C. Eberl

Tailored 3D Mechanical Metamaterials Made by Dip‐in Direct‐Laser‐Writing Optical Lithography

Dip-in direct-laser-writing (DLW) optical lithography allows fabricating complex three-dimensional microstructures without the height restrictions of regular DLW. Bow-tie elements assembled into mechanical metamaterials with positive/zero/negative Poissons ratio and with sufficient overall size for direct mechanical characterization aim at demonstrating the new possibilities with respect to rationally designed effective materials.

Mechanical properties of bulk single crystalline nanoporous gold investigated by millimetre-scale tension and compression testing

In this work, the mechanical behaviour of millimetre-scale, bulk single crystalline, nanoporous gold at room temperature is reported for the first time. Tension and compression tests were performed with a custom-designed test system that accommodates small-scale samples. The absence of grain boundaries in the specimens allowed measurement of the inherent strength of millimetre-scale nanoporous gold in tension. The elastic modulus and strength values in tension and compression were found to be significantly lower than values measured with nanoindentation-based techniques and previously reported in the literature, but close to those reported for millimetre-scale polycrystalline samples tested using traditional compression techniques. Fracture toughness was found to be very low, in agreement with the macroscopic brittleness of nanoporous gold, but this is due to the localization of deformation to a narrow zone of ligaments, which individually exhibit significant plasticity and necking.

A novel high-throughput fatigue testing method for metallic thin films

Abstract Thin films are used in a wide variety of computing and communication applications although their fatigue behavior and its dependence on alloying elements are not very well known. In this paper, we present an experimental implementation of a novel high-throughput fatigue testing method for metallic thin films. The methodology uses the fact that the surface strain amplitude of a vibrating cantilever decreases linearly from the fixed end to the free end. Therefore, a thin film attached to a vibrating cantilever will experience a gradient of strain and corresponding stress amplitudes along the cantilever. Each cantilever can be used to extract a lifetime diagram by measuring the fatigue-induced damage front that progresses along the cantilever during up to 108 load cycles.

Damage analysis in Al thin films fatigued at ultrahigh frequencies

A quantitative damage analysis provides insight into the damage mechanisms and lifetimes of aluminum thin films fatigued at ultrahigh frequencies. Surface acoustic wave test devices were used to test continuous and patterned Al thin films up to more than 1014cycles. The analysis revealed increasing extrusion and void formation concentrated at grain boundaries. This finding and the observed grain growth indicate a high material flux at the grain boundaries induced by the cyclic load. A correlation between device degradation and defect density is established which is explained by a theoretical model. For stress amplitudes as low as 14MPa lifetime measurements showed no fatigue limit for 420nm Al thin films.

Comparison of the Stress Distribution and Fatigue Behavior of 10- and 25-

The stress distribution and fatigue behavior of nominally identical kilohertz fatigue resonators with two different thicknesses, 10 and 25 μm, was compared in this study. The results highlight the non-uniform 3-D stress distribution of the micron-scale notched cantilever beams that depends on the thickness. The areas corresponding to the first principal stress being within 2% of the maximum value are much smaller than the overall notch area and are a function of device thickness. It is also shown that the non-negligible influence of small, nanometer-scale geometrical variations in the dimensions of nominally identical devices on the maximum stress values can be accounted for by measuring the devices resonant frequency (f0). The observed scatter in the fatigue results of these microresonators is in part associated with the challenge in accurately calculating the local stress amplitudes. Despite that large scatter, the fatigue behavior of the 10 and 25 μm thick devices is similar. Particularly, the overall relative decrease rates in f0 are well related to fatigue life (Nf) and can be used to predict Nf within a factor of 5, for Nf ranging from 104 to 1010 cycles.

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Freestanding vapor-deposited gold films 0.15 μm, 0.5 μm, and 1.0 μm thick were tested in tension with strain measured directly in the gage section by digital imaging. The average grain size of all three films was ~ 60 nm with some large grains in the 0.15 μm material. Youngs modulus decreased as the thickness decreased - from 73.1 ± 6.7 GPa to 64.7 ± 9.9 GPa to 51.4 ± 10.6 GPa respectively. The yield stress decreased from 365 ± 25 MPa to 328 ± 28 MPa, but was 371 ± 36 MPa for the 0.15 μm film, which was more brittle. Microstructural studies provide no clear explanation for this behavior.Copyright

-Thick Deep-Reactive-Ion-Etched Si Kilohertz Resonators

Uniaxial tensile tests were performed on Cu/Nb multilayered foils to investigate yield strength and grain boundary strengthening in the layered foils at room temperature and in fine-grained Nb at 600 °C. At room temperature, yielding in Cu/Nb multilayered foils is controlled by deformation in both layers, and grain boundary strengthening is observed with a Hall–Petch slope ( k RT ) of 198 ± 56 MPa·μm 1/2 at a strain rate of 10 −4 s −1 . At 600 °C, yielding in Cu/Nb multilayered foils is controlled by deformation in just the Nb layers. Hall–Petch strengthening is observed over a range of strain rates, but the Hall–Petch slope decreases from 197 ± 71 MPa·μm 1/2 for a strain rate of 10 −4 s −1 to only 25 ± 40 MPa·μm 1/2 for a strain rate of 10 −6 s −1 . The significant drop in the Hall–Petch slope for Nb with decreasing strain rate indicates a change in the controlling deformation mechanism from dislocation glide to dislocation creep.

Tensile Stress-Strain Curves of Gold Film

Thin film processing has been a driving technology in microelectronics and mechanics for years. The reliability of such devices is often limited by the failure of thin films. Therefore a deeper understanding of fatigue mechanisms of thin films through experiments is necessary to develop physical based lifetime models. Thus, this paper focuses on a novel setup for micro beam bending of thin metal films on Si cantilever substrate and first results will be presented.

Grain boundary strengthening in copper/niobium multilayered foils and fine-grained niobium

Abstract In the present work, a fatigue assessment of the electric motor within the electric axle drive (electric axle) was developed using virtual load data. Now that the electric axle with torque vectoring function has been integrated into multiple vehicle classes, it is possible to create simulation models which allow the drive train to be mapped in various driving scenarios. Validation is performed by means of optical measurement methods (object tracking) which allow to determine load data for subsystems and components under variable torque and speed scenarios (e. g., handling course). In this paper, a load spectrum was created and used for fatigue analysis of the subsystem, namely the traction motor (EM) and torque vectoring motor. Based on the local stress, a finite element analysis (FEA) was carried out on the rotor, and investigations of strength and fatigue were performed on thin sheets of electrical steel. These S-N diagrams consider various factors such as material, temperature, and influence of specimen dimensions and are the basis for fatigue assessment based on the load spectrum from simulation.

Fatigue Testing of Thin Films

Abstract Mechanical fatigue at frequencies in the GHz regime in submicron metal thin films is governed by size and frequency effects. The cyclic load can lead to damage formation also known as acoustomigration and determines the reliability of micro electro mechanical systems and surface acoustic wave devices. The size and frequency effects dramatically change the fatigue behavior compared to bulk material. The resulting damage structure is similar to electromigration experiments where void and extrusion formation lead to failure. Here, a dislocation based mechanism is presented which explains the damage formation. This mechanism is induced by gradients in cyclic shear stress which is induced by the short acoustic wave length at frequencies in the GHz regime. Discrete dislocation dynamic simulations are presented that reflect the dislocation behavior at these ultra high frequencies.