The influence of a multilayered metallic coating on fatigue crack nucleation

A thorough review of the literature on fatigue crack initiation indicates that for optimum resistance to fatigue crack initiation, a surface coating needs more than just a high hardness and that a combination of properties including toughness, cyclic work hardenability, residual compressive stresses, and adherence, in addition to a hardness higher than that of the substrate are required. Based on this assumption, it was hypothesized that nanometer-scale, multilayer coatings will posses a combination of these required properties enabling significant increases in fatigue crack initiation resistance. To test this hypothesis, fatigue experiments were conducted on Cu samples with different surface treatments including a nanoscale Cu–Ni multilayer. The fatigue lives of the multilayer coated samples were significantly greater than those of uncoated samples or samples coated with a monolithic coating of Cu or Ni indicating that the nanodimensional layering of the multilayer coating is responsible for retarding fatigue crack initiation and failure. The samples were examined with various analytical techniques including scanning and transmission electron microscopy and atomic force microscopy.