Load-induced dynamical transitions at graphene interfaces

Abstract

The structural superlubricity (SSL), a state of near-zero friction between two contacted solid surfaces, has been attracting rapidly increasing research interest since it was realized in microscale graphite in 2012. An obvious question concerns the implications of SSL for micro- and nanoscale devices such as actuators. The simplest actuators are based on the application of a normal load; here we show that this leads to remarkable dynamical phenomena in microscale graphite mesas. Under an increasing normal load, we observe mechanical instabilities leading to dynamical states, the first where the loaded mesa suddenly ejects a thin flake and the second characterized by peculiar oscillations, during which a flake repeatedly pops out of the mesa and retracts back. The measured ejection speeds are extraordinarily high (maximum of 294 m/s), and correspond to ultrahigh accelerations (maximum of 1.1×1010m/s2). These observations are rationalized using a simple model, which takes into account SSL of graphite contacts and sample microstructure and considers a competition between the elastic and interfacial energies that defines the dynamical phase diagram of the system. Analyzing the observed flake ejection and oscillations, we conclude that our system exhibits a high speed in SSL, a low friction coefficient of 3.6×10−6, and a high quality factor of 1.3×107compared with what has been reported in literature. Our experimental discoveries and theoretical findings suggest a route for development of SSL-based devices such as high-frequency oscillators with ultrahigh quality factors and optomechanical switches, where retractable or oscillating mirrors are required.

Microscale Schottky superlubric generator with high direct-current density and ultralong life

“…Structural superlubricity (SSL) is a state of ultrallow friction and wearless between two solid surfaces 22 . Since the first realization of microscale SSL in the atmospheric environment in 2012 23 , in addition to the realizations of high speed SSL (25 m/s ~ 293 m/s) 24,25 , SSL has aroused interest in practical application, bringing a dawn for the revolutionary solution of friction and wear probems 22 . As an application of SSL, several types of superlubric nanogenerators (SLNGs) based on capacitors or electrets were theoretically proposed recently by the authors 26 .…”

Rotational Friction Correlated with Moiré Patterns in Strained Bilayer Graphene: Implications for Nanoscale Lubrication

“…At strain 0%, the curve shows periodic sawtooth with high torque separated by flat profiles with near-zero torque, analogous to the stick−slip/smooth sliding characteristic in sliding motion. [8][9][10]12 This strong frictional anisotropy arises from the periodic appearance of the commensurate contact (θ = 0°), where the atoms get stuck in the trough of potential energy landscapes, leading to enhanced friction. Once the substrate stretching is imposed, the sawtooth gradually shrinks in both strain conditions, and the curve is characterized by high-frequency ripples.…”

“…The experimental evidence of ultralow friction found on the graphite/graphite interface demonstrates the reproducible superlubric motion at the incommensurate contact, which triggers extensive investigations of frictional dependence on sliding velocity, temperature, thickness, normal load, and misorientation angle. − For example, low sliding speed and high-temperature facilitate the reduction of friction by assisting atoms to overcome energy barriers; thickening materials diminishes surface puckering, thereby lessening friction. However, the strong anisotropy renders the superlubricity dynamically unstable. , How to eliminate frictional anisotropy becomes crucial for the application of structural superlubricity.…”

Layered Thin Film Deposition via Extreme Inter-Brush Slip in a Lamellar Block Copolymer

“…Nanometer thin 2D layered materials such as graphite readily transfer to substrates via mechanical forces due to the low friction between the atomically flat layers. − Solvent-swollen polymer brushes also exhibit very low friction that is essential, for example, in the articulation of bone joints. , Lamellar block copolymers (BCPs) are a type of layered material having a smectic A liquid crystal structure with 1D translational order. Subjecting a bulk lamellar BCP sample to low-rate, high-temperature, large amplitude oscillatory shear results in global alignment with the lamellar normal parallel to the velocity gradient due to easy shear as well as delamination across the mid-layer brush–brush slip planes. , When subjected to forces normal to the layers, a chevron pattern of sharp kink boundaries forms parallel to the force axis. − For glassy–rubbery lamellar BCPs deformed at quasi-static strain rates and room temperature, the lamellar normal tilts away from the force direction with increased strain.…”