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In-situ deformation of single crystal nanowires in TEM

SEM micrograph of the nanowire transfer process

In-situ transmission electron microscopy studies of the deformation of single crystal metal nanowires. – Recent studies have confirmed that “smaller is stronger” in metal samples down to sizes as small as several tens of nanometers. However, the reasons for this increase in strength remain highly controversial. Recent advances in nanowire fabrication, manipulation and handling of nano-size objects, and in-situ mechanical testing methods, are making direct observation of deformation at small length scales possible. This demanding project will involve the handling and transfer of nanowires in a focused ion beam microscope and the observation of these wires during tensile testing in a transmission electron microscope. The goal is to understand how deformation proceeds in small crystal volumes.

At present, Au and Cu wires are being supplied by two collaborators: Dr. Thomas Cornelius, GSI, Darmstadt, and Dr. Gunther Richter, MPI-MF, Stuttgart.

TEM weakbeam image of a dislocation in a deformed nanowire

Mechanical Characterization of Metal-Polymer-Nanocomposites

Multilayer thin films with dimensions at the nanometer scale represent a new class of materials which exhibit improved mechanical properties owing as a result of size effects. The choice of metals and polymers as multilayer materials promises the systematic manipulation of composite mechanical properties such as elastic modulus and hardness. Mechanical characterization of multilayer thin films is done by performing uniaxial compression tests on columns machined with a focused ion beam microscope. The compression tests are performed using a nanoindenter with a flat diamond punch. The figures show a column with a diameter of 2m before and after compression as well as the corresponding load-displacement data. In the future, a tensile testing machine will be developed to perform micro-tensile tests in the in-plane direction of freestanding multilayered films.

In-situ nanoindentation of thin films with STM-tips

The gerneral increase of the strength of metal with decreasing characteristic length is by now not completely understood. Especially the generalty over several orders of magnitude is difficlut to explain since the dislocation activity changes in nano scaled materials. One way to study dislocations is the investigation of the surface step that form when a dislocation reaches the surface of the sample. In this work we use an UHV scanning tunneling microscope (STM) to measure the surface topography of thin film on substrates before and after plastics deformation. The plastic deformation is done by performing (relativly uncontrolled) nanoindentations with the STM-Tip, using the same tip for imaging afterwards. By analysing step heights and directions one can gather information about the dislocation. In the future plastic deformation will also by done with an in-situ tensile loading system.
STM image of Nb thin film before and after plastic deformation

Thickness Effects in Very High Cycle Fatigue Behavior of Cu Films

It has recently been established that decreasing the thickness of metal films leads to an increase in fatigue life and a change in fatigue damage morphology. This has been correlated with the high strength of thin films and attributed to their limited dislocation activity. The current research extends fatigue studies below 100 nm film thickness and to ultra high cycle numbers. Such studies are of great relevance for predicting the reliability of metal components used in micro- and nanotechnology, where they often experience repeated, low-level mechanical or thermal-mechanical loading. A resonant cantilever method has been developed using an AFM to test and monitor the fatigue behavior of Cu films with thicknesses between 20 nm and 2 �m. Strain amplitudes up to 0.1 % and cycle numbers up to 1010 have been achieved. First results from a 200 nm thick film show evidence of narrow extrusions parallel and next to twin boundaries, as well as cracking and grooving at grain boundaries. This damage is similar to what is observed in low cycle fatigued films of similar thickness, giving first indications that the mechanisms of damage formation are determined predominately by the film thickness and not by the strain amplitude.