Liquid Superlubricity Enabled by the Synergy Effect of Graphene Oxide and Lithium Salts

Abstract

In this study, graphene oxide (GO) nanoflakes and lithium salt (LiPF₆) were utilized as lubrication additives in ether bond-containing dihydric alcohol aqueous solutions (DA(aq)) to improve lubrication performances. The apparent friction reduction and superlubricity were realized at the Si₃N₄/sapphire interface. The conditions and laws for superlubricity realization have been concluded. The underlying mechanism was the synergy effect of GO and LiPF₆. It was proven that a GO adsorption layer was formed at the interface, which caused the shearing interface to transfer from solid asperities to GO interlayers (weak interlayer interactions), resulting in friction reduction and superlubricity realization. In addition to the GO adsorption layer, a boundary layer containing phosphates and fluorides was formed by tribochemical reactions of LiPF₆ and was conducive to low friction. Additionally, a fluid layer contributed to friction reduction as well. This work proved that GO-family materials are promising for friction reduction, and provided new insights into realizing liquid superlubricity at macroscale by combining GO with other materials.

Keywords: LiPF6; friction reduction; graphene oxide; superlubricity.

Figure 1

(a) The structure of DAs, (b) layered structure of graphene oxide, and (c) diagram of testing model.

Figure 2

(a) HRTEM image showing the layered structure of GO nanoflakes with an interlayer spacing of approximately 0.5 nm, (b) AFM image showing the thickness of GO single layer is approximately 0.8 nm, (c) lateral size distribution of GO nanoflakes, (d) XPS spectrum of GO nanoflakes showing the C/O ratio is approximately 69:31, (e) XPS spectrum showing chemical bond within GO, and (f) Raman spectrum showing the typical D, G, 2D, and D + G bands of GO.

Figure 3

COFs of various DA(aq) acquired at 3 N, 100 mm/s, and 20 μL, (a) DEG(aq), (b) TEG(aq), (c) PEG(aq), and (d) effect of DA viscosities on COFs during wear−in and steady lubrication stages.

Figure 4

COFs of GO and LiPF₆ containing DEG(aq) under various conditions, (a) long run (3 N, 100 mm/s, and 20 μL), (b) sliding speed (3 N and 20 μL), (c) normal load (100 mm/s and 20 μL), and (d) solution volume (3 N and 100 mm/s).

Figure 5

(a–d) WSDs of the balls: (a) DEG(aq), (b) +LiPF₆, (c) +GO, and (d) +GO + LiPF₆; (e,f) worn area Ra lubricated with DEG(aq) + GO + LiPF₆ obtained by a white−light interferometer: (e) ball and (f) disk; (g,h) topographies of worn areas lubricated with DEG(aq) + GO + LiPF₆ obtained by SEM: (g) ball and (h) disk. The worn area was obtained under the condition of 3 N, 100 mm/s, and 20 μL.

Figure 6

Chemical state of worn areas on Si₃N₄ balls with the lubrication of various solutions.

Figure 7

(a,b) HRTEM images of GO adsorption layer on worn surfaces. (c,d) Raman spectra of GO adsorption layer on worn areas.

Figure 8

Result to prove GO nanoflakes further reduced COF based on tribochemical layer. Notably, DEG + GO solution contained no water. The conditions were 3 N, 100 mm/s, and 20 μL.

Figure 9

Lubrication model for GO and LiPF₆ containing DA(aq) solution at macroscale, (a) contact between asperities; (b) GO adsorption layer and tribochemical layer formed during wear−in stage; (c) GO adsorption layer, tribochemical layer, and fluid layer contributed to friction reduction during steady lubrication stage.