Mechanical properties and size effect in nanometric W/Cu multilayers
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Mechanical properties and size effect in nanometric W/Cu multilayers
P. Villain, D. Faurie, P.-O. Renault, E. Le Bourhis, P. Goudeau, K.-F. Badawi 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 mechanical behavior of W/Cu multilayers with periods ranging from 24 down to 3 nm prepared by ion beam sputtering was analyzed using a method combining X-ray diffraction and tensile testing, and instrumented indentation. Cracks perpendicular to the tensile axis observed by optical microscopy were generated in the films under the largest applied tensile stresses. These cracks may appear in the multilayer while W layers are still in a compressive stress state. Elastic modulus and hardness values were extracted from nano-indentation data. Crack initiation and elastic constants were observed to depend on the period of these multilayers. Keywords: Multilayers, X-ray diffraction, mechanical properties, cracks, size effects
INTRODUCTION In nanocrystalline materials or thin films with nanometer scale thickness, surface contribution becomes preponderant yielding deviations from the average elastic behavior of the material. Moreover, the mechanical properties are known to differ from those expected from the bulk state. Multilayers have attracted much attention since one dimension can be tailored down to the nano-scale, leading then to novel electronic, magnetic, optical and mechanical applications. A lot of studies [1-6] have been devoted for experimentally characterizing the length scale dependence of strength and also the theory and modelling of deformation mechanisms in nano-scale multilayers. Deviations from Hall–Petch extrapolation at nanoscale layer thickness are observed as well as very high yield strengths. These behaviors cannot be then explained by a simple extrapolation of scaling laws such as the Hall–Petch relationship. This indicates also the crucial role played by interfaces on the deformation behavior of nano-scale materials. Here, W/Cu multilayers with periods ranging from 24 down to 3 nm have been prepared by ion beam sputtering and characterized using X-ray reflectometry, X-ray diffraction, instrumented indentation and energy dispersive analysis in a scanning electron microscope. The elastic behavior of W sub-layers has been analyzed using a new method combining X-ray diffraction and tensile testing [7]. Young’s modulus softening has already been observed when reducing the period down to 3 nm. A similar effect is also observed for the stress free lattice parameter. Extended X-ray Absorption Fine Structure (EXAFS) measurements have revealed that surface alloying may occur in tungsten sub-layers for the smallest periods leading then to lattice parameter decrease [8].
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In the present paper, we will focus on the crack initiation under tensile testing and indentation, and the correlation between film damage and microstructural and elastic features.
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