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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.

[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.


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