Prof. Dr. Roland Bennewitz

Single asperity measurements on Si wafers with variable SiO2 layer thickness, yet identical roughness, revealed the influence of van der Waals (vdW) interactions on friction: on thin (1 nm) SiO2 layers, higher friction and jump-off forces were observed as compared to thick (150 nm) SiO2 layers. The vdW interactions were additionally controlled by a set of silanized Si wafers, exhibiting the same trend. The experimental results demonstrate the influence of the subsurface material and are quantitatively described by combining calculations of interactions of the involved materials and the Derjaguin-Müller-Toporov model.

Sliding friction experiments on graphene grown on SiC(0001) have been performed using a combination of a microtribometer with an atomic force microscope (AFM) allowing for the investigation of atomic-scale wear. The graphene layer delaminates within 10 sliding cycles starting from substrate step edges. After run in, friction is dominated by the interaction between a changing configuration of asperities at the probe sphere and a graphitic interface layer terminating the SiC substrate. Friction varies unpredictably due to changes in the contact configuration. However, the linear relation between friction and contact area can be confirmed and a shear strength as low as a few MPa is found for the contact between ruby and the graphitic layer on SiC, which remains intact under continuous sliding.

Light emission due to discharge in air is detected during and after the detachment of microstructured PDMS samples from glass surfaces, showing contact charging of the surfaces. The light emission provides information about the detachment process, like the velocity of the peeling front, which is difficult to obtain otherwise. While the work of separation exhibits the dependence on pulling velocity typically found for viscoelastic materials, the emission intensity exhibits almost no velocity dependence. We present a model for the rate-dependent contribution of a mosaic of contact charges to the work of separation. Also, the work of separation increases as expected with increasing aspect ratio of the microstructure, while the emission intensity shows a maximum for intermediate structures. Based on their different dependencies and on an upper-bound estimate of the energy emitted as light, we conclude that for the given system the contribution of electrostatic attraction to the work of separation is minor.

Time-dependent indentation plasticity experiments have been conducted with single-dislocation resolution on KBr(100) surfaces using atomic force microscopy (AFM) in ultrahigh vacuum. Discontinuous displacements of the the tip (pop-ins) with a typical distance on the order of 1 Å or less indicate the nucleation and glide of single dislocations within the sample. Pop-in events were observed to occur repeatedly for as long as 4 min while holding the indentation at constant load. These observations indicate that nucleation of dislocations below the indenting AFM tip is stress assisted and thermally activated. The rate of pop-in events decays with time in a power-law dependence with an exponent of −0.8. The characteristic decay of indentation creep in AFM indentation is much slower than in instrumented nanoindentation for comparable experimental conditions. Closed-loop load controlled and open-loop indentations result in the same pop-in displacement and rate, proving that in AFM-based indentation the influence of instrumental inertia is small compared to most instrumented nanoindentation experiments. A comparison between indentation with sharp silicon tips and with blunter diamond tips demonstrates the importance of the tip radius even at the nanometer length scale; sharper tips activate additional glide systems.

Noncontact atomic force microscopy provides access to several complementary signals, such as topography, damping, and contact potential. The traditional presentation of such data sets in adjacent figures or in colour-coded pseudo-three-dimensional plots gives only a qualitative impression. We introduce two-dimensional histograms for the representation of multichannel NC-AFM data sets in a quantitative fashion. Presentation and analysis are exemplified for topography and contact-potential data for graphene grown epitaxially on 6H-SiC(0001), as recorded by Kelvin probe force microscopy in ultrahigh vacuum. Sample preparations by thermal decomposition in ultrahigh vacuum and in an argon atmosphere are compared and the respective growth mechanisms discussed.

Having reached the quantum and thermodynamic limits of detection, atomic force microscopy (AFM) experiments are routinely being performed at the fundamental limit of signal to noise. A critical understanding of the statistical properties of noise leads to more accurate interpretation of data, optimization of experimental protocols, advancements in instrumentation, and new measurement techniques. Furthermore, accurate simulation of cantilever dynamics requires knowledge of stochastic behavior of the system, as stochastic noise may exceed the deterministic signals of interest, and even dominate the outcome of an experiment. In this article, the power spectral density (PSD), used to quantify stationary stochastic processes, is introduced in the context of a thorough noise analysis of the light source used to detect cantilever deflections. The statistical properties of PSDs are then outlined for various stationary, nonstationary, and deterministic noise sources in the context of AFM experiments. Following these developments, a method for integrating PSDs to provide an accurate standard deviation of linear measurements is described. Lastly, a method for simulating stochastic Gaussian noise from any arbitrary power spectral density is presented. The result demonstrates that mechanical vibrations of the AFM can cause a logarithmic velocity dependence of friction and induce multiple slip events in the atomic stick-slip process, as well as predicts an artifactual temperature dependence of friction measured by AFM.

Friction and wear of single layers of graphene have been studied at the micrometer scale. Epitaxial graphene grown by thermal decomposition on SiC-6H(0001) is found to have an initial friction coefficient of 0.02, significantly lower than graphite under the same experimental conditions. During reciprocal sliding the graphene layer is damaged. The evolving friction coefficient of 0.08 for the carbon-rich interface layer terminating the SiC layer is still lower than that of graphite and five times lower than that of the hydrogen-etched SiC substrate. Micrometer-sized patches within the sliding track retain the low friction coefficient of graphene even after hundred sliding cycles.

In this article, we present an analytical model that describes the plowing coefficient of friction for sliding, elastic-plastic contacts between a conical tip with a spherical extremity and a flat substrate. The model includes the effects of adhesion and bridges the gap between models which are based solely on dislocation activity and those based solely on interfacial effects scaling with the contact area. The Derjaguin-Muller-Toporov approximation for adhesive contact stress is used in our description of the contacts. Our model shows excellent agreement with large-scale molecular dynamics simulations and atomic force microscopy experiments of nanoscratching on copper single crystals. One important result of our study is that the model predicts coefficients of friction that are an order of magnitude higher than typically reported for nanoscale elastic contacts. Furthermore, the coefficients of friction described by the model are very close to values typical of macroscale sliding contacts.

The lubricating properties of an ionic liquid on gold surfaces can be controlled through application of an electric potential to the sliding contact. A nanotribology approach has been used to study the frictional behaviour of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4]FAP) confined between silica colloid probes or sharp silica tips and a Au(111) substrate using atomic force microscopy. Friction forces vary with potential because the composition of a confined ion layer between the two surfaces changes from cation-enriched (at negative potentials) to anion enriched (at positive potentials). This offers a new approach to tuning frictional forces reversibly at the molecular level without changing the substrates, employing a self-replenishing boundary lubricant of low vapor pressure.

Indentation experiments on the nanometre scale have been performed by means of atomic force microscopy in ultra-high vacuum on KBr(100) surfaces. The surfaces yield in the form of discrete surface displacements with a typical length scale of 1 Å. These surface displacements are detected in both normal and lateral directions. Measurement of the lateral tip displacement requires a load-dependent calibration due to the load dependence of the effective lateral compliance. Correlation of the lateral and normal displacements for each glide event allow identification of the activated slip system. The results are discussed in terms of the resolved shear stress in indentation experiments and of typical results in atomistic simulations of nanometre-scale indentation.