Effect of ZrO2 nanoparticles on thermophysical and rheological properties of three synthetic oils

1 Effect of ZrO2 Nanoparticles on Thermophysical and Rheological Properties of Three Synthetic Oils María J.G. Guimareya, Miguel R. Salgadoa, María J.P. Comuñasa*, Enriqueta R. Lópeza, Alfredo Amigob, David Cabaleiroc,d, Luis Lugod, Josefa Fernándeza a Laboratorio de Propiedades Termofísicas, Grupo NaFoMat, Departamento de Física Aplicada, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b Laboratorio de Propiedades Termofísicas y Superficiales de Líquidos, Departamento de Física Aplicada, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain c Institute of Construction Technologies, National Research Council, 35127 Padova, Italy; d Departamento de Física Aplicada, Facultade de Ciencias, Universidade de Vigo, 36310 Vigo; Spain * Corresponding author: M. J.P. Comuñas E-mail address: [email protected] 2 ABSTRACT This article presents an experimental study on some thermophysical properties (density, viscosity and adiabatic bulk modulus) of six synthetic oils-based ZrO2 nanolubricants. Twostep method with ultrasonic disruptor was used to prepare the nanodispersions. The morphology, crystalline degree and elemental composition of nanoparticles were analyzed by electron microscopy. Visual observation, temporal variation of refractive index and dynamic light scattering were used to analyze the stability of the nanolubricants and the average size of the aggregates. The presence of new interactions between nanoparticles and base oils was studied through Fourier transform infrared spectrometer. Vibrating tube densimeters, rotational viscometer and rheometer equipped with cone-plate geometry were used within the temperature range from 278.15 to 373.15 K. The ability of some theoretical simple models to predict densities and viscosities of these nanolubricants as a function of temperature and nanoparticle concentration was also checked. Keywords: synthetic oils; ZrO2 nanoparticles; nanolubricants; adiabatic bulk modulus; density; rheology 3 1. INTRODUCTION Lubrication is of great importance for the efficient use of machinery. The main purpose of lubrication is to reduce friction and wear in bearings or sliding components to prevent premature failure [1,2]. Frictional losses are the main source of energy loss in mechanical systems. It is crucial to investigate in the improvement of the thermophysical and tribological properties of lubricants which would lead to greater economic, social and environmental effectiveness. The variety of lubricating fluids has grown to satisfy the demands of new machines having more stringent requirements due to their operation under more severe conditions or in harsh environments [3]. Generally, the lubricants are formulated from synthetic, mineral or vegetable type base oils. Although mineral oils represent the majority of the market demand, many technological advances in equipment and machinery would not be possible without the benefits offered by synthetic oils [3]. To the base oils, additives are added (dissolved or suspended) to achieve the desirable characteristics of the final lubricant. Recently, Spikes published a review [4] to provide the state of art of friction modifier additives. Additives are organic or inorganic compounds that are used to improve properties (friction, viscosity grade and viscosity index, among others), to prevent wear and corrosion, or to reduce undesirable changes that appear during the service life of the lubricant (thermal degradation, for example). Hence, additives are used to improve the tribological performance of lubricants. In recent years, numerous articles investigated the use of nanoparticles as oil additives [5,6]. Shahnazar et al. [7] have published a review that compiles the results of other researchers showing that lubricants containing nanoadditives may have better tribological properties than those including traditional additives. Xiao and Liu [8] have published a review remarking the recent developments in using 2D nanomaterials to improve the friction and the anti-wear properties of base lubricants [4,7,9-12]. Nanoadditive dispersions in base 4 oils are usually called nanolubricants [13]. A priori due to their thermal conductivity and high dynamic viscosity, nanolubricants will have good ability to tolerate shear force and to heat transfer [14]. The published works report studies on metals and their oxides [14-24], molybdenum and tungsten disulfide [8,25,26], metal borates [27,28], fullerenes [29,30], graphitic nanoparticles [25,31,32] and nanodiamond particles [33] as nanoadditives. Representative properties such as viscosity, density, friction and wear coefficients, flow curves and thermal stability, among others, should be taken into consideration when evaluating lubricant performance. Then, it is necessary to study the effects of the nanoadditives on these properties. Hence, in this work we focus our attention on the thermophysical (density, speed of sound and adiabatic bulk modulus) and rheological properties (viscosity and flow curves) of six ZrO2 nanolubricants based on three synthetic base oils (a polyalkyleneglycol, PAG2, a biodegradable polymeric ester, BIOE, and isotridecyl trimellitate, TTM). Some authors have previously studied the ability of ZrO2 nanoparticles as lubricant additives. Thus, Hernández Battez et al. [16] have analyzed the antiwear behavior of ZrO2 nanoparticles dispersed at mass concentrations of 0.5%, 1.0% and 2.0% in PAO6. The effect of ZrO2 functionalized nanoparticles as additive of multialkylated cyclopentanes on the friction and wear behavior was investigated by Ma and Bai [34]. Rani et al. [35] have studied the tribological properties of ZrO2 nanoparticles dispersed in a vegetable oil. 2. MATERIALS AND METHODS 2.1. Materials: characterization We have used a dimethoxy-end-capped poly(propylene glycol) named as PAG2 that has been kindly provided by Croda. By using MALDI-TOF [36] mass spectrometry we have obtained for this fluid an average molecular mass, Mw, of 1717 g/mol, and a polydispersity 5 index Mw/Mn=1.022 [37] that indicates that this polymer can be considered as monodisperse. Taking into account that the molecular structure of this type of PAG is CH3-O-[CH2CH(CH3)-O]n-CH3, it can be concluded that, n, the average number of monomers is =29. The synthetic esters (isotridecyl trimellitate, TTM, and the biodegradable polymeric ester, BIOE) were provided by Verkol Lubricantes. General chemical structures of the three base oils are shown in Figure 1. Zirconium oxide nanoparticles stabilized with 3% HfO2 (30-60 nm nominal diameter and 5.9 g·cm-3 bulk density) with a purity of 99.9% were supplied by Iolitec (lot. INO059008). The ZrO2 nanopowder was characterized by X-ray diffraction (XRD) using a Philips type powder diffractometer. The instrument was equipped with a graphite diffracted beam monochromator and copper radiation source (λ(Kα1)=1.5406Å), operating at 40 kV and 30 mA. It is well known that ZrO2 has three polymorphs [38]: monoclinic, tetragonal and cubic phases/structures. The X-ray patterns of ZrO2 nanopowder present sharp and well defined peaks, evidence of a high degree of crystallinity as shown in Figure 2, where intensive diffraction patterns are observed at 2θ = 24.5º, 28.2º, 31.5º and 34.3º which correspond to monoclinic ZrO2 crystal phase [39]. Monoclinic ZrO2, baddeleyite, has the space group symmetry P21/c. The unit-cell dimensions are a = 0.5145 nm; b = 0.5210 nm; c = 0.5312 nm, and with angles α=γ=90º and β=99.226º (ICSD 98-008-2545). These values are very close to the those of the literature [40]. The morphology and size of ZrO2 nanoparticles were obtained by scanning electron microscopy (SEM, Zeiss FESEM Ultra Plus). As it is shown in Figure 3 these nanoparticles are spherical. Besides, with the help of an additional detector for energy dispersive X-ray microanalysis (EDX), the spectrum of ZrO2 nanoparticles was obtained. As expected, results indicate the predominant presence of zirconium and oxygen in the sample. The content of hafnium is due to the manufacturer uses HfO2 to stabilize the ZrO2 nanoparticles. The 6 aggregation state of ZrO2 nanoparticles dispersed in butanol was evaluated through transmission electron microscopy (TEM, JEOL JEM-2010) by using an accelerating voltage of 200 kV. Figure 4 reveals that ZrO2 nanoparticles appear as aggregates of individual particles. 2.2. Nanolubricant Preparation Two-step method was used to prepare the nanolubricants. Firstly, we mixed the ZrO2 nanoparticles, in form of dry powder, with the base oil (PAG2, TTM or BIOE). A high precision balance Sartorius MC 210P was used to determine the mass concentration of ZrO2 nanoparticles (around 1 wt % or 2 wt %, which correspond to 0.17% and 0.34% in volume fraction, considering a density value of 5.9 g∙cm‐3 for the ZrO2 nanopowder). The readability of the balance in the measured mass range is 0.00001 g. Secondly, we disperse the nanoparticles in the base oil by using an ultrasonic disruptor (HD 2200 Sonopuls). The sonicator probe conducts the acoustic energy from the transducer into the sample. The energy transferred to the nanodispersion depends on: the applied power, the total time that the dispersion is subjected to ultrasounds, the volume of the sample, the shape and diameter of the probe and its immersion depth. In this work we have used the same power (200 W), amplitude (302 m), diameter and shape probe (MS73, 3 mm), sonication time (30 minutes) and prepared the same sample volume (20 ml) for all studied nanolubricants. To minimize overheating during sonication, the samples were immersed in an ice-water bath. Fourier transform infrared spectrometer (FTIR, VARIAN 670-IR) analyses were conducted to study the formation of chemical bonds between nanoparticles and the base oils. FT-IR spectra of PAG2, TTM and BIOE base oils and their corresponding nanodispersion at 2 wt % are shown in Figure 5. For pure PAG2 typical bands [41,42] are visible at wavenumbers of 2867 cm-1 (CH2 stretching), 1373 cm-1 (CH3 bending) and 1093 cm-1 (C-O-C asymmetrical stretching). 7 For TTM typical bands appears at 2956, 2925 and 2871 cm-1 (C-H stretching), 1725 cm-1 (C=O stretching), 1461 (aromatic C=C stretching), 1234, 1112 and 1068 cm-1 (C-O stretching) [43]. For BIOE typical bands [44] take place at 2921 cm−1 and 2852 cm−1 which conform to -CH2 asymmetrical and symmetrical stretching respectively, 1743 cm-1 which confirms the formation of ester linkages in the methyl ester of the oil, and 1151 cm-1 which corresponds to a C-O link typical of esters. No new spectral peaks or shifts are found for the dispersions in comparison with the base oils, which indicates that no chemical bonds were formed. Visual observation is the simplest method to initially evaluate stability of nanolubricants. In this work, the nanolubricants were placed at room temperature without any disturbance. Subsequently, the samples were observed every hour until detecting the nanoparticles sedimentation at the bottom of the container. Figure 6 shows photographs of the ZrO2 nanoparticles dispersed in PAG2, TTM or BIOE as a function of the time for the dispersions prepared at the highest nanoparticle concentration. From the visual analysis it can be observed that dispersion based on BIOE base oil showed the fastest sedimentation. Separation layer of ZrO2 and base lubricant (BIOE) can be clearly seen 144 hours after sonication. Partially sedimentation appears after 48 h for the other two nanolubricants based on visual tests but the separation layer is not clearly observed at 192 h. For all nanolubricants prepared in this work the time interval necessary to visually observe partial sedimentation is much higher than that necessary to perform the thermophysical experiments (3 hours). In order to use a less qualitative method to analyze the stability of the nanolubricants we have performed some tests with an UV-Vis spectrophotometer (Varian Cary 50 Bio UV-Visible) and a turbidimeter (Hanna Instruments HI 88713). UV-Vis spectrophotometry is one of the most common techniques employed to investigate the stability of colloidal suspensions. Nevertheless, at the concentrations of interest (1 wt % and 2 wt %) signals from both devices 8 show saturation (dispersions are usually too opaque so that the absorbance and the turbidity cannot be analyzed). Thus, it was necessary to dilute the dispersions up to a mass concentration around 10-3 wt %. In our opinion, it is not rigorous to extrapolate the results found for 10-3 wt % to 2 wt %, and therefore both techniques have been discarded. We have also used dynamic light scattering (Zetasizer Nano ZS DLS) due to this technique offers the possibility of obtaining the average size of dispersed nanoparticles as well as valuable information of the stability of dispersions. For PAG2-based nanofluids we have obtained average diameters of 226 nm and 234 nm for 1 wt % and 2 wt % concentrations, respectively. For nanolubricants based on TTM, the mean diameters are 102 nm and 87 nm and for those based on BIOE 67 nm and 69 nm. As a result, DLS data reveal agglomeration of ZrO2 nanoparticles in the suspensions, the lowest agglomeration corresponding to the dispersions containing BIOE whereas for PAG2-based nanofluids the average cluster size is around four times the diameter of the primary ZrO2 nanoparticles. We observed that the highest average diameter was obtained for the base oil with the lowest viscosity grade (PAG2). This may be due to smaller probability of collision of nanoparticles in the more viscous fluids because a lower Brownian motion. Moreover, in the more viscous fluids can suspend particles easier because the higher hydrodynamic drag. Figure 7 represents the temporal evolution of the average diameter for the nanofluids at 2 wt %. The three nanolubricants present partial sedimentation at the initial stage (within 24 hours after sonication). This time interval is shorter that those estimated only from visual tests, but larger than those needed to measure the thermophysical properties. Furthermore, we have checked the possibility to use a refractometer (Mettler Toledo RA-510M) to analyze the stability of the nanolubricants. Thus, the temporal evolution of the refractive index (n) at 298.15 K has been registered over 99 hours for ZrO2/PAG2 and ZrO2/TTM nanolubricants at 2 wt %. For BIOE based nanolubricants it was not possible to 9 make this study due to its refractive index is outside the instrument working range. The interest of this study is not the quantitative value of the refractive index but its time dependence. For each nanolubricant, we have performed three replicates of the temporal evolution of the refractive index. That is, we sonicate the samples at the conditions detailed above (power, amplitude and sonication time), after that we measure the refractive index as a function of time. Several days later, we sonicate again the sample (under the same conditions) and we measure again the temporal dependence of n. As Figure 8 shows, 20 hours after sonication the refractive index increases with time for both PAG2 and TTMbased nanolubricants, which could mean that nanoparticle sedimentation occurs. Another interesting fact we observe for nanolubricants containing PAG2 is that the n(t) curve is practically the same for the three test performed, even if for one of them we have sonicated 120 minutes instead of 30 minutes as usual. This is not the case of TTM based nanolubricants, for which we observe different curves for each sonication process. This fact could mean that the agglomeration is more sensible to the sonication process for this last nanolubricant. Indirectly this fact could also be affected by the high viscosity of TTM base oil (1449 mPas at 293.15 K against 143 mPas for PAG2). The overall variation of the refractive index is smooth ( 0.003) during the 20 first hours after sonication for both PAG2 and TTM based nanolubricants. This result could confirm that these nanolubricants are relatively stable during this time interval. 2.3. Thermophysical characterization techniques Density at atmospheric pressure of the nanolubricants was measured by using a vibrating densimeter Anton Paar SVM 3000 Stabinger. We have performed the measurements from 278.15 to 373.15 K. The temperature of the cell is controlled through an integrated thermostat with cascaded Peltier elements and measured with a Pt100 16 being 4-6% these values for ZrO2/TTM samples. We have observed that this equation underpredicts the viscosity of the nanolubricants. It is necessary to highlight that Einstein equation is valid for dilute non-interacting suspensions of spherical particles. We have also tried to use the following empirical equations due respectively to Saito (equation 6), Brinkman (equation 7), Batchelor (equation 8), Wang et al. (equation 9) and Chen et al. [60,61,64-66]: bfn) -1 2.5 +(1=     f (6) ( ) bf 5.2 n1 1 =    − f (7) bf 2 n6.2+2.5+1=        f (8) bf 2 n123+7.3+1=        f (9) ( ) bf 2 n10.6+10.6+1=        f (10) These equations (5 to 10) are the most frequently applied in theoretical or empirical models for viscosity estimation of nanofluids. As it can be seen in Figure 16b the highest AADs were found for nanolubricants based on TTM oil. The results obtained with Saito [65], Brinkman [61] and Batchelor [60] equations are quite similar to those obtained with Einstein equation. For PAG and TTM based nanolubricants we observe that the deviations between the predicted and the experimental viscosity values increase with the concentration of nanoparticles for all the empirical equations used. For BIOE based nanolubricants the deviations remain almost constant (around 3%) with nanoparticles concentration for equations 5 to 8. However, with equations 9 and 10 the deviations decrease with concentration. The best results were obtained with the Chen et al. model [64] (equation 10), 17 for which AADs ranges from (0.29 to 0.33%) for PAG2 based nanolubricants, from (2.27 to 3.52%) for TTM nanolubricants and (0.50 to 1.54%) for BIOE based nanolubricants. 4. CONCLUSIONS AND FUTURE WORK Experimental studies of different additives and base oils combinations are needed to analyze the influence that nanoadditives have on the thermophysical properties, and subsequently to elucidate lubricating mechanism with nanoadditives. In this work we have observed that the nanolubricant dispersion stability was low and the ZrO2 nanoparticles started to suspend so early after sonication. Results show that density increases (around 2%) with the increase of the mass concentration of ZrO2 nanoparticles. The same occurs with viscosity for which increments up to 8% are observed for isotridecyl trimellitate ester based nanolubricants. Adiabatic bulk modulus has a smoothly variation with the addition of zirconium oxide nanoparticles. The best low temperature fluidity was observed for nanolubricants containing poly(propylene glycol). Linearity found in the flow curves of the three base oils is also present when nanoparticles are added; this fact confirms the Newtonian behavior of all the nanolubricants studied in this work at shear strain rates up to 1000 s-1. The predicted densities agree with the experimental data being the average absolute deviations lower or equal than 0.12%. For viscosities the Chen et al. predictive model showed reasonably agreement (absolute relative deviations lower than 4%) with the experimental viscosity data. In a future it will be interesting to analyze if the usage of dispersants or nanoparticle functionalization could improve the stability of these nanolubricants. Finally, we must point out that more efficient techniques for dispersing nanoadditives in base oils and for controlling the stability of the nanodispersions are still necessary. 18 NOMENCLATURE Mw Molecular mass (g·mol-1) n Refractive index T Temperature (K) p Pressure (Pa) ρ Density (g·cm-3) ρnp Density of nanoparticle (g·cm-3) ρbf Density of the base oil (g·cm-3) ρnf Density of nanofluid (g·cm-3) u Speed of sound (m·s-1) Ks Adiabatic bulk modulus (GPa) ĸs Adiabatic compressibility (GPa-1) α Pressure-viscosity coefficient 𝛾󰇗 Shear rate (s-1)  Shear stress (Pa)  Viscosity (Pa·s)  bf Viscosity of the base oil (Pa·s)  nf Viscosity of nanofluid (Pa·s)  Shear stress (Pa)  Particle volume fraction  Particle mass fraction AAD Average absolute deviations SUBSCRIPTS np Nanoparticle 19 bf Base fluid nf Nanofluid ACKNOWLEDGMENTS Authors acknowledge Croda and Verkol Lubricantes for the lubricants provided. This work was supported by Spanish Ministry of Economy and Competitiveness and the UE FEDER programme through ENE2014-55489-C2-1-R, ENE2014-55489-C2-2-R, ENE2017-86425C2-1-R and ENE2017-86425-C2-2-R projects. Moreover, this work was funded by the Xunta de Galicia (AGRUP2015/11 and GRC ED431C 2016/001). D.C. was recipient of a postdoctoral fellowship from Xunta de Galicia (Spain). REFERENCES [1] N.S. Ahmed, A.M. Nassar, Lubrication and Lubricants, Tribology-Fundamentals and Advancements, InTech, 2013. [2] Y. Zhang, Boundary lubrication—An important lubrication in the following time, J. Mol. Liquids 128 (2006) 56-59. [3] L.R. Rudnick, Synthetic, Mineral Oils and Bio-based Lubricants (2013). [4] H. Spikes, Friction Modifier Additives, Tribol. Lett. 60 (2015) 1-26. [5] M. Ivanov, O. Shenderova, Nanodiamond-based nanolubricants for motor oils, Curr. Opin. Solid St. M. 21 (2017) 17-24. [6] L. Kong, J. Sun, Y. Bao, Preparation, characterization and tribological mechanism of nanofluids, RSC Advances 7 (2017) 12599-12609. [7] S.Shahnazar, S.B.A. Bagheri, Enhancing lubricant properties by nanoparticle additives, Int. J. Hyd. Energy 41 (2016) 3153-3174. [8] H. Xiao, S. Liu, 2D nanomaterials as lubricant additive: A review, Materials & Design 135 (2017) 319-332. [9] V.N. Bakunin, A.Y. Suslov, G.N. Kuzmina, O.P. Parenago, Recent Achievements in the synthesis and application of the inorganic nanoparticles as lubricant componentsa Review, Lubr. Sci. 17 (2005) 127-145. 20 [10] B. Gupta, N. Kumar, K. Panda, S. Dash, A.K. Tyagi, Energy efficient reduced graphene oxide additives: Mechanism of effective lubrication and antiwear properties, Sci. Rep. 6 (2016) 18372. [11] G. Zhao, Q. Zhao, W. Li, X. Wang, W. Liu, Tribological properties of nano-calcium borate as lithium grease additive, Lubr. Sci. 26 (2014) 43-53. [12] J.M. Martin, N. Ohmae, Nanolubricants vol. 13, Wiley, New York (2008). [13] L. Joly-Pottuz, Nanolubricants. in: Q.J. Wang, Y.-W. Chung, (Eds.), Encyclopedia of Tribology, Springer US, Boston, MA, 2013, pp. 2364-2369. [14] M. Hemmat Esfe, M.R. Sarlak, Experimental investigation of switchable behavior of CuO-MWCNT (85%–15%)/10W-40 hybrid nano-lubricants for applications in internal combustion engines, J. Mol. Liquids 242 (2017) 326-335. [15] R. Chou, A.H. Battez, J.J. Cabello, J.L. Viesca, A. Osorio, A. Sagastume, Tribological behavior of polyalphaolefin with the addition of nickel nanoparticles, Tribol. Int. 43 (2010) 2327-2332. [16] A. Hernández-Battez, R. González, J.L. Viesca, J.E. Fernández, J.M.D. Fernández, A. Machado, R. Chour, J. Riba, CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants, Wear 265 (2008) 422-428. [17] A. Hernández-Battez, R. Gonzalez, D. Felgueroso, J.E. Fernandez, M.R. Fernández, M.A. García, I. Penuelas, Wear prevention behaviour of nanoparticles suspension under extreme pressure conditions, Wear 263 (2007) 1568-1574. [18] A. Hernández-Battez, J.E.F. Rico, A. Navas, J.L. Viesca, R. Chou, J.M. Diaz, The tribological behaviour of ZnO nanoparticles as an additive to PAO6, Wear 261 (2006) 256263. [19] A. Kotia, S.K. Ghosh, Experimental analysis for rheological properties of aluminium oxide (Al2O3)/gear oil (SAE EP-90) nanolubricant used in HEMM, Ind. Lub. Tribol. 67 (2015) 600-605. [20] Q. Xue, W. Liu, Z. Wang, Friction and wear properties of a surface-modified TiO2 nanoparticle as an additive in liquid paraffin, Wear 213 (1997) 29-32. [21] Z.S. Hu, J.X. Dong, G.X. Chen, Study of antiwear and reducing friction additive of nanometer ferric oxide, Tribol. Int. 31 (1998) 355-360. [22] K. Sepyani, M. Afrand, M. Hemmat Esfe, An experimental evaluation of the effect of ZnO nanoparticles on the rheological behavior of engine oil, J. Mol. Liquids 236 (2017) 198-204. 21 [23] S. Kannaiyan, C. Boobalan, A. Umasankaran, A. Ravirajan, S. Sathyan, T. Thomas, Comparison of experimental and calculated thermophysical properties of alumina/cupric oxide hybrid nanofluids, J. Mol. Liquids 244 (2017) 469-477. [24] M. Karimi-Nazarabad, E.K. Goharshadi, A. Youssefi, Particle shape effects on some of the transport properties of tungsten oxide nanofluids, J. Mol. Liquids 223 (2016) 828-835. [25] M. Farsadi, S. Bagheri, N.A. Ismail, Nanocomposite of functionalized graphene and molybdenum disulfide as friction modifier additive for lubricant, J. Mol. Liquids 244 (2017) 304-308. [26] L. Joly-Pottuz, F. Dassenoy, Nanoparticles Made of Metal Dichalcogenides, Nanolubricants, John Wiley & Sons, Ltd, 2008, pp. 15-92. [27] C. Boshui, G. Kecheng, F. Jianhua, W. Jiang, W. Jiu, Z. Nan, Tribological characteristics of monodispersed cerium borate nanospheres in biodegradable rapeseed oil lubricant, Appl. Surf. Sci. 353 (2015) 326-332. [28] Z.S. Hu, R. Lai, F. Lou, L.G. Wang, Z.L. Chen, G.X. Chen, J.X. Dong, Preparation and tribological properties of nanometer magnesium borate as lubricating oil additive, Wear 252 (2002) 370-374. [29] T. Jafari Behbahani, A.A.M. Beigi, Z. Taheri, B. Ghanbari, The effect of amino [60] fullerene derivatives on pour point and rheological properties of waxy crude oil, J. Mol. Liquids 211 (2015) 308-314. [30] J. Lee, S. Cho, Y. Hwang, H.-J. Cho, C. Lee, Y. Choi, B.-C. Ku, H. Lee, B. Lee, D. Kim, S.H. Kim, Application of fullerene-added nano-oil for lubrication enhancement in friction surfaces, Tribol. Int. 42 (2009) 440-447. [31] E.K. Goharshadi, Z. Niyazi, M. Shafaee, M.B. Moghaddam, R. Ludwig, M. Namayandeh-Jorabchi, Transport properties of graphene quantum dots in glycerol and distilled water, J. Mol. Liquids 241 (2017) 831-838. [32] A.K. Rasheed, M. Khalid, W. Rashmi, T.C.S.M. Gupta, A. Chan, Graphene based nanofluids and nanolubricants – Review of recent developments, Renew. Sustain. Energy Rev. 63 (2016) 346-362. [33] M.G. Ivanov, D.M. Ivanov, Nanodiamond Nanoparticles as Additives to Lubricants, Ultananocrystalline Diamond Second Edition (2012). [34] J. Ma, M. Bai, Effect of ZrO2 Nanoparticles Additive on the Tribological Behavior of Multialkylated Cyclopentanes, Tribol. Lett. 36 (2009) 191-198. 22 [35] S. Rani, The tribological behavior of TiO2, CeO2 and ZrO2 nano particles as a lubricant additive in rice bran oil, Int. J. Sci. Eng. Res. 7 (2016) 708-712. [36] H.J. Räder, W. Schrepp, MALDI-TOF mass spectrometry in the analysis of synthetic polymers, Acta Polymer. 49 (1998) 272-293. [37] O. Fandiño, L. Lugo, M.J.P. Comuñas, E.R. López, J. Fernández, Temperature and pressure dependences of volumetric properties of two poly(propylene glycol) dimethyl ether lubricants, J. Chem. Thermodyn. 42 (2010) 84-89. [38] P. Gao, L.J. Meng, M.P. dos Santos, V. Teixeira, M. Andritschky, Study of ZrO2– Y2O3 films prepared by rf magnetron reactive sputtering, Thin Solid Films 377-378 (2000) 32-36. [39] S.N. Basahel, T.T. Ali, M. Mokhtar, K. Narasimharao, Influence of crystal structure of nanosized ZrO2 on photocatalytic degradation of methyl orange, Nanoscale Res. Lett. 10:73 (2015). [40] J.D. McCullough, K.N. Trueblood, The crystal structure of baddeleyite (monoclinic ZrO2), Acta Crystallogr. 12 (1959) 507-511. [41] Y.-l. Su, J. Wang, H.-z. Liu, FTIR Spectroscopic Study on Effects of Temperature and Polymer Composition on the Structural Properties of PEO−PPO−PEO Block Copolymer Micelles, Langmuir 18 (2002) 5370-5374. [42] B. Tang, C. Wu, M. Qiu, X. Zhang, S. Zhang, PEG/SiO2–Al2O3 hybrid form-stable phase change materials with enhanced thermal conductivity, Mat. Chem. Phys. 144 (2014) 162-167. [43] H. Lai, Z. Wang, P. Wu, B.I. Chaudhary, S.S. Sengupta, J.M. Cogen, B. Li, Structure and Diffusion Behavior of Trioctyl Trimellitate (TOTM) in PVC Film Studied by ATR-IR Spectroscopy, Ind. Eng. Chem. Res. 51 (2012) 9365-9375. [44] S. Pramanik, K. Sagar, B.K. Konwar, N. Karak, Synthesis, characterization and properties of a castor oil modified biodegradable poly(ester amide) resin, Prog. Org. Coat. 75 (2012) 569-578. [45] I. Otero, E.R. López, M. Reichelt, M. Villanueva, J. Salgado, J. Fernández, Ionic Liquids Based on Phosphonium Cations As Neat Lubricants or Lubricant Additives for a Steel/Steel Contact, ACS Appl. Mat. Inter. 6 (2014) 13115-13128. [46] X. Paredes, Viscous Behaviour of Lubricants at High Pressures, Thesis, University of Santiago de Compostela (2012) 23 [47] S. Mia, N. Ohno, Relation between low temperature fluidity and sound velocity of lubricating oil, Tribol. Int. 43 (2010) 1043-1047. [48] N. Ohno, M.Z. Rahman, K. Kakuda, Bulk Modulus of Lubricating Oils as Predominant Factor Affecting Tractional Behavior in High-Pressure Elastohydrodynamic Contacts, Tribol. Trans. 48 (2005) 165-170. [49] S. Mia, Prediction of Tribological and Rheological Properties of Lubricating Oils by Sound Velocity, Saga University (2010) [50] A. Hernández Battez, J. Viesca, R. González Rodríguez, A. García Martínez, T. Reddyhoff, A. Higuera-Garrido, Effect of Shear Rate, Temperature, and Particle Concentration on the Rheological Properties of ZnO and ZrO2 Nanofluids, Tribol. Trans. 57 (2014) 489-495. [51] J. Meyer, S. Adio, M. Sharifpur, P. N. Nwosu, The Viscosity of Nanofluids: A Review of the Theoretical, Empirical, and Numerical Models, Heat Transfer Eng. 37 (2016) 387-421. [52] X. Paredes, A.S. Pensado, M.J.P. Comuñas, J. Fernández, How Pressure Affects the Dynamic Viscosities of Two Poly(propylene glycol) Dimethyl Ether Lubricants, J. Chem. Eng. Data 55 (2010) 4088-4094. [53] M. Kole, T.K. Dey, Effect of aggregation on the viscosity of copper oxide-gear oil nanofluids, Int. J. Therm. Eng. 50 (2011) 1741-1747. [54] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles, Exp. Heat Transf. Int. 11 (1998) 151-170. [55] E.J. Wasp, J.P. Kenny, R.L. Gandhi, Solid-liquid flow slurry pipeline transportation, Ser Bulk Mater Handl 1:56-58 (1977). [56] A. Kotia, A. Haldar, R. Kumar, P. Deval, S.K. Ghosh, Effect of copper oxide nanoparticles on thermophysical properties of hydraulic oil-based nanolubricants, J. Braz. Soc. Mec. Sci. Eng. 39 (2017) 259-266. [57] D. Cabaleiro, M.J. Pastoriza-Gallego, C. Gracia-Fernández, M.M. Piñeiro, L. Lugo, Rheological and volumetric properties of TiO2-ethylene glycol nanofluids, Nanoscale Res. Lett. 8 (2013) 286-286. [58] P.C. Mishra, S. Mukherjee, S.K. Nayak, A. Panda, A brief review on viscosity of nanofluids, Int. Nano Lett. 4 (2014) 109-120. [59] A. Barati-Harooni, A. Najafi-Marghmaleki, A. Mohebbi, A.H. Mohammadi, On the estimation of viscosities of Newtonian nanofluids, J. Mol. Liquids 241 (2017) 1079-1090. 24 [60] G.K. Batchelor, The effect of Brownian motion on the bulk stress in a suspension of spherical particles, J. Fluid Mech. 83 (1977) 97-117. [61] H.C. Brinkman, The Viscosity of Concentrated Suspensions and Solutions, J. Chem. Phys. 20 (1952) 571-571. [62] A. Einstein, Eine neue bestimmung der moleküldimensionen, Annalen der Physik 324.2 (1906) 289-306. [63] M.K. Meybodi, A. Daryasafar, M.M. Koochi, J. Moghadasi, R.B. Meybodi, A.K. Ghahfarokhi, A novel correlation approach for viscosity prediction of water based nanofluids of Al2O3, TiO2, SiO2 and CuO, J. Taiwan Inst. Chem.Eng. 58 (2016) 19-27. [64] H. Chen, Y. Ding, C. Tan, Rheological behaviour of nanofluids, New J. Phys. 9 (2007) 367. [65] N. Saitô, Concentration Dependence of the Viscosity of High Polymer Solutions. I, J. Phys. Soc. Japan 5 (1950) 4-8. [66] X. Wang, X. Xu, S.U. S. Choi, Thermal Conductivity of Nanoparticle - Fluid Mixture, J. Thermophys. Heat Trans. 13 (1999) 474-480. 25 Table 1. Experimental densities,  /gcm-3, determined with both Stabinger and DSA 5000 densimeters for the synthetic oils used as base fluids at different temperatures and 0.1 MPa. Stabinger DSA T (K) PAG2 TTM BIOE PAG2 TTM BIOE 278.15 1.0011 0.9642 0.9522 283.15 0.9976 0.9609 0.9490 0.99847 0.96206 0.95029 288.15 0.9938 0.9576 0.9457 0.99455 0.95850 0.94682 293.15 0.9900 0.9542 0.9425 0.99069 0.95483 0.94330 298.15 0.9862 0.9509 0.9392 0.98683 0.95128 0.93975 303.15 0.9824 0.9475 0.9360 0.98297 0.94788 0.93644 308.15 0.9787 0.9442 0.9327 0.97911 0.94449 0.93314 313.15 0.9749 0.9408 0.9295 0.97525 0.94110 0.92984 318.15 0.9711 0.9375 0.9262 0.97141 0.93769 0.92655 323.15 0.9673 0.9342 0.9230 0.96759 0.93436 0.92326 328.15 0.9635 0.9309 0.9198 0.96377 0.93104 0.91997 333.15 0.9598 0.9276 0.9166 0.95997 0.92771 0.91674 338.15 0.9560 0.9242 0.9134 0.95618 0.92438 0.91352 343.15 0.9521 0.9209 0.9101 348.15 0.9483 0.9176 0.9069 353.15 0.9445 0.9142 0.9037 358.15 0.9407 0.9109 0.9005 363.15 0.9369 0.9076 0.8972 368.15 0.9332 0.9042 0.8940 373.15 0.9294 0.9009 0.8908 32 Table 7. Experimental viscosities,  /mPas, determined with Stabinger rotational viscometer for nanolubricants at different temperatures and 0.1 MPa. T/K  /mPas T/K  /mPas T/K  /mPas 99 wt % PAG2 + 1 wt % ZrO2 278.15 358.9 313.15 59.77 348.15 20.76 283.15 258.3 318.15 49.76 353.15 18.46 288.15 191.8 323.15 41.96 358.15 16.51 293.15 146.1 328.15 35.79 363.15 14.85 298.15 113.6 333.15 30.83 368.15 13.43 303.15 90.17 338.15 26.81 373.15 12.20 308.15 72.83 343.15 23.50 99 wt % TTM + 1 wt % ZrO2 278.15 6931 313.15 312.8 348.15 47.61 283.15 3997 318.15 225.0 353.15 38.84 288.15 2417 323.15 165.6 358.15 32.08 293.15 1515 328.15 124.6 363.15 26.79 298.15 977.8 333.15 95.60 368.15 22.62 303.15 650.8 338.15 74.60 373.15 19.27 308.15 445.3 343.15 59.16 99 wt % BIOE + 1 wt % ZrO2 278.15 5803 313.15 478.98 348.15 97.54 283.15 3747 318.15 365.40 353.15 81.56 288.15 2504 323.15 283.33 358.15 68.85 293.15 1719 328.15 223.03 363.15 58.63 298.15 1209 333.15 178.05 368.15 50.35 303.15 869.77 338.15 144.00 373.15 43.56 308.15 639.22 343.15 117.86 98 wt % PAG2 + 2 wt % ZrO2 278.15 365.6 313.15 60.70 348.15 21.17 283.15 263.0 318.15 50.59 353.15 18.81 288.15 195.1 323.15 42.69 358.15 16.83 293.15 148.4 328.15 36.43 363.15 15.15 298.15 115.3 333.15 31.40 368.15 13.69 303.15 91.46 338.15 27.31 373.15 12.44 308.15 73.89 343.15 23.95 98 wt % TTM+ 2 wt % ZrO2 278.15 7151 313.15 320.1 348.15 48.56 283.15 4109 318.15 230.0 353.15 39.61 288.15 2482 323.15 169.1 358.15 32.70 293.15 1554 328.15 127.1 363.15 27.31 298.15 1003 333.15 97.50 368.15 23.05 303.15 667.6 338.15 76.10 373.15 19.65 308.15 456.4 343.15 60.36 33 Table 7. Continued. T/K  /mPas T/K  /mPas T/K  /mPas 98 wt % BIOE + 2 wt % ZrO2 283.15 5850 313.15 481.5 348.15 98.17 288.15 - 318.15 367.3 353.15 82.09 293.15 2512 323.15 284.9 358.15 69.30 298.15 1725 328.15 224.4 363.15 59.03 303.15 1214 333.15 179.2 368.15 50.71 308.15 873.7 338.15 144.9 373.15 43.86 278.15 642.4 343.15 118.6 34 Table 8. Parameters of Eq. (2) and average absolute deviation (AAD%) between experimental and correlated values for each base oil and nanolubricant. Sample A / mPa·s-1 B / K C / K AAD% PAG2 base oil 0.32487 710.02 176.50 0.2 99 wt % PAG2 + 1 wt % ZrO2 0.33250 708.60 176.70 0.1 98 wt % PAG2 + 2 wt % ZrO2 0.35400 696.40 177.80 0.2 TTM base oil 0.00876 1603.9 159.64 3.4 99 wt % TTM + 1 wt % ZrO2 0.00836 1625.0 158.91 3.7 98 wt % TTM + 2 wt % ZrO2 0.01131 1549.1 162.18 2.6 BIOE base oil 0.06050 1440.7 152.07 0.9 99 wt % BIOE + 1 wt % ZrO2 0.05600 1469.2 150.92 1.0 98 wt % BIOE + 2 wt % ZrO2 0.06520 1425.2 153.18 0.7 35 Figure 1. Chemical structures of the synthetic base oils: a) PAG2 (dimethoxy-endcapped poly(propylene glycol)), b) TTM (isotridecyl trimellitate ester) and c) BIOE (biodegradable polymeric ester). R represents alkyl groups. b) c) a) 36 Figure 2. X-ray patterns of ZrO2 nanopowder. 37 Figure 3. SEM micrograph and EDX microanalysis of spherically shaped ZrO2 nanoparticles. 38 Figure 4. TEM image of ZrO2 nanoparticles dispersed in butanol. 39 Figure 5. FTIR spectra of base oils (⎯⎯) and nanolubricants with 2 wt% of ZrO2 nanoparticles ( ). a) PAG2, b) TTM and c) BIOE. 40 Figure 6. Photographs captured to observe the sedimentation of ZrO2 nanoadditive at 2%wt mass concentration in PAG2, TTM and BIOE. 41 Figure 7. Average particle size diameter, Dm, obtained by DLS for dispersions at 2 wt% concentration of ZrO2 nanoparticles in: ( ) PAG2; ( ) TTM and ( ) BIOE base oils. 48 Figure 14. Dynamic viscosity (logarithmic scale) of the three base oils as a function of the temperature: ( ) PAG2; ( ) TTM and ( ) BIOE. 10 100 1000 10000 278 283 288 293 298 303 308 313 318 323 328 333 338 343 /mPa·s T /K 49 Figure 15. Relative variation of the viscosity of the nanolubricants of 2 wt% ZrO2 concentration (nl) with respect to the base oil (bf): (◼) PAG2; (◼) TTM and ( ) BIOE at 278.15 K, 323.15 K and 373.15K. 0 2 4 6 8 10 278.15 323.15 373.15 100 (nl−bf)/bf T/ K 50 Figure 16. Relative deviations between experimental density data (ρexp) and predicted values (ρpre): (; ; ) Pak and Cho [54] equation at 1 wt% ZrO2; (⚫; ◼; ) Pak and Cho [54] equation at 2 wt% ZrO2, ( ; ; ) Wasp et al. [55] equation at 1 wt% ZrO2 and , ( ; ; ) Wasp et al. [55] equation at 2 wt% ZrO2. 51 Figure 17. Absolute average relative deviations between experimental viscosity data (exp) and predicted values (pre) by using equations 5 to 10: ( ) 1 wt% ZrO2 and ( ) 2 wt% ZrO2.