The plasma spray process uses a DC electric to generate a stream of hi temperature ionised plasma gas ,which acts as the spraying heat source. The coating materials, in powder form ,is carried in an inert gas stream into the plasma jet where it is heated and propelled towards the substrate.
Because of the high temperature (15000°C） and high thermal energy of the plasma jet ,materials with high melting points can be sprayed .
The plasma spray gun comprises a copper anode and tungsten cathode ,both of which are water cooled .plasma gas (argon ,nitrogen ,hydrogen,helium) flows around the cathode and through the anode which is shaped as a constricting nozzle.
The plasma is initialized by high voltage discharge which causes localised ionisation and a conductive path for a DC arc to form between cathode and anode .The resistance heating from he arc causes the gas to reach extreme temperatures dissociate and ionise to form plasma. The plasma exists the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current) .
Plasma spraying produces a high quality coating by a combination of a high temperature ,high energy heat source a relatively inert spaying medium and quite high particle velocities ,typically 200-300m/s .
Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconium.plasma spray coating probably account for the widest range of thermal spray coatings and applications and make this process he most versatile .