Next Article in Journal
An Experimental Study on Ultrasonic Vibration-Assisted Turning of Aluminum Alloy 6061 with Vegetable Oil-Based Nanofluid Minimum Quantity Lubrication
Previous Article in Journal
Comparison of Rheological Methods to Measure Grease Degradation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Lubricants
Volume 11
Issue 11
10.3390/lubricants11110469
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

Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts

by
B. Depu Kumar Patro
1,
P. S. Suvin
1,
Raimondas Kreivaitis
2 and
Milda Gumbytė
2,*
1
Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India
2
Agriculture Academy, Vytautas Magnus University, K. Donelaičio Str. 58, 44248 Kaunas, Lithuania
*
Author to whom correspondence should be addressed.
Lubricants 2023, 11(11), 469; https://doi.org/10.3390/lubricants11110469
Submission received: 30 August 2023 / Revised: 12 October 2023 / Accepted: 30 October 2023 / Published: 1 November 2023
(This article belongs to the Special Issue Advances in Molecular Rheology and Tribology)
Browse Figures
Versions Notes

Abstract

:
This study aims to evaluate the tribological properties of two protic ionic liquids (PILs) under different tribological conditions as a sustainable alternative for mineral oil-based neat lubricants. The synthesis of PILs in this study uses a relatively simple and less expensive method. The Fourier transform infrared (FTIR) spectroscopy results help validate the synthesised PILs’ formation. Further, their physicochemical and tribological properties were investigated. The PILs as neat lubricants were tested on a ball-on-plate reciprocating tribometer using bearing steel–bearing steel and bearing steel–aluminium alloy friction pairs at 30 °C and 80 °C. The results show that the investigated PILs significantly reduced the coefficient of friction and wear. The dodecylamine-based PILs performed better in friction and wear reduction than the other investigated lubricants. The formation of the adsorption layer on the friction pairs was assumed to be the dominant friction and wear reduction mechanism.

1. Introduction

The energy losses in the form of friction during machining, the rubbing action of the cylinder liner surface and piston ring, the sliding of the balls present in the ball bearing inside the rotor or pump system, and many others can be estimated to be from 20 to 30% of the total power loss [1]. Applying lubricant not only achieves fuel economy by reducing friction and wear but also increases the heat transfer capability, reduces surface corrosion and oxidation activity, and removes wear debris and contamination simultaneously [2]. The demand for environmentally friendly and less toxic lubricants has risen due to the restricted use of common additives such as sulphur, zinc, and phosphorus in the base oil to improve the lubricant’s efficiency. As a result, the newly developed lubricants must be free from such toxic additives and should be environmentally friendly.
In the past decade, ionic liquids (ILs) were used as alternative lubricants to replace the consumption of mineral-based oil. ILs used for lubrication are room-temperature molten salts made by combining an appropriate ratio of organic cations and organic or inorganic anions. Most ILs generally exist in liquid forms at room temperature because their ions interact poorly with each other. Ionic liquids were first explored in the lubrication field in 2001; since then, they have attracted many researchers to use ILs as neat lubricants or as additives in base oils. Their unique properties make them special compared to conventional lubricants, such as their good thermal stability [3,4,5], low vapour pressure [6,7,8], non-flammability [7], low toxicity [9,10,11,12], high pour point and flash point [13], negligible volatility [5], inherent polarity [5,14], low melting point and molecular design flexibility [5,6,7], etc. The most popular cations reported for ionic liquids are imidazolium, ammonium, phosphonium, pyridinium, and some popular halogen anions such as PF6, BF4, NTf2, etc. [15].
The aprotic ionic liquids (AILs) are used as neat lubricants or as additives in base oil for better lubrication capability [16,17,18,19]. Most reported AILs lubricants are associated with halogen elements like chlorine or fluorine. In the lubricants, the halogen elements interact with the moisture in the atmosphere and liberate acids. Those acids come in contact with metal surfaces and corrode the metal surfaces [3,20,21,22,23]. Moreover, the complicated synthesis of AILs makes them more expensive for use as lubricants [24]. The interest in protic ionic liquids lubrications significantly increased due to their low toxicity, environmentally friendly nature, and simple one-step synthesis. The desired properties of PILs can be obtained by combining different cations and anions [25,26,27,28].
The current investigation reports the synthesis of two halogen-free neat PILs consisting of an acid anion and two dissimilar ammonia-based cations. Furthermore, the PIL’s physicochemical properties, such as thermal stability, wettability, and rheological properties, are analysed. Likewise, a ball-on-plate reciprocating tribometer evaluated the tribological performance of a PIL lubricant on bearing steel–bearing steel and bearing steel–aluminium alloy friction pairs at 30 °C and 80 °C. The results were compared with the fully formulated engine oil 20W40.

2. Materials and Methods

2.1. Materials and Synthesis of PILs

All the chemical reagents used to synthesise the PILs in this study were technical grade. Oleic acid CAS No. 112-80-1 (≥90%), tert-octylamine CAS No. 107-45-9 (≥95%), and dodecylamine CAS No. 124-22-1 (≥98%) were obtained from Sigma-Aldrich. Engine oil 20W40 was obtained from a local distributor for reference purposes.
The chemical structure of desired PILs is shown in Table 1. The two PILs were synthesised based on a simple acid/base neutralisation reaction, in which oleic acid is added to tert-octylamine or dodecylamine. The PILs are synthesised as per the literature [14]. The solutions were prepared by acid: base molar ratio 1:1. The following procedure was used to conduct the reaction: a three-necked round bottom glass reactor with a volume of 300 mL with a reflux condenser and dropping funnel was taken (Figure 1). The whole reactor was kept inside the paraffin oil bath. A thermometer was kept inside the oil bath, which helped to measure the reaction’s temperature. The required amount of amine was kept inside the glass reactor. Then, the organic acid was added dropwise (6 drops/min) into the glass reactor and stirred (400–600 rpm) by a magnetic stirrer continuously for 24 h at a temperature of 80 ℃. The acid and amine used to synthesise PILs are summarised in Table 2. The block diagram indicates the synthesis process and the visual appearance of synthesised PILs is shown in Table 1 and Figure S1, respectively. The Fourier transform infrared (FTIR) spectroscopy of Bruker IFS66V/S (Bruker, Karlsruhe, Germany) uses a range of scans of 4000–400 cm−1 at scanning intervals of 1 cm−1, and a resolution of 4 cm−1 helps to validate the formation of PILs. Kinematic viscosity measurements of PILs and 20W40 lubricants were performed in an Anton Paar Lovis 2000 M/ME viscometer (Anton Paar, Graz, Austria). The results are shown in Table 3.

2.2. Contact Angle Measurements

The contact angle represents the wettability of lubricant on a solid substrate. The contact angle is the angle between the tangent drawn on the surface of the liquid–vapour interface and the tangent drawn on the solid–liquid interface at their intersection. Young’s equation (Equation (1)) is used to find the contact angle of the lubricant on the metal surface. Young’s equation is applicable for an equilibrium system having a non-reactive, perfectly smooth, chemically homogeneous, and non-deformable surface:
cos θ = γ S V − γ S L γ L V
where θ is the contact angle; γ S V , solid–gas interface surface tension; γ S L , solid–liquid interfacial surface tension; γ L V , liquid–gas interfacial surface tension.
The contact angle/wettability measurements were obtained using KRUSS Drop shape analyser DSA 100 (KRÜSS GmbH, Hamburg, Germany) on the polished aluminium and bearing steel plate surfaces. The metal surfaces were cleaned with a cotton cloth wetted in n-heptane before each measurement. The digital microscope was tilted by 1.5 degrees to the horizontal surface of the slide table to obtain a clear image of the drop profile [29]. A droplet volume of approximately 10–20 μL was poured on the metal surface at room temperature. The contact angle image was recorded after 5, 15, 180, and 300 s. At least three repetitions were taken for each lubricant. In this study, a lower contact angle shows the better spreading ability of lubricants on the metal surface.

2.3. Thermal Analysis

The thermal stability of the PILs was determined according to their decomposition temperature using Perkin Elmer thermogravimetric analysis (PerkinElmer TGA 4000, Singapore) instruments in a nitrogen atmosphere of 20 mL/min flow rate. A weight of approximately 4.5 mg of the sample was taken in a crucible pan. The test was carried out in a temperature range which varied from 25 °C to 800 °C, including a 5 min temperature hold at 200 °C at a heating rate of 10 °C/min. The onset decomposition temperature (To) and endset decomposition temperature (Td) were measured at 5% and 95% mass loss, respectively [30]. The onset decomposition temperature (To) is calculated according to ASTM E2550-17 [31]. This is the region on the TGA curve where a first deviation is observed from the established baseline earlier to the thermal event [32]. TGA study gives the lubricant performance analysis at elevated temperatures.

2.4. Rheology

The PILs’ rheological behaviours were studied using a rotational rheometer (Anton Paar MCR 702, Graz, Austria) with plate–plate configurations. The rotational plate has a diameter of 20 mm, maintaining a 1 mm gap between the plates throughout the experiment. A Peltier system with an accuracy of 0.1 °C was used to control the temperature of the rheometer. Before starting each test, the rheometer equilibration time was set to 5 min to avoid any temperature difference. For a given temperature, the influence of shear flow on viscosity was evaluated while varying shear rates from 0.01 to 500 s−1. Each test was performed at four different temperatures of 25, 40, 80, and 100 °C. Each test was repeated at least three times to ensure reproducibility.

2.5. Tribological Tests

Tribological tests were examined by an Anton Paar (Graz, Austria) TRB3 Tribometer with a ball-on-plate configuration with reciprocating motion. The tests were conducted using a bearing steel ball of 6 mm diameter with a hardness of 697–900 HV and surface roughness of 0.05 μm, which was slid against a bearing steel and aluminium alloy plate. The chemical compositions of tribo-pairs used in these experiments are listed in Table 4. The experiments were conducted at 30 and 80 °C. The tribometer has a dedicated heating device. The device heated the BS and AL plates. The friction pairs were kept at the investigated temperatures for 10–15 min to ensure uniform heating. The lubricants were applied to the respective plates before heating. Before starting the tests, the balls, sample plates, and other involved parts were cleaned with acetone/isopropyl alcohol (99.5%) and then dried at room temperature; this cleaning process was repeated after each completed trial. The testing was conducted with the minimum amount of lubrication, using approximately 30–40 μL of lubricant between the friction pairs. The testing parameters are summarised in Table 5. Each lubricant sample test was repeated at least three times to ensure reproducibility. The coefficient of friction was continuously recorded with respect to the time and calculated as the average of each run.

2.6. Worn Surface Analysis

A Nanovea ST400 (Nanovea, Irvine, USA) 3D profilometer was used to study the three-dimensional wear scar image and evaluate wear volume. The wear volume is evaluated according to ASTM G133 [33], in which the cross-sectional area of the wear track profile is evaluated at different locations along the length of the wear scar. Then, the mean area of the cross-section was multiplied by the length of the wear scar to obtain the wear volume results. The surface characterisation image and surface elemental analysis of the wear track of frictional metal pairs were analysed using a Schottky field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDX) (FESEM-7610FPlus, Jeol Ltd., Tokyo, Japan).

3. Results and Discussion

3.1. Lubricants Characterisation and Thermal Stability

A simple acid/base neutralisation process between the equimolar portion of amine and organic acids was used to synthesise the PILs. The FTIR spectroscopic analyses show the formation of the PILs. The FTIR spectrum of the synthesised PILs is shown in Figure 2. A strong, broad-spectrum range of 3000–2800 cm−1 corresponds to the overlapping of the O-H stretch in the carboxylate group and the N-H stretching bands. The vibration modes of the C-H bending at 1462 cm−1 confirmed the alkyl chains in the PILs. The band range from 2921–2854 cm−1 is attributed to the symmetric and asymmetric -CH2 stretching, indicating the presence of an oleate anion in the PIL lubricants. Figure 2a shows a weak appearance of the C=O stretch in the carboxylate group of bands at 1720 cm−1, which might correspond to the presence of a carboxylic acid in the PIL 01 lubricant. The vibrational modes at 1561 cm−1 and 1552 cm−1 bands shown in Figure 2a,b is attributed to the NH2 scissoring of a primary amine in both the PILs. The bands at 722 cm−1 and 721 cm−1 correspond to the N-H wag vibration of a secondary amine. These results confirmed that the TA and DA cations were present in PILs [28,34,35]. The physical properties of the synthesised PILs are shown in Table 3. The two PILs show nearly similar densities. The viscosity index of PIL 01 is higher than PIL 02, which resembles the viscosity variation with respect to temperature, which was higher in PIL 02 than in PIL 01.
Table 6 summarises the contact angle measurements at room temperature for the PILs and reference oil on BS and AL plate surfaces. The wettability is generally attributed to the spreading of the lubricants over the substrate surface. A good wettability allows the lubricants to quickly penetrate a small clearance between the contact surfaces [36].
PIL 01 lubricant shows the lowest contact angle on both the contact substrates. It could be due to the higher polarity of the tert-octylamine cation-based PIL 01 lubricant, which helps to increase the overall surface adsorption. In general, the polarity of lubricant molecules leads to better wettability by reducing the surface tension. The contact angle of PIL 02 was higher than that of PIL 01 at all testing time intervals for the aluminium and bearing steel plates. It could be interpreted that, with an increase in the carbon chain length of the ionic liquid, the contact angle also increased, like the contact angle of PIL 02. With the increased exposure time, the droplets of the lubricants were spreading; thus, the contact angle was reduced. The motor oil, 20W40, had the lowest contact angle among the investigated lubricants. This might be because the engine oil contains an additive, which helps to minimize the viscosity and surface tension for a lower contact angle. The above results show that the more vital interaction between the lubricant molecules and the metal surface under static conditions could be the reason for forming the boundary layer [37,38].
In general, lubricants are not only used for the lubrication process but also as a carrier of the heat generated from the friction pair application. So, the thermal study of the PILs is needed to understand the sustainability of the lubricants under high temperatures. The thermal stability of the lubricants was investigated by thermogravimetric analysis (TGA), and the results are shown in Figure 3 and Table 7. The thermal stability experiment using TGA was conducted in a nitrogen-surrounding atmosphere to reduce the oxidation effect of the lubricants. This ensured the lubricants could sustain higher temperatures before degradation occurred.
The decomposition curve exhibits a two-stage degradation process [39]. The initial degradation phase refers to the loss of the absorbed moisture; the peak temperature at this stage for the PIL 01, PIL 02, and 20W40 lubricants was 85, 132, and 171 °C, respectively. The second decomposition stage refers to the primary decomposition of lubricants. The To and Td are the peak temperatures of this stage, as represented in Table 7. It was noticed that the thermal decomposition temperature of the PILs increases as the alkyl chain length of the cation increases, without any changes to the anion. A longer alkyl chain has a stronger intermolecular force. Breaking the neighbour bonds of a molecule, particularly breaking the C-C bond, requires a large amount of energy as it bonds with an inter- and intramolecular competitive structure [40]. Therefore, PIL 02 has higher thermal stability than PIL 01. The commercially available engine oil shows slightly higher thermal stability than the PILs. This could result from the reversal of the acid/base proton exchange equilibrium of the PILs and the consequent volatilisation of the acid/base components [41].

3.2. Rheological Behaviour

The rheological behaviour in the current study discusses how the viscosity changes with the shear rate at different temperatures. Figure 4 shows the viscosities at different temperatures as a function of the shear rate. It was noticed that PIL 02 has the highest viscosity among the investigated lubricants, except at 100 °C. In general, a highly viscous fluid could be helpful for a higher load-carrying capacity under difficult tribological conditions [38].
Furthermore, the higher viscosity of the lubricants resembles a high internal resistance, which in turn requires a higher torque to run the system, which leads to poor mechanical efficiency. At the same time, a viscous fluid has less internal leakage through the clearance in rotating components, which results in better volumetric efficiency. Therefore, an optimal viscosity of the lubricants is necessary to obtain better results [42].
It can be seen in Figure 4 that the behaviours of PIL 02 show that it behaves as a Newtonian fluid only at 40 °C, with a constant viscosity of 1108 mPa·s. At the other investigated temperatures, it behaves as a non-Newtonian fluid. The high viscosity at a lower temperature of the PILs could be due to the strong internal hydrogen bonding of the PILs. When the temperature of the lubricant rises, it results in a weakening of the hydrogen bonds. Also, it reduces the intermolecular interaction between the fluid molecules, reducing the viscosity of the PILs [43,44]. Thus, a drop in the viscosities of the lubricant was reported. The PIL 01 and 20W40 lubricants behave as Newtonian fluid at temperatures of 25 °C and 40 °C, with a constant viscosity of 957.6 mPa.s and 309 mPa.s at 25 °C and 329 mPa.s and 173.5 mPa.s at 40 °C, respectively. Furthermore, the behaviours differed, like non-Newtonian fluids, at 80 °C and 100 °C. The PIL 02 lubricant shows shear thinning behaviour at 25 °C, which was not observed in the other investigated lubricants.
In Figure 4c,d, the viscosity value was gradually increased with respect to the shear rate, showing the shear thickening behaviours of the lubricants. These non-Newtonian behaviours of the lubricants are attributed to the thermally disordered molecules at high temperatures. The shear thickening results are attributed to the van der Walls forces and ionic interaction, which is negligible at lower temperatures. However, hydrogen bond strength is reduced with increasing temperature due to its sensitivity towards temperature and pressure. Thus, the aggregate formation resulted from the intermolecular hydrogen bond networks [45]. Figure 4 also noted that when the temperature increased, the viscosity value decreased. However, the shear thinning and shear thickening behaviours were seen with increasing shear rates. These results might be attributed to the molecular orientation differences in the ILs when subjected to stress [46].

3.3. Friction and Wear Evaluation

The mean friction coefficient (µ) obtained for all the studied lubricants is shown in Figure 5. The order of the COF on the BS–BS and BS–AL friction pairs for 30 °C and 80 °C is as follows: —PIL 02 < PIL 01 < 20W40. Both the PILs show a low coefficient of friction in comparison to the 20W40 oil. Further, the PIL 02 lubricant has the lowest COF compared to PIL 01 and 20W40 at 30 °C and 80 °C. The PIL lubricant shows a decreased mean friction coefficient with increased temperature, except for PIL 01 in the case of the BS–AL friction pair. Also, both PILs show lower friction coefficients for the BS–BS friction pairs with increasing temperature. This could be due to the change in the viscosity and increased chemical reaction between the lubricants and friction pairs as the temperature increased. Moreover, the wear was considerably higher at the higher test temperatures, which resulted in reduced contact pressure.
The literature mentions several reasons for the low COF while using the PIL lubricants. The formation of a strongly adsorbed layer and thermally and/or mechanically induced chemical reactions are the primary lubrication mechanisms. It could be that the anion in the PILs, like the carboxylic group COOH, was firmly attached to the positively charged metal surface by electrostatic attraction. Above this layer, cations were attached by their alkyl chain to form the next subsequent layer. This leads to the formation of a closed-packed layer of the adsorbed molecules. Generally, for the PILs, the wear and friction coefficient reduction mainly depended on the molecular structure of the cation and anion and the interactive force between the molecules. The formation of a low-shear adsorption layer will decompose at a higher load and react with the nascent metal surface to form a protective tribofilm. This summarises the reason behind the reduction in the friction coefficient and wear on the tribo-pairs [47,48,49,50,51].
Figure 6 summarises the wear volume of the BS–BS and BS–AL friction pairs using PILs and 20W40 at 30 °C and 80 °C. The wear volume at high temperatures for the BS–BS tribo-pair is nearly the same as that at low temperatures for the BS–AL tribo-pair. The wear volume of the BS–BS frictional pair at two different temperatures follows the following order at 30 °C: PIL 02 < PIL 01 < 20W40, and at 80 °C: PIL 01 < PIL 02 < 20W40. Similarly, for the BS–AL pair at 30 °C, the order is as follows: PIL 02 < PIL 01 < 20W40, and at 80 °C: 20W40 < PIL 02 < PIL 01. The wear rate is much higher in both friction pairs at 80 °C. Moreover, in the BS–AL tribo-pair, the material loss at the high temperature increases manifold compared to the wear rate at the low-test temperature.
In the case of the BS–AL friction pair at 80 °C, the PILs show higher wear than when the reference oil 20W40 was used. This could result from tribo-corrosion, in which the decomposition products of the PILs were aggressive against the aluminium alloy. Under dynamic conditions at a high load and temperature, aluminium’s outmost passivated oxide layer becomes worn out, and the exposed nascent surface strongly reacts with the PILs [52,53]. Aluminium is a highly reactive metal with a stronger affinity for reacting with a functional group, such as the carboxylate group in the PILs. Therefore, different tribological behaviour of PILs was reported on the BS–AL friction pair compared to that of BS–BS [41].
At 30 °C for the BS–BS friction pair, the lubricant PIL 02 shows good anti-wear properties, leading to the lowest wear compared to the other tested lubricants. When the temperature rises to 80 °C, the wear of PIL 02 increases more than PIL 01, but still, the wear is less than the 20W40 lubricant. The worn surface roughnesses observed on the bearing steel and aluminium plates are summarised in Table 8.

3.4. Worn Surface Analysis

Figure 7 and Figure 8 show the 3D profilometer image of the wear track on the BS and Al surfaces. From these images, it can be noticed that the wear track of the BS and AL surfaces have different dimensions throughout their lengths.
For the PILs at 30 °C, both ends of the wear tracks were slightly broader and deeper on the BS and AL plate surfaces. These results are attributed to the change in the ball direction during the reciprocating motion. However, in the middle of the wear track, as the ball moves at high speed, the wear track shows a narrow and shallower trace compared to the ends. This phenomenon is due to the dynamic characteristics of the friction pairs, where the ball sliding speed changes from a high to low speed at the two ends of the reciprocating motion [52].
The surface morphology and surface elemental analysis of the wear track on the BS and AL plates were studied through SEM/EDX. Figure 9 and Figure 10 show the wear track SEM images observed on the BS and AL plates.
At 30 °C, the PILs-lubricated surfaces underwent a slight polishing and appeared relatively smooth (Table 8). The aluminium alloy disk lubricated with the PILs at 30 °C shows small pits and peelings, but the surface is also relatively smooth. No scuffing is observed. Due to the more intense wear, more severe surface damage occurred at the elevated test temperature. The damage is particularly evident on the aluminium disc surface, where directional marks of abrasive wear are seen (Figure 10). The temperature-induced wear differences could be due to changes in the viscosity with increasing temperature (Figure 4). The PILs’ interacting surfaces were separated by an oil film at 30 °C, resulting in low wear. The evidence for this can be observed in Figure 7a,c and Figure 8a,c, in which more significant wear occurs only at the ends of the wear trace. At 80 °C, the PILs’ viscosity decreased and could not form an oil film, resulting in increased wear. The lubrication with the reference oil 20W40 produced a smooth wear scar on the bearing steel specimens. However, it failed to lubricate the aluminium, where abrasive wear was observed after the tribo-test at both temperatures.
EDS analysis helps to evaluate the chemical composition of wear tracks due to surface interaction between the friction pairs and lubricants, and the results are shown in Figures S2 and S3 and Table 9 and Table 10.
The EDS result was obtained by measuring the surface composition inside and outside the wear track, followed by an inside and outside elemental difference calculation. The increase in the amount of oxygen in the wear trace shows worn surface oxidation, while a higher amount of carbon results from the tribo-film formation, which helps reduce friction and wear during the friction process [17,22,54].

4. Conclusions

This study evaluated the wettability, rheological and tribological properties, and thermal stability of two PILs and 20W40. At 30 °C, PIL 01 and PIL 02 were liquid in nature. The main observations and significant outcomes from the investigated results are as follows:
  • The wettability of the investigated PILs and 20W40 showed comparable results on the bearing steel surface, while the wettability of the engine oil on the aluminium alloy surface was superior to the PILs. Among the synthesised PILs, PIL 01 gave the maximum wettability on both surfaces.
  • PIL 02 exhibited a higher thermal stability than PIL 01, which could be related to the higher molecular weight of dodecylamine oleate PIL.
  • PIL 02 shows a higher viscosity than PIL 01, which might be attributed to the respective PIL’s closed-packed structure and carbon chain length.
  • The highly viscous PIL02 lubricant shows a shear thinning effect at 25 °C, and when the temperature rises above 40 °C, all the lubricants show a shear thickening effect.
  • The PIL 02 showed optimal results in reducing the mean friction coefficient for both friction pairs. In the BS–BS friction pair, the mean friction coefficient was reduced by 43.85% at 30 °C and 78.94% at 80 °C. Similarly, the BS–AL friction pair’s mean friction coefficient was reduced by 58.97% at 30 °C and 41.52% at 80 °C. All the results were compared with the 20W40 lubricant.
  • Regarding the wear, the PIL 02 performed better than the PIL 01 and 20W40 lubricants at 30 °C for both the BS–BS and BS–AL contact pairs.
  • At 80 °C, the PILs showed an excellent reduction in wear for the BS–BS tribo-pairs compared to the 20W40 lubricant.
  • The results showed that the lubricating capability of the PILs reduced while the temperature increased, but still, their performance in most of the obtained results is superior to that of 20W40.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/lubricants11110469/s1. Figure S1: The appearance of the investigated lubricating fluids. Figure S2: EDS analysis of wear scar surface at 30 °C and 80 °C on BS plate lubricated with (a,b) PIL01, (c,d) PIL02, (e,f) 20W40, respectively. Figure S3: EDS analysis of wear scar surface at 30 °C and 80 °C on AL plate lubricated with (g,h) PIL01, (k,l) PIL02, (m,n) 20W40, respectively.

Author Contributions

Conceptualisation, B.D.K.P. and P.S.S.; methodology, B.D.K.P. and P.S.S.; investigation, B.D.K.P.; supervision, P.S.S.; writing—original draft preparation, B.D.K.P., R.K., and M.G.; writing—review and editing, all authors; funding acquisition, M.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Davis, D.; Shah, A.F.; Panigrahi, B.B.; Singh, S. Effect of Cr2AlC Nanolamella Addition on Tribological Properties of 5W-30 Engine Oil. Appl. Surf. Sci. 2019, 493, 1098–1105. [Google Scholar] [CrossRef]
  2. Waqas, M.; Zahid, R.; Bhutta, M.U.; Khan, Z.A.; Saeed, A. A Review of Friction Performance of Lubricants with Nano Additives. Materials 2021, 14, 6310. [Google Scholar] [CrossRef] [PubMed]
  3. Barbera, E.; Hirayama, K.; Maglinao, R.L.; Davis, R.W.; Kumar, S. Recent Developments in Synthesizing Biolubricants—A Review. Biomass Convers. Biorefinery 2022, 1–21. [Google Scholar] [CrossRef]
  4. Xu, C.; Cheng, Z. Thermal Stability of Ionic Liquids: Current Status and prospects for future development. Processes 2021, 9, 337. [Google Scholar] [CrossRef]
  5. Shah, F.U.; An, R.; Muhammad, N. Editorial: Properties and Applications of Ionic Liquids in Energy and Environmental Science. Front. Chem. 2020, 8, 627213. [Google Scholar] [CrossRef]
  6. 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]
  7. Barulli, L.; Mezzetta, A.; Brunetti, B.; Guazzelli, L.; Vecchio Ciprioti, S.; Ciccioli, A. Evaporation Thermodynamics of the Tetraoctylphosphonium Bis(Trifluoromethansulfonyl)Imide([P8888]NTf2) and Tetraoctylphosphonium Nonafluorobutane-1-Sulfonate ([P8888]NFBS) Ionic Liquids. J. Mol. Liq. 2021, 333, 115892. [Google Scholar] [CrossRef]
  8. Bermúdez, M.D.; Jiménez, A.E.; Sanes, J.; Carrión, F.J. Ionic Liquids as Advanced Lubricant Fluids. Molecules 2009, 14, 2888–2908. [Google Scholar] [CrossRef]
  9. Kianfar, E.; Mafi, S. Ionic Liquids: Properties, Application, and Synthesis. Fine Chem. Eng. 2020, 22–31. [Google Scholar] [CrossRef]
  10. Flieger, J.; Flieger, M. Ionic Liquids Toxicity—Benefits and Threats. Int. J. Mol. Sci. 2020, 21, 6267. [Google Scholar] [CrossRef]
  11. Oliveira, M.V.S.; Vidal, B.T.; Melo, C.M.; de Miranda, R.C.M.; Soares, C.M.F.; Coutinho, J.A.P.; Ventura, S.P.M.; Mattedi, S.; Lima, Á.S. (Eco)Toxicity and Biodegradability of Protic Ionic Liquids. Chemosphere 2016, 147, 460–466. [Google Scholar] [CrossRef] [PubMed]
  12. Ortega Vega, M.R.; Baldin, E.K.; Pereira, D.P.; Martins, M.C.S.; Pranke, P.; Horn, F.; Pinheiro, I.; Vieira, A.; Espiña, B.; Mattedi, S.; et al. Toxicity of Oleate-Based Amino Protic Ionic Liquids towards Escherichia Coli, Danio Rerio Embryos and Human Skin Cells. J. Hazard. Mater. 2022, 422, 126896. [Google Scholar] [CrossRef] [PubMed]
  13. Song, J. Research Progress of Ionic Liquids as Lubricants. ACS Omega 2021, 6, 29345–29349. [Google Scholar] [CrossRef] [PubMed]
  14. Zhu, L.; Dong, J.; Zeng, Q.; Chao, M.; Gong, K.; Li, W.; Wang, X. A Comprehensive Study of Amino Acids Based Ionic Liquids as Green Lubricants for Various Contacts. Tribol. Int. 2021, 162, 107137. [Google Scholar] [CrossRef]
  15. Zhou, Y.; Qu, J. Ionic Liquids as Lubricant Additives: A Review. Appl. Mater. Interfaces 2017, 9, 3209–3222. [Google Scholar] [CrossRef] [PubMed]
  16. Liu, X.Q.; Zhou, F.; Liang, Y.M.; Liu, W.M. Tribological Performance of Phosphonium Based Ionic Liquids for an Aluminum-on-Steel System and Opinions on Lubrication Mechanism. Wear 2006, 261, 1174–1179. [Google Scholar] [CrossRef]
  17. Patel, A.; Guo, H.; Iglesias, P. Study of the Lubricating Ability of Protic Ionic Liquid on an Aluminum-Steel Contact. Lubricants 2018, 6, 66. [Google Scholar] [CrossRef]
  18. 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, 845–852. [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. Song, Z.; Liang, Y.; Fan, M.; Zhou, F.; Liu, W. Ionic Liquids from Amino Acids: Fully Green Fluid Lubricants for Various Surface Contacts. RSC Adv. 2014, 4, 19396–19402. [Google Scholar] [CrossRef]
  21. Mu, L.; Shi, Y.; Guo, X.; Ji, T.; Chen, L.; Yuan, R.; Brisbin, L.; Wang, H.; Zhu, J. Non-Corrosive Green Lubricants: Strengthened Lignin-[Choline][Amino Acid] Ionic Liquids Interaction via Reciprocal Hydrogen Bonding. RSC Adv. 2015, 5, 66067–66072. [Google Scholar] [CrossRef]
  22. 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. [Google Scholar] [CrossRef]
  23. Mitkare, S.S.; Lakhane, K.G.; Kokulwar, P.U. Ionic Liquids: Novel Applications in Drug Delivery. Res. J. Pharm. Technol. 2013, 6, 1274–1278. [Google Scholar]
  24. Ratti, R. Ionic Liquids: Synthesis and Applications in Catalysis. Adv. Chem. 2014, 2014, 729842. [Google Scholar] [CrossRef]
  25. Greaves, T.L.; Drummond, C.J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206–237. [Google Scholar] [CrossRef] [PubMed]
  26. Espinosa, T.; Sanes, J.; Jiménez, A.E.; Bermúdez, M.D. Protic Ammonium Carboxylate Ionic Liquid Lubricants of OFHC Copper. Wear 2013, 303, 495–509. [Google Scholar] [CrossRef]
  27. Duan, Z.; Gu, Y.; Zhang, J.; Zhu, L.; Deng, Y. Protic Pyridinium Ionic Liquids: Synthesis, Acidity Determination and Their Performances for Acid Catalysis. J. Mol. Catal. A Chem. 2006, 250, 163–168. [Google Scholar] [CrossRef]
  28. Vega, M.R.O.; Parise, K.; Ramos, L.B.; Boff, U.; Mattedi, S.; Schaeffer, L.; Malfatti, C.F. Protic Ionic Liquids Used as Metal-Forming Green Lubricants for Aluminum: Effect of Anion Chain Length. Mater. Res. 2017, 20, 675–687. [Google Scholar] [CrossRef]
  29. Ponomar, M.; Krasnyuk, E.; Butylskii, D.; Nikonenko, V.; Wang, Y.; Jiang, C.; Xu, T.; Pismenskaya, N. Sessile Drop Method: Critical Analysis and Optimization for Measuring the Contact Angle of an Ion-Exchange Membrane Surface. Membranes 2022, 12, 765. [Google Scholar] [CrossRef]
  30. Aldas, M.; Pavon, C.; López-Martínez, J.; Arrieta, M.P. Pine Resin Derivatives as Sustainable Additives to Improve the Mechanical and Thermal Properties of Injected Moulded Thermoplastic Starch. Appl. Sci. 2020, 10, 2561. [Google Scholar] [CrossRef]
  31. E2550-21 AI; Standard Test Method for Thermal Stability by Thermogravimetry. ASTM: West Conshohocken, PA, USA, 2017.
  32. 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]
  33. G133-05; Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear. ASTM: West Conshohocken, PA, USA, 2011.
  34. Ibarra, J.; Melendres, J.; Almada, M.; Burboa, M.G.; Taboada, P.; Juárez, J.; Valdez, M.A. Synthesis and Characterization of Magnetite/PLGA/Chitosan Nanoparticles. Mater. Res. Express 2015, 2, 95010. [Google Scholar] [CrossRef]
  35. Alcantara, M.L.; De Almeida Oliveira, G.; Lião, L.M.; Borges, C.P.; Mattedi, S. Amine/Carboxylic Acid Ionic Liquid Composite Membranes for CO2 separation. Ind. Eng. Chem. Res. 2021, 60, 4405–4419. [Google Scholar] [CrossRef]
  36. Edachery, V.; Ravi, S.; Badiuddin, A.F.; Tomy, A.; Kailas, S.V.; Suvin, P.S. Wetting Behaviour of a Green Cutting Fluid (GCF); Influence of Surface Roughness and Surface Energy of AA5052, Ti6Al4V and EN. Mater. Today Proc. 2022, 62, 7605–7609. [Google Scholar] [CrossRef]
  37. Pereira, M.M.; Kurnia, K.A.; Sousa, F.L.; Silva, N.J.O.; Lopes-Da-Silva, J.A.; Coutinho, J.A.P.; Freire, M.G. Contact Angles and Wettability of Ionic Liquids on Polar and Non-Polar Surfaces. Phys. Chem. Chem. Phys. 2015, 17, 31653–31661. [Google Scholar] [CrossRef] [PubMed]
  38. Avilés, M.D.; Pamies, R.; Sanes, J.; Arias-Pardilla, J.; Carrión, F.J.; Bermúdez, M.D. Protic Ammonium Bio-Based Ionic Liquid Crystal Lubricants. Tribol. Int. 2021, 158, 106917. [Google Scholar] [CrossRef]
  39. Ali, M.K.A.; Xianjun, H. Role of Bis(2-Ethylhexyl) Phosphate and Al2O3/TiO2 Hybrid Nanomaterials in Improving the Dispersion Stability of Nanolubricants. Tribol. Int. 2021, 155, 106767. [Google Scholar] [CrossRef]
  40. Keshapolla, D.; Srinivasarao, K.; Gardas, R.L. Influence of Temperature and Alkyl Chain Length on Physicochemical Properties of Trihexyl- and Trioctylammonium Based Protic Ionic Liquids. J. Chem. Thermodyn. 2019, 133, 170–180. [Google Scholar] [CrossRef]
  41. Guo, H.; Pang, J.; Adukure, A.R.; Iglesias, P. Influence of Hydrogen Bonding and Ionicity of Protic Ionic Liquids on Lubricating Steel–Steel and Steel–Aluminum Contacts: Potential Ecofriendly Lubricants and Additives. Tribol. Lett. 2020, 68, 1–10. [Google Scholar] [CrossRef]
  42. Wan Nik, W.B.; Zulkifli, F.; Ayob, A.F.; Kader, A.S.A.; Warikh, A.R.M. Rheology Study of Plant Oil for Marine Application. Procedia Eng. 2013, 68, 138–144. [Google Scholar] [CrossRef]
  43. Shi, Y.; Larsson, R. Non-Corrosive and Biomaterials Protic Ionic Liquids with High Lubricating Performance. Tribol. Lett. 2016, 63, 1–7. [Google Scholar] [CrossRef]
  44. Del Río, J.M.L.; Rial, R.; Nasser, K.; Guimarey, M.J.G. Experimental Investigation of Tribological and Rheological Behaviour of Hybrid Nanolubricants for Applications in Internal Combustion Engines. Tribol. Lett. 2023, 71, 25. [Google Scholar] [CrossRef]
  45. Burrell, G.L.; Dunlop, N.F.; Separovic, F. Non-Newtonian Viscous Shear Thinning in Ionic Liquids. Soft Matter 2010, 6, 2080–2086. [Google Scholar] [CrossRef]
  46. Moosavi, M.; Daneshvar, A. Investigation of the Rheological Properties of Two Imidazolium-Based Ionic Liquids. J. Mol. Liq. 2014, 190, 59–67. [Google Scholar] [CrossRef]
  47. 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]
  48. Mendonça, A.C.F.; Malfreyt, P.; Pádua, A.A.H. Interactions and Ordering of Ionic Liquids at a Metal Surface. J. Chem. Theory Comput. 2012, 8, 3348–3355. [Google Scholar] [CrossRef] [PubMed]
  49. Zhao, Q.; Zhao, G.; Zhang, M.; Wang, X.; Liu, W. Tribological Behavior of Protic Ionic Liquids with Dodecylamine Salts of Dialkyldithiocarbamate as Additives in Lithium Complex Grease. Tribol. Lett. 2012, 48, 133–144. [Google Scholar] [CrossRef]
  50. Yao, M.; Fan, M.; Liang, Y.; Zhou, F.; Xia, Y. Imidazolium Hexafluorophosphate Ionic Liquids as High Temperature Lubricants for Steel-Steel Contacts. Wear 2010, 268, 67–71. [Google Scholar] [CrossRef]
  51. Palacio, M.; Bhushan, B. A Review of Ionic Liquids for Green Molecular Lubrication in Nanotechnology. Tribol. Lett. 2010, 40, 247–268. [Google Scholar] [CrossRef]
  52. 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]
  53. Kreivaitis, R.; Gumbytė, M.; Kupčinskas, A.; Kazancev, K.; Jankauskas, V.; Jokubynienė, V. Lubricating Properties of Ammonium-Based Protolytic Ionic Liquids in Friction Pairs Based on Different Metals. J. Frict. Wear 2021, 42, 152–159. [Google Scholar] [CrossRef]
  54. Guo, H.; Smith, T.W.; Iglesias, P. The Study of Hexanoate-Based Protic Ionic Liquids Used as Lubricants in Steel-Steel Contact. J. Mol. Liq. 2020, 299, 112208. [Google Scholar] [CrossRef]
Lubricants 11 00469 g001
Figure 1. Block diagram of the synthesis process of PILs.
Figure 1. Block diagram of the synthesis process of PILs.
Lubricants 11 00469 g001
Lubricants 11 00469 g002
Figure 2. FTIR spectra for (a) PIL 01 and (b) PIL 02.
Figure 2. FTIR spectra for (a) PIL 01 and (b) PIL 02.
Lubricants 11 00469 g002
Lubricants 11 00469 g003
Figure 3. Thermogravimetric curve of PILs and 20W40 lubricants.
Figure 3. Thermogravimetric curve of PILs and 20W40 lubricants.
Lubricants 11 00469 g003
Lubricants 11 00469 g004
Figure 4. Viscosity vs shear rate at different temperatures for PIL 01, PIL 02, and 20W40: (a) 25 °C, (b) 40 °C, (c) 80 °C, (d) 100 °C, respectively.
Figure 4. Viscosity vs shear rate at different temperatures for PIL 01, PIL 02, and 20W40: (a) 25 °C, (b) 40 °C, (c) 80 °C, (d) 100 °C, respectively.
Lubricants 11 00469 g004
Lubricants 11 00469 g005
Figure 5. The average friction coefficient of the investigated samples at the temperatures of 30 °C and 80 °C.
Figure 5. The average friction coefficient of the investigated samples at the temperatures of 30 °C and 80 °C.
Lubricants 11 00469 g005
Lubricants 11 00469 g006
Figure 6. Wear volume of investigated sample for two different tribo-pairs at two different temperatures: (a) BS–BS, (b) BS–AL.
Figure 6. Wear volume of investigated sample for two different tribo-pairs at two different temperatures: (a) BS–BS, (b) BS–AL.
Lubricants 11 00469 g006
Lubricants 11 00469 g007
Figure 7. 3D profilometer images of wear tracks at 30 °C and 80 °C on the BS plate lubricated with (a,b) PIL01, (c,d) PIL02, and (e,f) 20W40, respectively.
Figure 7. 3D profilometer images of wear tracks at 30 °C and 80 °C on the BS plate lubricated with (a,b) PIL01, (c,d) PIL02, and (e,f) 20W40, respectively.
Lubricants 11 00469 g007
Lubricants 11 00469 g008
Figure 8. Three-dimensional profilometer images of wear tracks at 30 °C and 80 °C on the AL plate lubricated with (a,b) PIL01, (c,d) PIL02, and (e,f) 20W40, respectively.
Figure 8. Three-dimensional profilometer images of wear tracks at 30 °C and 80 °C on the AL plate lubricated with (a,b) PIL01, (c,d) PIL02, and (e,f) 20W40, respectively.
Lubricants 11 00469 g008
Lubricants 11 00469 g009
Figure 9. SEM morphology of wear scars at 30 °C and 80 °C on the BS plate lubricated with (a,b) PIL 01, (c,d) PIL 02, and (e,f) 20W40, respectively.
Figure 9. SEM morphology of wear scars at 30 °C and 80 °C on the BS plate lubricated with (a,b) PIL 01, (c,d) PIL 02, and (e,f) 20W40, respectively.
Lubricants 11 00469 g009
Lubricants 11 00469 g010
Figure 10. SEM morphology of wear scars surface at 30 °C and 80 °C on AL plate lubricated with (a,b) PIL 01, (c,d) PIL 02, and (e,f) 20W40, respectively.
Figure 10. SEM morphology of wear scars surface at 30 °C and 80 °C on AL plate lubricated with (a,b) PIL 01, (c,d) PIL 02, and (e,f) 20W40, respectively.
Lubricants 11 00469 g010
Table 1. Molecular structure of synthesised PILs.
Table 1. Molecular structure of synthesised PILs.
CationAnionAbbreviation
Lubricants 11 00469 i001Lubricants 11 00469 i002PIL 01
Lubricants 11 00469 i003PIL 02
Table 2. Acids and amines are used for the preparation of PILs.
Table 2. Acids and amines are used for the preparation of PILs.
ReactionReagentsMolecular Weight (g/mol)Density (g/mL)
PIL 01Tert-octylamine129.240.805
Oleic Acid282.460.887
PIL 02Dodecylamine185.350.806
Oleic Acid282.460.887
Table 3. Physical properties of investigated lubricant samples.
Table 3. Physical properties of investigated lubricant samples.
LubricantsDensity (g/cm3)VIKinematic Viscosity (mm2/s)Pour Point (°C)
40 °C100 °C
PIL 010.879105197.8118.6812.0
PIL 020.89089319.4322.969.0
20W400.853114108.75 12.96-
Table 4. Chemical composition of aluminium and bearing steel specimens.
Table 4. Chemical composition of aluminium and bearing steel specimens.
Friction Pair MaterialCSiCuMnMgCrNiZnTiAlFe
AISI 52100 (BS) Plate0.9370.318-0.386-1.4100.184---Balance
AW 6261 (AL) Plate-0.4510.1280.2390.4120.024-0.2360.014Balance0.431
AISI 52100 (BS) Ball0.93–1.050.21-0.37-1.43≤0.25---Balance
Table 5. Testing parameters for reciprocating ball-on-plate tribological experiments.
Table 5. Testing parameters for reciprocating ball-on-plate tribological experiments.
Test Parameter
Load (N)4
Max Hertz contact pressure (GPa)1.046 for BS-BS and
0.66 for BS-AL
Stroke length (mm)5
Frequency (Hz)5
Test duration (s)4001
Sliding speed (m/s)0.0785
Sliding distance (m)200
Test temperature (°C)30 and 80
Plate
Plate MaterialBearing steel (AISI 52100)Aluminium (AW 6261)
Surface roughness Ra (μm)0.120.55
Hardness (HV)25155
Table 6. Contact angle ( ° ) values of lubricants tested on bearing steel and aluminium alloy surfaces.
Table 6. Contact angle ( ° ) values of lubricants tested on bearing steel and aluminium alloy surfaces.
LubricantsBS PlateAL Plate
After 5 sAfter 15 sAfter 180 sAfter 300 sAfter 5 sAfter 15 sAfter 180 sAfter 300 s
PIL 0136.1 (± 2.2)32.1 (±0.1)25.5 (±1.02)23.3 (±0.85)38.9 (± 1.9)33.6 (± 0.2)21.6 (±0.19)21.5 (±1.04)
PIL 0237.9 (± 2.2)37.6 (±0.7)26.5 (±1.34)26.2 (±0.91)54.9 (± 1.4)54.1 (± 1.6)26.7 (±1.16)26.2 (±0.47)
20W4037.6 (± 0.7)36.5 (±0.4)19.4 (±0.24)19.4 (±0.28)26.8 (± 1.5)20.7 (± 2.7)21.2 (±1.16)20.7 (±1.23)
Table 7. Thermal degradation temperature of PILs and 20W40 evaluated by TGA.
Table 7. Thermal degradation temperature of PILs and 20W40 evaluated by TGA.
LubricantTo (°C)Td (°C)
PIL 01113.50294.83
PIL 02162.50387.66
20W40185.66399.54
Table 8. Surface roughness value of wear track.
Table 8. Surface roughness value of wear track.
Test TemperatureSampleBS PlateAL Plate
Ra/µmRz/µmRa/µmRz/µm
30 °CPIL 010.164 (±0.016)1.041 (±0.06)0.668 (±0.008)4.186 (±0.27)
PIL 020.123 (±0.010)1.004 (±0.12)0.576 (±0.025)3.922 (±0.42)
20W400.284 (±0.043)2.117 (±0.335)0.811 (±0.018)4.95 (±0.31)
80 °CPIL 010.296 (±0.004)1.777 (±0.0005)2.16 (±0.06)12.40 (±0.20)
PIL 020.278 (±0.003)1.702 (±0.106)2.04 (±0.075)11.46 (±0.28)
20W400.277 (±0.019)1.722 (±0.019)1.232 (±0.012)6.40 (±0.24)
Table 9. EDS results (wt.%) of elements difference between inside and outside wear scar surface on the BS plate.
Table 9. EDS results (wt.%) of elements difference between inside and outside wear scar surface on the BS plate.
Elements Difference Between Inside and Outside (wt.%)PIL 01PIL 0220W40
30 °C80 °C30 °C80 °C30 °C80 °C
O12.42.32.80.91.83.7
C23.610.20.90.21.6
Fe−32.5−2.2−3.2−4.1−0.5−0.4
Table 10. EDS results (wt.%) of elements difference between inside and outside wear scar surface on the AL plate.
Table 10. EDS results (wt.%) of elements difference between inside and outside wear scar surface on the AL plate.
Elements Difference Between Inside and Outside (wt%)PIL 01PIL 0220W40
30 °C80 °C30 °C80 °C30 °C80 °C
O5.15.60.44.22.44.4
C1.66.9345.415.2
Al−5.1−12.4−0.2−8.8−4.5−10.2
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

Patro, B.D.K.; Suvin, P.S.; Kreivaitis, R.; Gumbytė, M. Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts. Lubricants 2023, 11, 469. https://doi.org/10.3390/lubricants11110469

AMA Style

Patro BDK, Suvin PS, Kreivaitis R, Gumbytė M. Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts. Lubricants. 2023; 11(11):469. https://doi.org/10.3390/lubricants11110469

Chicago/Turabian Style

Patro, B. Depu Kumar, P. S. Suvin, Raimondas Kreivaitis, and Milda Gumbytė. 2023. "Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts" Lubricants 11, no. 11: 469. https://doi.org/10.3390/lubricants11110469

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