Tuning friction force and reducing wear by applying alternating electric current in conductive AFM experiments

Abstract

Reducing friction has been a human pursuit for centuries, and is especially important for the development of nanotechnology. Nowadays, with the atomic-level understanding of friction, it is possible to reduce friction by modulating the configuration and motion of interfacial atoms. However, how to further reduce friction by modulating the interfacial electronic properties is still unclear. Here we show a strategy to achieve friction and wear reduction through inducing dynamic electronic density redistribution via alternating electric current. The friction force between conductive Ir AFM tip and graphene on Ni substrate can be reduced to 1/4 under 1 kHz alternating current, and maintain for more than 70,000 s under 9.1 GPa contact pressure without any obvious wear. An electronic-level friction model (PTT-E model) is presented to unravel and quantify the tuning effect, showing that the alternating current induced dynamic electron density redistribution is the key to friction reduction. This work proposes a feasible and robust method to reduce friction and wear in nanomechanical devices, and advances the understanding and predicting of electronic contribution in friction tuning.

Fig. 1. Tuning friction force and reducing wear via applying alternating electric current in conductive AFM experiments.

a The c-AFM experimental setups and current density calculated by DFT + NEGF of Ir/Gr/Ni interface. b The averaged friction force of the scan region under 1 V alternating voltage with different frequencies, under 2.4 nm s⁻¹ sliding velocity. Error bars indicate the standard deviation of the friction force obtained from 1024 friction loops. c, d The lateral force maps and friction loops (along the black dashed lines) measured before (c) and after applying 1 V / 1 kHz alternating voltage (d). eThe friction force evolution of Ir/Gr/Ni interface under 1 V amplitude and 1 kHz frequency alternating voltage (red line) and no bias voltage (blue line). f The lateral force maps measured before (①③) and after 20 h friction (②④) under 840 nN normal load with (③④) and without (①②) alternating current. The SEM images of the AFM tips observed before (①③) and after (②④) 20 h friction under 840 nN normal load with (③④) and without (①②) alternating current. Source data are provided as a Source Data file.

Fig. 2. Influencing factors of friction under alternating current.

a Lateral force maps and (b) lateral force loops measured under different bias voltage amplitudes. The blue and red lines represent the lateral forces during the trace and retrace processes, respectively. c Variation of friction force with the bias voltage amplitude. Error bars indicate the standard deviation of the friction force obtained from 512 friction loops. d–f The relationship between friction force and alternating current frequency measured at 2 nm s⁻¹ (d), 500 nm s⁻¹ (e) and 1000 nm s⁻¹ (f) sliding velocity. g The relationship between friction force and AC frequency of Ir/h-BN/Au and Ir/Gr/Cu tribosystem measured by the c-AFM under 1 V alternating voltage amplitude. Error bars indicate the standard deviation of the friction force obtained from 512 friction loops. (h) The relationship between conductance and friction reduction ratio of the tribosystems measured in the experiment. The green, orange, and yellow colors represent the results of the Ir/Gr/Ni, Ir/Gr/Cu, and Ir/h-BN/Au, respectively. Source data are provided as a Source Data file.

Fig. 3. Charge density redistribution and atomic force change under various bias voltage.

a, b External electrostatic potential distributions at the tip-sample interface for different atomic configurations of the tip calculated via DFT + NEGF. c, d Charge density redistribution Δρ = ρ(V_b) – ρ(0) calculated via DFT + NEGF under a + 0.4 V bias voltage. e, f Profiles of the charge density redistribution along the dashed lines indicated in (c)(d). Red and blue curves represent the results under +0.4 V and –0.4 V bias voltages, respectively. g, h The charge density redistribution as a function of bias voltage amplitude. The charge density redistribution value is taken as the value with largest absolute magnitude around the interfacial Ir atoms. The red/blue curves correspond to the cases with positive/negative applied bias voltage. i, j The total force F_E on the contact region Ir atoms for different tip configurations as a function of bias voltage amplitude. The force perturbation ∆F_E(V_b) = (F_E(V_b)-F₀)/F₀, where F_E(V_b) and F₀ represent the forces under bias voltage V_b and zero bias voltage, respectively. Source data are provided as a Source Data file.

Fig. 4. Friction model under alternating electric current.

a The schematic of the electrically-thermally activated PT (PTT-E) model. The lateral force (b, c), x direction displacement of tip - sliding time (d, e) and the trajectory (f, g) of the tip apex (translucent blue cycle) sliding along the potential energy surface (PES, orange dash lines) pulled by the driver (gray square) with 10 nm s⁻¹ speed, under 500 Hz alternating current (left) and no external field (right). The red and gray curves in (b) and (c) represent the calculated results of the PTT-E and PT-E models, respectively. Source data are provided as a Source Data file.

Fig. 5. Investigation of friction influencing factors under applied alternating current using the PT-E and PTT-E models.

a, b Variation of friction with bias voltage amplitude at different temperatures. The sliding velocity is set to 10 nm s⁻¹, and the calculations are based on the tip contact (a) and flat contact configuration (b). The red and blue curves in (a) and (b) represent the calculated results of the PTT-E and PT-E models. c The relationship between the absolute value of atomic force change |ΔF_E| and change density redistribution |Δρ| for different interfaces. The green, cyan, orange, and yellow markers represent the calculated results for the Ir/Gr/Ni interface, the Ir/Gr/Ni interface with a layer of Ar atoms as adsorbates, the Ir/Gr/Cu interface, and the Ir/h-BN/Au interface, respectively. d The relationship between friction force and alternating current frequency calculated via PT-E and PTT-E model at 2 nm s⁻¹ (d), 500 nm s⁻¹ (e) and 1000 nm s⁻¹ (f). g The relationship between the AC frequency f corresponding to the minimum friction force and the sliding velocity v obtained from the experiment (gray stars) and calculated using PT-E model (blue triangles) and PTT-E model (red cycles). h–k The schematic of the tip apex sliding along the potential energy surface under different f v⁻¹a. Source data are provided as a Source Data file.