electric arc spraying, a thermal spray process in which an arc is
struck between two consumable electrodes of a coating material.
Compressed gas is used to atomize and propel the material to the
substrate. (1)
The electric arc spray process utilizes metal in wire form. This
process differs from other thermal spray processes in that there
are no external heat sources as in any the combustion gas/flame
spray processes. Heating and melting occur when two electrically
opposed charged wires, comprising the spray material, are fed together
in such a manner that a controlled arc occurs at their intersection.
The molten metal is atomized and propelled onto the prepared workpiece
by jets of compressed air or gas.
As early as 1914, Schoop in collaboration with
Bauerlin (2), an electrical engineer, experimented with
electrical heating for spraying. Initial attempts were unsuccessful
as they attempted to tailor their spray apparatus on the lines of
molten metal equipment rather than wire. One pole was a graphite
crucible, loaded with the consumable, the other a carbon rod. An
arc was struck between the crucible and the rod causing the metallic
consumable to melt and flow through an orifice. On exiting, the
molten metal was atomized by jets of compressed gas. Eventually,
a device was built utilizing two wires, insulated from each other,
made to advance and intersect at some point. Generally, the wires
were given a difference of electrical potential of about 89 V that
caused the wires to melt and; in the presence of a gas stream, spraying
was produced. Later guns, developed by Schoop, do not radically
differ from those used today.
The gun is relatively simple. Two guides direct the wires to an
arcing point. Behind this point a nozzle directs a stream of high-pressure
gas or air onto the arcing point where it atomizes the molten metal
and carries it to the workpiece as in the graphic above. Typically,
power settings of about 450 A can spray over 50 kg/hr (110 lb/hr).
Electric arc spray systems are offered that feed wire by either
an air or electrical motor. Some units push the wire to the gun
while others pull the wire into the arc. Controls include volt and
ampere meters and air regulators.
Electric arc spraying has the advantage of not
requiring the use of oxygen and/or a combustible gas; it has demonstrated
the ability to process metals at high spray rates; and is, in many
cases, less expensive to operate than either plasma and/or wire
flame spraying. "Pseudo" alloy coatings, or those constructed
by simultaneously feeding two different materials, are readily fabricated.
An example would be copper-tin coatings constructed by feeding pure
copper and tin wires into the arc to produce a heterogeneous mixture
of each in the coating. Also, the introduction of cored wires has
enabled the deposition of complex alloys (such as MCrAlYs) as well
as carbide-containing metal alloys that were only attainable using
powdered materials as feedstock. Some materials produce "self-bonding"
coatings that are sprayed in a "superheated" condition.
The overheated, hot particles tend to weld to many surfaces thereby
increasing the coatings' adhesive strength.
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