|
Electroplating
Tungsten Carbide/Cobalt-Nickel
Summary: Inframat Corporation (IMC) had demonstrated that high hardness and
wear resistant WC/Co-Ni nanocoating could be obtained via a co-electrodeposition
of WC/Co nanoparticles from a nickel sulfamate colloidal solution
[1]. Properties of this WC/Co-Ni nanocoating compared with conventional
thermal sprayed WC/Co and electroplated hard chrome are listed in
Table 1. Benefits derived from this nanocoating include high hardness
and wear resistance, low coefficient of friction, and a smooth deposited
surface. This previously conducted NSF project concentrated on the
electrodeposition of WC/Co-Ni nanocoatings to replace electroplating
hard chrome for tribosurfaces. With this success, IMC is currently
looking to establish a prototype process for commercial applications.
Other electrodeposition experience at IMC include the deposition of
hydroxyapatite nanocoatings on Ti prosthesis for artificial hip implant
[2] and lead coatings on Ti grids for Naval Air Warfare’s lightweight
V-22 battery [3] applications.
Table
1. Highlights of IMC’s electrodeposited WC/Co-Ni
nanocoatings compared to thermal sprayed WC/Co coatings [1]
| Coating
Processing Techniques |
WC (Vol.%) |
Hardness (VHN) |
Wear
Rate (mm3/Nm) |
Friction
coeff. |
Surface
roughness |
| Electroplated
n-WC/Co-Ni |
25-30% |
850 |
0.4 x 10-5 |
0.31 |
~ 0.7 µm |
| Thermal-sprayed
WC/Co |
88% |
~1050 |
0.1 x 10-5 |
0.5 |
~ 3 µm |
|
Introduction: Engineered tribocoatings offer a significant opportunity as they
enhance component reliability and reduce life-cycle cost. These
coatings are either thermal sprayed using chromium or tungsten carbide
particles or electroplated using hard chrome. Electrolytic hard
chrome, however, suffer from severe environmental problems due to
the emission of hexavalent chromium ions, which are carcinogenic.
Hard chrome coatings are now being replaced by High Velocity Oxy
Fuel (HVOF) coatings. It is estimated that at least 20-30% of the
DOD’s chromium plated parts have complex geometries and fall
in the category of NLOS applications that are not amenable to HVOF
technology. Therefore, an environmentally friendly electrodeposited
tribocoating having properties similar to those of hard chrome will
satisfy this NLOS market need and achieve the EPA’s goal by
offering a clean deposition technology.
Electroplating
Co-deposition process: WC nanoparticles are electrolytically
codeposited with a Ni-Co matrix, which potentially can offer the
desired hardness, friction/wear characteristics, and erosion and
corrosion resistance of hard chrome. The suspend WC nanoparticles
are in a colloidal solution, thus generating a superior surface
finish, and a uniform coating thickness on all sides of complex
shapes can be achieved without expensive mechanical finishing.
The already
demonstrated unique combination of low-friction and low-wear rates
minimizes heat generation at contact surfaces and enhances component
life and reliability. The integrity of electrodeposited nanocoating
is superior as the bonding will be metallurgical, and the coating
is 100% dense. Besides environmental issues, electroplated hard
chrome exhibits severe cracking, which promotes under coat oxidation,
and grain pull-out during service, the latter due to its large grain
characteristics. Because of the small WC grain characteristics,
which are finely dispersed in the Ni-Co metal matrix, all interphase
interfaces of the WC/Co-Ni nanocoatings have superior metallurgical
bonds preventing coating delamination and hard-particle pull-out.
The effect of particle size is schematically illustrated in Fig.
1.
The friction
coefficient has been found to decrease with grain size. The wear
tracks of nanophase WC coatings have been demonstrated to exhibit
a lower surface roughness compared to other electrodeposited coatings
containing 0.5-2µm silicon carbide (SiC) particles, currently
being used. Also, these particles are generally not metallurgically
bonded to the coating matrix causing particle pull-out. Both of
these phenomena are expected to increase the friction coefficient
and wear rate of the coating.
Fig.
1. Effect of particle size on wear track; a rougher wear
track resulting from a larger particle or grain size will exhibit
a higher friction coefficient and wear rate. |
A true colloidal solution is necessary for uniform codeposition
on all surfaces and hence the need for WC nanoparticles. IMC’s
knowledge of dispersants and surfactants and other commercially
available dispersants are critical to achieve a stable colloidal
solution with nanophase WC in suspension. This colloidal solution
mixes with nickel sulphamate solution to codeposit Ni and WC/Co
particles as shown in Fig. 2a. The resultant coating consists of
a finely dispersed 30 nm size WC grains in a Ni-Co matrix (Fig.
2b). These nanometer sized WC grains compared to microsize SiC particles
lead to a smoother wear track and hence low friction/low wear compared
to nickel coatings containing SiC (see Fig. 1).
Fig.
2. Schematic diagram of the electrolytic codeposition of (a) Ni and
n-Co/WC particles,
and (b) tribological coating containing n-WC/Co particles in a Ni
matrix. |
A uniform dispersion of nanoparticulate WC with volume fraction
of about 30% has been achieved. This system uses a highly efficient
Ni/Co sulfamate bath containing and nanoparticulate WC, resulting
in superior properties of surface tribocoatings. With about 30 vol%
WC nanograins, this WC/Co-Ni system shows a considerable promise
as a coating hardness of 850 Hv has already been achieved.
The typical
as-plated microstructure is shown in Fig. 3a scanning electron microscopy
studies, which illustrates a fine distribution nanoparticulate WC
in a 100% dense matrix of a Ni-Co alloy. Two types of morphologies
are present, (1) agglomerated n-WC particles (pockets of many WC
nanoparticulates with a bright contrast), and (2) a gray contrast
showing the cobalt – nickel matrix. Detailed microstructure
cannot be seen at this low magnification. Transmission electron
microscopy (“TEM”) thin foils were prepared using an
ion milling technique of the cross-sectioned WC/CoNi coating. Typical
TEM micrographs are shown in Fig. 3b. In the TEM studies, we paid
particular attention to the Ni matrix area as shown in Fig. 3a of
the SEM micrograph. This area, when magnified using TEM, again showed
two types of microstructure, including (1) areas of ~200 nm-size
WC agglomerated particles embedded in the CoNi matrix, and (2) 10-30
nm WC nanoparticulates embedded in the CoNi matrix.
Fig.
3. Microstructures of electroplated WC/Co-Ni nanocoating
showing two levels of WC nano-particle distribution in the 100%
dense Ni-Co alloy matrix, (a) SEM: agglomerated WC nano-particulates
(~ 5 mm agglomerates) and the Co-Ni matrix, and (b) TEM: dispersion
of 20 nm WC nanoparticles in Co-Ni matrix. |
References
[1]. Z.T. Zhang, A. Datta, and T.D. Xiao, “Electrochem. Codeposited
Nanocomposites Made from Nano-particles,” IMC invention disclosure,
supported by NSF SBIR Progr., Cont. No. DMI 9961316 (1999)
[2]. a). Z.T. Zhang & T.D. Xiao, “Multi-layer coating
useful for the coating of implants,” U.S. Pat. Appl. 2003099762
A1 20030529 (2003); b) NIH SBIR Progrs., Contract Nos. 1R43AR47278-02
(2002) and 2R44AR047278-02A1 (2003)
[3]. Z.T. Zhang, J.X. Dai, and T.D. Xiao, “Titanium grids
for lead acid cells,” IMC Internal invention disclosure, supported
by Navy SBIR Program, Contract No. N68335-03-C-0068, June 15, 2003.
|
