Electroplated chromium coatings have been widely used in the automotive, aerospace, manufacturing industries for applications such as internal combustion engine components, hydraulic cylinders and machine tools. Such coatings exhibit attractive properties such as high hardness and excellent wear resistance. Most coatings of this type are deposited from hexavalent chromium compounds, but there is significant environmental concern over such materials, and replacements are being sought. Alloy electrodeposits including nickel–tungsten, nickel–phosphorus and cobalt–tungsten have been considered as potential materials to replace conventional hard chromium deposits, and it has been shown that cobalt-rich alloys can exhibit much lower friction coefficient and higher wear resistance when compared with nickel-rich alloys. This work examines the tribological behaviour of nanostructured cobalt and cobalt–tungsten alloy electrodeposits sliding against a hard steel counterbody, and compares the behaviour of these couples with that associated with sliding of electrodeposited chromium against the hard steel. The wear rates of the cobalt–tungsten alloy electrodeposits are at least an order of magnitude lower than those of the unalloyed cobalt electrodeposits across the range of loads examined. The wear rate of the cobalt–tungsten is also lower than that of the chromium electrodeposit; moreover, at the highest loads examined, the chromium coating was seen to fail catastrophically by fracture whereas the cobalt–tungsten alloy was able to withstand sliding under the same conditions with little damage. In addition, when sliding against the cobalt–tungsten electrodeposit, the wear rates of the hard steel counterbody were two and three orders of magnitude lower than when sliding against the cobalt and chromium electrodeposits respectively. The results presented in this paper (along with an assessment of the literature concerning nickel and nickel–tungsten electrodeposits) show that electrodeposited cobalt–tungsten coatings are one of the strongest contenders for replacement of chromium electrodeposits that have been identified.

Friction and sliding wear behaviour of electrodeposited cobalt and cobalt-tungsten alloy coatings for replacement of electrodeposited chromium

Co–W alloys with 12–26 at.-%W (25–45 wt-%W) can be produced by electrodeposition from complexed aqueous plating baths. The deposits can be in the amorphous or nanostructured conditions, depending on the plating bath operations. Most recent attention has been focused on the nanocrystalline conditions which have hardness values in excess of 850 kg mm−2, which makes them competitive with hard engineering chromium. This may allow the coatings to replace hard Cr which is produced from hexavalent chromium baths having associated health and safety problems. Co–W coatings, 2–120 μm thick, can be deposited with a low incidence of cracking, with good corrosion and wear properties (dry and lubricated) when deposited on a variety of steel substrates. The underlying reasons, i.e. the amount of tungsten in solution, the density of grain boundaries and the reduction in bonding between coating and counterface under dry sliding conditions, which account for these enhanced properties are discussed. Potential applic...

Nanostructured Co–W coatings produced by electrodeposition to replace hard Cr on aerospace components

Microelectrodeposition of Co–Pt alloys for micromagnetic applications

The crystal structure and magnetic properties of Co-W (4–6 at % W) and Co-P (2–5 at % P) coatings produced by electrodeposition at various temperatures (Co-W coatings) or sodium hypophosphite concentrations in an electrolyte (Co-P coatings) are studied. The coatings consist of crystallites having an hcp structure with a [001] and/or a [100] texture depending on the deposition conditions. The structural characteristics, the magnetic nonuniformity, and the magnetic intercrystalline interaction are found to correlate with each other.

Structural characteristics, magnetic nonuniformity, and magnetic intercrystalline interaction in high-coercivity Co-W and Co-P coatings

Analysis is given of the effect of the dimensionality of structural inhomogeneities on the properties of the magnetic structure of ultradisperse ferromagnetic materials. The coordinate dependence of the magnetization dispersion of magnets with one-dimensional and two-dimensional inhomogeneities of magnetic anisotropy has been calculated. The linear dimension of a magnetic block for one-and two-dimensional inhomogeneities of the anisotropy field has been estimated. A method for exact calculation of the distribution function of magnetization rotations in the block is suggested.

On the Properties of a Stochastic Magnetic Structure of Low-Dimensional Ultradisperse Ferromagnets

Electrodeposited Co-W films having a columnar microstructure with a [001] texture and magnetic anisotropy of the easy-plane type were studied. Analysis of field dependences of the remanent magnetization and irreversible susceptibility showed that the films consisted of isolated single-domain particles coupled by dipole (magnetostatic) interaction.

Columnar structure and magnetic interaction in electrodeposited Co-W films

The structural and magnetic characteristics of Co-W coatings with a tungsten content from 0 to 20 wt % are compared. The coatings deposited at room temperature were found to consist of columnar crystallites with an hcp (cobalt) structure and the [001] axis perpendicular to the coating surface. It was suggested that the interaction between the crystallites is predominantly magnetostatic. In the coatings deposited at higher electrolyte temperatures, the [001 ] axis lies in the coating plane, the columnar type of the crystallites is replaced by a lamellar type, and the crystallites become larger. As a result, exchange interaction starts playing an important role in the magnetic interaction.

Structural and magnetic properties of electrodeposited Co-W coatings

The crystal structure and magnetic properties of Co-W (10-15 wt % W) and Co-P (2-5 wt % P) coatings obtained by electrochemical deposition onto copper substrates at various electrolyte temperatures (Co-W) and various concentrations of sodium hypophosphite in the electrolyte (Co-P) were studied. The coatings consist of individual crystallites with [001] and/or [100] textures determined by the deposition conditions. The structural and magnetic properties of the coatings were found to depend on the deposition conditions.

Structural properties and magnetic nonuniformity of high-coercivity coatings of Co-based alloys

Co-W and Co-Ni-W films with 15 to 17 wt % tungsten electrochemically deposited on copper substrates at different electrolyte temperatures were studied. The films were found to consist of individual grains of an hcp cobalt-based solid solution. The film structure and the character of magnetic interaction between the grains were determined in relation to electrolyte temperature.

R. I. Tagirov

Papers

Columnar structure and magnetic interaction in electrodeposited Co-W films

Structural and magnetic properties of electrodeposited Co-W coatings

Structural properties and magnetic nonuniformity of high-coercivity coatings of Co-based alloys

Intergranular magnetic interactions in electrochemically deposited Co-W films