Scientific publications

Sliding friction experiments on graphene grown on SiC(0001) have been performed using a combination of a microtribometer with an atomic force microscope (AFM) allowing for the investigation of atomic-scale wear. The graphene layer delaminates within 10 sliding cycles starting from substrate step edges. After run in, friction is dominated by the interaction between a changing configuration of asperities at the probe sphere and a graphitic interface layer terminating the SiC substrate. Friction varies unpredictably due to changes in the contact configuration. However, the linear relation between friction and contact area can be confirmed and a shear strength as low as a few MPa is found for the contact between ruby and the graphitic layer on SiC, which remains intact under continuous sliding.

In the present study a procedure to characterize the finite viscoelasticity and to simultaneously identify the influence of adhesion in nanoindentation experiments of soft polymers is developed. Silicone rubber, which is assumed to be an isotropic elastomer, is chosen to be examined. Different nanoindentation testing protocols are used to visualize and proof the viscoelastic properties and the adhesion behavior of a soft silicone rubber in contact with a Berkovich tip. It could be shown that the analytical solution of linear viscoelastic indentation has some limitations in order to predict the experimental data that contains finite viscoelasticity as well as adhesion effects. The inverse method is applied by using the finite element computation combined with a numerical optimization subroutine. A viscoelastic model at finite strain is chosen to represent the silicone rubber’s behavior. The default contact pressure-clearance relationship used in ABAQUS® is modified; a surface-based adhesive behavior in traction-separation law is incorporated into the contact pairs. The real geometry of the Berkovich tip is considered in order to minimize the systematic errors between the numerical model and the experiments. Finally, the parameters of the chosen viscoelastic constitutive model and the adhesive contact model are identified by matching the response of the numerical model with the experimental force-displacement curves. The present model contains the surface adhesion and is verified to show better reproducibility regarding the experiments than the analytical solution. There are several drawbacks of the analytical solution that are also presented in this work.

The basic dislocation processes responsible for plasticity of fcc and bcc metals are fundamentally different. Dislocation based deformation in fcc metals is not very sensitive to temperature whereas in bcc metals, deformation is strongly temperature dependent and controlled by the low mobility of screw dislocations. In bcc metals it has been observed that the mobility of screw dislocations can be enhanced in the proximity of the sample surface. In order to investigate this effect, a comparative study of small scaled samples with high surface to volume ratios and selected geometries was performed. The systems that were tested were made of the bcc metals Ta, Mo and Fe. Cu served as a reference material for fcc metals. Microcompression experiments were carried out on focused ion beam (FIB) machined samples. In the FIB machining process, the surfaces of cuboidal pillars were oriented relative to the expected active slip systems. For Ta and Mo the flow stress depended on the orientation of the pillar. This observation is interpreted in terms of the surface enhanced screw dislocation mobility of bcc metals.

The purpose of this study was to investigate increasing crystalline structure in polyetheretherketone (PEEK) via resistive heating. Multi-walled carbon nanotubes (MWCNT) were thoroughly mixed with PEEK powder, hot-pressed into a solid composite, then immediately quenched in ice water. Duplicate specimens were then reheated at 300 °C. 'Quenched' and 'heat treated' samples represented comparative minimum and maximum volume fraction crystallinity used in this study, respectively. Dynamic mechanical analysis, electrical conductivity measurements, scanning electron microscopy, optical microscopy, differential scanning calorimetry (DSC), and resistive heating tests were performed to characterize the composite properties as a function of MWCNT volume fraction. Results showed dispersion within the PEEK matrix is inherently limited by the powder-press technique. Nonetheless, MWCNT volume fractions as low as 1 vol% are above the percolation threshold and 5 vol% allow PEEK to be resistively heated above the glass transition temperature. DSC results of the quenched and reheated samples indicate that the MWCNTs do not inherently inhibit crystalline formation. Reheating the quenched samples via resistive heating is shown to increase the crystalline volume fraction. This study is the first to show an amorphous to crystalline phase transformation of PEEK via resistive heating, opening the possibility of changing mechanical properties and/or elastic deformation range in situ.

Micro-compression tests were performed on pre-strained nickel (Ni) single crystals in order to investigate the influence of the initial dislocation arrangement on the size dependence of small-scale metal structures. A bulk Ni sample was grown using the Czochralski method and sectioned into four compression samples, which were then pre-strained to nominal strains of 5, 10, 15 and 20%. Bulk samples were then characterized using transmission electron microscopy (TEM), micro-Laue diffraction, and electron backscatter diffraction. TEM results show that a dislocation cell structure was present for all deformed samples, and Laue diffraction demonstrated that the internal strain increased with increased amount of pre-straining. Small-scale pillars with diameters from 200 nm to 5 μm were focused ion beam (FIB) machined from each of the four deformed bulk samples and further compressed via a nanoindenter equipped with a flat diamond punch. Results demonstrate that bulk pre-straining inhibits the sample size effect. For heavily pre-strained bulk samples, the deformation history does not affect the stress-strain behavior, as the pillars demonstrated elevated strength and rather low strain hardening over the whole investigated size range. In situ TEM and micro-Laue diffraction measurements of pillars confirmed little change in dislocation density during pillar compression. Thus, the dislocation cell walls created by heavy bulk pre-straining become the relevant internal material structure controlling the mechanical properties, dominating the sample size effect observed in the low dislocation density regime.

By reevaluating the experimental study of Zhang et al. (2005), here we demonstrate that the extent of grain growth, previously proposed to be solely driven by external stress, may have been significantly overestimated. A new physical mechanism, termed as free surface assisted stress-driven grain growth (or self-mechanical annealing), is proposed and discussed in detail. Representing the cooperative effect of free surface and heterogeneous residual internal stress, the proposed mechanism is consideredmore favorable than the traditional pure stress-driven mechanism for interpreting the abnormal grain growth widely observed in deforming nanocrystalline metals at room temperature.

An analytical model of peeling of an elastic tape from a substrate is presented for large deformations and scenarios where sliding occurs in the adhered regions, with this motion resisted by interfacial shear tractions. Two geometries are considered: the first has a detached segment of the tape forming the shape of an inverted letter 'V' between adhered sections (double-sided peeling), and the second has a free end of the tape being pulled (single-sided peeling). The mechanics of peeling is analyzed in terms of the applied force, displacement of the load point and the angle that the peeled tape makes with the substrate. Formulae are provided for the energy released per unit area of peeling that explicitly and separately account for the work done by frictional sliding. Assuming that peeling occurs when the energy released per unit area equals the work of separation for purely normal separation, it is shown that the critical force to propagate peeling can be significantly higher with sliding as compared to pure sticking. Similarly, due to frictional dissipation, the amount of work done by the applied force needed to propagate peeling can be significantly greater than the work of separation. For the single-sided peel test, an effective mixed-mode interface toughness is presented to be used with purely sticking models when sliding is not explicitly modeled: the closed-form result closely mirrors common empirical forms used to predict mixed-mode delamination.

In the last years, there has been a rising interest to find an objective method to estimate listening effort. Previously, we have shown that the wavelet phase synchronization stability (WPSS), gained from the instantaneous phase of auditory late responses (ALRs), could serve as a feasible measure for listening effort related factors.In the current study, we examined if the WPSS reflects the listening effort in young as well as middle-aged subjects and in persons with a different degree of hearing loss. To evoke ALR sequences, we generated syllabic paradigms with a different level of difficulty to evoke ALR sequences. We expected, due to the varying task demand, that the subjects require a measurable difference in the amount of effort to solve the paradigms. Additionally, a possible age and/or hearing loss related effect on the neural correlates of listening effort was investigated.The results indicate, that WPSS reflects the listening effort related factors needed to solve an auditory task. A further finding was that the reaction time data and the N1 wave amplitude information hardly yield any correlate of the invested listening effort. In addition, we noticed an age as well as hearing sensitivity related effect on the listening effort.

During the cycling of a lithium-ion battery, the active storage materials experience a volume change due to the intercalation process, often causing fracture, loss of contact among the active particles, and finally the degradation of the whole electrode. Here, we present a model for the lithium diffusion and stress generation in a particle of active material having a phase change. In our approach the driving force for diffusion is deduced from basic thermodynamics and statistical physics. The parameters of our model may be obtained from experimental measurements, or from ab-initio density functional theory calculations if the values are experimentally not accessible, as in the evaluation of new, as yet unsynthesized, computer-designed materials. We present results from simulations representing graphite known to experience phase-changes or staging. In some of our simulations, the particles are coupled to a battery simulator to apply conditions experienced in a functioning cell. We find that staging causes a significant increase in particle stress in comparison to when it is absent.

We utilize a model for birefringence/permittivity based on the statistical mechanics of a Gaussian polymer chain to construct a relationship for the dependence of the dielectric permittivity of an elastomer on a general 3-dimensional state of deformation. The model, due to Kuhn and Grün (1942 [1]), expresses the birefringence/permittivity of a Gaussian polymer chain elastomer as a function of the end-to-end distance of the chains, and assumes that the motions of the chains are affine to the overall deformation. The outcome is an expression for the permittivity tensor of the elastomer as a function of its stretch ratios. The permittivity is isotropic in the undeformed state and under pure dilatation, but otherwise becomes anisotropic during deformation. With this model, we use the free energy of the elastomer to compute the response of a neo-Hookean thin film in an actuator configuration subject to electric and mechanical loading for conditions where the permittivity in the through thickness direction is allowed to increase or decrease with the in-plane extension of the thin film. With such an approach, we study the deformation characteristics of the actuator and its stability under through thickness electric fields. Our calculations show that the deformation dependent permittivity can hasten or postpone an electromechanical instability that can cause a sudden thinning of the dielectric, accompanied by in-plane stretching, when the through thickness electric field is raised above a critical magnitude. Specifically, we consider the case of an actuator exhibiting a through thickness permittivity that decreases with in-plane extension. We observe that in such an actuator the instability is delayed to a higher electric field than would be the case if the dielectric permittivity were independent of strain. Furthermore, we establish that upon removal of the electric field the system follows a different path in terms of potential versus charge, and so develops a hysteresis loop, similar to that identified by Zhao et al. (2007 [2]) for dielectric elastomers with constant isotropic permittivity, but that stiffen during straining.