Room-temperature ionic liquids: a novel versatile lubricant

Abstract

Alkylimidazolium tetrafluoroborates are promising versatile lubricants for the contact of steel/steel, steel/aluminium, steel/copper, steel/SiO₂, Si₃N₄/SiO_2, steel/Si(100), steel/sialon ceramics and Si₃N₄/sialon ceramics; they show excellent friction reduction, antiwear performance and high load-carrying capacity.

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Room-temperature ionic liquids were initially developed by electrochemists for use as electrolytes in batteries or for metal electrodeposition. As a non-conventional class of novel solvents, ionic liquids are becoming increasingly important and of particular interest. This is because they have a number of characteristics including negligible volatility, non-flammability, high thermal stability, low melting point, broad liquid range, and controlled miscibility with organic compounds, especially some heterocycle compounds.^1–4 Therefore they have attracted enormous attention as media for green synthesis and been successfully used to realize many important reactions.^5–8

On the other hand, the above-mentioned properties of ionic liquids make them potent excellent lubricants. The lubricants currently used in industry are restricted in their application. For instance, lubricants for steel/steel contact may be unsuitable for aluminium/ceramics. For example, while alcohol has been successfully used as a lubricant for ceramics and aluminium it behaves poorly for steel. Clearly versatile lubricants would be of great value for the industrial community.

Bearing this in mind and taking into consideration melting point, thermal stability, hygroscopicity and viscosity, we chose 1-methyl-3-hexylimidazolium tetrafluoroborate (denoted L106) and 1-ethyl-3-hexylimidazolium tetrafluoroborate (L206) (see Fig. 1) to evaluate their tribological behavior using an Optimol SRV (SRV is the abridged name for German Schwingung, Reibung, Verschleiss) oscillating friction and wear tester with ball-on-disc configuration.⁹

Fig. 1 Molecular structures of the ionic liquids L106 and L206 and traditional lubricants X-1P and PFPE.

The SRV test results at a medium load are listed in Table 1. It can be seen that the ionic liquid L106 exhibits excellent tribological performance for steel (SAE-52100), aluminium (Al2024), copper, single crystal SiO₂, single crystal Si(100) and sialon (Si–Al–O–N) ceramics.¹⁰ It exhibits the lowest friction coefficients as compared with the two fluorine-containing lubricants phosphazene (X-1P)¹¹ and perfluoropolyether (PFPE)¹² which are widely used as lubricants for head/disk interface or space applications. Thus it can be anticipated that ionic liquids might be promising versatile lubricants.

Table 1 Friction coefficients for several frictional pairs lubricated with various lubricants (SRV tester, load 50 N, frequency 25 Hz, amplitude 1 mm)

	Friction coefficient
Frictional pair (ball/disk)	L106	X-1P	PFPE
Steel/steel	0.065	0.098	0.145
Steel/Al	0.040	0.128	—
Steel/Cu	0.025	0.117	0.145
Steel/SiO2	0.060	0.110	0.132
Si3N4/SiO2	0.083	0.115	0.132
Steel/Si(100)	0.050	0.102	0.145
Steel/sialon	0.065	0.100	0.120
Si3N4/sialon	0.065	0.105	0.130

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Table 2 shows the SRV test results for the ionic liquid L206, X-1P and PFPE under relatively high loads (≥200 N). The friction coefficient and wear volume loss of a steel disc lubricated by L206 remains at a very low level with increasing load, and the load-carrying capacity of L206 reaches up to 1000 N, much higher than that of X-1P (300 N) or PFPE (400 N).

Table 2 Tribological properties of ionic liquid L206, X-1P and PFPE for steel/steel contact (SRV tester, load 50 N, frequency 25 Hz, amplitude 1 mm, duration 30 min)

	Friction coefficient			Wear volume/×10^−4 mm^3
Load/N	L206	X-1P	PFPE	L206	X-1P	PFPE
‘—’ lubrication failure.
200	0.060	0.070	0.120	0.05	0.07	0.60
300	0.055	0.065	0.110	0.22	2.21	1.90
400	0.050	—	0.105	0.39	—	5.03
500	0.045	—	—	0.45	—	—
600	0.045	—	—	0.53	—	—

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The ionic liquids L106 and L206 are non-hygroscopic, are air- and water-stable under ambient conditions, and slightly soluble in water; the solubility of L106 in water is ca. 0.28 g l⁻¹ at 20 °C while L206 has a similar solubility. The solubility of the ionic liquids in water has little effect on the tribological behavior. Moreover, the addition of a small amount of water (≤5 wt%) to the ionic liquids is helpful to improve the antiwear ability for various frictional pairs (steel/steel, steel/aluminium and steel/ceramic) but has little effect on the friction reduction performance.

A question then arises: why do ionic liquids exhibit superior tribological behavior? Noticing the unique dipolar structure, we propose that ionic liquids can be easily adsorbed on the sliding surface of frictional pairs. Subsequently an effective boundary film would form so as to reduce friction and wear. Furthermore, under severe friction conditions, the tetrafluoroborate anion will decompose to form anti-scratch components such as fluoride, B₂O₃ and BN. This is confirmed by the B 1s XPS spectra of worn surfaces under different testing conditions (Fig. 2). Only BN is detected in the sliding area of steel/sialon contact lubricated with L106 under 300 N, while both B₂O₃ and BN are detected in the sliding area of Si₃N₄/sialon ceramic contact lubricated with L106 under 80 N.

Fig. 2 B 1s spectra of neat L106 on Au (a) and in wear scar of sialon sliding against steel lubricated with L106 under 300 N (b) and of sialon sliding against Si₃N₄ under 80 N (c).

In addition, ionic liquids L106 and L206 have pour points below −55 °C and show no weight loss below 320 °C. Thus, the prominent low temperature fluidity, high temperature stability, low vapor pressure and excellent lubricity makes them an attractive alternative to conventional liquid lubricants.

This work was financially supported by the National Natural Science Foundation of China (Grant No. 59825116) and Chinese Academy of Sciences.

Notes and references

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