A comparative study on the tribological behavior of nanocrystalline nickel and cobalt coatings correlated with grain size and phase structure

Nanocrystalline (nc) nickel and cobalt coatings with different grains size were produced by the control of current density during pulse electrodeposition process. Effects of grain size reduction on the tribological properties of nanocrystalline Ni and Co coatings associated with phase structure were investigated comparatively. An important and interesting result is that the wear volume loss of all the Co coatings are more than one order of magnitude lower and the friction coefficient are nearly two times smaller than that of Ni coatings with almost the same grain size under the same wear conditions. For the nanocrystalline Ni coatings with face-centered cubic (fcc) structure, hardness related to grain size plays a dominating role in the tribological behavior, whereas in the case of nanocrystalline Co coatings with hexagonal close-packed (hcp) structure, the tribological behavior was slightly influenced by the hardness. The difference in friction and wear behavior between Ni and Co coatings as a function of grain size or hardness can be attributed to their different phase structure and the corresponding wear mechanism.