Stress-induced, Debris-Modulated Friction and Wear Resistance Performance of Nanostructured Ni–Co Coatings

Owing to their HCP structure and inherent resistance to deformation, Cobalt-containing alloys are often used for low-friction and wear-resistant purposes. Here, the tribological performance of nanostructured Nickel-Cobalt coatings with 0-83 wt% Co was investigated. Three Co-rich coatings with similar microhardness and grain sizes showed different friction and wear behaviors that cannot be solely attributed to the HCP phase fraction. The coating with the highest Co content of 83 wt% achieved the lowest wear rate. However, the lowest surface friction was observed in Ni-76 wt% Co with a large amount of wear debris covering the contact surface of its counterpart. Nevertheless, these debris led to micro-cutting of the coating and a high wear rate. Detailed surface investigation of these coatings and their counterparts revealed a significant difference in material transfer, distribution of wear products, and wear mechanisms. Our analysis showed that the tribological performance of Ni-Co coatings can be dominated by surface-related features (such as surface oxide integrity, tribofilm, and wear debris) and lattice internal stress over the underlying phase fractions.