Halogen-free imidazolium/ammonium-bis(salicylato)borate ionic liquids as high performance lubricant additives

Article abstract of DOI:10.1039/c3ra43052a

Halogen-free bis(salicylato)borate anion based ionic liquids having imidazolium and ammonium cations, were designed, synthesized, characterized and then evaluated as potential lubricant additives. Owing to the high polarity, high London dispersive forces

Full text of DOI:10.1039/c3ra43052a

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Halogen-free imidazolium/ammonium-
bis(salicylato)borate ionic liquids as high
Cite this: RSC Adv., 2014, 4, 1293
performance lubricant additives†
*
Rashi Gusain, Raghuvir Singh, K. L. N. Sivakumar and Om P. Khatri
Halogen-free bis(salicylato)borate anion based ionic liquids having imidazolium and ammonium cations,
were designed, synthesized, characterized and then evaluated as potential lubricant additives. Owing to
the high polarity, high London dispersive forces and high rigidity associated with the bulk size and ring
structure of the bis(salicylato)borate anion, these ionic liquids possess very high viscosities. The
interaction of these ionic liquids with contact surfaces has been found to be strongly influenced by the
van der Waals interactions in the ionic liquids, which increases with an increase of the alkyl chain length,
particularly, for the imidazolium cations. As lubricant additives, both imidazolium and ammonium ionic
liquids markedly improve the friction-reducing and anti-wear properties of PEG 200 lube base.
Microstructural images and element mapping of the worn surfaces of steel balls reveal that these ionic
liquids form tribo-films, which reduces both the friction and wear. Being halogen-, phosphorus-, and
sulfur-free, bis(salicylato)borate ionic liquids not only protect contact surfaces from tribo-corrosive
events but also keep the environment green and clean. These ionic liquids offer an environmentally
friendly alternative to conventional halogenated ionic liquids being currently developed for lubricant
applications.
Received 18th June 2013
Accepted 10th October 2013
DOI: 10.1039/c3ra43052a
www.rsc.org/advances
considering the nature of the sliding surfaces and the lube base
stock. A rst report by Ye et al. demonstrated the potential of
Introduction
ionic liquids as versatile lubricants for the contact surfaces of
steel, aluminium, copper, SiO2 and ceramics.5 They found
excellent friction reduction and anti-wear performance. Since
then, the interest in ionic liquids as novel lubricants, lubricant
additives and lubricious thin lms has gradually increased.
Different types of ionic liquids with imidazolium, ammonium,
phosphonium, pyridinium, pyrrolidinium, etc. as cations and
halides, PF6ꢁ, BF4ꢁ, NO3ꢁ, ClO4ꢁ, Tf2Nꢁ, BETIꢁ, FAPꢁ, etc. as
anions have been studied for their tribological potential.5–13 The
inherent polar nature of ionic liquids facilitates their interaction
with sliding surfaces, resulting in lube lm formation. Such lube
lms of ionic liquids avoid metal-to-metal contact, and conse-
quently result in a reduction in friction and wear. Most of the
ionic liquids studied for lubricant applications, contain halo-
gens, sulphur and phosphorus either in simple or complex
form (halides, tetrauroborate, hexaurophosphate, bis(tri-
uoromethanesulfonyl)amide, triuoromethanesulfonate, phos-
phates, sulfates, etc. as anions and phosphonium, etc. as cation),
which are prone to interact with moisture. In particular, hydro-
philic ionic liquids having halides, phosphates and sulphates are
more prone to hydrolyze.14–17 Eventually, such interactions facil-
itate corrosive events, under tribological conditions (high
temperature and pressure) and can damage the contact surfaces
in mechanical systems. Therefore, the development of halogen-,
sulphur- and phosphorus-free ionic liquids is urgently required
The need to conserve energy and to protect the environment has
propelled an interest in the development of green and energy
efficient lubricants and lubricant additives, which can keep the
surfaces of working parts separate under all loads, temperatures
and speeds, reduce the friction and wear signicantly, dissipate
the heat from the contact surfaces, and increase the life of
sliding machine tools by protecting contact surfaces. In this
context, ionic liquids have been explored in recent years as
lubricants and lubricant additives.1–4 Ionic liquids are salts of
organic cations and inorganic/organic anions, where the
charges on these ions are highly diffuse, consequently, most of
them are liquids at <100 ꢀC. Ionic liquids possess a combination
of unique and tunable physico-chemical characteristics such as
low volatility, good thermal stability, non-ammability, and
excellent conductivity, which makes them potent candidates for
tribological applications. The exibility in molecular structure
with a diverse range of cations and anions makes ionic liquids
versatile lubricants for different engineering surfaces. There-
fore, ionic liquids can be designed for task-specic purposes,
CSIR – Indian Institute of Petroleum, Mohkampur, Dehradun-248005, India. E-mail:
opkhatri@iip.res.in; Fax: +91 135 2660202
† Electronic supplementary information (ESI) available: Information on wear scar
diameter and FTIR spectra of all the ionic liquids used in this study. See DOI:
10.1039/c3ra43052a
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for industrial lubricant applications, not only to reduce friction 2 hours. This led to the formation of an organic layer composed
and loss of material from contact surfaces but also to protect the of TBA-BScB ionic liquid in the reaction product, which was
environment.
extracted using dichloromethane. This was followed by
The miscibility of ionic liquids with lube base is very impor- washing of TBA-BScB with pure water until no more bromide
tant for their efficient performance as lubricant additives and is ions were detected in the water. Finally, dichloromethane was
dependant on their molecular structure. Smolenski removed under reduced pressure and the extracted product was
et al.developed an oil-miscible trihexyltetradecylphosphonium dried in a vacuum oven at 80 ꢀC under reduced pressure for 48
1
bis(2-ethylhexyl) phosphate ionic liquid as a potential additive, hours. The preparation of TBA-BScB was conrmed by H and
which showed signicant reduction of both friction and wear.18 13C NMR analyses. 1H NMR (CDCl3, ppm): d 0.8–0.9 (t, 12H,
The presence of long alkyl chains facilitates its miscibility with N(CH2)3CH3), 1.3 (m, 8H, N(CH2)2CH2), 1.5 (q, 8H, NCH2CH2),
various hydrocarbon oils. Recently, Antzutkin et al. have designed 3.1 (t, 8H, NCH2), 6.8–6.9 (m, 4H, C6H4), 7.29–7.39 (m, 2H,
halogen-free chelated orthoborate–phosphonium ionic liquids as C6H4), 7.86–7.95 (dd, 2H, C6H4). 13C NMR (ppm): 173.30,
lubricants.19 These ionic liquids exhibited considerably better 165.19, 161.77, 159, 30, 136.43, 130.60, 118.73, 118.26, 115.36,
anti-wear and friction-reducing properties, compared with fully 58.5, 23.5, 19, 13.
formulated 15W-40 engine oil. However, from the environmental
Tetraoctylammonium bis(salicylato)borate (TOA-BScB). Tet-
viewpoint, the presence of phosphorus is of serious concern. In raoctylammonium bis(salicylato)borate (TOA-BScB) was
further development, tricyanomethanide [C(CN)3]ꢁ and tetra- prepared by following the TBA-BScB preparation procedure
cyanoborate [B(CN)4]ꢁ anions based ionic liquids have been using tetraoctylammonium bromide (10 mmol) as a cationic
designed to eliminate halogen, sulphur and phosphorous.20 precursor, instead of tetrabutylammonium bromide. The
However, these ionic liquids as lubricants showed no signicant preparation of TOA-BScB was conrmed by 1H and 13C NMR
improvement in their tribological properties.
analyses. 1H NMR (CDCl3, ppm):
d 0.84–0.87 (t, 12H,
Herein, we report the synthesis, characterization and the N(CH2)7CH3), 1.19–1.24 (m, 40H, NCH2CH2(CH2)5CH3), 1.46–
tribo-evaluation of bis(salicylato)borate anion based ionic 1.52 (m, 8H, NCH2CH2(CH2)5CH3), 3.0–3.11 (t, 8H,
liquids. Two series of ammonium and imidazolium ionic NCH2(CH2)6CH3), 6.8–6.9 (m, 4H, C6H4), 7.28–7.34 (m, 2H,
liquids were evaluated as lubricant additives using PEG 200 as a C6H4), 7.87–7.89 (dd, 2H, C6H4). 13C NMR (ppm): 173.09, 165.33,
lube base on a four-ball tribo-test machine. These ionic liquids 161.89, 159.47, 134.54, 130.71, 129.73, 58.57, 31.61, 28.97,
have shown signicant reduction in both friction and wear. The 26.12, 22.58, 21.58, 21.77, 14.07.
microstructural and elemental analysis of the worn surfaces of
Dioctylmethylpentylammonium
bis(salicylato)borate
steel balls, based on FESEM and EDX measurements, respec- (DOMPA-BScB). Dioctylmethylpentylammonium bis(salicylato)-
tively, revealed the role of the ionic liquids in the improvement borate (DOMPA-BScB) was prepared by following the TBA-BScB
of lubrication characteristics.
preparation procedure using dioctylmethylpentylammonium
bromide (10 mmol) as a cationic precursor, instead of tetrabu-
tylammonium bromide. Prior to this, dioctylmethylpentyla-
mmonium bromide was prepared by heating a mixture of
formic acid (0.55 mol, 21 ml), dioctylamine (0.4 mol, 32 ml) and
formaldehyde (1.2 mol, 23 ml) in a round bottom ask at 100 ꢀC
for 8 hours in the presence of boiling stones.22 Finally, an
equimolar amount of 1-bromopentane was added to the freshly
prepared dioctylmethylamine and the reaction mixture was
heated at 100 ꢀC for 24 hours under continuous stirring to
afford dioctylmethylpentylammonium bromide. The prepara-
Experimental
Materials
1-Methylimidazole (99%, Sigma Aldrich), tetrabutylammonium
bromide (GR, Merck Chemicals), tetraoctylammonium bromide
(98%, Sigma Aldrich), dioctylamine (98%, Sigma Aldrich), for-
mic acid (GR, Merck Chemicals), formaldehyde (41%, SD Fine
Chemicals), 1-bromopentane (purum, Fluka), boric acid (99.5%,
Loba Chemie), lithium carbonate (99.9%, Sigma Aldrich), sali-
cylic acid (99.8%, Merck Chemicals) were used without further
purication as precursors for the synthesis of the ionic liquids.
tion of DOMPA-BScB ionic liquid was conrmed by 1H and 13
C
NMR analyses. 1H NMR (CDCl3, ppm): d 0.815–0.874 (m, 9H,
CH2CH3), 1.19–1.29 (m, 24H, NCH2CH2(CH2)5CH3 and
NCH2CH2(CH2)2CH3), 1.52–1.54 (m, 6H, NCH2CH2(CH2)5CH3
and NCH2CH2(CH2)2CH3), 2.951 (s, 3H, NCH3), 3.10–3.14 (t, 6H,
NCH2(CH2)6CH3 and NCH2(CH2)3CH3), 6.8–6.9 (m, 4H, C6H4),
7.27–7.40 (m, 2H, C6H4), 7.86–7.89 (dd, 2H, C6H4). 13C NMR
(ppm): 165.46, 159.39, 134.78, 129.66, 119.06, 118.41, 115.34,
61.57, 55.50, 48.38, 31.58, 28.95, 28.07, 26.08, 23.74, 22.56,
1-Butyl-3-methylimidazolium
tris(pentauoroethyl)tri-
uorophosphate (BMI-FAP, Merck Chemicals), and 1-butyl-3-
methylimidazolium dibutylphosphate (BMI-DBP, Sigma
Aldrich) ionic liquids were used without further purication.
Synthesis and NMR characterization of ionic liquids
Tetrabutylammonium bis(salicylato)borate (TBA-BScB). 22.10, 21.82, 14.07, 13.73.
First, the lithium salt of bis(salicylato)borate was prepared by
mixing salicylic acid (2.762 g, 20 mmol) in an aqueous solution BScB).
3-Ethyl-1-methylimidazolium bis(salicylato)borate (EMIM-
3-Ethyl-1-methylimidazolium bis(salicylato)borate
of lithium carbonate (0.734 g, 10 mmol) and boric acid (0.619 g, (EMIM-BScB) was prepared by mixing an equimolar (10 mmol)
10 mmol).21 The solution was heated at 60 ꢀC for 2 hours under quantity of lithium salt of bis(salicylato)borate and 3-ethyl-1-
continuous stirring. Then tetrabutylammonium bromide methylimidazolium bromide (10 mmol) at 60 ꢀC for 2 hours
(10 mmol) was added to this solution and heated for a further under continuous stirring. The prepared EMIM-BScB ionic
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liquid was extracted using dichloromethane and washed a temperature were measured up to 363 K. Field emission scan-
couple of times with pure water to remove the inorganic salt. ning electron microscopy (FESEM) images of the worn surface
Finally, dichloromethane was removed under reduced pressure areas of the steel balls were taken using an FEI Quanta 200F.
and the extracted product was dried in a vacuum oven at 80 ꢀC The elemental composition and mapping of the worn surface
under reduced pressure for 48 hours to afford the EMIM-BScB area of the steel balls were carried out by using energy disper-
ionic liquid. Prior to this, 3-ethyl-1-methylimidazolium bromide sive X-ray spectroscopy (EDX) to probe the chemical composi-
was prepared by mixing an equimolar amount of ethyl bromide tion of the tribo-lm.
and 1-methylimidazoleꢀin a round bottom ask and heating the
reaction mixture at 75 C for 36 hours under a nitrogen atmo-
Friction and wear tests
sphere with continuous stirring. The prepared 3-ethyl-1-meth-
ylimidazolium bromide was puried by washing with ethyl
Lubrication characteristics in terms of friction coefficient and
wear scar diameter (WSD) for ionic liquids blended with PEG
200 were evaluated on a four-ball test machine. Details of the
WSD are provided in the ESI.† PEG 200 is an additive-free lube
base oil. In a typical experiment, a 12.7 mm steel ball under the
load was rotated against three stationary steel balls clamped in
the holder. All the tests were carried out as per the ASTM D4172
standard test method at a load of 392 N with a rotating speed of
1200 rpm for an hour. The temperature of the sample was
acetate and then used for the synthesis of EMIM-BScB. The
1
preparation of EMIM-BScB was conrmed by H and 13C NMR
analyses. 1H NMR (CDCl3, ppm): d 1.25–1.35 (t, 3H, NCH2CH3),
3.15 (s, 3H, NCH3), 3.8–3.9 (t, 2H, NCH2CH3), 6.78–6.9 (m, 4H,
C6H4), 7.15–7.27 (d, 2H, CH), 7.3–7.4 (m, 2H, C6H4), 7.75–7.89
(dd, 2H, C6H4), 8.9 (s, 1H, CH). 13C NMR: 165.82, 161.67, 159.11,
135.72, 135.37, 123.46, 121.67, 119.44, 119.02, 117.09, 114.87,
44.92, 35.98, 14.88.
ꢀ
maintained as 75 C throughout the experiment. During these
3-Butyl-1-methylimidazolium bis(salicylato)borate (BMIM-
experiments, the four balls were covered with a lube sample,
which was used for the wear and friction evaluation. The
morphological features of the worn areas of the steel balls,
lubricated with different types of ionic liquid samples were
examined by eld emission scanning electron microscopy.
BScB).
3-Butyl-1-methylimidazolium
bis(salicylato)borate
(BMIM-BScB) was prepared by following the EMIM-BScB prep-
aration procedure using 3-butyl-1-methylimidazolium bromide
(10 mmol) as
a precursor, instead of 3-ethyl-1-methyl-
imidazolium bromide. The preparation of BMIM-BScB was
conrmed by 1H and 13C NMR analyses. 1H NMR (CDCl3, ppm):
d 0.79–0.82 (t, 3H, N(CH2)3CH3), 1.15–1.19 (h, 2H, N(CH2)2CH2),
1.61–1.67 (q, 2H, NCH2CH2), 3.73 (s, 3H, NCH3), 3.96–3.99 (t,
2H, NCH2), 6.8–6.9 (m, 4H, C6H4), 7.124–7.128 (d, 2H, CH),
7.36–7.39 (m, 2H, C6H4), 7.82–7.86 (dd, 2H, C6H4), 8.9 (s, 1H,
CH). 13C NMR (ppm): 173.29, 165.80, 161.73, 159.21, 136.32,
135.10, 129.59, 123.42, 121.93, 119.30, 118.37, 116.99, 114.97,
49.63, 36.10, 31.67, 19.26, 13.24.
Results and discussion
Bis(salicylato)borate,
a chelated orthoborate anion, was
synthesized by using a mixture of salicylic acid, boric acid and
lithium carbonate. Tetraalkylammonium bis(salicylato)borate
3-Hexyl-1-methylimidazolium bis(salicylato)borate (HMIM-
BScB).
3-Hexyl-1-methylimidazolium
bis(salicylato)borate
(HMIM-BScB) was prepared by following the EMIM-BScB prep-
aration procedure using 3-hexyl-1-methylimidazolium bromide
(10 mmol) as
a precursor, instead of 3-ethyl-1-methyl-
imidazolium bromide. The preparation of HMIM-BScB was
conrmed by 1H and 13C NMR analyses. 1H NMR (CDCl3, ppm):
d
0.80–0.83 (t, 3H, N(CH2)5CH3), 1.17–1.22 (h, 6H,
N(CH2)2(CH2)3), 1.685–1.712 (q, 2H, NCH2CH2), 3.79 (s, 3H,
NCH3), 4.0–4.03 (t, 2H, NCH2), 6.85–6.95 (m, 4H, C6H4), 7.09–
7.10 (d, 2H, CH), 7.37–7.39 (m, 2H, C6H4), 7.83–7.88 (dd, 2H,
C6H4), 9.23 (s, 1H, CH). 13C NMR (ppm): 172.54, 165.83, 161.84,
159.26, 136.39, 135.08, 130.64, 129.61, 123.40, 121.90, 119.26,
118.41, 117.23, 114.99, 49.93, 36.13, 30.91, 29.79, 29.73, 25.73,
22.27, 13.88.
Characterization of ionic liquids
Fourier transform infrared (FTIR) spectra of all the ionic liquids
were recorded using a Thermo-Nicolet 8700 Research spectro-
photometer with a 4 cmꢁ1 resolution. The viscosity and density
of the ionic liquids were measured simultaneously in a Sta-
binger viscometer (Anton Paar, model SVM3000). The changes
Fig.
1 Structural illustration of (a) ammonium and imidazolium
in viscosities and densities of the ionic liquids as a function of cations, and (b) bis(salicylato)borate anion.
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Table 1 Infrared vibration frequencies of ionic liquids with their vibrational assignment
Ammonium cation
TBA-BScB TOA-BScB
Imidazolium cation
EMIM-BScB BMIM-BScB
DOMPA-BScB
HMIM-BScB
Vibrational assignment
3069w, 3041w 3065w, 3041w 3065w, 3041w, 3015w 3151s, 3112s,b 3150s, 3110m, 3083m 3149s, 3111s,b n(C–H), aromatic
groups
2964s, 2938s,
2876s
2955s, 2927s,
2856s
2956s, 2928s, 2857s
2982w, 2939w, 2961s, 2933m, 2872s
2853w
2960s, 2935s,
2869s
na & ns(C–H), CH2/CH3
groups
1683s
1611s
1577w
—
1685s
1611s
1580w
—
1684s
1610s
1579w
—
1681s
1609s
—
1680s
1608s
—
1685s
1610s
—
1575m
n(C]O)
na(COO)
n(C]C), aromatic
n(C–N/C–C), imidazolium
ring, n(C]C), aromatic
d C–H, CH2 groups
ns(COO), split peaks
na(B–O)/n(C–O)
ns(B–O)
1575m
1575w
1465s
1466s
1468s
1467s
1469s
1468s
1269s, 1244s
1200–900
768s, 699s
1269s, 1242s
1200–900
759s, 698s
1268s, 1242s
1200–900
759s, 698s
1268s, 1243s
1200–900
762s, 699s
1267s, 1241s
1200–900
761s, 700s
1266s, 1244s
1200–900
753s, 697s
and alkylmethylimidazolium bis(salicylato)borate ionic liquids bonding, ion size, polarizability of the ions, degree of freedom
were prepared by metathesis of lithium bis(salicylato)borate within the ion, etc.28 Herein, the high viscosities of BScB based
with tetraalkylammonium and alkylmethylimidazolium ionic liquids was found to be due to (a) high dipole–dipole
halides, respectively. A total of six ionic liquids, three under attractions determined by the high polarity (containing two
each category, having variable alkyl groups (Fig. 1) attached to carbonyl groups), (b) high London dispersion forces, deter-
ammonium and imidazolium ionic liquids were prepared to mined by the bulkier size of bis(salicylato)borate anion and (c)
probe their effect on tribological properties. The preparation of high rigidity (comprising two six-membered aromatic rings and
these ionic liquids was conrmed by their NMR and FTIR
analyses. The vibrational peak assignments of ammonium and
imidazolium bis(salicylato)borate ionic liquids are presented in
Table 1. FTIR spectra (Fig. S2, ESI†) of these ionic liquids
showed strong vibrational peaks in the range of 3000–2800
cmꢁ1 associated to alkyl groups,23–25 of ammonium and imida-
zolium cations. Ammonium bis(salicylato)borate ionic liquids
exhibited weak vibrations in the range of 3080–3000 cmꢁ1
,
which are attributed to n(C–H), revealing the presence of an
aromatic ring in the bis(salicylato)borate anion. However, in
imidazolium ionic liquids, in addition to these weak signatures,
new strong vibrational peaks appeared in the range of 3160–
3080 cmꢁ1 which are attributed to n(C–H) of the imidazolium
rings.23 The other characteristic vibrational peaks at 1685 and
1610 cmꢁ1 are attributed to n(C]O) and na(COO) functional-
ities. The ns(COO) vibrational signature was divided into two
peaks at 1268 and 1242 cmꢁ1, revealing the complex carboxylate
group in the bis(salicylato)borate anion.26 Further, the strong
vibrations in the range of 1200–900 cmꢁ1 are attributed to the
na(B–O) in tetrahedral boron complexes. Two additional vibra-
tions were present at ꢂ760 and ꢂ698 cmꢁ1 representing the
ns(B–O) signature in bis(salicylato)borate anion. As a whole,
these vibrational characteristics show the preparation of
ammonium and imidazolium bis(salicylato)borate ionic
liquids.
Ionic liquids based on chelated orthoborate anions were
found to have high glass transition temperatures and very high
viscosities at room temperature than those with peruorinated
anions such as bis(triuoromethane)sulfonimide, tri-
uoromethanesulfonyl, tetrauoroborate, etc.27 In general, the
viscosity of ionic liquids is governed by multiple factors
including van der Waals interactions, coulomb force, hydrogen
Fig. 2 (a) Density and (b) viscosity of DOMPA-BScB and EMIM-BScB
ionic liquids as a function of temperature.
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Fig.
3 Changes in friction coefficient and WSD for TOA-BScB
ionic liquid as a function of concentration blended with PEG 200
under a load of 392 N, speed of 1200 rpm, and temperature of 75 ꢀC
for one hour.
two ve membered rings) of the bis(salicylato)borate anion. The
structure of the cation also inuences the viscosity of the ionic
liquids. In bis(salicylato)borate ionic liquids, aromatic imida-
zolium cations have a higher density and viscosity than
aliphatic ammonium cations (Fig. 2). The imidazolium ionic
liquids signicantly loose their uidity with an increase in the
alkyl chain length attached to the imidazolium ring. The longer
the chain length the further the van der Waals interaction force,
resulting in a change from a wax-to-solid form of the HMIM-
BScB. Fig. 2 shows the temperature dependence of the density
and viscosity of the DOMPA-BScB and EMIM-BScB ionic liquids,
selected as representative examples from the imidazolium and
ammonium cation series. The densities were found to reduced
linearly with increasing temperature as reported earlier.29 The
viscosity of bis(salicylato)borate anion based ionic liquids was
found to change signicantly with temperature (Fig. 2b). The
Fig. 4 Changes in friction coefficient versus time for (a) ammonium
ionic liquids (TBA-BScB, TOA-BScB and DOMPA-BScB) and (b) imi-
dazolium ionic liquids (EMIM-BScB, BMIM-BScB and HMIM-BScB)
blended with PEG 200. Concentration of ionic liquids: 2% w/v, load:
392 N, rotating speed: 1200 rpm, temperature: 75 ꢀC, and test dura-
tion: 1 hour.
thermal kinetic energy of ionic liquids molecules increase with absence of friction and wear modier in PEG 200 led to direct
an increase of temperature, as a result, the viscosities of EMIM- contact between the steel balls under the high tribo-stress,
BScB and DOMPA-BScB decrease markedly with an increase of which promoted not only high friction but also severe wear and
temperature.
scuffing of materials. Probably the severe wear and high scuff-
The synthesized halogen-free BScB anion based ionic liquids ing increased with time and made the materials more uneven,
were further evaluated for their lubrication potential using PEG consequently, the friction increased with time. In both ammo-
200 as the lube base stock. Fig. 3 shows the average friction nium and imidazolium ionic liquids, the alkyl chain structure
coefficient and WSD for different doses of TOA-BScB ionic was changed to study their effect on tribo-characteristics. There
liquid blended with PEG 200 under a load of 392 N. The average was a signicant reduction in the friction coefficient for both
friction coefficient and WSD for PEG 200 were about 0.123 and ammonium and imidazolium ionic liquids blended samples
797 mm, respectively. A 1% (w/v) dose of TOA-BScB resulted in a compared to that of PEG 200. DOMPA-BScB ionic liquid showed
signicant reduction of both the friction coefficient and WSD . A a comparatively higher friction coefficient than the corre-
further increase of the TOA-BScB concentration resulted in no sponding TBA-BScB and TOA-BScB ionic liquids (Fig. 4a). In
signicant change in friction coefficient, however, the WSD was tribology, the viscosity index (VI) and friction reducing additives
found to be at a minimum at a concentration of 2%. Therefore, play a crucial role in reducing the friction between contacting
a 2% dose was considered to be the optimum concentration for surfaces. In general, the viscosity of lubricant decreases with a
further studies. These results reveal that TOA-BScB as an addi- increase of temperature and at high temperature the lubricant
tive, played a positive role in remarkably improving the lubri- becomes too thin and unable to force the two contacting
cation properties of PEG 200.
surfaces apart, resulting in severe wear. Hence, for good lubri-
Fig. 4 illustrates the variation in friction coefficient with time cants the viscosity should not be reduced signicantly at high
for steel balls lubricated with ammonium and imidazolium- temperatures, which means that the VI should be high. As is
BScB ionic liquids blended with PEG 200. The friction coeffi- shown in Table 2, the VI of PEG 200 was improved by the
cient of PEG 200 was found to gradually increase with time. An addition of TBA-BScB and TOA-BScB ionic liquids, whereas
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Table 2 Physico-chemical properties of lubricants used for tribolog-
ical evaluation
DOMPA-BScB molecules were able to reduce the friction,
however, friction was still higher than that for the TBA and TOA
cations based ionic liquids.
Kinematic
viscosity/ mm2 sꢁ1
Likewise, imidazolium ionic liquids (EMIM-BScB, BMIM-
BScB, HMIM-BScB) having variation in associated alkyl chain
length (Cn ¼ 2, 4 and 6), show a considerable reduction in
friction compared to PEG 200 (Fig. 4b). Importantly, it was
noted that the friction coefficient gradually reduced with time.
Fig. 5a shows the WSD for PEG 200 and alkylmethylimidazo-
lium-BScB ionic liquids blended with PEG 200. An increase in
the alkyl chain length associated with the imidazolium ring,
increases the WSD. An EMIM-BScB ionic liquid having the
shortest chain length shows ꢂ33% reduction in WSD of PEG
200, whereas HMIM-BScB ionic liquid with the longest chain
length shows ꢂ24% reduction. These results follow an opposite
trend to the results obtained for of imidazolium tetra-
uoroborate ionic liquids, where the highest friction and wear
were found for the shortest alkyl chain ionic liquid, EMIM-BF4,
owing to the corrosive events under the tribo-stressed area.7
Here, alkylmethylimidazolium-BScB ionic liquids are halogen-
free and possess high interactions associated with the bulk size
of the BScB anion, which provides hydrophobic and reduced
polar character. And such a phenomenon, further increases
with an increase of the chain length of associated imidazolium
cations. As a whole, the interaction of these ionic liquids on
tribo-surfaces could subside with an increase of the chain
length. Therefore, EMIM-BScB ionic liquid, possessing
maximum polar character among this series shows more
interaction with contacting surfaces and forms the tribo-lm,
which protects metal-to-metal contact and provides very good
anti-wear properties compared to the BMIM-BScB and HMIM-
BScB. The gradual reduction in friction coefficient with
increasing contact time (Fig. 4b), further revealing the reduced
polar character of alkylmethylimidazolium-BScB ionic liquids.
These ionic liquids with reduced polar character take a longer
time to form a tribo-lm, and probably the degree of such tribo-
lm formation increases with increasing tribo-contact time,
hence there is a gradual reduction in friction. Another plausible
reason could be wearing of the steel balls to some extent even in
the presence of ionic liquid. This worn material from steel balls
(primarily Fe and Cr) makes the structure complex with ionic
liquids, which function as a complex tribo-lm on the steel
surfaces. However, such a mechanism is slow compared to
direct interaction of ionic liquids on the contact surfaces. As a
result, the friction coefficient gradually reduces with time as is
shown in Fig. 4. The tetraalkylammonium-BScB ionic liquids
follow an interesting trend for WSD with a change of associated
alkyl chain length (Fig. 5b). TBA-BScB ionic liquids possess very
good packing and exist in a semi-solid form at room tempera-
ture. Probably, the good packing of TBA-BScB ionic liquids
results in a solid like tribo-lm and signicantly reduces the
Viscosity Pour
index
Density/
Lubricant
At 40 ꢀ
C
At 100 ꢀ
C
point/ ꢀ g mlꢁ1
C
PEG 200
2% TBA-BScB
2% TOA-BScB
2% DOMPA-BScB 23.77
2% EMIN-BScB
2% BMIM-BScB
22.4
23.38
23.46
4.1
70
85
85
50
79
80
ꢁ27
ꢁ45
ꢁ45
ꢁ45
ꢁ51
ꢁ48
1.13
1.13
1.13
1.14
1.13
1.13
4.33
4.34
4.13
4.34
4.26
23.94
23.08
DOMPA-BScB ionic liquid reduced the VI. This reveals that the
low VI of DOMPA-BScB blended PEG 200 at high temperature
(75 ꢀC), might be unable to support the load, hence there is
more chance of metal-to-metal contact. Therefore, DOMPA-
BScB ionic liquids have a higher friction coefficient (Fig. 4a and
5b) and wear scar diameter compared to those of TBA-BScB and
TOA-BScB. It was also noted that aer a couple of minutes,
Fig. 5 WSD of (a) imidazolium ionic liquids (EMIM-BScB, BMIM-BScB WSD. Whereas, a further increase of alkyl chain length associ-
and HMIM-BScB) and (b) ammonium ionic liquids (TBA-BScB, TOA-
BScB and DOMPA-BScB) lubricated steel balls. A 2% blend of each
ionic liquids in PEG 200 is used as lubricant. Load: 392 N, rotating
speed: 1200 rpm, temperature: 75 ꢀC, test duration: 1 hour. WSD
ated with the ammonium ionic liquid results in steric
hindrance and the ionic liquids are found to be viscous liquids.
We presume that the steric hindrance due to long alkyl chain
length is unable to provide a good quality tribo-lm, hence the
values are based on measurement of worn area on three different balls
and the provided error bars are the standard deviation of these values. WSD is a little high compared to that of TBA-BScB.
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Table 3 Influence of anion structure of ionic liquids on the friction
coefficient and WSDa
Chemical structure
of anion
Avg. friction
coefficient
Lubricant
PEG 200
WSD/mm
—
0.123 ꢃ 0.014 798 ꢃ 48
0.042 ꢃ 0.007 583 ꢃ 6
2% BMIM-BScB
2% BMIM-DBP
0.116 ꢃ 0.004 478 ꢃ 19
2% BMIM-FAP
0.147 ꢃ 0.007 746 ꢃ 15
Fig. 6 FESEM images of the worn surfaces of steel balls lubricated with
(a and b) PEG 200, (c and d) DOMPA-BScB blended with PEG 200, (e
and f) TOA-BScB blended with PEG 200, (g and h) TBA-BScB blended
with PEG 200, (i and j) EMIM-BScB blended with PEG 200, (k and l)
BMIM-BScB blended with PEG 200, and (m and n) HMIM-BScB
blended with PEG 200. Load: 392 N, rotating speed: 1200 rpm,
temperature: 75 ꢀC, tribo-test duration: one hour, concentration of
ionic liquid: 2% w/v.
a
Load: 392 N, speed: 1200 rpm, test time: 1 hour.
To examine the effect of halogen and phosphorus based
ionic liquid on lubrication properties, two different ionic
liquids; BMIM-DBP and BMIM-FAP were selected and their
lubrication properties were compared with BMIM-BScB. These
ionic liquids possess different anions and their chemical
structures are shown in Table 3. The friction coefficient and
WSD for BMIM-DBP, BMIM-FAP and BMIM-BScB ionic liquids
blended with PEG 200 are also presented in Table 3. The BMIM-
DBP shows little reduction in friction coefficient compared to
PEG 200, however, a signicant improvement in anti-wear
property is revealed. This could be due to the formation of a
phosphate tribo-lm on the contact surfaces, which provides
excellent anti-wear properties under tribo-stress.11,18,30 However,
phosphorus based additives are known to be toxic in nature and
are hazardous to the environment, hence there is growing
interest in replacing the phosphorus with an environmentally-
friendly element. Halogen containing ionic liquids are known
to be sensitive to moisture and may produce acids (hydrogen
halides) under tribological conditions, which can damage
engineering surfaces. Fluorine rich BMIM-FAP ionic liquid
shows no signicant wear reduction but high friction compared
to that of PEG 200. The presence of uorine in the FAP anion
might lead to tribo-corrosive events, therefore, high friction and
no signicant improvement in anti-wear properties. In-contrast,
BMIM-BScB reveals signicant reduction in both friction and
wear owing to the formation of ionic liquid tribo-lm having
lubricous boron chemistry and a lack of tribo-corrosive
elements.
Table 4 Elemental composition on the worn area of lubricated steel
balls under a load of 392 N
Lubricant
Fe%
Cr%
C%
O%
N%
PEG 200
72.45
74.49
74.33
69.33
69.61
1.42
1.60
1.48
1.24
1.93
12.00
11.26
12.76
20.64
13.06
14.13
7.64
6.83
4.09
8.83
—
2% DOMPA-BScB
2% TBA-BScB
2% EMIM-BScB
2% HMIM-BScB
5.00
4.60
4.70
6.56
lubricated with ionic liquids blended samples were notably
smaller than with PEG 200 (Fig. 5). The worn area lubricated
with PEG 200 shows severe plastic deformation with scuffing
damage on the steel surface; illustrating the metal-to-metal
contact because of poor lubrication. As shown in Fig. 6c–n, the
wear scars of the steel balls lubricated with 2% ionic liquids
(DOMPA-BScB, TOA-BScB, TBA-BScB, EMIM-BScB, BMIM-BScB
and HMIM-BScB) were comparatively smoother with shallow
friction scratches, and severe scuffing damage was greatly alle-
viated in the presence of ionic liquids. The shallow scratches
and grooves on the worn area and reduction of WSD indicate
undoubtedly the role of ionic liquids as anti-wear and anti-
scuffing additives.
Furthermore, EDX analysis of worn surfaces was carried out
to examine the surface composition aer tribo-tests. Because of
the instrument detection limit, we couldn’t detect boron on the
worn area, though this is an important constituent element of
BScB anion; hence, the role of ionic liquids in tribo-lm
formation was conrmed by the presence of nitrogen, which is a
main constituent element of imidazolium and ammonium
Fig. 6 shows the FESEM images of the worn surface of steel
balls lubricated with PEG 200 and ionic liquids blended
samples at a load of 392 N. The WSD of all the steel balls
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These ionic liquids are found to be hydrophobic in nature and
possess good miscibility with PEG 200, which is very important
for their efficient performance. BScB ionic liquids showed very
high viscosities at room temperature, associated with high
polarity, high London dispersive forces, high rigidity, bulk size
and ring structure of bis(salicylato)borate anion, which is
further increased with an increase of the alkyl chain length
attached to the imidazolium ring. The BScB ionic liquids as
additives in PEG 200 exhibited excellent friction-reduction and
anti-wear properties. The WSD for imidazolium-BScB ionic
liquids were found to increase with an increase of the alkyl
chain length of imidazolium cations. This could be due to a
reduction of their polar character as a result of an increase of
the van der Waals interactions and such phenomena slow
down the tribo-lm formation. The microstructural examina-
tion of the worn surfaces of steel balls lubricated with PEG 200
and BScB ionic liquids revealed the anti-wear and anti-scuffing
properties of these ionic liquids owing to the boron based
chemistry of the tribo-lm composed of BScB ionic liquids, as
conrmed by their elemental analyses. The high viscosities,
hydrophobic nature, good miscibility with lube base stock and
outstanding tribological properties of these halogen-, phos-
phorus- and sulfur-free bis(salicylato)borate anion based ionic
liquids suggest their potential as environment friendly lubri-
cant additives.
Fig.
7 FESEM micrograms and corresponding element (carbon,
oxygen and nitrogen) maps of the worn surface of steel ball lubricated
with (a) TBA-BScB and (b) BMIM-BScB ionic liquids blended PEG 200.
cations associated with BScB anion. Table 4 shows the
elemental composition of the worn surfaces of steel balls
lubricated with PEG 200 and BScB ionic liquids blended
samples. The absence of nitrogen on the worn surface area of
the steel ball lubricated with PEG 200, reveals the absence of
ionic liquids, and the resulting very high friction and severe
wear. However, a 2% dose of imidazolium and ammonium ionic
liquids was good enough to prepare the tribo-lm on the worn
area as revealed by the presence of nitrogen (4.6 to 6.6%). The
tribo-lm composed of ionic liquids, not only reduced friction,
but also prevented direct contact of the steel balls, resulting in
signicant reduction in wear. The FESEM micrographs and
corresponding element maps of the worn surface of the steel
ball aer tribo-test with TBA-BScB and BMIM-BScB ionic liquids
blended samples, illustrates the uniform distribution of
nitrogen (Fig. 7), which is an integral part of imidazolium and
ammonium based ionic liquids, revealing the formation of
tribo-lm composed of ionic liquids. Although, the exact
mechanism of the role of these ionic liquids in reduction of
friction and wear is expected to be very complex because of
boron’s chemistry. In general, boron compounds are known to
be lubricious materials because of their remarkable tribo-
chemical and mechanical properties.31–33 Recently, it was found
that under high pressure boron has a partial negative charge,34
which may facilitate it’s interaction with metallic surfaces
under boundary lubrication conditions and forms a tribo-lm.
The very hard nature of the boron35 tribo-lm probably protects
the contacting surfaces and provides the anti-wear and anti-
scuffing properties. As a whole imidazolium and ammonium
BScB ionic liquids not only enable the marked reduction of both
friction and wear, but also protect the tribo-surfaces from the
corrosive events, which makes the lubricant system green and
energy efficient.
Acknowledgements
We kindly acknowledge the Director of IIP for his kind
permission to publish these results. The authors are thankful
for nancial support from DST, India for this work. We are also
thankful to the Analytical Sience Division of IIP for providing
help in the analyses of the samples. Author R.G. thanks the
CSIR, India for nancial support.
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