Proceedings of WTC2005-63744
World Tribology Conference iii
September 12-16, 2005, Washington, D.C., USA
Proceedings of WTC2005
World Tribology Congress III
September 12-16, 2005, Washington, D.C., USA
WTC2005-63744
WTC2005-63744
NOVEL IONIC LIQUID LUBRICANTS FOR AEROSPACE AND MEMS
Kinzig, Barbara J.1, Sutor, Paul1 , Sawyer, Gregory W.2 , Rennie, Allison2, Dickrell, Pamela2,
Gresham, Jennifer3
1
2
Surfaces Research, Lenexa, KS
University of Florida, Dept. of Mechanical and Aerospace Engineering., Gainesville, FL
3
U. S. Air Force Office of Scientific Research, Arlington, VA
INTRODUCTION
Room temperature ionic liquids (RTILs) are molten salts
with melting points at or below room temperature. RTILs have
recently been recognized as novel lubricants. Only a few have
previously been evaluated.
RTILs have unique properties ideal for lubricants. Their
negligible vapor pressure allows interoperability in vacuum and
air. They have very low friction on both metallic and nonmetallic substrates, and high electrical conductivity advantageous for
electrical contacts. Until now, RTILs did not have better wearreduction properties than conventional formulated lubricants.
Careful selection of RTILs is necessary; not all are suitable
for tribological use. Some are moisture-sensitive, corrosive
toward metals and/or have poor surface wettability.
In this work, new ionic liquid lubricants were successfully
predicted and demonstrated. Ionic structural features that
determine surface interactions and lubricating behavior were
evaluated with fundamental surface science, tribological testing
and ionic shape analysis/modeling, and were shown to correlate
with the performance of RTILs as lubricants in both standard
and microscale (for MEMS) tribological evaluations.
Ionic liquid lubricants with low friction, excellent wear
prevention properties (equivalent to formulated engine oil), and
excellent lubricity on noble metals and silicon, as well as on
ferrous and non-ferrous metals, are described.
IONIC LIQUID SELECTION
Ionic Liquid Compositions. In RTILs, one or both of the
ions are organic species. Compositions with relatively large
and/or asymmetric ions reduce their packing density, accounting for their liquid state. Typical cations include imidazolium,
pyridinium, ammonium, phosphonium and sulfonium, illustrated below.
+
+
R N
N R
Imidazolium
N
NR4
+
SR3
+
R
Pyridinium
Ammonium
Sulfonium
Typical anions include tetrafluoroborate, BF4-, hexafluorophosphate, PF6-, (CF3SO2) 2N = “triflamide”, Tf2N-, and toluene4-sulfonate = “tosylate”, Ts. These are by no means exhaustive.
Selection. In the previous tribological literature, only RTILs
with dialkyl imidazolium cations and BF4- and PF6- anions have
been reported [1-4]. In addition, we selected many new nonimidazolium and non-fluoroanion ionic liquids for study, based
on prediction of the effects that cation and anion chemical
composition and shape would have on expected lubrication
behavior. Imidazolium cations are relatively flat rings. Other
ion shapes and sizes included both larger and smaller ions with
both rigid and flexible components.
From among 300 ILs, we selected 17, in a systematic series
with common anions and common cations. Most were liquids
with typical lubricant viscosities and viscosity indices (VI).
Some low-melting solids completed several structural series.
RTILs were custom-synthesized if not available.
Surface wettability, material interactions, thermal stability,
macro- and microtribology for MEMS were evaluated.
SURFACE WETTING AND MATERIAL INTERACTIONS
The formation and retention of a lubricating film at surfaces in contact implies wetting and spreading on the mating
surfaces without deleterious etching or corrosion. RTILs were
screened for wetting and compatibility with 11 materials.
Small (ca 2mm) drops were applied to clean flat surfaces
and photographed at several hour intervals. Figure 1 shows
wetting and non-wetting RTILs on 440C steel, gold, and glass.
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Copyright ©2005 by ASME
Figure 2 summarizes and ranks wetting behavior on eleven
substrates.
Anions with surface interactions analogous to those of
conventional lubricants were among those with the lowest
wear. Thus, the proposed anion effects may well occur, but are
secondary to the cation effect.
Figure 1. RTILs on (a) 440C steel , (b) gold,
(c) non-wetting glass and (d) wetting glass.
Imidazolium M50
440C
with
BF4
0
0
PF6
0
0
N(CN)2
0
0
SCN
0
0
Oxy 1
2
3
Oxy 2
1
3
52100
Inconel
Al
Ti
Au
Si3N4
Si
Glass
SiO2/
Si
0
0
0
0
0
1
1
1
0
0
0
0
0
0
1
1
1
0
0
0
0
0
0
1
1
1
0
0
0
0
0
0
1
1
1
0
0
2
0
1
1
2
1
2
0
1
1
1
4
3
4
2
1
1
Var Cations
with
BF4
N(CN)2
Ts
Tf2N
Oxy 3
Tf2N
M50
440C
52100
Inconel
Al
Ti
Au
Si3N4
Si
Glass
SiO2/
Si
1
3
2
0
2
2
1
3
2
2
1
2
1
1
3
1
1
4
4
1
2
2
1
1
1
2
1
2
3
4
1
3
1
2
2
2
4
1
2
4
4
2
4
3
1
3
2
0
2
2
2
3
3
3
2
1
1
1
2
1
1
2
4
1
1
1
Figure 3. Wear scar diameters from 4-ball for RTILs.
Consistently Low Friction. All ionic liquids had friction
coefficients lower than all conventional lubricants not containing friction modifier additives. Friction for the 17 RTILs in
Figure 3 are shown in Figure 4, with PFPE and engine oil again
as references. Friction for RTILs did not show a trend, but all
RTILs had extremely low friction without any additives.
Figure 2. Relative wettability of 12 ionic liquids on 11
surfaces. Data ranked and cells colored from 0=gray
=nonwetting to 4=red =complete spreading.
Cation Effect. Surface wetting characteristics of ionic
liquids were most strongly dependent on the nature of the
cation. Non-imidazolium RTILs, shown in the lower section of
Figure 2, wet surfaces better than comparable imidazolium salts
on all surfaces—metals, noble metal, ceramics, and silicon.
Among imidazolium salts, there is an anion effect, in which
oxyanions significantly increase wettability.
Substrate Effect. In general, both imidazolium and nonimidazolium ionic liquids wet noble metal and nonmetal
surfaces better than conventional metal surfaces. Wetting and
spreading were better on gold, silicon nitride, glass and silicon
than on stainless steels, Inconel, aluminum or titanium.
TRIBOLOGY STUDIES
Tribology Methods. Four-ball wear/friction tests were
conducted according to ASTM D-4172, in air: 52100 steel balls
40 kg load (3.4 GPa pressure), 75°C, and 1200 rpm (0.46 m/s).
Microtribometer pin-on-disk friction was obtained with
lubricated gold pins on a polysilicon counterface, in both air
and argon at 0.15 GPa pressure, 25°C, oscillating at 1.5 mm/s.
Repeatability of friction in all tests was ± 0.01.
Cation Effect. Wear prevention by ionic liquids was
found to be strongly dependent on the cation. RTILs with nonimidazolium cations prevented wear of steel much better than
any imidazolium salt. The negative anion had been predicted to
more strongly influence interfacial behavior. Wear of 17 RTILs
is shown by cation type in Figure 3, where the consistent cation
effect on lubrication with ionic liquids is easily observed.
Wear Prevention Without Additives. Several new
RTILs, without antiwear additives, gave extremely low wear of
steel, comparable to fully-formulated engine oils. The best
imidazolium salts had wear an order of magnitude greater than
the most effective new RTILs.
Figure 5. Friction from 4-ball tests for RTILs.
Friction Independent of Surfaces or Atmosphere. Friction coefficients of all ionic liquids were remarkably similar,
and independent of the materials lubricated or the environment.
All friction coefficients were in the narrow range of 0.06 to
0.08, lubricating steel-on-steel or gold-on-silicon contacts, in
air or in inert atmosphere, 75°C or 25°C, at high or low load.
RTIL friction appears independent of contact material.
ACKNOWLEDGMENTS
Support by U.S. Air Force Office of Scientific Research,
Contract FA9550-04-C-0135 is gratefully acknowledged.
REFERENCES
1. Ye, C., Liu, W., Chen, Y. and Yu, L., “Room-temperature
ionic liquids: a novel versatile lubricant,” Chem. Commun.,
Nov 2001, Vol. 7, No. 21, 2244-5.
2. Chen Y., Ye, C., Wang H., and Liu, W., “Tribological
Performance of an Ionic Liquid as a Lubricant for Steel/
Aluminum Contacts,” J. Synth. Lubr. Vol. 20-3, 2003, 217-25.
3. Reich, R. A., Stewart, P. A., Bohaychick, J. and Urbanski, J.
A., “Base Oil Properties of Ionic Liquids,” Lubr. Eng., Vol. 59,
No.7, Jul. 2003, 16-21.
4. Wang, H., Lu, Q., Ye, C., Liu, W., and Cui, Z., Wear 256,
2004, 44-5.
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