Models for saturation damage state and interfacial shear strengths in multilayer coatings

The present work investigates the saturation damage state of a two-layer coating on a substrate (layer 1/layer 2/substrate) under uniaxial tensile loading in order to derive expressions for the interfacial strength between layer 1 and layer 2, and between layer 2 and substrate. It is based on experimental data on specimens where layer 1 is an inorganic film, layer 2 is an organic coating and the substrate is a polymer. The analysis is relevant to the cases where layer 1 cracks first, followed by layer 2, in which cracks appear due to stress concentrations caused by the cracks in layer 1. It considers the cases where at least one interface is completely yielded with shear stress equal to the interfacial shear stress, and where the crack density in layer 1 is equal to or higher than the crack density in layer 2. The possible situations depend on the relative shear strengths between layers 1 and 2 and between layer 2 and the substrate. The interfacial shear strength between layer 1 and layer 2, and between layer 2 and substrate are derived for elastic and yielded stress transfer cases and found to frame experimental values obtained with single-layer coatings.