Abstract2D or 3D layered materials, such as graphene, graphite, and molybdenum disulfide, usually exhibit superlubricity properties when sliding occurs between the incommensurate interface lattices. This study reports the superlubricity between graphite and silica under ambient conditions, induced by the formation of multiple transferred graphene nanoflakes on the asperities of silica surfaces after the initial frictional sliding. The friction coefficient can be reduced to as low as 0.0003 with excellent robustness and is independent of the surface roughness, sliding velocities, and rotation angles. The superlubricity mechanism can be attributed to the extremely weak interaction and easy sliding between the transferred graphene nanoflakes and graphite in their incommensurate contact. This finding has important implications for developing approaches to achieve superlubricity of layered materials at the nanoscale by tribointeractions.
Superlubricity between graphite and silica is achieved under ambient conditions, induced by the formation of multiple transferred graphene nanoflakes (GNFs) on a silica surface. As reported by Jinjin Li and co‐workers in article number 1700616, the friction coefficient could be reduced to 0.0003 at a maximal local contact pressure of 700 MPa because of the extremely weak interaction and easy sliding between transferred GNFs and graphite in the incommensurate contact.
Applied
potential can control the friction on electrical conductor
surfaces in both air and liquid environments. Using a three-electrode
electrochemical cell filled with aqueous ionic solution under atomic
force microscopy, a potential-dependent friction phenomenon was observed
on a graphitic surface. With an applied positive potential on the
graphitic surface, the friction force increased over that observed
under neutral applied potential and increased with increasing positive
potential. With the applied negative potential on the graphitic surface,
lubricity was observed compared to the case with the neutral potential
on the graphitic surface. The applied potential also changed the interaction
force between the probe and graphitic surface, where the positive
potential resulted in a stronger attractive force, while the negative
potential induced a stronger repulsive force. The mechanism of the
friction and interaction force modulation under the applied potential
was ascribed to the rearrangement of hydrated cations and anions on
the graphitic surface under different potentials, resulting in electrostatic
forces and hydrogen bonds varying as a function of potential. This
study demonstrates that the friction on a graphitic surface can be
modulated by the surface potential in aqueous ionic solutions, which
may provide a novel method for reducing friction in aqueous environments.
An extremely low friction state was observed on the gold surface induced by applying a specific negative potential in cationic surfactant solution. The friction force showed a remarkable reduction from 8.3 to 3.5 × 10−2 nN (reduced by 99.6%) with increasing the period of negative applied potential, and the final friction coefficient could reduce down to 3 × 10−4. The extremely low friction state was robust, and it also exhibited an excellent load bearing capacity, which cannot be damaged by a high load. Moreover, the extremely low friction state achieved under negative applied potential could keep stable even after the removal of potential, but failed in a short time, once a specific positive potential was applied. It was demonstrated that there was a stable electro-adsorption of surfactant molecules on the gold surface induced by applying a negative potential, leading to the formation of a bilayer structure on the gold surface. The hydration layers of the bilayer on the gold surface and micelles on the silica probe provided a shear plane with an extremely low shear strength, leading to the extremely low friction state on the gold surface. This study provides a method to achieve extremely low friction state by applied potential.
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