The Plasma Spray Process is basically the spraying of molten or heat softened
material onto a surface to provide a coating. Material in the form of powder
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 forming a coating. This plasma spray process
carried out correctly is called a "cold process" (relative to the
substrate material being coated) as the substrate temperature can be kept
low during processing avoiding damage, metallurgical changes and distortion
to the substrate material.
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 initiated by a 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 the arc causes the gas to reach extreme
temperatures, dissociate and ionise to form a plasma. The plasma exits the
anode nozzle as a free or neutral plasma flame (plasma which does not carry
electric current) which is quite different to the Plasma Transferred Arc coating
process where the arc extends to the surface to be coated. When the plasma
is stabilised 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 be in the order of 25 to 150 mm. Plasma Flame Theory
The plasma spray process is most commonly used in normal atmospheric conditions and referred as 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 VPS or LPPS.
Plasma spraying has the advantage that it can spray very high melting point
materials such as refractory metals like tungsten and ceramics like zirconia
unlike combustion processes. Plasma sprayed coatings are generally much denser,
stronger and cleaner than the other thermal spray processes with the exception
of HVOF and detonation processes. Plasma spray coatings probably account for
the widest range of thermal spray coatings and applications and makes this
process the most versatile.
Disadvantages of the plasma spray process are relative high cost and complexity
of process.