Elsevier

Applied Surface Science

Volume 567, 30 November 2021, 150777
Applied Surface Science

Full Length Article
Chemisorption mechanism of defluorinated fluorine on bcc Fe surface during formation of PTFE transfer film

https://doi.org/10.1016/j.apsusc.2021.150777Get rights and content

Highlights

  • Defluorinated fluorine is chemisorbed on Fe surface to generate iron fluoride.

  • Adsorption on Fe (1 0 0) surface at bridge site exhibits preferable configuration.

  • Electrons are transferred from Fe surface to accumulate around the adsorbed F atom.

  • Only the electronic structure of the topmost Fe layer is affected by the adsorbed F.

  • The Fe-F bond formed on the surface reveal high degrees of ionic character.

Abstract

Since defluorination is most likely to occur during the formation of polytetrafluoroethylene (PTFE) transfer film, understanding the adsorption mechanism of defluorinated fluorine can help describe the interfacial tribochemical reaction. XPS analysis showed that the defluorinated fluorine was chemisorbed on the Fe surface to generate iron fluoride. Density functional theory (DFT) calculations revealed that the adsorption of fluorine on the Fe (1 0 0) surface at the bridge site with 0.25 monolayer coverage exhibited the lowest adsorption energy (−4.71 eV), indicating a preferable adsorption configuration. At an identical adsorption site, the adsorption strength decreases with increasing coverage, which is attributed to the repulsive interaction of adsorbed fluorine. The chemisorption of fluorine on the Fe surface induces a considerable redistribution of charges, i.e. electrons are transferred from the Fe surface and accumulate around the adsorbed fluorine. The adsorbed fluorine mainly interacted with the topmost Fe layer. The two Fe atoms located at the bridge sites of the topmost layer play significant roles in the formation of Fe–F bonds that have high degrees of ionic character. The electronic states of Fe–F bond are derived from the hybridisation of Fe 4s, Fe 4p, Fe 3d, F 2s, and F 2p orbitals.

Introduction

Polytetrafluoroethylene (PTFE) is extensively used as a solid lubricant with remarkable self-lubricating properties derived from the PTFE transfer film formed on a metal counterface [1]. Accordingly, the investigation of PTFE transfer film is beneficial for revealing the self-lubricating mechanism of PTFE. The morphology and properties of PTFE transfer films under various sliding conditions have been extensively studied experimentally [2]. These studies revealed that the thickness of PTFE transfer film ranges from 1 to 1000 nm [3]. A thin and uniform PTFE transfer film is more beneficial to the antifriction and wear-resisting performances of PTFE composites than a thick and discontinuous one [4], [5].

In addition to experimental techniques, molecular simulation techniques involving of molecular dynamics (MD) and density functional theory (DFT) calculations, have also been used to elucidate the self-lubricating mechanism of PTFE transfer films on an atomic scale. Sawyer et al. used MD simulations to reveal that both the crosslink density and sliding orientation can influence the tribological performance of PTFE [6], [7]. MD simulations were also employed to study the effect of the external load and the PTFE fragments on the tribological properties of PTFE [8]. Using DFT and MD methods, Onodera et al. analysed the mechanisms of formation and chemical reactions PTFE transfer films on aluminum surfaces, and they found that polar gas molecules and polar groups play important roles in forming a PTFE transfer film [9], [10], [11]. They also simulated the working mechanism of a polyetheretherketone/PTFE composite transfer film based on DFT and MD calculations [12].

Our previous DFT transition state studies [13] had revealed that PTFE defluorination occurred easily during the formation of the PTFE transfer film. The defluorinated fluorine was adsorbed on the Fe surface to form metal fluorides, which exerted a positive effect on the tribological properties of PTFE [11], [14]. Nevertheless, the mechanism by which defluorinated fluorine interacted with the Fe surface remains unclear, and the details of the chemisorption of defluorinated fluorine on the Fe surface have not been reported. The aim of this study was to elucidate the chemisorption mechanism of defluorinated fluorine on the Fe surface, which is a crucial step in revealing the details of the tribochemical reaction that occur during the formation of the PTFE transfer film. In addition, the chemisorption mechanism of fluorine might provide guidance for the surface modification of metals, especially for the preparation of fluorinated nanocoatings with excellent properties of antifriction, moisture proof, hydrophobic, and antifouling.

Because the computational efficiency of DFT is strongly dependent on the atom number of models, DFT was applied on smaller length scales. Moreover, DFT calculations provide more insight than MD simulations into the electronic structure of the adsorption system, which plays a key role in revealing the chemisorption mechanisms. Thus, DFT is well suited for studying the chemisorption mechanism of small molecules or atoms. In this study, DFT calculations were performed to study the mechanism of chemisorption of defluorinated fluorine on the Fe surface, in terms of various sites and coverages. The electronic structure of the most favourable adsorption configuration is discussed in detail, in terms of the surface dipole moment, atomic charge, bond population, electron density difference, electronic partial density of state, and electron localisation function of the adsorption system.

Section snippets

Preparation of PTFE sample

PTFE powder with a 200-mesh particle size (CGM031) was supplied by the Zhonghao Chenguang Research Institute of Chemical Industry (Sichuan, China). First, the PTFE powder was pressed under 40 MPa at room temperature for 10 min. Then, the pressed PTFE was placed in a muffle furnace, and the chamber temperature was increased to 375 °C at a heating rate of 1 °C/min and maintained at 375 °C for 90 min. Finally, the chamber was cooled naturally to room temperature.

Tribological tests

Ring-on-disk tribological tests

Tribochemical reaction between defluorinated fluorine and worn Fe surface

The friction coefficient of PTFE sliding against Q235 steel under normal load of 400 N and sliding velocity of 0.5 m/s are depicted in Fig. 1. Apparently, the friction coefficient of PTFE was stable within 60 min, indicating that the steady friction stage of PTFE was reached. In other words, a stable PTFE transfer film was formed on the steel surface. To determine the tribochemical reaction products formed between the Fe surface and the adsorbed fluorine, the worn steel surface sliding against

Conclusion

This paper presents the chemisorption mechanism of defluorinated fluorine on the bcc Fe surface during the formation of a PTFE transfer film. XPS revealed that the defluorinated fluorine was chemisorbed on the worn Fe surface to generate iron fluorides. On this basis, density functional theory (DFT) calculations were performed to reveal the chemisorption details of the defluorinated fluorine on the Fe surface. The adsorption of fluorine on the Fe (1 0 0) surface at the bridge site and 0.25 ML

CRediT authorship contribution statement

Zhen Zuo: Conceptualization, Methodology, Software, Data curation, Supervision, Writing – original draft, Writing - review & editing, Funding acquisition. Xin Jin: Investigation, Visualization, Writing – original draft. Chaojiang Li: Validation, Writing - review & editing, Data curation. Zhijing Zhang: Validation, Writing - review & editing. Yulin Yang: Software.

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.

Acknowledgements

This work was supported by China Postdoctoral Science Foundation (grant number 2020M670152).

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