Interactive Surfaces

The anisotropy of friction on graphitic surfaces is investigated by a combined friction force microscopy and modeling study. Friction vectors deviate up to 15° from pulling directions. The strongest deviations are found for pulling directions which lie almost along one zigzag direction of the honeycomb structure, the preferred sliding direction on graphite surfaces and epitaxial graphene grown on SiC(0001). Atomic stick-slip events along and across molecular rows determine direction and magnitude of friction. Simulation and modeling reveal the role of temperature and of the two-dimensional character of the surface potential for the friction anisotropy.

We describe an interdisciplinary class offered to undergraduate university students of Arts and Sciences, with most participants majoring in philosophy or physics. The class combines learning about the theory of atomic force microscopes (AFMs), using them for gathering data and processing images out of the data in hands-on exercises with (1) understanding important do's and don'ts of image processing and (2) with philosophical reflection on microscopy and image theory guided by philosophers' texts, written between 1690 and 2010, on microscopes, on images and on the suitable-realist or antirealist-interpretation of microscopic images.

Friction involves a complex set of phenomena spanning a large range of length scales, but experiments assessing the evolution of the slip-front between two dry sliding bodies now reveal that slip can be reasonably well described by linear fracture mechanics theory.

Reibung lässt sich elektrochemisch kontrollieren. Mit ionischen Flüssigkeiten elektrochemisch gesteuerte Reibvorgänge könnten in kleinskaligen, miniaturisierten Kontakten herkömmliche Schmiermittel ersetzen.

The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1 1 1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip-sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface.

We study nanoindentation and scratching of graphene-covered Pt(111) surfaces in computer simulations and experiments. We find elastic response at low load, plastic deformation of Pt below the graphene at intermediate load, and eventual rupture of the graphene at high load. Friction remains low in the first two regimes, but jumps to values also found for bare Pt(111) surfaces upon graphene rupture. While graphene substantially enhances the load carrying capacity of the Pt substrate, the substrate's intrinsic hardness and friction are recovered upon graphene rupture.

An attempt has been made to develop a new metallic glass (MG) that combines high hardness with wear resistance. Refractory metallic films of W33Ni32B35 (at.%) have been deposited on stainless steel and Si substrates by dc magnetron sputtering. The alloy films are glassy, have a high crystallization temperature of 873 °C and rank among the very hard metallic materials (∼24 GPa). Importantly, this MG also shows excellent wear resistance, approaching that of standard tribological materials like TiN and hence it represents one of the most wear-resistant known metallic materials. Based on its unique combination of high strength and low elastic modulus, other potential applications are also discussed.

Young's modulus, fracture stress, and Poisson's ratio are important mechanical characteristics for micromechanical devices. The Poisson's ratio of a material is a good measure to elucidate its mechanical behavior and generally is the negative ratio of transverse to axial strain. A nanocrystalline (NCD) and an ultrananocrystalline (UNCD) diamond sample with grain boundaries of different chemical and structural constitutions have been investigated by an ultrasonic resonance method. For both samples, the elastic moduli are considerably reduced, compared with the elastic modulus of single crystal diamond (sc-diamond). Depending on the chemical and structural constitution of grain boundaries in nano- and ultrananocrystalline diamond different values for Poisson's ratio and for the fracture strength are observed. We found a Poisson's ratio of 0.201 ± 0.041 for the ultrananocrystalline sample and 0.034 ± 0.017 for the nanocrystalline sample. We discuss these results on the basis of a model for granular media. Higher disorder in the grain boundary leads to lower shear stiffness between the single grains and ultimately results in a decrease of Young's and shear modulus and possibly of the fracture strength of the material.

In this study, polyetheretherketone composites were compounded using a two-screw extruder followed by injection moulding. The effects of multi-fillers on the mechanical properties and crystallization performances were investigated. Differential scanning calorimetry results indicate that the addition of fillers slightly increases the crystallization temperature and crystallinity. Compared to neat polyetheretherketone, the incorporation of inorganic filler leads to a significant improvement in matrix hardness, matrix stiffness and a slight increase in tensile strength. However, the material ductility, the impact strength and the fracture toughness of polyetheretherketone composites decrease. Fractography analyses show that the addition of fillers restraints the ductile deformation of polymers, which is responsible for the reduction of material ductility, impact strength as well as fracture toughness of polyetheretherketone composites.

The tribological properties of Poly-ether-ether-ketone (PEEK) were studied systematically by multiple scratch tests in both unidirection and bidirection mode on the micro- and nano-length scale. The tip geometry has a strong influence on the scratch friction behavior, in particular on the scratch initiation and on the resulting damage patterns. Plowing contributions to friction are significant for nano-scale tips during the initial scratch cycles. Shear contributions dominate for both nano- and micro-scale once a groove has been established by multiple scratches. While the damaging mechanisms are the same for both micro- and nano-scratch tests, the resulting damaging patterns differ depending on the scratching mode (unidirection or bidirection) and the normal load. Patchy layers of material formed by scratching are torn into fracture by nano-scale tips, while they are stretched to flakes by the micro-scratch indenter.