Tribological performance of silicon nitride and carbon black Ionanofluids based on 1-ethyl-3-methylimidazolium methanesulfonate

*To whom correspondence should be addressed (Tel.: +34 986813771, Email: [email protected]) Tribological performance of silicon nitride and carbon black 1 Ionanofluids based on 1-ethyl-3-methylimidazolium 2 methanesulfonate 3 4 Javier P. Vallejo1,*, José M. Liñeira del Río2, Josefa Fernández2, Luis Lugo1 5 6 7 1Departamento de Física Aplicada, Facultade de Ciencias, Universidade de Vigo, E-36310 Vigo, Spain 8 2Laboratory of Thermophysical Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics, University of 9 Santiago de Compostela, 15782 Santiago de Compostela, Spain 10 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 2 Abstract 11 Development of nano-lubricants by the dispersion of nano-particles in current lubricants has contributed to improving energy 12 efficiency reducing wear and friction. During the last two decades, ionic liquids have evolved as novel lubricants or lubricant 13 additives, especially for high vacuum and high temperature. Nevertheless, the number of experimental studies regarding the 14 tribological properties of Ionanofluids, defined as dispersions of nano-particles in ionic liquids, is limited. 1-ethyl-315 methylimidazolium methanesulfonate, [EMIM][MS], is a promising candidate for lubrication applications due to its wide liquid 16 range, high thermal conductivity, low friction coefficients and low compressibility in comparison with commercial mineral and 17 synthetic hydrocarbon based lubricants. In this work, Ionanofluid lubricants based on dispersions of nano-additives (silicon 18 nitride and carbon black) at mass concentration between 0.10 to 1.0 wt% in [EMIM][MS] were designed. The nearly spherical 19 morphology of the nano-additives, silicon nitride and carbon black, was described by using scanning electron microscopy. The 20 stabilities of the resulting nano-particle dispersions in ionic liquids were analyzed by dynamic light scattering measurements of 21 size for one month. Tribological characterization was performed by a rotational rheometer coupled with a tribology cell with 22 ball-on-three-pins configuration (100Cr6 steel) in sliding conditions at 298.15 and 353.15 K. Afterwards, the wear track 23 morphology of the worn pins was analyzed by a 3D optical profiler and Raman Spectroscopy. The dispersions at the optimal 24 nano-additive concentrations for lubrication reached friction coefficient decreases of up to 16% and wear tracks with a volume 25 28 times lower. Additionally, the density and dynamic viscosity of [EMIM][MS] and the optimal Ionanofluids for lubrication 26 applications measured with a rotational Stabinger visco-densimeter in the 278.15 to 373.15 K temperature range show small 27 increases with almost no dependence on temperature. 28 Keywords 29 tribological properties; 1-ethyl-3-methylimidazolium methanesulfonate; friction; Ionanofluids; nanolubricants. 30 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 3 1. Introduction 31 Nanofluids, dispersions of nano-sized solid particles in a conventional fluid, were initially conceived as a solution to enhance 32 the thermal properties of classic heat transfer fluids [1, 2]. Nevertheless, the use of nano-particles as additives has also contributed 33 to improving the tribological efficiency of common lubricants [3, 4]. Several studies have experimentally verified important 34 reductions of the friction coefficient and wear volume by dispersing diverse nano-materials (metals, oxides, nitrides, carbon35 based structures) in conventional lubricants in the past decade, as summarized various recent reviews [4-6]. Singh et al [6] 36 summarized the physical phenomena described in the literature to improve the lubrication performance of the base fluid by the 37 nano-particle addition: rolling mechanism, mending mechanism, polishing mechanism and protective film (Figure 1). Rolling 38 effect assumes that nano-additives work like ball bearings, rolling between the two solid surfaces. Mending effect considers that 39 the nano-additives fill the small grooves and repair microdamages of friction surfaces. Polishing mechanism considers that hard 40 nano-additives act as polishers by eliminating asperities, hence, diminishing surface roughness. Finally, protective film effect 41 assumes that nano-particles create a lubricating layer on the friction surface, preventing the direct contact between metallic 42 elements [6, 7]. 43 44 Figure 1. Schematic diagram representing the lubrication physical phenomenon and the lubrication mechanisms of nano45 particles as additives. 46 Ionic liquids, IL, are organic salts in liquid state composed only of ions and with melting points lower than 100 ºC [8]. Those 47 called room-temperature ILs melt below room-temperature. Their low melting points and negligible vapor pressure made them 48 revolutionary in industry. They can be found in several applications such as heat transfer, absorption, sealant, lubricant or 49 pressure transmission agents [9]. Specifically over the last two decades, ILs have been examined as novel lubricants [10] or 50 lubricant additives [11, 12] due to their greater thermal stability, broader liquid range, lower volatility, lower flammability, higher 51 thermal conductivity, and lower sensitivity to environmental variations in rheological behavior than conventional lubricants, 52 among other interesting characteristics [12, 13]. ILs have been considered adequate as lubricants for extreme environments such 53 as high vacuum and high temperature. Moreover, some ILs may constitute an environmental and health friendly alternative in 54 comparison to traditional anti-wear additives (ZDDP) [13, 14]. 55 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 4 The combination of the concepts of nanofluid and ionic liquid leads to the idea of Ionanofluids, which can be defined as 56 dispersions of nano-particles in ionic liquids. This term was first proposed in 2009 [15] and since then, the scientific concern 57 about these nano-dispersions has continuously increased [16, 17], mainly being used as heat transfer fluids [18, 19]. Nevertheless, 58 many of the favorable lubrication mechanisms and properties described for both nanofluids (potential rolling, mending, polishing 59 and protective film effects) and ionic liquids (protective film, interesting thermal and rheological properties, high vacuum and 60 temperature adequacy, less harmful alternative for the environment and health) can show promising synergistic results. 61 There are few studies in the literature regarding the use of Ionanofluids for lubrication. Concerning studies examining 62 Ionanofluids directly as lubricating agents, some literature examples and their main conclusions are summarized as follows. 63 Initially, different authors [20-23] used ILs with hexafluorophosphate ([PF6]) and tetrafluoroborate ([BF4]) anions, but their 64 reactivity with water produces corrosive hydrogen fluoride, so the applicability of these Ionanofluids as lubricants is restricted 65 [24-26]. Khare et al. [27] synthesized and tribologically characterized at room temperature two different dispersions of graphene 66 in 1-butyl-3-methylimidazolium iodide, [BMIM][I], obtaining slight reductions of the friction coefficient, wear, and roughness 67 surface, in comparison to the neat IL. These behaviors were attributed to three main mechanisms: rolling, sliding, and exfoliation 68 and film effects. The authors conclude that the rolling (due to nano-particles) and sliding (due to aggregates) phenomena compete 69 between them when aggregates begin to appear and that the ball rolling effect is less effective for square-shaped nano-particles 70 than for spherical nano-particles. Kheireddin et al. [28] studied the tribological properties at room temperature of SiO2 nano71 particles as additives of 1-butyl-3-methylimidazolium (trifluoromethysulfony)imide, [BMIM][TFSI], observing that the best 72 anti-friction and anti-wear behavior were achieved for the optimum NP concentration (0.05 wt%), with a wear reduction of 24%. 73 These improvements were ascribed to the enhanced loading capacity of the IL by the presence of SiO2 nano-particles and to the 74 capacity of the nano-particles to fill valleys between asperities (polishing mechanism). Yegin et al. [29] investigated the 75 tribological behavior at room temperature of functionalized SiO2 dispersions in 1-butyl-3-methylimidazolium 76 bis(trifluoromethylsulfonyl)imide, [BMIM][NTf2], reaching the higher friction coefficient reduction, 37%, for the 0.1 wt% 77 concentration. This enhancement was attributed to the roller bearing effect of spherical SiO2 nano-particles. Carrión et al. [30] 78 and Espejo et al. [31] analyzed the tribological properties at room temperature of 0.5 wt% single-walled CNT and 0.5 wt% multi79 walled CNT in 1-octyl-3-methylimidazolium chloride ([OMIM][Cl]) and 1-ethyl-3-methylimidazolium tosylate 80 ([EMIM][TOS]), respectively. These authors obtained negligible wear surfaces and friction coefficients, with great reductions 81 of 66 and 54% with respect to neat IL, respectively. These reductions were attributed to the improved load-carrying capacity of 82 the dispersion and its enhanced ability to separate the sliding surfaces (explained by the interactions between nano-additives and 83 IL molecules). From the same group, Saurín et al. [23, 32] obtained tribological characterizations at room temperature for 0.1 84 wt% few layer graphene and 0.1 wt% nano-diamonds dispersions in protic ammonium carboxylate tri-[bis(285 hydroxyethylammonium)] citrate [32]. Graphene dispersion led to slight reductions of the IL friction coefficient for full-fluid 86 and thin layer lubrication, while nano-diamond dispersions lead to a 30% reduction. The graphene-IL dispersion covers the 87 sliding path with a protecting layer that prevents wear, whereas this anti-wear effect is not observed for nano-diamonds. Besides, 88 Pamies et al. [33] studied the tribological performance of 0.5 to 1 wt% graphene dispersions in [EMIM] dicyanamide ([DCA]) 89 and [EMIM][TFSI] at room temperature. They obtained maximum reductions of 11 and 40% in friction and wear, respectively, 90 for [EMIM][DCA], and a maximum friction reduction of 11% for [EMIM][TFSI]. The higher lubrication was attributed to three 91 main reasons: the superior load-carrying ability of the dispersions, the formation of a surface layer of graphene deposited on the 92 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 5 wear track, and the ability of graphene sheets to retain the nano-sized wear debris, preventing the creation of larger abrasive 93 agglomerates. 94 As observed, the literature of Ionanofluids for lubrication applications usually describe tribological properties at room 95 conditions. Therefore, comprehensive experimental analyses are needed to evaluate the effect of temperature variation. 96 Moreover, most of the research on Ionanofluid lubricants has focused on the use of halogen-containing ionic liquids, which can 97 cause negative effects on the environment [8, 34]. The delay in the development of industrial applications of the very promising 98 research results obtained with Ionanofluid lubricants could be attributed to several factors, one of the most relevant is doubtless 99 the problems of agglomeration of the nano-particles [34, 35], which can change the lubrication regime and increase wear due to 100 abrasion. For this reason, as Avilés et al. [34] pointed out, there is an urgent need to optimize the nano-additive concentration, 101 not only to achieve long-term stability, but also to control its influence on the thermophysical and tribochemical properties of the 102 nanofluid. Hence, the aim of this work is to obtain the optimal concentration of two nano-additives for a non-halogenated ionic 103 liquid, analyzing the tribological and thermophysical behavior at different temperatures. 104 The non-halogenated IL 1-ethyl-3-methylimidazolium methanesulfonate, [EMIM][MS], has been chosen due to the much 105 lower isothermal compressibility and lower friction coefficients than those of commercial hydraulic fluids and compressor 106 lubricants [36]. Furthermore, its thermal conductivity, 0.20 W·m-1·K-1 at 273 K [37], is higher than that usually reported for 107 mineral and synthetic hydrocarbon based lubricants, 0.14 W·m-1·K-1 at 273 K [38], which implies a greater capacity to dissipate 108 heat, one of the main functions of the lubricants. Recently, Bioucas et al. [37] concluded that the chemical structure and 109 intermolecular interactions that characterize this IL lead to exceptional properties, which allow for heat transfer, among other 110 applications. In addition, [EMIM][MS] has a wide liquid temperature range, with its freezing point at around 250 K [39]. The 111 value of its kinematic viscosity at 313 K has been reported as 54.4 cSt [40]. 112 To the best of the authors’ knowledge, there are no previous experimental works in the literature analyzing the performance 113 of [EMIM][MS] as a novel lubricant. In this work, the friction coefficient in sliding conditions between steel surfaces and the 114 corresponding wear tracks of [EMIM][MS] were analyzed at two different controlled temperatures, 298.15 and 353.15 K, to 115 study the temperature dependence. Furthermore, silicon nitride and carbon black, two nano-additives with different promising 116 characteristics to improve the tribological performance of a lubricant and a relatively low production cost, have been employed 117 to design [EMIM][MS]. Silicon nitride nano-particles are characterized by their resistance to oxidation at high temperatures, 118 wear, and corrosion resistance. In addition, Çöl et al. [41] have found that silicon nitride nano-particles reduce the friction 119 coefficient of an engine oil up to around 30% for 0.8 wt % concentration and the specific worn rate up to 40% for 0.1 wt% 120 concentration. Ionanofluids in the 0.10 to 1.0 wt% concentration range. The appearance of the optimal concentration for each 121 nano-additive and the different physical mechanisms involved in the improvement of tribological performance is one of the 122 objectives of this work. Moreover, the morphology of the nano-additives and the stability of the resulting dispersions were 123 analyzed. Then, the Ionanofluid lubricants were tribologically characterized, analyzing the friction coefficient at 298.15 and 124 353.15 K and characterizing the corresponding wear tracks. Elemental mapping and Raman spectra of the worn surfaces 125 contributed to the physical interpretation. Additionally, the density and dynamic viscosity of the base IL and the optimized nano126 additive dispersion for lubrication applications were also experimentally determined in a wide temperature range. 127 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 6 2. Materials and methods 128 2.1. Design 129 The ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate, [EMIM][MS], was provided by Merck (Darmstadt, 130 Germany) with a purity ≥ 95% and a water content < 0.5 wt%, see Table 1. Silicon nitride, Si3N4, and carbon black, CB, were 131 provided by PlasmaChem (Berlin, Germany) with purities > 99%, see Table 1. The Ionanofluid lubricants, dispersions of Si3N4 132 and CB at three mass concentrations (0.10, 0.25 and 1.0 wt%) in [EMIM][MS], were prepared following a two-step method. The 133 amounts of IL and each nano-additive used were weighted in a CPA225 balance from Sartorius AG (Goettingen, Germany) with 134 0.1 mg uncertainty. Subsequently, the dispersions were sonicated through an ultrasonic bath Fisherbrand FB11201 from Thermo 135 Fisher Scientific (Waltham, MA, USA) for 120 min, operating in continuous shaking mode with an effective power of 180 W 136 and a sonication frequency of 37 kHz. 137 Table 1. Main characteristics of materials used, according to the manufacturer1,2. 138 Ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate1, [EMIM][MS] CAS Number 145022-45-3 Other common nomenclature [EMIM] [MeSO3] [EMIM] [CH3SO3] [C2MIM][MS] [C2mim][CH3SO3] Chemical structure C7H14N2O3S Flash point 559.15 K Purity ≥ 95% Water content < 0.5 wt% Nano-additive Silicon nitride2, Si3N4 Average particle size 25 nm Specific surface area 75 m²·g-1 Purity > 99% Other contents Fe < 0.05, Ca < 0.05, Al < 0.1% Nano-additive Carbon black2, CB Average particle size 13 nm Specific surface area 550 m²/g Purity > 99% Other contents Ash < 0.02% 1Merck (Darmstadt, Germany) 139 2PlasmaChem (Berlin, Germany) 140 2.2. Experimental 141 The nano-particles used were morphologically characterized through scanning electron microscopy (SEM). A drop of each 142 dispersion composed of each nano-powder in analytical grade methanol was dried at room temperature on a silica support. 143 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 7 Backscattering electron images over the specimens were obtained by a JEOL JSM-6700F field emission scanning electron 144 microscope from JEOL (Tokyo, Japan) at an operating accelerator voltage of 10.0 kV. 145 The stability of the ionic dispersions was analyzed by dynamic light scattering (DLS) technique through a Zetasizer Nano 146 ZS from Malvern Instruments (Malvern, United Kingdom). The temporal evolution of the apparent size of the nano-additives 147 was characterized with the procedure previously described [42, 43]. The present study was carried out for the 0.25 wt% 148 Ionanofluids, the least concentrated dispersions, for 31 days at 298.15 K with a scattering angle of 173º. Two types of samples 149 were analyzed: dispersions in static conditions since their preparation (from now on referred to as “static” samples) and 150 dispersions to which mechanical agitation was applied before the DLS measurement (from now on referred to as “shaken” 151 samples). The mechanical agitation of the shaken samples was performed with a ZX3 Advanced Vortex Mixer from VELP 152 Scientifica (Usmate Velate, Italy) during 1 min at 2000 rpm. 153 The tribological characterization was conducted by a rotational rheometer MCR 302 from Anton Paar (Graz, Austria) 154 coupled with a tribology cell T-PTD 200 at 298.15 and 353.15 K [44, 45]. The temperature was controlled with a Peltier system 155 H-PTD 200 with 0.1 K accuracy. A ball-on-three-pins configuration was employed for the tests. The 100Cr6 steel ball is 12.7 156 mm in diameter and the 100Cr6 steel pins have a diameter of 6 mm and are 6 mm high. Before each test, all materials were 157 cleaned by means of hexane and dried in atmospheric conditions. 1.3 mL of sample per test was used, ball and pins being 158 completely submerged. The operating parameters were 45 N total axial force of the rheometer (21.20 N normal force at each pin159 ball surface contact, 0.7 GPa average contact pressure), 0.1 m·s-1 constant sliding velocity and 340 m sliding distance. Three 160 replicates were run for each dispersion. More information about the tribological cell and the procedure can be found elsewhere 161 [44, 45]. 162 The surface morphology of the worn pins generated after the tribological tests was analyzed by a 3D optical profiler S neox 163 from Sensofar (Tarrasa, Spain) [46, 47]. 3D images of the wear of the pins as well as the wear scar diameter, the wear track depth 164 and wear hole volume were obtained in confocal mode with a 10x objective. This apparatus was also used to evaluate the 165 roughness (Ra) of the worn surfaces of the pins lubricated with the studied samples. For this task, the ISO4287 standard 166 (International Organization for Standardization, Vernier, Switzerland) was followed, employing a Gaussian filter with a long 167 wavelength cut-off of 0.08 mm and 0.25 mm. The presented values of wear scar diameter, the wear track depth, wear hole volume 168 and roughness were obtained as the average of the three replicates for each nano-additive concentration. Elemental mapping and 169 Raman spectra of the worn surfaces were recorded with a confocal Raman microscope alpha 300R+ from WITec (Ulm, Germany) 170 at a wavelength of 532 nm in order to obtain information about the composition in the wear track. 171 Dynamic viscosities were determined by a rotational Stabinger viscometer SVM 3000 from Anton Paar (Graz, Austria) at 172 atmospheric pressure and in the temperature range from 278.15 to 373.15 K, 5 K step. This device also includes a vibrating tube 173 densimeter that determines the densities at the same conditions. More information of this setup was previously described [48, 174 49]. The expanded uncertainty of these measurements (0.95 level of confidence) was previously established as 1% for dynamic 175 viscosity, 0.5 kg·m-3 for density and 0.02 K for the temperature. 176 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 8 3. Results 177 3.1. Nano-powder characterization and Ionanofluid stability 178 SEM images of Si3N4 and CB nano-powders (Figure 2) indicate that both types of nano-particles present a nearly spherical 179 shape. The creation of the observed aggregates can be attributed to the drying process. The perceived particle sizes are in 180 agreement with the information provided by the manufacturer (Table 1), with Si3N4 nano-particles being about twice as large as 181 CB nano-particles. 182 183 Figure 2. SEM images of Si3N4 (a) and CB (b) nano-particles at ×100000 magnification 184 With regard to the stability characterization, it is worth mentioning that the size value by DLS measurements is assumed as 185 the diameter of a perfect sphere that presents a translational diffusion coefficient equal to that of the dispersed particle. Thus, 186 these values are commonly referred to as hydrodynamic diameter or apparent size. On the other hand, the average Z-size is the 187 intensity-weighted mean size calculated from a cumulants fit of the intensity autocorrelation function resulting from the DLS 188 measurement. Therefore, it should be taken into account that the stability analysis is focused on the temporal evolution of the 189 DLS measurements rather than the values of the sizes themselves. Figure 2 shows the average Z-size dependence on time after 190 preparation for “static” and “shaken” 0.25 wt% Ionanofluids. 191 a) b) J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 9 192 Figure 3. Average Z-size dependence on time after preparation at 298.15 K for the 0.25 wt% dispersions: Si3N4 193 Ionanofluids (a) and CB Ionanofluids (b). Error bars mean standard deviation (k=2) of the experimental measurements and 194 reference lines (- - -) indicate the calculated average value for each sample. 195 Figure 3 a) indicates a quasi-constant average Z-size value during one month for “static” (~ 103 nm) and “shaken” (~ 113 196 nm) Si3N4 Ionanofluids, which is a sign of the excellent long-term stability of the dispersions. Furthermore, it should be noted 197 that the differences between the obtained values are covered by the experimental expanded uncertainty. On the other hand, Figure 198 3 b) shows a marked decrease of the average Z-size with time for the “static” CB Ionanofluid while a practically constant value 199 for the “shaken” CB Ionanofluid (~ 286 nm) during the one-month period is obtained. From these results we conclude that the 200 largest dispersed CB nano-particles or particle agglomeration tend to precipitate in static conditions, but the initial dispersion 201 state is easily recoverable by means of a brief mechanical agitation. 202 3.2. Tribological characterization 203 A first tribological test was performed for the six designed dispersions and the IL, in order to characterize the nano-additive 204 mass concentration dependence on the friction coefficient,  . Figure 4 gathers the experimental friction coefficients as a function 205 of the sliding distance for all the samples at 353.15 K, showing that both 0.25 wt% Ionanofluid lubricants reach the highest 206 friction coefficient reduction among those analyzed. This decrease is higher for the 0.25 wt% CB dispersion (0.089 average 207 value, 16 % decrease) than for the 0.25 wt% Si3N4 dispersion (0.097 average value, 8.5 % decrease), as is also shown in Table 2 208 and Figure 5. Both 0.10 wt% samples (Si3N4 and CB-based Ionanofluids) exhibit practically the same friction behavior than the 209 IL (an average  value of 0.106 for [EMIM][MS] while average  values of 0.105 and 0.103 for the corresponding Si3N4 and 210 CB-based Ionanofluids, respectively). The 1.0 wt% CB dispersion presents an average  value of 0.105, very similar to that of 211 the IL (0.106), while the 0.25 wt% Si3N4 dispersion shows an average  value of 0.123, which implies a 16 % worsening with 212 respect to the neat IL. 213 The existence of an optimal nano-additive concentration for which a minimum friction coefficient is achieved was previously 214 pointed out for other nano-lubricants [20, 21, 28, 29, 33, 46, 47, 50]. For instance, Pamies et al.[33] analyzed the tribological 215 0 50 100 150 200 250 0 4 8 12 16 20 24 28 32 Average Z-ize [nm] Time [days] Static Shaken a) 0 100 200 300 400 500 0 4 8 12 16 20 24 28 32 Average Z-size [nm] Time [days] Static Shaken b) J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 16 298 Figure 9. Raman spectra and elemental map of the worn surface obtained with the 0.25 wt% Si3N4 Ionanofluid (a) and the 0.25 299 wt% CB Ionanofluid (b). 300 3.3. Density and dynamic viscosity 301 Figure 10a shows that experimental density values for [EMIM][MS] (reported in Table 4) are in agreement with the 302 corresponding literature data [37, 40, 56-68], with absolute average deviations within 0.4%. It should be noted that the content 303 of water is slightly different among the reported literature, but always less than 1% [37, 40, 56-68]. The best agreements were 304 found with the data from Domanska et al. [60], Rabari et al. [64], Krannich et al. [40], Marium et al. [66], and Chereches et al. 305 [68], with absolute average deviations of 0.03%, 0.10%, 0.09%, 0.02% and 0.05%, respectively. 306 307 Figure 10. Relative deviations between experimental, exp, and literature, lit, data as a function of temperature for 308 [EMIM][MS] for density, ρ, [37, 40, 56-68] (a), and dynamic viscosity, ƞ, [37, 56, 58, 59, 65, 69-71] (b). 309 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 273.15 293.15 313.15 333.15 353.15 373.15 (ρexp-ρlit) / ρlit·100 [%] Temperature [K] Hasse et al. Ficke et al. Freire et al. Stark et al. Domanska et al. Seki et al. Anantharaj et al. Shah et al. Rabari et al. Harris et al. Krannich et al. Marium et al. Safarov et al. Bioucas et al. Chereches et al. a) -20 -15 -10 -5 0 5 10 15 20 273.15 293.15 313.15 333.15 353.15 373.15 (ƞexp-ƞlit) / ƞlit·100 [%] Temperature [K] Hasse et al. Freire et al. Stark et al. Singh et al. Tenney et al. Harris et al. Bioucas et al. Safarov et al. b) b) J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 17 Table 4 shows the experimental density values of the optimal nano-additive loading dispersions for the friction reduction, the 310 0.25 wt% Ionanofluids. The base fluid and both Ionanofluids show a density decrease of 5.0% with the temperature rising from 311 278.15 to 373.15 K. On the other hand, the dispersion of Si3N4 and CB caused almost temperature independent increases in the 312 densities around 0.14% and 0.10%, respectively. Increases in mass per unit volume can lead to additional difficulties in fluid 313 flow. The reported density increases for the selected improved dispersions are less than 0.15% in both cases, a slight variation 314 that does not involve a noticeable change in relation to the base fluid. 315 Table 4. Experimental densities, ρ, for [EMIM][MS], 0.25 wt% Si3N4 Ionanofluid and 0.25 wt% CB Ionanofluid at 316 temperatures, T, from 278.15 to 373.15 K and fitting parameters, A0, A1, A2, standard deviations, s, and absolute average 317 deviations, AAD, from Eq. (1). 318 T [K] ρ [kg·m-3] [EMIM][MS] 0.25 wt% Si3N4 0.25 wt% CB 278.15 1252.8 1254.9 1254.3 283.15 1249.3 1251.3 1250.7 288.15 1245.8 1247.9 1247.2 293.15 1242.5 1244.5 1243.8 298.15 1239.2 1241.1 1240.5 303.15 1236.0 1237.8 1237.2 308.15 1232.7 1234.5 1234.0 313.15 1229.5 1231.2 1230.7 318.15 1226.2 1227.9 1227.4 323.15 1222.9 1224.6 1224.1 328.15 1219.7 1221.2 1220.8 333.15 1216.4 1217.9 1217.5 338.15 1213.1 1214.6 1214.2 343.15 1209.8 1211.3 1210.9 348.15 1206.5 1208.0 1207.6 353.15 1203.2 1204.7 1204.3 358.15 1199.9 1201.4 1201.1 363.15 1196.7 1198.2 1197.8 368.15 1193.4 1194.9 1194.5 373.15 1190.2 1191.7 1191.3 A0 [kg·m-3] 1435.1 1439.1 1437.5 -A1 [kg·m-3·K-1] 0.6566 0.6636 0.6602 109·A2 [kg·m-3·K-2] 2.824 5.891 7.324 s [kg·m-3] 0.11 0.14 0.14 AAD 0.006% 0.008% 0.008% The temperature dependence on density was correlated by the following quadratic equation: 319 01 2··)( ATATAT 2++=  (1) 320 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 18 where ρ and T means density and temperature, respectively, and A2, A1 and A0 are the fitting parameters. The values of the fitting 321 parameters reported in Table 4 allow for a correlation of the experimental density data with standard deviations lower than 0.15 322 kg·m−3 for all samples. Isobaric thermal expansivity at atmospheric pressure was obtained by the following equation [72, 73]: 323 p pdT d      −=    · 1 (2) 324 Figure 11 gathers the obtained isobaric thermal expansivities in the temperature range from 283.15 to 368.15 K. Rises with 325 the temperature increase of 4.7% were detected for base fluid and Ionanofluids. The nano-additive dispersion leads to higher 326 thermal expansivity values. Increases for the 0.25 wt% Si3N4 and CB loadings of around 0.92 % and 0.45% were obtained, 327 respectively. Isobaric thermal expansivity values are useful to determine the size of the container when the fluid is heated. As 328 observed, the differences between Ionanofluids and base IL are lower than 1% in both cases. 329 330 Figure 11. Isobaric thermal expansivity, αp, as a function of temperature, T, for [EMIM][MS], 0.25 wt% Si3N4 331 Ionanofluid and 0.25 wt% CB Ionanofluid. 332 Figure 10b shows the relative deviations between the experimental dynamic viscosities for [EMIM][MS] (Table 5) and the 333 corresponding literature data [37, 56, 58, 59, 65, 69-71]. It should be noted that some data sets present large deviations from the 334 rest of the literature data, with significant dependence on temperature. The absolute average deviations between the experimental 335 dynamic viscosity data reported in this work and those from Tenney et al. [70] and Harris et al. [65] are within 4.2%. The best 336 agreements were found with Hasse et al. [56], Bioucas et al. [37] and Safarov et al [71], with absolute average deviations of 337 2.5%, 1.5% and 1.4%, respectively. 338 Table 5 also reports the experimental dynamic values of the optimal nano-additive loading dispersions for the friction 339 reduction. The increasing temperature from 278.15 to 373.15 K leads to a dynamic viscosity reduction of 99% for [EMIM][MS] 340 and both Ionanofluids. Contrastingly, increases in the ranges 3.5-6.3% and 5.2-12% at constant temperature were observed 341 because of the dispersion of Si3N4 and CB in the base fluid, respectively. These increases tend to be lower as the temperature 342 rises. Viscosity is directly related to the energy required to make a fluid flow. Small increases in dynamic viscosity were obtained 343 for both selected Ionanofluids, where the increment for Si3N4 Ionanofluid is half of that for CB Ionanofluid. The CB dispersion, 344 5.20 5.25 5.30 5.35 5.40 5.45 5.50 5.55 5.60 273.15 293.15 313.15 333.15 353.15 373.15 αp[10-4·K-1] T [K] 0 wt% 0.25 wt% Si3N4 0.25 wt% CB Si3N4 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 19 which allows for a greater improvement in the friction coefficient and scar wear, implies a higher increase in viscosity and, 345 therefore, a slightly higher pumping power. 346 Table 5. Experimental dynamic viscosities, ƞ, for [EMIM][MS], 0.25 wt% Si3N4 Ionanofluid and 0.25 wt% CB 347 Ionanofluid at temperatures, T, from 278.15 to 373.15 K and fitting parameters, η0, A and T0, standard deviations, s, and 348 absolute average deviations, AAD, from Eq. (3). 349 T [K] ƞ [mPa·s] [EMIM][MS] 0.25 wt% Si3N4 0.25 wt% CB 278.15 689.8 732.9 772.3 283.15 439.7 465.6 492.0 288.15 295.0 311.0 329.6 293.15 206.2 216.7 230.1 298.15 149.1 156.2 166.2 303.15 111.1 116.1 123.5 308.15 84.98 88.59 94.26 313.15 66.49 69.15 73.52 318.15 53.07 55.09 58.43 323.15 43.07 44.67 47.26 328.15 35.51 36.78 38.82 333.15 29.70 30.73 32.29 338.15 25.13 26.01 27.19 343.15 21.50 22.26 23.14 348.15 18.57 19.23 19.91 353.15 16.19 16.77 17.28 358.15 14.22 14.73 15.11 363.15 12.59 13.05 13.32 368.15 11.22 11.64 11.82 373.15 10.06 10.44 10.58 η0 0.2392 0.2339 0.2301 A [K] 3.447 3.492 3.608 T0 [K] 194.1 194.0 192.6 s [mPa·s] 0.33 0.36 0.42 AAD 0.4% 0.9% 0.6% The temperature dependence on the dynamic viscosity was correlated by the Vogel–Fulcher–Tammann (VFT) equation [74350 76], or Vogel–Fulcher–Tammann–Hesse equation: 351 0 0 T A·T 0·− =T e  (3) 352 where ƞ and T means dynamic viscosity and temperature, respectively, and ƞ0, A and T0 are the fitting parameters. The values of 353 the fitting parameters gathered in Table 5 allow for the correlation of the experimental dynamic viscosity data with absolute 354 average deviations of less than 1%. 355 J.P. Vallejo et al. / Journal of Molecular Liquids xxx (2020) xxxxx 20 4. Conclusions 356 In this work the following features were achieved: 357 1. New Si3N4 and CB dispersions in [EMIM][MS] at nano-particle mass concentration between 0.1 to 1 % were 358 conveniently designed for lubrication purposes. 359 2. DLS measurements indicated the excellent long-term stability of the Si3N4 Ionanofluids and the easily recoverable initial 360 dispersion conditions of the CB Ionanofluids. 361 3. Ionanofluids at 0.25 wt% mass concentration achieved the highest friction coefficient reductions at both analyzed 362 temperatures, 293.15 and 353.15 K. At the highest temperature, these decreases reached 16% and 8.5% for the CB and 363 Si3N4 dispersions, respectively. 364 4. The maximum wear reductions were produced at 353.15K: the 0.25 wt% CB Ionanofluid achieved 46% and 85% in the 365 diameter and depth of the wear, respectively, whereas 36% and 73% reductions were respectively obtained for the 0.25 366 wt% Si3N4 Ionanofluid. The maximum volume reduction was achieved for the 0.25 wt% CB Ionanofluid with 28 times 367 lower wear volume than that for the base IL. 368 5. The roughness of the worn surfaces achieves 75% and 61% reductions by using the 0.25 wt% CB and Si3N4 new 369 lubricants, respectively. 370 6. Raman mapping indicates that mending effect occurs using the optimal CB Ionanofluid, but does not show this effect 371 for the optimal Si3N4 dispersion. 372 7. Dynamic viscosity rises lower or equal to 6.3% and 12% for the 0.25 wt% Si3N4 and CB Ionanofluids were reached, 373 whereas density increases up to around 0.14% and 0.10%, respectively, with almost no dependence on temperature. 374 CRediT authorship contribution statement 375 Javier P. Vallejo: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. José M. Liñeira del 376 Río: Investigation, Methodology, Writing - original draft. Josefa Fernández: Conceptualization, Supervision, Validation, Writing - review & 377 editing. Luis Lugo: Conceptualization, Supervision, Validation, Writing - review & editing. 378 Declaration of competing interest 379 There is no conflict of interest. 380 Acknowledgements 381 This work was supported by the “Ministerio de Economía y Competitividad” (Spain) and the ERDF program through ENE2017-86425-C2382 1/2-R projects, and by the “Xunta de Galicia” (ED431E2018/08, ED431D 2017/06 and GRC ED431C 2016/001). J.P.V. acknowledges the FPI 383 Program of the “Ministerio de Economía y Competitividad”. 384 References 385 [1] J.P. Vallejo, E. Álvarez-Regueiro, D. Cabaleiro, J. Fernández-Seara, J. Fernández, L. 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