Universität Stuttgart
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Item Open Access Interrelationships of microstructure, stress and diffusion(2008) Kuru, Yener; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Extensive research has been performed on thin metal films due to their interesting mechanical, electrical and magnetic properties. They can exhibit very high residual, internal stresses arising from the film growth and/or external effects. Apart from direct mechanical consequences, several processes such as grain growth and diffusion can be affected by these stresses and their gradients. As a result, it is of cardinal importance to measure and control the residual stresses in thin films. X-ray diffraction (XRD) is one of the most frequently used approaches for (residual) stress measurement. It is non-destructive, highly accurate (stress (variation) of some MPa can be detected) and the stress states of all crystalline phases in a layered structure can be obtained separately. Moreover, additional microstructural information, as the crystallographic texture, the density of crystalline defects, such as dislocations, and the crystal size can be acquired from the collected XRD data. This thesis is dedicated to the investigation of microstructural changes, residual stresses and interdiffusion in thin films by in-situ XRD. A focal point of interest is methodological aspects of in-situ measurements, which are discussed in detail in Chapter 2 and come to application in the following Chapters 3 and 4.Item Open Access Wet chemical synthesis and characterization of organic/TiO2 multilayers(2008) Tucic, Aleksandar; Aldinger, Fritz (Prof. Dr.)The low-temperature deposition of oxide-base thin films from solution induced by organic templates is inspired by the process of biomineralization. Biominerals, i.e. inorganic materials synthesized by living organisms, show highly controlled micro- and nanostructures and in many cases physical properties superior to their manmade counterparts. In bio-inspired processes thin oxide films can be deposited from aqueous solutions on organic self-assembled monolayers or polyelectrolytes (PE). Liquid flow deposition (LFD) for the synthesis of TiO2 is based on the continuous flow of a precursor solution along the substrate. Whereas the concentration of the precipitating species within the reaction solution decreases with increasing deposition time, LFD provides a means to keep the concentration within the solution constant. Consequently, also the growth rate of the film is not affected by such aging effects. The deposition technique for the synthesis of PE layers is based on the electrostatic attraction between oppositely charged polyions layer by layer. The spontaneous sequential adsorption of dissolved anionic and cationic polyelectrolytes leads to the formation of ordered multilayer assemblies on a solid substrate. In this work, both techniques were combined in order to synthesize composite, multilayer PE/TiO2 thin films by wet chemical processing is investigated. The main aim is to mimic the architecture of nacre, which is present for instance in sea-shells in order to achive ceramic-based system with enhanced mechanical performances. The properties of the deposited films were characterized by means of SEM, XRD, AFM in order to establish the optimum parameters of the reaction process concerning the film homogeneity, thickness, structure and surface roughness. Polyelectrolyte (PE) films were synthesized applying the layer-by-layer deposition technique. The film thickness was determined applying AFM, TEM cross-sections and the quartz-crystal microbalance (QCM) technique. AFM investigations of the surface morphology of the PE layers showed densely packed globular aggregates of the deposited polymer. In order to investigate the dependence of the morphology and structure of the TiO2 films on the surface modification, depositions on Si substrates modified with polyelectrolytes (PE) were carried on. Thickness of the deposited TiO2 films, estimated by SEM cross-sections, is slightly higher than that of films deposited unmodified Si. Homogeneous films with the same microstructure were deposited on unmodified Si substrate and on PE-covered silicon substrates. Composite PE/TiO2 films were synthesised by applying the layer-by-layer deposition technique for the synthesis of PE layers and the static deposition techniques for synthesis of TiO2 layers. Auger electron spectroscopy (AES) was used to determine the concentrations of Ti and O (as the main constituent of the inorganic phase), C (as the main constituent of the organic phase) and Si (substrate), as a function of depth below the film surface. The AES profile clearly demonstrates the presence of a multilayered structure of alternating TiO2-enriched and C-enriched layers; i.e. it provides proof for the existence of an ordered composite structure of well-defined inorganic and organic layers. SEM, TEM and STEM cross-sections were used for the characterization of the microstructure of the multilayers. Analytical TEM investigations were performed using the energy-dispersive X-ray spectroscopy (EDX) and electron-energy loss spectroscopy (EELS) method. The nanoindentation technique was employed to determine the mechanical properties of obtained composite films, with the emphasis on their hardness and Young’s modulus. The comparison of the nanoindentation data obtained from the TiO2 single layer and the (PE/TiO2)2 and (PE/TiO2)3 multilayer samples reveals that the incorporation of organic layers improves the mechanical properties of CBD-derived TiO2 films. This enhanced mechanical performances can be attributed to the hardening by the differences in shear modulus between the organic and the inorganic phase and the interaction between the incorporated TiO2 particles and the PE, which is stronger then the one between the TiO2 particles within the oxide layers.Item Open Access Mechanics of soft polymer indentation(2008) Deuschle, Julia; Arzt, Eduard (Prof.)Nanoindentation has become a fast and reliable technique for the mechanical characterization of engineering materials. The applicability of this technique to small or confined volumes and the sub- µN and sub- nm resolution of the instruments are unique features in the field of mechanical testing and make nanoindentation a promising, yet not fully accepted tool for the investigation of soft materials. Since these materials are of high importance for technological and biomedical purposes, a strong interest in the application of nanoindentation to this class of materials exists. The present work deals with nanoindentation studies on various polymers in order to address the two crucial factors for testing of soft materials, which are surface detection and contact area determination. An improved surface detection criterion was established, which allows testing of materials with elastic moduli below 1 MPa, whereas before only materials stiffer than several GPa could be tested. The advances in the surface identification are based on the usage of dynamically acquired instead of quasi-static quantities, which exhibit a much better signal-to-noise ratio, thus allow a more accurate surface detection. The improvements of this method were successfully demonstrated for polymers ranging over 4 orders of magnitude in modulus. For a quantitative determination of the contact area, comparative finite element simulations, in-situ indentation tests and tensile tests were performed. Several factors like viscoelasticity and adhesion can influence the contact area of polymeric materials; thus they must be taken into consideration, which is not done in the widely used Oliver & Pharr method for contact area determination. Through this comparative approach, individual sources of error were identified and their contributions quantified. The Oliver & Pharr method was found to underestimate the contact area for shallow indentations, because the contact increase due to adhesive forces is neglected. For high penetrations, the sink-in effect is underestimated slightly. This leads to the conclusion that common indentation techniques are not applicable to soft polymeric materials without modifications. A methodology with alterations necessary for achieving accurate indentation results is provided in the present study. Thus, nanoindentation is confirmed as a usable tool for the mechanical characterization of materials with elastic moduli below 1 MPa.Item Open Access Thermoanalytische und konstitutionelle Charakterisierung des Systems Si3N4-Y2O3-Al2O3-SiO2(2008) Ludwig, Thomas; Aldinger, Fritz (Prof. Dr.)Die gängigsten Additive für das Flüssigphasensintern von Si3N4 sind Y2O3 und Al2O3. Diese Oxide bilden mit dem immer auf der Oberfläche von Si3N4 befindlichen SiO2 eine flüssige Phase, die bei Temperaturen ab 1200°C Stickstoff löst. Nach der Abkühlung liegt ein Teil dieser Schmelzphase als amorphe stickstoffhaltige Korngrenzenphase vor, die in einer weiteren Wärmebehandlung teilweise auskristallisiert werden kann. Hierbei entstehen stickstoffhaltige Sekundärphasen wie zum Beispiel die Oxinitride Apatit, Y10(SiO4)6N2, Melilith, Y2Si3O3N4 und Wöhlerit Y4Si2O7N2. Da nur wenig thermodynamische Daten dieser oxinitridischen Phasen in der Literatur zu finden sind, wurden erstmals Wärmekapazitätsdaten auf direktem Wege mittels dynamischer Hochtemperaturkalorimetrie ermittelt. Im Fall von Apatit und Wöhlerit liegen die Messdaten bis 15% über der Neumann-Kopp-Abschätzungen, für Melilith 17% unterhalb der Abschätzung. Die Bildungsenthalpien der Verbindungen Yttrium-Monosilikat Y2SiO5, Ytterbium-Monosilikat (Yb2SiO5), YAM (Y4Al2O9), Apatit (Y10(SiO4)6N2) und Wöhlerit (Y4Al2O7N2) wurden mit Hochtemperatur-Einwurflösungskalorimetrie (Alkali-Borat-Schmelze als Lösungsmittel) gemessen. Die Verbindungen sind stabil bezüglich ihrer Ausgangsverbindungen und den Elementen. Eine Ausnahme liegt für die YAM Phase Y4Al2O9 vor. Die Bildungsenthalpie bezüglich den binären Oxiden ist nur schwach exotherm. Im System Y2O3-Al2O3-SiO2 wurde die eutektische Temperatur des SiO2-reichen Eutektikums überprüft und die Schmelzenthalpie bestimmt. Die eutektische Temperatur liegt mit 1371±5°C über den experimentellen Literaturwerten und unterhalb des von Gröbner [94Grö] berechneten Wertes. Desweiteren konnte eine quasiternäre Phase mit der Zusammensetzung Y0,9Al1,3Si1,7O7 (auf 7 Sauerstoff-Atome normiert) detektiert werden. Eine Strukturbestimmung war bisher nicht möglich. Auch ist nicht geklärt, ob die Phase nur metastabil vorliegt oder ob aus kinetischen Gründen die Bildung nur aus der Glasphase erfolgt. Geht man von der fremdionenstabilisierten y-Modifikation des Disilikats aus, so lässt sich die Stöchiometrie YAlSi2O7 als Endglied ableiten. Damit wäre auch die Elektroneutralitätsbedingung erfüllt. Es wurden Aluminosilikatgläser mit unterschiedlichem Stickstoffgehalt mit temperaturabhängiger Röntgenpulverdiffraktometrie untersucht, um die Kristallisation in situ zu verfolgen. Die Kristallisationstemperaturen nahmen mit steigendem Stickstoffgehalt zu. Dies ist auf die stärkere Vernetzung durch den Einbau des trivalenten Stickstoffanions zurückzuführen. Es erfolgt immer zuerst die Kristallisation einer Yttriumdisilikat-Modifikation, gefolgt von Aluminatphasen. Im Gegensatz zu Auslagerungsexperimenten kristallisiert das Disilikat in der für den entsprechenden Temperaturbereich stabilen Modifikation und wandelt dann mit zunehmender Temperatur in die nächst stabilere Modifikation um. Die Umwandlungstemperaturen stimmen mit den Literaturangaben überein. In klassischen Experimenten (Auslagerung eines Festkörpers im Ofen) findet meist zuerst eine Primärkristallisation zweier unterschiedlicher Disililikat-Modifikationen statt, wobei oft Modifikationen auskristallisieren, die bei den entsprechenden Temperaturen nicht stabil sein dürften. Da das Kristallisationsverhalten von der Beweglichkeit der Baugruppen oder Ionen im Glas abhängt und damit von der Viskosität, werden hier Strukturen bevorzugt die ähnlich der Struktureinheiten im Glas aufgebaut sind, obwohl sie thermodynamisch eigentlich nicht stabil sind.