The effect of the mechanical polishing of the substrate surface on which thin films were deposited was investigated by 'in situ' tensile tests. These tests, performed in a Scanning Electron Microscope, showed different responses through cracking and debonding behaviour of the films, which could be related to the roughness of the substrate surfaces fixed by the granulometry of the polishing paste. The rougher surfaces produced an activation of film cracking and debonding earlier than the smoother ones. We also observed that the crack activation was dependent on the direction of the scratches which were produced before the deposition on the samples surfaces. Two directions of scratches were investigated, perpendicular and parallel to the sample tensile axis. A simple energy balance gave apparent values of the critical cracking energy of the film/substrate inter-faces.