Adhesion-dependent negative friction coefficient on chemically modified graphite at the nanoscale

Area of Science:

• Tribology
• Nanotechnology
• Materials Science

Background:

• Friction typically increases with normal load, a principle observed from macroscopic to nanoscale studies.
• Nanoscale friction can exhibit nonlinear behavior, deviating from classical models.

Purpose of the Study:

• To investigate nanoscale friction on chemically modified graphite using atomic force microscopy.
• To explore the emergence of a negative friction coefficient under specific tip-sample adhesion conditions.

Main Methods:

• Nanoscale friction force microscopy (FFM) experiments on graphite.
• Atomic force microscopy (AFM) for precise load and friction measurements.
• Atomistic and continuum-based simulations to model the observed phenomena.

Main Results:

• Friction increased with decreasing load during tip retraction when tip-sample adhesion exceeded graphite's exfoliation energy.
• An effectively negative coefficient of friction was observed in the low-load regime.
• The magnitude of the negative friction coefficient correlated with the ratio of tip-sample adhesion to exfoliation energy.

Conclusions:

• Reversible partial delamination of graphite's topmost atomic layers, mimicking few- to single-layer graphene, explains the negative friction coefficient.
• This phenomenon challenges existing continuum mechanical models for nanoscale tribology.
• The lamellar structure of graphite leads to unique nanoscale tribological properties.