Photo courtesy of Westaim Ambeon
by Frank J. Hermanek
On the eve of celebrating the one hundred anniversary of its discovery,
thermal spraying looks back to its roots - early experiments in
which liquids were broken up into fine particles by a stream of
high-pressure gas. Efforts more directed at producing powders
rather than constructing coatings. It fell to one Dr. Max Ulrick
Schoop of Zurich who recognized the possibility that a stream
of molten particles impinging upon themselves could create a coating.
His work, and that of his collaborators, resulted in the establishment
of the thermal spray process. This process has fostered a worldwide
industry serving over thirty technology sectors and generating
sales of over two billion dollars per year. This article traces
the history and development of the principal flame and electrical
thermal spray processes.
thermal spraying, a group
of coating processes in which finely divided metallic or nonmetallic
materials are deposited in a molten or semimolten condition to form
a coating. The coating material may be in the form of powder, ceramic-rod,
wire, or molten materials. (1)
In the early 1900's Dr. M. U. Schoop and his associates
developed equipment and techniques for producing coatings using
molten and powder metals. Several years later, in about 1912, their
efforts produced the first instrument for the spraying of solid
metal in wire form. This simple device was based on the principle
that if a wire rod were fed into an intense, concentrated flame,
(the burning of a fuel gas with oxygen), it would melt and, if the
flame were surrounded by a stream of compressed gas, the molten
metal would become atomized and readily propelled onto a surface
to create a coating. This process was initially referred to as metallizing.
Currently the technique is known as oxy-fuel or flame spraying.
Other oxy-fuel methods include wire, powder (metallic and ceramic),
molten metal, ceramic-rod, detonation and high velocity oxy-fuel
(HVOF).
In addition to using chemical means to plasticize
the in-put consumables electrical currents are also used. Typically,
electrical energy is used to create a heat source into which powder,
and more recently wires, are fed, melted/plasticized and conveyed
onto the surface to be coated. Major, commercially employed electrical
methods, used to construct coatings include non-transferred arc
plasma, RF plasma, and wire arc. Based upon the two (2)
heat sources a "family tree" of thermal spray methods
can be constructed as noted below.
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