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Das tribologische Verhalten wird neben den chemischen und physikalischen Eigenschaften der Reibpartner auch von deren Oberflaechentopographie bestimmt. Um deren Beitrag auf die Reibkraft zu untersuchen, wurden daher definierte Rauheitsstrukturen in Siliziumwafer geaetzt. Reibversuche mit einem Mikrotribometer [1, 2] gaben Aufschluss ueber den Einfluss dieser Strukturen auf das Deformationsverhalten im Kontaktbereich. Mit Hilfe der gewonnenen Daten wurde ein Deformationsmodell entwickelt, das eine Abschaetzung der Reibkraft in Abhaengigkeit von der Rauheit erlaubte. Treten starke Adhaesionskraefte im Reibkontakt auf, ist ein inverter Einfluss der Rauheit auf die Reibkraft zu verzeichnen. Um diesen Verlauf weiter zu charakterisieren, wurden mit einem UHV-AFM Adhaesionsmessungen an strukturierten Siliziumwafern durchgefuehrt. Diese zeigen den Verlauf der Adhaesionskraft in Abhaengigkeit von der Groesse der Kontaktflaeche auf. Ein Vergleich dieser experimentell gefundenen Korrelation mit den klassischen Kontaktmodellen machte dabei den Einfluss der Oberflaechenrauheit auf die Groesse der Adhaesionskraft deutlich. Danach ergab sich, dass bei gleicher Kontaktflaeche, aber unterschiedlicher Rauheit sich die Adhaesionskraefte um bis zu einem Faktor 3 unterscheiden. [1] K. Meine, T. Schneider, D. Spaltmann, E. Santner, Wear, 253, 2002, 725-732 [2] K. Meine, T. Schneider, D. Spaltmann, E. Santner, Wear, 253, 2002, 733-738
Spacers and supports belong to the critical elements in the structure of a nuclear fusion device, particularly, when they are installed in the vacuum or superconducting magnet system of the reactor. These supports have to sustain high loads while allowing a certain amount of tangential movement with low friction and without any stick-slip motion. Vacuum and cryogenic environments are extremely hostile for smooth sliding systems, because conventional liquid lubricants cannot be employed, and only a very limited number of materials for dry sliding systems under these conditions are available. In order to test the friction behaviour of support elements in the field coils of the fusion experiment WENDELSTEIN 7-X, a test rig was build which allows testing in liquid helium environment at 4.2 K, with a normal force up to 150 kN, and a sliding velocity of 0.1 mm/s in oscillating motion. First tests have shown that the apparatus is appropriate for simulating the sliding behaviour of support elements. As an example, a result of an earlier model test of an MoS2-coating is shown.
Simple, fast, and versatile methods for the quantification of thiol groups are of considerable interest not only for protein analysis but also for the characterization of the surface chemistry of nanomaterials stabilized with thiol ligands or bearing thiol groups for the subsequent (bio-) functionalization via maleimide−thiol chemistry. Here, we compare two simple colorimetric assays, the widely used Ellman’s assay performed at alkaline pH and the aldrithiol assay executed at acidic and neutral pH, with respect to their potential for the quantification of thiol groups and thiol ligands on different types of nanoparticles like polystyrene nanoparticles, semiconductor nanocrystals (SC NC), and noble metal particles, and we derive criteria for their use. In order to assess the underlying reaction mechanisms and to obtain stoichiometry factors mandatory for reliable thiol quantification, both methods were studied photometrically and with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), thereby demonstrating the influence of different thiols on the reaction mechanism. Our results underline the suitability of both methods for the quantification of directly accessible thiol groups or ligands on the surface of 2D- and 3D-supports, here exemplarily polystyrene nanoparticles. Moreover, we could derive strategies for the use of these simple assays for the determination of masked (i.e., not directly accessible) thiol groups like disulfides such as lipoic acid and thiol stabilizing ligands coordinatively bound to Cd and/or Hg surface atoms of II/VI and ternary SC NC and to gold and silver nanoparticles.
In order to obtain a better understanding of the role of tribofilms during automotive braking, their structures were investigated and the essential features identified.
Next, different ingredient combinations were studied by preparing some model materials with simpler compositions than real tribofilms. A test method was developed for verifying the tribological properties of the model materials and for comparison with results obtained with numerical sliding simulations of such structures. Prerequisites of good brake Performance properties were identified. Although the coefficient of friction could be varied in a wide ränge, smooth sliding conditions could only be achieved with values smaller than 0.4. Tests with artificial third body powders turned out to be useful as screening method for the selection of raw materials for brake pad formulations.
MoS2 is a suitable solid lubricant for environments free of oxygen or water vapor (i.e. vacuum). Humid air degrades film properties due to oxidation accompanied by high wear and increasing coefficients of friction. The present study aims at the further development of sputtered pure MoS2 coatings, extending their applicability to varying environmental conditions by increasing the resistance against humidity. The systematic coating development process is supported by using an experimental Box–Behnken design with variations of the deposition parameters cathode voltage, target/substrate distance, temperature and argon gas pressure. In contrast to common one-factor-at-a-time (OFAT) studies, this approach enables a determination of interactions between deposition process parameters and tribological–mechanical MoS2 film properties. The tribological improvement focuses on a maximization of wear resistance in air and vacuum measured in ball-on-disk experiments. The evaluated mechanical properties are hardness, elastic modulus and residual stresses. These stresses were determined by the substrate curvature method. The study reveals that the residual stress state in the films and the hardness-to-modulus ratio are crucial for their tribological performance in humid air and vacuum environments. After a detailed determination of the relationships between deposition conditions and film properties, some selected microstructural analyses are presented which show that a substantially basal orientation of the lattice has positive effects on wear but also causes anisotropic film properties which result in fissile fracture of the coating if strong shock or point loads occur.
Aus der Literatur zusammengefasste experimentelle Untersuchungen mit der Methodik der Oberflächenphysik sowie den aufgeführten neuen Verfahren bilden den zentralen Teil, Ergänzend hierzu werden einige grundlegende theoretische Modelle sowie Simulationsrechnungen aus jüngerer Zeit vorgestellt. Ziel dieser Arbeit ist es, Werkstoffwissenschaftlern, Ingenieuren und Physikern eine Einführung in das relativ neue Forschungsgebiet " Mikrotribologie" zu geben und in einem größeren Kontext seinen Zusammenhang mit der (Makro-)Tribologie aufzuzeigen.