Understanding the effects of intercalated molecules on structural superlubric contacts

Yao Cheng and Ming Ma

Phys. Rev. Materials 4, 113606 – Published 17 November 2020

Abstract

Third bodies are ubiquitous in meso- and nanoscopic friction pairs. So far, few researchers have investigated their effects on structural superlubric contact which is a promising approach in reducing friction and wear. In this work, using molecular dynamics simulations we find that intercalated molecules, a typical type of third body, have multiple effects in graphite contacts. While for commensurate contacts the friction decreases with the coverage of intercalated molecules in general, for an incommensurate interface a complex dependence is observed. With state-of-the-art detailed analysis, we reveal that with coverage increases, for commensurate contacts the change in contact area accounts for the variation in friction. For incommensurate contacts, the resistance due to the direct interaction between intercalated molecules and the slider dominates the energy dissipation. For both kinds of contacts, complex transitions in the structures and kinetic behaviors of intercalated molecules are observed. Our results provide an overall picture for the effects of intercalated molecules on friction in structural superlubric systems.

Received 16 July 2020
Accepted 6 October 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.113606

Physics Subject Headings (PhySH)

Friction Surface diffusion Tribology

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yao Cheng¹ and Ming Ma ^1,2,*

¹State Key Laboratory of Tribology, Department of Mechanical Engineering, and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
²Institute of Superlubricity Technology, Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China

^*maming16@tsinghua.edu.cn

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Vol. 4, Iss. 11 — November 2020

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Images

Figure 1
(a) Side view of simulation model when misfit angle $θ$ is $0^{\circ}$ . X, Y, and Z axes are represented by red, green, and blue arrows, respectively; graphene layers (black and cyan) are labeled as layer 1 to layer 6 from bottom to top, while single-atom molecules (ice blue) are between layer 3 and layer 4. Layer 1 (L1), depicted in black, is held fixed throughout the simulation. The topmost layer, i.e., the sixth layer, is attached with a point moving at constant velocity $V = 2 m / s$ by a spring with spring constant $K = 10 N / m$ . The interlayer springs with total spring constant being 10 N/m are depicted as six sets of polylines in vertical orientation. In the X direction, an auxiliary spring is connected to the sixth layer to mimic mechanical constraints exerted by the driving tip. (b) Friction stress $τ$ versus coverage $α$ when misfit angle $θ$ between layer 3 and layer 4 is $0^{\circ}$ (solid black squares) and $30^{\circ}$ (hollow black squares), which corresponds to commensurate and incommensurate contacts, respectively. (c,d) The friction stress $τ$ (black, left axis) and normalized contact area (NCA) (red, right axis) variation with molecule coverage $α$ when $θ = 0^{\circ}$ (c) and $30^{\circ}$ (d). Red dotted lines in (c),(d) are fitted for NCA at small coverages $(α \leq 20 %)$ .
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Figure 2
(a),(b) Friction stress $τ$ (black) and $τ_{m}$ (blue) variation with coverage $α$ when $θ = 0^{\circ}$ [(a), solid marks] and $30^{\circ}$ [(b), hollow marks]. $τ_{m}$ is the total lateral force along the pulling direction between intercalated molecules and the slider divided by nominal contact area. The inset in (a) is a magnification for $α \geq 20 %$ . The error bars of $τ$ for incommensurate cases are shown in Fig. 1, and error bars of $τ_{m}$ are similar to those of $τ$ , which are not shown in Fig. 2 for clarity.
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Figure 3
(a)–(d) Top view of layer 3 (topmost layer of substrate, cyan) and intercalated molecules (ice blue) at the end of the driving process. X, Y, and Z directions are indicated by red, green, and blue (outward) arrows. (e)–(h) The corresponding transient two-dimensional structure factor of intercalated molecules. Gray cross markers represent the virtual structure factor that corresponds to the centers of the hexagons within the surface of the substrate. From left to right are commensurate cases with coverage of 5%, 10%, 20%, and 45%, respectively.
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Figure 4
Evolution of lateral stress $τ_{l}$ during sliding processes for commensurate cases with coverage of 0, 5%, 10%, 20%, and 45%. One slip event is marked in the 5% case. A transition from characteristic stick-slip phenomena to erratic behavior can be observed.
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Figure 5
(a)–(d) Slip displacements (SDs) for cases with coverage of 0, 1%, 5%, and 10% are portrayed as arrows based on the same point. Seven hexagonal graphene lattices are selected as background to show the relative lengths of SDs.
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Figure 6
(a) Evolution of $l_{e}^{⊥}$ during driving processes for commensurate cases with coverage of 0, 5%, 10%, 20%, and 45%. (b) Friction stress $τ$ versus coverage, for original systems (black squares) and restricted cases (red circles) where the motion of the slider in the transversal direction is restricted.
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