Elastic behavior of fibre-textured gold films by combining synchrotron X-ray diffraction and in-situ tensile testing
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Elastic behavior of fibre-textured gold films by combining synchrotron X-ray diffraction and in-situ tensile testing D. Faurie, P.-O. Renault, E. Le Bourhis, P. Goudeau Laboratoire de Métallurgie Physique, UMR 6630 CNRS - Université de Poitiers, SP2MI, Bd Marie et Pierre Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France. ABSTRACT The elastic behavior of gold thin films deposited onto Kapton substrate has been studied using in-situ tensile tester in a four-circle goniometer at a synchrotron beam line (LURE facility, France). Knowing the stress tensor in the film, the strong {111} fibre texture was taken into account using the Crystallite Group Method (CGM). CGM strain analysis allows predicting a non linear relationship between strain and sin2 Ψ obtained for the thin films due to the strong anisotropy of gold. In contrast, the average of strains in longitudinal and transversal directions varies linearly with sin2 Ψ. The evolution of the slope of these curves as a function of the applied stresses in the film allowed determining the single-crystal elastic constant s44 of thin gold films. INTRODUCTION Synchrotron X-ray diffraction is well-known as a trustworthy and powerful tool to determine the stress state of small-sized crystalline materials such as thin films [1]. It is a phase selective and a non destructive technique which allows determining both the mechanical behavior and the microstructure of diffracting phases. The elastic change in interplanar distance is used to determine the residual or applied stresses, owing that the elastic constants are known [2,3]. Indeed, thin film elastic constants may differ from the bulk references as a result of the particular microstructure (nanometer grain size, high defects density, constraints caused by the substrate) [4-6]. Combining X-ray diffraction and in-situ uniaxial tensile tester, it is possible to determine X-ray elastic constants of diffracting phases in a thin film deposited on substrate. The first experiments were performed using metallic substrates [7,8]. More recently, the elasticplastic behaviors of Cu and Au thin films have been characterized using compliant polyimide substrates (Kapton®) [1,9,10]. Also, Badawi et al. [11] have determined elastic constants of non textured tungsten thin films studying the linear d-sin2 Ψ curves as a function of applied load, assuming an isotropic elastic behavior of both the compliant substrate and the thin film. For a fibre-texture thin film that is made of elastically anisotropic crystals, the Crystallite Group Method (CGM) [3], based on Reuss assumptions, can be used to determine the stresses from Xray diffraction, provided the dispersion of texture is not elevated (not higher than 10°) [12]. This method consists in idealizing the texture as a set of crystallites with the same orientation [3,13]. When we consider the case of a uniaxial tensile load applied on a film/substrate system, transversal stress is induced by the difference between the Poisson’s ratio of the thin film and that of the substrate. There
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