Ionic Liquids as Lubricant Additives: A Review - ACS Publications

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Ionic Liquids as Lubricant Additives: A Review Yan Zhou and Jun Qu* Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States ABSTRACT: In pursuit of energy efficiency and durability throughout human history, advances in lubricants have always played important roles. Ionic liquids (ILs) are roomtemperature molten salts that possess unique physicochemical properties and have shown great potential in many applications with lubrication as one of the latest. While earlier work (2001−2011) primarily explored the feasibility of using ILs as neat or base lubricants, using ILs as lubricant additives has become the new focal research topic since the breakthrough in ILs’ miscibility in nonpolar hydrocarbon oils in early 2012. This work reviews the recent advances in developing ILs as additives for lubrication with an attempt to correlate among the cationic and anionic structures, oil-solubility, and other relevant physicochemical properties, and lubricating behavior. Effects of the concentration of ILs in lubricants and the compatibility between ILs and other additives in the lubricant formulation on the tribological performance are described followed by a discussion of wear protection mechanism based on tribofilm characterization. Future research directions are suggested at the end. KEYWORDS: ionic liquids, lubricant additives, oil-solubility, tribofilm, friction, wear

1. INTRODUCTION Lubricants have been essential throughout human history with their purpose gradually shifting from “mobility” in ancient era to “durability” in modern times and then most recently to “energy efficiency.” The earliest documented use of lubricants, which described the use of water or oil to lubricate wooden planks for moving large stones,1 was found in the grave of Egyptian king Tehuti-Hetep (ca. 1650 B.C.). Lubricants had predominantly been animal and vegetable fats/oils until the modern petroleum industry was born with Drake’s well in Titusville, PA, USA, in 1859.1 Petroleum-based lubricants have since been the standard in almost all industries: transportation, manufacturing, and power generation, etc. There is a consensus that savings of 1.0−1.4% of a country’s GDP may be achieved through lubrication R&D,2 which has prompted the relentless pursuit of advances in lubricants in order to increase both energy efficiency and durability. The two major approaches are to develop better base oils and more effective additives. A typical commercial lubricant contains a blend of base oils and several categories of additives including antioxidants, detergents, dispersants, friction modifiers, antiwear and/or extreme-pressure additives, and viscosity modifiers. Ionic liquids (ILs) are room-temperature molten salts that consist of cations and anions. They possess unique physical and chemical properties such as inherent polarity (ions) for strong surface adsorption, low flammability, high thermal stability, and low sensitivity in rheological behavior to environmental changes compared to conventional oil lubricants. ILs were first explored for lubrication in 2001.3 In the next decade, many studies had utilized ILs as neat or standalone lubricants.4−9 While such an approach takes full advantage of the unique physicochemical properties of ILs, it is economically restrictive. © 2016 American Chemical Society

There were a few scattered studies exploring the feasibility of using ILs as lubricant additives4−8 but encountered a major technical barrier in that most ILs studied earlier had very low solubility in common nonpolar hydrocarbon lubricating oils (≪1%). The breakthrough came in early 2012 when a couple of oil-miscible ILs were first reported along with promising antiscuffing/antiwear functionalities.10,11 Since then, using ILs as oil additives has become the new central topic for ILs lubrication. There were several reviews on the topic of ILs lubrication in 2009−2013, but they all focused on the earlier approach of using ILs as neat or base lubricants.4−9,12,13 Therefore, there is a great need to review the recent advances in studying ILs as lubricant additives and discuss future research directions. This review starts with ILs’ oil-solubility and other important physicochemical properties with correlations to the molecular structure, followed by the tribological performance as lubricant additives with regard to the cationic and anionic chemistry, concentrations, and compatibility with other commonly used additives. The IL tribofilm formation process is then discussed based on the tribofilm characterization results in the literature to provide insights for the wear protection and friction reduction mechanisms. Future research directions are suggested at the end of this review.

2. OIL-SOLUBILITY OF ILS Common IL cations are ammonium-, phosphonium-, imidazolium-, or pyridinium-based, whereas there are a larger variety of anions, either organic or inorganic, as shown in Figure 1. In Received: September 30, 2016 Accepted: December 28, 2016 Published: December 28, 2016 3209

DOI: 10.1021/acsami.6b12489 ACS Appl. Mater. Interfaces 2017, 9, 3209−3222

Review

ACS Applied Materials & Interfaces

Figure 1. Structures and abbreviations of cations and anions of the ILs used as lubricant additives in the literature (2001−2016).

concentrations of ILs in nonpolar base oils15−22 while others used a polar base stock for a better compatibility with ILs.23,24 [N12,H,H,H][Cl] possibly was the first IL used as an oil additive, at a 1% concentration which was beyond its oil-solubility, to lubricate an aluminum−steel contact.25 Table 1 summarizes the earlier literature of using insoluble ILs as oil additives, with the cations, anions, concentrations, base stocks, and tribo-pairs listed. The more frequently tested cations were ammoniumand imidazolium-based while pyridinium-, pyrrolidinium-, thiazolium-, and thiouronium-based cations were also explored. The anions included [Cl]−, [BF4]−, [PF6]−, [Tf2N]−, [sulfate]−, and [sulfonate]−. In 2012, Qu’s group first reported two oil-soluble ILs, trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P6,6,6,14][DEHP])10 and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)alkylphosphinate ([P 6 , 6 , 6 , 1 4 ][BTMPP]).11 Interestingly, these two ILs are mutually miscible with most common nonpolar hydrocarbon oils either mineralbased or synthetic. Both the cations and anions of these two ILs have quaternary structures with relatively long hydrocarbon chains (high steric hindrance), which led to the hypothesis that reducing the charge density of ions would increase the IL’s compatibility with neutral oil molecules.10,11 A three-dimen-

general, ions and nonpolar neutral molecules are immiscible because ions are attracted by ionic forces and sometimes also hydrogen bonding while nonpolar molecules are held together by van der Waals forces. The solubility of an IL in an oil usually is judged by visual inspection (an oil−IL blend would appear cloudy if insoluble) followed by centrifugation given most ILs possess higher densities than hydrocarbon oils. Heating and/or freezing may help determine the oil−IL stability. In this review, for ILs whose oil-solubility was not explicitly stated in the literature, we attempt to categorize them based on molecular structures. Measuring the viscosity of the oil−IL blend10,11 was also used to check an IL’s solubility based on the Refutas equation.14 A simplified equation for oil−IL blends is shown as follows, where υ is the kinematic viscosity in centistokes and χoil and χIL represent the mass fractions of oil and IL in the blend, respectively. υ blend = exp(exp(χoil ln(ln(υoil + 0.8)) + χIL ln(ln(υIL + 0.8)))) − 0.8

(1)

Most ILs that were studied for lubrication before 2012 had poor solubility (≪1%) in nonpolar hydrocarbon oils. Some of these early studies used either unstable oil−IL emulsions or low 3210


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ACS Applied Materials & Interfaces Table 1. Oil-Insoluble ILs as Lubricant Additives (2001−2011) cation ammonium

anion Cl Tf2N

concn (wt %)

base stock

tribo-pair

ref

1 10 5 2.5 0.3 2.5 0.6−8 2.5 1 1 1 2, 4 1

base oil MO MO glycerol PAO + ester glycerol glycerol glycerol PAO PAO PAO MO PAO

steel on Al steel on Al piston ring on flat steel on steel steel on steel steel on steel steel on steel steel on steel steel on CrN steel on TiN steel on Cr-DLC steel on steel steel on TiN steel on CrN steel on DLC

25 16 19 29 21 29 30 29 27 28 31 32 33

sulfate

1 1 1 1 1 0.1−2 1 1 1 0.3 5 1 1 3 1 1 1 1 1 1 1

base oil base oil propylene glycol MO base oil liquid paraffin propylene glycol PEG propylene glycol PAO + ester MO PEG base oil polyamide propylene glycol base oil MO PEG propylene glycol base oil base oil

steel on Al steel on Al steel on Al steel on steel steel on SiC steel on steel steel on Al alloys steel on steel steel on Al alloys steel on steel piston ring on flat steel on steel steel on Al steel on steel steel on Al steel on Al steel on steel steel on steel steel on Al steel on Al steel on Al

34 17 18 35 20 36 37 24 37 21 19 24 34 38 18 17 35 24 18 17 17

bisimidazolium

[BF4]2/[PF6]2/[Tf2N]2

3

PEG

steel on steel

23

pyridinium pyrrolidinium

imide sulfate FAP

1 0.6−8 1 1 2, 4

base oil glycerol PAO PAO MO

steel steel steel steel steel

17 30 31 39 32

thiazolium, thiouronium

Tf2N

0.3

PAO + ester

steel on steel

sulfate sulfonate FAP

imidazolium

BF4

Tf2N

PF6

fluorosulfate

sional structure with relatively large molecules, e.g., long alkyls, has been found to improve an IL’s oil-solubility. It is believed that both the cation and anion need to be oil-soluble in order to make the IL oil-soluble. This solubility theory may explain why most ILs in early work had low oil-solubility. For instance, the 2D-structured imidazolium-based cations and small inorganic anions, such as [BF4]−, [PF6]−, or [Tf2N]−, cannot be dissolved in nonpolar oils. Later studies generally agreed with the solubility hypothesis, though it could not explain the abrupt change in oil-solubility for some “subtle” difference in the ion structure reported in the literature.11,40,41,43

dioleate

dioleate dioleate

dioleate

dioleate

on on on on on

Al steel Cr-DLC TiN/CrN/DLC steel

21

Several studies were carried out to correlate the cation structure and the oil-solubility. The [P1,4,4,4][dimethyl phosphate] with relatively short-chain alkyls was reported to be insoluble in multialkylated cyclopentanes (MACs) but miscible in water; the larger sized [P4,4,4,8][DEHP] and [P8,8,8,8][DEHP] showed limited solubility in MACs, and the largest [P8,8,8,14][DEHP] was fully miscible in MACs.42 A similar trend was observed for alkylphosphonium cations of different alkyl chain lengths when paired with [DEHP]−. In a synthetic nonpolar oil, the shorter chain [P4,4,4,4][DEHP], [P4,4,4,8][DEHP], and [P4,4,4,14][DEHP] had solubilities of 10%, 2−5%, 3212


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ACS Applied Materials & Interfaces Table 3. Ammonium-Based ILs as Lubricant Additives (2012−2016)a cation

anion

concn (wt %)

base stock

tribo-pair

thermal stability

ref

Td = 288 °C Td = 275 °C

59

N1,1,1,12 N1,1,18,18

DOSS

1

PAO

steel on steel

N1,4,8,8

BScB

1−2.5

PEG

steel on steel

N1,8,8,8

DOSS dibutyl phosphate BTMPP

1 5

PAO MO

steel on steel steel on steel

N1,8,9,10

dioctyl phosphite didodecyl phosphite

0.5−3

PAO

steel on steel

N1,12,12,12

DOSS dibutyl phosphate BTMPP

1 5

PAO MO


Td = 204 °C Td = 261 °C Td = 273 °C

59 45

N4,H,H,H

dibutyl phosphate

0.5

ester

steel on Al alloy

Td = 225 °C

64

N4,4,4,H

DEHP

1.31

base oil, EF w/o AW

T90% = 114 °C

41

N4,4,4,4

dibutyl phosphate BScB BScB BF4 bis(mandalato)borate bis(malonato)borate bis(oxalato)borate n-C17H35COO oleic acid linoleic acid

0.5 1−2.5 1−3

ester PEG PEG

steel on Al alloy steel on steel steel on steel

Td = 205 °C

64 62 65

0.5−2.5

POE

steel on steel

N6,6,6,H

DEHP

1.53

base oil, EF w/o AW

N8,8,8,H

DEHP

0.87−1.74 1.74

GTL base oil, EF w/o AW

steel on cast iron steel on cast iron

62 Td = 282 °C Td = 239 °C Td = 252 °C

59 45

63

T50% = 361 °C T50% = 376 °C T50% = 383 °C T50% = 226 °C

66

T90% = 156 °C

41

T90% = 210 °C

51 41

N8,8,8,8

BScB

1−2.5

PEG

steel on steel

62

choline

Tf2N

5

steel on steel

60

DEHP

0.1 P

steel on steel

61

dibutyl dithiophosphate

0.1 P 0.1 P 0.1 P

FF PAO FF MO MO FFO MO MO FFO MO


Td = 258 °C

47 61

steel on steel

Td = 172 °C

47

bis(2-hydroxyethyl) ammonium

adipate

1

PAO

Cu on Cu

T50% = 328 °C

67

bis(N3,3,3,5) bis(N8,8,8,5) bis(N11Cy65)

[BScB]2

2

PEG

steel on steel

T50% = 344 °C T50% = 312 °C T50% = 370 °C

68

a

Note: Numbers in ammonium Nx,x,x,x represent the length of the alkyl chain; T50% and T90% represent the temperature where weight reduces to 50% and 90%, respectively.

simulation, and the results showed that δ ≤ 25 served as a good indication of solubility for phosphonium-based ILs.40 Mineral oils (MOs) are known to accommodate additives better than PAOs, which seem to be applicable to ILs as well. Studies suggested that [P1,4,4,4]+ and [P2,4,4,4]+ were possibly soluble in

and >10%, respectively, and the solubilities of [P6,6,6,14][nC7H15COO] and [P6,6,6,14][n-C9H19COO] were both 1% in ester base oils and VOs.46 Table 2 summarizes the work of literature since 2012 on using phosphonium-based oil-soluble ILs as oil additives, categorized by the cation structure and anion chemistry. Besides phosphonium-based ILs, ammonium-based ILs have also been investigated as lubricant additives, as listed in Table 3. Similarly, longer alkyls improve the oil-solubility for ammonium cations. Fan et al. synthesized a range of ILs of [ammonium][DOSS] and determined their solubility in a PAO base oil.59 [N1,1,1,1]+, [N4,4,4,4]+, [N8,8,8,8]+, [N1,1,1,8]+, [N1,1,8,8]+, [N1,2,2,2]+, [N1,4,4,4]+, [Nb,1,1,1]+, [Nb,2,2,2]+, and [Nb,4,4,4]+ (b-benzyl) were insoluble, whereas, [N1,1,1,12]+, [N1,1,18,18]+, [N1,8,8,8]+, and [N1,12,12,12]+ had solubility ≥ 1%.59 Additionally, it was found that protic ammonium cations usually possess better oilsolubility than the aprotic versions. 41 For example, [Nx,x,x,x][DEHP] (x = 4, 6, or 8) has solubility < 1% in a synthetic base oil while [N4,4,4,H][DEHP] and

T50% = ∼440 °C T50% = ∼400 °C

77

[N6,6,6,H][DEHP] can stably dissolve into the same oil by >2% and [N8,8,8,H][DEHP] is mutually miscible with the oil. This is likely a result of hydrogen bonding between the cation and anion to form a quasi-neutral ion pair, which is more compatible with the nonpolar neutral base oil molecules. Also, ILs composed of choline cations and various anions were mixed with MO or fully formulated (FF) oils at concentrations up to 5%, but solubility was not reported.47,60,61 [Imidazolium]+ and [pyrrolidinium]+ have been paired with many anions, and the resulted ILs were only dissolvable in polar base oils, such as poly(ethylene glycol) (PEG), rapeseed oil, glycerol, castor oil, and trimethylolpropane (TMP) oleate, or in FF oils with the help from other additives. These studies are summarized in Table 4. Organophosphates currently are the most popular anions used in synthesizing oil-miscible ILs including but not limited to [DEHP]−, [diethyl phosphate]−, [dibutyl phosphate]−, and [diphenyl phosphate]−. Other common oil-soluble anions include [phosphinate]−, [phosphite]−, [sulfosuccinate]−, [adipate]2−, and [carboxylate]−. Generally, anions with longer alkyls have been reported with a better oil-solubility.11,40 In addition, borate-based anion [BScB]− has been paired with 3214


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Figure 2. TGA curves of a conventional ZDDP and 6 ILs. (Reproduced with permission from ref 47. Copyright 2016 Royal Society of Chemistry.)

[N1,4,8,8]+, [N8,8,8,8]+, [EMIM]+, [BMIM]+, or [HMIM]+ to produce ILs to be used in the polar PEG oils.62 Dicationic and dianionic ILs were also explored as oil additives.67,68 [Bis(2-hydroxyethyl)ammonium]2[adipate] was mixed with a PAO 6 cSt base oil at 1% and stirred for 30 min immediately before the test, but the solubility was not studied.67 [Bis(N3,3,3,5)][BScB]2, [bis(N8,8,8,5)][BScB]2, [bis(N11Cy65)][BScB]2, [MIm5][BScB]2, [MMIm5][BScB]2, and [BIm5][BScB]2 were reported to be soluble in the polar PEG at 2%.68

shows TGA plots of six ILs along with a conventional antiwear additive, ZDDP. The onset temperature of decomposition, Td, is the intersection of the baseline weight with the tangent of the weight−temperature curve. Except [choline][dibutyl dithiophosphate], all other five ILs exhibited higher Td than the ZDDP.47 As compared in Tables 2 and 3, phosphonium− phosphate ILs consistently showed higher thermal stability than phosphonium−carboxylate and ammonium−phosphate ILs. While the cationic alkyl chain length has little effect on the thermal stability of imidazolium- and pyridinium-based ILs,79 interestingly the thermal stability of ILs decreases as the alkyl chain length increases for [ammonium][DOSS] ILs.59 3.3. Corrosion Behavior. The corrosivity of an IL is usually experimentally characterized using exposure tests by directly applying ILs on Cu,59,66,71,76 brass,45 steel,45,76 and cast iron10,11,40 surfaces at different temperatures. While corrosion is a known problem for some halogen-containing ILs, most oilsoluble ILs are halogen-free and do not corrosively attack ferrous alloys. Specifically, phosphonium−phosphate and ammonium−phosphate ILs have shown strong passivation to the iron surface in electrochemical measurements.10,41,43

3. PHYSICOCHEMICAL PROPERTIES OF OIL-SOLUBLE ILS 3.1. Density and Viscosity. While the density of ILs is in a relatively narrow range, 1.0−1.6 g/mm3,78 the viscosity varies substantially. Both the cationic and anionic structures significantly affect the rheological behavior. Larger and symmetric cations are reported to increase viscosity, which has been observed for phosphonium-43,44 and ammoniumbased ILs,41 which is attributed to longer alkyl chains increasing the interionic interactions and higher symmetry leading to closer packing. Such a trend was not as obvious in other studies. In one case, paired with the same ammonium cations, phosphite anions with alkyl chains of 8 and 12 carbons showed little difference in viscosity,63 and in another study of carboxylate ILs, no clear correlation was observed between the viscosity and the anionic alkyl chain length or structure.40 In addition to the molecular size and symmetry, protic ammonium-based ILs were found to be less viscous compared with aprotic ones,41 which may be due to the strong H-bonding between the protic cations and their counterions. For anions, phosphate and phosphinate ILs seem to be more viscous than carboxylate ILs.11,40 3.2. Thermal Stability. Thermogravimetric analysis (TGA) is normally used to determine the thermal stability of the oilsoluble ILs in N2,10,11,47,54,61,65,66,68 inert gases,45,60 O2,54 and air.40,43,46,48,59,64,67,71,74 Air is a more realistic environment for most lubrication applications where oxidation is inevitable. Many ILs are more thermally stable than hydrocarbon base oils which normally start decomposing around 250 °C. Figure 2

4. TRIBOLOGICAL FUNCTIONALITIES OF ILS AS OIL ADDITIVES Oil-soluble ILs, when used as lubricant additives, have repeatedly exhibited effective wear and friction reductions in tribological bench tests10,11,40,41,43 and demonstrated improved engine mechanical efficiency in engine dynamometer tests.58 The lubricating performance has shown a strong correlation with the IL’s chemistry, concentration, compatibility with other oil additives, material compositions of the contact surfaces, and rubbing conditions. 4.1. IL Chemistry. Phosphorus-containing ILs have been the focus of many studies which repeatedly demonstrated effective friction and wear reductions.10,11,40,41,43 While some results simply showed improvement over the base oils,45,56 others had direct comparisons with commercial antiwear additives. In a study to understand the impact of anions,40 three groups of phosphonium-based ILs were blended into a PAO base oil at the same 800 ppm P content. These blends 3215


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Figure 3. Wear and friction performance of ILs. (A) Antiwear performance of four ILs, [P6,6,6,14][DEHP], [P8,8,8,8][DEHP], [N8,8,8,H][DEHP], and [P6,6,6,14][BTMPP], and their combinations with ZDDP compared with that of ZDDP when added into a GTL base oil at 800 ppm P content. (Adapted with permission from ref 51. Copyright 2015 John Wiley and Sons.) Results suggest synergistic effects in wear protection between phosphonium−phosphate ILs and ZDDP. (B) [P6,6,6,14][BTMPP] and [P6,6,6,14][DEHP] benchmarked against a ZDDP for wear protection in a YUBASE base oil at various P contents. (Reproduced with permission from ref 54. Copyright 2015 Elsevier.) (C) Multiple Stribeck curve scans of experimentally formulated (EF) engine oils showing effective friction reduction for using [P8,8,8,8][DEHP] + ZDDP compared with using ZDDP alone as the AW additives (800 ppm P in both oils). (Adapted with permission from ref 58. Copyright 2015 Barnhill, Gao, Kheireddin, Papke, Luo, West and Qu (Open Access)).

cations [N4,4,4,4]+, [N8,8,8,8]+, [N1,4,8,8]+, [EMIM]+, [BMIM]+, and [HMIM]+. At a 2% treat rate in PEG, the wear in the steel−steel four ball test was significantly reduced by the ILs. On the other hand, for the same cation [BMIM]+, [BScB]− generated a lower friction than [FAP] − or [dibutyl phosphate]−, while [dibutyl phosphate]− had the most wear reduction.62 In the following study, wear and friction were significantly reduced when two [BScB]− anions were paired with dicationic [bis(imidazolium)]2+ and [bis(ammonium)]2+.68 For the same cation [N4,4,4,4]+, anions [bis(mandelato)borate]−, [bis(malonato)borate]−, and [bis(oxalato)borate]− were compared with [BScB]− and [BF4]−. Under similar testing conditions, [N4,4,4,4][bis(mandelato)borate] outperformed other ILs by 50% or more in wear reduction.65 [N4,4,4,4]+ was paired with fatty acids (stearic acid, oleic acid, and linoleic acid with unsaturation levels 0, 1, and 2, respectively) and added to polyolester (POE) at 2%.66 Friction and wear were lowered in all cases while the unsaturation level showed little effect on the performance. The wear protection of these ILs was inferior to ZDDP, but the friction coefficient was much lower.66 It has recently been reported that some ILs could be synthesized in situ in oil.76 Methoxy tris-ethoxy methylene benzotriazole (BTAG3) only become soluble in MACs when an equal molar of lithium salt (LiBF4, LiPF6, LiSO3CF3, and LiTf2N) was added at the same time, indicating onset formation of the ILs. The solubility of Li-containing ILs in MACs were: [Li(BTAG3)][BF4] < 0.5%, [Li(BTAG3)][PF6] < 0.5%, [Li(BTAG3)][SO3CF3] < 2%, and [Li(BTAG3)][Tf2N] > 5%. When tested in steel−steel sliding under 300 N at room temperature, all ILs significantly reduced wear.76 Certain ILs may serve as both a lubricant additive and a catalyst

were then tested by using a steel−steel ball-on-flat reciprocating sliding contact at 100 °C. All three groups outperformed a commercial ashless antiwear additive regarding wear reduction. The effectiveness, from high to low, was phosphonium− phosphate, phosphonium−carboxylate, and phosphonium− sulfonate.40 When compared with ZDDP, studies suggested that [P8,8,8,8][DEHP], [N 8,8,8,H][DEHP], and [P6,6,6,14][BTMPP] provided similar or better surface protection for both steel−steel and steel−iron contacts, as shown in Figure 3A,B.51,54 [P6,6,6,14][DEHP] seemed to perform slightly worse than ZDDP in wear reduction (Figure 3) but demonstrated stronger scuffing prevention.48 In another study of six Pcontaining ILs, ([choline][DEHP], [choline][dibutyl dithiophosphate], [P6,6,6,14][BTMPP], [P6,6,6,14][Tf2N], [P6,6,6,14][dimethyl phosphate], and [P6,6,6,14][diethyl phosphate]), in an MO at 0.1% P content, all ILs showed wear reduction to some extent compared with the base oil, but only [choline][DEHP] and [P6,6,6,14][Tf2N] reduced wear as much as ZDDP.47 Imidazolium-, pyrrolidinium-, and phosphoniumbased ILs were used as additives in ester base oils and a VO, but only [P2,4,4,4][diethyl phosphate] and [P6,6,6,14][FAP] showed a stable >1% oil-solubility. Under low loads (14 and 22 N) for a steel−steel ball-on-flat contact, these two ILs showed comparable wear protection to ZDDP at the same concentration 1%.46 It also was reported that the maximum nonseizure load in the four-ball test was slightly higher for [N1,8,9,10][dioctyl phosphite] and [N1,8,9,10][didodecyl phosphite] than that for ZDDP when added to PAO at a 1% treat rate.63 Boron-containing and ammonium-containing ILs are also of great interest. By using the same chelated orthoborate anion, [BScB]−, Gusain et al. synthesized a series of ILs with the 3216


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content of 800 ppm.51 This IL + ZDDP combination resulted in 30% friction and 70% wear reductions when compared with using either the IL or ZDDP alone, as shown by the wear results in Figure 3A. Anion exchange between the IL and ZDDP was discovered to produce a new compound that strongly preferred to reside at the interface between the lubricant and other media, which is believed to be responsible for the superior tribological performance. Synergy with ZDDP, however, was not observed with phosphonium−phosphinate or ammonium−phosphate ILs, even though they share either the same cation or the same anion with phosphonium−phosphate ILs. This implies that both the cation and anion play critical roles. Such a synergy was further demonstrated in a FF experimental engine oil with the additive package specifically designed for the IL + ZDDP pair, which reduced friction in boundary and mixed lubrication regimes in bench tests (Figure 3C) and improved fuel economy in full-size engine dynamometer tests.58 Another study applied 0.6% antioxidant together with 0.25% commercial amine phosphate, [P1,4,4,4][diphenyl phosphate], or [P6,6,6,14][diphenyl phosphate] in various base oils (VO, TMP, and POE), and [P1,4,4,4][diphenyl phosphate] + antioxidant tolerated seizure up to 350 N while the amine phosphate + antioxidant tolerated only until 200 N.44 In another study, 6% [P6,6,6,14][BTMPP] or [P6,6,6,14][DEHP] was added to fresh, used, and in-service FF oils to lubricate Crcoated piston ring segments on cast iron flats. The addition of ILs improved the wear protection for the used FF oil and had a detrimental effect on both fresh and in-service FF oils, suggesting compatibility issues between the ILs and that particular additive package.50

simultaneously. In an interesting study, 3−7% [imidazolium][DEHP] was used to enhance the esterification of TMP with oleic acid, and the resulted IL-containing TMP oleate reduced friction in lubricating a steel−steel contact.74 4.2. Effects of ILs’ Concentrations. Based on the reports in the literature, using a higher concentration of ILs may not necessarily improve the tribological performanceall depending on the IL chemistry and the tribosystem. In the study of [P6,6,6,14][DEHP] and [P6,6,6,14][BTMPP] additized Yubase oils, the wear was reduced as the P content increasing from 200 to 800 ppm, as shown in Figure 3B.54 For [crown diimidazolium][DEHP], the wear decreased by increasing the IL concentration from 1 to 7% in PEG.69 In a study on a series of [N1,8,8,8][DOSS], the wear reduction was enhanced by increasing the IL treat rate from 0.5 to 3% in PAO.59 In contrast, some studies showed saturation of the antiwear performance of certain ILs even at relatively low concentrations. For example, when increasing [N8,8,8,8][BScB] from 1 to 2.5%, appreciable change in wear rate was not observed.62 Similarly, the wear and friction were not further reduced by increasing the treat rate from 0.5% to 3% for [N1,8,9,10][dioctyl phosphite], [N1,8,9,10][didodecyl phosphite], or ZDDP.63 Furthermore, a higher IL concentration could worsen the performance. Imidazolium-based ILs with alkyl sulfate showed 50% wear reduction at 0.63% addition in glycerol, but more material loss was observed at 8% concentration.72 In another case of rapeseed oil lubrication, surface protection decreased with the treat rate of [crown diimidazolium][DEHP] increasing from 1 to 3%.70 4.3. Lubricating Nonferrous Materials. While most studies focused on steel−steel or steel−cast iron contacts, others explored the feasibility of using ILs as oil additives to lubricate nonferrous alloys and ceramics. In one case of lubricating aluminum, wear was effectively reduced by using up to 20% phosphonium ILs in the oil,56 though the increased blend viscosity might be a factor. [N4,H,H,H][dibutyl phosphate] or [N4,4,4,4][dibutyl phosphate] in an ester oil reduced the wear of 2024 Al alloy against a steel ball at various speeds and loads. 64 For a Cu−Cu contact, [bis(2-hydroxyethyl)ammonium]2[adipate] reduced the disc wear by 34% at a 1% treat rate in PAO.67 In a Si3N4−steel contact, adding 1% [P6,6,6,14][DEHP] into a base oil reduced the steel flat wear effectively but increased the ceramic ball wear by preventing forming of a protective material transfer layer on the ceramic surface.49 ILs have also been studied as additives in lubricating nonmetallic coatings. The addition of 1% [ammonium][FAP] or [pyrrolidinium][FAP] in PAO reduced the wear of a CrDLC coating with the effectiveness similar to that of ZDDP.31 In contrast, [P6,6,6,14][DEHP] and ZDDP caused a significant increase in the wear rate of the steel ball when rubbing against DLC coatings. Such detrimental effects only existed when phosphate anions and carbon coatings were used together, which suggested carbon catalyzed an undesirable high tribochemical reaction rate between the phosphate anions and the steel surface.52 4.4. Compatibility with Other Additives. There are many additives that coexist in commercial lubricants; hence, it is important to understand the compatibility between a candidate IL and other compounds in the package. Interesting synergistic effects have been observed when using phosphonium−phosphate ILs ([P8,8,8,8][DEHP] and [P6,6,6,14][DEHP]) and a ZDDP together in a base oil at a combined P

5. ILS AS ADDITIVES IN WATER AND GREASE ILs have also been added to aqueous solutions and greases. For Si3N4−Si3N4 contact, [phosphazene][Tf2N] and [EMIM][BF4] were used in water-based lubricants as additives at concentrations of 0.25% and 2%, respectively, and significantly reduced the friction in the running-in period.80,81 [BMIM][PF6] showed its potential in the following test conditions: 2% for Si3N4− Si3N4 contact in water;81 2−14.4% for steel−steel contact in a surfactant−water solvent.82 [Imidazolium][BF4] was tested at various concentrations in water for multiple contacts: SiO2 on Si3N4, poly-Si on Si3N4, and Si3N4 on Si3N4.83 Fluoride-based ILs26 and lithium-based ILs77 were investigated as candidate additives for greases and showed potential benefits in both friction and wear reductions. Protic [2-hydroxyethylammonium]2[succinate] was added at 1% in water to shorten the running-in period and to reduce the friction and wear.84 6. TRIBOFILM CHARACTERIZATION AND WEAR PROTECTION MECHANISM It has been repeatedly demonstrated that ILs, when used as lubricant additives, tend to form a protective tribofilm on the contact area upon rubbing. While the exact mechanism is not yet known, it is thought that ILs and/or their decomposition products tribochemically react with the contact surfaces and/or wear debris at the lubricating interface to grow a compliant and protective tribofilm. A significant amount of surface characterizations has been conducted to confirm the existence of IL tribofilms and to reveal their morphology, thickness, nanostructure, chemical composition, and mechanical properties. Optical and electron microscopes have widely been used to examine the worn surface morphology and sometimes the 3217


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Figure 4. (A) TEM imaging and EDS element mapping on a cross-section of the near surface zone of a cast iron worn surface lubricated by PAO + 5% [P6,6,6,14][DEHP]. (Reproduced with permission from ref 10. Copyright 2012 American Chemical Society.) (B) Red spectra represent binding energies for the initial 2−3 nm of the surface including the top of the tribofilm and any lubricant residue. Green spectra were produced after 60 s of ion sputtering of the tribofilm to eliminate the surface contaminants. (Reproduced with permission from ref 43. Copyright 2014 American Chemical Society.) The tribofilm was formed on a cast iron surface by base oil + 1% [P8,8,8,8][DEHP]. (C) XANES results show slightly increased a/c (a′/c′ for ZDDP) ratio of phosphorus L edge from the TEY spectra to the FY spectra for ZDDP, [choline][DEHP], [P6,6,6,14][BTMPP], and [P6,6,6,14][Tf2N], which indicates polymerization with the phosphates changing from short chains to medium chains. (Reproduced with permission from ref 47. Copyright 2016 Royal Society of Chemistry.).

suggest iron phosphates and iron oxides as the main compounds in the tribofilm formed by base oil containing 1% [P8,8,8,8][DEHP] on a cast iron surface.43 In Figure 4C, XANES analyses of IL tribofilms showed a slightly increased a/c ratio of phosphorus L edge from the TEY spectra (signals from the top 5 nm tribofilm surface) to the FY spectra (signals collected from as deep as 60 nm into the tribofilm), indicating polymerization with the phosphates changing from short chains to medium chains.47,61 [choline][DEHP] (0.23 to 0.49) and [P6,6,6,14][Tf2N] (0.17 to 0.32) were the best performers in wear reduction and also had big a/c increases.47 Unlike phosphorus, which is always seen in the tribofilms formed by P-containing IL anions, boron has not been convincingly detected on the worn surfaces lubricated by Bcontaining IL anions.62,65 Aided by argon ion sputtering, the compositional transition throughout the thickness of a tribofilm could also be obtained by XPS40,41,43,51−53 or AES,53 and it is another way to determine the tribofilm thickness.

tribofilm coverage. The lateral force on the adsorbed IL molecules on a surface could be determined by using AFM,85 which has also been used in surface morphology characterization86 and growth mechanism determination of ZDDP tribofilms.87 FIB enables cross-sectional TEM/STEM imaging that could be coupled with EDS/EELS elemental mapping to reveal the thickness, nanostructure, and elemental information on the tribofilm and its interface with the substrate. One example, as shown in Figure 4A,10 with the TEM image clearly shows a 120−180 nm thick amorphous tribofilm formed on a cast iron worn surface lubricated by a PAO base oil containing a phosphonium−phosphate IL. The EDS elemental maps of the tribofilm show significant contents of iron, oxygen, and phosphorus, thus indicating contributions from both the iron and the IL during the tribofilm formation process. Spectroscopy techniques, such as EDS, 10 EELS, 52 XPS,40,43,49,52,53 AES,52,53 XANES,47,61,63 SIMS,88 and APT,89 provide insights into the chemical compositions of IL tribofilms. For example, the XPS core spectra in Figure 4B 3218


Review

ACS Applied Materials & Interfaces Nanoindentation has been used to measure mechanical properties of IL tribofilms. The hardness increases from top tribofilm surface to the interface with the substrate which supports the polymerization observation in XANES.47,61 Nanoindentation of tribofilms formed by phosphate ILs and their combinations with ZDDP revealed that the tribofilm hardness has little impact on the tribological behavior.51,90 On the other hand, a lower resistance-to-plastic-deformation (P/S2 ratio) tends to reduce friction and improve surface protection,51,90 which is opposite to what was observed on bulk materials91 and is likely attributable to the dynamic, selfhealing characteristics of tribofilms. Based on the tribofilm characterization results in the literature, a hypothetical IL tribofilm formation process is proposed here. First, the initial layer of the tribofilm is formed as a result of tribochemical reactions between the metallic contact surface and IL anions and their decomposition products as well as oxygen. Such a layer serves as the foundation for the tribofilm growth with good bonding to the substrate, but, in the same time, it also could become a barrier between the IL and the metal surface for further reactions. There are three sources for supplying the metal cations to react with the IL to continue the tribofilm self-healing and growth: (a) wear removes the tribofilm and exposes fresh metal surface areas; (b) wear debris acts as an alternative metal supply and the nucleated tribochemical reaction products deposit onto the surface; (c) metallic diffusion from metal surface through the tribofilm barrier. As supportive evidence, IL-induced tribofilms were detected on the contact area of boride coatings that rubbed against a steel ball.92 The presence of iron compounds in the tribofilm on the nonmetallic, iron-free coating surface unambiguously indicates the involvement of the wear debris (from the steel ball) in the tribofilm formation. Unreacted or oxidized wear debris particles may be enclosed in the tribofilm via mechanical mixing. Similar to ZDDP tribofilm formation, phosphate cross-linking and phosphate−oxide bonding could be critical for the IL tribofilms’ integrity, which requires further investigation.

Although the proposed charge−dilution oil-solubility concept for ILs has a general agreement with the experimental observations, this hypothesis is imperfect because it cannot explain the abrupt change in oil-solubility for some “subtle” difference in the ion structure reported in the literature.11,40,41,43 Tailoring the IL molecular structure is foreseen as the key for optimizing the physicochemical and tribological properties. Most studies thus far blended ILs into base oils without an additive package, though a few involved fully formulated engine oils. Except for limited investigation into combining ILs with ZDDP, little is known about the compatibility between ILs and other surface-adsorbing additives such as detergents, dispersants, and friction modifiers. This should be an important future research topic for formulating ILs into lubricant packages for actual applications. ILs’ longterm stability in a lubricant formulation and their interactions with lubricant decomposition products and contaminants, e.g., soot, acids, fuels, water, and wear particles, also needs to be studied. Another area requiring future study is the compatibility between the IL chemistry and the materials pair in contact. The recent report52 of increased steel ball wear when rubbing against DLC coatings in lubricants containing phosphate ILs is a good example. Last but not least, examining the toxicity and biodegradability of candidate ILs should be included in the future research portfolio.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jun Qu: 0000-0001-9466-3179 Funding

This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paidup, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

7. CONCLUSIONS AND OUTLOOK In the last round of reviews for ILs lubrication, the major concerns included corrosion, thermal oxidation, oil-miscibility, toxicity, and cost.4−9,12,13 The recent successful development of noncorrosive, thermally stable, and oil-soluble ILs has largely addressed these technical barriers. Since the introduction of oilmiscible ILs in early 2012, the mainstream research of ILinvolved lubrication has been shifted from using ILs as neat or base lubricants to using them as lubricant additives. Various oilsoluble ILs have been developed in the past 5 years, and some key physicochemical properties have been characterized and correlated to their cation and anion structures. The addition of relatively low concentrations of ILs into common lubricating oils has repeatedly shown effective wear and friction reduction in both laboratory-scale tribological evaluations and full-sized engine dynamometer tests, suggesting potential feasibility and affordability for actual industrial applications. Furthermore, the latest discovery of unique synergistic effects between certain ILs and the conventional ZDDP opened the door for a new research direction. Sophisticated surface characterizations have revealed a wealth of morphological, nanostructural, and compositional information on IL tribofilms and provided fundamental insights into the wear protection mechanism.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research was sponsored by the Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE).

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3219

ACRONYM AES = auger electron spectroscopy AFM = atomic force microscopy APT = atom probe tomography AW = anti-wear BF4 = tetrafluoroborate BIm5 = 1,1′-(pentane-1,5-diyl)bis(3-butylimidazolium) BMIM = 1-butyl-3-methylimidazolium BMP = 1-butyl-1-methylpyrrolidinium BScB = bis(salicylato)borate BTAG3 = methoxy tris-ethoxy methylene benzotriazole BTMPP/BMPP = bis(2,4,4-trimethylpentyl) phosphinate COF = coefficient of friction DEHP = bis(2-ethylhexyl)phosphate DOI: 10.1021/acsami.6b12489 ACS Appl. Mater. Interfaces 2017, 9, 3209−3222

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as a Lubricant Additive. ACS Appl. Mater. Interfaces 2012, 4, 997− 1002. (11) Yu, B.; Bansal, D. G.; Qu, J.; Sun, X. Q.; Luo, H. M.; Dai, S.; Blau, P. J.; Bunting, B. G.; Mordukhovich, G.; Smolenski, D. J. OilMiscible and Non-Corrosive Phosphonium-Based Ionic Liquids as Candidate Lubricant Additives. Wear 2012, 289, 58−64. (12) Bermudez, M. D.; Jimenez, A. E.; Sanes, J.; Carrion, F. J. Ionic Liquids as Advanced Lubricant Fluids. Molecules 2009, 14, 2888−2908. (13) Xiao, H. Ionic Liquid Lubricants: Basics and Applications. Tribol. Trans. 2017, 60, 20−30. (14) Maples, R. E. Petroleum Refinery Process Economics, 2nd ed.; PennWell: Tulsa, OK, USA, 2000. (15) Qu, J.; Truhan, J. J., Jr.; Dai, S.; Luo, H.; Blau, P. J. Lubricants or Lubricant Additives Composed of Ionic Liquids Containing Ammonium Cations. U.S. Patent 7,754,664, Jul. 13, 2010. (16) Qu, J.; Truhan, J. J.; Dai, S.; Luo, H.; Blau, P. J. Ionic Liquids with Ammonium Cations as Lubricants or Additives. Tribol. Lett. 2006, 22, 207−214. (17) Jimenez, A. E.; Bermudez, M. D.; Carrion, F. J.; MartinezNicolas, G. Room Temperature Ionic Liquids as Lubricant Additives in Steel-Aluminium Contacts: Influence of Sliding Velocity, Normal Load and Temperature. Wear 2006, 261, 347−359. (18) Jimenez, A. E.; Bermudez, M. D. Imidazolium Ionic Liquids as Additives of the Synthetic Ester Propylene Glycol Dioleate in Aluminium-Steel Lubrication. Wear 2008, 265, 787−798. (19) Qu, J.; Blau, P. J.; Dai, S.; Luo, H.; Meyer, H. M. Ionic Liquids as Novel Lubricants and Additives for Diesel Engine Applications. Tribol. Lett. 2009, 35, 181−189. (20) Mistry, K.; Fox, M. F.; Priest, M. Lubrication of an Electroplated Nickel Matrix Silicon Carbide Coated Eutectic Aluminium-Silicon Alloy Automotive Cylinder Bore with an Ionic Liquid as a Lubricant Additive. Proc. Inst. Mech. Eng., Part J 2009, 223, 563−569. (21) Schneider, A.; Brenner, J.; Tomastik, C.; Franek, F. Capacity of Selected Ionic Liquids as Alternative EP/AW Additive. Lubr. Sci. 2010, 22, 215−223. (22) Lu, R.; Nanao, H.; Kobayashi, K.; Kubo, T.; Mori, S. Effect of Lubricant Additives on Tribochemical Decomposition of Hydrocarbon Oil on Nascent Steel Surfaces. J. Jpn. Pet. Inst. 2010, 53, 55−60. (23) Yao, M. H.; Liang, Y. M.; Xia, Y. Q.; Zhou, F. Bisimidazolium Ionic Liquids as the High-Performance Antiwear Additives in Poly(Ethylene Glycol) for Steel-Steel Contacts. ACS Appl. Mater. Interfaces 2009, 1, 467−471. (24) Cai, M. R.; Liang, Y. M.; Yao, M. H.; Xia, Y. Q.; Zhou, F.; Liu, W. M. Imidazolium Ionic Liquids as Antiwear and Antioxidant Additive in Poly(Ethylene Glycol) for Steel/Steel Contacts. ACS Appl. Mater. Interfaces 2010, 2, 870−876. (25) Iglesias, P.; Bermudez, M. D.; Carrion, F. J.; Martinez-Nicolas, G. Friction and Wear of Aluminium-Steel Contacts Lubricated with Ordered Fluids-Neutral and Ionic Liquid Crystals as Oil Additives. Wear 2004, 256, 386−392. (26) Cai, M. R.; Zhao, Z.; Liang, Y. M.; Zhou, F.; Liu, W. M. Alkyl Imidazolium Ionic Liquids as Friction Reduction and Anti-Wear Additive in Polyurea Grease for Steel/Steel Contacts. Tribol. Lett. 2010, 40, 215−224. (27) Blanco, D.; Battez, A. H.; Viesca, J. L.; Gonzalez, R.; FernandezGonzalez, A. Lubrication of CrN Coating with Ethyl-Dimethyl-2Methoxyethylammonium Tris(Pentafluoroethyl)Trifluorophosphate Ionic Liquid as Additive to PAO 6. Tribol. Lett. 2011, 41, 295−302. (28) Blanco, D.; Gonzalez, R.; Hernandez Battez, A.; Viesca, J. L.; Fernandez-Gonzalez, A. Use of Ethyl-Dimethyl-2-Methoxyethylammonium Tris(Pentafluoroethyl)Trifluorophosphate as Base Oil Additive in the Lubrication of TiN PVD Coating. Tribol. Int. 2011, 44, 645−650. (29) Kronberger, M.; Pejakovic, V.; Gabler, C.; Kalin, M. How Anion and Cation Species Influence the Tribology of a Green Lubricant Based on Ionic Liquids. Proc. Inst. Mech. Eng., Part J 2012, 226, 933− 951. (30) Pejakovic, V.; Kronberger, M.; Mahrova, M.; Vilas, M.; Tojo, E.; Kalin, M. Pyrrolidinium Sulfate and Ammonium Sulfate Ionic Liquids

DLC = diamond-like carbon DOSS = dioctyl sulfosuccinate EDS = energy-dispersive spectroscopy EELS = electron energy loss spectroscopy EF = experimentally formulated EMIM = 1-ethyl-3-methylimidazolium FAP = tris(pentafluoroethyl)trifluorophosphate FF = fully formulated FIB = focused ion beam GTL = gas to liquid HMIM = 1-hexyl-3-methylimidazolium i(C8)2PS2 = bis(2,4,4-trimethylpentyl) dithiophosphinate IL = ionic liquid MAC = multialkylated cyclopentane MIm5 = 1,1′-(pentane-1,5-diyl)bis(3-methylimidazolium) MMIm5 = 1,1′-(pentane-1,5-diyl)bis(2,3-dimethylimidazolium) MO = mineral oil N 11 Cy 65 = pentane-1,5-diylbis(dimethylcyclohexylammonium) PAO = poly-α-olefin PE = polyester PEG = poly(ethylene glycol) PF6 = hexafluorophosphate POE = polyolester SEM = scanning electron microscopy SIMS = secondary ion mass spectrometry SSi = 3-(trimethylsilyl)propan-1-sulfonate STEM = scanning transmission electron microscopy TEM = transmission electron microscopy Tf2N/NTf2/NTf/TFSI = bis(trifluoromethanesulfonyl)imide TMP = trimethylolpropane VO = vegetable oil XANES = X-ray absorption near edge structure spectroscopy XPS = X-ray photoelectron spectroscopy ZDDP = zinc dialkyldithiophosphate

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(48) Qu, J.; Luo, H. M.; Chi, M. F.; Ma, C.; Blau, P. J.; Dai, S.; Viola, M. B. Comparison of an Oil-Miscible Ionic Liquid and ZDDP as a Lubricant Anti-Wear Additive. Tribol. Int. 2014, 71, 88−97. (49) Cai, Z. B.; Meyer, H. M.; Ma, C.; Chi, M. F.; Luo, H. M.; Qu, J. Comparison of the Tribological Behavior of Steel-Steel and Si3N4-Steel Contacts in Lubricants with ZDDP or Ionic Liquid. Wear 2014, 319, 172−183. (50) Anand, M.; Hadfield, M.; Viesca, J. L.; Thomas, B.; Hernandez Battez, A.; Austen, S. Ionic Liquids as Tribological Performance Improving Additive for in-Service and Used Fully-Formulated Diesel Engine Lubricants. Wear 2015, 334-335, 67−74. (51) Qu, J.; Barnhill, W. C.; Luo, H. M.; Meyer, H. M.; Leonard, D. N.; Landauer, A. K.; Kheireddin, B.; Gao, H.; Papke, B. L.; Dai, S. Synergistic Effects between Phosphonium-Alkylphosphate Ionic Liquids and Zinc Dialkyldithiophosphate (ZDDP) as Lubricant Additives. Adv. Mater. 2015, 27, 4767−4774. (52) Zhou, Y.; Leonard, D. N.; Meyer, H. M.; Luo, H. M.; Qu, J. Does the Use of Diamond-Like Carbon Coating and Organophosphate Lubricant Additive Together Cause Excessive Tribochemical Material Removal? Adv. Mater. Interfaces 2015, 2, 1500213. (53) Qu, J.; Meyer, H. M., III; Cai, Z.-B.; Ma, C.; Luo, H. Characterization of ZDDP and Ionic Liquid Tribofilms on NonMetallic Coatings Providing Insights of Tribofilm Formation Mechanisms. Wear 2015, 332-333, 1273−1285. (54) González, R.; Bartolomé, M.; Blanco, D.; Viesca, J. L.; Fernández-González, A.; Battez, A. H. Effectiveness of Phosphonium Cation-Based Ionic Liquids as Lubricant Additive. Tribol. Int. 2016, 98, 82−93. (55) Li, H.; Somers, A. E.; Howlett, P. C.; Rutland, M. W.; Forsyth, M.; Atkin, R. Addition of Low Concentrations of an Ionic Liquid to a Base Oil Reduces Friction over Multiple Length Scales: A Combined Nano- and Macrotribology Investigation. Phys. Chem. Chem. Phys. 2016, 18, 6541−6547. (56) Somers, A. E.; Khemchandani, B.; Howlett, P. C.; Sun, J. Z.; MacFarlane, D. R.; Forsyth, M. Ionic Liquids as Antiwear Additives in Base Oils: Influence of Structure on Miscibility and Antiwear Performance for Steel on Aluminum. ACS Appl. Mater. Interfaces 2013, 5, 11544−11553. (57) Grace, J.; Vysochanska, S.; Lodge, J.; Iglesias, P. Ionic Liquids as Additives of Coffee Bean Oil in Steel-Steel Contacts. Lubricants 2015, 3, 637. (58) Barnhill, W. C.; Gao, H.; Kheireddin, B. A.; Papke, B. L.; Luo, H.; West, B. H.; Qu, J. Tribological Bench and Engine Dynamometer Tests of a Low Viscosity SAE 0W-16 Engine Oil Using a Combination of Ionic Liquid and ZDDP as Anti-Wear Additives. Front. Mech. Eng. 2015, 1, 12. (59) Fan, M. J.; Yang, D. S.; Wang, X. L.; Liu, W. M.; Fu, H. Z. DOSS- Based Qails: As Both Neat Lubricants and Lubricant Additives with Excellent Tribological Properties and Good Detergency. Ind. Eng. Chem. Res. 2014, 53, 17952−17960. (60) Monge, R.; Gonzalez, R.; Hernandez Battez, A.; FernandezGonzalez, A.; Viesca, J. L.; Garcia, A.; Hadfield, M. Ionic Liquids as an Additive in Fully Formulated Wind Turbine Gearbox Oils. Wear 2015, 328-329, 50−63. (61) Sharma, V.; Gabler, C.; Doerr, N.; Aswath, P. B. Mechanism of Tribofilm Formation with P and S Containing Ionic Liquids. Tribol. Int. 2015, 92, 353−364. (62) Gusain, R.; Singh, R.; Sivakumar, K. L. N.; Khatri, O. P. Halogen-Free Imidazolium/Ammonium-Bis(Salicylato)Borate Ionic Liquids as High Performance Lubricant Additives. RSC Adv. 2014, 4, 1293−1301. (63) Fu, X. S.; Sun, L. G.; Zhou, X. G.; Li, Z. P.; Ren, T. H. Tribological Study of Oil-Miscible Quaternary Ammonium Phosphites Ionic Liquids as Lubricant Additives in PAO. Tribol. Lett. 2015, 60, 23. (64) Qiao, D.; Wang, H. Z.; Feng, D. P. Tribological Performance and Mechanism of Phosphate Ionic Liquids as Additives in Three Base Oils for Steel-on-Aluminum Contact. Tribol. Lett. 2014, 55, 517−531. 3221


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Ionic Liquids as Lubricant Additives: A Review - ACS Publications

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