Origin of High Friction at Graphene Step Edges on Graphite.

On graphite, friction is known to be more than an order of magnitude larger at step edge defects as compared to on the basal plane, especially when the counter surface slides from the lower terrace of the step to the upper terrace. Very different mechanisms have been proposed to explain this phenomenon, including atomic interactions between the counter surface and step edge (without physical deformation) and buckling or peeling deformation of the upper graphene terrace. Here, we use atomic force microscopy (AFM) and reactive molecular dynamic (MD) simulations to capture and differentiate the mechanisms proposed to cause high friction at step edges.

Identifying Physical and Chemical Contributions to Friction: A Comparative Study of Chemically Inert and Active Graphene Step Edges.

Friction has both physical and chemical origins. To differentiate these origins and understand their combined effects, we study friction at graphene step edges with the same height and different terminating chemical moieties using atomic force microscopy (AFM) and reactive molecular dynamics (MD) simulations. A step edge produced by physical exfoliation of graphite layers in ambient air is terminated with hydroxyl (OH) groups.

Effect of Atomic Corrugation on Adhesion and Friction: A Model Study with Graphene Step Edges.

This Letter reports that the atomic corrugation of the surface can affect nanoscale interfacial adhesion and friction differently. Both atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that the adhesion force needed to separate a silica tip from a graphene step edge increases as the side wall of the tip approaches the step edge when the tip is on the lower terrace and decreases as the tip ascends or descends the step edge. However, the friction force measured with the same AFM tip moving across the step edge does not positively correlate with the measured adhesion, which implies that the conventional contact mechanics approach of correlating interfacial adhesion and friction could be invalid for surfaces with atomic-scale features.

Effect of Ambient Chemistry on Friction at the Basal Plane of Graphite.

Graphite is widely used as a solid lubricant due to its layered structure, which enables ultralow friction. However, the lubricity of graphite is affected by ambient conditions and previous studies have shown a sharp contrast between frictional behavior in vacuum or dry environments compared to humid air. Here, we studied the effect of organic gaseous species in the environment, specifically comparing the adsorption of phenol and pentanol vapor.

Chemical and physical origins of friction on surfaces with atomic steps.

Friction occurs through a complex set of processes that act together to resist relative motion. However, despite this complexity, friction is typically described using a simple phenomenological expression that relates normal and lateral forces via a coefficient, the friction coefficient. This one parameter encompasses multiple, sometimes competing, effects.

Mechanochemical Association Reaction of Interfacial Molecules Driven by Shear.

Shear-driven chemical reaction mechanisms are poorly understood because the relevant reactions are often hidden between two solid surfaces moving in relative motion. Here, this phenomenon is explored by characterizing shear-induced polymerization reactions that occur during vapor phase lubrication of α-pinene between sliding hydroxylated and dehydroxylated silica surfaces, complemented by reactive molecular dynamics simulations. The results suggest that oxidative chemisorption of the α-pinene molecules at reactive surface sites, which transfers oxygen atoms from the surface to the adsorbate molecule, is the critical activation step.

Mechanical characterization of diesel soot nanoparticles: in situ compression in a transmission electron microscope and simulations.

Incomplete fuel burning inside an internal combustion engine results in the creation of soot in the form of nanoparticles. Some of these soot nanoparticles (SNP) become adsorbed into the lubricating oil film present on the cylinder walls, which adversely affects the tribological performance of the lubricant. In order to better understand the mechanisms underlying the wear caused by SNPs, it is important to understand the behavior of SNPs and to characterize potential changes in their mechanical properties (e.