Effects of Morphology on the Decohesion of Compressed Thin Films

Residually compressed thin films are susceptible to spalling from substrates. A prerequisite for this to occur is that a separation develops at the interface large enough to allow buckling. Thereafter, the mechanisms of spalling are well-established. In this article, the mechanics of formation of the initial separation are addressed. Perturbations on the interface are deemed responsible for this process. Calculations of energy release rates for various interface morphologies have revealed that aperiodic perturbations can initiate and extend the separations to a length sufficient for buckling. Conversely, periodic perturbations trap separations at dimensions too small to buckle. Illustrations are given for an alumina film (scale) on Ni-based superalloys. Implications for life prediction models are explored.