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Article

Tribological Properties of Borate-Based Protic Ionic Liquids as Neat Lubricants and Biolubricant Additives for Steel-Steel Contact

by
Alfonso Sierra
1,
Michael G. Coleman
2 and
Patricia Iglesias
1,*
1
Department of Mechanical Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, NY 14623-5603, USA
2
School of Chemistry and Materials Science, College of Science, Rochester Institute of Technology, Rochester, NY 14623-5604, USA
*
Author to whom correspondence should be addressed.
Lubricants 2022, 10(10), 269; https://doi.org/10.3390/lubricants10100269
Submission received: 29 September 2022 / Revised: 13 October 2022 / Accepted: 16 October 2022 / Published: 19 October 2022
(This article belongs to the Special Issue New Developments and Future Trends of Ionic Liquids as Lubricants)
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Abstract

:
A series of four novel halogen-free borate-based protic ionic liquids were synthesized with identical organoborate anions but dissimilar ammonium cations, to allow systematic discussion of the effects of cation alkyl chain group length on their physicochemical and tribological properties. The ionic liquids (ILs) studied showed up to a 30% friction reduction compared to a biolubricant and even displayed negligible wear when tested as neat lubricants. Blends of 1 wt.% ionic liquid and biolubricant were also investigated, leading up to maximum friction and wear reductions of 25% and 96%, respectively, compared to the base oil. Interestingly, the ionic liquid that performed least effectively as a neat lubricant due to the solidification of the fluid achieved the best tribological response as a lubricant additive. The tribological improvements were attributed to the formation of a self-assembled adsorbed layer that separated the frictional surfaces. This theory was supported by rheological studies and elemental analysis.

1. Introduction

Tribological interactions are the source of about a quarter of the world’s total energy consumption. Most of these energy losses are due to friction, and a smaller fraction is due to wear as a consequence of the replacement of mechanical parts. This circumstance not only has a significant economic impact but contributes to the detriment of the environment in the form of considerable CO2 emissions [1]. Nevertheless, researchers have estimated that advanced lubrication could substantially mitigate these effects by up to 40% as the years go by [2].
However, it is estimated that around half of the lubricant sold worldwide ends up in the environment. Nowadays, most of them are mineral-oil-based, thus highly ecotoxic and poorly biodegradable, representing a substantial threat to the environment [3]. On the other hand, biolubricants, currently mainly based on vegetable oils, completely degrade in a short time, hence serving as a renewable and sustainable alternative [4,5,6,7].
Ionic liquids (ILs) have been highlighted among the research community not only as environmentally friendly neat lubricants [8,9,10] but also as additives to traditional lubricants and biolubricants [11,12,13]. ILs are substances composed of cations and anions, which melt at or below 100 °C. Even though the existence of ILs was first reported in 1914 by Paul Walden [14], it was not until the early 2000s that the application of these fluids was first introduced in tribology [15]. Since then, due to their outstanding physical and chemical properties, such as high thermal stability, nonflammability, and negligible volatility [16,17], the number of studies on IL has grown rapidly, not only in tribology but in many other fields [18]. In addition to these excellent properties, the potential for combining different cations and anions could lead to up to 1018 possibilities of different ILs [18]. These diverse configurations give rise to a broad range of behavior that can be employed to satisfy specific lubrication requirements. The excellent tribological performance of ILs has been attributed to two main mechanisms [19,20,21]: first, the formation of interfacial layers that keep the surfaces apart by adsorption of ILs on polar surfaces; second, the promotion of a tribolayer at the sliding interface by tribochemical reactions of ILs with the sliding surfaces. As these processes depend on such an ionic and amphiphilic molecular structure of ILs, to date generalizations are not possible and experimental studies of these fluids are still needed.
It is well known that IL properties originate from complex ion interactions [22], including Coulombic, van der Waals (Keesom, Debye, and London dispersion forces), hydrogen bonding [23,24,25,26], solvophobic [27], π–π [28], and steric [29] interactions. Even though the complicated behavior of ILs at interfaces is not completely understood, it has been extensively reported that ionic liquids experience strong surface forces that enable them to spontaneously self-assemble on the lubricating surfaces [30,31,32,33]. This feature originates in a high polarity and is reflected in the ability of ILs to create an adsorbed ordered layer that protects the surfaces in motion [34,35]. Some studies suggest that the anions present in ILs are adsorbed onto positively charged sites of metallic surfaces, and that consequently, Pauling’s principle of electroneutrality causes the cations to assemble into ion pairs [36,37,38].
ILs are generally divided into two classes: aprotic ionic liquids (APILs) and protic ionic liquids (PILs). The latter are formed by proton transfer from a Brønsted acid to a Brønsted base, resulting in a characteristic free proton on their cation. Thus, this easier synthesis route allows them not only to be more affordable but to avoid halogens in their molecular composition. Halogens have been proved to form hydrogen halides when they come into contact with water; therefore, avoiding them provides advantages in the form of a reduction in their toxicity and an improvement in their corrosive interaction with metals [39,40]. On the other hand, boron-containing lubricants have been widely employed in the past [41]. However, most of the ILs studied that contain boron also include halogens, as do those based on the extensively researched [BF4]− anion [15].
In the present work, the tribological ability of a series of four novel halogen-free borate-based PILs with identical organoborate anions but dissimilar ammonium cations for steel-steel contact is studied. Likewise, the PILs synthesis route and the main physicochemical properties of lubricants, namely thermal stability and viscosity, are also reported. Synchronously, in continuation to the efforts of reducing the environmental impact of lubrication, the lubricating ability of blends of biolubricant and 1 wt.% IL is investigated.

2. Materials and Methods

2.1. Materials

The reagents ethanolamine (≥99.0%), N-methylethanolamine (≥98%), N,N′-dimethylethanolamine (≥99.5%), N-butylethanolamine (≥98%), and boric acid (≥99.5%) were purchased from Sigma-Aldrich (St. Louis, MO, USA), and 1,2-dodecanediol (≥ 90%) was purchased from Santa Cruz Biotechnology (Dallas, TX, USA). Base bio-oil, BO, and the commercially available lubricant BOA were provided by Repsol (Madrid, Spain). All the reagents and materials were used as received without any further treatment.
316 stainless steel disks sliding against 52,100 steel balls with a diameter of 1.5 mm were employed as contact surfaces. Tables S1 and S2 show the elemental compositions of the disk and ball, respectively.
The disks were put through a 3-step grinding process and subsequently polished in two additional steps on a Struers Tegra Mechanical Polishing System machine (Struers, Ballerup, Copenhagen, Denmark) as summarized in Table S1. The surface finish of the disks was examined on a NANOVEA PS50 3D profilometer (NANOVEA, Irvine, CA, USA), as shown in Figure S1, resulting in an average roughness of 0.28 μm.

2.2. Synthesis of PILs

Four novel protic ionic liquids synthesized with identical organoborate anions but dissimilar ammonium cations were investigated in this study: ethanolamine 1,2-dodecanediol borate (IL1), N-methylethanolamine 1,2-dodecanediol borate (IL2), N’N-dimethylethanolamine 1,2-dodecanediol borate (IL3), and N-butylethanolamine 1,2-dodecanediol borate (IL4).
The PILs were synthesized through a stoichiometric acid-base Brønsted reaction using the following general procedure. First, 0.1 mol of boric acid (6.18 g), 1,2-dodecanediol (20.23 g), and the corresponding amine (ethanolamine (6.11 g), N-methylethanolamine (7.51 g), N’N-dimethylethanolamine (8.91 g), or N-butylethanolamine (11.72 g) were precisely weighed to achieve a 1:1:1 ratio and transferred respectively to a 250 mL flask. Subsequently, 150 mL of toluene was added as a solvent. The reaction mixture was held with stirring at around 125 °C for 24 h, while the byproduct water was removed continuously through azeotropic distillation using a Dean–Stark apparatus. The solvent was evaporated using a rotatory evaporator and a vacuum line. Finally, to remove any remaining water, they were heated to 100, 150, and 170 °C for ~1 h at each temperature using a Kugelrohr vacuum distillation apparatus to obtain ILs 1–4 as viscous fluids. The molecular structure and synthesis routes of the PILs are shown in Table 1.

2.3. Thermal Analysis, Rheology, and Solubility

The PILs’ thermal stability was determined through thermogravimetric analysis (TGA) using a TA Instruments TGA-G500 in the range of 10 to 600 °C with a heating rate of 10 °C/min under air atmosphere.
The PILs’ rheological behavior was studied using a TA Instruments Discovery HR-2 with a plate-plate configuration; the diameter of the rotational plate was 20 mm, and the gap between plates was 500 micrometers. The temperature control was carried out with a Peltier system, whose accuracy was 0.1 °C. The thermal effect on the viscosity was characterized by temperature ramps from 25 to 100 °C with a heating rate of 3 °C/min and a constant shear rate of 50 s−1. The shear flow influence on the steady-state viscosity was investigated at 25 °C by increasing the shear rate from 0.1 to 100 s−1.
Solubility tests were conducted based on visual inspection after the centrifugation of the BO + IL blends at 6500 rpm for 3 min.

2.4. Friction and Wear Tests

The tribological tests were performed at room temperature using a custom-designed reciprocating ball-on-flat tribometer that complied with ASTM G133-05. Live friction coefficients were recorded over time, and the results were averaged. The experimental parameters selected for this study are collected in Table 2.
About 1 mL of lubricant was placed at the beginning of each test. For each condition, at least 3 tests were performed to guarantee the repeatability of the results. Before and after each test, the specimen was cleaned with isopropyl alcohol (99.5%). Subsequently, optical images and measurements of wear tracks were taken using an Olympus BH2 Optical microscopy (Olympus, Shinjuku City, Tokyo, Japan). Wear volume was calculated using stroke length, and the average width of the wear track was determined using the Formulas (1) and (2) given by Qu, J. and Truhan, J.J. [42].
V f = L S [ R f 2 a r c s i n ( W 2 R f ) − W 2 ( R f − h f ) ] + π 3 h f 2 ( 3 R f − h f )
h f = R f − R f 2 − W 2 4
where:
  • V f = wear volume
  • L S = stroke length
  • R f = contact ball radius
  • W = wear track width
  • h f = wear track depth
A TESCAN Mira3 scanning electron microscope (SEM) (TESCAN, Brno, South Moravian, Czech Republic) along with an energy-dispersive X-ray spectrometer (EDX) (BRUKER, Billerica, MA, USA) with an accelerating secondary electron voltage of 5 keV were used to study the wear mechanism and surface interactions of the worn steel disks after lubrication.

3. Results

3.1. Thermal Analysis, Rheology, and Solubility

3.1.1. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) results of the PILs are shown in Figure 1. Thermal stability is characterized by the onset temperature, which can be observed in Table 3. For the PILs investigated, the addition of substituent groups to the cation was found to increase their thermal stability, following the order IL1 < IL2 < IL3. An increase in the alkyl chain length of the cationic substituent group also increased the PIL corresponding thermal stability as IL2 < IL4. These results are in accordance with previous research [43].
Furthermore, from Figure 2, the addition of 1 wt.% of any PILs to BO decreased its thermal stability. The thermal stability of the mixtures seems to be highly related to the molecular weight of the cation, resulting in increased thermal stability for the longer alkyl-chained PIL, in the case of BO + IL4. Interestingly, BO + IL1 and BO + IL2 exhibited a second step in their decomposition process at 423.2 and 440.4 °C, respectively, which suggests poor solvation of the ions at decomposing temperatures.

3.1.2. Solubility

Solubility pictures of 1 wt.%. PILs with BO are shown in Figure 2. Only BO + IL1 was found to precipitate directly after centrifuging; the rest of the blends appeared to be soluble over a one-month period. This observation reflects the difference between ion–ion and ion–solvent interactions, suggesting higher solvophobic interactions between the ILs and the BO [44,45].
Additionally, this amphiphilic self-assembly feature may originate from the formation of a three-dimensional hydrogen-bond network [46], whereas in the rest of the ionic liquids studied, the BO used as solvent would solvate the ions complicating the process of an ion approaching another.

3.1.3. Rheology

Dynamic viscosity results from 25 to 100 °C of neat PILs and BO + PIL mixtures are presented in Figure 3 and Figure 4, respectively. In the figures, viscosity is displayed in log 10 and log 2 scales, respectively, for improved visualization. The data are fitted to the Arrhenius Equation (3):
η = A · e ( E a R · T )
where A is the pre-exponential factor related to the viscosity at a theoretically infinite temperature, Ea is the activation energy of the flow and R is the ideal gas universal constant. As can be seen from the viscosity data and Arrhenius parameters collected in Table 4, all fluids showed satisfactory fitting. The temperature-dependence of the viscosity of the fluids is inferred from the activation energy parameter.
Ionic liquid viscosity is determined by two main mechanisms: short-range Coulomb compaction between ions, and long-range ordered charged networks [47]. In this manner, IL2 and IL4 showed higher viscosity than the rest of the ILs studied. These results may be a consequence of a reduction in the free volume due to the achievement of a closely packed structure on that particular cationic distribution [48]. This phenomenon was first described by Doolittle [49] and later was connected to ionic liquids [50]. The free volume theory has been discussed recently for ionic liquid characterization [51]. IL2 also experienced a higher dependency on temperature, followed by IL3, IL4 and IL1.
The solvation of the ions by the biolubricant greatly influenced the viscosity of the fluids investigated, even at 1 wt.% concentration of ILs. The solvation shells stiffened the solution and reduced the free volume of the fluid by localizing themselves around ions; in this way, BO + IL2, BO + IL3, and BO + IL 4 increased the viscosity of the BO. In the case of BO + IL1, which precipitated during the solubility test, as observed in Figure 2, this lower solvation phenomenon resulted in a viscosity reduction in the blend.
Variation of viscosity with shear rate is presented in Figure 5, using logarithmic scales on both axes. From the figure, IL1 and IL3 exhibited a shear thinning behavior, whereas this behavior was diminished for IL2, and in the case of IL4, Newtonian plateau followed. The non-Newtonian performance of the PILs studied may be related to the existence and breakage of nanostructures, as has been previously described in the literature [52,53,54,55]. Furthermore, as stainless steel was the material used in both plates to confine the fluid, the similar system enabled an approach close to the ordination process that took place in the tribological contact studied.
Nevertheless, as depicted in Figure S2, all of the 1 wt.% BO + IL mixtures displayed a Newtonian nature, as well as BO and BOA. Therefore, the use of ILs as additives may improve the tribological behavior of the BO without altering its bulk response to shear.

3.2. Friction and Wear Tests

3.2.1. Ionic Liquids as Neat Lubricants

Figure 6 shows the average friction coefficients for each neat lubricant. Except for IL1, the rest of the PILs performed better than BO and the commercially available BOA. Specifically, IL3 showed a reduction in the coefficient of friction of 30%, whereas IL2 and IL4 reduced its value by about 20% compared to BO. Except for IL1, all of them exhibited slightly better performance than BOA.
In the case of IL1, the lowest average friction coefficient (0.084) was archived just before a consistent ‘‘squeeze out’’ phenomena [56,57,58,59,60] at 17 m, as depicted in Figure 7. The solidification process that took place was observed for each of the 3 trials and resulted in the inability of the film to remain in place when sliding surfaces were compressed.
As previously described in Figure 5, IL1 experimented with the higher shear thinning behavior, thus perceiving a more intense aligning effect from the solid surfaces [61,62]. Likewise, preliminary studies showed that IL3 exhibited similar behavior when higher duration tests were performed, suggesting that the kinetics of this process also plays a crucial role.
Figure 8 shows the wear volume of the steel disks after tests lubricated with BO, BOA, and the neat PILs. Except for IL1, the rest of the ionic liquids performed better than BO and BOA. According to the optical micrographs in Figure 9, even though BOA effectively reduced wear compared to BO, darker abrasive marks were found along the sliding direction, whereas in the case of IL2 and IL3, only superficial scratches were displayed. IL4 was also found capable of preventing wear but presented darker abrasive marks along the center of the wear track, and plastically deformed material accumulated at the borders.
SEM/EDX results of BO and IL2 with an accelerating voltage of 5keV were shown in Figure 10. It can be noted that the wear width is wider for BO, but also contains more carbon and oxygen than IL2, as a result of oxidation and wear.
Additionally, no boron traces were found for IL2, and only a minor difference in the iron composition was detected. These results suggest that the lubrication mechanism for the PILs investigated may follow a non-sacrificial solid-like film deposition that prevents wear that may be due to physical adsorption or self-assembly of the PILs, instead of a tribochemical reaction with the metal surface [63,64]. Unfortunately, the vacuum condition required for the use of SEM and the cleaning process of the samples precludes this phenomenon from being confirmed through this technique.

3.2.2. Ionic Liquids as Biolubricant Additives

Figure 11 shows the average friction coefficient for BO and BOA, and as 1 wt.% PILs + BO mixtures over the tribological tests. All PIL mixtures resulted in lower coefficients of friction compared to BO and BOA. Particularly, BO + IL 1 displayed the highest friction reduction, with a value of 30%, followed by BO + IL 3 and BO + IL4 with a friction reduction of 25% and BO + IL 2 with 20%, compared to the COF of BO.
The wear volume of the steel disks after tests lubricated with BO, BOA, and the 1wt.% PILs +BO mixtures can be observed in Figure 12. Interestingly, steel disks lubricated with BO + IL1 exhibited the lowest wear for all the lubricants studied, reducing wear of steel disk by 96% compared to BO and by 48% compared to BOA. This great lubricating ability of IL1 as an additive may be attributed to the formation of adsorbed protective layers of ILs on the steel surface.
The optical images of the wear tracks displayed in Figure 13 also show how the aforementioned mixture exhibited fewer abrasive marks and no plastic deformation compared to all other lubricants tested. A slight correlation between the molecular weight and the wear results for the rest of the blends can be observed, where a higher number of hydrogen substituents and shorter alkyl chain cation groups led to lower wear results, following the series IL1 < IL2 < IL3 < IL4.
SEM/EDX results of BOA and BO + IL1 shown in Figure 14 illustrate how the latter contained less wear particles and oxidation, supporting an adsorption mechanism.

4. Conclusions

This work studied the lubricating abilities of four novel halogen-free ammonium-organoborate protic ionic liquids as neat lubricants and 1 wt.% additive to a biolubricant, with the following major results:
  • The thermal stability of the PILs increased with the molecular weight, following the series IL1 < IL2 < IL3 < IL4. Thermal stability of the mixtures was slightly reduced by the addition of the PILs.
  • IL2 and IL4 showed higher viscosity than IL1 and IL3, which was attributed to a closely packed structure for that particular cationic configuration.
  • BO + IL mixtures increased the viscosity of the base oil, except for BO + IL1, due to the poor solvation of the blend illustrated during the solubility study.
  • IL1 and IL3 exhibited shear-thinning rheological behavior linked to the breakage of nanostructures when confined with stainless steel plates. This phenomenon was diminished in the case of IL2 and disappeared with the increment of the alkyl chain with IL4.
  • When tested as neat lubricants, IL2 and IL3 prevented wear, as only superficial scratches were observed. Additionally, the PILs greatly reduced friction by 20–30% compared to BO. In the case of IL1, a solidification process led to the squeeze-out phenomena of the film.
  • The 1 wt.% PILs mixtures reduced friction by 20–25% compared to BO. BO + IL1 reduced wear by 96% compared to BO and by 48% compared to BOA, performing the best among ionic liquids studied as biolubricant additives.
  • The proficient performance of these PILs was mainly attributed to an adsorption phenomenon rather than a tribochemical reaction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/lubricants10100269/s1. Figure S1: 3D Profile of a polished disk; Figure S2: Variation of viscosity with shear rate for BO, BOA, BO + IL1, BO + IL2, BO + IL3, and BO + IL4; Table S1. Elemental composition (wt. %) of 316 stainless steel disks; Table S2. Elemental composition (wt. %) of AISI 52,100 steel balls; Table S3: Mechanical polishing steps for the 316 stainless steel disks.

Author Contributions

Conceptualization, P.I.; methodology, A.S., P.I. and M.G.C.; software, A.S.; validation, A.S., P.I. and M.G.C.; formal analysis, A.S. and P.I.; investigation, A.S. and P.I.; resources, P.I.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and editing, P.I. and M.G.C.; visualization, A.S.; supervision, P.I. and M.G.C.; project administration, P.I.; funding acquisition, P.I. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by Kate Gleason College of Engineering at Rochester Institute of Technology.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank REPSOL (Spain) for providing the biodegradable oil BIO TELEX 46 and the biodegradable base oil. In addition, Alfonso Sierra wishes to thank Gleason Corporation for the Gleason Doctoral Fellowship.

Conflicts of Interest

The authors declare no conflict of interest.

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Lubricants 10 00269 g001 550
Figure 1. Thermogravimetric analysis (TGA) curves of the ionic liquids.
Figure 1. Thermogravimetric analysis (TGA) curves of the ionic liquids.
Lubricants 10 00269 g001
Lubricants 10 00269 g002 550
Figure 2. Solubility study on the BO + 1 wt%. ILs mixtures. (a) 1 day BO + IL1, (b) 1 day BO + IL2, (c) 1 day BO + IL3, (d) 1 day BO + IL4, (e) 1 week BO + IL1, (f) 1 week BO + IL2, (g) 1 week BO + IL3 (h) 1 week BO + IL4, (i) 1 month BO + IL1, (j) 1 month BO + IL2, (k) 1 month BO + IL3, (l) 1 month BO + IL4.
Figure 2. Solubility study on the BO + 1 wt%. ILs mixtures. (a) 1 day BO + IL1, (b) 1 day BO + IL2, (c) 1 day BO + IL3, (d) 1 day BO + IL4, (e) 1 week BO + IL1, (f) 1 week BO + IL2, (g) 1 week BO + IL3 (h) 1 week BO + IL4, (i) 1 month BO + IL1, (j) 1 month BO + IL2, (k) 1 month BO + IL3, (l) 1 month BO + IL4.
Lubricants 10 00269 g002
Lubricants 10 00269 g003 550
Figure 3. Variation of viscosity with temperature for BO, BOA, and PILs.
Figure 3. Variation of viscosity with temperature for BO, BOA, and PILs.
Lubricants 10 00269 g003
Lubricants 10 00269 g004 550
Figure 4. Variation of viscosity with temperature for BO, BOA, and BO + PILs mixtures.
Figure 4. Variation of viscosity with temperature for BO, BOA, and BO + PILs mixtures.
Lubricants 10 00269 g004
Lubricants 10 00269 g005 550
Figure 5. Variation of viscosity with shear rate for IL1, IL2, IL3, and IL4.
Figure 5. Variation of viscosity with shear rate for IL1, IL2, IL3, and IL4.
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Lubricants 10 00269 g006 550
Figure 6. Average friction coefficient for BO, BOA, and PILs.
Figure 6. Average friction coefficient for BO, BOA, and PILs.
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Lubricants 10 00269 g007 550
Figure 7. Friction coefficient for IL1. Images illustrate the squeeze out phenomenon over distance.
Figure 7. Friction coefficient for IL1. Images illustrate the squeeze out phenomenon over distance.
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Lubricants 10 00269 g008 550
Figure 8. Average wear volume for BO, BOA, and PILs.
Figure 8. Average wear volume for BO, BOA, and PILs.
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Lubricants 10 00269 g009 550
Figure 9. Optical images of wear tracks on the steel disks lubricated with (a) BO, (b) BOA, (c) IL1, (d) IL2, (e) IL3, and (f) IL4.
Figure 9. Optical images of wear tracks on the steel disks lubricated with (a) BO, (b) BOA, (c) IL1, (d) IL2, (e) IL3, and (f) IL4.
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Lubricants 10 00269 g010 550
Figure 10. SEM/EDX results of (a) BO, and (b) IL2.
Figure 10. SEM/EDX results of (a) BO, and (b) IL2.
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Lubricants 10 00269 g011 550
Figure 11. Average friction coefficient for BO, BOA, and BO + PILs mixtures.
Figure 11. Average friction coefficient for BO, BOA, and BO + PILs mixtures.
Lubricants 10 00269 g011
Lubricants 10 00269 g012 550
Figure 12. Wear volume of BO, BOA, and BO + PILs mixtures.
Figure 12. Wear volume of BO, BOA, and BO + PILs mixtures.
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Lubricants 10 00269 g013 550
Figure 13. Optical images of wear tracks on the steel disks lubricated with (a) BO, (b) BOA, (c) BO + IL1, (d) BO + IL2, (e) BO + IL3, and (f) BO + IL4.
Figure 13. Optical images of wear tracks on the steel disks lubricated with (a) BO, (b) BOA, (c) BO + IL1, (d) BO + IL2, (e) BO + IL3, and (f) BO + IL4.
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Lubricants 10 00269 g014 550
Figure 14. SEM/EDX results of (a) BOA, and (b) BO + IL1.
Figure 14. SEM/EDX results of (a) BOA, and (b) BO + IL1.
Lubricants 10 00269 g014
Table
Table 1. Molecular structure of PILs and their corresponding synthesis routes.
Table 1. Molecular structure of PILs and their corresponding synthesis routes.
Lubricants 10 00269 i001
CodeR1R2Cation *Anion *Name
IL1Lubricants 10 00269 i002Lubricants 10 00269 i003Lubricants 10 00269 i004Lubricants 10 00269 i005Ethanolamine
1,2-dodecanediol borate
IL2Lubricants 10 00269 i006Lubricants 10 00269 i007Lubricants 10 00269 i008Lubricants 10 00269 i009N-methylethanolamine
1,2-dodecanediol borate
IL3Lubricants 10 00269 i010Lubricants 10 00269 i011Lubricants 10 00269 i012Lubricants 10 00269 i013N’N-dimethylethanolamine
1,2-dodecanediol borate
IL4Lubricants 10 00269 i014Lubricants 10 00269 i015Lubricants 10 00269 i016Lubricants 10 00269 i017N-butylethanolamine
1,2-dodecanediol borate
* Lubricants 10 00269 i018 Boron Lubricants 10 00269 i019 Nitrogen Lubricants 10 00269 i020 Oxygen Lubricants 10 00269 i021 Carbon Lubricants 10 00269 i022 Hydrogen.
Table
Table 2. Tribometer Experimental Parameters.
Table 2. Tribometer Experimental Parameters.
ParameterValue
Normal load (N)3
Max. Hertz contact pressure (GPa)2.36
Stroke length (mm)3
Frequency (Hz)3
Duration (s)3600
Sliding speed (m/s)0.03
Sliding distance (m)64.8
Table
Table 3. Onset temperature of the lubricants studied.
Table 3. Onset temperature of the lubricants studied.
LubricantTonset (°C)
BO348.7
BOA348.5
IL1268.0
IL2273.6
IL3293.7
IL4263.1
BO + IL1330.5
BO + IL2324.5
BO + IL3344.0
BO + IL4349.2
Table
Table 4. Viscosity at 25, 40, and 100°C, and Arrhenius parameters.
Table 4. Viscosity at 25, 40, and 100°C, and Arrhenius parameters.
Dynamic Viscosity (Pa s)Arrhenius Parameters
Lubricant25 °C40 °C100 °CA (106) (Pa·s)Ea (kJ/mol)r2
BO0.06440.03920.00881.8226.020.997
BOA0.07080.04270.00962.3025.680.998
IL10.52940.22800.02602.27 × 10−242.020.998
IL297.4824.420.16071.75 × 10−572.480.998
IL32.7230.86610.01941.11 × 10−565.200.999
IL413.654.9380.05562.86 × 10−461.340.992
BO + IL10.04610.02810.00601.3225.950.995
BO + IL20.08620.05120.01132.0026.440.997
BO + IL30.07350.04390.00961.7026.470.997
BO + IL40.08110.04910.01092.4225.840.998
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Sierra, A.; Coleman, M.G.; Iglesias, P. Tribological Properties of Borate-Based Protic Ionic Liquids as Neat Lubricants and Biolubricant Additives for Steel-Steel Contact. Lubricants 2022, 10, 269. https://doi.org/10.3390/lubricants10100269

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Sierra A, Coleman MG, Iglesias P. Tribological Properties of Borate-Based Protic Ionic Liquids as Neat Lubricants and Biolubricant Additives for Steel-Steel Contact. Lubricants. 2022; 10(10):269. https://doi.org/10.3390/lubricants10100269

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Sierra, Alfonso, Michael G. Coleman, and Patricia Iglesias. 2022. "Tribological Properties of Borate-Based Protic Ionic Liquids as Neat Lubricants and Biolubricant Additives for Steel-Steel Contact" Lubricants 10, no. 10: 269. https://doi.org/10.3390/lubricants10100269

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