Werner Österle

Toxicity of amorphous silica nanoparticles on eukaryotic cell model is determined by particle agglomeration and serum protein adsorption effects

Cell cultures form the basis of most biological assays conducted to assess the cytotoxicity of nanomaterials. Since the molecular environment of nanoparticles exerts influence on their physicochemical properties, it can have an impact on nanotoxicity. Here, toxicity of silica nanoparticles upon delivery by fluid-phase uptake is studied in a 3T3 fibroblast cell line. Based on XTT viability assay, cytotoxicity is shown to be a function of (1) particle concentration and (2) of fetal calf serum (FCS) content in the cell culture medium. Application of dynamic light scattering shows that both parameters affect particle agglomeration. The DLS experiments verify the stability of the nanoparticles in culture medium without FCS over a wide range of particle concentrations. The related toxicity can be mainly accounted for by single silica nanoparticles and small agglomerates. In contrast, agglomeration of silica nanoparticles in all FCS-containing media is observed, resulting in a decrease of the associated toxicity. This result has implications for the evaluation of the cytotoxic potential of silica nanoparticles and possibly also other nanomaterials in standard cell culture.

Investigation of white etching layers on rails by optical microscopy, electron microscopy, X-ray and synchrotron X-ray diffraction

Abstract Patches of white etching layers on rail surfaces were investigated using sophisticated techniques like cross-sectional transmission electron microscopy (XTEM) and synchroton X-ray diffraction. Optical microscopy failed to resolve the microstructure, but in the TEM submicron grains with high dislocation densities and occasional twins, which are characteristic features of high carbon martensite, were observed. The martensitic structure was confirmed by evaluation of synchroton X-ray diffraction line profiles. The latter technique also allowed to determine dislocation densities of the order of 10 12 cm −2 and residual compressive stresses of about 200 MPa.

Modelling the orientation and direction dependence of the critical resolved shear stress of nickel-base superalloy single crystals

Abstract The orientation and direction dependence of the critical resolved shear stress was determined experimentally for the chromium-rich superalloy SC 16 at 650, 750 and 850°C and a constant strain rate of 10−3 s−1. The results are used to establish an extended Schmid law for octahedral slip in the temperature and orientation range in which cross-slip pinning of dislocation pairs in the γ′ phase takes place. Normal Schmid behaviour was assumed for orientations near [111], for which cube slip was activated on a macroscopic level. Differences between some commercial superalloys are worked out and can be attributed to morphology and volume fraction of the γ′ phase. The orientation dependence and asymmetry effects increase in the order NIMONIC 105, SC 16, Rene N4. The orientation range where macroscopic cube slip can be expected increases in the same order. A close inspection of the parameters which are responsible for non-Schmid behaviour suggests that, in addition to cross-slip pinning, a matrix effect must be operating as well, partly counteracting the behaviour expected for mono-phase γ′ crystals.

Ti/TiN multilayer coatings : deposition technique, characterization and mechanical properties

Abstract Ti/TiN multilayer coatings with multilayer periods in the range 5–50 nm and a final thickness of 2 μm were deposited on steel substrates by cyclic modulation of nitrogen gas flow into the chamber of a PVD sputtering device. Coating characterization was performed by cross-sectional transmission electron microscopy, glancing-angle X-ray diffraction and instrumental indentation testing. Individual α-titanium and titanium nitride layers were always observed, although for the finer microstructures, the TiN layers were thicker than the Ti layers by a factor three. The plastic hardness of the films increased steadily with decreasing layer spacing, following a Hall–Petch relationship. Finally, a hardness value of 42 GPa was reached, which is similar to that of a thick TiN monolayer, prepared under the same coating conditions.

Impact of polymer shell on the formation and time evolution of nanoparticle–protein corona

The study of protein corona formation on nanoparticles (NPs) represents an actual main issue in colloidal, biomedical and toxicological sciences. However, little is known about the influence of polymer shells on the formation and time evolution of protein corona onto functionalized NPs. Therefore, silica-poly(ethylene glycol) core-shell nanohybrids (SNPs@PEG) with different polymer molecular weights (MW) were synthesized and exhaustively characterized. Bovine serum albumin (BSA) at different concentrations (0.1-6 wt%) was used as model protein to study protein corona formation and time evolution. For pristine SNPs and SNPs@PEG (MW=350 g/mol), zeta potential at different incubation times show a dynamical evolution of the nanoparticle-protein corona. Oppositely, for SNPs@PEG with MW≥2000 g/mol a significant suppression of corona formation and time evolution was observed. Furthermore, AFM investigations suggest a different orientation (side-chain or perpendicular) and penetration depth of BSA toward PEGylated surfaces depending on the polymer length which may explain differences in protein corona evolution.

On the role of surface composition and curvature on biointerface formation and colloidal stability of nanoparticles in a protein-rich model system.

The need for a better understanding of nanoparticle-protein interactions and the mechanisms governing the resulting colloidal stability has been emphasised in recent years. In the present contribution, the short and long term colloidal stability of silica nanoparticles (SNPs) and silica-poly(ethylene glycol) nanohybrids (Sil-PEG) have been scrutinised in a protein model system. Well-defined silica nanoparticles are rapidly covered by bovine serum albumin (BSA) and form small clusters after 20min while large agglomerates are detected after 10h depending on both particle size and nanoparticle-protein ratio. Oppositely, Sil-PEG hybrids present suppressive protein adsorption and enhanced short and long term colloidal stability in protein solution. No critical agglomeration was found for either system in the absence of protein, proving that instability found for SNPs must arise as a consequence of protein adsorption and not to high ionic environment. Analysis of the small angle X-ray scattering (SAXS) structure factor indicates a short-range attractive potential between particles in the silica-BSA system, which is in good agreement with a protein bridging agglomeration mechanism. The results presented here point out the importance of the nanoparticle surface properties on the ability to adsorb proteins and how the induced or depressed adsorption may potentially drive the resulting colloidal stability.

Microstructural characterization of binary and ternary hard coating systems for wear protection. Part II : Ti(CN) PACVD coatings

Abstract A series of Ti(CN) coatings including TiN and TiC were produced by plasma-assisted chemical vapour deposition (PACVD). Systematic changes of microstructure and texture were investigated by analytical transmission electron microscopy (TEM) and X-ray diffraction (XRD). With increasing carbon content, a transition from an equiaxed grain structure with random orientation distribution to a fan-like structure showing a weak 〈111〉 texture, and finally a very fine columnar structure with 〈110〉 texture was observed. Most coatings had a mono-phase cubic TiN or TiC structure, except that with the highest carbon content which was composed of TiC columns and a graphite-like constituent in intercolumnar regions. Ball on disk reciprocated sliding tests revealed a significant reduction of the coefficient of friction for the bi-phase structure compared to mono-phase Ti(CN).

Investigation of surface film nanostructure and assessment of its impact on friction force stabilization during automotive braking

Abstract The unique nanostructure formed during severe as well as moderate braking on the surface of brake discs was investigated by conventional and analytical Transmission Electron Microscopy. In both cases nanocrystalline magnetite mixed with carbon nanoinclusions and minor amounts of other pad constituents were identified. On the basis of these observations the friction performance of a single micro-contact was simulated with the method of Movable Cellular Automata. Inspite of a simplified nanostructure which was examined in two dimensions only, the calculated mean coefficient of friction fitted well to the value usually demanded for automotive braking. Furthermore, the model predicts that oxide films without soft nanoinclusions are not capable of providing smooth velocity accommodation at the pad–disc interface and thus lead to unstable friction behaviour.

Modelling the Sliding Behaviour of Tribofilms Forming During Automotive Braking: Impact of Loading Parameters and Property Range of Constituents

The impact of pressure, sliding velocity and property variation of constituents on the sliding behaviour of a model tribofilm was studied with the method of movable cellular automata (MCA). Whereas a clear pressure dependency of the coefficient of friction (COF) was always observed and could be correlated with the structure formation in terms of varying thickness of a mechanically mixed layer, the impact of the other parameters was either negligible or rather weak. Only if a brittle-to-ductile transition of the oxide-based tribofilm was assumed, a significant decrease in the COF level was predicted. Temperature-dependent property changes can be neglected during MCA modelling, unless this transition takes place. For magnetite-based tribofilms, the transition temperature is beyond 800°C, i.e. a temperature leading to fading effects during braking anyway. Thus, it could be concluded that, except for very severe braking conditions, sliding simulations with the MCA method yield meaningful results without considering temperature-dependent mechanical properties.

Numerical Simulation of Mechanically Mixed Layer Formation at Local Contacts of an Automotive Brake System

Processes taking place at local contacts in an automotive brake system are analyzed on the basis of computer simulation with the method of movable cellular automata. The conditions of a mechanically mixed layer (MML) formation on the tribosurfaces and the influence of the layer on the friction coefficient are investigated. The results show that the MML formation leads to the stabilization of the coefficient of friction at a convenient range (0.3–0.4) for brake application. The presence of graphite particles in the MML decreases the critical value of local normal stress for switching from stick-slip to smooth relative motion with MML formation.