Atomic-Scale Wear of Amorphous Hydrogenated Carbon during Intermittent Contact: A Combined Study Using Experiment, Simulation, and Theory

Abstract

In this study, we explore the wear behavior of amplitude modulation atomic force microscopy (AM-AFM, an intermittent-contact AFM mode) tips coated with a common type of diamond-like carbon, amorphous hydrogenated carbon (a-C:H), when scanned against an ultra-nanocrystalline diamond (UNCD) sample both experimentally and through molecular dynamics (MD) simulations. Finite element analysis is utilized in a unique way to create a representative geometry of the tip to be simulated in MD. To conduct consistent and quantitative experiments, we apply a protocol that involves determining the tip-sample interaction geometry, calculating the tip-sample force and normal contact stress over the course of the wear test, and precisely quantifying the wear volume using high-resolution transmission electron microscopy imaging. The results reveal gradual wear of a-C:H with no sign of fracture or plastic deformation. The wear rate of a-C:H is consistent with a reaction-rate-based wear theory, which predicts an exponential dependence of the rate of atom removal on the average normal contact stress. From this, kinetic parameters governing the wear process are estimated. MD simulations of an a-C:H tip, whose radius is comparable to the tip radii used in experiments, making contact with a UNCD sample multiple times exhibit an atomic-level removal process. The atomistic wear events observed in the simulations are correlated with under-coordinated atomic species at the contacting surfaces.

Matching Atomistic Simulations and In Situ Experiments to Investigate the Mechanics of Nanoscale Contact

“…These continuum models and their various extensions rely on underlying assumptions that may be violated at nanometer length scales. An alternative to continuum models is atomistic simulations, which are able to track the position of each atom and have been employed to study nanocontacts mimicking experimental conditions [15][16][17][18][19]. Some such simulations have suggested that contact mechanics may be applied when atomic-scale surface roughness is considered in the model [20] or when modifications of these theories are used, such as thin-coating contact mechanics [16,21].…”

Interface Engineering and Controlling the Friction and Wear of Ultrathin Carbon Films: High sp3 Versus High sp2 Carbons

“…In general, ultrathin amorphous carbon films have gathered great technological interest as overcoats for magnetic storage devices, MEMS/NEMS, and AFM tips, owing to their excellent mechanical and tribological properties, and the capability for room‐temperature deposition . However, at the ultrathin film level, the role of the substrate and the film–substrate (FS) interface also become very important, and the wear and friction mechanisms at this level are not yet well understood.…”

Direct mechanochemical cleavage of functional groups from graphene

“…However, despite extensive study, these wear studies have produced only empirical relationships between mechanical load and film chemistry, due in part to poor chemical definition of the starting material, film inhomogeneity and environmental contaminants 10 . Consequently, there has been a growing interest in using nanometre scale, single asperity sliding contacts to probe mechanochemical bond scission at the tip-substrate interface, and recent work has shown volumetric resolution approaching a few atoms 11,12 . It remains unclear, however, what role surface topography plays, which bond or bonds are being broken and which properties-length, energy, polarization and so on-of the bond are important.…”