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| Cross
sectional photomicrograph of a composite EN coating with more
than 40% by volume particles in EN at 1,000X magnification.
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The
finishing industry is facing a greater challenge to reduce the environmental
impact of its processes than ever before. Perhaps the paramount
challenge is to replace chrome plating due to its negative environmental
and health effects. The EPA has found chromium to potentially cause
skin irritation and ulceration during short-term exposures. Long-term
effects include damage to the liver, kidneys, circulatory system
and nerve tissue, as well as skin damage and cancer.
In 1972, Congress passed the Clean Water Act, which protects our
lakes, rivers, aquifers and coastal areas. It was amended in 1977.
Under this law, most chromium limits were set by state and local
environmental agencies. Under the recently proposed Metal Products
and Machinery Rule (MP&M), the maximum daily limit for chromium
would be 1.3 mg/liter and 0.55 mg/liter maximum monthly average.
Not necessarily easy numbers to reach.
Due to this, chromium reduction has been a key focus of companies,
the military, industry conferences, academia and legislation. Many
applications have already been converted from chrome plating to
other finishing operations. Because chrome is used so widely for
varying purposes, it is impractical to expect to find a single replacement
that will work in all applications.
While questions exist about the environmental ramifications of nickel,
it is still clearly less problematic than chrome. For this reason,
electroless nickel (EN) has been used to replace chrome in many
decorative as well as functional applications, such as for corrosion
and wear resistance. In applications requiring hardness and wear
resistance, composite EN coatings have been even more successful
in not only replacing chrome but actually surpassing the performance
of hard chrome plating.
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| Yarnline
Abrasive Wear Test Results
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Composite
EN coatings have codeposited particles dispersed throughout the
coating layer as in Figure 1. These coatings, therefore, have all
of the inherent features of electroless nickel as well as the properties
of whatever particles are selected, such as hardness, wear resistance,
lubricity, heat transfer, light absorption, etc. For this reason,
composite EN coatings are better than chrome or any electrolytic
or spray processes for non-line-of-sight applications.
Recent analysis has further demonstrated that these composite EN
coatings not only have tremendous potential to replace chrome, but
actually can be used to reduce nickel use and pollution as well.
This interesting opportunity exists on three levels.
No Chrome
Composite EN coatings use no chrome. The environmental problems
inherent with plating and using chrome are therefore entirely eliminated.
Less
Nickel Used
Composite EN coatings can be routinely produced with up to 40% by
volume of codeposited particles. The implications are significant
in four aspects:
- Most simply,
this means that at least 40% less nickel is required to produce
composite coatings of equal thickness to a conventional coating
without such particles.
- Given the
greater wear resistance of composite EN coatings versus conventional
coatings, the deposit thickness of composite coatings can be significantly
less than conventional EN coatings. This means even less nickel
needs to be used.
- As such,
composite EN coatings last longer, parts will need to be recoated
or replaced less frequently. Again, resulting in even less nickel
used.
- The less
nickel the plating shop uses, the longer the baths will last.
This means less baths required, less waste treatment and less
waste.
Less
Nickel Released
Concern about the release of chrome, nickel and other metals into
the environment does not stop at the plating shop’s door. As
coatings wear, their constituents are released. Depending on the
application, they can be released into work areas, food applications,
sensitive assemblies and the environment as a whole. Composite EN
coatings have the further advantage, therefore, of preventing the
release of such metals based on the following four principles:
- Greater
wear resistance of the composite EN coatings reduces the release
of the coating into the environment.
- As the composite
coating can be up to 40% inert particles, the coating released
into the environment will be up to 40% less metal.
- As parts
last longer, they are not discarded into the environment as often,
and less replacement parts are required.
- Because
composite EN coatings can be chemically stripped, used parts can
be stripped and recoated, thereby reclaiming the nickel metal
in solution form for recycling.
There is one
other aspect worth considering. Chrome is often “over-plated”
on parts with complicated geometries to achieve the correct deposit
thickness in areas with lower current densities. Not only is this
an excessive use of chrome plating, it also requires grinding the
plated parts to the proper dimensional tolerances. This grinding
naturally releases chrome metal into the environment. Over-plating
and grinding also require additional and wasted energy consumption.
Here is a simple
analogy showing that “less can be more” in performance
and environmental terms. In the past, the government has required
the inclusion of various additives to gasoline. These additives
such as ethanol or oxygenated fuel serve to reduce the amount of
gasoline used and, subsequently, the amount of gasoline released
into the environment. This same principle is achieved by adding
inert particles to EN plating, as well as significant performance
advantages provided by the particles for hardness, wear resistance,
impact resistance, lubricity, etc., depending on the particles incorporated.
Background on Composite EN
Composite EN is intriguing because it intentionally introduces insoluble
particulate matter into the plating solution for codeposition into
the coating. The stability ramifications to the plating bath are
significant. One gram of 1.0-micron sized diamond particles, for
instance, contains 310,000,000,000 particles.1 This creates a surface
area loading near 100,000 cm2/liter, approximately 800 times the
preferred loading of a conventional EN bath.2
This natural
incompatibility between an inherently unstable, surface-area-dependent
plating bath and an extraordinary loading of insoluble particles
has been overcome by the precise addition of particulate matter
stabilizers or PMSs.3 The methods disclosed therein have made composite
EN plating reliable and commercially viable by modifying the Zeta
potential of particles in a plating system. Zeta potential is an
effect of electrostatic charge. A wide variety of particulate matter
is capable of codeposition in EN coatings. In each instance, the
plating bath must be modified to accept the specific particles and
produce an optimal coating.
Composite EN
coatings are regenerative because of the uniform manner with which
the particles are dispersed throughout the entire plated layer,
as observable in the cross sectional Figure 1. Particle matter suitable
for composite EN incorporation can be from nanometers up to approximately
10 microns in size. A narrow particle size range is specified for
each application. Certain performance benefits have been discovered
when a composite coating is generated simultaneously using two distinct
particle sizes. It is theorized that the smaller particles fill
the spaces between the larger particles.4 This also further increases
the percent by volume of the particulate matter and further reduces
the amount of nickel used.
Coating thickness
specifications are typically set on a value between 10 and 25 microns
(0.0005 -0.001 inch) for most applications. Very tight coating thickness
specifications can be established for particular applications and
routinely reproduced within a few microns by the plating shop. As
with conventional EN, composite EN coatings can be heat treated
after plating to enhance their hardness and adhesion to the substrate.
Depending on
the particle sizes and certain plating conditions, coatings can
be produced with a particle density of up to 40% by volume. Lesser
densities may not provide the maximum benefit available from the
particulate matter, and significantly higher densities risk premature
wear of the coating since there may not be enough of the metal “glue”
to prevent the particles from being removed. This observation indicates
that the typical wear mechanism of composite EN coatings is not
wear to the particles themselves, but rather wear to the surrounding
metal matrix that eventually allows the particles to be removed.
To date, coatings
designed for increased wear resistance have proven to be the most
widely used composite EN coatings. As this category of composite
EN coatings has the greatest potential to replace and surpass hard
chrome plating, and provide the health and environmental benefits
presented above, we will focus on this category. Within the wear-resistance
category, an extensive array of suitable particles can be used,
including diamond, silicon carbide, aluminum oxide, tungsten carbide,
boron carbide and chromium carbide. These materials differ not only
in hardness and wear resistance but also in their shape. Any of
these factors can affect surface and performance characteristics.
Table
I5 includes hardness measurements for various materials and
coatings. Due to the mechanism of standard indentation hardness
testing, true hardness evaluation of composite EN coatings is a
bit elusive. Because of this limitation of the test method, and
that such coatings are primarily employed for wear resistance (a
feature not necessarily directly correlated to hardness), a review
of various wear resistance testing is more useful. It should be
noted, however, that standardized wear testing methods are instructive
but cannot substitute for controlled testing of various composites
under the actual intended use conditions.
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Various test
methods have been employed to evaluate wear resistance of different
materials and coatings. Perhaps the most common test method is the
Taber abrasive wear test. In the Taber test method, a coated panel
turns under two rotating abrasive wheels. Wear is measured as the
weight loss of the panels following a specified number of rotating
cycles. The lower the wear index, the lower the wear to the coating.
The coatings and materials in Table II6 were
tested by 1,000 cycles on the Taber test device.
Table
III7 presents Taber abrasion test results for Nano-PlateTM
150 (a composite electroless nickel deposit with nano-sized diamond
particles) and hard chrome plating. These results are based on an
extensive test of 10,000 cycles.
Other test
methods also demonstrate the enhanced wear resistance of composite
EN coatings in comparison to hard chrome plating. It is instructive
to see the performance of materials under various wear conditions.
Figure 28 includes the results of the Yarnline Abrasive Wear Test,
where an abrasive yarnline under constant tension is drawn across
a material sample at a constant speed and force. Results are measured
in material removal over time as cu mil per hour and show the dramatic
difference between hard chrome plating and a composite EN coating
with silicon carbide particles.
Composite EN
coatings can offer excellent wear resistance and hardness compared
to hard chrome plating. Other application and performance benefits
of composite EN coatings over hard chrome plating have also been
presented. Composite EN coatings are available to replace and perhaps
surpass hard chrome plating. There are significant health and environmental
benefits created by the elimination of chrome. As composite EN coatings
can be reliably produced with up to about 40% by volume of codeposited
particles, such coatings further have the ability to reduce the
amount of nickel used and released into the environment.
References:
- Mypolex
Micropolycrystalline Diamond Powder, E.I. DuPont de Nemours
& Company, Inc., page 17.
- Feldstein,
N.; Lancsek, T.; Lindsay, D; Salerno, L.; Electroless Composite
Plating; Metal Finishing, August, 1983, pgs. 35-51.
- U.S.
Patents 4,997,686, 5,145,517, 5,300,330, and 5,863,616.
- U.S.
Patents 4,547,407 and 4,906,532.
- N.
Feldstein, Composite Coatings, Materials Engineering, Cleveland,
Ohio, (1981).
- N.
Feldstein, Composite Coatings, Materials Engineering, Cleveland,
Ohio, (1981).
- “Composite
Electroless Coatings with Nanometer Diamond Particles”,
Michael Feldstein, Nanomaterials Workshop, December 11,
2002.
- N.
Feldstein, Composite Coatings, Materials Engineering, Cleveland,
Ohio, (1981)
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