Experimental investigation of tribological and rheological behaviour of hybrid nanolubricants for applications in internal combustion engines

Vol.:(0123456789) 1 3 Tribology Letters (2023) 71:25 https://doi.org/10.1007/s11249-023-01697-5 ORIGINAL PAPER Experimental Investigation ofTribological andRheological Behaviour ofHybrid Nanolubricants forApplications inInternal Combustion Engines JoséM.LiñeiradelRío1,2· RamónRial3· KhodorNasser1· MaríaJ.G.Guimarey1,4 Received: 12 November 2022 / Accepted: 22 January 2023 / Published online: 9 February 2023 © The Author(s) 2023 Abstract In this study, the improvement in SAE 10W-40 engine oil tribological performance after the addition of magnesium oxide (MgO) nanoadditive and two different phosphonium-based ionic liquids (ILs) was investigated. Besides, the rheological behaviour of MgO-based nanolubricant and IL-based hybrid nanolubricants at the temperature range from 293.15 to 363.15K was studied. The nanoparticle characterization was performed by means of transmission electron microscopy (TEM), Fouriertransform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The tribological properties, friction coefficients and wear parameters of the formulated oil modified with 0.01wt% MgO and 1wt% ILs compared with the neat 10W-40 oil were performed and analysed using a ball-on-three-pins tribometer and a 3D optical profilometer, respectively. Further analysis on the worn surface was shown by Raman spectroscopy and SEM images illustrating the formation of the protective IL and MgO tribo-films as hybrid additives. In friction tests with sliding steel-steel tribo-pairs, IL3-based hybrid nanolubricant decreased the coefficient of friction and wear volume by 7% and 59%, respectively, in comparison with the neat SAE 10W-40, hence better positive synergies were found for MgO and IL3 as hybrid additives. Interestingly, the thermophysical characterization by rheology also revealed that the nanoparticle and ionic liquids addition did not affect neither the viscosity response nor the Newtonian behaviour of the engine oil, adequately meeting the requirements for their use in internal combustion engines. * María J. G. Guimarey [email protected] 1 Laboratory ofThermophysical andTribological Properties, Nafomat Group, Department ofApplied Physics, Faculty ofPhysics, University ofSantiago de Compostela, 15782SantiagodeCompostela, Spain 2 FEUP, Universidade doPorto, Rua Dr. Roberto Frias S/N, 4200-465Porto, Portugal 3 Soft Matter andMolecular Biophysics Group, Department ofApplied Physics, University ofSantiago de Compostela, 15782SantiagodeCompostela, Spain 4 Department ofDesign andEngineering, Faculty ofScience & Technology, Bournemouth University, Talbot Campus, PooleBH125BB, UK Tribology Letters (2023) 71:25 1 3 25 Page 2 of 16 Graphical Abstract of commercial oils which plays a key role in the wear and friction processes of materials [3–6]. Thus, nanolubricants can enable major technological breakthroughs and cost savings in a variety of domestic and industrial applications, including transport, electronic devices, chemical engineering applications, aerospace, wind turbines and automotive [5]. In addition, viscosity is considered a significant thermophysical characteristic that also influences lubricant performance. The flow behaviour of fluids is determined by viscosity variations with shear rate, shear stress and temperature [7]. Moreover, a fluid is considered to be Newtonian if shear stress changes linearly with shear rate, and non-Newtonian if its response depends on the movement. In terms of viscosity, it means that in a Newtonian fluid its viscosity will remain constant regardless of the applied forces. An optimum viscosity range ensures proper lubricant film formation and therefore effective lubrication, so it is crucial not to alter this property outside the operating range for a given application. The tribological and rheological properties of lubricants are the main factors to be considered to prevent wear under operating conditions. The addition of nano-sized spherical particles can improve these properties, making the lubricating oil performance stable under severe operating conditions; however, the selection of suitable nanoparticles is Keywords Lubricant· Nanoparticles· Phosphonium-based ionic liquids· Tribological mechanisms· Rheology 1 Introduction In automotive engines, the high frictional energy loss between the tribo-pairs especially at the piston rings and cylinder liner surface contact was estimated to be from 20 to 30% of its total power loss [1]. This matter has led researchers to investigate various additives as possible components of conventional lubricating oil. A multifunctional engine lubrication system is considered more critical and effective since it could have various characteristics to reduce friction, attenuate wear, enhance heat transfer capabilities, inhibit corrosion and oxidation and remove wear debris and contamination, simultaneously. Thus, engine oil plays a key role in an internal combustion engine to achieve greater fuel economy by reducing friction and wear of the interacting surfaces [2]. Due to certain limitations of the base oils physicochemical properties as to fulfil the desired requirements, a further improvement is required in order to enhance their performance. Recently, research has focused on the use of nanomaterials to improve the lubricity and durability Tribology Letters (2023) 71:25 1 3 Page 3 of 16 25 very important [8]. Friction and wear behaviour of nanoparticles content are associated with hardness, size and deposition efficiency of nanoparticles on worn surfaces [9]. Much work has been done on organic and inorganic nanoparticles for application as extreme pressure (EP) and anti-wear agents [2, 10, 11]. Among all of them, the metal oxides are commonly used as additives for anti-friction and anti-wear applications, and they can be composed of a variety of diverse materials, including titanium, copper, zirconium, zinc, cerium, aluminium and iron oxides. Wu etal. [12] have studied the behaviour of two different metal oxides, TiO2 and CuO, as nanoadditives in two lubricating oils, API-SF engine oil (SAE30 LB51153) and a base oil (SAE30 LB51163-11), revealing a decrease in the friction of both lubricants when these nanoadditives were added, in particular for CuO nanolubricants. Hernández Battez etal. [13] have discussed the tribological behaviour of CuO, ZrO2 and ZnO nanoparticles as anti-wear additives of a polyalphaolefin (PAO6), concluding that all nanolubricants reduce the friction coefficient and increase the anti-wear ability of base oil (PAO6). The mechanism by which the lubricating behaviour for the nanolubricants is based on metal oxides (Al2O3, SiO2, CuO, ZnO, ZrO2 and TiO2) is improved and based on the formation of a layer formed by mechanical compaction [12, 14–16]. Among the wide variety of existing nanoparticles, magnesium oxide has better properties than many of them, as it is easily soluble and has lower density [17]. However, MgO has only recently been studied for tribological purposes by Singh etal. [17] and Loo etal. [18], demonstrating excellent anti-wear results, reducing wear rate by almost 50% in vegetable oil and 7% of wear scar diameter in biofuel, respectively. These outstanding tribological results were obtained using MgO concentrations of 0.2, 0.6 and 1.2 [17] and 0.1 and 0.2 [18], respectively. The use of low concentrations of additives in engine oils is more effective in reducing the coefficient of friction in piston rings and cylinder lines [19]. Therefore, the need to further study the ability of MgO at low concentrations as a lubricant additive is of great interest. However, a main challenge in this field is the stability of nanoadditives in oils along time, being a vital matter that affects the nanolubricant lubrication performance [20]. A way to elongate stability and avoid agglomerates in Fig. 1 Raman spectrum of 10W-40 formulated oil 600 800 1000 1200 1400 1600 1800 2000 100600 1100 1600 2100 2600 3100 Absorbance Wavenumber /cm -1 2852 cm -1 1451 cm -1 Fig. 2 Transmission electron microscopy (TEM) image of MgO nanoparticles [34] Tribology Letters (2023) 71:25 1 3 25 Page 4 of 16 nanolubricants as in other nanofluids is the use of coated nanoparticles and organic or hybrid nanomaterials [21, 22]. Another alternative to improve the stability of the nanoadditives in the lubricant is the addition of surfactants as ionic liquids (ILs). ILs are excellent dispersants for the stabilization of well-characterized nanomaterials. ILs are molten salts at room temperature that possess unique physicochemical properties and have shown good oil-soluble and excellent lubricating properties, especially protic ILs with long alkyl chains [23–26]. Among the myriad of existing protic ILs, phosphonium cation-based ILs exhibit outstanding solubility in non-polar liquids, thus facilitating their use as ILs in conventional lube oils [27]. Since the International Lubricants Standardization and Approval Committee specification (ILSAC GF-5) restricts the phosphorus content to a range of 0.06–0.08wt% [28], considering the molecular weight of phosphonium ILs, previous work has used a concentration of 1wt% of these ILs in engine lubricants [24, 28, 29]. The Fig. 3 Infrared spectrum of MgO nanopowders Fig. 4 Raman spectrum of MgO nanopowders 490 510 530 550 100600 1100 1600 2100 2600 3100 Absorbance Wavenumber/cm -1 446cm -1 281 cm -1 Tribology Letters (2023) 71:25 1 3 Page 5 of 16 25 double action in the case of IL-modified hybrid nanoadditives reveals that nanomaterials are beneficial for increasing wear resistance due to their good mechanical strength, and ILs can improve their dispersion in lubricating oils [26]. This work focuses on the study of the synergistic effects of metal oxide nanoparticles, magnesium oxide (MgO), with two phosphonium ionic liquids on the tribological and rheological behaviour of a 10W-40 commercial engine oil. In previous studies, the application of a commercial engine oil with the effect of MgO combined with ionic liquids has never been considered for tribological applications. Most of the studies have reported the rheological behaviour by using MWCNT (COOH-functionalized)/MgO as lubricant additives to engine oils [30, 31]. The authors have performed this study and found better results that are beneficial to the readers of this journal. 2 Materials andMethods 2.1 Oil, Nanoadditive andIonic Liquids Commercial SAE 10W-40 oil was used as the base oil to prepare the nanolubricant and hybrid nanolubricants in this work. 10W-40 oil, supplied by Repsol, is a semi-synthetic oil characterized by being a mixture of mineral and synthetic oils. This engine oil has a density, r, of 0.8498g cm−3 at 298.15K, a kinematic viscosity, ν, of 96.45 and 14.54 mm2 s−1 at 303.15 and 373.15K, respectively, and a viscosity index, VI, of 162. 10W-40 has several advantages over other types of oils such as improving the engine performance, increasing its lifespan and keeping it more viscous, which means that it is a cleaner oil than others. Furthermore, it is considered less expensive than synthetic oils. Fig. 5 XRD pattern of MgO nanopowders Fig. 6 SEM images MgO nanopowders Tribology Letters (2023) 71:25 1 3 25 Page 6 of 16 Raman spectrum of formulated 10W-40 oil was performed with a WITec alpha300R + confocal Raman microscopy at a wavelength of 532nm. Raman spectrum of 10W-40 is shown in Fig.1, which detects an intense band around 2800–3000 cm−1 with a peak of 2852 cm−1 corresponding to C–H stretching [32] and other peak at 1451 cm−1 that corresponds to δ(CH3) vibrations [33]. Concerning the additives, magnesium (II) oxide (MgO) nanoparticles, CAS:1309-48-4, ~ 30nm size, 99.9% purity, were supplied by Iolitec. Transmission electron microscopy (TEM) was provided by the manufacturer to determine the nanoparticle average particle size. Figure2 confirms the average particle size of 30nm. In addition, these nanopowders were characterized by means of different techniques, such as Infrared (FTIR), Raman spectroscopy and X-ray diffraction (XRD). As can be observed in Fig.3, FTIR reveals at 684 cm−1 the stretching vibration mode of Mg–O–Mg bonds, the bending vibration of surface hydroxyl group at 1483 cm−1 and the presence of hydroxyl group at 3698 cm−1 [35–37]. Raman spectroscopy shows the presence of two characteristic bands at 281 and 446 cm−1 (Fig.4). The first one is related to a TA phonon in the boundary zone while the latter one corresponds with a TA phonon at the centre zone, being T: transversal (in-plane) waves and A: acoustic modes. The found Raman lines are closely paired with previous characterizations of MgO [38, 39]. X-ray diffraction, XRD, method was employed to find the crystalline nature of MgO nanoparticles, for this aim a Bruker D8 Advance was utilized. XRD analysis (Fig.5) shows important peaks at angles 36.99°, 42.94° and 62.33° associated with (1 1 1), (0 0 2) and (2 0 2) planes (JCPDS No. 87-0653) that suggests the existence of a cubic polycrystalline structure for MgO nanoparticles [35]. Additionally, the XRD pattern exhibits a high strong (0 0 2) orientation peak displaying a high crystallinity of MgO nanoparticles [35]. Scanning electron microscopy studies (SEM) were performed to get clear information of MgO nanoparticles morphology and shape. Figure6 displays the surface topography of nanoparticles, observing that they are spherical, uniform in size and with agglomeration [40, 41]. The ionic liquids used as lubricant additives were trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate, Fig. 7 Nanolubricant preparation scheme Measuringand rotating shaft Sample holder (1 ml of lubricant) 3Pins Ball Fig. 8 Tribology cell T-PTD200 in combination with a Peltier hood H-PTD 200 mounted on MCR 302 rheometer the experimental test setup Tribology Letters (2023) 71:25 1 3 Page 7 of 16 25 [P6,6,6,14][DEHP], (IL1, CAS number: 1092655-30-5 and purity > 0.98), and trihexyltetradecylphosphonium bis(2,4,4- trimethylpentyl) phosphinate, [P6,6,6,14][(iC8)2PO2], (IL3, CAS number: 465527-58-6 and purity > 0.9), both were supplied by Iolitec and previously characterized by Nasser etal. [29]. 2.2 Formulation ofNanolubricant andHybrid Nanolubricants An engine oil-based nanolubricant was prepared with a concentration of 0.01wt% of MgO nanopowders. A low concentration of MgO was selected to interpret its effect on friction and consequently wear rather than high concentrations as in previous performed studies [17, 18]. Hybrid nanolubricants were prepared with the same MgO concentration and 1wt% of two different ionic Iiquids: trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate (IL1) or trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate (IL3) dispersed in the commercial oil 10W-40. The concentration of 1wt% of each ionic liquid was selected on the basis of the maximum permitted phosphorus content, which has beenused in previous studies [24, 25]. Therefore, the subsequent nanolubricant and hybrid nanolubricants were formulated: 10W-40 + 0.01wt% MgO, 10W-40 + 0.01wt% MgO + 1wt% IL1 and 10W-40 + 0.01wt% MgO + 1wt% IL3. The formulation scheme of the nanolubricant and hybrid nanolubricants is shown in Fig.7. For the nanolubricant, a traditional two-step process was employed: dry MgO nanopowders were added to the 10W-40 oil after reaching the desired mass concentration using a Sartorius balance, with a readability of 0.01mg. Afterwards, the homogenization of the oil and the nanopowders (nanolubricant) was performed by an ultrasound bath (Fisherbrand) for a duration of 4h continuously, at 180 W power and 37kHz frequency. Concerning the hybrid nanolubricant with MgO and IL, an improved two-step method was used [29, 42, 43], as a replacement for the directly mixing nanoparticles with oil. For this purpose, suitable quantities of MgO nanoadditives were set in an agate mortar and mechanically mixed with the appropriate quantity of ILs for 10min with no stop to get a final prepared mixture of MgO and IL. Indeed, the lubricity of the IL is governed by the structure of the anion [44] which is in this case [DEHP] for IL1 and [(iC8)2PO2] for IL3. The method used might enhance the lubricity of the ILs by introducing the nanoadditives MgO separately with continuous mixing before its addition as hybrid additive to 10W-40 oil. This technique obtains more stable mixture, instead of individual addition of IL and MgO. Subsequently, this mixture was added to the 10W-40 oil and homogenized using an ultrasonic bath, similarly as in the preceding process. Two sets of the prepared sample were made: one to study the stability against time and the other one to perform friction tests. 2.3 Friction Tests andWear Analysis Tribological tests of the neat oil (10W-40), the nanolubricant (with MgO) and the hybrid nanolubricants (with MgO/ILs) were performed with a MCR 302 rheometer from Anton Paar (Graz, Austria) coupled with a tribology cell T-PTD200 equipped by a Peltier hood H-PTD200 for temperature control, being the tribological configuration ball-on-three pins. In this configuration, the ball (upper specimen) is mounted on a vertical shaft that is driven by the rheometer motor, while the three pins (lower specimens) are set inside a holder at a 45° angle of contact to the rotating shaft. To fully cover the top level of the pins mounted into the sample holder, around 1ml of tested lubricant was added. Through every tribological trial, the ball rotates against the three pins under a static force applied by the rheometer. This axial force is relocated into three normal forces acting perpendicularly to the surfaces of the pins [45]. Figure8 shows the real tribometer with the T-PTD200 tribo cell; more details about the tribometer can be seen in a previous research [29]. The specimens used were polished AISI 52100 (100Cr6) steel balls (Ra = 20nm) and pins (Ra = 50nm) with hardness of 62–66 HRC. AISI 52100 steel is one of the most widely used materials in bearings [46], one of the components of internal combustion engines where significant mechanical power losses occur [47]. Regarding the test conditions, 213rpm rotational speed (0.1m s−1), under 20 N axial load (9.43 N in each pin) implying a maximum Hertzian pressure of 1.1GPa, sliding distance of 340m, sliding time of 3500s and optimum operating temperature for internal combustion engines temperature of 363.15K were chosen. Three trials were performed for each friction test to obtain accurate experimental average values. Concerning the wear evaluation produced on the surfaces of the pins during the aforementioned friction tests, a high-resolution 3D profiler S Neox Sensofar was employed to quantify the generated wear scar through different wear parameters, such as wear track width (WTW), wear track depth (WTD), wear area and volume. Wear measurements were taken in confocal mode using a 10 × objective and the SensoMAP software, integrated with the profilometer, which provides a fast and accurate analysis of the surface geometry. By means of these wear measurements for all the tests (with base oil, nanolubricant and hybrid nanolubricants) a good comparison of the wear produced by the nano- and hybrid nanolubricants with respect to the formulated oil without nanoadditives and/or ILs can be determined. Regarding the measurements of the worn area, the 3D profiler first generates a cross-sectional profile of the worn surface and then the software determines the area as the subtraction of the worn Tribology Letters (2023) 71:25 1 3 25 Page 8 of 16 area (valley) minus the sum of the areas of the displaced material profiles on both sides of the worn track (peaks). The wear volume was automatically determined by the SensoMAP software by pre-setting the level of the unworn surface of the pin, and the software detects the coordinates of the points of the wear surface profile and calculates the volume of the hole below this level. The 3D profiler was also utilized to evaluate the worn track roughness (Ra) on the surface of the pins used in tribotests to illustrate the anti-wear skill of each nano- and hybrid nanolubricant. For this reason, ISO4287 standard was applied using a Gaussian filter (cut-off: 0.08mm wavelength). In addition, a Raman microscope WITec alpha300R + was used to examine the internal worn track surface of the pins and acquire information about the nanolubricant and hybrid nanolubricants components (10W-40, MgO nanoparticles and ILs) and their distribution inside the worn scar as well as the tribological mechanisms occurred to discuss their influence in enhancing the tribological properties compared to the based oil. To better understand the anti-wear mechanisms of the nanoparticles in the engine oil, the wear scar of the tested steel pins was also scanned and analysed by using FESEM Ultra plus model from ZEISS. An InLens secondary electron detector was used to record the SEM images at 3kV. The equipment works in ultra-high vacuum, so the sample is metallized with Iridium (Ir), to avoid charging, in this case a thickness of 10nm is deposited. To determine the composition of the surface of the sample, the EDX detector model INCA-X act of the commercial company OXFORD was used, all controlled with the AZtec software, the acquisition conditions are 20kV and a working distance (WD) of 8.5mm. These are the optimal conditions for capturing and collecting beads for a more accurate analysis. 2.4 Rheological Measurements Rheological measurements were performed with the previous Anton Paar Rheometer Physica MCR 302 (Graz, 0.01 wt%MgO 0.01 wt%MgO +IL1 0.01 wt%MgO +IL3 0h 120 h 456 h Fig. 9 Visual stability of 10W-40 nanolubricants with and without ILs Fig. 10 Temporal evolution of the refractive index, n, at T = 293.15K for engine oil (10W-40), for nanolubricant and for hybrid nanolubricants Tribology Letters (2023) 71:25 1 3 Page 9 of 16 25 Fig. 11 Comparison between friction coefficients (μ) and wear volume reduction reached with 10W-40, IL-lubricants, MgO nanolubricant and hybrid nanolubricants Table 1 Average friction coefficients, μ, as well as average wear parameters: width (WTW), depth (WTD), area and volume and their standard deviations for the analysed lubricants Lubricant µ σ WTW/mm σ/mm WTD/mm σ/mm Area/ mm2σ/102mm2Volume/mm3σ/mm3 10W-40 0.1341 0.0002 234 10 0.323 0.040 35.38 4.90 2347.7 200.1 + 1wt% IL1 0.1377 0.0001 242 5 1.082 0.040 80.52 1.98 3119.6 181.1 + 1wt% IL3 0.1371 0.0001 236 4 0.553 0.029 64.79 5.27 2816.8 152.4 + 0.01wt% MgO 0.1325 0.0009 199 20 0.245 0.031 28.29 4.38 1369.3 188.0 + 0.01wt% MgO + 1wt% IL1 0.1260 0.0007 201 13 0.253 0.050 29.90 5.29 1422.5 222.7 + 0.01wt% MgO + 1wt% IL3 0.1251 0.0001 219 14 0.220 0.019 26.83 3.67 962.0 86.3 Fig. 12 Cross section profiles comparison of worn tracks for the 10W-40 oil and the developed lubricants Tribology Letters (2023) 71:25 1 3 25 Page 16 of 16 23. 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