Plasma Spray Coating – Process
The plasma spray process uses a dc electric arc to generate a stream of high temperature ionized 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˚ cel) 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 s constricting nozzle.
The plasma is initialized by a high voltage discharge which causes localized ionization and a conductive path for a dc arc to form between cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperatures dissociate and ionize to form plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current).
Plasma spray produces a high-quality coating by a combination of a high temperature, high energy source, a relatively inert spraying medium and quite high particle velocities, typically 200-300 m/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 coatings probably account for the widest range of thermal spray coatings and applications and make this process the most versatile.
Benefits
The great advantage of the plasma spray coating technique is its ability to spray a wide range of materials, from metals to refractory ceramics, on both small and large components offering:
- corrosion protection
- wear resistance
- clearance control – abrasives and Abradables
- heat and oxidation resistance
- temperature management
- electrical resistivity and conductivity
Application & Materials:
- Due to its versatility and excellent characteristics the plasma spray coating process is selected by many technologists as a process which offers the widest choice of coating materials.
- Fretting wear, spraying of tungsten carbide/cobalt seal ring grooves – gas turbines;
- High temperature protection, thermal barrier coatings -TBCS onto gas turbine combustion hardware;
- Wear resistance, spraying of chrome oxide ceramic onto printing rolls for laser engraving;
- Anti-galling, spraying of molybdenum alloys onto industrial diesel engine piston rings; and
- Erosion/abrasion resistance, corrosion protection – down hole tool – oil and gas product.
- Carbides, metallics, ceramics, Abradables.