Measurement of applied strains in thin films deposited onto polymer by synchrotron X-ray diffraction

Abstract

The objective of the present work is to study the elastic behaviour of composites exhibiting strong mechanical contrast components, namely, nanostructured metallic thin films deposited onto polymeric substrates. The stiff film/compliant substrate composites are submitted to complex stress fields during elaboration or applications and the use of a biaxial tensile device allows us mimicking the stress field of in-service thin films. The mechanical characterization of such structures and the relationship with the micro structure still require for further understanding both from fundamental and technological point of view. In such a way, the strain measurements at the macroscopic level have to be correlated with those at the microscopic level. In the present study, the applied strains were measured in situ both by X-ray diffraction (at the microscopic scale) and by digital image correlation (at the macroscopic scale). We used the biaxial testing machine dedicated to the DiffAbs beam line at the French synchrotron SOLEIL, which allows for controlling equi- or non-equi-biaxial loading on thin films supported by polymeric substrates. The metallic thin films were produced via Physical Vapour Deposition onto the centre of a cross-shaped polyimide substrate (Kapton®). The film-substrate composites were then deformed in the DiffAbs X-ray goniometer. Grain interaction models are very simple in the case of perfect local elastic isotropic materials such as tungsten. Such a material allows for directly comparing micro-strains (X-ray elastic strains) to macro-strains (DIC strains). The first results concerning W/Cu thin films deformed in situ with this biaxial testing machine are presented.