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A research team led by Prof. Tian-Bao Ma from the Department of Mechanical Engineering at Tsinghua University has proposed a novel strategy to reduce friction and wear by inducing dynamic electronic density redistribution through the application of an alternating electric current.

This method enables flexible and instantaneous modulation of friction by adjusting the amplitude and frequency of the alternating current. Remarkably, it maintains low friction and wear over long durations under high contact pressure and current density, requiring only a low driving voltage.

The findings are published in the journal .

Friction and wear are ubiquitous in daily life and are major sources of energy loss and component degradation. Reducing friction has been a human pursuit for centuries. With today's atomic-level understanding of friction, researchers have been able to lower friction by controlling the configuration and motion of interfacial atoms. However, how to further reduce friction by modulating interfacial electronic properties remains unclear.

The team discovered that, in conductive atomic force microscopy (c-AFM) experiments, applying alternating current (AC) at specific frequencies can reduce the friction force between an Ir-coated conductive AFM tip and graphene (on a Ni substrate) by approximately 75%, and maintain this low friction level for over 70,000 seconds under a contact pressure of 9.1 GPa and a current density of 100 GA/m², with no observable wear.

Unlike mechanical vibration-induced friction reduction, the optimal AC frequency for minimizing friction does not correspond to the resonance frequency of the system, but is instead related to the washboard frequency. Moreover, the electrostatic force induced by the alternating current (on the order of 1 nN) is significantly smaller than the normal load (approximately 840 nN), indicating a different underlying mechanism.

First-principles calculations reveal that the dynamic redistribution of electronic density triggered by the AC is the key mechanism for friction reduction, which is fundamentally different from mechanical vibration or thermal activation methods.

When a bias voltage is applied, the interfacial electrostatic potential distribution changes, causing electrons at the interface to redistribute, which in turn alters the interfacial atomic forces.

To quantitatively understand why this dynamic redistribution of electrons affects friction, the team derived the PTT-E (Electrically-Thermally Activated Prandtl-Tomlinson) model.

This model shows that the force perturbation induced by electron redistribution provides an additional driving force for atoms to overcome the sliding energy barriers, thereby lowering the friction force.

The model predictions regarding how friction varies with AC amplitude, frequency, and sliding velocity match well with the experimental results, and shed light on understanding and predicting the electronic contribution to friction modulation.

The team further demonstrated that this electronically activated effect also applies to other 2D material–metal interfaces, such as Ir/graphene/Cu and Ir/h-BN/Au systems. In highly conductive systems, the interfacial electronic redistribution is more pronounced, resulting in greater friction reduction.

Prof. Ma commented, "This mechanism of electrically activated friction reduction is applicable to various conductive 2D material interfaces, providing a theoretical framework for understanding and predicting electronic contributions to friction modulation.

"It could offer a new solution to one of the key challenges that has hindered the practical application of MEMS/NEMS for decades—namely, severe wear under high mechanical load."