Zenta Group (Chile)

The understanding of the mechanical properties of coated materials is a key factor in a number of technological applications. In particular, the delamination of compressed thin films is one of most limiting factor for the structural performance of the material and presents various interesting problems in physics and mechanics. The delamination patterns result from relaxation phenomenon of high residual compressive stresses in the thin film mainly due to the deposition process and involve the propagation, from an initial buckle, of cracks at the film/substrate interface. They have been extensively investigated in various experimental and theoretical studies [1][2] However, only a few experimental set-ups have been developed to measure the buckle growth dynamics. In fact, the additional compressive stress can be produced in the adherent thin film by using an experimental apparatus which allows the in-situ observation by AFM of the surface during deformation. Buckling patterns generated during the stress experiments are then described and discussed taking advantage of the high resolution offered by the atomic force microscopy (AFM). It has been observed that buckling structures may evolve from straight-sided wrinkles to either worm-like or varicose patterns when the applied stress is released. The formation of these two structures from the initial straightsided wrinkles has been characterized. In particular, it has been found that one of the driving forces of delamination is the relaxation of the stresses along the longitudinal axis of the initial wrinkles. In the case of bubbles, the film does not recombine with the substrate; in situ picosecond acoustic experiments [2] would allow studying the adhesion between film and substrates in this region of interest. Finite element simulations have been done in order to investigate the delamination evolution Confrontations with in situ stress mapping measurements have to be achieved for validating and then improving these simulations. In this work, we proposed to use the Scanning X-Ray microdiffraction technique developed at the ALS to perform in-situ spatially resolved stress measurements [1, 3] during a compression test on a film/substrate set. These measurements are expected to provide new insights on the first stage of the delamination process. Preliminary experiments have been done at ALS on gold thin films deposited on LiF substrates.