Friction and wear are detrimental to functionality and reduce the service life of products with mechanical elements. Here, we unveil the atomic-scale friction of a single tungsten asperity in real time through a high-resolution transmission electron microscopy investigation of a nanocontact in countermotion, induced through a piezo actuator. Molecular dynamics simulations provide insights into the sliding pathway of interface atoms and the dynamic strain/stress evolution at the interface. We observe a discrete stick–slip behaviour and an asynchronous process for the accumulation and dissipation of the strain energy together with the non-uniform motion of interface atoms. Our methodology allows for studying in situ atomic-friction phenomena and provides insights into friction phenomena at the atomic scale.
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S.X.M. acknowledges support from the National Science Foundation (NSF CMMI 1824816) through the University of Pittsburgh. G.W. acknowledges the computational resources provided by the University of Pittsburgh Center for Research Computing.
The authors declare no competing interests.
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Three force components of the interface atoms in MD simulation with 7 contact atoms. (Fig. 2) When the interface atoms completed the second slipping along the zig-zag route, the accumulated stress along z-direction in the first slipping would release, leading to the drop of the friction force. In experiments, since the shear force was obtained by measuring shear strains of seven/eleven atoms along x-direction in contact, the effect from the stress release along z-direction on the friction force might not be noticed.
The friction process within one period in MD simulation. (a-g) The structure evolution of the single asperity W-W contacts during friction, corresponding to the points of numbers 1–7 in Fig. 2a.
Discrete stick-slip behavior between tungsten asperities revealed by molecular dynamics (MD) simulation (The width of the contact region is 11 atoms’ space). (a) The function of the lateral force with the sliding displacement of the tip. (b-h) The snapshots of the dynamic movement of atoms in the top layer of the tip with respect to the substrate. The cyan balls represent the atoms in the bottom of the substrate. Four selected atoms were colored in yellow, orange, red, dark respectively. (i) The motion traces of the selected four atoms marked by the broken circles within one friction period.
Dynamic evolution of shear stress field on the bottom layer of the substrate in MD simulation. (a-e) The evolution of σxy (σzy) distribution on the bottom layer of the substrate and the corresponding sequences are indicated by red Roman numbers in Extended Data Fig. 4a.
Supplementary Figs. 1–6, Discussion 1–3, Tables 1 and 2 and references.
In situ TEM observation of atomic friction.
In situ TEM observation of atomic friction containing 11 contact atoms.
Another in situ atomic friction test.
In situ TEM observation of contact between two tungsten asperities.
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Wang, X., Liu, Z., He, Y. et al. Atomic-scale friction between single-asperity contacts unveiled through in situ transmission electron microscopy. Nat. Nanotechnol. 17, 737–745 (2022). https://doi.org/10.1038/s41565-022-01126-z