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Thin films and coatings are used in a wide range of technological applications, such as microelectronics, packaging or corrosion barriers for engines. Thin films elaborated by sputtering methods often develop high residual stresses during the deposition process, sometimes about few GPa in compression. Such large residual compressive stresses may cause the nucleation and growth of buckling structures, resulting in the lost of functional properties that were conferred on the film/substrate composites. Spontaneous buckling patterns observed just after the deposition process are of various dimensions and structures, mainly depending on the internal stresses and film thickness. In many cases, the delaminated part of the film adopts a sinusoidal shape behind a propagation tip and looks like telephone cords. Anisotropy was at one point thought to be essential for the phenomenon. However, it has been experimentally established that undulated blisters propagate also in films under isotropic stress. Instabilities resulting from the mixed-mode nature of the interfacial crack are established at the crack tip and are responsible for such characteristic buckling patterns. In this context, we have experimentally demonstrated that both the telephone cords and varicose structures can result from morphological instabilities, referred to as secondary buckling. This latter transition has been fully characterized in the frame of non-linear finite element simulations by determining the successive equilibrium states of the film under stress. A mapping of the various stable post-critical buckling equilibriums has been established. The computed phase diagram is of great interest to interpret the spontaneous buckling structures commonly observed just after the deposition process. It is shown that the telephone cord patterns are stable for strong stress levels. The state of high isotropic stresses is located in the telephone cord area. This explains why this buckling structure spontaneously develops in most of the observed highly stressed thin films and coatings, mainly deposited by sputtering methods. The straight-sided and bubbles patterns are more probably seen on films submitted to moderate stress levels associated with a strong anisotropy of stresses and/or elastic properties. |