocities usually achieved in plasma spray deposition range from about 300–460 m/s. Temperatures are usually at or slightly above the melting point. Generally, higher particle velocities and temperatures above the melting point, but without excessive superheating, yield coatings with the highest densities and bond strengths.
Plasma spraying done in an inert atmosphere and/or low-pressure chamber has become a widely accepted practice, particularly in the aircraft engine industry.
Plasma spray systems have proven to be an effective means for applying complex, hot corrosion-resistant coatings of the types Ni-Co-Cr-Al-Y to high-temperature aircraft engine components without oxidation of the highly reactive constituents. In fact, plasma spraying can be used to produce coatings of virtually any metallic, cermet, or ceramic material.