A molecular dynamics study on the synergistic lubrication mechanisms of graphene/water-based lubricant systems

https://doi.org/10.1016/j.triboint.2021.107356Get rights and content

Highlights

  • Water can protect the structural integrity of graphene by inducing its movement.

  • Graphene can enhance the diffusions of water molecules and promote their rolling.

  • Low friction is due to better water replenishments and pairs isolation by graphene.

  • Water amount highly changes the friction force by affecting the contact interface.

  • Friction fluctuation with water-based lubricants is insensitive to various loadings.

Abstract

Water-based lubricants with graphene have excellent lubrication behaviors, but the synergistic mechanism involving the activities of both water and graphene isblue still unclear. This work investigates such mechanism by considering the effect of different factors such as the number of water molecules, loading, surface roughness and sliding velocity via molecular dynamics simulation. It is found that the friction reduction by water-based lubricants is due to the enhanced isolation between friction pairs by the graphene presence and the improvement of water replenishment. The synergistic lubrication mechanism works as that the water molecules may protect the structural integrity of graphene by inducing its movement to release lateral stresses and the graphene can enhance the movement of water molecules and promote their rolling like bearings.

Introduction

Water-based lubricants play a very important role in lubricating joints of mammals and have been used to reduce the friction and wear of mechanical components since 2400 BCE in Egypt. Nevertheless, their strong corrosions to these components have highly limited their development and have been gradually replaced by oil-based lubricants [1]. Recently, with the rising of environmental pollution problems, water-based lubricants have re-entered people's vision because oil lubrication is usually hard to degrade in natural environments.

Lubricant additives have been commonly employed to improve the stability in the performance of water-based lubricants. Their most widely used additives, such as compound lipids of borate and carboxylate containing nitrogen, triethanolamine oleate, borate ester and synthesis of thiourea, are corrosion inhibitors to reduce their corrosive activities without adding polluting elements. Moreover, interests have also been paid to pursue new additives to improve the lubrication performance of water-based lubricants. The emerging two-dimensional (2D) materials such as graphene (GR) and its derivatives [2] (graphene oxide, graphene fluoride, etc.) have excellent mechanical strengths, chemical inertness and super-low coefficient of friction [3], [4]. Moreover, multilayer graphene even shows ultra-low friction due to the incommensurate contact between layered structure [5] and the thermal escape motion[6], [7]. Therefore, they have become the ideal additives for water-based lubricants.

Recent studies about 2D materials as lubricant additives have been mainly focused on their dispersibility in water-based lubricants. Song et al. used an improved Hummers and Offeman method to improve the dispersibility of graphene oxide, and found that the well-dispersed graphene oxide can avoid the direct contact between friction pairs by adhering to the friction surface [8]. Ye et al. covalently functionalized fluorographene to obtain hydrophilicity and this change made the prepared ultrafine particles dispersed well in water, which makes it an effective lubricating additive and improves the tribological properties of water [9]. Ge et al. obtained well-dispersed graphene oxide nanoflakes by Hummers method and confirmed that the synergistic effect between them and ionic liquids plays a leading role in the realization of liquid super lubrication under extreme pressure [10].

Despite the rapid progress in the dispersibility control of 2D materials, their roles in reducing the friction and wear at sliding interfaces still lack good understanding, especially due to the difficulty in directly observing the lubricating behaviors in experiments. Some recent studies have employed theoretical approaches such as molecular dynamics (MD) simulation and ab initio simulation to investigate the interfacial evolution with the incorporation of 2D materials into water-based lubricants. Solanky et al. observed that the graphene wraps up water molecules after withdrawing from them and such wrapping depends on the size of graphene and the number of water molecules [11]. Restuccia et al. [12] found that the lubricity origin of graphene in a humid environment is closely related to the passivation of the edges of graphene and the increase of its hydrophilicity. During the friction process, the water at the edges decomposes at a high rate, which prevents the direct interaction of graphene edges with other graphene ribbons/sheets. The study by Sobrino et al. [13] further demonstrated that the water molecules at the interface of graphene show the formation of a flat ice layer. Wijn et al. [14] pointed out that the ultra-low friction of water-graphene system is due to the non-covalent contact on the contact surface.

It is evident that the previous studies about the friction mechanisms of graphene in water-based lubricants are mainly focused on the interaction between the graphene and water molecules [13], [15], [16], [17], [18], [19], [20], [21], [22], [23]. However, the mechanisms of the synergistic lubrication of both graphene and water are still unclear.

This work aims to investigate the mechanisms of the synergistic lubrication for the graphene/water-based lubricant system by using MD simulation. Diamond-like carbon (DLC) is employed as friction pair materials, since it has excellent tribological behaviors as well as wide applications in the water environment and its amorphous microstructure with C element as dominant compositions can make the friction-interfacial evolution easy to analyze [24]. In order to mimic the composition of practical water-based lubricants, triethanolamine is used as corrosion inhibitors. Considerations are given to the influence of different factors such as graphene presence, the number of water molecules, loading, sliding velocity and friction surface roughness. It is hoped that this work could help to understand the lubricating mechanisms of graphene in water-based lubricants and promote the optimizations of their lubrication performances in the future.

Section snippets

Modelling

The friction simulation is conducted by relatively sliding DLC films, and water-based lubricants are placed between them. The atomic configuration before the simulation commencement is shown in Fig. 1. The triethanolamine molecules are added as corrosion inhibitors into the lubricants. The ratio of molecule amount of triethanolamine to water is kept as 1:10. The graphene layer with its edge passivated by H atoms is also introduced into the lubricants. Along the loading direction (the z

Effect of the number of water molecules and loading

Fig. 2 shows that water-based lubricants can significantly reduce the friction resulting the low friction coefficients (FC) about 0–0.2 between smooth DLC films, and a super-low FC (< 0.1) is present with a small Fn (4 GPa). In all the cases, the friction force Ff is quite high at the beginning since the lubricants stay static and need some time for becoming sheared flowing to make the sliding happen inside them. As the sliding going on, the Ff gradually stabilizes and fluctuates within a

Conclusions

This work investigates the lubrication performance of water-based lubricants with single-layer graphene for DLC films by using MD simulation. Considerations are given to the effects of different factors such as the number of water molecules, loading, surface roughness, sliding velocity and the presence of graphene. It is found that the friction force highly depends on the number of water molecules, since water with a large amount can prevent DLC films from direct contact and result in the fewer

CRediT authorship contribution statement

Chenjie Li: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft, Visualization. Weiwei Tang: Validation, Data curation, Writing – review & editing. Xiu-Zhi Tang: Visualization, Writing – review & editing. Xiu-Zhi Tang: Visualization, Writing – review & editing. Linyan Yang: Resources, Writing – review & editing. Lichun Bai: Conceptualization, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We acknowledge financial support of the National Natural Science Foundation of China (51905548), and the open project of State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University (SV2019-KF-24). The simulations in this work are supported by the High Performance Computing Center of Central South University.

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