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What
is ENHP?
Electroless nickel high phosphorus - a unique brand of electroless
nickel
phosphorus chemistries offering 10 - 12% phosphorus in the plated
deposits,
the remainder being nickel.
What
is the ENHP process?
ENHP is a chemical redox reaction process whereby a spontaneous
chemical
reaction in weakly acidic solution produces a completely uniform
metallic coating
over the substrate surface. It can be applied to virtually any
material to enhance
the hardness, lubricity, wear resistance, and corrosion protection
of the surface
and thus increase the useful life of the product. 
What
makes it unique?
As a chemical redox reaction process any configuration of substrate
can be
coated. Unlike chroming and the creation of electrical transfer,
chemical redox
allows the plating of any configuration of substrate, from blind
holes and bolt
threads to impellers. As plated, ENHP has superior strength and
ductility to
chrome; post-plating heat treatment has produced hardness values
equivalent to
hard chrome. Most noteworthy, ENHP has superior corrosion resistance,
especially in acidic environments.
What
is the precipitated bake operation and how does it increase
the
hardness of the ENHP coating even more?
In some applications, after coating, the parts are exposed to
a precipitation
hardening process. This process precipitates nickel phosphide
throughout the
structure of the coating to increase the hardness and thus the
wear resistance. For
maximum hardness, this is done at 400 degrees C for one hour.
A post
plating bake not involving precipitation hardening can also be
done at 250 - 270
degrees C for 5 to 8 hours to offer improved hardness and adhesion
while
maintaining a structure conducive to corrosion resistance.
Is
the ENHP process the same for every product or do you tailor
it to suit the requirements of the product and the customer?
Each new application requires a certain amount of engineering.
It is important that
the background of the substrate and the environment that the coating
will be
operating in be clearly understood. The coating process and requirements
can be
tailored to the specific application both from an economic and
performance
point-of-view. To best meet customer needs we have developed an
Engineering
Data Sheet, that once completed, provides the full spectrum of
information
required by Chetana.
What
gives ENHP such superior corrosion resistance?
The key to the ENHP coating performance is the amorphous non-crystalline
structure. A completely uniform amorphous structure is compressively
stressed,
eliminating any solution migration to the substrate surface. The
strong bond
between the coating and the substrate surface as well as the ductility
of the coating
preserve the amorphous structure even in abusive environments.
What
is the temperature resistance of ENHP?
The final melting point of ENHP is 880 degrees C. Applications
are useful up to
temperatures of 400 degrees C as the deposit hardness will increase
up to this
temperature.
Can
you coat any kind of substrate metal or alloy using the
ENHP process?
Rather than describe what alloys can be coated, it is easier to
list what cannot be
coated: tungsten carbide and pure magnesium. In addition to metals
carbon,
graphite, and plastics have also been coated. Chetana is presently
configured for
ferrous alloys and non-ferrous alloys..
Does
the ENHP coating have to be machined or ground after
application?
In most applications, the as-plated condition of ENHP is dimensionally
correct.
This means that no secondary machining operations are usually
required to true up
the dimensions. ENHP is an electroless process, as compared to
hard chrome,
which is an electrolytic process. (In the electrolytic hard chrome
process, the
chrome is deposited from an anode, the part being the cathode.
As the current
travels the path of least resistance, the coating typically builds
up preferentially on
corners and edges with very little accumulation in recessed areas.)
ENHP does }
not use an external power source (the process being chemical not
electrical), so
anywhere that the part is exposed to the chemical bath, provided
the chemical
solution can be agitated uniformly, the same coating thickness
will result. ENHP
generally deposits to within 5% of total coating thickness specified.
Are
there any limitations on how thin, or how thick, the ENHP coating
can be?
Minimum of 5 um and maximum of 75 um can be comfortable
coated.
How
do you measure the hardness of ENHP?
The hardness of ENHP is measured using a micro-hardness tester.
It is an
industry standard piece of equipment, using a very small indentation,
done
under a microscope. Results are reported using the Knoop or Vickers
scale.
Unlike the hardness of an uncoated material, the hardness of a
coating is difficult
to measure. Generally, the measurement must be made on the cross-section
of a
sample. Conversion of micro-hardness measurements to other scales
can be
done on a theoretical basis, but with the validity of such conversions
subject to
debate. ENHP hardness, as converted to Rockwell, measures approximately
Rc 48 as plated and as high as Rc 68 after heat treatment.
Can
the ENHP coating, once applied, ever be removed?
ENHP can be chemically removed, or stripped, if required. For
example, if
ENHP is used on the inside cavity of a mould, the thickness of
the coating can
be monitored. Once the coating thickness is reduced to a predefined
value, the
remaining coating can be stripped off and a new coating applied.
In this way, the
coating is used as a long-life sacrificial surface and no damage
to the mould
occurs. Traditionally the surface of a mould is hardened using
a process such as
nitriding. When the hardened surface wears, the mould is eventually
rendered
unusable.
Does
the entire part have to be coated or is there some way to coat
only specific portions of it?
In applications where only portions of a part are to be coated,
a masking agent is
applied to the surface areas where the coating is not required.
Masking can be
labour intensive and therefore costly. In high volume production
and repair part
applications, it is often better to design the part to accept
full plating.
Is
ENHP as inflexible as chrome?
No. Due to the superior ductility of ENHP and greater ultimate
strength, a plated
surface has much greater resistance to an abusive environment.
Chrome, on the
other hand, can de-laminate from a flexing substrate.
In
what applications has ENHP been most successful?
ENHP is most successful in a corrosive environment, especially
acidic. The
amorphous nature of the as plated deposit affords better barrier
protection to
many industrial parts including ball, gate, plug, and check valves,
blow out
preventers, chokes, heat exchange equipment, pumps, compressors,
tubing
vessels, packers, and more. Its high ultimate tensile strength
and acceptable
ductility afford success in an abusive environment. It often replaces
stainless
steel and more exotic alloys used in compressor blades, turbines,
valves, pumps,
extruders, and blowers. For unmatched wear, composite coatings
of aluminium
oxide, boron carbide, boron nitride, or tungsten carbide in an
ENHP matrix have
achieved this success. For sliding wear reduction, a dispersion
of 20 % PTFE in
the ENHP matrix has proven effective. For hardness values orders
of magnitude
superior to hard chrome, composite coatings including dispersions
of diamond
and silicon carbide in an ENHP matrix have proven successful on
metal and
ceramic grinding and cutting tools.
What
advantages does ENHP have over hard chrome?
Unlike the crystalline structure of hard chrome, ENHP is amorphous
and thus
prevents migration of solution to the substrate surface. As such,
ENHP is an excellent coating for corrosive environments, especially
acidic. ENHP has
greater ultimate strength than chrome and superior ductility making
it much less
susceptible to fracture in abusive environments. Due to the high
hardness of
chrome, galling (the wear of a mated surface) often occurs; with
ENHP, however,
the part can be heat treated to render a hardness within 150 HV100
of the mated
surface, thus minimizing galling. Apart from its own properties,
ENHP has been
enhanced for particular applications through proven techniques
which include
duplex coatings, baking, alloying, and composite coatings.
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