Effect of Interface Undulations on the Thermal Fatigue of Thin Films and Scales on Metal Substrates

Oxide scales that form on superalloys eventually spall, especially upon thermal cycling. This phenomenon is motivated by the large residual compression in the oxide. Buckling and interface crack propagation are known aspects of this behavior, but the origin of the interface separation that precedes and activates buckling is not understood. One mechanism is described and analyzed in this article. It relies on the observation that the interfaces are typically non-planar. Such non-planarity can result in cyclic straining of the substrate, near the interface, leading to crack initiation by fatigue. The conditions that lead of cyclic plasticity are analyzed for a typical range of parameters.