Influence of Alkali Ions on Tribological Properties of Silicon Surface

Tribological properties of surfaces (friction, adhesion and wear) provide challenging limitations to the design of reliable machines on the micro- and nanometer scale as the surface to volume area increases and volume, mass and inertia of the mobile parts decrease. This study reports on the reduction in the friction force of silicon surfaces after the alkali metal ion exposure in the form of aqueous solutions. A scanning force microscope equipped with a liquid cell was used to investigate the friction force and the pull-off force of a flat silicon surface immersed in water and in different alkali metal chlorides solutions: LiCl, NaCl and CsCl. The concentration ranged from 0.1 up to 1000 µmol/l. The changes in the free surface energy of the initial surface and of the modified surfaces after drying were determined from contact angle measurements and from the acid–base adhesion theory. In both cases, in the liquid environment and after drying of the exposed silicon substrates in air, the friction force is reduced by approximately 50 %. Our results provide new, fundamental insight into the exchange of surface termination layers in particular for tribology. Also it is suggested to use the procedure as a low-cost alternative to improve the tribological properties of the silicon surface in particular in applications where lubricating fluids are not appropriate, e.g., in nanomachines and devices

High optical contrast nanoimprinted speckle patterns for digital image correlation analysis

For the characterization of the mechanical deformation of materials at microscopic length scales, image processing of a high-quality surface pattern was used. We imprinted speckle patterns onto a thin polymer film attached to the surface of flat and curved metal substrates using flexible molds and soft-thermal nanoimprint lithography. High optical contrast was achieved by mixing black dye into the film generating high absorption in the elevated structures, and by adding titania nanoparticles as fillers to the recessed areas to induce diffuse scattering. For accessing resolution suitable to detect deformation at an individual grain level, the structure sizes were scaled down from 20 μm to 2 μm. For both structure sizes imaging was tested using a digital image correlation setup, that enables 3D imaging of samples with angles of up to 10° of inclination

Hot embossing of Au- and Pb-based alloys for x-ray grating fabrication

Grating-based X-ray phase-contrast interferometry has a high application impact in materials science and medicine for imaging of weakly absorbing (low Z) materials and soft tissues. For absorbing gratings, casting of highly X-ray absorbing metals, such as Au and Pb alloys, has proven to be a viable way to generate large area periodic high aspect ratio microstructures. In this paper, the authors review the grating fabrication strategy with a special focus on a novel approach of casting low temperature melting alloys (Au-Sn and Pb-based alloys) into Si grating templates using hot embossing. This process, similar to nanoimprint lithography, requires particular adjusting efforts of process parameters as a function of the metal alloy and the grating feature size. The transition between the solid and liquid state depends on the alloy phase diagram, the applied pressure can damage the high aspect ratio Si lamellas, and the microstructure of the solid metal can affect the grating structure. The authors demonstrate that metal casting by hot embossing can be used to fabricate gratings on a large area (up to 70 × 70 mm2) with an aspect ratio of up to 50:1 and a pitch in the range of 1–20 μm

Disposable Polymeric Micro-Cantilever Arrays for Sensing

AbstractTo fabricate low-cost polymeric cantilever arrays, we have applied injection molding. For polymers, including polypropylene and polyvinylidenfluoride, cantilever dimensions in the micrometer range with an aspect ratio as large as 10 were successfully manufactured. The cantilevers show a performance similar to the established silicon cantilevers. Combined with functionalization, the cantilever arrays show a great potential in biomedical applications