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

Abstract

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.