Thermal
spray and HVOF
The most
commonly used technology for replacing hard chrome plating
is thermal spray (primarily, but not exclusively, HVOF).
While this is the technology of choice for most applications
it is not necessarily the best choice for all.
There are a large number of
companies around the US and the world who supply thermal
spray services, and the equipment and supplies (thermal
spray powders) are available from a number of international
companies.
Thermal spray is the general
term for a number of processes in which particles of coating
material are heated and sprayed using high speed gas onto
the surface to be coated. It is a fully commercial
industrial process that is used for coating everything from
bridges and radar towers to aircraft parts and prosthetic
knee joints. Since thermal spray coatings are readily
sprayed up to 0.020” (500 microns) thick, they are good for
rebuilding worn components, which is the primary use for
hard chrome plate.
If you are currently using hard chrome
plating you will find thermal spray to be a considerable culture
shock. Thermal spray is not all that difficult, but it means doing
things in a very different way.
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If
done well, thermal spray will give you a higher quality,
more reliable coating with much longer service life. If
done poorly, coatings will flake off or be very porous,
or they will turn into efficient cutting tools that tear
up seals and mating surfaces.
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Unlike hard chrome, where there are only minor choices
of plating additives and hard chrome is what you always
get, there is a large choice of coating materials,
starting powders, gases, types of equipment and
deposition conditions. The most common coating
materials for chrome replacement are chrome
carbide-nickel chrome (Cr3C2-NiCr)
and tungsten carbide-Co (WC-Co and WC-CoCr). (Note that
the Cr in these materials is not hexavalent, despite the
misinformation in the California CARB regulations.)
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Hard
chrome plating is very forgiving and makes acceptable
coatings without a lot of attention. Thermal sprays
require proper process definition and careful control.
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Workers require a lot more training and skill to do
thermal spray reliably that to do chrome plating.
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You
cannot always use the same finishing methods. Chrome
plate is usually ground with an aluminum oxide wheel to
a finish of about 16m”
Ra for bearing surfaces, but the carbide sprays must be
ground with a diamond wheel or belt, and sometimes
superfinished with a stone or belt to <6m” Ra.
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If
you need to coat internal diameters or complex shapes,
HVOF is not a good choice, and you may need to use
plasma spray for larger diameters or alternative
electroplates or electroless nickel for smaller
diameters.
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The
way thermal spray fits into the production system is
different. Chrome plating requires products to be
cleaned and masked, then left in the plating bath for a
day or more. Large items are sprayed one at a time
for15 minutes to an hour, while batches of small items
can be sprayed in special fixtures. Thus thermal spray
is much faster, but requires constant attention.
Because
most users are not very familiar with all these choices, we
offer consulting assistance to make the best choices, as
well as training and coating optimization for a consistently
high quality product.
Click here
for technology description. |
