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Article

Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding

by
Hong Guo
,
Brandon Stoyanovich
,
Junru Pang
and
Patricia Iglesias
*
Mechanical Engineering Department, KGCOE, Rochester Institute of Technology, Rochester, NY 14623, USA
*
Author to whom correspondence should be addressed.
Lubricants 2023, 11(8), 329; https://doi.org/10.3390/lubricants11080329
Submission received: 28 June 2023 / Revised: 24 July 2023 / Accepted: 26 July 2023 / Published: 3 August 2023
(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)
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Abstract

:
Although aluminum alloys are widely used in the automotive and aerospace industries due to their excellent strength-to-weight ratio and good corrosion resistance, the poor tribological performance and low compatibility of these materials with lubricant anti-wear and anti-friction additives in conventional mineral oils are major limitations. In addition, environmental awareness has increased the need for more environmentally friendly lubricants. Ionic Liquids (ILs) have exhibited significant potential as lubricants and lubricant additives. One of the more interesting properties of ILs is that they can form physically-adsorbed or chemically-reacted layers that reduce friction and wear of the surfaces in contact. Among ILs, Protic Ionic Liquids (PILs) have received more attention recently because of their simple and economic synthesis route. Furthermore, the anions and cations of PILs can be selected to be considered environmentally benign. In this article, the tribological behavior of a family of six PILs are studied as additives to a biodegradable oil (BO), under aluminum-steel contact. Al2024 disks slid against AISI52100 steel balls under a normal load of 3 N and a frequency of 5 Hz at room temperature and using a ball-on-flat reciprocating tribometer. PILs used in this study, were synthesized using two strong acids, with short and long hydrocarbon chains, and three weak bases with different propensities to hydrogen bonds. Results show that, although adding just 1 wt.% of any PIL to BO reduced friction and wear, the alkyl chain length influenced the lubricating ability of these ordered fluids. Wear mechanisms and surface interaction are discussed on the basis of 3D profilometry, SEM-EDX and RAMAN spectroscopy.

1. Introduction

Friction and wear are undesirable phenomena commonly found in machines elements. Friction and wear losses are responsible for 23% of the world’s total energy consumption [1], and have indirectly a negative impact on the environment [2,3,4,5]. According to Holmberg and Erdemir [1], the implementation of advanced tribological technologies, such as new surface treatments [6] and high-performance lubricants, has the potential to reduce energy losses by up to 40%. Furthermore, the adoption of these advanced tribological technologies can significantly decrease CO2 emissions, leading to substantial economic savings.
Ionic Liquids (ILs) have been at the center of the research community spotlight as potential advanced lubricants and additives since 2001, when Ye et al. presented a group of alkylimidazolium tetrafluoroborates ILs as promising and versatile lubricants for several contacts [7]. A couple of years later, Jimenez et al. found that just adding 1 wt.% of an imidazolium IL to a mineral oil, friction coefficient and wear volume of the aluminum disk were significantly reduced compared to the base oil [8]. Since then, research interest in these ordered fluids as lubricants [9,10,11] and additives [12,13,14] has exponentially grown. The great lubricating ability of ILs can be attributed to their ability to form physically adsorbed layers on the interacting surfaces [15,16,17] and, if the conditions are appropriate (temperature, load, speed, etc.), the formation of a tribolayer [18,19] that reduces friction and wear of the sliding system. Early works were mainly focused on conventional Aprotic Ionic Liquids (AILs) with halogen-containing anions [20,21,22] with promising results. Hernandez Battez et al. [23] conducted a study on two imidazolium-based AILs utilizing tetrafluoroborate and hexafluorophosphate as anions. These AILs were used as additives to mineral oil, and the findings demonstrated significant reductions in friction and wear compared to using the neat lubricant alone. The favorable outcomes were attributed to the reactivity of the halogenated anions with the metal surfaces. However, it is now widely acknowledged that anions containing halogens are prone to hydrolysis in the presence of water or moisture, leading to the generation of toxic and corrosive substances [24,25]. Furthermore, corrosion of machine parts can potentially result in critical structural failures [26]. The use of AILs as neat lubricants is also restricted due to their complex synthesis route and high cost. In the last few years, some promising research has started on a subgroup of ILs, known as Protic Ionic Liquids (PILs), with a simple synthesis route and a natural tendency to form hydrogen bonds [27,28]. PILs are easily synthesized by proton transfer from an acid to base [29] and have shown favorable results as neat lubricants [15] and additives [30,31]. Furthermore, PILs can be synthesized avoiding halogens and other undesirable elements to be considered environmentally friendly. A previous study [32] that investigated the environmental properties of several PILs, along with other characteristics, concluded that these fluids exhibit significantly superior environmental properties when compared to the industry-standard reference additive (ZDDP). However, the study also revealed certain limitations regarding the miscibility of PILs with base oils. PILs have shown potential as neat lubricants and additives for steel contacts [11,30,33,34,35,36,37], but studies investigating the use of these ordered fluids in aluminum contacts are still scarce [18,38].
The global consumption of lubricants amounts to an average of 38 million tons per year. While the demand is projected to decelerate with the rise of Electric Vehicles (EVs) entering the market, lubricant consumption is still anticipated to rise in the forthcoming years [39]. A significant portion of these lubricants are derived from mineral oils, which are sourced from limited petroleum reserves. However, these petroleum-based lubricants exhibit toxicity and possess poor biodegradability. Environmental awareness and ever-growing restrictive regulations on contamination have increased the need for more environmentally-friendly lubricants. Synthetic and vegetable oils present a viable solution as they offer superior biodegradability and reduced toxicity compared to the presently utilized mineral oils [40].
Due to their excellent strength-to-weight ratio and good corrosion resistance, aluminum alloys are widely used in automotive and aerospace applications [41]. However, their poor tribological performance, in particular when paired with steel, and the low compatibility of aluminum and aluminum alloys with lubricant anti-wear and anti-friction additives in conventional mineral oils [42,43] are major drawbacks. Consequently, new lubricant and additive alternatives for these alloys are needed, and PILs are a potential and environmentally friendly option.
In this work, six PILs, synthesized from two strong acids, with short and long hydrocarbon chains, and three weak bases with different propensities to hydrogen bonds, are investigated as 1 wt.% additives to a biodegradable oil under reciprocating aluminum-steel contact.

2. Materials and Methods

2.1. Materials and Preparation of PILs

Three hydroxyalkyl-amines, ethanolamine (≥99%), N-methyl ethanolamine (≥98%), and N, N-dimethylethanolamine (≥99.50%) with differing abilities to form hydrogen bonding, and two strong acids (p-Toluenesulfonic and 4-dodecylbenzenesulfonic acids), with different alkyl chain lengths were purchased from Sigma Aldrich (Saint Louis, MO, USA). All chemicals for synthesizing PILs were used as received, except for the p-Toluenesulfonic acid. The biodegradable oil with additives (BOA) and its corresponding base oil (BO) used in this work were kindly supplied by Repsol SA (Spain). The main properties of both fluids are described in Table 1.
The six PILs used in this study were synthesized in our laboratory according to the procedures described in previous articles [30,32]. Briefly, anhydrous p-toluenesulfonic acid (p-tolSO3H) was obtained firstly by azeotropic distillation. Then, stoichiometric amounts of p-tolSO3H were added incrementally to each hydroxyalkyl-amine, stirring the reaction mixture at less than 80 °C for 1 h under Argon. Similarly, stoichiometric quantities of 4-dodecylbenzenesulfonic acid were added incrementally to each amine and stirred the reaction mixture under Argon for 1 h with a temperature of less than 120 °C. In addition, each reaction mixture was heated for 1 h under vacuum and kept at 100 °C overnight. Last, each neutral (PH~7) and water content < 1% (Karl fisher), reaction mixture was transferred for subsequent analysis and tribological studies. The abbreviation, and molecular structure of the six PILs are depicted in Table 2.

2.2. Spectroscopic Characterization and Ionicity of PILs

Nuclear magnetic resonance (1H NMR) was utilized to verify the stoichiometry of the PILs employing a Bruker spectrometer. The 1H NMR results, which can be found in our previous work [30], confirm the molecular structures of the six PILs.
A Shimadzu IRPrestige-21 Fourier Transform Infrared Spectrophotometer was used to obtain the Fourier transform infrared (FTIR) spectra of the PILs. Furthermore, the ionic conductivity of the six PILs was measured from 40 °C to 90 °C, since they are not in a liquid phase at temperatures below 40 °C.

2.3. Solubility of PILs in BO

Adding small amounts of ILs to a base oil has been proven to be very effective in reducing friction and wear of surfaces in contact [10,37,38]. In fact, some research has shown that ILs as additives could outperform their use as neat lubricants [5]. Considering economic constraints and results from previous studies, in this study 1 wt.% of each PIL was added to the polar base oil. One hour of magnetic stirring and one hour of ultrasonicating were taken for each PsIL-BO mixture to form a homogeneous mixture. After that, the stability of each mixture was evaluated following the procedure previously described [30].

2.4. Thermal Stability and Viscosity

The thermal stability of neat PILs, 1%PIL + BO, BO, and BOA were characterized using a TA Instruments Q500 with TG analyzer software. Around 11 mg of each sample was deposited in a pan and heated from room temperature to 550 °C at a heating rate of 10 °C/min under an air atmosphere.
The dynamic viscosity of BO, BOA, PILs and 1%PIL + BO was measured from 25 to 100 °C using a Viscometer (Brookfield DV2T-LV) and a Thermosel System.

2.5. Friction and Wear Tests

The lubricating ability of the PIL + BO mixtures was studied at room temperature using a ball-on-flat reciprocating tribometer in aluminum-steel contact under a boundary lubrication regime. The chemical compositions of 2024 aluminum alloy are tabulated in Table 3. Each aluminum disk was polished until Ra ≈ 0.01 μm. Before each running, a small quantity of lubricant was deposited on top of the disk surface without adding more lubricant during the test. Except for the various lubricants, tribological tests were conducted under the same conditions of normal load (3N-corresponding to a maximum Hertz contact pressure of 1.56 GPa), sliding speed (0.03 m/s), stroke length (0.003 m), frequency (5 Hz), and sliding distance (108 m). At least three frictional tests were carried out for each lubricant to minimize test measurement errors and maintain standard deviations lower than 5%.
After the tests, each aluminum sample was cleaned for wear-trach width measurement using an optical microscope (Olympus BH-2). 20 measurements of the width of each track were taken and the wear volume of the aluminum samples were found using the equations in [39]. Also, the 2D and 3D profiles of the worn surfaces were assessed through the NANOVEA PS50 profilometer.

2.6. Surface Analysis

The aluminum samples were characterized after each frictional test using a scanning electron microscope (Tescan-Bruker Vega 3) with energy dispersive X-ray spectrometer and a Raman microscope (Bruker Senterra II) to investigate the wear mechanisms and surface interactions between aluminum and PIL mixtures. For Raman spectroscopy, a laser with a wavelength of 532 nm was chosen based on the Raman scattering intensity and the composition of the resulting tribo-film, which mainly consists of inorganic materials. To enhance the spatial resolution of the laser spot both inside and outside the wear track, a 100× objective and 25 mV power were utilized. Each measurement involved an integration time of 2 s and 6 co-additions over time to optimize the quality of the obtained Raman spectra.

3. Results

The FTIR spectra for the family of six PILs and their corresponding bases can be found in [30], along with a comprehensive discussion. Notably, the Ethanolamine-derived PIL, Ets, exhibits a significantly higher absorption intensity than Mts and Dts, suggesting a stronger tendency of hydrogen bonding among its molecules compared to the other two p-toluenesulfonate-based PILs. Similarly, Eds derived from ethanolamine also demonstrate a higher propensity for hydrogen bonding among their molecules compared to Mds and Dds. This highlights the important role of the amine base selection in influencing the intermolecular hydrogen bonding in the corresponding PILs, following the order of ethanolamine (Base1) > N-methylethanolamine (Base2) > N,N′-dimethylethanolamine (Base3) [30].
The six PILs studied as additives in this work are salts synthesized with three weak bases (Ethanolamine, N-methyl ethanolamine, N, N′-dimethyl ethanolamine) and two strong acids with different carbon chain lengths (p-toluenesulfonic acid and 4-dodecylbenzenesulfonic acid). The equilibrium of the proton transfer reaction in these PILs (ionization) depends on the basicity of the three amines and the acidity of the two sulfonic acids [15]. The ionic conductivity of the PILs used was studied at different temperatures and the results can be found in our previous article [30]. Temperature increased the ionic conductivity of all PILs. It is important to note that Ets, Mts and Dts were solid at some temperature at which their conductivities were not accessible. Within the same family of PILs (p-toluenesulfonate-based and 4-dodecylbenzenesulfonate-based PILs), the ionic conductivity follows the order N, N′-dimethyl ethanolamine > N-methyl ethanolamine > Ethanolamine. In general, molecules with greater ionic character tend to exhibit weaker hydrogen bonding tendencies. In addition, the long alkyl chain on the anionic moiety of the 4-dodecylbenzenesulfonic-based PILs caused lower conductivity than those of the corresponding p-toluenesulfonate-based PILs [30].
Solubility and stability images for six PILs in the polar BO are shown in Figure 1. As expected, the presence of longer oleophilic hydrocarbon group length in the PILs improved the solubility in the polar base oil. It is also important to know that, although the presence of the long alkyl chain length in the anion improves the solubility of the 4-dodecylbenzenesulfonate-based PILs in BO; in general both families of PILs were not totally soluble in BO. As seen in the figure, some deposits appear on the bottom of the container after just 24 h, probably due to the benzenesulfonate aromatic group present in both anionic moieties.
The thermal stability of both base biodegradable oils (BO and BOA), and mixtures was characterized and the results are presented in Table 4. The decomposition temperature of the neat PILs can be found in our previous work [30]. It is noteworthy that the PILs synthesized with the weakest amine (Ethanolamine) showed the lowest onset temperature, and the PILs synthesized with the stronger amine showed the higher onset temperature. In general, the decomposition temperatures of all six PILs are very close to the onset temperature of BO (see Table 4). However, PIL-BO mixtures showed higher thermal stability than BO and their corresponding neat PIL, presenting thermal stabilities very close to BOA. This phenomenon may be due to the interactions of the PIL molecules and the polar BO.
The average dynamic viscosities of both base biodegradable oils (BO and BOA), and each mixture at different temperatures are summarized in Table 5. The average dynamic viscosity of the six neat PILs was only calculated at 100 °C, since these ionic fluids were semi-solid at lower temperatures. Even at this high temperature, the family of PILs with longer alkyl chain length anions exhibit significantly higher viscosity values. Furthermore, within the same family of PILs, the propensity of the amine group to form hydrogen bonds also plays a crucial role in determining viscosity. This indicates that hydrogen bonds may create additional intermolecular connections, limiting molecular mobility within the liquid and thereby increasing its viscosity [44]. In any case, the viscosity of BO showed a slight increase at all temperatures, with a more significant effect observed at room temperature, upon the addition of 1 wt.% of any PIL. Under the experimental conditions used in this study, the lambda ratio (λ) between the lubricant film thickness and the combined roughness of the contacting surfaces for all the lubricants is smaller than 1 as can be seen in Table S1, which corresponds with the boundary lubrication regime. In this regime, full film lubrication is difficult to maintain due to the contacts between asperities in relative motion. Therefore, under the experimental conditions studied the viscosity of the PILs is not a key factor, and the potential formation of a tribofilm when PILs are used as additives may be the main reason for their good lubricating ability [40].
Figure 2a shows the average values of the friction coefficient of the steel-aluminum contact lubricated with both biodegradable oils and the mixtures. When the family of PILs with the longer hydrocarbon group (4-dodecylbenzenesulfonate-based PILs) were used as additives to BO showed the best anti-friction ability, showing a maximum friction reduction of 58% with respect to BO when the Dds is present in the base oil. Within the family of 4-dodecylbenzenesulfonate-based PILs, the ionicity of the PILs played also a role, and the friction coefficient obtained under the experimental conditions studied was inversely related to the corresponding PIL’s ionicity. The addition of any p-toluenesulfonate-based PIL to BO also reduced the friction coefficient with respect to BO, reaching values close to that obtained with BOA.
Figure 2b shows the friction vs. time curve for each lubricant. A friction value of around 0.1 was obtained when BO was used as a lubricant, with some peaks reaching values of 0.12 or higher. These peaks likely correspond to instances when wear debris gets trapped between the surfaces in contact, resulting in elevated friction values. As the debris is subsequently swept out of the interphase, the friction values decrease. In contrast, the addition of 1 wt.% of any PILs to BO resulted in lower and more consistent friction values. Among them, 1 wt.% Dds exhibited the lowest friction values, averaging around 0.045. In no case was a break-in period observed under the studied experimental conditions.
Under the experimental conditions employed in this study, only the aluminum disks exhibited wear, whereas the steel balls showed negligible signs of wear. Wear rates (and standard deviation) of the aluminum disks after tests lubricated with both biodegradable oils, and mixtures are summarized in Figure 3. The figure shows that the addition of any PIL as an additive efficiently reduced the wear rate of BO, with a maximum wear reduction of 76.42% when 1% Eds + BO was employed as a lubricant. In all cases, the use of the new PIL-BO mixtures resulted in aluminum wear rates that are half of that obtained when BOA was used. Overall, the family of PILs characterized by longer alkyl chain lengths in the anion (4-dodecylbenzenesulfonate-based PILs) exhibited lower wear rates when compared to the PILs featuring shorter alkyl chain lengths (p-toluenesulfonated-based PIL). Within the same family, the tendency of forming hydrogen bonding in the amide did not exhibit a discernible effect on wear. This finding is consistent with our previous research [30], wherein the same PIL families were employed as additives to a mineral oil under steel-steel contact. It is worth noting that if these PILs had been utilized as standalone lubricants, a different outcome may have been observed. However, due to their semi-solid nature at room temperature, conducting such tests was impractical. Notably, in our previous study [15], a distinct PIL family employing the same amides as cations revealed wear values that directly correlated with the propensity of the amide to form hydrogen bonding.
Figure 4 displays the optical micrographs of the aluminum wear scars after tests lubricated with the biodegradable base oils, and mixtures. Severe wear was observed on the aluminum disks when the samples were lubricated with BO or BOA. The addition of any PIL to BO resulted in milder wear on the aluminum disks although the wear mechanism is pretty similar for all samples. Abrasive wear is observed in all cases, particularly after lubrication with BOA and BO + PIL mixtures. In addition to the plowed grooves inside the wear tracks, characteristic of abrasive wear, all wear tracks showed adhesive wear with plastically deformed materials piled up on the scar boundaries. The three samples lubricated with the longer alkyl chain length family (4-dodecylbenzenesulfonate-based PILs) of PILs showed narrower wear tracks with a smaller amount of plastically deformed material than those lubricated with the corresponding p-toluenesulfonated-based PIL.
Since the 1% Eds + BO mixture resulted in lower wear volumes of the aluminum disks, the samples that used this lubricant mixture were further characterized to gain some insight into its superior antiwear effect. The 2D and 3D wear scar profiles on the aluminum disks represented in Figure 5 show that the wear track lubricated with 1% Eds + BO is not only narrower but also shallower and smoother than those lubricated with BO and BOA. Based on the figure, it can be observed that the addition of a mere 1% Eds to BO resulted in a wear depth of 10 μm. This reduction represents a significant decrease of approximately 67% compared to neat BO and about 60% compared to the commercially available BOA.
In order to get further information on the wear mechanisms and the chemical reactions between the lubricants and the aluminum surfaces, SEM images and EDS analysis were obtained on the worn aluminum surfaces. Figure 6 shows the SEM images and EDS results on worn disks after tests lubricated with BO, our baseline, and 1 wt.% Eds + BO, the lubricant mixture with the best anti-wear effect. On the higher magnification of SEM images, plastically deformation material was observed along the edges of the worn surfaces when BO was used as lubricant. When 1 wt.% of Eds is added to BO and used to lubricate the aluminum disk, the wear track is narrower and no plastic deformation was detected on the borders. In both cases, but particularly for 1%Eds + BO, abrasive marks along the sliding motion are observed.
As can be seen in the EDS spectra of Figure 6, the addition of 1 wt.% of Eds to BO resulted in higher carbon and oxygen content on the worn surfaces. The elevated levels of carbon and oxygen suggest the formation of a protective tribolayer [15,45] within the aluminum and steel surfaces. This tribolayer formation contributes to a noticeable reduction in friction and wear experienced during the tribological process. In addition to EDS, Raman analysis was used to verify the presence of a carbon and oxygen enriched tribolayer on the aluminum surfaces when 1%EDS + BO was used as a lubricant (Figure 7). From the figure, peaks of Al2O3, appear at approximately 276 cm−1 [41] in both cases, but with higher intensity when 1 wt.% Eds is present in the lubricant mixture. When the sample was lubricated with 1%Eds + BO, in addition to Al2O3, is confirmed the presence of D and G bands of carbon materials at 1338 cm−1 and 1568 cm−1 [42]. It is important to mention that Raman analysis was also conducted on the surfaces lubricated with the other PILs as additives. However, the presence of the D and G bands of carbon was exclusively detected in the 1% Eds + BO sample. Consequently, the observed reduction in friction and wear when the other PILs are used as 1% additives to BO may be attributed to the formation of a physically adsorbed layer that could be washed out after the lubricant is removed post-tests. This could potentially explain the absence of D and G bands when the other additives are used.

4. Conclusions

In this work, the lubricating ability of six PILs, synthesized from two strong acids (with short and long hydrocarbon chains) and three weak bases with a different propensity to hydrogen bond, is investigated as 1 wt.% additives to a biodegradable oil under reciprocating aluminum-steel contact. The main conclusions from this experimental work are as follows:
  • None of the PILs studied in this work was completely soluble in the polar BO, probably due to the presence of the benzenesulfonate aromatic group present in the anion. However, PILs containing the longer alkyl chain length showed better solubility.
  • Although the viscosity values of the neat PILs were greatly influenced by the alkyl chain length of the anions and the propensity for hydrogen bonding of the cations, no major differences were found in the viscosity values of the mixtures in this work since the PILs were used as only 1% additives to BO. The decomposition temperatures of all six PILs were very close to the onset temperature of BO. However, each PIL-mixture showed higher thermal stabilities of their corresponding neat PIL and BO.
  • The addition of any PIL to BO significantly reduced the friction coefficient and wear volume of the steel disks, probably due to the formation of an oxygen- and carbon-rich tribolayer. The family of PILs with the longer hydrocarbon group length (4-dodecylbenzenesulfonate-based PILs) in the anion showed higher friction and wear reductions, outperforming the commercially available oil (BOA). Since the PILs were used as only 1% additives, no major effect on friction and wear was found in the tendency for hydrogen bonding of the amide.
  • The significant reduction in friction and wear observed by adding only 1 wt.% of these PILs presents a promising opportunity for substantial economic and environmental benefits worldwide.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/lubricants11080329/s1, Table S1: Lubrication regime estimation for each lubricant at aluminum-steel contacts. See [46].

Author Contributions

Conceptualization, P.I.; methodology, P.I. and H.G.; validation, H.G. and P.I.; formal analysis, H.G.; investigation, H.G. and P.I.; resources, P.I.; data curation, H.G., B.S. and J.P.; writing—original draft preparation, H.G.; writing—review and editing, P.I.; supervision, P.I. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data is contained within the article and supplementary material.

Acknowledgments

Hong Guo acknowledges the Gleason Corporation for the Gleason Doctoral Fellowship.

Conflicts of Interest

The authors declare no conflict of interest.

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Lubricants 11 00329 g001
Figure 1. Pictures of stability of 1 wt.% PILs in BO after 1 day, 1 week and 1 month. (a)-1, Ets; (a)-2, Mts; (a)-3, Dts; (b)-1, Eds; (b)-2, Mds; (b)-3, Dds.
Figure 1. Pictures of stability of 1 wt.% PILs in BO after 1 day, 1 week and 1 month. (a)-1, Ets; (a)-2, Mts; (a)-3, Dts; (b)-1, Eds; (b)-2, Mds; (b)-3, Dds.
Lubricants 11 00329 g001
Lubricants 11 00329 g002
Figure 2. (a) Average values of friction coefficients and (b) real time friction curves of Al-steel pairs lubricated with both biodegradable oils and mixtures.
Figure 2. (a) Average values of friction coefficients and (b) real time friction curves of Al-steel pairs lubricated with both biodegradable oils and mixtures.
Lubricants 11 00329 g002
Lubricants 11 00329 g003
Figure 3. Wear rate of aluminum after tests lubricated with biodegradable oils (BO and BOA), and mixtures.
Figure 3. Wear rate of aluminum after tests lubricated with biodegradable oils (BO and BOA), and mixtures.
Lubricants 11 00329 g003
Lubricants 11 00329 g004aLubricants 11 00329 g004b
Figure 4. Optical micrographs of wear tracks of aluminum disks after tests lubricated with biodegradable oils (BO and BOA) and mixtures.
Figure 4. Optical micrographs of wear tracks of aluminum disks after tests lubricated with biodegradable oils (BO and BOA) and mixtures.
Lubricants 11 00329 g004aLubricants 11 00329 g004b
Lubricants 11 00329 g005
Figure 5. 2D profiles (bottom) and 3D images (up) of the wear tracks on aluminum disks after test lubricated with biodegradable oils (BO and BOA), and 1%Eds + BO.
Figure 5. 2D profiles (bottom) and 3D images (up) of the wear tracks on aluminum disks after test lubricated with biodegradable oils (BO and BOA), and 1%Eds + BO.
Lubricants 11 00329 g005
Lubricants 11 00329 g006
Figure 6. SEM images (left) of aluminum worn surfaces and EDS spectra (right) of worn surfaces after test lubricated with BO and 1%Eds + BO.
Figure 6. SEM images (left) of aluminum worn surfaces and EDS spectra (right) of worn surfaces after test lubricated with BO and 1%Eds + BO.
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Lubricants 11 00329 g007
Figure 7. Raman spectra of aluminum worn surfaces lubricated with (a) BO and (b) 1%Eds + BO.
Figure 7. Raman spectra of aluminum worn surfaces lubricated with (a) BO and (b) 1%Eds + BO.
Lubricants 11 00329 g007
Table 1. Main characteristics of BOA and BO.
Table 1. Main characteristics of BOA and BO.
LubricantDensity (g/mL)
at 15 °C		Kinematic Viscosity (mm2/s)Viscosity IndexFlash Point
(°C)		Pour Point
(°C)
40 °C100 °C
BO0.9249.229.87192290−4.50
BOA0.9149.909.91190312−4.50
Table 2. Name, abbreviation, and molecular structure of each PIL in this study.
Table 2. Name, abbreviation, and molecular structure of each PIL in this study.
AbbreviationStructure
CationAnion
EtsLubricants 11 00329 i001Lubricants 11 00329 i002
MtsLubricants 11 00329 i003
DtsLubricants 11 00329 i004
EdsLubricants 11 00329 i005Lubricants 11 00329 i006
MdsLubricants 11 00329 i007
DdsLubricants 11 00329 i008
Table 3. Chemical compositions of 2024 aluminum alloy.
Table 3. Chemical compositions of 2024 aluminum alloy.
ElementAlCuMgMnSiCrFe
Content (%)90.70–94.703.80–4.901.20–1.800.30–0.90Max 0.50Max 0.10Max 0.50
Table 4. Onset temperature of BO, BOA, neat PILs, and PIL mixtures with BO.
Table 4. Onset temperature of BO, BOA, neat PILs, and PIL mixtures with BO.
LubricantOnset Temperature (°C)
BO323.00
BOA333.01
1 wt.% Ets + BO334.25
1 wt.% Mts + BO336.08
1 wt.% Dts + BO332.99
1 wt.% Eds + BO330.92
1 wt.% Mds + BO331.86
1 wt.% Dds + BO336.10
Table 5. Values of dynamic viscosity of both biodegradable oils, neat PILs, and mixtures. The uncertainty of each value of viscosity was consistently maintained below 0.5%.
Table 5. Values of dynamic viscosity of both biodegradable oils, neat PILs, and mixtures. The uncertainty of each value of viscosity was consistently maintained below 0.5%.
LubricantAverage Dynamic Viscosity (cP)
25 °C40 °C100 °C
BO85.0245.488.76
BOA86.4445.868.89
* Ets--70.66
* Mts--29.23
* Dts--26.86
* Eds--1.907 × 105
* Mds--9.965 × 104
* Dds--3.081 × 104
1 wt.% Ets + BO90.3448.499.06
1 wt.% Mts + BO89.6747.589.03
1 wt.% Dts + BO89.1048.318.93
1 wt.% Eds + BO98.8550.348.95
1 wt.% Mds + BO97.7950.128.95
1 wt.% Dds + BO111.252.248.98
* PILs were solid at this temperature.
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MDPI and ACS Style

Guo, H.; Stoyanovich, B.; Pang, J.; Iglesias, P. Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding. Lubricants 2023, 11, 329. https://doi.org/10.3390/lubricants11080329

AMA Style

Guo H, Stoyanovich B, Pang J, Iglesias P. Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding. Lubricants. 2023; 11(8):329. https://doi.org/10.3390/lubricants11080329

Chicago/Turabian Style

Guo, Hong, Brandon Stoyanovich, Junru Pang, and Patricia Iglesias. 2023. "Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding" Lubricants 11, no. 8: 329. https://doi.org/10.3390/lubricants11080329

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