Microstructural dependence of the fracture toughness of metallic thin films: A bulge test and atomistic simulation study
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Microstructural dependence of the fracture toughness of metallic thin films: A bulge test and atomistic simulation study on single-crystalline and polycrystalline silver films Eva I. Preiß1, Hao Lyu1, Jan P. Liebig1, Gunther Richter2, Florentina Gannott3, Patric A. Gruber4, Mathias Göken1, Erik Bitzek1, Benoit Merle5,a) 1
Materials Science & Engineering, Institute I, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen D-91058, Germany Max-Planck-Institute for Intelligent Systems, Stuttgart 70569, Germany Technology Development and Service Unit for Nanostructuring, Max-Planck-Institute for the Science of Light, Erlangen 91085, Germany 4 Institute for Applied Materials, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany 5 Materials Science & Engineering, Institute I, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen D-91058, Germany; and Interdisciplinary Center for Nanostructured Films (IZNF), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen D-91058, Germany a) Address all correspondence to this author. e-mail: [email protected] 2 3
Received: 6 June 2019; accepted: 2 August 2019
The microstructure contribution to the very low fracture toughness of freestanding metallic thin films was investigated by bulge fracture tests on 200-nm-thick {100} single-crystalline and polycrystalline silver films. The single-crystalline films exhibited a significantly lower fracture toughness value (KIC = 0.88 MPa m1/2) than their polycrystalline counterparts (KIC = 1.45 MPa m1/2), which was rationalized by the observation of an unusual crack initiation behavior—characterized by twinning in front of the notch tip—during in situ testing in the atomic force microscope. Twinning was also observed as a dominant deformation mechanism in atomistic simulations. This twinning tendency is explained by comparing the resolved shear stresses acting on the leading partial dislocation and the full dislocation, which allows to develop a size- and orientation-dependent twinning criterion. The fracture toughness of polycrystalline samples was found to be higher because of the energy dissipation associated with full dislocation plasticity and because of crack meandering along grain boundaries.
Introduction Thin films, even when made out of the most ductile metals, exhibit an extremely low fracture toughness compared with their bulk counterparts [1, 2, 3, 4, 5]. As an example, the fracture toughness of 200-nm-thick gold membranes was measured to be only ;2 MPa m1/2 [6], far off from the 50– 100 MPa m1/2 reported in the literature for standard bulk specimens [7]. The scaling relationship between thickness and fracture toughness of thin films has already been investigated in detail in [1, 2, 3, 4, 8, 9]. The focus of the present study is, therefore, on identifying additional factors contributing to the low toughness of thin metallic films. A preliminary study [10] evidenced that this low toughness is connected to the strong confinement of the plastic deformation in front of the notch, which strongly limits
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