Interactive Surfaces

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.

We introduce a molecular toolkit for studying the dynamics in friction and adhesion from the single molecule level to effects of multivalency. As experimental model system we use supramolecular bonds established by the inclusion of ditopic adamantane connector molecules into two surface-bound cyclodextrin molecules, attached to a tip of an atomic force microscope (AFM) and to a flat silicon surface. The rupture force of a single bond does not depend on the pulling rate, indicating that the fast complexation kinetics of adamantane and cyclodextrin are probed in thermal equilibrium. In contrast, the pull-off force for a group of supramolecular bonds depends on the unloading rate revealing a non-equilibrium situation, an effect discussed as the combined action of multivalency and cantilever inertia effects. Friction forces exhibit a stick-slip characteristic which is explained by the cooperative rupture of groups of host-guest bonds and their rebinding. No dependence of friction on the sliding velocity has been observed in the accessible range of velocities due to fast rebinding and the negligible delay of cantilever response in AFM lateral force measurements.

Friction and adhesion between two [small beta]-cyclodextrin functionalized surfaces can be switched reversibly by external light stimuli. The interaction between the cyclodextrin molecules attached to the tip of an atomic force microscope and a silicon wafer surface is mediated by complexation of ditopic azobenzene guest molecules. At the single molecule level, the rupture force of an individual complex is 61 +/- 10 pN.

We report on a friction study at the nanometer scale using atomic force microscopy under electrochemical control. Friction arises from the interaction between two surfaces functionalized with cyclodextrin molecules. The interaction is mediated by connector molecules with (ferrocenylmethyl)ammonium end groups forming supramolecular complexes with the cyclodextrin molecules. With ferrocene connector molecules in solution, the friction increases by a factor of up to 12 compared to control experiments without connector molecules. The electrochemical oxidation of ferrocene to ferrocenium causes a decrease in friction owing to the lower stability of ferrocenium-cyclodextrin complex. Upon switching between oxidative and reduction potentials, a change in friction by a factor of 1.2-1.8 is observed. Isothermal titration calorimetry reveals fast dissociation and rebinding kinetics and thus an equilibrium regime for the friction experiments.

We combine non-contact atomic force microscopy (AFM) imaging and AFM indentation in ultra-high vacuum to quantitatively and reproducibly determine the hardness and deformation mechanisms of Pt(111) and a Pt57.5Cu14.7Ni5.3P22.5 metallic glass with unprecedented spatial resolution. Our results on plastic deformation mechanisms of crystalline Pt(111) are consistent with the discrete mechanisms established for larger scales: Plasticity is mediated by dislocation gliding and no rate dependence is observed. For the metallic glass we have discovered that plastic deformation at the nanometer scale is not discrete but continuous and localized around the indenter, and does not exhibit rate dependence. This contrasts with the observation of serrated, rate-dependent flow of metallic glasses at larger scales. Our results reveal a lower size limit for metallic glasses below which shear transformation mechanisms are not activated by indentation. In the case of metallic glass, we conclude that the energy stored in the stressed volume during nanometer-scale indentation is insufficient to account for the interfacial energy of a shear band in the glassy matrix.