Electroplates, electroless nickel and composite plating
As drop-in replacements for chrome plating, electroless-
and electro-plates have the advantage that they are bath
processes able to be done with the same sort of technology
with which chrome plating users are familiar. Most
non-chrome hard electroplates are based on nickel, although
there are some plates that are based on other materials:
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Ni-W,
Ni-W-B and Ni-W-B with SiC (the Takada process). All of
these are alloy coatings, which typically are more
complicated to deposit than the simple elemental
coatings. Coatings with particles co-deposited (such as
SiC, diamond, PTFE, etc.) are particularly difficult as
particle suspension and deposition uniformity are
difficult to control, and in some cases the particles
have been known to wreak havoc with air handling
systems.
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Cobalt-based electroplates avoid the Ni ESOH problems,
although Co is a more expensive metal and often termed a
strategic metal because its sources lie mostly in
unstable areas of the world, with little available in
North America or Europe. Co-based electroplates include
Co-P nanophase alloys developed specifically as chrome
replacements and only just now becoming commercially
available, and Co-SiC composites, which have been used
in some aircraft engine applications for some years in
the UK.
Electroless nickel (often just called EN) is a standard and widely
available plating process. There are a great many platers
offering electroless Ni, and the plating equipment and
solutions are commercially available.
Unlike chrome plating and other electroplating methods,
which use an electric current to deposit the coating,
electroless Ni is an “autocatalytic” process – i.e. the Ni
deposits out of the bath onto the parts just because they
are there, without any need for anodes and robbers, and all
the associated fixturing used in electroplating. This makes
it a very flexible process, able to coat even very complex
parts and small internal spaces easily and uniformly. But
because the bath is a delicate balancing act of keeping the
Ni in solution, but allowing it to deposit out onto anything
put into the bath, it requires careful attention to bath
chemistry and cleanliness of the surface being coated.
Unlike chrome plating baths, which last essentially for
ever, electroless Ni baths typically require complete
replacement periodically.
There are several types of electroless Ni available
commercially:
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Electroless Ni-P is the most common type, with a typical
hardness up to.
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Electroless Ni-B is used for some aerospace and other
specialized applications. It is much less widely
available than Ni-P. Because lead or thallium are
required for the bath chemistry, Ni-B coatings usually
contain from 0.1-1wt% of these toxic materials.
Therefore these coatings are not always RoHS-compliant.
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Electroless Ni-P composite coatings are available with a
variety of additions to the base Ni-P coating. These
include SiC or diamond for wear resistance, PTFE for
lubricity, or even both for a hard, lubricious coating.
Because it is a plating process electroless Ni is not a big
step for most companies used to using chrome plating,
although it is a much less forgiving process:
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Electroless Ni is not usually as wear resistant as hard
chrome and must be heat treated at 400°C
(750°F) to create the phosphide precipitates that give the
coating its hardness. As-deposited the hardness of
electroless Ni-P is 500-700 HV as deposited and up to
1,100 HV after heat treating, while Ni-B is typically
650-750 HV as-deposited and up to 1,200 HV after heat
treating. This makes it unsuitable for alloys and
products that cannot take this temperature.
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The
coating is very corrosion resistant as-deposited,
provided there are no holes or scratches, but loses some
corrosion resistance on heat treating.
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It is
usually a thin coating (<0.003” (75mm),
although some bath chemistries now allow coatings that
are much thicker), and so it is not usually used for
rebuilding worn items, which is the primary use for hard
chrome.
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Electroless Ni coatings can take a very smooth finish,
and are often used for mirrors and very smooth dies and
molds. The coating is not microcracked as hard chrome
is.
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Electroless Ni is most useful for small components,
although some companies are able to plate items several
feet across weighing several tons. While small
components can usually be cleaned and plated fairly
easily, it is often difficult to prepare and clean large
items perfectly over their whole surface. As a result
large components frequently exhibit small poorly adhered
areas.
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Because the process requires a fine chemical balance it
is critical to maintain bath chemistry accurately. Some
users set up systems to do this automatically to
maintain consistent quality.
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Especially with large items the cleanliness and finish
of the surface to be plated is critical as contamination
can prevent the autocatalytic reaction and leave holes
in the coating.
The
presence of Pb in electroless Ni-B coatings makes them
non-compliant with RoHS. The alternative, thallium, is not
covered under RoHS, but it is also a heavy metal cumulative
poison.
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