AVS2013 Session TR+AS+NS+SS-MoA: Molecular Origins of Friction

The nanoscale properties of two bodies in contact cannot be fully analyzed on an atomistic level using experimental methods or understood solely using continuum mechanics. Molecular dynamics (MD) simulations allow nanoscale behavior to be modeled by resolving the positions, velocities, and forces of discrete atoms in the system. Diamond has been of interest as both an object of scientific study and as an ideal material for applications such as, cutting tool coatings, waste water purifiers, chemical sensors, electronic devices, and micro- and nanoelectromechanical systems (M/NEMS) because of its unique electrical, mechanical, and tribological properties. Due to its high fracture strength and chemical robustness, it can withstand exposure to harsh environments and resist mechanical wear. It can be grown in nanocrystalline form with nearly equivalent mechanical performance to the crystalline form. Silicon, due to the ability to create atomically sharp tips, is frequently used in scanning probe microscopy. Recently, carbon implantation of preformed Si-tips has been used to improve wear properties. In this work, MD was used to simulate the nanoscale adhesion and tribological behavior between diamond, diamond-like carbon (DLC) surfaces and silicon, and silicon carbide tips. Work of adhesion values from the MD simulations with axisymmetric tips are compared to, and discussed within the context of, complementary AFM experiments where available, finite element simulations, and continuum mechanics-based analytical models. MD simulations show that the work of adhesion is sensitive to the identity of the contacting materials because they have inherent roughness differences. In addition, work of adhesion values obtained from continuum mechanics-based analytical models are consistently higher than values obtained using the atomic-force microscope, which are higher than the simulated values. A recently developed bond-order potential for C-, H-, and Si-containing systems was used to carry out these simulations. The novel aspects of this model will be discussed.

The atomic force microscope (AFM) has been used widely to study nanoscale tribological phenomena, but a unified model for the mechanics of the tip-sample interaction is lacking. Experimental data show that nanoscale friction depends strongly on interfacial chemistry, but these correlations are not explained adequately by existing models. Here we report measurements of interactions between hydrogen bond-forming molecules adsorbed onto solid surfaces and AFM tips. By making measurements in liquid mixtures, we demonstrate a quantitative correlation between the surface shear strength in a nanoscale contact and the free energy of solution-phase hydrogen bonding interactions, uniting classical contact mechanics with equilibrium thermodynamics. We demonstrate that the thermodynamics of intermolecular interactions may be determined quantitatively from nanoscale friction measurements. It has been found that the contact mechanics are best modeled by treating the friction force as the sum of a load-dependent term (attributed to “molecular plowing”) and an area-dependent term attributed to shearing (adhesion). The relative contributions of plowing and shearing are determined by the coefficient of friction, µ, and the surface shear strength τ. The transition from adhesion- to load-determined friction is controlled by the solvation state of the surface: solvated surfaces represent a limiting case in which the shear term approaches zero, and the friction-load relationship is linear, while in other circumstances, the friction-load relationship is non-linear and consistent with Derjaguin-Muller-Toporov (DMT) mechanics. A striking correlation has been observed between the concentration-dependence of the association constant (K_a) for the formation of 1:1 hydrogen-bonded complexes and the pull-off force F_a and surface shear strength τ for the same molecules when one partner is immobilized by attachment to an AFM probe and the other is adsorbed to a surface. Analysis of the concentration-dependence of F_a and τ enables the prediction of K_S with remarkably high precision, indicating that for these hydrogen bonding systems, the tip-sample adhesion is dominated by the H-bond thermodynamics. For hydrocarbon surfaces, we have found that friction-load relationships are also fitted by DMT mechanics, and experimentally determined works of adhesion correlate closely with predictions from Lifshitz theory. For polymer brushes, a broader range of behavior is observed, but this may also be understood if the contact mechanics are modeled by treating the friction force as the sum of a load-dependent term and an area-dependent term attributed to shearing.

We report the first stages of an experiment to lower dry, unlubricated friction by use of a tailor-made nanostructure. In previous studies, we have demonstrated that random thermal fluctuations change the familiar, atomic stick-slip motion of the tip of a friction force microscope (FFM) into a thermal drifting motion, when these fluctuations are strong enough with respect to the barriers in the energy landscape. Based on a two-mass-two-spring model of an FFM [1], we know that these excitations are concentrated in the last nanometers of the tip, because of its extremely small mass and its flexibility. To achieve similar behavior in a macroscopic contact with an area well beyond that of the very specific tip-surface geometry of an FFM, we have shaped one of the two, macroscopic contacting surfaces in the form of a micro-fabricated array of Si nanopillars, each with a well-defined spring coefficient, equal to that of a standard FFM tip. This pattern can be regarded as a large multitude of FFM-like tips, each one exhibiting the thermal fluctuation motion that we identified as a lubricating effect in an FFM. Since the density of these asperities is high, the forces on each individual asperity are always low enough to keep its deformations completely elastic. We expect that each of the tip-like pillars will fluctuate independently under the load of a ball-shaped AFM tip and as a result of this, it should be possible to translate the entire, multi-asperity contact by an extremely low lateral force. This approach should lead not only to low friction, but also to a characteristic, strong dependence of the sliding motion and the friction force on temperature and on sliding velocity. We will also explore how the thermal fluctuation behavior changes as a function of the dynamic properties of the individual nanopillars and how it evolves when we scale up the total number of asperities that are simultaneously in contact.

[1] S.Yu. Krylov, J.W.M. Frenken, Phys. Rev. B80, 235435 (2009).

We present frictional measurements involving controlled lateral manipulation of antimony nanoparticles on graphite featuring atomically smooth particle-substrate interfaces via tapping- and contact-mode atomic force microscopy. As expected from earlier studies, the power required for lateral manipulation as well as the frictional forces recorded during the manipulation events exhibit a linear dependence on contact area over a wide size range from 2000 nm² to 120,000 nm². However, we observe a significant and abrupt increase in frictional force and dissipated power per contact area at a value of about 20,000 nm², coinciding with a phase transition from amorphous to crystalline within the antimony particles. Our results suggest that variations in the structural arrangement and stoichiometry of antimony oxide at the interface between the particles and the substrate may be responsible for the observed effect.