Tribological behavior of nanocrystalline nickel

During the last decade, an intensive investigative effort around the globe has been devoted to the understanding of scale effects on materials properties. In spite of their importance, nanoscale effects on tribological properties have attracted little attention. Such effects are of utmost importance to small scale devices such as nano and micro electromechanical systems that contain nanostructured dynamic components that would be difficult to replace or repair. The significant increase in strength arising from the grain size reduction in the nano domain is expected to impact on mechanical processes at asperity contacts that are dominating wear behavior. In the present work, nanocrystalline Ni produced by electroplating was used as a model system to study scale effects on tribological behavior. It was found that compared to bulk (microcrystalline), nanocrystalline Ni can cause a significant reduction in both, the coefficient of friction and wear rate. A consistent relationship was found between grain size, hardness and tribological behavior. It is suggested that the improved tribological behavior of the nanocrystalline Ni is due to the refinement of mechanical processes inhibiting plastic deformation by extensive dislocation motion leading to fracture events.