In-Situ Observation and Mechanical Criterion on Interface Cracking in Nano-Components
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In-Situ Observation and Mechanical Criterion on Interface Cracking in Nano-Components T. Kitamura1 and T. Sumigawa1 1 Department of Mechanical Engineering & Science, Kyoto University, Kyoto, Kyoto 606-8501, Japan. ABSTRACT We have investigated the criterion of interfacial crack initiation in nanometer-scale components (nano-components) by means of a loading facility built in a transmission electron microscope (TEM). Three types of experiments are conducted in this project. (1) In order to clarify the applicability of conventional continuum mechanics to the nano-components, we prepare cantilever specimens with different size, which introduce different stress fields, containing an interface between a 20 nm-thick copper (Cu) thin film and a silicon (Si) substrate. These demonstrate the validity of the “stress” criterion even for the nano-scale fracture. (2) In order to examine the effect of microscopic structure on the mechanical property, we fabricate a bending specimen in the nano-scale with thin Cu bi-crystal (the thickness of about 100 nm) formed on Si substrate, of which understructure can be observed in situ by means of a TEM during the mechanical experiment. The initial plastic deformation takes place near the interface edge in a grain with a high critical resolved shear stress and expands preferentially in the grain. Then, the plasticity appears near the between Cu grain boundary and Cu/Si interface, and this development brings about the interfacial cracking from the junction. These indicate the governing influence of understructure on the mechanical property in the nano-components. (3) In order to investigate the fatigue behavior of metal in a nano-component, a cyclic bending experiment is carried out using nano-cantilever specimens with a 20 nm-thick Cu constrained by highly rigid materials (Si and SiN). The high strain region is in the size of 20-40 nm near the interface edge. The specimen breaks along the Cu/Si interface before the maximum load under the fatigue loading. The load-displacement curve shows nonlinear behavior and a distinct hysteresis loop, indicating plasticity in the Cu film. Reverse yielding appearing after the 2nd cycle suggests the development of a cyclic substructure in the Cu film. These indicate that the crack is caused by characteristic understructure owing to fatigue cycles. INTRODUCTION MEMS (Micro electro mechanical system) and NEMS (Nano electro mechanical system), which are composed of large number of submicron- or nano-scale dissimilar materials, often experience extrinsic loads due to operational and environmental conditions and these bring about local fracture. In order to make an accurate life prediction, the mechanical properties of components (e.g., deformation, fracture, and fatigue behavior) have to be understood well. However, since there are numerous uncertainties in the mechanical property of micro-nano scale components, we have to specify it experimentally. The strength evaluation of nano-component involves major challenges in experimental set-up, specimen fabrication
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