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Velocity dependence of kinetic friction by multi-scale Quantum Mechanics/Green's Function molecular dynamics

Alberto Pacini, Seiji Kajita, Gabriele Losi, Maria Clelia Righi

TL;DR

This work tackles the velocity dependence of kinetic friction at a buried, hydrogen-passivated diamond interface by introducing a hybrid QM-GF molecular dynamics framework that couples a DFT-treated interfacial region to a semi-infinite Green's-function bath for explicit energy dissipation. Applying this method to diamond interfaces with 100%, 75%, and 50% hydrogen coverage under a vertical load of $5~\text{GPa}$ and lateral shear corresponding to $1~\text{GPa}$ and $5~\text{GPa}$, the authors observe a net friction decrease with increasing sliding speed, with the effect diminishing as coverage decreases and vanishing at 50% coverage. Two sliding regimes are identified: stick-slip at low velocity, where trajectories follow the minimum-energy path (MEP) on the interfacial PES and produce a sawtooth force signal, and continuous sliding at high velocity, where external shear dominates and forces cancel out; the washboard frequency follows $\nu_{wash} = v_{slide}/a$, yielding $\nu_{wash} = 0.19~\text{THz}$ at low-shear and $1.12~\text{THz}$ at high-shear. The study demonstrates first-principles access to velocity-dependent friction in a multiscale setting and suggests broad applicability of the QM-GF approach to other interfacial dissipation problems.

Abstract

Atomistic simulations are powerful tools for investigating tribological phenomena at a fundamental level; however, simulating a tribological system remains challenging due to the multiscale nature of frictional processes. Recently, we introduced a hybrid method, QM-GF, that enables an accurate description of both interfacial chemistry and phononic dissipation in semi-infinite bulks. In this work, we apply this simulation scheme to study the dependence of kinetic friction on sliding velocity. Using a prototypical diamond interface with varying hydrogen coverages, we find that the friction force decreases with increasing sliding velocity, revealing two distinct sliding regimes at low and high speeds. We provide a physical interpretation of this velocity dependence based on the modulation of the frictional force by the sliding motion over the periodic potential energy surface of the interface. High velocities lead to force cancellation, while low velocities result in a net frictional force characterized by a distinctive sawtooth profile.

Velocity dependence of kinetic friction by multi-scale Quantum Mechanics/Green's Function molecular dynamics

TL;DR

This work tackles the velocity dependence of kinetic friction at a buried, hydrogen-passivated diamond interface by introducing a hybrid QM-GF molecular dynamics framework that couples a DFT-treated interfacial region to a semi-infinite Green's-function bath for explicit energy dissipation. Applying this method to diamond interfaces with 100%, 75%, and 50% hydrogen coverage under a vertical load of and lateral shear corresponding to and , the authors observe a net friction decrease with increasing sliding speed, with the effect diminishing as coverage decreases and vanishing at 50% coverage. Two sliding regimes are identified: stick-slip at low velocity, where trajectories follow the minimum-energy path (MEP) on the interfacial PES and produce a sawtooth force signal, and continuous sliding at high velocity, where external shear dominates and forces cancel out; the washboard frequency follows , yielding at low-shear and at high-shear. The study demonstrates first-principles access to velocity-dependent friction in a multiscale setting and suggests broad applicability of the QM-GF approach to other interfacial dissipation problems.

Abstract

Atomistic simulations are powerful tools for investigating tribological phenomena at a fundamental level; however, simulating a tribological system remains challenging due to the multiscale nature of frictional processes. Recently, we introduced a hybrid method, QM-GF, that enables an accurate description of both interfacial chemistry and phononic dissipation in semi-infinite bulks. In this work, we apply this simulation scheme to study the dependence of kinetic friction on sliding velocity. Using a prototypical diamond interface with varying hydrogen coverages, we find that the friction force decreases with increasing sliding velocity, revealing two distinct sliding regimes at low and high speeds. We provide a physical interpretation of this velocity dependence based on the modulation of the frictional force by the sliding motion over the periodic potential energy surface of the interface. High velocities lead to force cancellation, while low velocities result in a net frictional force characterized by a distinctive sawtooth profile.

Paper Structure

This paper contains 4 sections, 4 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. METHODS
  3. RESULTS
  4. Conclusion

Figures (3)

  • Figure 1: Representation of a frictional interface described by the QMGF scheme. The chemical active region is given by the QM atoms in red, GF atoms representing the semi-infinite bulk where the lateral shear is applied are indicated by yellow arrows, while cap hydrogen atoms are indicated by green arrows.
  • Figure 2: Color maps of the static interfacial PES calculated under 5 GPa of vertical load for 100% (top), 75% (middle), and 50% (bottom) hydrogen passivated systems. Dynamic trajectories at high (5 GPa applied shear, red) and low (1 GPa applied shear, black) sliding velocities are superimposed on the PES together with the MEP (yellow curves). A common energy scale for different PES is used.
  • Figure 3: PES-induced forces per area along the direction of sliding for the 100% hydrogenated interface. The black curve represents stick-slip regime (1 GPa applied shear) while the red one is continuous sliding (5 GPa applied shear). The green dashed lines represent the time averages of the signals.