Next Article in Journal / Special Issue
Compatibility Study of Polyamide (PA6) with Lubricant Bases for Electric Vehicle Applications
Previous Article in Journal
Effect of Graphene Nanoplatelet Content on Mechanical and Elevated-Temperature Tribological Performance of Self-Lubricating ZE10 Magnesium Alloy Nanocomposites
Previous Article in Special Issue
Classification of Progressive Wear on a Multi-Directional Pin-on-Disc Tribometer Simulating Conditions in Human Joints-UHMWPE against CoCrMo Using Acoustic Emission and Machine Learning
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Lubricants
Volume 12
Issue 2
10.3390/lubricants12020053
lubricants-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

A Comparison of the Tribological Properties of Two Phosphonium Ionic Liquids

by
Jeng-Haur Horng
1,
Thi-Na Ta
1,
Raimondas Kreivaitis
2,
Jolanta Treinytė
2,
Artūras Kupčinskas
2 and
Milda Gumbytė
2,*
1
Department of Power Mechanical Engineering, National Formosa University, Yunlin 632, Taiwan
2
Agriculture Academy, Vytautas Magnus University, K. Donelaičio Str. 58, 44248 Kaunas, Lithuania
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(2), 53; https://doi.org/10.3390/lubricants12020053
Submission received: 9 January 2024 / Revised: 8 February 2024 / Accepted: 12 February 2024 / Published: 14 February 2024
(This article belongs to the Special Issue Current and Future Trends in Tribological Research: Fundamentals and Applications–The 10th Anniversary of Lubricants)
Browse Figures
Review Reports Versions Notes

Abstract

:
For over two decades, ionic liquids have been among the most exciting lubrication topics. Ionic liquids were investigated by using them as neat lubricants and lubricity-enhancing additives. However, new and unique features were revealed by introducing new ionic liquids. This paper compares the tribological properties of two ionic liquids with the same trihexyltetradecylphosphonium [P 6,6,6,14] cation and different anions—dicyanamide [DCN] and bis(2,4,4-trimethylpentyl)phosphinate. The widely investigated 1-Butyl-3-methylimidazolium hexafluorophosphate [BMIM] [PF6] ionic liquid was used as a reference. The lubricity was comprehensively investigated using two testing modes: reciprocation in a ball-on-plate tribometer and continuous sliding in a ball-on-disc tribometer. The tests were performed at temperatures of 30 and 80 °C. The friction, wear, and film thickness were evaluated, and a worn surface analysis was conducted. It was found that in the case of reciprocation, anion has a significant effect on the lubricity. The difference was particularly evident when the results at two temperatures were compared. The ability to build a low-friction tribo-film was suggested as the primer source of lubricity. In the case of continuous sliding, the differences were not as noticeable. In this case, viscosity was assigned to be the leading property.

Graphical Abstract

1. Introduction

Room-temperature ionic liquids (RTILs) are salts which exist in liquid form at room temperature [1,2]. These substances were found to be helpful in many applications, including tribology. Due to unique properties, such as lubricity [3,4], adsorption to surfaces [5,6], thermal and electrochemical stability [7,8], and low volatility [9], RTILs have been among the most exciting lubrication topics for over two decades. There was an immense number of papers published on this topic [3,4,10,11,12,13,14]. However, with the appearance of new cation/anion combinations and different application conditions, new and unique features of ILs could be revealed [15,16,17,18].
RTILs can be used as neat lubricants and lubricity-improving additives for various base oils and lubricating greases. Although many scholars recommend using ILs as additives, their unique properties are first revealed by using them as neat lubricants [4]. Moreover, many ILs are insoluble or poorly soluble in nonpolar or even polar base oils [19,20,21].
ILs comprise cations and anions, the combination of which provides them with unique properties. In early studies, halogen-containing ILs were shown to have superior lubricity; however, the effect was very complex [22,23]. Besides enhanced lubricity, in some cases, halogen-containing ILs caused severe corrosion, which was attributed to anion decomposition, reaction with ambient moisture, and the formation of an acidic environment. Based on many studies, a more complex picture involving test conditions, cation structure, and lubricated material composition was revealed [12,24,25,26,27].
Scholars usually focus on investigating one or a few ILs in different conditions. Monge et al. [28] studied the tribological performance of two ILs with a bis(trifluoromethylsulfonyl)imide [NTf2] anion combined with choline and pyrrolidinium cations as additives in fully formulated wind turbine gearbox oil. It was shown that combining NTf2 with the pyrrolidinium cation was superior to choline. The result was explained based on the tribo-film composition, where a higher ratio of iron oxides and iron hydroxy-oxides resulted in better lubricity. They also revealed that an increased test temperature decreases the ratio, decreasing ILs’ positive effect on lubricity. Kawada et al. made a great effort to uncover the lubrication mechanisms of different cyano-based anions with imidazolium and phosphonium cations [29,30,31,32,33,34]. One of their studies demonstrated how different ciano-based anions combined with an imidazolium cation can alter the lubrication mechanisms of H-free DLC [34]. It was shown that the dicyanamide [DCN] anion provides stable friction due to the adsorption of both the anion and cation. On the other hand, an IL with a tricyanomethane [TCC] anion facilitated the formation of tribo-film containing only anions, which resulted in even lower friction. Several studies focused on the influence of the alkyl chain length of a cation or anion [35,36]. By summarising their findings, it could be stated that (i) with longer alkyl chains, a better adsorption ability could be achieved; (ii) increasing the chain length leads to better lubricity; and (iii) short chain possessing moieties can cause corrosion. Some studies reported only a marginal effect of the IL composition [37,38].
Generally, the lubricity of ILs lies in the ability to form an adsorption layer and/or react with the interacting surfaces to form chemically bonded layers [12]. In a mild friction regime, adsorption is enough to prevent metal-to-metal contact. The higher loads and temperatures facilitate the decomposition of IL, followed by a reaction with the metal surface [6,13,36,39]. In most cases, the ambient conditions are of great importance, too. It was shown that hydrophilic ILs lose their lubricity in specific ambient humidity [29,31,40]. On the other hand, some ILs demonstrated extremely low friction when humidity was present [41,42,43]. The behaviour of ILs depends on the nature of the cation and anion and could be revealed through systematic studies.
This paper compares the tribological properties of two phosphonium ionic liquids with the same [P 6,6,6,14] cation and two different anions, namely, DCN and phosphinate. The widely investigated hydrophobic imidazolium IL [BMIM] [PF6] was used as a reference. Their lubricity was comprehensively investigated using two testing modes.

2. Materials and Methods

2.1. Materials

The investigated ionic liquids, Trihexyltetradecylphosphonium dicyanamide, Trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, and 1-Butyl-3-methylimidazolium hexafluorophosphate, were obtained from Sigma Aldrich and used without any preparation. Their chemical structures and purity are presented in Table 1. [BMIM] [PF6] was chosen as a reference due to its hydrophobicity, which ensures stable lubricity results in different ambient conditions [31,44].

2.2. Physical Properties

The kinematic viscosity and density of investigated ILs were measured using Anton Paar Stabinger viscometer SVM 3000. At least three measurements were performed, and the mean value was reported. The viscosity index was calculated according to the standard test method ASTM D2270.

2.3. Lubrication Performance

The lubricity of investigated ionic liquids was compared using two tribometers. The Ducom TR-282 ball-on-plate tribometer acting in reciprocating mode represented harsh friction conditions when changing sliding speed and direction. The PCS Instruments MTM 2 ball-on-disc tribometer worked in continuous sliding and had more opportunities to use viscosity to separate interacting surfaces. The tests were carried out at two temperatures, namely, 30 and 80 °C, referred to as low- and high-temperature tests. The lubricity test conditions are summarised in Table 2.
Three dimensional spacer layer imaging (SLIM) images were taken every 30 min during the ball-on-disc test to measure lubricating film thickness. In addition, the coefficient of friction (COF) variation in time was recorded and analysed.
Before the experiments, balls, plates, and all of the appropriate parts in contact with the lubricating fluids were cleaned using an ultrasonic bath in toluene and acetone. All of the tests were repeated at least twice.

2.4. Wear Evaluation and Analysis of Worn Surface

The worn surfaces were inspected using a Nikon ECLIPSE MA 100 optical microscope (OM) and a Hitachi 3400N scanning electron microscope (SEM). Before the inspection, the surfaces were wiped with acetone. After the ball-on-plate tribo-test, the cross-section profiles of wear traces on the plate were measured using a Mahr GD-25 tip profilometer. The wear volume was calculated using the cross-section profile measurements. Using OM, the diameter of the wear scar on the ball was measured in two diagonal directions, and its average was reported. Bruker Quad 5040 energy-dispersive spectroscopy (EDS) analysis was used to investigate the compositions of worn surfaces.

3. Results

3.1. Physical Properties

The physical properties of the investigated ionic liquids are presented in Table 3. The investigated phosphonium ILs had substantially different viscosities, which sharply varied with the temperature. Due to the large anion [P 6,6,6,14] [phosphinate], the IL has a 1.89 times higher kinematic viscosity than [P 6,6,6,14] [DCN]. However, the difference diminishes at a higher temperature, leaving a difference of 1.4. Because viscosity plays a significant role in lubrication, one could expect differences in performance—a higher performance difference can be expected at a lower test temperature. The reference ionic liquid [BMIM] [PF6] has the lowest viscosity. It also possesses the lowest viscosity index, leading to a high difference in viscosity between the investigated temperatures.

3.2. Friction Reduction Performance

The friction force between sliding surfaces occurs due to viscosity and surface physical interaction. The viscosity-induced friction force requires energy to move the surfaces but usually does not cause wear. On the other hand, the interaction between surface asperities requires energy and produces wear. Moreover, it causes tribo-reactions on the worn surface. Therefore, comparing lubricity in different sliding conditions could lead to substantially different results, which gives a broader understanding of the lubrication performance.

3.2.1. Friction in Reciprocating Tribo-Test

Despite the differences in viscosity, the investigated ILs showed similar COFs at the low test temperature (Figure 1). However, significant differences occurred with the increased test temperature. Surprisingly, phosphonium IL containing a DCN anion showed much lower friction, while the other investigated ILs, including the reference, demonstrated a higher coefficient of friction.
The variations in the friction coefficient during the tests revealed the lubrication process in more detail (Figure 2). It was found that the temperature influences the lubrication process in different ways. At the test temperature of 30 °C, both phosphonium ILs have similar COF variation patterns. However, different friction variations occur at the higher test temperature of 80 °C. In the case of [P 6,6,6,14] [DCN], the COF increases at the onset of the test and then decreases, reaching a value as low as 0.03 (Figure 2a). This variation pattern was confirmed by performing five repetitions in the same conditions, and its variation curves are presented in the Supplementary Material (Figure S1). On the other hand, lubrication with [P 6,6,6,14] [phosphinate] at the temperature of 80 °C resulted in higher friction, which had an increasing tendency and higher fluctuation. In the case of imidazolium IL, the higher temperature increased but stabilised the COF. From these results, it is hypothesised that temperature-induced friction changes predict different lubrication mechanisms.

3.2.2. Friction in Continuous Sliding Tribo-Test

As a result of a higher viscosity, both phosphonium ILs have higher COFs at a lower test temperature in the case of continuous sliding (Figure 3). At the test temperature of 80 °C, the COF was lower at the onset of the test and had a continuously increasing tendency. Therefore, in about 100 testing minutes, both the low- and high-temperature COF became similar. Furthermore, the IL containing a phosphinate anion has a higher viscosity, which results in higher COFs at both investigated temperatures.

3.3. Wear Reduction and Analysis of Worn Surfaces

3.3.1. Wear Observed after Reciprocating Tribo-Test

The wear scar diameters and wear volumes are listed in Table 4. The summarised OM images and cross-section profiles observed after lubrication with [P 6,6,6,14] [DCN], [P 6,6,6,14] [phosphinate], and [BMIM] [PF6] are presented in Figure 4, Figure 5 and Figure 6, respectively. The wear reduction behaviour corresponded to the friction results—higher COF-possessing ILs underwent more intense wear. However, there are many more differences between the investigated ILs and temperatures than in the case of the COF.
The OM images show that, in the cases of low wear, the sliding velocity plays a significant role in lubrication. This is evident in the case of [P 6,6,6,14] [phosphinate] at the temperature of 30 °C (Figure 5a). On the other hand, less pronounced phenomena were also observed after the test of [P 6,6,6,14] [DCN] and [BMIM] [PF6] at the temperatures of 80 and 30 °C, respectively. In these cases, the depth of the wear trace on the plate differed through the length. According to the cross-section profiles, the wear was more profound at both ends and less in the middle. The low speed and changed sliding direction caused more intense wear at the ends of wear traces. The sliding speed reached its maximum in the middle of the wear trace, resulting in less intensive wear.
As the coefficient of friction predicted, an increased test temperature caused changes in the lubrication mechanism for all of the investigated ILs. Interestingly, the influence of temperature was different for the investigated phosphonium ILs. The [P 6,6,6,14] [DCN]-lubricated surfaces underwent higher wear at the test temperature of 30 °C, while [P 6,6,6,14] [phosphinate] showed the opposite performance—the lower wear was observed at the lower test temperature.
A wear trace with many minor scratches was formed during lubrication with [P 6,6,6,14] [DCN] at a low test temperature (Figure 4a). According to the cross-section profiles, the wear trace has a similar depth throughout its length. Substantially different lubrication mechanisms occurred at the higher test temperature of 80 °C. A shallow wear trace with a smooth surface was formed (Figure 4b). The roughness measurements of the wear trace indicate surface polishing (Table 5). A performance difference is also evident when comparing the wear scars on the balls at different test temperatures. Only slight polishing occurred at the high test temperature, while the low-temperature test produced many scratches. It is evident from the wear volume that the tribo-test temperature significantly influences the lubricity of the [P 6,6,6,14] [DCN] ionic liquid (Table 4). At the temperature of 30 °C, relatively high wear was produced, while at the test temperature of 80 °C, the wear volume was near zero. It also must be noted that the ratio between worn and build-up material in the wear trace on the plate after the high-temperature test is close to one, which is evidence of tribo-film formation.
The analysis of the [P 6,6,6,14] [DCN] ionic liquid lubricated surface composition indicated that a nitrogen-rich layer was formed at both test temperatures (Figure 7a,b). Furthermore, a high amount of oxygen was observed on the surface after the low-temperature test. On the other hand, phosphorus was found only in the case of the low-temperature tests.
The lubrication with the phosphonium IL with a phosphinate anion resulted in an opposite temperature–lubricity relation. At the test temperature of 30 °C, low wear and relatively smooth worn surfaces were observed (Figure 5a, Table 5). The ball also underwent only slight polishing. In this case, the sliding velocity plays a vital role because there is a considerable difference between the wear depth throughout the length of the wear trace. At both ends, it is more profound and has less build-up material at the edges of the wear trace. Unfortunately, the wear was raised with the increased test temperature and became abrasive (Figure 5b). The ball also underwent abrasive wear, which can result from three-body wear when wear debris appears in the contact. According to the worn surface composition, the oxygen-rich layer was built during the high-temperature tribo-test (Figure 7d). A small amount of phosphorus was found after the test at both temperatures.
The imidazolium-based [BMIM] [PF6] ionic liquid also showed better lubricity at the lower test temperature. The increased test temperature led to much higher wear and different lubrication mechanisms. At the test temperature of 30 °C, the wear trace on the plate was minimal. It was smooth and had different depths throughout its length. The wear scar on the ball was also smooth, with only slight polishing. However, there were a few tiny scratches, which can result from three-body wear. At the high test temperature of 80 °C, high wear occurred. However, its appearance was different from those observed in the case of phosphonium. The wear scars remained relatively smooth and had a lot of build-up material at the edges of the wear trace (Figure 6). The SEM image of the wear trace on the plate confirms the smooth surface (Figure 7f). High-magnification images reveal many pits of different sizes. According to the literature, there can be tribo-corrosion-based wear [22,45]. The low friction in this case also confirms tribo-corrosion. The only difference in the EDS worn surface analysis was a higher amount of oxygen observed after the high-temperature test.
The roughness of the wear traces observed on the plate reported in Table 5 suggests that smoother surfaces give lower friction coefficients and wear.

3.3.2. Wear and Tribo-Film Thickness Observed in Continuous Sliding Tribo-Test

When comparing the width of the wear trace on the plate, we did not observe a significant difference between the investigated phosphonium ionic liquids (Figure 8). The only difference occurred between the low- and high-temperature tests, where a higher temperature caused a higher wear width. In addition, the OM images of the worn surfaces reveal only slight polishing (Figure 9). More small scratches were formed after the high-temperature test, which aligned with wear.
The tribo-film thickness measurements show that the DNC anion containing IL had a more stable tribo-film built within the first 30 test minutes (Figure 10). It produced tribo-film with a similar thickness at both investigated temperatures. According to the SLIM images, the evenly distributed tribo-film was formed during the low-temperature test. On the other hand, the tribo-film was not so even at a high test temperature of 80 °C.
The film thickness varied widely in the case of the phosphinate-containing IL (Figure 10). At the low test temperature, the tribo-film thickness rose initially, and then decreased. As a result, the SLIM images showed an uneven tribo-film distribution and scratches along the contact zone. The tribo-film formation at 80 °C was the opposite—the thickness increased from the onset and stabilised in 60 min. Again, the SLIM images show an uneven tribo-film distribution and a few scratches along the contact zone. Interestingly, both ILs possess a similar tribo-film thickness after 90 min up to the end of the test.

4. Discussion

The investigated ILs showed considerably different performances at different test temperatures. The most significant difference occurred in the case of reciprocation sliding, where both the lubricants’ viscosity and ability to build tribo-film are essential. On the other hand, the continuous sliding tests showed the prevailing influence of viscosity. The cyano anion with a [P 6,6,6,14] [DCN] IL performed better at the higher test temperature. According to the findings of Kawada et al., cyano-based ILs undergo catalytic decomposition on freshly rubbed metal surfaces, resulting in anion adsorption [29]. It could be that a higher temperature is required for the decomposition process. This hypothesis is supported by surface composition, where more nitrogen and no phosphorus were found in the wear trace after a high-temperature test (Figure 7a,b). In the case of the [P 6,6,6,14] [phosphinate] tests, a small amount of phosphorus was found at both temperatures in the reciprocating tribo-test (Figure 7c,d). Additionally, the thickness of the tribo-film in the test of [P 6,6,6,14] [phosphinate] at a higher test temperature was increased. It could contribute to phosphate tribo-film formation due to tribochemical reactions with rubbing surfaces [46]. The surfaces which underwent intensive wear had considerably more oxygen due to surface oxidation (Figure 7). The oxidised wear debris produced three-body wear, leading to rough, worn surfaces. On the other hand, the wear traces obtained after continuous sliding tribo-tests had only slightly polished surfaces. The SLIM images show the formation of tribo-films with thicknesses of 4 to 6 nm, which was way less than that of ZDDP reported by Dawczyk et al. [47]. In their study, a tribo-film of more than 150 nm was reported. Therefore, it was concluded that while viscosity governs lubricity in continuous sliding conditions, the tribo-film is not growing. While viscosity plays a significant role in lubricity, and the viscosity of ILs is strongly dependent on the water content [48,49], it would be worth studying this phenomenon in the future.

5. Conclusions

The tribological properties of two phosphonium ILs with the same cation and different anions were compared using reciprocation and continuous sliding tribo-test modes. The lubricity evaluation criteria were the coefficient of friction, wear, and worn surface analysis. In light of the obtained results, the following conclusions could be drawn:
  • The anion undoubtedly changes the lubricity of the phosphonium ionic liquid. According to the results of the reciprocation tribo-test, the phosphinate anion-containing ionic liquid performed better at the low test temperature. At the same time, the dicyanamide anion provided phosphonion with better high-temperature lubricity. The lubricity in the continuous sliding tribo-test was governed by the viscosity, where a lower viscosity possessing a dicyanamide-containing ionic liquid was superior in both investigated temperatures.
  • Due to the temperature-dependent lubricity, the investigated phosphonium ionic liquids can outperform the reference imidazolium ionic liquid only at specific temperatures, and the dicyanamide anion-containing ionic liquid was particularly superior at a high test temperature. In contrast, the lubricity of the phosphinate anion-containing ionic liquid was comparable at the low test temperature.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/lubricants12020053/s1, Figure S1: CoF variations observed during the reciprocation tests when lubricating with [P 6,6,6,14] [DCN] at the temperatures of 30 °C (a) and 80 °C (b).

Author Contributions

Conceptualisation, J.-H.H. and R.K.; methodology, J.-H.H., T.-N.T. and R.K.; validation, J.-H.H. and R.K.; formal analysis, T.-N.T. and R.K.; investigation, T.-N.T., J.T., A.K. and M.G. resources, J.-H.H. and M.G.; writing—original draft preparation, J.-H.H., T.-N.T. and R.K.; writing—review and editing, J.-H.H., T.-N.T. and R.K.; visualisation, T.-N.T., J.T. and R.K.; supervision, J.-H.H. and R.K.; funding acquisition, J.-H.H., R.K. and M.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grant no. S-MIP-21-61 from the Research Council of Lithuania.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Austen Angell, C.; Ansari, Y.; Zhao, Z. Ionic Liquids: Past, Present and Future. Faraday Discuss 2012, 154, 9–27. [Google Scholar] [CrossRef]
  2. Ye, C.; Liu, W.; Chen, Y.; Yu, L. Room-Temperature Ionic Liquids: A Novel Versatile Lubricant. Chem. Commun. 2001, 21, 2244–2245. [Google Scholar] [CrossRef]
  3. Zhou, Y.; Qu, J. Ionic Liquids as Lubricant Additives: A Review. ACS Appl. Mater. Interfaces 2017, 9, 3209–3222. [Google Scholar] [CrossRef] [PubMed]
  4. Song, J. Research Progress of Ionic Liquids as Lubricants. ACS Omega 2021, 6, 29345–29349. [Google Scholar] [CrossRef] [PubMed]
  5. Mero, A.; Mezzetta, A.; Nowicki, J.; Łuczak, J.; Guazzelli, L. Betaine and L-Carnitine Ester Bromides: Synthesis and Comparative Study of Their Thermal Behaviour and Surface Activity. J. Mol. Liq. 2021, 334, 115988. [Google Scholar] [CrossRef]
  6. Yang, S.; Zhang, D.; Wong, J.S.S.; Cai, M. Interactions between ZDDP and an Oil-Soluble Ionic Liquid Additive. Tribol. Int. 2021, 158, 106938. [Google Scholar] [CrossRef]
  7. Cao, Y.; Mu, T. Comprehensive Investigation on the Thermal Stability of 66 Ionic Liquids by Thermogravimetric Analysis. Ind. Eng. Chem. Res. 2014, 53, 8651–8664. [Google Scholar] [CrossRef]
  8. Piatti, E.; Guglielmero, L.; Tofani, G.; Mezzetta, A.; Guazzelli, L.; D’Andrea, F.; Roddaro, S.; Pomelli, C.S. Ionic Liquids for Electrochemical Applications: Correlation between Molecular Structure and Electrochemical Stability Window. J. Mol. Liq. 2022, 364, 120001. [Google Scholar] [CrossRef]
  9. Chen, Y.; Han, X.; Liu, Z.; Li, Y.; Sun, H.; Wang, H.; Wang, J. Thermal Decomposition and Volatility of Ionic Liquids: Factors, Evaluation and Strategies. J. Mol. Liq. 2022, 366, 120336. [Google Scholar] [CrossRef]
  10. He, Y.; Li, H.; Qu, C.; Cao, W.; Ma, M. Recent Understanding of Solid-Liquid Friction in Ionic Liquids. Green Chem. Eng. 2020, 2, 145–157. [Google Scholar] [CrossRef]
  11. Santhosh Kumar, S.; Ramesh Kumar, S. Ionic Liquids as Environmental Friendly Cutting Fluids—A Review. Mater. Today Proc. 2020, 37, 2121–2125. [Google Scholar] [CrossRef]
  12. Xiao, H. Ionic Liquid Lubricants: Basics and Applications. Tribol. Trans. 2017, 60, 20–30. [Google Scholar] [CrossRef]
  13. Kreivaitis, R.; Kupčinskas, A.; Žunda, A.; Ta, T.N.; Horng, J.H. Effect of Temperature on the Lubrication Ability of Two Ammonium Ionic Liquids. Wear 2022, 492–493, 204217. [Google Scholar] [CrossRef]
  14. Kreivaitis, R.; Gumbytė, M.; Kupčinskas, A.; Kazancev, K.; Ta, T.N.; Horng, J.H. Investigation of Tribological Properties of Two Protic Ionic Liquids as Additives in Water for Steel–Steel and Alumina–Steel Contacts. Wear 2020, 456–457, 203390. [Google Scholar] [CrossRef]
  15. Donato, M.T.; Colaço, R.; Branco, L.C.; Saramago, B. A Review on Alternative Lubricants: Ionic Liquids as Additives and Deep Eutectic Solvents. J. Mol. Liq. 2021, 333, 116004. [Google Scholar] [CrossRef]
  16. Reeves, C.J.; Kasar, A.K.; Menezes, P.L. Tribological Performance of Environmental Friendly Ionic Liquids for High-Temperature Applications. J. Clean Prod. 2021, 279, 123666. [Google Scholar] [CrossRef]
  17. Del Sol, I.; Gámez, A.J.; Rivero, A.; Iglesias, P. Tribological Performance of Ionic Liquids as Additives of Water-Based Cutting Fluids. Wear 2019, 426–427, 845–852. [Google Scholar] [CrossRef]
  18. Nasser, K.I.; Liñeira del Río, J.M.; López, E.R.; Fernández, J. Hybrid Combinations of Graphene Nanoplatelets and Phosphonium Ionic Liquids as Lubricant Additives for a Polyalphaolefin. J. Mol. Liq. 2021, 336, 116266. [Google Scholar] [CrossRef]
  19. Yu, B.; Bansal, D.G.; Qu, J.; Sun, X.; Luo, H.; Dai, S.; Blau, P.J.; Bunting, B.G.; Mordukhovich, G.; Smolenski, D.J. Oil-Miscible and Non-Corrosive Phosphonium-Based Ionic Liquids as Candidate Lubricant Additives. Wear 2012, 289, 58–64. [Google Scholar] [CrossRef]
  20. Somers, A.E.; Khemchandani, B.; Howlett, P.C.; Sun, J.; 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. [Google Scholar] [CrossRef] [PubMed]
  21. Barnhill, W.C.; Qu, J.; Luo, H.; Meyer, H.M.; Ma, C.; Chi, M.; Papke, B.L. Phosphonium-Organophosphate Ionic Liquids as Lubricant Additives: Effects of Cation Structure on Physicochemical and Tribological Characteristics. ACS Appl. Mater. Interfaces 2014, 6, 22585–22593. [Google Scholar] [CrossRef]
  22. Arora, H.; Cann, P.M. Lubricant Film Formation Properties of Alkyl Imidazolium Tetrafluoroborate and Hexafluorophosphate Ionic Liquids. Tribol. Int. 2010, 43, 1908–1916. [Google Scholar] [CrossRef]
  23. 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. [Google Scholar] [CrossRef]
  24. Qu, J.; Blau, P.J.; Dai, S.; Luo, H.; Meyer, H.M.; Truhan, J.J. Tribological Characteristics of Aluminum Alloys Sliding against Steel Lubricated by Ammonium and Imidazolium Ionic Liquids. Wear 2009, 267, 1226–1231. [Google Scholar] [CrossRef]
  25. Somers, A.E.; Biddulph, S.M.; Howlett, P.C.; Sun, J.; MacFarlane, D.R.; Forsyth, M. A Comparison of Phosphorus and Fluorine Containing IL Lubricants for Steel on Aluminium. Phys. Chem. Chem. Phys. 2012, 14, 8224–8231. [Google Scholar] [CrossRef]
  26. Somers, A.E.; Howlett, P.C.; Sun, J.; MacFarlane, D.R.; Forsyth, M. Transition in Wear Performance for Ionic Liquid Lubricants under Increasing Load. Tribol. Lett. 2010, 40, 279–284. [Google Scholar] [CrossRef]
  27. Minami, I.; Inada, T.; Sasaki, R.; Nanao, H. Tribo-Chemistry of Phosphonium-Derived Ionic Liquids. Tribol. Lett. 2010, 40, 225–235. [Google Scholar] [CrossRef]
  28. Monge, R.; González, R.; Hernández Battez, A.; Fernández-González, A.; Viesca, J.L.; García, A.; Hadfield, M. Ionic Liquids as an Additive in Fully Formulated Wind Turbine Gearbox Oils. Wear 2015, 328–329, 50–63. [Google Scholar] [CrossRef]
  29. Kawada, S.; Sasaki, S. Tribological Properties of Cyano-Based Ionic Liquids under Different Environments. Tribol. Online 2018, 13, 152–156. [Google Scholar] [CrossRef]
  30. Kawada, S.; Sasaki, S.; Miyatake, M. In-Situ Observation of Tribo-Decomposition Behavior of Ionic Liquids Composed of Phosphonium-Cation and Cyano-Anion Using Quadrupole Mass Spectrometer. Tribol. Int. 2021, 153, 106547. [Google Scholar] [CrossRef]
  31. Kawada, S.; Sasaki, S.; Miyatake, M. Effect of Surrounding Atmosphere on Friction Properties of Hydrophobic and Hydrophilic Ionic Liquids. Tribol. Online 2019, 14, 285–292. [Google Scholar] [CrossRef]
  32. Okubo, H.; Kawada, S.; Watanabe, S.; Sasaki, S. Tribological Performance of Halogen-Free Ionic Liquids in Steel–Steel and DLC–DLC Contacts. Tribol. Trans. 2018, 61, 71–79. [Google Scholar] [CrossRef]
  33. Kawada, S.; Watanabe, S.; Tadokoro, C.; Tsuboi, R.; Sasaki, S. Lubricating Mechanism of Cyano-Based Ionic Liquids on Nascent Steel Surface. Tribol. Int. 2018, 119, 474–480. [Google Scholar] [CrossRef]
  34. Kawada, S.; Watanabe, S.; Tsuboi, R.; Sasaki, S.; Prakash, B. Lubrication Mechanism of Halogen-Free Ionic Liquids. Tribol. Online 2017, 12, 155–161. [Google Scholar] [CrossRef]
  35. Pejaković, V.; Tomastik, C.; Dörr, N.; Kalin, M. Influence of Concentration and Anion Alkyl Chain Length on Tribological Properties of Imidazolium Sulfate Ionic Liquids as Additives to Glycerol in Steel–Steel Contact Lubrication. Tribol. Int. 2016, 97, 234–243. [Google Scholar] [CrossRef]
  36. Huang, G.; Fan, S.; Ba, Z.; Cai, M.; Qiao, D. Insight into the Lubricating Mechanism for Alkylimidazolium Phosphate Ionic Liquids with Different Alkyl Chain Length. Tribol. Int. 2019, 140, 105886. [Google Scholar] [CrossRef]
  37. Bartolomé, M.; Gonçalves, D.; Tuero, A.G.; González, R.; Battez, A.H.; Seabra, J.H.O. Greases Additised with Phosphonium-Based Ionic Liquids—Part I: Rheology, Lubricant Film Thickness and Stribeck Curves. Tribol. Int. 2021, 156, 106851. [Google Scholar] [CrossRef]
  38. Matczak, L.; Johanning, C.; Gil, E.; Guo, H.; Smith, T.W.; Schertzer, M.; Iglesias, P. Effect of Cation Nature on the Lubricating and Physicochemical Properties of Three Ionic Liquids. Tribol. Int. 2018, 124, 23–33. [Google Scholar] [CrossRef]
  39. Lu, R.; Mori, S.; Kobayashi, K.; Nanao, H. Study of Tribochemical Decomposition of Ionic Liquids on a Nascent Steel Surface. Appl. Surf. Sci. 2009, 255, 8965–8971. [Google Scholar] [CrossRef]
  40. Minami, I.; Kita, M.; Kubo, T.; Nanao, H.; Mori, S. The Tribological Properties of Ionic Liquids Composed of Trifluorotris(Pentafluoroethyl) Phosphate as a Hydrophobic Anion. Tribol. Lett. 2008, 30, 215–223. [Google Scholar] [CrossRef]
  41. Han, T.; Yi, S.; Zhang, C.; Li, J.; Chen, X.; Luo, J.; Banquy, X. Superlubrication Obtained with Mixtures of Hydrated Ions and Polyethylene Glycol Solutions in the Mixed and Hydrodynamic Lubrication Regimes. J. Colloid. Interface Sci. 2020, 579, 479–488. [Google Scholar] [CrossRef] [PubMed]
  42. Espinosa, T.; Jiménez, M.; Sanes, J.; Jiménez, A.-E.; Iglesias, M.; Bermúdez, M.-D. Ultra-Low Friction with a Protic Ionic Liquid Boundary Film at the Water-Lubricated Sapphire–Stainless Steel Interface. Tribol. Lett. 2014, 53, 1–9. [Google Scholar] [CrossRef]
  43. Hua, J.; Björling, M.; Larsson, R.; Shi, Y. Controllable Superlubricity Achieved with Mixtures of Green Ionic Liquid and Glycerol Aqueous Solution via Humidity. J. Mol. Liq. 2022, 345, 117860. [Google Scholar] [CrossRef]
  44. Huddleston, J.G.; Visser, A.E.; Reichert, W.M.; Willauer, H.D.; Broker, G.A.; Rogers, R.D. Characterization and Comparison of Hydrophilic and Hydrophobic Room Temperature Ionic Liquids Incorporating the Imidazolium Cation. Green Chem. 2001, 3, 156–164. [Google Scholar] [CrossRef]
  45. Jiménez, A.E.; Bermúdez, M.D.; Iglesias, P.; Carrión, F.J.; Martínez-Nicolás, G. 1-N-Alkyl -3-Methylimidazolium Ionic Liquids as Neat Lubricants and Lubricant Additives in Steel–Aluminium Contacts. Wear 2006, 260, 766–782. [Google Scholar] [CrossRef]
  46. Luiz, J.F.; Spikes, H. Tribofilm Formation, Friction and Wear-Reducing Properties of Some Phosphorus-Containing Antiwear Additives. Tribol. Lett. 2020, 68, 75. [Google Scholar] [CrossRef]
  47. Dawczyk, J.; Morgan, N.; Russo, J.; Spikes, H. Film Thickness and Friction of ZDDP Tribofilms. Tribol. Lett. 2019, 67, 34. [Google Scholar] [CrossRef]
  48. Seddon, K.R.; Stark, A.; Torres, M.-J. Influence of Chloride, Water, and Organic Solvents on the Physical Properties of Ionic Liquids. Pure Appl. Chem. 2000, 72, 2275–2287. [Google Scholar] [CrossRef]
  49. Marták, J.; Schlosser, Š. Density, Viscosity, and Structure of Equilibrium Solvent Phases in Butyric Acid Extraction by Phosphonium Ionic Liquid. J. Chem. Eng. Data 2017, 62, 3025–3035. [Google Scholar] [CrossRef]
Lubricants 12 00053 g001
Figure 1. The mean coefficient of friction observed in the reciprocation tribo-test.
Figure 1. The mean coefficient of friction observed in the reciprocation tribo-test.
Lubricants 12 00053 g001
Lubricants 12 00053 g002
Figure 2. CoF variations observed during the reciprocation tests when lubricating with [P 6,6,6,14] [DCN] (a), [P 6,6,6,14] [phosphinate] (b), and [BMIM] [PF6] (c) at both investigated temperatures.
Figure 2. CoF variations observed during the reciprocation tests when lubricating with [P 6,6,6,14] [DCN] (a), [P 6,6,6,14] [phosphinate] (b), and [BMIM] [PF6] (c) at both investigated temperatures.
Lubricants 12 00053 g002
Lubricants 12 00053 g003
Figure 3. COF variations observed during the continuous sliding tribo-tests when lubricating with [P 6,6,6,14] [DCN] and [P 6,6,6,14] [phosphinate] at both investigated temperatures.
Figure 3. COF variations observed during the continuous sliding tribo-tests when lubricating with [P 6,6,6,14] [DCN] and [P 6,6,6,14] [phosphinate] at both investigated temperatures.
Lubricants 12 00053 g003
Lubricants 12 00053 g004
Figure 4. OM images of the wear trace on the plate (a,b), wear scar on the ball (a1,b1), and corresponding cross-section profiles of wear trace on the plate produced during the test of [P 6,6,6,14] [DCN] at the temperatures of 30 and 80 °C, respectively.
Figure 4. OM images of the wear trace on the plate (a,b), wear scar on the ball (a1,b1), and corresponding cross-section profiles of wear trace on the plate produced during the test of [P 6,6,6,14] [DCN] at the temperatures of 30 and 80 °C, respectively.
Lubricants 12 00053 g004
Lubricants 12 00053 g005
Figure 5. OM images of the wear trace on the plate (a,b), wear scar on the ball (a1,b1), and corresponding cross-section profiles of wear trace on the plate produced during the test of [P 6,6,6,14] [phosphinate] at the temperatures of 30 and 80 °C, respectively.
Figure 5. OM images of the wear trace on the plate (a,b), wear scar on the ball (a1,b1), and corresponding cross-section profiles of wear trace on the plate produced during the test of [P 6,6,6,14] [phosphinate] at the temperatures of 30 and 80 °C, respectively.
Lubricants 12 00053 g005
Lubricants 12 00053 g006
Figure 6. OM images of the wear trace on the plate (a,b), wear scar on the ball (a1,b1), and corresponding cross-section profiles of wear trace on the plate produced during the test of [BMIM] [PF6] at the temperatures of 30 and 80 °C, respectively.
Figure 6. OM images of the wear trace on the plate (a,b), wear scar on the ball (a1,b1), and corresponding cross-section profiles of wear trace on the plate produced during the test of [BMIM] [PF6] at the temperatures of 30 and 80 °C, respectively.
Lubricants 12 00053 g006
Lubricants 12 00053 g007
Figure 7. SEM images and EDS spectra of wear scars on the plate produced during the test of [P 6,6,6,14] [DCN] (a,b), [P 6,6,6,14] [phosphinate] (c,d), and [BMIM] [PF6] (e,f) at the temperatures of 30 and 80 °C, respectively. Other elements refer to Fe, Cr, and Al for [P 6,6,6,14] [DCN] and [P 6,6,6,14] [phosphinate]-lubricated surfaces and Fe, Cr, Al, Na, and K for [BMIM] [PF6]-lubricated surfaces. The red dashed line in the SEM image separates worn and intact regions.
Figure 7. SEM images and EDS spectra of wear scars on the plate produced during the test of [P 6,6,6,14] [DCN] (a,b), [P 6,6,6,14] [phosphinate] (c,d), and [BMIM] [PF6] (e,f) at the temperatures of 30 and 80 °C, respectively. Other elements refer to Fe, Cr, and Al for [P 6,6,6,14] [DCN] and [P 6,6,6,14] [phosphinate]-lubricated surfaces and Fe, Cr, Al, Na, and K for [BMIM] [PF6]-lubricated surfaces. The red dashed line in the SEM image separates worn and intact regions.
Lubricants 12 00053 g007
Lubricants 12 00053 g008
Figure 8. Width of the wear trace on the disc formed during continuous sliding tribo-test.
Figure 8. Width of the wear trace on the disc formed during continuous sliding tribo-test.
Lubricants 12 00053 g008
Lubricants 12 00053 g009
Figure 9. Appearance of the wear traces on the disc formed during continuous sliding tribo-test.
Figure 9. Appearance of the wear traces on the disc formed during continuous sliding tribo-test.
Lubricants 12 00053 g009
Lubricants 12 00053 g010
Figure 10. Tribo-film formation during the ball-on-disc continuous sliding test when lubricating with [P 6,6,6,14] [DCN] and [P 6,6,6,14] [phosphinate].
Figure 10. Tribo-film formation during the ball-on-disc continuous sliding test when lubricating with [P 6,6,6,14] [DCN] and [P 6,6,6,14] [phosphinate].
Lubricants 12 00053 g010
Table 1. Summary of investigated ionic liquids.
Table 1. Summary of investigated ionic liquids.
Ionic Liquid LubricantsMolecular StructureAppearance ColourPurity, %Water
Trihexyltetradecylphosphonium dicyanamide
[P 6,6,6,14] [DCN]
CAS No. 701921-71-3		Lubricants 12 00053 i001Very dark yellow>95 *0.2% *
Trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate
[P 6,6,6,14] [phosphinate]
CAS No. 465527-59-7		Lubricants 12 00053 i002Light yellow>90 *≤1.0% *
1-Butyl-3-methylimidazolium hexafluorophosphate
[BMIM] [PF6]
CAS No. 174501-64-5		Lubricants 12 00053 i003Very faintly brownish yellow99.7 *n.d.
* Values obtained from the manufacture certificate.
Table 2. Summary of test conditions in performed tribo-tests.
Table 2. Summary of test conditions in performed tribo-tests.
Ball-on-Plate Reciprocating ModeBall-on-Disc Continuous Sliding
BallDiameter—6 mm; Roughness (Ra)—0.05 μm; Hardness (HV)—750…800; Material—E 52100BallDiameter—19.05 mm; Roughness (Ra)—0.02 μm; Hardness (HV)—800…920; Material—E 52100
PlateDiameter—10 mm; Thickness—3 mm; Roughness (Ra)—0.02 μm; Hardness (HV)—190…200; Material—E 52100DiscDiameter—46 mm; Roughness (Ra)—0.02 μm; Hardness (HV)—720…780; Material—E 52100
Tribo-test parameters
Reciprocation frequency, Hz15Sliding speed, m/s0.1
Load, N4 40
Contact pressure, GPa1.05 1
Test duration, min120 min 120 min
Test temperature, °C30 and 80 30 and 80
Stroke length, mm1
Table 3. Physical properties of investigated ionic liquids.
Table 3. Physical properties of investigated ionic liquids.
Ionic Liquidν (mm2/s)VIρ (g/cm3)
at 25 °C		MW (g/mol)
3080
[P 6,6,6,14] [DCN]358.244.2147.80.8997549.9
[P 6,6,6,14] [phosphinate]677.461.9139.40.8933773.3
[BMIM] [PF6]133.617.9120.51.3642284.2
ν, kinematic viscosity; VI, viscosity index; ρ, density; MW, molecular weight.
Table 4. The mean wear scar diameter on the ball and wear volume on the plate observed after tribo-tests.
Table 4. The mean wear scar diameter on the ball and wear volume on the plate observed after tribo-tests.
Ionic LiquidWear Scar Diameter [μm]Wear Volume [μm3] × 103
30 °C80 °C30 °C80 °C
[P 6,6,6,14] [DCN]168106110.200.94
[P 6,6,6,14] [phosphinate]1162845.42685.04
[BMIM] [PF6]1152061.93162.30
Table 5. The roughness of the wear traces on the plate measured perpendicular to the sliding direction.
Table 5. The roughness of the wear traces on the plate measured perpendicular to the sliding direction.
Ionic LiquidsRa, μm
30 °C80 °C
[P 6,6,6,14] [DCN]0.0330.002
[P 6,6,6,14] [phosphinate]0.0050.151
[BMIM] [PF6]0.0040.022
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Horng, J.-H.; Ta, T.-N.; Kreivaitis, R.; Treinytė, J.; Kupčinskas, A.; Gumbytė, M. A Comparison of the Tribological Properties of Two Phosphonium Ionic Liquids. Lubricants 2024, 12, 53. https://doi.org/10.3390/lubricants12020053

AMA Style

Horng J-H, Ta T-N, Kreivaitis R, Treinytė J, Kupčinskas A, Gumbytė M. A Comparison of the Tribological Properties of Two Phosphonium Ionic Liquids. Lubricants. 2024; 12(2):53. https://doi.org/10.3390/lubricants12020053

Chicago/Turabian Style

Horng, Jeng-Haur, Thi-Na Ta, Raimondas Kreivaitis, Jolanta Treinytė, Artūras Kupčinskas, and Milda Gumbytė. 2024. "A Comparison of the Tribological Properties of Two Phosphonium Ionic Liquids" Lubricants 12, no. 2: 53. https://doi.org/10.3390/lubricants12020053

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop