A Review of Recent Developments of Friction Modifiers for Liquid Lubricantspdf

• Periodical Listing
• Nanomaterials (Basel)
• v.x(4); 2020 Apr
• PMC7221784

Nanomaterials (Basel). 2020 Apr; 10(four): 683.

Tribological Behavior of Nanolubricants Based on Coated Magnetic Nanoparticles and Trimethylolpropane Trioleate Base Oil

José M. Liñeira del Río

¹Laboratory of Thermophysical Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics, Universidade of Santiago de Compostela, 15782 Santiago de Compostela, Kingdom of spain; se.csu@arienil.leunamesoj (J.M.L.d.R.); se.csu@zepol.ateuqirne (E.R.L.)

Enriqueta R. López

¹Laboratory of Thermophysical Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics, Universidade of Santiago de Compostela, 15782 Santiago de Compostela, Spain; se.csu@arienil.leunamesoj (J.M.50.d.R.); se.csu@zepol.ateuqirne (E.R.L.)

David Eastward. P. Gonçalves

³Plant for Scientific discipline and Innovation in Mechanical Applied science and Industrial Engineering (INEGI), Universidade do Porto, Dr. Roberto Frias St., 4200-465 Porto, Portugal; tp.pu.igeni@sevlacnoged

Jorge H. O. Seabra

⁴Kinesthesia of Engineering of the Academy of Porto (FEUP), Dr. Roberto Frias St., 4200-465 Porto, Portugal; tp.pu.igeni@arbaesj

Josefa Fernández

¹Laboratory of Thermophysical Backdrop, Nafomat Group, Department of Applied Physics, Kinesthesia of Physics, Universidade of Santiago de Compostela, 15782 Santiago de Compostela, Spain; se.csu@arienil.leunamesoj (J.M.L.d.R.); se.csu@zepol.ateuqirne (E.R.50.)

Received 2020 Feb 27; Accepted 2020 Mar 27.

Abstruse

The primary task of this work is to study the tribological functioning of nanolubricants formed by trimethylolpropane trioleate (TMPTO) base oil with magnetic nanoparticles coated with oleic acrid: Fe₃O_four of 2 sizes six.three nm and 10 nm, and Nd alloy compound of 19 nm. Coated nanoparticles (NPs) were synthesized via chemical co-atmospheric precipitation or thermal decomposition by adsorption with oleic acid in the aforementioned step. Three nanodispersions of TMPTO of 0.015 wt% of each NP were prepared, which were stable for at least 11 months. 2 unlike types of tribological tests were carried out: pure sliding conditions and rolling conditions (v% slide to roll ratio). With the aim of analyzing the wear by ways of the clothing scar bore (WSD), the wear rail depth and the volume of the wear rails produced after the first type of the tribological tests, a 3D optical profiler was used. The best tribological performance was found for the Nd alloy chemical compound nanodispersion, with reductions of 29% and 67% in friction and WSD, respectively, in comparison with TMPTO. On the other paw, rolling conditions tests were utilized to written report friction and film thickness of nanolubricants, determining that Iron₃O₄ (6.3 nm) nanolubricant reduces friction in comparison to TMPTO.

Keywords: friction, wear, movie thickness, magnetic nanoparticles and nanolubricant

1. Introduction

In 2014, the worldwide energy consumption was around 396 EJ for dissimilar uses such as transport, residential consumption or industrial activeness [ane]. In all these sectors, mechanical systems endure energy losses, mostly due to friction and wear, and therefore their reduction is decisive. Thus, in economic terms the almanac total losses originating from tribological contacts are estimated to be 2,536,000 million euros, beingness 73% due to friction and 27% due to wear [2]. Friction and habiliment losses tin be reduced substantially by using new tribological solutions. In recent years, nanotechnology has been introduced in the development of numerous applications due to the physical and chemic properties of the nanomaterials being quite different from those of the bulk materials [three,4]. Recently several articles showed that the addition of nanoparticles (NPs) to electric current lubricants of mechanical elements can considerably reduce both friction and wear [iv,five,six,vii]. The primary advantages of using NPs as additives with respect to other materials are due the higher capabilities to reduce friction and vesture and even to repair the worn surface. This is due to the modest size of NPs that allows them to enter the contact area, resulting in a positive lubrication issue [8]. Moreover, the nanoparticles used every bit additives can be less chemically reactive than the mutual additives since the NP films are formed mechanically, so they are more than durable and less reactive than those with other additives [9]. In addition, NPs interact less with other additives nowadays in a lubricant and since their picture formation is largely mechanical, they may form films on many unlike types of surface [nine] Another of the advantages of using NPs as lubricant additives is their low volatility which avert losses at high temperature conditions [9]. Accordingly, NPs not only provide the nanolubricants with higher anti-wear and anti-friction capabilities and simply also a higher power for heat dissipation. NPs as friction modifier additives tin can work by dissimilar lubrication mechanisms: rolling effect and tribofilm formation, owing to the direct effect of the nanoparticle on the surface, and mending and polishing effects, due to surface enhancement [ten].

Stribeck curves are used to explain how nanoparticles play a role in friction over elasto-hydrodynamic lubrication (EHL), mixed lubrication (ML) and purlieus lubrication (BL) regimes. Ordinarily, purlieus lubrication occurs under low-speed and high-load conditions, so nanolubricant additives are essential in this lubrication regime due to the high friction coefficient in these conditions [11,12]. The improvement produced past the nanoparticles is due to their loftier affinity for the metal surfaces, so they adhere to them preventing the metal–metallic contact. Zin et al. [xiii] studied the tribological properties through Stribeck curves of nanolubricants formed by an engine oil and carbon nano-horns as additives. These authors conclude that nanolubricants containing carbon nano-horns exhibit meliorate anti-friction properties in all the unlike lubrication regimes (purlieus, mixed and elasto-hydrodynamic lubrication). Furthermore, Ghaednia et al. [14] analyzed the tribological beliefs of nanolubricants besides through Stribeck curves. These lubricants consist of CuO nanoparticles in mineral base oil using sodium oleate as a surfactant. These authors deduced that CuO additives decrease the friction coefficient deeply into the boundary lubrication authorities in comparison to the mineral base oil.

Despite the numerous studies and advances that have been made with nanoparticles in the field of lubricants, there is even so a serious problem with the stability of the nanodispersions against sedimentation, since nanoparticles tend to agglomerate with each other [fifteen]. It is well known that dispersants have been used in lodge to increase the stability time of nanoparticles [16,17,18,19]. Better results can exist obtained modifying the nanoparticles' surface past reacting with a surfactant. The resulting nanoparticles are known equally coated or functionalized nanoparticles. Chen et al. [fifteen] recently reviewed the time stability of numerous nanolubricants. For this task, these authors analyzed different characteristics of nanoparticles such every bit material nature, particle size, and surface modification, concluding that surface modification is essential to disperse nanoparticles into a lubricating oil. This modification tin can exist done mainly through reaction with surfactants [20] or alkoxysilanes [21] every bit surface modifiers.

Carbon-based and metallic-based nanoparticles are two of the most utilized nanomaterials for anti-friction and anti-wearable applications [22,23,24,25]. Magnetic metal nanoparticles are broadly studied due to their numerous applications such as magnetic records, drug-commitment agents and anti-clothing additives of lubricants, among others [26]. In the case of lubricants, at that place are several manufactures about iron oxide nanoparticles with skillful anti-friction and anti-clothing properties: Hu et al. [16] analyzed the tribological properties of non-coated ferric oxide (Fe_iiO_iii, particle size around 20–50 nm), used every bit an additive in a lubricating mineral oil (SN500) using sorbitol monostearate as dispersing agent. These authors observed that the anti-article of clothing properties of the base of operations oil were improved slightly (11.six%) by the addition of Fe₂O_three. Zhou et al. [26] analyzed the tribological beliefs of coated Fe₃O_four magnetic nanoparticles (particle size around 10 nm) with oleic acid dispersed in a liquid alkane, showing that the coefficient of friction and the article of clothing tin can exist effectively reduced upwards to 25% and 65%, respectively, with the add-on of the nanoparticles. Recently, Zhang et al. [27] studied the tribological performance of PAO 6 (a polyalphaolefin, whose kinematic viscosity at T = 373.15 K is around 6 mm² southward^-1) with graphene oxide/Fe₃O₄ nanocomposites as additives obtaining reductions of seven% and 52% in friction and wearable, respectively, in comparison to PAO 6.

The aim of this work is to clarify the tribological behavior in unlike lubrication regimes of nanolubricants composed past unlike types and sizes of functionalized superparamagnetic nanoparticles with an oleic acid coating. For this purpose, Fe₃O₄ with sizes of 6.3 and ten nm and Nd blend compound with average sizes of xix nm and trimethylolpropane trioleate (TMPTO) were called. This biodegradable base oil is a not-combustible hydraulic fluid, which is known as a proficient boundary lubricant [28] with a high viscosity index [29]. In addition, there is expected to be a good affinity between the trioleate group of the oil and the oleic coating of the nanoparticles. In that location are no previous studies about the anti-friction and anti-article of clothing properties of the Nd alloy chemical compound nanoparticles equally a lubricant additive. For this work, brawl-on-three-plates tests were performed at 20 °C to decide the friction coefficient at boundary conditions also as to quantify the vesture past means of a 3D profiler. Moreover, the habiliment track surface was analyzed through confocal Raman microscopy in society to know the role that nanoparticles play in the reduction of the habiliment. For mixed and elastohydrodynamic lubrication regimes, Stribeck curves were measured in a ball-on-disk tribometer under rolling conditions at operating temperatures of 30, 50 and 80 °C. These conditions were selected because rolling elements unremarkably work at temperatures around eighty °C while the behavior at low temperatures can occur if the equipment needs to brand a cold kickoff. Additionally, motion picture thickness measurements of prepared nanolubricants were performed at different temperatures in order to analyze their lubrication capacity. There are no previous research works on Stribeck curves or film thickness measurements with ferrites equally additives of lubricants. Rolling bearing tests were also performed for the coated Fe_iiiO₄ (6.3 nm) nanolubricant.

two. Materials and Methods

two.one. Base of operations Oil

The TMPTO base of operations oil sample was provided past Croda (Snaith, United Kingdom). A high-performance liquid chromatograph (HPLC) coupled to a quadrupole orthogonal acceleration fourth dimension-of-flight mass spectrometer micrOTOFQ™ (Bruker Daltonics Inc., Billerica, MA, USA) equipped with an electrospray ionization source (ESI) was used to characterize the sample. The procedure used is the aforementioned equally that previously reported for a different lot [30]. The analysis shows that the base oil used in the present work is composed by 69.ii% of trimethylolpropane trioleate (Figure S1), 26.5% of a very similar compound with a C–C bond more than than TMPTO (i.e., two H atoms less) and 4.3% of another compound with two extra C–C bonds (i.e. four H atoms less).

2.two. Synthesis and Label of Nanoparticles

2.2.one. Chemical and Materials

All chemicals used were of belittling course and without purification. Iron (Three) acetylacetonate (Iron(acac)₃), ane,2-hexadecanediol, oleic acrid, phenyl ether, cyclohexane, tri-n-octylamine (TOA), ferrous sulfate (FeSO_four·7H₂O), hydrochloric acid (HCl) and ammonium hydroxide (NH₄OH) were purchased from Sigma (Saint Louis, MO, United states of america). Oleylamine was obtained from Acros Organics (Geel, Kingdom of belgium). Ethanol was purchased from Panreac (Madrid, Spain) and ferric chloride (FeCl₃·6H_iiO) was obtained from Alfa Aesar (Madrid, Espana).

2.2.ii. Synthesis

Ii different sets of Fe₃O_iv@Oleic_acid NPs were prepared co-ordinate to a modification of Sun'southward method [31] in order to produce samples with tailored size, with average diameters around half-dozen nm and ten nm. The oleic acid was used as a surfactant to stabilize the NPs.

To produce Fe₃O_four@Oleic_acid NPs with an boilerplate bore effectually half dozen nm (S1), 10 mmol of Atomic number 26(acac)_three, 50 mmol of ane,2-hexadecanediol, 30 mmol of oleic acrid, 20 mmol of oleylamine, and 100 mL of phenyl ether, were mixed and magnetically stirred under a flow of nitrogen. The mixture was heated to 250 °C for 4 h earlier being cooled down to room temperature. Afterward the addition of ethanol to induce the atmospheric precipitation of Atomic number 26_iiiO₄ nanoparticles, the individually formed dispersed nanoparticles were magnetically separated and washed several times with ethanol (four×) and cyclohexane (iii×). Finally, NPs were resuspended in cyclohexane.

In the example of the coated Fe_threeO_iv@Oleic_acid NPs with average bore of 10 nm (S2), a coprecipitation procedure following Massart´due south method [32] was employed with some modifications. Fe₃O₄ NPs of 10 nm, the coprecipitation method was used, according to the Massart´s method with some modifications. A solution containing 45 mmol of FeCl₃·6H₂O and 30 mmol of FeSO₄·7H₂O and 100 mL of HCl (0.01 M) were mechanically stirred (250 rpm) and heated to 60 °C. And so, 750 mmol of NH₄OH and 8 mmol of oleic acid were added to obtain the hydrophobic character of the magnetic nano-particles and the reaction was stirred for 1 h. After, the mixture was transferred to a chalice and placed on a hot plate at 100 °C to allow for precipitation. The magnetic nanoparticles obtained were washed with Milli-Q h2o (3×) and were resuspended in cyclohexane.

The Nd alloy compound was prepared past thermal decomposition. Typically, half-dozen mol of Nd oleate were added to a flask containing oleic acid and TOA. After being filled with nitrogen temper, the flask was then heated at 330 °C for 72 h. Nanoparticles were precipitated magnetically separated and washed with ethanol (4×) and cyclohexane (iv×). Finally, the NPs were resuspended in cyclohexane.

All prepared nanoparticles were fully characterized with different techniques. The morphology and size of coated Fe₃O₄ and Nd alloy compound (Figure 1) were determined by manual electron microscopy (TEM), using a JEOL JEM-1011 microscope operating at 100 kV (JEOL, Tokyo, Nippon). The micrographs of Fe₃O₄ and Nd blend compound NPs are presented in Effigy 1a–c, together with their corresponding histograms from Effigy 1e to Effigy aneg. Information technology tin can be clearly observed that Fe₃O₄@Oleic_acid NPs have a spherical cadre, are individually dispersed, and show a moderate size distribution with averaged sizes around 6.iii nm, in one instance, and 10 nm in the other case. Nd alloy chemical compound nanoparticles have a nigh cubic shape with narrow size distribution and an boilerplate size around xix nm.

(a–c) Transmission electron microscopy (TEM) micrographs and (due east–1000) size distributions of the Atomic number 26₃O_iv@Oleic acrid with average sizes of 6.3 nm (a and e); x nm (b and f) and Nd alloy chemical compound with size of 19 nm (c and 1000) nanoparticles. The size distribution was performed using Image J software.

The Fourier transform infrared spectroscopy (FT-IR) technique was used to characterize the nanoparticles using a Thermo Nicolet Nexus spectrometer (Thermo Fisher Scientific, Madrid, Spain) and the attenuated total reflectance (ATR) method in the range 400–4000 cm^−ane. Every bit tin be seen in Figure two, the FT-IR spectra of coated Iron₃O₄ nanoparticles present the feature bands of oleic acid groups and Fe₃O_iv. For both samples, an assimilation peak appears around 572 cm^−ane which corresponds to the Atomic number 26-O vibration of the magnetic Fe_threeO_iv phase [33]. Ii assimilation peaks can be observed at 2925 and 2848 cm^−one that tin can be attributed to the asymmetric and symmetric CH₂ stretching in oleic acrid, respectively [33,34]. Besides, the CH_three umbrella manner appears around 1406 cm⁻¹ [34]. In addition, two peaks appear around 1598 and 1521 cm^−ane which correspond to the asymmetric and symmetric stretching vibrations of COO− groups of the oleic acid [34]. Furthermore, the wavenumber separation (∆) between the V _as(COO−) and V _s(COO−) bands can exist used to study the type of the interaction (monodentate, bridging bidentate, chelating bidentate or ionic interaction) between the carboxylate head and the metallic atom [35]. In this example, the ∆ of 77 cm⁻¹ is ascribed to a chelating bidentate interaction between the COO⁻ group and the Fe or Nd atoms. This result is in concordance with previous works, in which a strong chemic bail betwixt carboxylic acid and the amorphous atomic number 26 oxide or neodymium alloy nanoparticles' surface has been shown [36,37].

Fourier transform infrared spectroscopy (FT-IR) spectra of coated Fe₃O₄ nanoparticles: (S1) Fe₃O_iv (half dozen.3 nm), (S2) Fe₃O₄ (ten nm) and (S3) Nd alloy compound (nineteen nm).

Also, thermogravimetric analysis (TGA) was used to judge the oleic acid content in the synthesized nanoparticles equally well equally the number of oleic acid molecules per surface area of each nanoparticle (Tabular array 1). For this aim, a Perkin Elmer Pirys vii TGA (Perkin, Waltham, MA, United states of america) was employed heating from l to 850 °C at x °C/min under a nitrogen menstruum of 20 mL/min. Effigy three shows a schematic representation of the oleic acrid-particle construction.

Representation of the oleic acid-particle structure.

Table 1

Oleic acid content in each synthesized nanoparticle.

Sample	Oleic Acid Content (wt%)	Number of Oleic Acid Molecules per NP Surface Area in nmtwo
Fe3O4@OA (half-dozen.3 nm)	29.10	five
Fe3O4@OA (ten nm)	xix.30	half dozen
Nd alloy	fourteen.45	1

Additionally, crystallinity and the phase structures of coated Fe₃O₄ nanoparticles were too characterized past X-ray powder diffraction (XRD) using a Philips PW1710 diffractometer (Cu Kα radiation source, λ = 1.54186 Å) with a stride size of 0.02° and counting time of two s per step from 10 to 80° (2θ). In Figure 4, the diffractograms of both samples S1 and S2, show a set of peaks corresponding to those of the crystalline Iron_threeO_four phase (Joint Committee on Powder Diffraction Standards, JCPDS 79-0417) [36]. Moreover, the widening of the peaks, indicate the ultra-fine nature and minor crystallite size of the particles [37]. An additional broad band appearing at depression angles 2θ< 20° is related to the presence of oleic acid. The Nd alloy compound is formed past a mixture of NdFe₁₂B_half-dozen and Fe₃O₄.

Ten-ray powder diffractogram (XRD) for the coated F_threeO₄ nanoparticles: (S1) 6.iii nm and (S2) ten nm.

Magnetization of the coated NPs was also measured. For this chore, stale samples were magnetically characterized using a vibrating sample magnetometer (VSM) (DMS/ADE Technologies, Massachusetts, Usa) at room temperature with practical magnetic fields between −10 and ten kOe. In Figure v the typical hysteresis loops of the coated nanoparticles, normalized to their magnetite mass content, can be observed. It was ended that these species were superparamagnetic owing to their almost zero coercivity and zero remanence in the magnetization curve. Saturation magnetization (M_S) of these Atomic number 26₃O₄ coated NPs (S1 and S2) is lower than bulk magnetite saturation (~92 emu/g). This fact is in accordance to that expected for the reduction of magnetization caused by a dead magnetic layer in small magnetic nanoparticles and the non-magnetic contribution of the oleic acid blanket beat out. The magnetization of Nd alloy NPs (S3) shows a nearly superparamagnetic behavior and a saturation magnetization around 20 emu/g.

Magnetization curves at room temperature for the coated F₃O₄ nanoparticles: (S1) half dozen.3 nm and (S2) 10 nm and (S3) Nd alloy.

2.iii. Preparation of the Nanolubricants

In order to obtain homogeneous nanolubricants, the nanoparticles suspended in cyclohexane afterward the synthesis were added to the oil through the elimination of the solvent past humid with a rotary evaporator. For this aim, the start pace was to notice a solvent which can be easily and completely separated from the oil. Dissimilar organic solvents with lower boiling betoken than that of the TMPTO base oil, such as cyclohexane, ethanol, ether or chloroform, were tested. For this purpose, TMPTO was mixed with each one of these solvents and so was separated with the rotary evaporator at different temperature conditions. In order to know whether the solvent was completely removed, the viscosity of the TMPTO obtained was measured with a rotational viscometer Stabinger SVM3000 (Anton Paar, Graz, Austria) and compared with that of the neat TMPTO. Chloroform was the solvent for which the viscosity of the base of operations oil remained unchanged. Hence, it has been concluded that the all-time solvent to comprise the nanoparticles in TMPTO is chloroform. Therefore, the nanoparticles that after synthesis were dispersed in cyclohexane were transferred to chloroform. Subsequently, concentration is determined past thermogravimetry and the chloroform dispersion was added to the TMPTO base oil and mixed by ultrasonic agitation for fifteen min. Then, the chloroform was eliminated from the oil using a rotary evaporator. Using this procedure, three nanodispersions of TMPTO with 0.015 wt% of each NP were obtained. Once all the solvent has been removed, the dispersion of oil and nanoparticles are sonicated with a homogenizer Fisherbrand ultrasonic bath, in a continuous shaking mode for 240 min with an effective ability of 180 Due west and a fixed sonication frequency of 37 kHz to obtain the nanolubricants. Density and viscosity of the nanolubricants and TMPTO base of operations oil measured with the Stabinger SVM3000 deviceare shown in Table 2 at reference temperatures and Tables S1 and S2 from 278.15 K to 373.15 One thousand. For this device, the expanded uncertainties (k = 2) are 0.02 K for the temperature from 288.xv to 378.xv Yard and 0.05 K outside this range, 0.0005 chiliad·cm⁻³ for density and ane% for dynamic viscosity.

Tabular array 2

Main physical properties of trimethylolpropane trioleate (TMPTO) base oil and the iii nanodispersions of TMPTO with 0.015 wt% of each coated nanoparticle (NP).

Physical Property	TMPTO Base Oil	0.015 wt% (FeiiiO4-6.3 nm)	0.015 wt% (Atomic number 26threeO4-x nm)	0.015 wt% (Nd blend-nineteen nm)
Density at 293.15 K/chiliad cm−3	0.9161	0.9172	0.9167	0.9177
Viscosity at 313.fifteen K/mPa s	45.32	46.04	45.81	46.22

2.4. Tribological Behavior: Ball-on-Three-Pins Exam

Tribological tests were carried out under pure sliding weather in a modular compact rheometer MCR 302 from Anton-Paar equipped with a tribology cell T-PTD 200, in combination with a Peltier hood H-PTD 200 for a precise temperature control. This rheometer has an outstanding speed and torque control, an accurate normal force detector besides as a fast temperature command in a broad range. All these characteristics are utilized for the tribology tests. In this work, the test configuration used was ball-on-3-pins. The ball is fixed on a shaft and driven by the MCR rheometer motor, then rotating on the 3 pins under the stock-still normal forcefulness. The resulting torque is associated with the friction forcefulness by using simple geometric calculations. The axial strength of the rheometer is transferred into a normal forcefulness acting perpendicular to the bottom pins at the contact points. Nil distance is adamant by a 'zero-gap measurement', in which the measuring organization (ball) is lowered past the rheometer until softly touching the sample surface (i.east., a normal force will be detected). The height at which the normal forcefulness is sensed corresponds to zero height. The distance resolution is better than 0.01%. More details can be found in the literature [38]. The diameter of the ball is 12.7 mm, the cylindrical-shaped pins have vi mm in both diameter and height. Balls and pins were cleaned with a stream of hexane and dried with air before the tests. Samples were fully submerged by adding 1.3 mL of lubricant. The ball rotates on the 3 pins with a total normal force of 45 Due north, resulting in a force of 21 N acting normally to each pin surface during the tests, which corresponds to a maximum contact pressure level of about 1 GPa and a mean contact pressure around 0.vii GPa [39]. Tests were performed at a constant rotational speed 213 rpm (corresponding to a linear speed 0.1 m/s) and a sliding distance of 340 thou at xx °C. Iii replicates were run for each concentration of lubricant.

In society to analyze the worn surface after the brawl on three-pins tests, a 3D Optical Profiler Sensofar Due south Neox was used to quantify the wear of pins in different terms such as habiliment scar diameter (WSD), wear track depth (WTD) and the volume of the wear hole. In order to obtain representative average values, these parameters have been measured in three dissimilar zones using a confocal mode with a 10× objective. Furthermore, a WITec alpha300R+ confocal Raman microscopy was used to obtain information about the composition in the pins worn track.

Roughness (Ra) of worn surfaces of pins lubricated with the studied nanolubricants and the base of operations oil were also analyzed to characterize the anti-clothing capability of the nanolubricants with this device. For this purpose, the ISO4287 standard (International Organization for Standardization, Vernier, Switzerland) was used employing a Gaussian filter with a long wavelength cut-off of 0.08 mm.

2.5. Film Thickness Measurements

A brawl-on-disc tribometer, EHD2 model from PCS Instruments, equipped with optical interferometry, was employed in order to measure out the lubricant film thickness of the contact formed between a steel ball (19.05 mm in bore) and a rotating drinking glass disc (coated with a 20 nm chromium and 500 nm silica layer). The disc and ball are driven by two electric motors for carrying out tests under rolling/sliding conditions, the load-applying system beingness based on moving the ball against the disc. Thus, the normal load is measured with a load cell, positioned below the ball wagon, measuring the normal load practical on the ball against the disc. The friction force is as well measured on the ball, through a torque jail cell mounted on the ball shaft with the disc rotating faster than the ball, and subsequently, at the same entrainment/rolling speed, the friction force is measured over again with the ball rotating faster than the disc. Therefore, the friction coefficient is then calculated from the normal force and the friction force. The central film thickness is obtained by optical interferometry through the wavelength of the light returned from the primal plateau of the contact [40]. The glass disc can be tested upwards to approximately 0.7 GPa of maximum Hertz pressure level and the ball specimen presents a high-form surface finish. The ball and disc characteristics (provided past the manufacturer) are presented in Table 3.

Tabular array three

Main characteristics of the ball and discs.

Parameters	Ball	Glass Disc	Steel Disc
			Polished	Rough
Elastic modulus/GPa	210	75	210
Poisson ratio	0.29	0.20	0.29
Radius/mm	xix.05	50	50	50
Surface roughness, Ra/nm	20	5	50	500

The zero distance is evaluated before each moving-picture show thickness examination. The ball is loaded against the disc (same load equally the test load 50 N), without lubricant between the disc and the brawl. The infinite layer coating thickness is and then evaluated through interferometry. The corresponding wavelength is recorded and set equally the cipher indicate. Measured wavelengths above that value ways that the film thickness is higher than zero, and below that value mean the infinite layer is worn out.

Film thickness measurements were carried out under fully flooded lubrication (130 mL of lubricant oil) for the base oil and the magnetic nanolubricants at iii operating temperatures (30, 50 and 80 °C) using a load of 50 N which corresponds to a maximum Hertz pressure of 0.66 GPa and for 5% slide-to-curlicue ratio (SRR) divers equally:

SRR ( % ) = 2 × ( U disc − U brawl ) ( U disc + U ball ) × 100 ,

(1)

being U_disc and U_brawl the speed of disc and brawl on the contacting surfaces respectively.

The entrainment speed (U_{due south} ) is defined by the following equation:

For each temperature the same entrainment speed ramp was used: 0.01 m/s to 2 thousand/s. These conditions allow a very thin lubricant motion picture thickness to be worked with. The lowest brawl speed is 0.097 thousand/s and the highest ball speed is 1.950 m/s for the three studied temperatures. On the other mitt, the lowest disc speed is 0.102 yard/s and the highest disc speed is ii.049 m/due south. The tribometer adjusts disc and brawl speeds automatically, existence the disc speed faster in lodge to become positive SRR and the disk slower to obtain negative SRR, while the entrainment speed is maintained constant. The result is the average of iii measurements, two ramps increasing speed and one decreasing.

2.6. Tribological Behavior: Stribeck Curves

Friction coefficient measurements were besides performed with the EHD2 ball-on-disc described in the previous section. For these measurements, the ball runs against a steel disc under fifty North load producing contact pressures up to one.11 GPa. The used balls and discs are made of carbon steel (American Atomic number 26 and Steel Institute, AISI 52100 100Cr6) with nineteen.05 mm and 100 mm diameters, respectively. The disc properties were provided by the manufacturer and the surface roughness was measured with a Hommelwerke Profiler for both polished and rough samples (Tabular array 3), while the characteristics of the ball are the same as for the lubricant film thickness measurements. Friction coefficients of nanolubricants and base of operations oil were likewise measured at 30, 50 and 80 °C. For both discs (rough and polished) the friction properties have been studied through Stribeck curves for a five% SRR value and entrainment speeds from 0.01 to ii grand/s. For this test, the range of the brawl and disc speeds and the SSR are the aforementioned as for the picture thickness measurements. The friction coefficient values are also given by the average of measurements with 5% SSR.

2.vii. Rolling Bearing Test Rig

To carry out the rolling bearing tests a modified four-ball auto by Marques et al. [41] was employed. The machine configuration was replaced past a rolling begetting associates in order to test a different kind of rolling bearings [41]. This device consists mainly of two different parts, the upper part is directly connected to the automobile shaft and the second part is where the bearing arrangement is fitted, 50 mL of sample book assures that the lubricant level reaches the centre of the rollers. In this piece of work, SKF 51107 thrust ball bearings take been used. The friction torque and operating temperature at different points were continuously measured during the tests; the starting time i was measured with a piezoelectric torque prison cell Kistler 9339, which ensured high-accurateness measurements even at very low friction torque [41,42]. The temperature was measured by three thermocouples in real fourth dimension at different strategic locations: inside the oil sump, most the rolling bearing raceway and the lubricant, and the 3rd measured the room temperature. More details were reported by Marques et al. [41].

The car initially worked at 500 rpm for 10 min at very low load (5 kg applied in a dead weight lever system, the resulting force being approximately 1000 N) in society to ensure the warm-upward of the arrangement. Afterward, an centric load of vii kN was practical, producing a maximum contact pressure of around ii.3 GPa in each ball contact. At the same time the heater was switched on to reach the required oil bath temperature of 70 °C. Afterwards, five friction torque measurements were carried out for each of the 5 speed steps (100, 200, 500, 1000 and 1500 rpm) for each lubricant in order to obtain better repeatability. Friction torque measurements were taken after xxx min in one case the chosen speed, load and temperature had been reached.

3. Results

three.1. Stability of the Nanolubricants

In a first step, the stability of the nanolubricants against sedimentation was studied past visual ascertainment. For the three prepared nanodispersions no signs of sedimentation appeared for eleven months just after sonication as can be seen in Figure 6. In the Chen et al. [15] review almost the stability of numerous nanolubricants, nevertheless, none of them showed time stabilities every bit large as those for the nanolubricants presented in this work. Indeed, the stability times were much longer than those required to perform ball on three pins and rolling test bearing, 4 and 6 h respectively.

Visual observation of the stability of the nanolubricants TMPTO + 0.015 wt% NP: (a) just after sonication of 240 min; (b) 11 months after sonication.

Refractometry is the second method to analyze the stability against sedimentation of prepared nanodispersions. For this aim a Mettler Toledo Refractometer was used to measure the refractive index of nanodispersions over time. This procedure was previously explained [43]. Figure 7 shows an astonishing stability over time for all the nanodispersions, showing an important improvement of the stability in comparison with other nanodispersions previously studied [44,45]. Specifically, for TMPTO-based nanolubricants, the refractive index increases 0.44 and 0.08 for graphene oxide, Become and reduced graphene oxide, rGO nanoparticles after 50 h [45]. Meanwhile for this period, in this work the refractive alphabetize development shows increases of 0.02, 0.03 and 0.02 for Fe₃O_four (vi.3 nm), Fe_iiiO₄ (ten nm) and Nd alloy (19 nm) nanolubricants, respectively. These great stabilities can be attributed to the oleic acid coating of the nanoparticles likewise every bit to its affinity with the trioleate group of the TMPTO.

Temporal evolution of the refractive index, n, for TMPTO + 0.015 wt% (Iron₃O_iv (6.3 nm), Fe₃O_four (ten nm) or Nd alloy (19 nm), TMPTO + 0.05% (graphene oxide, Go or reduced graphene oxide, rGO) [45].

iii.2. Friction Behavior in Ball-on-Three-Pins

Friction coefficients (μ) obtained with the tribology cell T-PTD 200 from Anton-Paar for the three nanolubricants equanimous by coated Iron₃O₄ (6.3 nm), Iron_threeO₄ (10 nm) and Nd alloy (19 nm) with TMPTO base oil over time are presented in Figure eight. Information technology can be seen clearly that for the three nanodispersions the friction coefficients obtained are lower than those of the base of operations oil without additives, the greatest friction reduction beingness obtained for the nanolubricant formed by the Nd alloy nanoparticles in TMPTO base oil. The boilerplate friction values are given in Table 4 and plotted in Figure nine. The lowest average friction coefficient is 0.067. This upshot leads to a friction reduction of 29% due to the Nd alloy additives. In the instance of coated magnetites, reductions of 4% and eighteen% in the average coefficient of friction were obtained for Fe_threeO₄ (6.iii nm) and Fe₃O₄ (10 nm), respectively.

Evolution of friction coefficient values for TMTPO base of operations oil and nanolubricants of TMPTO and 0.015 wt% of each NP with the sliding altitude at 20 °C.

Friction coefficient, μ, (■) and WSD, (■) obtained with the neat oil TMPTO and with the iii nanolubricants with 0.015 wt% of each NP. Error bars betoken the standard deviation of the hateful friction coefficient or of the WSD.

Tabular array 4

Hateful values of friction coefficient (µ) and of the bore (habiliment scar diameter, WSD), depth (wear track depth, WTD) and volume of the vesture hole and their respective standard deviations (σ) for all nanolubricants for the TMPTO base oil and the iii nanolubricants with 0.015 wt% of each NP.

Lubricant	µ	σ	WSD/μm	σ/μm	WTD/μm	σ/μm	Vol/10iiiμm3	σ/103μm3
Neat TMPTO	0.094	0.006	358	15	1.57	0.xi	58.1	three.4
Atomic number 263O4 (6.iii nm)	0.090	0.005	226	17	1.52	0.21	23.seven	1.1
Iron3O4 (ten nm)	0.078	0.005	145	17	1.17	0.thirteen	thirteen.7	0.viii
Nd alloy (19 nm)	0.067	0.006	119	xiii	ane.02	0.eleven	0.56	0.5

three.3. Surface Assay of Worn Pins

Profiles of the wear tracks produced in the pins due to the ball-on-three-pins tests for the studied nanolubricants and TMPTO oil are shown in Figure 10. The wear in the pins was evaluated through the diameter, depth and volume below the unworn surface of the track (Table 4). Figure 11 shows cross-section profiles of the clothing tracks in the pins lubricated with TMPTO base oil and with the nanolubricants. It can be observed that, using the nanolubricants, the obtained vesture is lower than for the TMPTO base oil, in terms of bore, depth and book of habiliment scar. For diameter and depth of the wear track, the maximum reductions were obtained with the Nd blend nanolubricant, beingness 67% and 35% respectively. For the magnetite nanolubricants, improved results were also observed in comparing with the base oil. Thus, for Fe₃O₄ (6.iii nm) reductions of 37% and 3% were observed for diameter and depth of the wear scar respectively whereas for Fe₃O_four (ten nm) reductions of 59% and 25% were obtained respectively. These results show a close relationship between friction and article of clothing behaviors, as information technology can exist seen in Figure 9.

Three-dimensional (3D) profile (ten×) and second images (10×) of the article of clothing tracks of pins for: (a) TMPTO, (b) TMPTO + 0.015 wt% Fe_iiiO_four (6.3 nm), (c) TMPTO + 0.015 wt% Fe₃O₄ (10 nm) and (d) TMPTO + 0.015 wt% Nd blend (nineteen nm).

Cross section profiles of the wearable tracks on the pins lubricated with (a) TMPTO, (b) TMPTO + 0.015 wt% Fe_threeO₄ (half dozen.3 nm), (c) TMPTO + 0.015 wt% Fe_threeO_four (10 nm) and (d) TMPTO + 0.015 wt% Nd blend (nineteen nm).

Roughness (Ra) of worn surfaces of pins has been besides analyzed to characterize the anti-clothing capability of the nanolubricants. Tabular array 5 shows that the worn surfaces lubricated with the nanolubricants are smoother than those lubricated with TMPTO. An Ra value of 61 nm was obtained for the worn surface lubricated with TMPTO whereas for the track corresponding to the Nd alloy nanolubricant the lowest Ra was found (37 nm), which leads to a 54% reduction in roughness. As a result, information technology can be concluded that a polishing issue occurs due to the presence of nanoparticles. In the case of the Nd alloy this effect seems to exist stronger than for the other nanoparticles. As regards magnetites, the smoother surface was obtained with the bigger size of the nanoparticles (10 nm).

Table 5

Roughness parameters, Ra, and their uncertainties σ of worn surfaces lubricated with the three studied nanodispersions and neat TMPTO.

Lubricant	Ra/nm	σ/nm	Gaussian Filter/mm
TMPTO	60.ix	five.2	0.08
TMPTO + 0.015 wt% Fe3Ofour (6.3 nm)	47.9	4.1	0.08
TMPTO + 0.015 wt% Fe3O4 (10 nm)	41.8	4.two	0.08
TMPTO + 0.015 wt% Nd alloy (xix nm)	37.3	3.8	0.08

Raman spectra of the base oil (Figure S2) and the three studied magnetic nanopowders (Figures S3 and S4) as well as elemental mapping of the worn pin surfaces lubricated with the three nanolubricants (Effigy 12, Figure 13 and Figure 14) were carried out with the confocal Raman microscope at a wavelength of 532 nm to know the role that nanoparticles play in the reduction of surface wear of pins. The Raman spectrum of both magnetites (Effigy S4) exhibits feature bands at 289 cm^−ane which is due to the E_g phonon mode and at 682 cm⁻¹ that is associated to the A_1g phonon mode [46]. In improver, the peaks observed at 1580 cm^-1 and 1309 cm⁻ⁱ are consistent with the presence of the oleic acid blanket, specifically these peaks are associated to C=C alkyl stretching and C-H ethylene, respectively [47]. As regards the Nd alloy (Figure S4), there is no previous characterization nigh this compound.

Raman spectra and elemental mapping of the worn surface obtained with the nanolubricant of Fe₃O₄ (vi.three nm) nanoparticles.

Raman spectra and elemental mapping of the worn surface obtained with the nanolubricant of Fe_threeO_iv (10 nm) nanoparticles.

Raman spectra and elemental mapping of the worn surface obtained with the nanolubricant of Nd alloy (xix nm) nanoparticles.

Figure 12, Figure 13 and Figure xiv evidence the mapping of worn surfaces lubricated with the nanolubricants containing both magnetites and Nd blend. Important tribofilms are evidenced owing to a significant presence of nanoparticles (cherry color). Furthermore, the presence of TMPTO base oil (blue color) is observed. Both magnetite and Nd alloy nanoadditives are mainly placed along several furrows on the worn surface, especially in the example of the Fe_iiiO_iv (10 nm)-based nanolubricant. This fact indicates that a mending consequence takes place [43]. Moreover, the major presence of nanoparticles can exist observed in Figure 12, which corresponds with the smallest nanoparticle size (6.iii nm). This fact seems to point that these nanoparticles have a loftier affinity to the surface although the tribofilm is less effective at protecting it which is evidenced by the poorer anti-friction and anti-wear performances. Taking into account the Raman and roughness results it can exist ended that the mechanisms which explain the part of these nanoparticles equally lubricant additives are the tribofilm formation, which is a direct effect of the nanoparticle on the worn surface, likewise as mending and polishing effects which are related to surface enhancement [43].

3.4. Flick Thickness

Figure 15 shows the typical graph log(film thickness) versus log(entrainment speed). As expected, the higher is the entrainment speed, the thicker is the motion-picture show. All studied lubricants (base of operations oil and nanolubricants) testify similar film thickness values for each operating temperature (thirty °C, 50 °C and 80 °C) during the tests performed, due to their very similar viscosities (Tabular array ane and Table S2). Specifically, the maximum viscosity increase is only ii.8%, which was obtained for the Nd blend at 278.15 K in comparison to the base of operations oil. Viscosity increment is common when a base oil is condiment with nanoparticles [44]. On the other hand, the viscosity, pressure-viscosity coefficient and film thickness of the lubricants decrease when temperature rises [48]. Like movie thickness values of the nanolubricants with respect to those of the base oil is an advantage when information technology is needed to replace the oil past the nanolubricant in the mechanical elements such equally gearboxes or bearings.

Film thickness of the TMPTO base oil and prepared nanolubricants at slide-to-roll ratio (SRR) of 5%.

3.5. Friction Behaviour: Stribeck Curves

The Stribeck curves for all lubricant samples were studied at temperatures of thirty, 50 and 80 °C and SRR of 5%. The coefficient of friction is presented confronting the specific film thickness, Λ, which in this work, is given by:

where h 0 is the central film thickness and σ is the average roughness given by σ = ( σ disc ) two + ( σ ball ) 2. Other authors use the minimum flick thickness instead to calculate Λ, but for the purpose of this analysis, this would only shift the curves to the left.

Total Stribeck curves (Effigy 16) were obtained for nanolubricants and base oil: boundary to mixed (for crude disc) and full film lubrication (smooth disc). As expected, the friction tests made with rough discs produced higher friction values than those made with smooth discs. At operating temperature of 30 °C it can be observed that the addition of the studied nanoparticle hardly changed the coefficient of friction of the nanolubricants in comparison with the TMPTO base oil, and there is no difference since viscosity of all dispersions is very like. As temperature increases, at 50 and 80 °C the Fe_threeO₄ (six.iii nm) nanolubricant shows smaller friction than the base oil for both polished and rough discs, in accordance with the slightly higher film thicknesses (Figure 16). This effect is mainly interesting for oil engine applications, habitually operating at high temperatures (around 100 °C) [49]. Considering these facts, the tribological behavior of the Fe_threeO₄ (vi.iii nm) nanolubricant was as well analyzed in a real awarding of rolling bearings (Section three.6).

Stribeck curves of the base oil and dispersions with magnetic nanoparticles tested for crude and polished discs at 5% SRR.

3.6. Friction Behavior: Rolling Bearing Test Rig

Figure 17 shows the friction torque results for rolling bearings lubricated with TMPTO base oil and Fe₃O_four (six.three nm) nanolubricants at lxx °C under 7000 N. The surface roughness of the cylindrical roller thrust bearing is 0.14 μm so that, because Equation (iii), tests were performed under purlieus lubrication conditions (Λ ≈ 0.06) for both lubricants. The central pic thickness (h ₀) at this temperature has been predicted through Hamrock and Downson's equation for elliptical contacts [l]. As Figure 17 shows, friction torque values obtained for the nanolubricant with Fe_threeO₄ (6.3 nm) at 70 °C are much lower than those for TMPTO base of operations oil, especially at very low speed (200 rpm) likely due to the fact that at boundary conditions the nanoparticles play an anti-friction and anti-wear office. Equally the speed is increased (500 rpm and 1000 rpm) the behavior is nevertheless better than that for the base oil but not as much every bit for the lowest speed. Finally, at the highest speed (1500 rpm) there is a hardly divergence in the friction torque between both lubricants due to the thick lubricant flick. The friction torque reduction is about 35% for the start studied speed (200 rpm), 23% for both the speeds of 500 rpm and 1000 rpm.

Friction torque results for the rolling bearing lubricated with the TMPTO base oil and TMPTO + 0.015 wt% Fe₃O₄ (six.3 nm) nanolubricant.

4. Conclusions

In this work the following features were accomplished:

• -

  Magnetic nanoparticles: Fe₃O_four (six.3 nm), Iron_threeO₄ (10 nm) and Nd alloy (19 nm) functionalized with oleic acrid have been synthesized.

• -

  Dispersions of TMPTO base oil with 0.015 wt% of Iron₃O_iv (6.3 nm), Iron₃O₄ (x nm) and Nd alloy (19 nm) were prepared showing that through chemical modification with oleic acid, a greater stability of the nanodispersions is achieved, which tin validate their employ for many industrial applications. This is one of the highest time stabilities in the literature for nanolubricants [15].

• -

  For pure sliding/boundary tests at xx °C, for the three nanolubricants the friction coefficient is lower than that obtained with the base oil, the best friction behavior beingness obtained with the Nd blend nanolubricant with a friction reduction of 29% in comparison with the base of operations oil.

• -

  The diameters and depths of the habiliment scar obtained with the three nanolubricants are lower than those corresponding to the base of operations oil, obtaining maximum wear reductions for the Nd blend nanolubricant, beingness 67% and 35% in terms of diameter and depth of the wear scar, respectively

• -

  Protective tribofilm germination was confirmed by confocal Raman microscopy on the worn surfaces.

• -

  Film thickness values for all studied nanolubricants are very like due to their similar viscosities, so lubrication chapters will exist coordinating for all lubricants.

• -

  Nether rolling weather of 5% SRR and 30 °C, the total Stribeck curves for all lubricants are similar whereas at the higher temperatures the Atomic number 26_threeO₄ (6.iii nm) nanolubricant shows lower friction coefficient than the base oil and the other nanolubricants.

• -

  Fe₃O₄ (6.3 nm) nanolubricant leads to a lower friction torque in comparison with the base oil, particularly at low speed when the moving-picture show is thin and the nanoparticles play an important role in the reduction of friction.

Acknowledgments

Authors would like to thank Alfredo Amigo from Practical Physics Department, University of Santiago de Compostela for kindly allowing to utilise a refractometer and to RIAIDT-USC analytical facilities, especially to Ezequiel Vázquez for his useful advice. Authors would like also to thank Croda for providing the TMPTO base oil. JMLdR is grateful for a IACOBUS grant to the European Group for Territorial Cooperation Galicia-Norte de Portugal (GNP-EGTC).

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/10/4/683/s1: Figure S1: Chemic construction of trimethylolpropane trioleate, Figure S2: Raman spectrum of trimethylolpropane trioleate (TMPTO), Effigy S3: Raman spectrum of both Fe₃O₄ (6.3 nm) and Atomic number 26₃O₄ (10 nm) nanoparticles coated with oleic acid, Figure S4. Raman spectrum of Nd blend (nineteen nm) nanoparticles coated with oleic acrid. Tabular array S1: Experimental density, ρ/thousand⋅cm⁻³ of the nanodispersions and base oil as a function of temperature, Tabular array S2: Experimental dynamic viscosity, η/mPa⋅southward of the nanodispersions and base oil as a part of temperature.

Author Contributions

Conceptualization, J.F., J.R. and J.H.O.Due south., methodology, E.R.L., Y.P. and D.E.P.G.; validation, J.Chiliad.L.d.R., D.East.P.G. and S.Y.5.; formal analysis, J.M.L.d.R. and E.R.Fifty.; investigation, J.M.Fifty.d.R. and South.Y.V.; resources, J.F., J.H.O.S. and Thou.G.G.; writing—original draft training, J.One thousand.L.d.R.; writing—review and editing, D.Due east.P.K., S.Y.V., J.F. and E.R.L.; visualization, J.M.L.d.R. and J.F.; project administration, J.F., J.R. and J.H.O.S.; funding acquisition, J.F. and E.R.L. All authors take read and agreed to the published version of the manuscript.

Funding

This research was supported by the Spanish Ministry of Economy and Competitiveness and the ERDF programme through the ENE2017-86425-C2-2-R project. Moreover, this piece of work was funded by the Xunta de Galicia (ED431E 2018/08, GRC ED431C 2017/22 and GRC ED431C 2016/001). These three funders also financed the acquisition of the 3D Optical Profile Sensofar Southward Neox (UNST15-DE-3156).

Conflicts of Interest

The authors declare no disharmonize of interest. The funders had no office in the design of the study; in the drove, analyses, or estimation of data; in the writing of the manuscript; or in the determination to publish the results.

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7221784/

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