Halogen-free ionic liquids: Effect of chelated orthoborate anion structure on their lubrication properties

Article abstract of DOI:10.1039/c5ra03092g

In transportation vehicles, a large portion of energy is consumed to overcome friction in the engine and associated components. An efficient lubricant system has a direct impact on saving energy and material loss by reducing the friction, wear, and corrosion. Herein, halogen-free chelated orthoborate ionic liquids were designed, synthesized and then evaluated as potential lubricant additives. The effect of the orthoborate anion structure on the thermal stability, corrosion, friction, and wear properties of ionic liquids were studied. The copper strip tests revealed the non-corrosiveness of bis(mandalato)borate (BMdB), bis(salicylato)borate (BScB), and bis(malonato)borate (BMlB) anion constituted ionic liquids. Whereas, the bis(oxalato)borate (BOxB) anion, eventually developed micro-pits of corrosion owing to its lower stability and acidic nature of the decomposed product. These ionic liquids as additives to the synthetic lube base oil significantly reduced both friction and wear. The degree of friction and wear reduction was influenced by the structure of associated anions. BMdB and BScB anion constituted ionic liquids exhibited excellent thermal stability, friction-reduction, and antiwear properties, which are attributed to their compact and chemically stable structure driven by higher intermolecular interactions and rigidity of aromatic rings. The chemical analysis of tribo-interfaces suggested the formation of an ionic liquid composed tribochemical product and that enhanced the lubrication properties. Being halogen-free, these ionic liquids could be energy efficient and environmentally-friendly substitutes to the conventional friction-reducing and antiwear additives. This journal is

Full text of DOI:10.1039/c5ra03092g

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Halogen-free ionic liquids: effect of chelated
orthoborate anion structure on their lubrication
Cite this: RSC Adv., 2015, 5, 25287
properties†
*
Rashi Gusain and Om P. Khatri
In transportation vehicles, a large portion of energy is consumed to overcome friction in the engine and
associated components. An efficient lubricant system has a direct impact on saving energy and material
loss by reducing the friction, wear, and corrosion. Herein, halogen-free chelated orthoborate ionic
liquids were designed, synthesized and then evaluated as potential lubricant additives. The effect of the
orthoborate anion structure on the thermal stability, corrosion, friction, and wear properties of ionic
liquids were studied. The copper strip tests revealed the non-corrosiveness of bis(mandalato)borate
(BMdB), bis(salicylato)borate (BScB), and bis(malonato)borate (BMlB) anion constituted ionic liquids.
Whereas, the bis(oxalato)borate (BOxB) anion, eventually developed micro-pits of corrosion owing to its
lower stability and acidic nature of the decomposed product. These ionic liquids as additives to the
synthetic lube base oil significantly reduced both friction and wear. The degree of friction and wear
reduction was influenced by the structure of associated anions. BMdB and BScB anion constituted ionic
liquids exhibited excellent thermal stability, friction-reduction, and antiwear properties, which are
attributed to their compact and chemically stable structure driven by higher intermolecular interactions
and rigidity of aromatic rings. The chemical analysis of tribo-interfaces suggested the formation of an
ionic liquid composed tribochemical product and that enhanced the lubrication properties. Being
halogen-free, these ionic liquids could be energy efficient and environmentally-friendly substitutes to the
conventional friction-reducing and antiwear additives.
Received 18th February 2015
Accepted 4th March 2015
DOI: 10.1039/c5ra03092g
www.rsc.org/advances
growing interest to reduce the use of ZDDP based lubricant
additives because of their toxicity to aquatic wildlife, adverse
Introduction
effects to human-health and poisoning of automotive exhaust
gas catalyst components.1
Energy and the environment play important roles in everyone's
life and are inextricably linked with each other. The global
awareness on the sustainability of fuels, need for energy-
efficient systems, and utilization/conversion of energy in envi-
ronmentally benign practices have been increasing immensely
because of high volatility in fuel prices and stringent environ-
mental regulations. In this context, the energy and material
losses due to high friction and wear in the transportation
system leads to high fuel consumption, frequent failure of
engine parts, increased greenhouse gas emission, discharge of
partially oxidized fuels etc. An efficient lubricant can reduce
these undesirable events. Friction and wear are considered to be
the most damaging events to the mating surfaces of the engine.
Zinc dialkyldithiophosphate (ZDDP) has been a well established
antiwear and friction-reducing additive for iron based compo-
nents since 1960s. Over the last two decades there has been a
Recently, ionic liquids have been studied as versatile
lubricants and lubricant additives for various engineering
surfaces, because of their unique and tunable physico-
chemical properties like negligible vapour pressure, wide
liquid range, excellent conductivity, good thermal stability,
inherent polarity etc.2,3 Ionic liquids are task specic in nature
and their properties can be tailored by changing the structure
of constituted cation and anion. The molecular thin lms of
ionic liquids on solid surfaces mediate efficient lubrication
relative to conventional non-polar hydrocarbon liquids and is
attributed to the geometric and charge characteristics of the
ionic liquids. The irregular shapes of cation and anion leads to
low shear stress, revealing low friction than non-polar molec-
ular liquids.4 Inherent polar nature of ionic liquids facilitates
their interaction with engineering surfaces under tribo-stress,
resulting in boundary thin lm formation. Such protective
thin lm of ionic liquids avoids the direct contact between
mating surfaces and is believed to be responsible for the
antiwear property.5
Chemical Science Division, CSIR
– Indian Institute of Petroleum, Mohkampur,
Dehradun-248805, India. E-mail: opkhatri@iip.res.in; Fax: +91 135 2660200
† Electronic supplementary information (ESI) available: FTIR, NMR (1H, 13C)
spectra of ionic liquids, physico-chemical properties of ionic liquids blends in
PEG 200. See DOI: 10.1039/c5ra03092g
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Boron compounds are established as lubricious materials
because of their remarkable tribochemical and high pressure
properties.6,7 The boron constituted materials, such as
organoboron compounds, h-BN powder, metal borate nano-
particles etc. exhibit excellent friction-reducing and antiwear
properties.6,8,9 The rst report on lubrication properties of
ionic liquids was based on theꢀboron constituted alkylimi-
dazolium tetrauoroborate (BF4 ) ionic liquid.10 Under tribo-
stress, alkylimidazolium-BF4 ionic liquid developed a tribo-
thin lm composed of FeF2 and B2O3 over the steel surface,
which avoids direct contact between tribo interfaces, conse-
quently reduction in both friction and wear.11 Phillips et al.
have revealed that BF4ꢀ anion based ionic liquids break down
into several reaction products mainly consisting of FeF2, and
this leads to the degradation of the lubricant uid and
corrosion of the substrate surface.12 Over last one decade,
Experimental section
Materials
Tetrabutylammonium bromide (99%, Loba Chemie), boric acid
(99.5%, Loba Chemie), lithium carbonate (99.9%, Sigma
Aldrich), salicylic acid (99.8%, Merck), malonic acid (99%,
Sigma Aldrich), mandelic acid (99%, Sigma Aldrich), oxalic Acid
(99%, Merck Chemicals) and sodium tetrauoroborate (97%,
Alfa Aesar) were used without further purication for the
synthesis of ionic liquids. The dichloromethane (DCM; AR
grade, Loba Chemie) was used for ionic liquid extraction. The
polyethylene glycol (mol. weight: 200; PEG 200) was procured
from Alfa Aesar was used as synthetic lube base oil.
Synthesis and NMR characterization of ionic liquids
several ionic liquids composed of imidazolium, ammonium,
Tetrabutylammonium bis(salicylato)borate (TBA-BScB). The
lithium salt of BScB, an anionic precursor was prepared by
phosphonium, pyridinium etc. as cations and Xꢀ, PF6
,
ꢀ
ꢀ
CF3SO3ꢀ, (CF3SO2)2Nꢀ, (C2F5)3PF3 etc. as anions have been mixing salicylic acid (2.762 g, 20 mmol) in an aqueous solution
studied as lubricant and lubricant additives for diversied of lithium carbonate (0.367 g, 5 mmol) and boric acid (0.619 g,
range of engineering surfaces and have exhibited structure 10 mmol) at 60 ꢁC temperature for 2 hours. In the next step,
dependent lubrication properties.2,3,13 These halogenated TBA-Br (3.224 g, 10 mmol), a cationic precursor, was gradually
ionic liquids have hazardous and toxic effects to the envi- added to the freshly prepared solution of lithium-BScB and
ronment and are corrosive to the engineering surfaces.14,15 heated the reaction mixture at 50 ꢁC for 2 hours. This led to the
The decomposition of halogenated ionic liquids under tribo- formation of an organic layer composed of TBA-BScB ionic
conditions and/or via hydrolysis by trapped water molecules liquid. The TBA-BScB was extracted using DCM and then
evolve toxic and corrosive products, which contaminate the washed with distilled water several times to remove all un-
environment and corrode the engineering surfaces.15,16 reacted bromide content. The DCM was then removed under
Besides that, high cost of halogens, particularly, uorine reduced pressure and TBA-BScB ionic liquid was dried in
based precursors and disposal/discharge of halogenated vacuum oven at 80 ꢁC for 48 hours. The preparation of TBA-BScB
ionic liquids are big challenges for their penetration to the was conrmed by H and 13C NMR analyses. H NMR (CDCl3,
industrial applications. Thus, halogen-free ionic liquids have ppm): 0.8–0.9 (t, 12H, N(CH2)3CH3), 1.3 (m, 8H, N(CH2)2CH2),
become a topic of great research interest for the energy effi- 1.5 (q, 8H, NCH2CH2), 3.1 (t, 8H, NCH2), 6.8–6.9 (m, 4H, C6H4),
1
1
cient and environmentally-friendly lubricant development.
7.29–7.39 (m, 2H, C6H4), 7.86–7.95 (dd, 2H, C6H4). 13C NMR
Recently, amino acids, dicyanamide, tricyanomethanide and (ppm): 173.30, 165.19, 161.77, 159, 30, 136.43, 130.60, 118.73,
fatty acids anions based ionic liquids were explored as halogen- 118.26, 115.36, 58.5, 23.5, 19, 13. Yield: 95%, physical state (at
ꢁ
free ionic liquids. These ionic liquids showed good tribological 24 C): off-white wax.
performance compared to the synthetic lube oils.17–19 However,
Tetrabutylammonium bis(mandelato)borate (TBA-BMdB).
low thermal stability and high cost of precursors limit their TBA-BMdB was prepared by following the TBA-BScB preparation
usage from application perspective. Antzutkin et al. have procedure using mandelic acid (3.043 g, 20 mmol) as an
developed halogen-free orthoborate anions based phospho- ingredient to anionic precursor, instead of salicylic acid. The
1
nium ionic liquids. These hydrophobic ionic liquids as lubri- preparation of TBA-BMdB was conrmed by H and 13C NMR
cants exhibited better antiwear and friction-reducing properties analyses. 1H NMR (CDCl3, ppm): 0.85–0.88 (t, 12H,
as compared to fully formulated engine oil.20 Nevertheless, the N(CH2)3CH3), 1.2–1.27 (m, 8H, N(CH2)2CH2), 1.36–1.39 (q, 8H,
stringent regulations on environment protection drives to NCH2CH2), 2.92–2.95 (t, 8H, NCH2), 5.2 and 5.3 (s, 2H, CH2–
replace the phosphorus from the lubricant composition C6H5), 7.22–7.26 (m, 2H, C6H4), 7.27–7.32 (m, 4H, C6H4), 7.58–
because of its hazardous and toxic effects to environment.21 The 7.65 (m, 4H, C6H4). 13C NMR (ppm): 177.95, 40.29, 140.01,
scope of chelated orthoborate anions based ionic liquids were 131.79, 129.65, 128, 127.32, 126.44, 126.32, 126.23, 126.14,
further extended with imidazolium, bis-imidazolium and pyr- 78.26, 78.01, 58.26, 23.64, 19.49, 13.58. Yield: 94%, physical
rolidinium cations.22–24 Herein, we report synthesis, corrosion state (at 24 C): off-white gel.
ꢁ
and tribo-evaluation of chelated orthoborate anions based
Tetrabutylammonium bis(malonato)borate (TBA-BMlB).
ammonium ionic liquids, which are halogen- and phosphorous- TBA-BMlB was prepared by following the TBA-BScB preparation
free. These ionic liquids having variable anion structure are procedure using malonic acid (2.081 g, 20 mmol) as an ingre-
evaluated for non-corrosiveness, friction-reducing and antiwear dient to anionic precursor, instead of salicylic acid. The prep-
properties. Results are further compared with halogenated aration of TBA-BMlB was conrmed by 1H and 13C NMR
analogue.
analyses. 1H NMR (CDCl3, ppm): 0.92–0.95 (t, 12H,
N(CH2)3CH3), 1.35–1.42 (m, 8H, N(CH2)2CH2), 1.58–1.64 (q, 8H,
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NCH2CH2), 3.2–3.3 (t, 8H, NCH2), 3.35 (s, 4H, CO–CH2_CO). 13
C
rotated over these stationary balls under specic load and
NMR (ppm): 173.55, 58.83, 38.43, 24.03, 22.13, 19.69, 13.68. rotating speed. PEG 200 was used as a reference lube base oil
ꢁ
Yield: 91%, physical state (at 24 C): off-white semi-solid.
throughout the study. All ionic liquids were kept in vacuum at
ꢁ
Tetrabutylammonium bis(oxalato)borate (TBA-BOxB). TBA- 80 C for 48 hours, prior to their tribo-evaluation. The known
BOxB was prepared by following the TBA-BScB preparation quantity of each ionic liquid was mixed with PEG 200 by normal
procedure using oxalic acid (1.800 g, 20 mmol) as an ingredient stirring at ambient condition. All ionic liquids exhibited excel-
to anionic precursor, instead of salicylic acid. The preparation lent miscibility with PEG 200. Blend of each ionic liquid in PEG
1
1
of TBA-BOxB was conrmed by H and 13C NMR analyses. H 200 was used for lubrication test. Friction coefficient as a
NMR (CDCl3, ppm): 0.92–0.95 (t, 12H, N(CH2)3CH3), 1.34–1.42 function of time for each sample was measured at a load of
(m, 8H, N(CH2)2CH2), 1.58–1.64 (q, 8H, NCH2CH2), 3.26–3.33 392 N and rotating speed of 1200 for the one hour. Temperature
(t, 8H, NCH2). 13C NMR (ppm): 158.77, 58.88, 24.05, 19.70, of lube sample was maintained as 75 ꢁC throughout the
13.68. Yield: 86%, physical state (at 24 ꢁC): off-white waxy-solid. experiment. The structural features of the worn area of the steel
Tetrabutylammonium tetrauoroborate (TBA-BF4). TBA-BF4 balls, lubricated with different types of ionic liquid samples,
was prepared by the metathesis of TBA-Br (3.224 g, 10 mmol) were examined by FESEM using an FEI Quanta 200F. The
ꢁ
and NaBF4 (1.098 g, 10 mmol) in water for two hours at 60 C elemental composition and distribution of elements on
under continuous stirring. The organic layer of reaction product surfaces of the worn balls were examined by Energy Dispersive
composed of TBA-BF4 was extracted using DCM and then X-ray Spectroscopy (EDX). Coefficient of friction and wear scar
washed with distilled water several times to remove all bromide diameter values reported here are the mean of three different
content. The DCM was separated by distillation under the tests of each sample. Furthermore, the error bar of each result is
reduced pressure and TBA-BF4 ionic liquid was dried in vacuum provided to understand the repeatability of test results.
oven for 48 hours at 80 ꢁC. The preparation of TBA-BF4 was
conrmed by 1H and 13C NMR analyses. 1H NMR (CDCl3, ppm):
Results and discussion
0.91–0.94 (t, 12H, N(CH2)3CH3), 1.33–1.39 (m, 8H, N(CH2)2CH2),
1.51–1.58 (q, 8H, NCH2CH2), 3.11–3.14 (t, 8H, NCH2). 13C NMR The chelated orthoborate ionic liquids comprising variable
(ppm): 58.47, 23.77, 19.61, 13.60. Yield: 89%, physical state (at anions were prepared by metathesis of lithium salt of chelated
ꢁ
24 C): white solid.
orthoborate with TBA-Br in equimolar amount. Total four ionic
liquids with aromatic and aliphatic structures (Fig. 1) were
prepared to probe their structural effects on the thermal
stability, corrosion and tribo-physical properties. The TBA-BF4
ionic liquid was prepared as a halogenated analogue for
comparison study. The synthesis of these ionic liquids were
conrmed by their FTIR and NMR (1H and 13C) characteriza-
tions. The vibrational peak assignments of all ionic liquids are
extracted in Table 1 and their respective spectra are shown as
Fig. S6 (ESI†). FTIR spectra of TBA-BMdB and TBA-BScB
ionic liquids showed weak vibrational peaks in the range of
Characterization of ionic liquids
The preparation of all ionic liquids was conrmed by their
Fourier Transform Infrared (FTIR) and 1H and 13C Nuclear
Magnetic Resonance (NMR) analyses. FTIR spectra of all ionic
liquids were recorded using Nicolet 8700 research spectrometer
at 4 cmꢀ1 resolution. The H and 13C NMR spectra of all ionic
1
liquids were collected using Bruker Av III 500 MHz Spectrom-
eter. Thermal decomposition properties of these ionic liquids
were probed by a thermal analyzer (Diamond, PerkinElmer)
under the ow of nitrogen (50 ml minꢀꢁ1) at thermal rate of 10 ꢁC
minꢀ1. The viscosities at 40 and 100 C, and viscosity index of
each ionic liquid (2 wt%) in polyethylene glycol (PEG 200)
synthetic lube base oil were measured as per the ASTM D445
and ASTM D2270 standard test methods, respectively. The
corrosion properties of these ionic liquids were carried out
using a copper strip as a substrate. In each test, a freshly pol-
ished copper strip was dipped in a vial containing 2 wt% of
ionic liquid in PEG 200 at 100 ꢁC for 3 hours. Aer the test,
copper strip was washed thoroughly to remove the traces of
ionic liquids. Corrosion pits, eventually developed on the
copper strips, and changes in color were thoroughly examined
by optical microscopy, Field Emission Scanning Electron
Microscopy (FESEM) and elemental mapping.
3068–3030 cmꢀ1 and
a strong peak in the range of
1577–1557 cmꢀ1, which are ascribed to n(C–H) and n(C]C),
respectively, revealing the presence of aromatic rings. These
vibrational peaks are absent in rest of the ionic liquids because
of their aliphatic nature. The strong vibrational features in the
range of 3000–2800 cmꢀ1 are attributed to the symbolic signa-
ture of methylene and methyl units of alkyl groups present in
the TBA cation of all ionic liquids. The vibrational signatures in
the region of 1800–1600 cmꢀ1 are assigned to n(C]O) and
na(O–C]O) stretching mode of COO group.25 The ns(O–C]O)
vibrational signature was splitted into two peaks in the range of
1275–1240 cmꢀ1, revealing the complex nature of carboxylate
groups in the chelated orthoborate anions. The broad peaks in
the range of 1200–900 cmꢀ1 are attributed to na(B–O) and
n(C–O), representing the tetrahedral chelated orthoborate
complex. Two additional vibrations in the range of 801–768 and
725–696 are assigned to the ns(B–O) signature, and suggested
Tribo-evaluation of ionic liquids
All lubrication tests were conducted on four-ball tribo tester the presence of chelated orthoborate complex.26 These charac-
(Ducom, India) following the ASTM D4172 standard test teristic vibrational peaks conrmed the synthesis of chelated
method. In a typical test, three steel balls (f ¼ 12.7 mm) were orthoborate ionic liquids. Besides that, the broad vibrational
xed in a pot containing the lube sample and fourth ball was peaks at 1110–1036 cmꢀ1, attributed to the stretching modes of
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Fig. 2 Thermal decomposition of TBA-BMdB, TBA-BScB, TBA-BOxB
and TBA-BF4 ionic liquids at thermal rate of 10 ꢁC minꢀ1 under the
nitrogen flow.
Fig. 1 Structural illustration of boron based anions (a) BMdB, (b) BScB,
(c) BMlB, (d) BOxB and (e) BF4. (f) TBA cation.
aromatic ring constituent TBA-BMdB and TBA-BScB ionic
liquids compared to their aliphatic analogues.
Presence of halogen, phosphorus, and sulfur constituent
components in the lubricant system facilitates the corrosion
events and damages the engineering surfaces. Herein, corro-
sion property of each ionic liquid was probed by copper strip
test method using 2 wt% blend of each ionic liquid in PEG 200.
A freshly polished copper strip was exposed to ionic liquid
B–F and it conrmed the presence of BF4ꢀ anion in the TBA-BF4
ionic liquid.27
The thermal decomposition properties of synthesized ionic
liquids were probed over the temperature range of 35 to 450 ꢁC.
Fig. 2 explicitly shows that TBA-BMdB and TBA-BScB ionic
liquids exhibited higher thermal stability than TBA-BOxB ionic
liquid. The higher thermal stability of TBA-BMdB and TBA-BScB
ionic liquids are attributed to the presence of aromatic rings in
their chelated structure, which provides higher intermolecular
interactions and rigidity to their anionic moieties. It was noted
that TBA-BMdB and TBA-BScB ionic liquids thermally decom-
posed in the range of 300–400 ꢁC, whereas TBA-BOxB ionic
liquid decomposed at moderately low thermal range,
i.e. 200–270 ꢁC. Thermal stability of ionic liquids were also
measured in terms of nucleophilicity and coordinating nature
of anions.28 The BF4 anion is considered as less nucleophilic
and non-coordinating in nature, hence exhibited good thermal
stability. Table 2 shows the TGA temperatures for 10, 20, 30 and
50% thermal loss of each ionic liquid, which are higher for
ꢁ
blend for three hours at 100 C. Aer thoroughly washing, the
changes in structural features of each sample was examined by
optical and electron microscopy. Fig. 3 shows the optical and
electron microscopic images of the copper strips exposed to
PEG 200 and 2 wt% blend of each ionic liquid. The copper strip,
exposed to TBA-BOxB, exhibited corrosion pits (Fig. 3ei)
distributed throughout the substrate. These pits couldn't be
revealed in optical images, however, electron microscopic
images unambiguously demonstrated the sub-micron-to-few
microns size pits. Furthermore, the elemental distribution
images revealed that these pits are mainly composed of copper
and oxygen (Fig. 4b). Owing to poor stability, BOxB tends to
develop acidic products (oxalate), which might have interacted
Table 1 Infrared vibration frequencies of synthesized ionic liquids and their vibrational assignment
TBA-BScB
TBA-BMdB
TBA-BMlB
TBA-BOxB
TBA-BF4
Vibrational assignment
3068, 3042
2964, 2938, 2876
1700, 1684
1611
3062, 3030
2963, 2935, 2876
1738
—
—
—
n(C–H), aromatic ring
na & ns(C–H), CH2/CH3 groups
n(C]O)
2989, 2959, 2874
1728
1626
2962, 2937, 2875
1805, 1778
1625
2987, 2960, 2875
—
—
1601
na(COO)
1577
1557
1486, 1453, 1381
1271
1200–900
—
—
—
—
n(C]C), aromatic
dC–H, CH2/CH3 groups
ns(COO)
na(B–O)/n(C–O)
n(B–F)
1478, 1466
1268, 1244
1200–900
—
1473, 1456, 1381
1286, 1240
1200–900
—
1487, 1473, 1383
1275, 1244
1200–900
—
1472, 1379
—
1108–1036 b
768, 698
770, 739, 694
796, 738
801, 740, 725
—
ns(B–O)
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Table 2 Thermal loss of ionic liquids as deduced from thermogravi-
metric pattern of respective ionic liquid
TGA temperature for % weight loss, ꢁC
Ionic liquids
10%
20%
30%
50%
TBA-BMdB
TBA-BScB
TBA-BOxB
TBA-BF4
293.4
297.1
203.1
329.9
361.7
333.9
210.2
351.3
373.5
347.3
215.2
361.9
382.8
361.4
225.9
375.9
Fig. 4 Overlay elemental mapping and corresponding FESEM images
with copper substrate and developed the corrosion pits. The
copper strip, exposed to TBA-BF4 was signicantly corroded and
developed plenty of corrosion pits (Fig. 3fi–ii). The element
of copper strips exposed to 2 wt% blends of (ai–ii) TBA-BScB, (bi–ii
)
TBA-BOxB and (ci–ii) TBA-BF4 ionic liquids in PEG 200 at 100 ꢁC for
3 hours. The green, red and blue pixels represent copper, oxygen and
overlay images (Fig. 4c) explicitly showed that the corrosive pits fluorine, respectively.
are composed of uorine, oxygen and copper, and are distrib-
uted throughout the substrate. However, the surface features of
dose optimization in PEG 200. Fig. 5 show gradual reductions in
copper strips, exposed to TBA-BMdB, TBA-BScB and TBA-BMlB
ionic liquids were found to remain intact without any
damage. These results suggested that TBA-BMdB, TBA-BScB
and TBA-BMlB ionic liquids, which are halogen-free, do not
corrode the copper strips, whereas, presence of uorine in
TBA-BF4, facilitated the corrosive events. Considering the non-
corrosive properties, chelated orthoborate ionic liquids were
then examined for their lubrication properties.
All tribological tests were conducted using steel balls in
rolling contact geometry under the load of 392 N for one hour at
75 ꢁC temperature. The average coefficient of friction and WSD
for the PEG 200 were found as ꢂ0.17 and 1275 mm, respectively.
The TBA-BScB was used as a representative ionic liquid for the
both friction and WSD with increasing the dose of TBA-BScB,
and no further measurable changes were observed beyond the
2 wt% dose. These results revealed that 2 wt% dose of TBA-BScB
is good enough to provide the efficient lubrication performance
by interacting with tribo-interfaces and reduced both friction
and WSD by ꢂ60 and ꢂ47%, respectively, compared to PEG 200.
An addition of 2 wt% dose of each ionic liquid in PEG 200 has
slightly improved the kinematic viscosities and viscosity index
(Table S1, ESI†). This is attributed to the high viscosities of
these ionic liquids and their excellent miscibility, which are
important for their efficient performance in lubricant
applications.
Fig. 6 compare the changes in coefficient of friction as a
function of contact time for steel balls lubricated by PEG 200
and 2 wt% blends of TBA-BMdB, TBA-BMlB and TBA-BF4 ionic
liquids. PEG 200 is an additive-free lube base oil and under the
tribo-stress, it is unable to hold the lube thin lm between steel
balls interfaces, resulting in high friction. The presence of
boron based ionic liquids showed signicant reductions in
friction coefficient (Fig. 6). The inherent polarity of these ionic
Fig. 3 FESEM and optical images of copper strips exposed to (a) PEG
200, and 2 wt% blends of (b) TBA-BMdB, (c) TBA-BScB, (d) TBA-BMlB, Fig. 5 Changes in coefficient of friction and WSD as a function of
(e) TBA-BOxB, and (f) TBA-BF4 ionic liquids in PEG 200 for 3 hours at TBA-BScB ionic liquid dose in PEG 200. Load: 392 N, rotating speed:
100 ꢁC.
1200 rpm, temperature: 75 ꢁC, test duration: 1 hour.
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liquids is believed to provide strong adsorption to the steel
surfaces and forms the tribo-thin lm of low shear strength,
which reduces the friction signicantly. Fig. 7 revealed that the
degree of friction and WSD reductions are strongly monitored
by the chemical structure of constituted anions. Among these
ionic liquids, TBA-BMdB provided maximum reduction in both
friction (ꢂ65%) and WSD (ꢂ55%) and was slightly better than
the performance of TBA-BScB ionic liquid. The excellent
friction-reducing and antiwear properties of TBA-BMdB and
TBA-BScB ionic liquids were attributed to the compact, rigid
and stable structure of BMdB and BScB anions, provided by two
six-membered aromatic rings in each anion. The BMlB and
BOxB anions based ionic liquids too reduced both friction and
WSD, however, their tribo-performance was relatively poor.
Furthermore, it was revealed that TBA-BOxB ionic liquid has
poor thermal stability and its decomposed acidic (oxalic acid)
product leads to corrosive events. As a result, TBA-BOxB showed
higher friction and WSD compared to other chelated orthobo-
rate ionic liquids.
Fig. 7 Comparison of (a) coefficient of friction and (b) WSD of steel
balls lubricated with PEG 200 and 2 wt% blend of TBA ionic liquids
consisting variable anions, in PEG 200. Load: 392 N, rotating speed:
1200 rpm, temperature: 75 ꢁC, test duration: 1 hour.
Fig. 8 shows FESEM images of worn surface of steel balls
lubricated by PEG 200 and 2 wt% blend of each ionic liquid. The
worn area lubricated by PEG 200 exhibited very high WSD
(ꢂ1275 mm) with lot of uneven features attributed to the plastic
deformation on the steel surface under the tribo-stress, sug-
gesting the direct contact between the steel balls. The 2 wt%
dose of ionic liquids in PEG 200 reduced the WSD signicantly
and generates comparatively smoother features on the worn
surfaces of steel balls (Fig. 8). Among all ionic liquids,
maximum antiwear property was achieved by TBA-BMdB ionic
liquid and was attributed to the compact, rigid and stable
structure of BMdB anion. Furthermore, the EDX analyses of
steel balls were carried out to probe the elemental composition
of tribo-chemical thin lm developed on the worn surfaces. The
FESEM micrographs and corresponding elemental distribution
images of worn area of steel balls lubricated with 2 wt% of each
ionic liquid, explicitly illustrated uniform distribution of boron
and nitrogen (Fig. 9a–d). In these ionic liquids, boron and
nitrogen are characteristics elements of anion and cation,
respectively, and are uniformly distributed on the worn area of
steel balls. These results suggested that the tribo-chemical thin
lm on the steel balls are composed of ionic liquids or its tribo-
chemical product.
In order to probe the effect of halogen, the friction and wear
properties of uorine constituted TBA-BF4 ionic liquid was
examined under identical condition (Fig. 7). Though, TBA-BF4
showed reduction in both friction and wear, the tribo-
performance was noted poorer compared to the chelated
orthoborate ionic liquids. The elemental images (Fig. 9e) illus-
trated regular distribution of uorine along with boron and
nitrogen, throughout the worn surface. Usually, BF4 anion
based ionic liquid developed a tribo-thin lm composed of FeF2
and B2O3 over the steel surface,11 and such tribo-chemical thin
lm reduced both the friction and wear. The corrosion results
(Fig. 4) and poor triboꢀ-performance suggested corrosive prod-
ucts generation by BF4 anion constituted ionic liquids, which
could be further facilitated by trapped water molecules in the
lubricant. Probably, the corrosive events compromised the
tribo-performance of TBA-BF4 ionic liquid, and thus, poorer
lubrication properties compared to the chelated orthoborate
ionic liquids.
Elemental mapping results showed that chelated orthobo-
rate ionic liquids have generated a tribo-chemical thin lm on
the steel interfaces, which was composed of tribo-chemical
product of ionic liquids with the steel. The exact mechanism
and role of ionic liquids in tribo-chemical thin lm formation is
believed to be complex because of inherent polarity of ionic
liquids, rich boron chemistry, and hard nature of the boron.
Usually, under the tribo-stress, metal surfaces induced the
positive charge,29 where the chelated orthoborate anions could
be adsorbed and then counter cation TBA would have provided
the layering structure through electrostatic attractions and
Fig. 6 The evolution of coefficient of friction with contact time for
PEG 200, 2 wt% of TBA-BMdB, TBA-BMlB, and TBA-BF4 ionic liquids
individually blended in PEG 200. Load: 392 N, rotating speed:
1200 rpm, temperature: 75 ꢁC, test duration: 1 hour.
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Fig. 8 FESEM images of the worn surfaces of steel balls lubricated with (ai–ii) PEG 200 and 2 wt% blends of (bi–ii) TBA-BMdB, (ci–ii) TBA-BScB,
(di–ii) TBA-BMlB (ei–ii) TBA-BOxB and (fi–ii) TBA-BF4 ionic liquids in PEG 200. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 ꢁC, test
duration: 1 hour.
generates partial negative charge,7 which might facilitate the
interaction of chelated orthoborate anions with steel surfaces
and forms the tribo-thin lm. Furthermore, the very hard
nature of boron30 is understood to provide durable tribo-thin
lm, which protects the steel interfaces and reduces wear
signicantly. Being halogen- and phosphorus-free, these ionic
liquids offer an energy-efficient, environment-friendly, and
economic alternative to the conventional halogenated ionic
liquids for the lubrication applications.
Conclusion
The focus of this work was (a) to develop halogen- and
phosphorus-free ionic liquids as energy efficient and
environment-friendly lubricant additives for the steel based
engineering surfaces, and (b) to established the correlation
between structure of anion and tribo-physical properties of
ionic liquids. In this study, chelated orthoborate ionic liquids,
having structural variations in their anionic moieties, were
synthesized and then thoroughly characterized by NMR (1H and
13C) and FTIR. The aromatic rings constituted TBA-BMdB and
Fig. 9 Elemental mapping of worn area lubricated with 2 wt% of (a)
TBA-BMdB, (b) TBA-BScB, (c) TBA-BMlB, (d) TBA-BOxB and (e)
TBA-BF4 in PEG 200. The uniform distribution of boron, and nitrogen
revealed the presence of ionic liquids or its decomposed products in
the tribo-chemical thin film. Furthermore, strong and uniform abun-
dance of fluorine on TBA-BF4 lubricated worn ball revealed the exis-
tence of fluorine based product on the tribo-interfaces. Load: 392 N,
rotating speed: 1200 rpm, temperature: 75 ꢁC, test duration: 1 hour.
TBA-BScB ionic liquids exhibited higher thermal stability
compared to their aliphatic analogous (TBA-BMlB and TBA-
BOxB). The higher stability of former ionic liquids was attrib-
uted to higher intermolecular interactions and rigidity to their
anionic moieties. The copper strip test results revealed non-
corrosive nature of TBA-BMdB, TBA-BScB and TBA-BMlB ionic
liquids, whereas, halogen-constituted TBA-BF4 ionic liquid
developed several corrosion pits under identical conditions.
The hydrolysis and decomposed product of TBA-BF4 ionic liquid
were found to facilitate the corrosion events, as revealed by
elemental mapping of corroded areas. All synthesized ionic
liquids are miscible with synthetic lubricant PEG 200. The
2 wt%, as an optimized dose of TBA-chelated orthoborate ionic
liquids in PEG 200, remarkably improved the friction-reducing
and antiwear properties. Among chelated orthoborate ionic
liquids, TBA-BMdB and TBA-BScB showed better lubrication
properties. Elemental mapping on the worn surfaces suggested
the formation of tribo-chemical thin lm composed of ionic
generated the physico-chemically adsorbed tribo-thin lm. This
mechanism is further supported by the layering structure of
ionic liquids conned between the two surfaces,4 which
provided low shear strength, and thus a signicant reduction in
the friction. The tribo-chemical thin lm developed by chemical
interaction of ionic liquids and their decomposed products with
steel interfaces could be an alternate to justify the tribo-
mechanism. Furthermore, elemental mapping results support
the development of such tribo-chemical thin lm. Recently,
Oganov et al. have revealed that under the high pressure boron
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We kindly acknowledge the Director IIP for his kind permission
to publish these results. Authors are thankful to Analytical
Science Division of CSIR – IIP for providing help in analyses of
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25294 | RSC Adv., 2015, 5, 25287–25294
This journal is © The Royal Society of Chemistry 2015