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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.
Molybdenum disulfide (MoS₂) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD) process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS₂coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS₂ coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper.
A modified pin-on-disc test was applied to determine tribological properties of typical brake pad constituents. Ball-milling of these ingredients together with iron oxide and graphite provided model materials displaying the main features of real third bodies. Solid lubricants like graphite affected the friction and wear behaviour of Fe3O4 powders considerably whereas further addition of hard nanoparticles induced only minor effects. This was corroborated by comparison with modelling results. MoS2 played a dual role. Depending on special conditions, this ingredient either reduced or increased friction. The latter could be explained, after nanoscopic characterization, by oxidation and destruction of the wear-protecting tribofilm.
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.
A model based on movable cellular automata has been applied to study the sliding behaviour of tribofilms formed during automotive braking. Since it is not possible yet to determine the composition of real tribofilms quantitatively, final verification of modelling results is needed. This was done by preparing artificial third bodies with compositions and nanostructures matching the ones assumed for modelling. Pin-on-disc tests were performed while applying the artificial third bodies to the contact. The results revealed that not only the structure of the third body but also the amount of the applied normal pressure determines the COF obtained by modelling and that much better correlation between experimental and modelling results was obtained while assuming high normal pressures at asperity contacts.
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.
Der Festschmierstoff Molybdändisulfid ist insbesondere für Extremanwendungen ein gerne eingesetzter Werkstoff, problematisch ist jedoch der negative Einfluss von Feuchtigkeit. Bei mittels PVD-Verfahren abgeschiedenen MoS2-Schichten ergibt sich jedoch die Möglichkeit, durch Anpassung der Beschichtungsparameter vorwiegend basal ausgerichtete Schichten herzustellen, die auch an Luft ein verbessertes tribologisches Verhalten zeigen.
The solid lubricant molybdenum disulfide is a well known material for extreme conditions where conventional oil or grease lubrication is not feasible (vacuum, high pressures). However, under reactive environments e.g. high humidity or hydrogen MoS2 is prone to premature failure. By using a PVD process, deposition parameters can be optimized to gain coatings with a more textured structure orienting the basal planes parallel to the Substrate surface, A similar concept has been used by Koch et al. for coatings developed to withstand the extreme operational conditions of the WENDELSTEIN W7-X fusion experiment. Therefore, an improved tribological behavior even under adverse conditions can be achieved.