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- ItemSurviving the surf: The tribomechanical properties of the periostracum of Mytilus sp(Amsterdam : Elsevier, 2014) Wählisch, Felix C.; Peter, Nicolas J.; Torrents Abad, Oscar; Oliveira, Mariana V.G.; Schneider, Andreas S.; Schmahl, Wolfgang; Griesshaber, Erika; Bennewitz, RolandWe investigated the friction and wear behavior as well as the mechanical properties of the periostracum of Mytilus sp. Tribological properties were determined with a reciprocal sliding microtribometer, while mechanical characterization was performed using a nanoindenter. Measurements were performed in dry and wet conditions. On the dry periostracum we found a low friction coefficient of 0.078 ± 0.007 on the young parts and a higher one of 0.63 ± 0.02 on the old parts of the shell. Under wet, saline, conditions we only observed one average coefficient of friction of 0.37 ± 0.01. Microscopic ex situ analysis indicated that dry periostracum wore rather rapidly by plowing and fatigue, while it exhibited a high wear resistance when immersed in salt water. The Young’s modulus and hardness of the periostracum were also investigated in both dry and wet conditions. Under dry conditions the Young’s modulus of the periostracum was 8 ± 3 GPa, while under wet conditions it was 0.21 ± 0.05 GPa. The hardness of dry periostracum samples was 353 ± 127 MPa, whereas the hardness of wet samples was 5 ± 2 MPa. It was found that, in the wet state, viscous behavior plays a significant role in the mechanical response of the periostracum. Our results strongly indicate that the periostracum can provide an important contribution to the overall wear resistance of Mytilus sp. shell.
- ItemTemperature-dependent size effects on the strength of Ta and W micropillars(Amsterdam : Elsevier, 2016) Torrents Abad, Oscar; Wheeler, Jeffrey M.; Michler, Johann; Schneider, Andreas S.; Arzt, EduardThe strength of metals increases with decreasing sample size, a trend known as the size effect. In particular, focused ion beam-milled body-centered cubic (BCC) micropillars exhibit a size effect known to scale with the ratio of the test temperature to the critical temperature (Tc) of the BCC metal, a measure of how much the yield stress is governed by the lattice resistance. In this paper, this effect is systematically studied by performing high-temperature compression tests on focused ion beam-manufactured Ta and W single crystal pillars ranging in diameter from 500 nm to 5 μm at temperatures up to 400 °C, and discussed in the context of bulk strength and size dependent stresses. Both metals show larger size effects at higher temperatures, reaching values that are in the range of FCC metals at temperatures near Tc. However, it is demonstrated that size effects can be considerably affected by material parameters such as dislocation density and lattice friction, as well as by the yield criterion used. Furthermore, for W, a change from uniform wavy deformation to localized deformation is observed with increasing temperature and pillar size, further indicating that the temperature ratio strongly influences the relative motion of screw and edge dislocations.
- ItemOn the process of co-deformation and phase dissolution in a hard-soft immiscible CuCo alloy system during high-pressure torsion deformation(Amsterdam : Elsevier, 2016) Bachmaier, Andrea; Schmauch, Jörg; Aboulfadl, Hisham; Verch, Andreas; Motz, ChristianIn this study, dual phase Cusingle bondCo composites with a total immiscibility in the solid state and a very different initial phase strength are deformed by severe plastic deformation. Nanocrystalline supersaturated solid solutions are reached in all Cusingle bondCo composites independent of the initial composition. The deformation and mechanical mixing process is studied thoroughly by combining scanning electron microscopy, transmission electron microscopy, three-dimensional atom probe tomography and nanoindentation. The indentation hardness of the Cu and Co phase and its evolution as a function of the applied strain is linked to deformation and mechanical mixing process to gain a better understanding how the phase strength mismatch of the Cu and Co phase effects the amount of co-deformation and deformation-induced mixing. Our results show that co-deformation is not a necessary requirement to achieve mechanical mixing.
- ItemDatasets from a vapor diffusion mineral precipitation protocol for Dictyostelium stalks(Amsterdam : Elsevier, 2016) Eder, Magdalena; Muth, Christina; Weiss, Ingrid M.Datasets from a slow carbonate vapor diffusion and mineral precipitation protocol for Dictyostelium ECM and cellulose stalks show examples for composite materials obtained by an in vitro approach, which differs substantially from the in vivo approach reported in The Journal of Structural Biology, doi: 10.1016/j.jsb.2016.03.015 [1]. Methods for obtaining the datasets include bright field transmitted light microscopy, fluorescence microscopy, LC-PolScope birefringence microscopy, variable pressure scanning electron microscopy (VP-SEM/ESEM), and Raman imaging spectroscopy.
- ItemIn-situ nanodiamond to carbon onion transformation in metal matrix composites(Amsterdam : Elsevier, 2018) Suarez, Sebastian; Reinert, Leander; Zeiger, Marco; Miska, Patrice; Grandthyll, Samuel; Müller, Frank; Presser, Volker; Mücklich, FrankIn the present study, nickel matrix composites reinforced with a fine distribution of nanodiamonds (6.5 vol%) as reinforcement phase are annealed in vacuum at different temperatures ranging from 750 °C to 1300 °C. This is carried out to evaluate the in-situ transformation of nanodiamonds to carbon onions within a previously densified composite. The resulting materials are thoroughly analyzed by complementary analytical methods, including Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The proposed in-situ transformation method presents two main benefits. On one hand, since the particle distribution of a nanodiamond-reinforced composite is significantly more homogenous than in case of the carbon onions, it is expected that the transformed particles will preserve the initial distribution features of nanodiamonds. On the other hand, the proposed process allows for the tuning of the sp3/sp2 carbon ratio by applying a single straightforward post-processing step.
- ItemVacuum or flowing argon: What is the best synthesis atmosphere for nanodiamond-derived carbon onions for supercapacitor electrodes?(Amsterdam : Elsevier, 2015) Zeiger, Marco; Jäckel, Nicolas; Weingarth, Daniel; Presser, VolkerWe present a comprehensive study on the influence of the synthesis atmosphere on the structure and properties of nanodiamond-derived carbon onions. Carbon onions were synthesized at 1300 and 1700 °C in high vacuum or argon flow, using rapid dynamic heating and cooling. High vacuum annealing yielded carbon onions with nearly perfect spherical shape. An increase in surface area was caused by a decrease in particle density when transitioning from sp3 to sp2 hybridization and negligible amounts of disordered carbon were produced. In contrast, carbon onions from annealing nanodiamonds in flowing argon are highly interconnected by few-layer graphene nanoribbons. The presence of the latter improves the electrical conductivity, which is reflected by an enhanced power handling ability of supercapacitor electrodes operated in an organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile). Carbon onions synthesized in argon flow at 1700 °C show a specific capacitance of 20 F/g at 20 A/g current density and 2.7 V cell voltage which is an improvement of more than 40% compared to vacuum annealing. The same effect was measured for a synthesis temperature of 1300 °C, with a 140% higher capacitance at 20 A/g for argon flow compared to vacuum annealing.
- ItemStructural evolution and strain induced mixing in Cu-Co composites studied by transmission electron microscopy and atom probe tomography(Amsterdam : Elsevier, 2015) Bachmaier, Andrea; Aboulfadl, H.; Pfaff, Marina; Mücklich, Frank; Motz, ChristianA Cu–Co composite material is chosen as a model system to study structural evolution and phase formations during severe plastic deformation. The evolving microstructures as a function of the applied strain were characterized at the micro-, nano-, and atomic scale-levels by combining scanning electron microscopy and transmission electron microscopy including energy-filtered transmission electron microscopy and electron energy-loss spectroscopy. The amount of intermixing between the two phases at different strains was examined at the atomic scale using atom probe tomography as complimentary method. It is shown that Co particles are dissolved in the Cu matrix during severe plastic deformation to a remarkable extent and their size, number, and volume fraction were quantitatively determined during the deformation process. From the results, it can be concluded that supersaturated solid solutions up to 26 at.% Co in a fcc Cu–26 at.% Co alloy are obtained during deformation. However, the distribution of Co was found to be inhomogeneous even at the highest degree of investigated strain.
- ItemImproved development procedure to enhance the stability of microstructures created by two-photon polymerization(Amsterdam : Elsevier, 2018) Purtov, Julia; Verch, Andreas; Rogin, Peter; Hensel, RenéNatural functional surfaces often rely on unique nano- and micropatterns. To mimic such surfaces successfully, patterning techniques are required that enable the fabrication of three-dimensional structures at the nanoscale. It has been reported that two-photon polymerization (TPP) is a suitable method for this. However, polymer structures fabricated by TPP often tend to shrink and to collapse during the fabrication process. In particular, delicate structures suffer from their insufficient mechanical stability against capillary forces which mainly arisein the fabrication process during the evaporation of the developer and rinsing liquids. Here, we report a modified development approach, which enables an additional UV-treatment to post cross-link created structures before they are dried. We tested our approach on nanopillar arrays and microscopic pillar structures mimicking the moth-eye and the gecko adhesives, respectively. Our results indicate a significant improvement of the me- chanical stability of the polymer structures, resulting in fewer defects and reduced shrinkage of the structures.
- ItemDetachment of an adhered micropillar from a dissimilar substrate(Amsterdam : Elsevier, 2015) Khaderi, S.N.; Fleck, N.A.; Arzt, E.; McMeeking, R.M.Abstract The mechanics of detachment is analysed for 2D flat-bottomed planar pillars and 3D cylindrical pillars from a dissimilar elastic substrate. Application of an axial stress to the free end of the pillar results in a singularity in stress at the corner with the substrate. An eigenvalue analysis reveals that the stress field near the corner is dominated by two singular eigenfields having eigenvalues ( λ 1 , λ 2 ) with corresponding intensities ( H 1 , H 2 ) . The asymptotic stress field σij is of the form σ ij = H 1 r λ 1 − 1 f ij ( λ 1 , θ ) + H 2 r λ 2 − 1 f ij ( λ 2 , θ ) , where fij describe the angular dependence θ of σij, and r is the radial distance from the corner. The stress intensities ( H 1 , H 2 ) are calculated numerically, using a domain integral approach, as a function of the elastic mismatch between the pillar and substrate. The singular zone extends across approximately 10 of the pillar diameter (in 3D) or pillar width (in 2D). Interfacial failure is predicted for an assumed crack emanating from the corner of pillar and substrate. For the case of an interfacial crack that resides within the domain of corner singularity, a boundary layer analysis is performed to calculate the dependence of the interfacial stress intensity factor K upon ( H 1 , H 2 ) . When the crack extends beyond the domain of corner singularity, it is necessary to consider the full geometry in order to obtain K. A case study explores the sensitivity of the pull-off stress to the flaw size and to the degree of material mismatch. The study has implications for the optimum design of adhesive surface micropatterns, for bonding to either stiffer or more compliant substrates.
- ItemAdhesion and relaxation of a soft elastomer on surfaces with skin like roughness(Amsterdam : Elsevier, 2018) Fischer, Sarah; Boyadzhieva, Silviya; Hensel, René; Kruttwig, Klaus; Arzt, EduardFor designing new skin adhesives, the complex mechanical interaction of soft elastomers with surfaces of various roughnesses needs to be better understood. We systematically studied the effects of a wide set of roughnesscharacteristics, film thickness, hold time and material relaxation on the adhesive behaviour of the silicone elastomer SSA 7–9800 (Dow Corning). As model surfaces, we used epoxy replicas obtained from substrates with roughness ranging from very smooth to skin-like. Our results demonstrate that films of thin and intermediate thickness (60 and 160 μm) adhered best to a sub-micron rough surface, with a pull-off stress of about 50 kPa. Significant variations in pull-off stress and detachment mechanism with roughness and hold time were found. In contrast, 320 μm thick films adhered with lower pull-off stress of about 17 kPa, but were less sensitive to roughness and hold time. It is demonstrated that the adhesion performance of the siliconefilms to rough surfaces can be tuned by tailoring the film thickness and contact time.
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