RF plasma, a system in which the torch is a water-cooled, high frequency
induction coil surrounding a gas stream. On ignition a conductive
load is produced within the induction coil, which couples to the
gas, ionizing it to produce a plasma. (1)
Inducing electricity to flow through a conductor causes heating
to occur. This occurs primarily as a result of resistance to the
flow of the induced current and is proportional to the square of
the current (I) and directly proportional to resistance (R) and
time (t) or (I2Rt) (8).
Induction occurs when a conductor is placed in
an alternating magnetic field. When the effect is sufficient, great
eddy currents are set up in the conductor, which rapidly gets hot
or even melts, the magnetic linkages necessary being increased with
the frequency. To be used for thermal spraying, a water cooled helix
of several turns is fashioned from OFHC copper. It is wrapped around
a quartz tube that is closed at its top end and fitted with two
inlet ports to feed a spray material and a plasma forming gas. Releasing
gas into the tube and energizing the copper helix by a high frequency
current that sets up an intense magnetic field inside the tube causing
ionization of the gas. Continuous feeding of the gas causes it to
escape through the open bottom of the tube. Powder fed into the
plasma filled tube is melted and relying on either gravity or the
plasma flow is conveyed to the work surface.
Coatings produced using RF plasma has shown to
be generally homogeneous and not porous. This method, using neutral
atmospheres, can deposit reactive and toxic metals including calcium,
uranium, niobium and titanium.
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