Research Projects in the Nanomechanics Group

The impact of high current densities on the microstructure of nanocrystalline iron-based alloys and related effects during spark plasma sintering of these alloys

The spark plasma sintering method for preparing high quality oxide materials is applied here to the preparation of high performance nanocrystalline metals. Just as for the oxide materials, we aim to use electric fields and currents to enhance densification of metal powders while limiting grain growth. The goal of our study is to test how useful this method is for sintering metal powders and to obtain a fundamental understanding of the mechanisms controlling sintering with and without electric fields.

To start with, the advantage of electric field processing of metal powders is tested by characterizing the microstructure and mechanical properties of conventionally sintered and spark plasma sintered Fe(C) powders. The expectation is that the electric currents lead to electromigration-induced impurity and surface fluxes, thereby modifying the dynamics of morphology evolution and grain growth of the metal powders. The mechanisms underlying these effects are investigated in a set of control experiments of thin Fe(C) films where we can investigate the evolution of the microstructure, morphology, and impurities, with and without applied fields. The goal is to understand how impurity redistribution and resultant grain growth are affected by high electric current densitites.

Support of Priority Programme (SPP 1959) "Manipulation of matter controlled by electric and magnetic fields: Towards novel synthesis and processing routes of inorganic materials" by the DFG is gratefully acknowledged.

In-Situ Electron Microscopy Studies of Electric Field Assisted Sintering of Oxide Ceramics

A wide range of studies show a dramatic effect of applied electric fields or currents on the sintering behavior of oxide ceramic powders. However, the mechanisms accounting for these strong effects remain elusive despite the wide application potential.

By using in-situ scanning and transmission electron microscopy, material changes during sintering of model oxide ceramics (such as ZnO) will be directly observed. The evolution of microstructure and morphology, including grain/void morphology, segregation, and precipitation, will be tracked, both with and without applied fields and currents. The results will be compared to models to determine the dominant driving forces and mechanisms for sintering, and how these are affected by fields and currents. In addition, in-situ high resolution and electron holography studies, EELS, and in-situ studies under oxidizing and reducing atmospheres will be used to gain insight into the atomic origins of sintering behavior, as well as of high conductivity states that occur in conjunction with flash events during field-assisted sintering.

Support of Priority Programme (SPP 1959) "Manipulation of matter controlled by electric and magnetic fields: Towards novel synthesis and processing routes of inorganic materials" by the DFG is gratefully acknowledged.

Grain refinement in ball-milled nanocrystalline iron-boron

In this project, iron-boron alloys were produced and the influence of the concentration of boron on the grain size was investigated. In theory, the relatively small boron atoms should segregate to the grain boundaries of iron. This reduces the surface energy of the grains and therefore favors the formation of grain boundaries and reduces grain growth. To investigate this, Fe and FeB powder were ball-milled to produce alloys with different boron concentrations. The grain size was determined by transmission electron microscopy and x-ray diffraction. It could be shown that in the investigated interval up to 17,3 at.% the grain size decreases with increasing boron concentration. The thermal stability of the powder was examined by differential scanning calorimetry, where a phase transition while heating and the formation of a Fe2B phase were observed. The results were connected to previous works on iron-oxygen and iron-carbon systems.

Understanding and controlling friction on the nanoscale

In-situ TEM study of LiMn2O4 lithiation

Quantitative heat capacity measurements of Au nanoparticles using a nanocalorimeter

As sample length scales reach down to nanometers, and the surface influence becomes large, thermal properties such as the heat capacity change. The involved heat in such samples is very small and can only be measured via convent. calorimetry by pressing milligrams of sample material together, whereby the surface influence of the sample is reduced. However, nanoscale samples can be directly measured by MEMS-based differential nanocalorimeters which have sufficient res. to measure with nJ sensitivity. Nonetheless, it is well known that heat losses of the MEMS sensors may change in the presence of a sample, leading to large changes in the nanocalorimetric signal and quantitative determination of heat capacity. In this study, FEA is used to investigate the interplay of heat losses (rad. and cond.), sample properties (therm. emissivity and conductivity), and temp. distr. on the sensor. On the basis of this analysis, the influence of exp. issues (misaligned sample, amb. temp.) can be evaluated. Furthermore, the sample geometry could be adapted to minimize the influence of the sample prop. on the measurement. With this knowledge, the heat capacity of Au nanoparticles (1 nm to 20 nm) was measured from 30 K to 300 K with a heating rate of 18,000 K/s with a resolution of better than 1 nJ/K.
Together with TEM investigations, this reveals an enhancement in the heat capacity at low temp. that decr. with incr. particle size.

Identifying structural changes in the medium range order of metallic glasses by Fluctuation Electron Microscopy (FEM)

The structural heterogeneity of a metallic glass (MG) is known to have tremendous impact on many important properties of the system, such as the mechanical performance and the formation of shear bands. While it is commonly understood how to qualitatively alter the structure by thermal and mechanical treatments, it is still extremely difficult to precisely quantify the subtle changes in the real-space order induced by structural alterations. For the characterization of medium range order (MRO) of an amorphous system, the method of Fluctuation Electron Microscopy (FEM) has been found useful in the past. In this method, very small volumes of the MG structure are probed by an electron beam in the Transmission Electron Microscope (TEM). Information about changes in the MRO is subsequently inferred from pixel variances of diffraction patterns taken at many different sample sites. We have used this technique to characterize the structure of a Pd77.5Cu6Si16.5 MG at different structural states induced by thermal annealing, cooling to cryogenic temperatures, and mechanical rejuvenation. The results will be compared with nanoindentation pop-in studies to understand the influence of structure on the onset of mechanical deformation.

Effect of hydrogen on the pop-in behavior in a metallic glass

Hydrogen is a detrimental element in structural materials which usually causes obvious reduction of fracture toughness. For example, hydrogen embrittlement is a well-known reason for failure of steels. In contrast, in metallic glasses, which are considered to be brittle materials,hydrogen (H) can play a beneficial roll by preventing shear band formation. In this contribution, the effect of H content on the popin behavior of a Zr based metallic glass (M) is studied. Samples are charged to different H content (H/M) with electrochemical hydrogen loading. Then, nanoindention with a spherical indenter is carried out on the surface of the samples. While the load of the first pop-in increases with increasing H content, the possibility of pop-ins occurring decreases. A transition from shear band dominated to homogeneous deformation is observed. This result is consistent with a study of the compression of submicron pillars in hydrogen gas environment. The suppression of shear band initiation is attributed to changes in the local environment induced by H doping, such as local strain.

Nanoscale mechanical properties of wood: Effects of heat treatment and sample preparation

The mechanical properties of wood depend on both the complex hierarchical cellular structure and on the mechanical behavior of the constituent materials. Here, we use nanoindentation to investigate the local mechanical properties of the cell walls, with the goal of understanding how they contribute to the bulk mechanical behavior of wood. Quantitative nanoindentation studies require however a smooth specimen surface. Up until now, this has usually been achieved by using specific embedding materials that unfortunately enter the cell wall, thereby changing their mechanical properties. Here, we investigate methods to prepare suitable nanoindentation specimens using (ultra) microtome from Pine wood and Eucalyptus wood that is either unembedded or embedded with materials such as paraffin that do not enter the cell walls. The Eucalyptus wood is an untreated state and heattreated at various temperatures and water vapor pressures.We observe that the wood samples become highly heterogeneous after heat treatment: In so e regions the elastic modulus is high (~100 GPa), while in other regions it is low (~5 GPa). A possible explanation is that the cellulose becomes spatially localized after heat treatment.

In-situ SEM / TEM fracture tests on (modified) pine sap wood

There is evidence that intrawall failure of wood occurs at the interface between the S1 and S2 cell wall layer. However, up to now, the fracture mechanisms of the cell wall remain unexplored. Therefore, we performed fracture tests of pine wood tracheid cell walls which are important because pine ? mostly made of tracheid cells ? is very common. Our experimental setup enables us to create a crack in the cell wall and to observe the crack propagation in-situ with an electron microscope. We have observed that crack propagation is not continuous, but intermittently starts and stops accompanied with a change in propagation direction. We attribute this intermittent behavior to the abrupt change of the microfibril angle at the interface between the S1 and S2 cell wall layer and propose that the resultant increase in toughness is a driving force for the natural adaptation of the layered structure. Additionally, we will investigate pine sap wood that has been heattreated or modified with the resin DMDHEU. The treated wood has higher durability but reduced bending and tensile strength relative to untreated wood. Here, the goal is to understand the reasons for the poorer mechanical properties on the basis of the crack behavior.

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.