Plasma Spray Process
Plasma Spray Process involves spraying semi-molten material onto a substrate surface to provide a functional coating. Material in powder form is injected into a very high temperature plasma spray flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools to form the coating. This process is sometimes referred to as a cold process(relative to the substrate material) since the plasma sprayed substrate temperature can be kept low during processing thus avoiding damage. The Plasma Spray Process can help avoid unwanted metallurgical changes and distortion to the substrate material.
Thermal Spray coating involves the use of torch to heat the material in powder or wire form to a mole or near molten state and use of a gas to propel the material to the target substrate, creating a completely new surface. The coating material may be a single element, alloy or compound with unique physical properties that are in most cases achievable only through the thermal spray process.
The plasma spray gun is comprised of a anode and cathode, both of which are water-cooled. The plasma spray requires gas, typically argon, nitrogen, hydrogen, or helium. This gas flows around the cathode and through the anode, which is a constricted nozzle. The plasma spray is initiated by a high voltage discharge that causes localized ionization and a conductive path for a DC arc to form between the cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperatures, dissociate and ionize to form plasma. The plasma spray exits the anode nozzle as a free or neutral plasma flame or plasma which does not carry electric current. This is different than the Plasma Transferred Arc coating process, in which the arc extends to the surface to be coated. When the plasma is stabilized and ready for spraying, the electric arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water-cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its temperature and velocity. Powder is fed into the plasma spray flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can reach as much as 25 to 150mm.
The plasma spray process is most commonly used in normal atmospheric conditions and referred as Atmospheric Plasma Spray (APS). Some plasma spraying is conducted in protective environments using vacuum chambers normally back filled with a protective gas at low pressure. This is referred as Vacuum Plasma Spray (VPS) or Low Pressure Plasma Spray (LPPS).
Advantages and Disadvantages of Plasma
Advantages of Plasma Spray
Plasma spray is a medium is a lower temperature process compared to HVOF. Plasma Spray technology won't damage, create metallurgical changes or distort most substrate materials.
Plasma spray provides a smoother finish with a low RA.
The porosity of Plasma Spray can be as low as 1%, a positive attribute regarding any functionality.
Plasma Spray does not carry electric current.
Overall significantly stronger, cleaner, and denser than the other thermal spray processes.
Application of very high melting point materials such as refractory metals like tungsten, and ceramics like zircon unlike combustion processes.
Disadvantages of Plasma
Potentially higher relative cost and complexity of process compared to Twin Wire Arc Spray
Plasma Spray temperature is not low enough to be sprayed onto most plastic substrates