Vacuum Coatings
For most
of the applications for which chrome plating is used, vacuum
coatings are not the answer because of their relatively high
cost and their inability to build the dimensions of worn or
damaged components back to spec. However, for some
applications these types of coatings can be cost-effective,
especially for small components that must last a long time.
There are
two primary types of vacuum deposition methods:
-
Physical vapor deposition (PVD) – In this method the
coating material is created from a solid, either
evaporated by an electric arc or an electron beam, or
sputtered. It goes through a plasma (to allow the ions
to be accelerated) and lands on the component surface,
forming a very thin but very hard layer.
-
Chemical vapor deposition (CVD) – CVD processes have a
lot in common with PVD, including cost and the size of
items that can be coated. However, in the CVD method
the coating material comes from gases that combine on
the hot surface to form a coating.
These
types of coatings are usually expensive because they are
relatively complex technologies that require vacuum
chambers. This limits the sizes of products that can be
coated to less than a foot or so (30cm).
Facts
about PVD coatings:
-
PVD
coatings are usually deposited at temperatures of 250°C
(480°F).
They can be deposited at lower temperatures, but,
depending on the method coating quality and adhesion
tend to suffer at low temperature. This limits the
alloys that can be readily coated.
-
PVD
coatings are highly sensitive to contaminants on the
surface being coated, which must be scrupulously clean
and must not expose the vacuum to water, oils, paints,
or other contaminants that will contaminate the process.
-
They
are only a few microns thick (typically 3 microns, or
0.0001”) and cannot be used for rebuilding worn
components. Thick PVD coatings often have such high
stress internal stress that they spall off. Some
Japanese automotive companies do, however, make PVD CrN
coatings 60mm
(0.0025”) thick.
-
PVD
nitride and carbide coatings are hard – 2,000 - 3,000 HV
(three times as hard as chrome plate), and some can even
be up to 5,000 HV (half as hard as diamond). For this
reason they are used as erosion coatings in some
aircraft engines
-
PVD
is not in general a good method for coating internals,
although some techniques have been developed
specifically to coat the insides of tubes, pipes and gun
barrels.
-
Among
the most commonly-used PVD coatings are TiN, CrN, and
the various diamond-like coatings (DLCs).
-
PVD
nitride coatings are essentially inert and do not
corrode, but they do not provide any protection once
scratched or damaged.
-
IVD
(Ion Vapor Deposition) aluminum is an example of a PVD
process that is used quite widely in the aircraft
industry. While there have been attempts to modify the
method to deposit hard coatings, none of these have gone
beyond the R&D stage.
-
PVD
coatings are often used for decorative surfaces, to
create “lifetime coatings” on door hardware and plumbing
fixtures. Many of the nitrides have gold or other
attractive colors.
-
PVD
is environmentally benign in that the only gases that go
through the process generally are nitrogen and argon,
and the source materials are solid metals, although some
processes do use methane or acetylene gas. Vacuum
chambers do have to be cleaned, and care must be taken
to avoid worker exposure to fine metal dusts.
Facts
about CVD coatings:
-
CVD
is used to create many of the same hard nitride coatings
as PVD, with similar hardness.
-
The
primary advantage of CVD coatings is that, because they
use gases, they can be used to coat very complex
objects, including internals, even down to micron sizes.
-
For
CVD coatings to grow on a surface the surface must
usually be red hot (around 1,000°C,1,830
°F),
which limits what types of alloys it can be used to
coat. As a result it is seldom used for coating high
strength alloys, which are heat-sensitive. CVD can be
done at lower temperatures by using plasmas or more
expensive metalorganic gases. This lowers to temperature
to around 500°C
(930°F),
which is still above the tempering temperature of many
alloys.
-
Coatings are usually a few microns thick, although the
method can be used to create materials several
centimeters thick (such as infrared windows).
-
CVD
materials are usually hazardous (some highly so) –
poisonous, pyrophoric, explosive, or combine with water
vapor in the air to create hazardous fumes such as HCl.
The process must therefore be used with proper safety
systems and methods of trapping the used gas.
If you
are used to using chrome plating, you will find adopting
vacuum coating methods to be very challenging. Very few
companies outside the cutting tool industry use CVD coatings
because of the safety issues. PVD coatings have been
brought in-house by manufacturers of plumbing products and
some aircraft engine parts manufacturers and overhaulers.
However, the cost of equipment can be very high ($1-3million
for a coating system, with additional costs for a cleaning
line). Running a coating operation also requires a
sophisticated workforce able to handle and troubleshoot
vacuum systems, cooling systems, gas handling systems and
electronics. PVD coatings require very careful attention to
cleanliness, both of the product to be coated and of the
vacuum chamber, which requires proper equipment and a
well-trained workforce.
In
general, unless production volumes are high, most users will
find it most cost-effective to a coating service vendor
rather than bringing the process in-house. Vendors can be
found in most areas of the world.
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