Prof. Dr. Roland Bennewitz

Self-assembled monolayers introduce chemical functionalities to material surfaces, providing a route to tune their equilibrium and dynamical properties. We report on atomic force microscopy measurements and simulations of adhesion and friction forces caused by a macromolecular host–guest system, where the host molecules are attached to silicon oxide surfaces by means of self-assembled silane layers. Different preparation routes for the silane layers lead to different flexibility of the molecular attachment. The velocity dependencies of the work of separation and of friction vary significantly for attachments with different flexibility. Stiff attachment leads to low pull-off forces at low pulling velocity and to vanishing friction forces in the limit of low sliding velocity. Flexible attachment enhances cooperative contribution of multiple molecular bonds to adhesion and friction and causes significant friction at low sliding velocity. The latter observation can be explained by the contribution of intermittent contact aging to the friction force.

In single-molecule force spectroscopy, the unbinding force is often used to quantify the interaction strength of single molecular bonds. We analyze force spectroscopy of fast reversible bonds probed in thermodynamic equilibrium by considering the dynamics of force probe and molecular linker. The effect of cantilever and linker dynamics is systematically addressed by measuring the unbinding force of single cyclodextrin inclusion complexes by atomic force spectroscopy for a variety of molecular linkers and varying force probe stiffness. The unbinding force of individual bonds probed in thermodynamic equilibrium is not unique for the molecular system but scales with , the square root of the force probe stiffness, and is largely independent of the molecular linker stiffness. The observations are explained by an effective potential resulting from fast linker fluctuations and fast rebinding kinetics which is probed by an AFM cantilever. The slow cantilever dynamics in the kHz range act as mechanical low pass filter, allowing for fast rebinding kinetics of the molecular complex in the order of 106 kHz. The binding energy of the complex can be estimated from the unbinding force as a function of cantilever stiffness, however with some uncertainty arising from lack of a model in three dimensions.

The nanoscale lubrication properties of graphene depend on chemical functionalization, adsorbants, and bonding to the substrate. The dependence of friction on the rotational orientation of graphene on a Pt(111) surface was studied by high-resolution friction force microscopy in ultra-high vacuum and interpreted through complementary simulations. Lateral forces reveal an atomic-scale stick-slip motion with the periodicity of the graphene structure. Additionally, the lateral forces were modulated by a Moiré pattern, which depends on the rotation of the respective domain. Comparing the experimental results to simulations of the friction process based on the Prandtl-Tomlinson model and to atomistic simulations of the tip-sample interactions, it was found that the modulation of lateral forces originated from a structural mismatch between Pt(111) and graphene domains that were locally pinned to the substrate. Domains with preferred orientations, such as the R1° and the R24°, appeared to exhibit lower average friction than those with orientations less frequently observed, such as the R10° domain.

Thermally grown surface oxide layers dominate the single-asperity tribological behavior of a Zr60Cu30Al10 glass. Increase in oxidation time leads to an increased contribution of shearing and a corresponding decreased contribution of ploughing to friction. This change in the dominating friction and wear mechanism results in an overall minor decrease of the friction coefficient of oxidized surfaces compared to the metallic glass sample with native surface oxide. Our results demonstrate the importance of creating a stable oxide layer for practical applications of metallic glasses in micro-devices involving sliding contact.

The confinement of liquids in nanometer-scale gaps can lead to changes in their viscous shear properties. For liquids of polar molecules, the charge state of the confining surfaces has a significant influence on the structure in the confined liquid. Here we report on the implementation of dynamic shear force microscopy in an electrochemical cell. Lateral oscillations of the tip of an atomic force microscope were magnetically activated at a frequency of about 50 kHz. The damping of the lateral tip oscillation was recorded as a function of the tip-sample distance and of the electrode potential at the surface of a Au(100) single crystal electrode. The influence of surface charges on the shear response of the nano-confined liquids was demonstrated for the ionic liquid [EMIM][NTf2] and for aqueous Na2SO4 solution.

Platelets as fillers in polymer coatings contribute to corrosion resistance by increasing the diffusion path of gases. The authors demonstrate that the same platelets can improve tribological properties and, thus, open a new way to design multifunctional polymer coatings. Improved corrosion resistance, low friction, and low wear are reported for polyimide composite coatings filled with a combination of boron nitride, pigment platelets, perfluoropolyether, and Si3N4 particles. Contributions of different fillers to the tribological performance are explored for coatings with different filling protocols. The synergy of four components leads to the excellent tribological performance of the fully formulated coatings, while they cannot impart significant improvement in friction and wear when used separately.

Hexadecane exhibits pronounced molecular layering upon confinement to gaps of a few nanometer width which is discussed for its role in boundary lubrication. We have probed the mechanical properties of the confined layers with the help of an atomic force microscope, by quasi-static normal force measurements and by analyzing the lateral tip motion of a magnetically actuated torsional cantilever oscillation. The molecular layering is modeled by a oscillatory force curve and the tip approach is simulated assuming thermal equilibrium correlations in the liquid. The shear response of the confined layers reveals gradually increasing stiffness and viscous dissipation for a decreasing number of confined layers.

In order to reveal fundamental tribological mechanisms in polymer/steel sliding pairs, the pin-on-flat configuration of classical macroscopic tribotests was transferred into a high-resolution tribometer designed for scratch tests. Experiments were performed with a polyetheretherketone (PEEK) pin sliding on a steel disk in straight unidirectional movement mode. The surface morphology was monitored by interrupting the tests every 10,000 sliding strokes. The evolving surface morphology of PEEK was correlated with the transfer layer formed on steel counter surface. Scratching grooves in the PEEK surface were induced by asperities at the counter steel surface covered with transfer layers. Transfer layers were composed of lumpy polymer material accompanied by fine wear debris in areas of lower roughness. These smooth areas limit the penetration of large asperities and distinguish the scratching mechanism in macroscopic sliding from typical single-asperity scratching tests. The results reveal the mechanisms leading to inhomogeneity in the transfer layers as consequence of the asperity distribution.

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation.

Carbon fibers are widely used as reinforcements in poly-ether-ether-ketone (PEEK). In recent years, these materials have also been used for tribological applications. For further optimization of these tribo-materials, the contribution and action mechanisms of carbon fiber reinforcements to the tribological performance of PEEK composites need to be understood. Toward this goal, we have studied carbon fibers in a PEEK composite by scratching experiments using Berkovich and conical indenters and friction imaging using contact atomic force microscopy. For comparison, scratching was extended into the PEEK matrix surrounding the carbon fibers. It is found that shearing dominates the friction and wear behavior of carbon fibers alone, while both shearing and plowing contribute to the overall friction of PEEK composites. There is no local variation in friction across a carbon fiber surface. The wear reduction by carbon fibers originates from their effective load-bearing capability. For the first time, fatigue of individual carbon fibers is revealed, as well as the dependence of interfacial debonding or delamination on the contact configuration between fibers and scratching asperities.