The Environmental
Benefits of Composite Electroless Nickel Coatings
Eliminating Chrome, and
Reducing Nickel
By Michael D. Feldstein
Surface Technology, Inc.
Trenton, New Jersey
The
metal 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
under short-term exposures. Long-term effects include damage to the
liver, kidneys, circulatory 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, 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 those 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.
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.
Figure
1 – Cross sectional photomicrograph of a composite EN coating with
over 40% by volume particles in EN at 1,000X magnification.
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:
1.
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.
2.
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.
3.
As such, composite EN coatings last longer,
parts will need to be recoated or replaced less frequently. Again, resulting in even less nickel used.
4.
The less nickel the plating shop uses, the longer
their 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:
1.
Greater wear resistance of the composite EN
coatings reduces the release of the coating into the environment.
2.
As the composite coating can be up to 40% inert
particles, the coating released into the environment will be up to
40% less metal.
3.
As parts last longer, they are not discarded
into the environment as often, and less replacement parts are required.
4.
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 requires 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 Electroless Nickel
Composite EN
is intriguing as 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 their 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 date 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 I 5 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. Due to 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.
Table I – Hardness Measurements
of Various Materials and Coatings
Material
Hardness
P2 Steel
400 VHN
Electroless Nickel 950
VHN
Hard Chrome Plating 1,000 VHN
Composite EN with 2 Micron Diamond Particles 1,161
VHN
Diamond
10,000 VHN
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 II 6 were tested
by 1,000 cycles on the Taber test device.
Table II – Taber Abrasion Testing of Various Materials and
Coatings
Coating
or Material Wear Index
Composite diamond-EN
0.0115
Cemented tungsten
carbide
0.0274
Grace C-9 (88WC,
12 Co)
Electroplated
hard chromium
0.0469
Tool steel,
hardened Rc 62 0.1281
Table III 7
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.
Table III –
Taber Abrasion Testing of Various Materials and Coatings
Material
Wear Index
Nano-PlateTM
150 0.0013
Hard chrome
plating
0.04
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 2 8 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 against the test piece.
Results are measured in material removal over time as mil3
per hour, and show the dramatic difference between hard chrome plating
and NiPlate 700TM, a composite EN coating with silicon
carbide particles.
Figure
2 - Yarnline Abrasive Wear Test Results
Conclusion
Composite
EN coatings can offer superior 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, therefore, are available to replace
and likely surpass hard chrome plating.
There are significant health and environmental benefits created
by this 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. PF
Notes
1. Mypolex â Micropolycrystalline Diamond Powder, E.I. DuPont de Nemours & Company,
Inc., page 17.
2. Feldstein, N.; Lancsek, T.; Lindsay, D; Salerno, L.; Electroless
Composite Plating; Metal Finishing, August, 1983, pgs. 35-51.
3. U.S. Patents 4,997,686, 5,145,517,
5,300,330, and 5,863,616.
4. U.S. Patents 4,547,407 and
4,906,532.
5. N. Feldstein, Composite Coatings, Materials Engineering, Cleveland,
Ohio, 1981.
6. N. Feldstein, Composite Coatings, Materials Engineering, Cleveland,
Ohio, 1981.
7. “Composite Electroless Coatings
with Nanometer Diamond Particles”, Michael Feldstein, Nanomaterials
Workshop, December 11, 2002.
8. N. Feldstein, Composite Coatings, Materials Engineering, Cleveland,
Ohio, 1981.
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