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In this study, polyetheretherketone composites were compounded using a two-screw extruder followed by injection moulding. The effects of multi-fillers on the mechanical properties and crystallization performances were investigated. Differential scanning calorimetry results indicate that the addition of fillers slightly increases the crystallization temperature and crystallinity. Compared to neat polyetheretherketone, the incorporation of inorganic filler leads to a significant improvement in matrix hardness, matrix stiffness and a slight increase in tensile strength. However, the material ductility, the impact strength and the fracture toughness of polyetheretherketone composites decrease. Fractography analyses show that the addition of fillers restraints the ductile deformation of polymers, which is responsible for the reduction of material ductility, impact strength as well as fracture toughness of polyetheretherketone composites.
Ionic liquids (ILs) represent an important class of liquids considered for a broad range of applications such as lubrication, catalysis, or as electrolytes in batteries. It is well-known that in the case of charged surfaces, ILs form a pronounced layer structure that can be easily triggered by an externally applied electrode potential. Information about the time required to form a stable interface under varying electrode potentials is of utmost importance in many applications. For the first time, probing of relaxation times of ILs by friction force microscopy is demonstrated. The friction force is extremely sensitive to even subtle changes in the interfacial configuration of ILs. Various relaxation processes with different time scales are observed. A significant difference dependent on the direction of switching the applied potential, i.e., from a more cation-rich to a more anion-rich interface or vice versa, is found. Furthermore, variations in height immediately after the potential step and the presence of trace amounts of water are discussed as well.
Plasticity experiments have been conducted with single dislocation resolution in both indentation and wear studies using atomic force microscopy (AFM). The high force sensitivity and the small tip radii in AFM permit the measurement of the nucleation of single dislocations in plastically deformed nanoscale volumes. Nanoscale mechanical testing in an ultra-high vacuum (UHV) environment allows for the preparation of oxide-free surfaces, required for direct comparison between atomistic simulation and experiment. Moreover, nanoscale mechanical testing often shows increased strength compared to what is observed in macroscale testing, motivating the use of atomistic simulation to gain insight into new deformation mechanisms. Indentation experiments show that it is possible not only to observe single dislocation events but also determine the glide vector of the dislocation in three dimensions on KBr(001). Discontinuous displacements of the tip during indentation in both normal and lateral directions are indicative of yielding events, referred to as pop-ins. The measured displacement of the tip into the material during these events is on the order of one Ångström or less when blunt diamond coated tips are used as indenters. Larger pop-in displacements are measured with sharper probes, resulting from the localization of stress near the surface. Only with the use of AFM can such small, Ångström-sized pop-in displacements be observed. Indentation creep studies indicate that creep in nanoscale volumes is accommodated only through dislocation nucleation and glide. A comparison between creep measured with AFM-based indentation and instrumented nanoindentation highlights the importance of dislocation nucleation and glide at this length-scale. High resolution imaging of the indented structure on KBr(001) allows for the identification of dislocations and charges associated with them. Wear experiments have demonstrated the contribution of dislocations to wear on the atomic scale. The role of dislocations in wear experiments has been observed through the similar dislocation structures typically surrounding scratches and indents, as well as in pop-ins observed while scratching. The measured friction coefficient in nanoscale wear experiments is closer to those typically reported in macroscopic experiments. This finding suggests that while single-asperity experiments at low loads on flat surfaces may produce no or little wear, friction of real rough surfaces always involves some wear and plastic deformation of microscopic contacts between the two surfaces.
Ein neuartiges Messgerät wurde durch die Kombination von einem AFM und zwei Tribometern entwickelt. Dieses Gerät ist in der Lage eine Probe mechanisch und tribologisch auf verschiedenen Längenskalen zu charakterisieren. Die Leistungsfähigkeit wird anhand von drei sehr unterschiedlichen Proben und Messaufgaben demonstriert, die im Folgenden beschrieben werden. Als Erstes wird das Reibungs- und Verschleißverhalten von Graphenlagen auf SiC und Cu untersucht. Die makroskopische Reibung auf SiC ist nach dem Abscheren des Graphens durch die Grenzflächenschicht bestimmt, auf Cu dominiert die Deformation der Oberfläche. Diese Arbeit zeigt außerdem, dass Wasser eine wichtige Rolle in der Reibung von Graphen auf der mikroskopischen Skala spielt. In der zweiten Untersuchung wird ein transparenter, viskoelastischer Kunststoff deformiert und die anschließende Formrelaxation profilometrisch vermessen. Zusätzlich wird mittels Polarisationsmikroskopie die Spannungsrelaxation bestimmt. Aus beidem zusammen werden die Kriechfolgefunktion und das Relaxationsspektrum berechnet. Zuletzt wird das Reibungs- und Verschleißverhalten des Periostracums der Miesmuschel im nassen und trockenen Zustand untersucht. Diese Arbeit zeigt, dass es im nass sehr abriebsresistent ist, während es im trockenen Fall schnell verschleißt. Zusätzlich wurden E-Modul und Härte mittels Nanoindentation bestimmt. Alle drei Projekte belegen die Wichtigkeit des skalenübergreifenden Ansatzes in mechanischen Untersuchungen.
Glabrous skin is hair-free skin with a high density of sweat glands, which is found on the palms, and soles of mammalians, covered with a thick stratum corneum. Dry hands are often an occupational problem which deserves attention from dermatologists. Urea is found in the skin as a component of the natural moisturizing factor and of sweat. We report the discovery of dendrimer structures of crystalized urea in the stratum corneum of palmar glabrous skin using laser scanning microscopy. The chemical and structural nature of the urea crystallites was investigated in vivo by non-invasive techniques. The relation of crystallization to skin hydration was explored. We analysed the index finger, small finger and tenar palmar area of 18 study participants using non-invasive optical methods, such as laser scanning microscopy, Raman microspectroscopy and two-photon tomography. Skin hydration was measured using corneometry. Crystalline urea structures were found in the stratum corneum of about two-thirds of the participants. Participants with a higher density of crystallized urea structures exhibited a lower skin hydration. The chemical nature and the crystalline structure of the urea were confirmed by Raman microspectroscopy and by second harmonic generated signals in two-photon tomography. The presence of urea dendrimer crystals in the glabrous skin seems to reduce the water binding capacity leading to dry hands. These findings highlight a new direction in understanding the mechanisms leading to dry hands and open opportunities for the development of better moisturizers and hand disinfection products and for diagnostic of dry skin.
The presence of mechanoreceptors in glabrous skin allows humans to discriminate textures by touch. The amount and distribution of these receptors defines our tactile sensitivity and can be affected by diseases such as diabetes, HIV-related pathologies, and hereditary neuropathies. The quantification of mechanoreceptors as clinical markers by biopsy is an invasive method of diagnosis. We report the localization and quantification of Meissner corpuscles in glabrous skin using in vivo, non-invasive optical microscopy techniques. Our approach is supported by the discovery of epidermal protrusions which are co-localized with Meissner corpuscles. Index fingers, small fingers, and tenar palm regions of ten participants were imaged by optical coherence tomography (OCT) and laser scan microscopy (LSM) to determine the thickness of the stratum corneum and epidermis and to count the Meissner corpuscles. We discovered that regions containing Meissner corpuscles could be easily identified by LSM with an enhanced optical reflectance above the corpuscles, caused by a protrusion of the strongly reflecting epidermis into the stratum corneum with its weak reflectance. We suggest that this local morphology above Meissner corpuscles has a function in tactile perception.
Epitaxial graphene on SiC(0001) exhibits superlow friction due to its weak out-of-plane interactions. Friction-force microscopy with silicon tips shows an abrupt increase of friction by one order of magnitude above a threshold normal force. Density-functional tight-binding simulations suggest that this wearless high-friction regime involves an intermittent sp3 rehybridization of graphene at contact pressure exceeding 10 GPa. The simultaneous formation of covalent bonds with the tip's silica surface and the underlying SiC interface layer establishes a third mechanism limiting the superlow friction on epitaxial graphene, in addition to dissipation in elastic instabilities and in wear processes.
Die Kontrolle von Reibung auf kleiner Skala ist von fundamentaler Bedeutung, insbesondere im Hinblick auf die fortschreitende Miniaturisierung von mechanischen Bauteilen. Im Rahmen dieser Arbeit wurden hochaufgelöste Experimente zur Reibung in ultrasauberen Flüssigkeiten durchgeführt, um so die Möglichkeiten der Kontrolle von Reibungskräften auf Gold in wässrigen Elektrolyten und ionischen Flüssigkeiten auf atomare Mechanismen zurückführen zu können. Die Kombination der Rasterkraftmikroskopie mit elektrochemischen Methoden erlaubt es, die Oberflächeneigenschaften reversibel und in situ zu variieren. Die vorliegende Arbeit zeigt, dass die Reibungskraft mit der atomaren Oberflächenrauigkeit skaliert. Auf reinen, rekonstruierten Goldoberflächen ist die Reibung sehr gering und nur schwach von der Normalkraft abhängig. Mit der Modifikation der Oberfläche durch Aufhebung der Rekonstruktion, durch Oxidation oder durch ionische Adsorbate wird eine signifikante Zunahme der Reibungskraft beobachtet. Die Prozesse sind reversibel und erlauben eine aktive Kontrolle und Schaltbarkeit der Reibung. Ionische Flüssigkeiten werden genutzt, um das effektive elektrochemische Fenster zu vergrößern. Es wird gezeigt, dass die Reibung über das Verhalten eingeschlossener ionischer Schichten anstatt durch die Modifikation der Oberfläche an sich geschaltet werden kann. Die Ergebnisse dieser Arbeit belegen eine breite Anwendbarkeit des Konzepts der elektrochemischen Kontrolle von Reibungskräften.
Stacked hetero-structures of two-dimensional materials allow for a design of interactions with corresponding electronic and mechanical properties. We report structure, work function, and frictional properties of 1 to 4 layers of MoS2 grown by chemical vapor deposition on epitaxial graphene on SiC(0001). Experiments were performed by atomic force microscopy in ultra-high vacuum. Friction is dominated by adhesion which is mediated by a deformation of the layers to adapt the shape of the tip apex. Friction decreases with increasing number of MoS2 layers as the bending rigidity leads to less deformation. The dependence of friction on applied load and bias voltage can be attributed to variations in the atomic potential corrugation of the interface, which is enhanced by both load and applied bias. Minimal friction is obtained when work function differences are compensated.
Electron microscopy of native biological materials is usually hampered by sample preparation procedures such as dehydration and freezing, and by electron beam damage. Liquid phase electron microscopy (LP-EM) allows the observation of biological samples in liquid environments without conventional preparation procedures. However, electron beam damage also occurs in LP-EM, and thresholds for biological samples are not yet fully explored. In this work, the electron dose tolerance of green fluorescent protein (GFP) was analyzed in LP-EM. Protein damage was studied with increasing electron dose, using fluorescence degradation as an indication. Dc < 0.01 e-/Ų and Dc < 0.1 e-/Ų were observed for GFP on silicon nitrite in transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM), respectively. In TEM, the dose tolerance was increased by three orders of magnitude when GFP was encapsulated in graphene liquid cells. The dose tolerance of more complex systems was investigated by binding GFP to actin filaments in fixed SKBR3 cells, which showed Dc < 0.1 e-/Ų in TEM and ESEM. In fixed SKBR3 cells, radiation damage was also studied based on the displacement of labeled membrane proteins. At electron doses of D = (7.8 ± 0.4) ∙ 10³ e-/Ų these labels showed a displacement of 0.8%. Procedures for studying biological materials such as proteins and fixed cells in LP-EM are presented in this thesis. Strategies to study and mitigate beam damage are demonstrated.