Multilayered metallic thin films often display enhanced mechanical properties, such as hardness and yield stress, as the bilayer repeat length is reduced below about 100 nm. This behavior suggests that surface coatings of such films on metallic substrates may lead to other mechanical property improvements, such as enhanced fatigue life. Unlike other hard coatings that have been used to improve fatigue life, which are often brittle, multilayered thin films display a substantial amount of ductility, and this feature may also have a significant impact on the surface deformation that is produced by cyclic loading.
This research evaluates whether thin, multilayered surface films can be used to alter the surface deformation and crack initiation mechanisms during cyclic loading and thereby extend the fatigue life. A model system consisting of annealed polycrystalline copper substrates and electrodeposited copper-nickel films was evaluated in bending fatigue to examine the relationships between the fatigue properties of a well characterized substrate material and of this same material with different surface coatings including thin, multilayered films. The results will be presented in relation to the fundamental mechanisms of fatigue crack initiation, the influence of the multilayered surface film, and the potential applications.