Channel cracking during thermal cycling of thin film multi-layers

Abstract

This paper describes mechanisms that can lead to film channel cracking, even in scenarios when layer thickness is small. Computational models are used to illustrate conditions that lead to the introduction of channel cracks and their subsequent cycle- or time-dependent propagation. Results for elastic structures with periodic features are briefly reviewed to illustrate that small low-modulus sections promote cracking in adjacent layers because they allow for the release of strain energy in adjacent sections with high residual stress. Inelastic deformation in layers adjacent to the cracked layer may also act to increase the channel crack driving force, by allowing for increasing displacements that serve to release strain energy. Two inelastic mechanisms form the primary focus in this effort: rate-independent plasticity and creep. Analyses of a cracked film on an elastic-plastic layer reveal pronounced cyclic displacements (as known as ratcheting) when the misfit thermal strain amplitude in the ductile layer exceeds twice its yield strain. Similar behavior occurs in cracked films on layers susceptible to creep. Simulations are presented for both isolated cracks and periodic arrays of cracks, and illustrate that the likelihood of cracking grows dramatically with time. In both inelastic regimes, the upper limit on deformation is dictated by the residual stress in the elastic layer and the substrate dimensions. These results are discussed in the context of analytical models developed elsewhere and potential experiments.