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Plasma Spray Process Technology
Plasma Spray Process involves spraying molten or heat-softened material
onto a substrate surface to provide a coating. Material in powder form is injected into
a very high temperature plasma 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 substrate temperature can be kept low during processing thus avoiding
damage, metallurgical changes and distortion to the substrate material.
The plasma gun is comprised of a copper anode and tungsten cathode, both of which
are water-cooled. The plasma gas, typically argon, nitrogen, hydrogen, or helium, flows
around the cathode and through the anode, which is a constricted nozzle. The plasma
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 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 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).
Plasma spray coatings may account for the widest range of thermal spray coatings
and applications making this process the most versatile.
Advantages
- Cold process avoids damage, metallurgical changes and distortion to substrate material.
- Plasma 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.
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Limitations
- Potentially higher relative cost and complexity of process.
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