Mechanical Properties of Metallic Thin Films: Tensile Tests vs. Indentation Tests
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Mechanical Properties of Metallic Thin Films: Tensile Tests vs. Indentation Tests Nian Zhang, Changjin Xie, and Wei Tong Department of Mechanical Engineering, Yale University, New Haven, CT ABSTRACT The existing interpretations of indentation test data (either theoretical or numerical approaches) have been largely based on isotropic plasticity models of polycrystalline materials while most of the metallic thin films widely used in many microelectronic and MEMS applications are strongly textured with a few grains or only a single grain running through the thickness of the films. The multicrystalline nature of the thin films on correlating their indentation and tensile properties is the focus of our investigation. Using multicrystalline aluminum and copper alloy thin sheets as model material systems, both tensile tests and indentation tests were performed and the testing results were compared based on a 3D crystal plasticity finite element analysis. The correlation between the indentation data and the tensile test data (at an effective or equivalent strain) is critically examined for these two multicrystalline materials. INTRODUCTION Metallic thin films are widely used in many microelectronic and MEMS applications. While instrumented indentation techniques are commonly used to measure mechanical properties of these thin films, direct tensile testing of thin films is rather difficult if not impossible. A classical relationship has been established that the ratio between the Vickers hardness and the tensile flow stress at 8% plastic strain is about 3.0 for polycrystalline metals [1]. However, this may not be true for thin films because of their highly anisotropic microstructures that have only one or few grains along thickness direction but a much larger dimension in the other two directions. This paper reports the experimental observations and crystal plasticity finite element modeling results of the relationships between microhardness and tensile strength for aluminum and copper alloy multicrystals. These multicrystals share the same microstructure characteristics as thin films in that they have only one grain running through the thickness direction and lots of grains lying in the plane normal to thickness direction. The multicrystals are much larger in size (14.5mm-by-3mm-by-1mm and 20mm-by-4.5mm-by0.5mm for aluminum and copper alloys respectively) than the thin films and are thus much easier for performing both tensile and indentation tests and for characterizing the crystallographic orientations of individual grains by the electron backscattered diffraction (EBSD) pattern analysis. EXPERIMENTAL PROCEDURE AND RESULTS Two coarse-grained multicrystalline samples were used in our investigation: Al-0.5wt%Mg (grown by directionally casting) and α-brass C260 (70%wt Cu and 30%wt Zn, annealed for 1 hour at 850°C of the as-received material) with a grain size of about 1mm and 0.5mm respectively [2,3]. The polycrystalline samples made of the same α-brass C260 (annealed 1 hour at 500°C of the as-received material) were
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