Study of multi cracking of brittle thin films and brittle/ductile multilayers on compliant substrate
Thin films coatings are used in many high technology applications particularly in microelectronics devices. Using flexible polymers as a substrate on which thin films (metal, oxides or organic) are bonded can give rise to new industrial applications such as OLED, flexible electronics or flexible photovoltaics devices. The mechanical stability and failure behavior of multilayer structures deposited on flexible substrate has been extensively studied both experimentally and theoretically [1,2,3] Several relaxation mechanisms in thin films have been identified such as channel crack, debonding or telephon cord. The objective of this study is to understand the multi cracking phenomenon of the silver and/or zinc oxide layers of various thicknesses coated on elastoplastic substrates (ETFE) The mechanical behavior of these systems is highly dependent on the elastic modulus mismatch between the film and the substrate, the plasticity of the substrate and the ductility/brittleness of the film. In order to clarify this mechanism, we present additional experimental study using a uniaxial tensile tests, similar to a recent work results [4] These tests confirm the existence of different fracture regimes and of a saturation in cracks density at high strain with large opening of the existing cracks. Conventional models in literature such as Xia Hutchinson model [3] and the "Shear lag" formalism [1,2] do not account for those experimental observations. We show that taking into account the plasticity in the substrate permit to extend Xia Hutchinson [3] model to an elasto plastic model that capture the saturation regime in crack density. In order to further validate this new model and the experimental observations, we present a numerical study which uses a cohesive zone model for the interface and takes into account the plastic behavior of the substrate. The redistribution of stresses during film cracking sheds a light upon the stage of saturation observed experimentally.