Influence of Operating Parameters on the Tribological Performance of

Article pubs.acs.org/IECR

Influence of Operating Parameters on the Tribological Performance of Oleic Acid-Functionalized Cu Nanofluids Ajay Kumar, Gananath D. Thakre,* P. K. Arya, and A. K. Jain Advanced Tribology Research Centre, CSIR-Indian Institute of Petroleum, Dehradun, 248005, India ABSTRACT: This paper reports the influence of operating parameters on the tribological performance of Cu nanofluids. The nanofluids have been prepared by blending oleic acid-functionalized Cu nanoparticles in mineral base oil and commercial multigrade lubricant. The functionalized Cu nanoparticles have been characterized by analytical techniques to ascertain chemical composition and structure of particles. The UV results indicate stable dispersion of nanofluids. The experiments have been performed to investigate the influence of load, speed, and temperature on the triboperformance behavior of nanofluids. The tribological performance reveals that the Cu nanofluids help in reducing friction and wear more significantly within the contact as compared to the base fluid under studied operating conditions.

the friction and wear between the contacting surfaces.19,20 Thus, a new domain of lubricant and lubrication has been opened for the Cu nanofluids. The Cu nanoparticles have been effectively used as additive in both the aqueous and oil-based lubricants. The negative charged functionality such as the hydroxyl group creates electrostatic repulsion that enhances the dispersion of Cu nanoparticles in water-based lubricants While in oil-based lubricants, a weak van der Waals interaction enhances the dispersion stability of nanoparticles.18 In recent times Zhang et al.19 developed the Cu water based lubricant and studied its tribo-performance. The study revealed that the addition of merely 4% of Cu nanoparticles in waterbased lubricant reduced the friction and wear by 51% and 15.7%, respectively. On similar grounds Sahu et al.21 developed the aqueous nanofluids of Cu in deionized water with and without polyethylene glycol as stabilizing agent. The study majorly focused on investigation of the thermal conductivity, thermal diffusivity, and specific heat of developed aqueous nanofluids. Zin et al.22 studied the tribological behavior of aqueous nanofluids of Cu prepared using the polyol method. The study mainly focused on developing the stribeck curves for the synthesized nanofluids. Li et al.18 studied the tribological properties of Cu nanofluids developed using mineral base fluid. The study reported that the 0.02% Cu nanofluids improved the friction and wear as compared with base fluids. A major challenge in the use of Cu nanofluids has been the dispersion stability. Usually, the nanofluids are prepared by dispersing the nanoparticles in the base fluids using mechanical stirring, an ultrasound probe, or the combination of both.

1.0. INTRODUCTION Lubricants over the ages have proved their vitality in reducing friction and wear.1 The early lubricants in the form of animal fats and oils have slowly and steadily evolved into the present day mineral-based lubricants. Chemical processes such as hydrotreating, catalytic hydrocracking, catalytic dewaxing, and modern wax hydroisomerization has advertently improved specific properties of lubricants making them suitable for more robust and stringent operating conditions.2 However, the ever increasing demand for energy efficient lubricants with increased drain intervals have entrusted the lubricant manufacturers to develop new and competitive products suitable for the modern day machinery. In this context nanofluids have emerged as a suitable option for enhanced lubrication performance. Nanofluids are prepared by blending the nanoparticles of different elements and compounds into the base fluids to impart the required property of the lubricant.3−12 The nanoparticles when suspended in lubricating oil act as ball bearings reducing the friction between the contacting surfaces.13 By and far researchers around the globe have studied a variety of nanomaterials namely CNT, diamond, MoS2, TiO2, SiO2, Au, Ag, Cu, CuO, etc. for their use as catalysts and or lubricant additives.7,8,14−17 Among all those studied Cu nanoparticles have gained a significant importance in the lubrication domain because of their enhanced thermal and electrical conductivity.18 The Cu nanofluids have been extensively studied as heattransfer fluids in chillers, domestic refrigerators, engine coolants, etc. The early research in Cu nanofluids can be delineated to the development of processes for synthesis of nanoparticles and investigating their thermal and electrical properties.19 Over time, the studies matured and new applications in the domain of tribology were targeted. The studies undertaken have established that the Cu nanofluids have tendencies not only to enhance the heat transfer capabilities but also reduce © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

November 10, 2016 February 3, 2017 March 17, 2017 March 17, 2017 DOI: 10.1021/acs.iecr.6b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

to the lubricant manufacturers to incorporate the Cu nanoparticles as additives for lubricant synthesis.

It has been observed that over time the nanoparticles agglomerate and settle, showing poor dispersion stability and reduced shelf life of the nanofluids.23−25 In recent times numerous attempts have been made to chemically functionalize the nanoparticles to enhance their dispersion stability in the lubricant blends. Li et al.18 studied the dispersion stability of Cu nanoparticles with and without surfactant in aqueous medium. The aqueous suspensions of Cu nanoparticles were developed using hexadecyl trimethylammonium bromide (CATB) and sodium dodecylbenzenesulfonate (SDBS) surfactants. Different chemical routes have been followed by the researchers to functionalize the nanoparticles and reported in the open literature.26−32 Morioka et al.,26 Lee et al.,27 Deng et al.,28 and Zhon et al.29 functionalized the Cu nanoparticles using polyvinylpyrrolidone, dialkyldithiophosphate, and some organic acids. It was observed that the organic compounds stabilized the Cu nanoparticles in various base fluids. Li et al.30 performed the surface modification of Cu nanoparticles using O,O-di-ncetyl-dithiophosphate and developed the nanofluids using kerosene, toluene, and decahydronapthalene as base fluids without dispersant. Studies using functionalized Cu nanoparticles have been restricted to investigation of either the dispersion stability or the variation in the physicochemical properties. Very little efforts have been made to determine the tribological performance of the Cu nanofluids. The studies pertaining to the tribological performance of Cu nanofluids have however been limited to the determination of the change in the friction and wear behavior of the lubricants using Cu nanofluids.19−22,29−32 With reference to this Zhou et al.32 studied the tribological performance of Cu nanofluids developed using P and S containing organic compounds. It was reported that Cu nanofluids have good load carrying capacity, antiwear, and antifriction properties. Zhou et al.29 studied the anit-wear performance of Cu nanofluids developed using dialkyldithiophosphate modified Cu nanoparticles. It was reported that the dialkyldithiophosphate modified Cu nanofluids improved the wear resistance effectively by 41.74%. On a similar note Viesca et al.31 studied the wear behavior of nanofluids synthesized using carboncoated Cu nanoparticles. However, not much emphasis on the tribological studies of Cu nanofluids has been made. On the basis of the literature review it has been observed that a significant work has been undertaken that discusses the use and application of Cu nanofluids as heat transfer fluids. However, a detailed approach on synthesis, functionalization, characterization, and investigation on influence of operating parameters on the performance of Cu nanofluids has not yet been presented. Hence, the present work is a systematic approach involving (i) functionalization of Cu nanoparticles using oleic acid, (ii) characterization of nanoparticles, (iii) lubricant blend preparation, (iv) physico-chemical characterization of blends, (v) tribological investigations of synthesized Cu nanofluids, (vi) influence of operating parameters on performance of nanofluids, and finally (vii) post-experimental analysis of the test specimens used. For a better understanding on the role of Cu nanofluids on the friction and wear reduction, results have also been presented for the Cu nanofluid blends prepared using commercial, fully formulated oil. The work undertaken shall provide vital information on how the Cu nanofluids behave in a wide range on operating conditions and when the base oils are changed. The functionaliztion of Cu nanoparticles using oleic acid enhances the dispersion stability and the results on dispersion characteristics too have been presented. The results reported in the present paper will be of significant help

2.0. EXPERIMENTAL SECTION 2.1. Materials. The Cu nanoparticles (size, 45 nm) were purchased from Reinste Nano Ventures, and oleic acid required for the surface coating of the Cu nanoparticles was purchased from Sigma-Aldrich, India, and used as such. It has been

Figure 1. Digital images of Cu nanofluids using virgin nanoparticles.

Figure 2. Schematic of functionalization route for Cu nanoparticles.

Figure 3. Four-ball tribo-tester experimental set up.

Table 1. Combination Matrix for Experimental Test Conditions experiment

load (N)

speed (RPM)

temp (°C)

influence of load influence of speed influence of temperature

98, 196, 392 392 392

1200 600, 1200, 1800 1200

75 75 25, 50, 75

Figure 4. Determination of wear volume using wear scar on ball specimen. B

DOI: 10.1021/acs.iecr.6b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Reaction scheme for functionalization of Cu nanoparticles.

reported in the literature that the smaller is the size of the nanoparticles, the better is the tribo-performance. Hence, the smallest possible particle size that was commercially available (i.e., 45 ± 5 nm) has been used in the present study. 2.2. Surface Coating of Copper Nanoparticles. Nonfunctionalized Cu nanoparticles show poor dispersibility in base fluids as shown in Figure 1. To enhance the dispersibility of Cu in mineral oil, the particles need to be either functionalized or surface coated, especially by an organic compound. Here we report the surface-functionalized Cu nanoparticles by an organic compound (oleic acid). The organic compound present on the surface of the Cu nanoparticles increases flexibility and stability of Cu nanoparticles in the lubricating oil. The functionalization results into capping of nanoparticles with oleic acid. Further, the hydrocarbon chain of the oleic acid becomes attached to the hydrocarbon chain of the mineral oil. Moreover, due to capping, the attractive forces between the two neighboring Cu nanoparticles decrease. As a result of this the agglomeration of nanoparticles is avoided, thus, resulting in the stable dispersion of Cu nanofluids. The hydrocarbon chain of oleic acid provides flexibility to the attached Cu nanoparticles due to which they remain suspended in the lubricating oil even when the lubricant is static or is in flowing mode.33 The functionalization was performed by treating Cu nanoparticles with oleic acid in the molar ratio of 1:10 at low temperatures for 48 h. During the reaction, hydrogen evolved in a stochiometric amount yielding the desired product. The schematic for functionalization of Cu nanoparticles is shown in Figure 2. 2.3. Preparation of Lubricant Blends. The functionalized Cu nanoparticles were blended in varying concentrations (0.10%, 0.15%, 0.20%, 0.25% and 0.3%) with mineral base oil N-150 and commercial multigrade SAE 5W40 lubricating oil to prepare the Cu nanofluids. To obtain an optimum dose of Cu nanoparticles that result into best performance of the lubricant blend, a range of concentrations varying from 0.1% to 0.3% by weight was selected. The lowest concentration was selected as one that reported better performance than the base oil. On the contrary the highest concentration selected was one that revealed increase in the values of friction and wear as compared to its predecessor concentration. To facilitate proper dispersion the blends were sonicated in an ultrasonic bath, model Powersonic 405 from M/s. Hwashin Technology Co. Korea. The Cu nanoparticles are in solid phase are when they are blended into the mineral oil, and they are prone to settle (precipitate) due to gravity. This phenomenon is shown using digital image in Figure 1, using the virgin Cu nanoparticles

without any functionalization. Such settling of nanoparticles is not desirable for efficient lubrication. Hence, the nanoparticles were functionalized using oleic acid. To check if the functionalized nanoparticles would settle or not, these nanofluids were filled in various sample bottles and kept under observation for the precipitation of Cu nanoparticles. In the case of precipitation of the nanoparticles, a layer of clear oil would be observed on the upper portion of the sample bottle (as shown in Figure 1; oil after 3 days). Hence, in order to ascertain proper stable dispersion these observations were made. Here and after the nanofluids prepared by blending of Cu nanoparticle in

Figure 6. FT-IR spectra of (a) oleic acid and (b) functionalized Cu nanoparticles.

Figure 7. XRD spectra of functionalized Cu nanoparticles. C


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in the temperature range of 40−900 °C with a scan speed of 10 °C min−1. The dispersion stability of Cu nanofluids was assessed by UV−visible technique over a span of 30 days with a UV-2600 spectrometer. 2.4.2. Physico-chemical Characterization. The physicochemical properties of the base fluids and the nanofluids were determined using standard test procedures proposed in ASTM and Bureau of Indian Standards (BIS) as follows. (i) Density: IS, 1448; P, 32/ASTM D, 1298. (ii) Viscosity and viscosity index: ASTM D: 445. (iii) Flash point: IS, 1448; P, 69 and (iv) pour point, ASTM D: 97. 2.5. Tribo-performance Investigation. The tribo-performance investigation of oleic acid-functionalized Cu nanofluids was performed on a four ball tribo-tester to assess the friction and wear behavior at the contact point. The diagrammatic sketch of the four-ball geometry used for the experimental work is shown in Figure 3. The standard 12.7 mm, AISI E-52100 steel balls with a hardness of 60−62 HRC were used as the test specimens. The friction torque encountered during the entire test duration was monitored and converted into a coefficient of friction (COF). The wear scar obtained on the test specimens at the end of the test was observed using an industrial stereozoom microscope, and the wear scar diameter (WSD) was measured. Five repetitive experiments were performed for each concentration of nanoparticles to establish the repeatability of the test results. The used ball test specimens were later analyzed using SEM and EDX to study the morphology of the worn out surface. 2.5.1. Experimental Procedures. The tribological tests for the assessment of friction and wear behavior of Cu nanofluids were performed under EHL conditions as per the standard ASTM D: 4172B procedure. The experimental procedure involved testing the lubricant sample at 75 °C at 392N load and 1200 rpm for 1 h duration. The optimum concentration of nanoparticles into the base fluid was determined on the basis of the tribo-performance of the lubricant blends. The influence of operating parameters on the performance of the nanofluid (with optimum dose of Cu nanoparticles) was determined by conducting experiments at different load, speed,

Figure 8. TGA of functionalized Cu nanoparticles.

mineral base oil are referred to as base oil nanofluids and those prepared by the blending of Cu nanoparticle in commercial multigrade SAE 5W40 oil are coded as commercial oil nanofluids. 2.4. Characterization. 2.4.1. Characterization by Instrument. Functionalized Cu nanoparticles were characterized by analytical techniques such as Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. FTIR spectra were recorded with a Nicolet 8700 FTIR spectrometer, equipped with XT-KBr beam splitter and utilizing a DTGS TEC detector in the region of 4000−400 cm−1 with 4 cm−1 spectral resolution and 36 kHz scanning speed. X-ray diffraction patterns were recorded using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA in the range of 2θ = 2−80° with a XRD diffractometer (D8 advance, Bruker, Germany). Diffraction data were collected by setting a proportional counter detector at 1° min−1 with an increment of 0.01° for 2θ values. Scanning electron micrographs were taken using a field emission (FE) SEM, (Quanta 200 F, Netherlands) at a voltage of 10−30 kV. The TGA analysis was performed using PerkinElmer TG/DTA diamond instrument under N2 atm. Approximately 2−4 mg of sample was mounted on the sample holder and tested

Figure 9. Characterization of Cu nanoparticles (a) SEM and (b) EDX analysis. D


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Figure 10. Dispersion stability of Cu nanofluids: (a) spectral absorbance and (b) maximum UV absorbency.

Table 2. Physico-chemical Properties of Base Oil, Commercial Oil, and Cu Nanofluids kinematic viscosity (mm2·s−1) sample descriptiona at 40 °C at 100 °C BO BO + 0.20% Cu CO CO + 0.2% Cu a

viscosity index

flash point °C

pour point °C

density at 15 °C g cm−3

−3.0 −3.0

0.8812 0.8836

33.30 33.33

5.49 5.55

100 103.2

208 212

83.68 85.58

13.28 14.31

162 173.9

216 218

−27 −27

0.8655 0.8637

Notation: BO, base oil; CO, commercial oil.

and temperature combinations. The combination matrix for the test conditions is shown in Table 1. The lubricants in actual industrial and automotive applications experience different lubrication regimes ranging from boundary lubrication to elastohydrodynamic and hydrodynamic lubrication. The lubrication regimes are demarcated on the basis of the ratio of fluid film thickness to composite surface roughness. Thus, for a similar surface roughness, the lubricant film thickness becomes the factor that determines the lubrication regime. The lubricant film thickness apart from the lubricant chemistry is by and far influenced by the operating parameters. Hence, for realistic prediction of lubricant behavior, the performance has to be assessed over a wide range of operating parameters that extends from boundary lubrication to elastohydrodynamic and hydrodynamic lubrication. Hence, in the present case the range in between lowest and the highest possible values of operating parameters that can influence the thickness of the lubricant film formed is considered. The operating temperature was selected from room temperature (25 °C) to 75 °C which is considered as the temperature range of normal operating machinery. The selected load resembles Hertzian contact pressures in the range of 2.16−3.42 GPa, which significantly reflects the contact pressures observed in actual engineering contacts. Thus, the range of operating parameters was selected for simulation of realistic contact conditions. The friction coefficient over the entire test duration and the wear scar diameter at the end of the test was monitored, measured, and reported. 2.5.2. Wear Rate Determination. The wear rate for the ball specimens was calculated using Archard’s formula and

Figure 11. Friction behavior as per ASTM D: 4172 B of Cu nanofluids prepared using (a) base oil and (b) commercial oil.

the WSD. The wear depth of the ball was deduced as shown in Figure 4. ⎡⎛ WSD ⎟⎞⎤ wear depth (h) = DB = R − R cos⎢⎜sin−1 ⎥ ⎣⎝ 2R ⎠⎦

(1)

where R is the radius of the ball. The wear volume and wear rate of the ball is given by wear volume (V ) = E

⎤ πh ⎡ 3(WSD)2 + h2 ⎥ ⎢ 6 ⎣ 4 ⎦

(2)


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Table 3. Percentage Change in COF and WSD for Lubricant Blends Tested as per ASTM D 4172B % change (w.r.t. BO)

Figure 12. Wear scar observed on the used ball test specimens. a

Figure 13. Tribological behavior of Cu nanofluids as per ASTM D 4172B procedure in (a) base oil and (b) commercial oil.

wear rate (Q ) =

wear volume (V ) sliding distance (X )

πDNt = sliding distance, mm 60

lubricant blendsa

COF (μ)

WSD (mm)

1 2 3 4 5 6 7 8 9 10 11

BO BO + 0.1% Cu BO + 0.15% Cu BO + 0.2% Cu BO + 0.25% Cu BO + 0.3% Cu CO CO + 0.1% Cu CO + 0.15% Cu C.O + 0.2% Cu C.O + 0.25% Cu

0.133 0.118 0.115 0.112 0.127 0.128 0.090 0.087 0.086 0.085 0.092

0.919 0.572 0.521 0.454 0.573 0.604 0.391 0.385 0.380 0.379 0.382

μ

WSD

10.49 13.34 15.46 4.42 3.37

37.80 43.27 50.59 37.65 34.29

3.33 4.44 5.55 −2.22

1.53 2.81 3.06 2.30


Figure 14. Variation in COF and WSD with change in concentration of Cu nanoparticles in (a) base oil and (b) commercial oil.

each Cu has a face centered cubic structure and possesses a polar moiety such as OH on the surface of the nano Cu. When Cu reacts with oleic acid, hydrogen is evolved in a stoichiometric amount and oleic acid is bonded with oleate on the surface of Cu nanoparticles. Thus, the oleic acid forms a coating on the surface of Cu nanoparticles as shown in Figure 5. The oleic acid aids in attachment of a long alkyl carbon chain with the Cu nanoparticles, thereby resulting in stable dispersion of the Cu nanofluids. Figure 6 shows the FT-IR spectra of virgin oleic acid and oleic acid functionalized copper nanoparticles. The FT-IR analysis was undertaken to confirm the fictionalization of Cu nanoparticles with oleic acid. The functional groups present in the chemical compound being tested resonate at particular IR stretching frequencies. These main IR stretching frequencies

(3)

where X is the sliding distance and calculated using X=

sample no.

(4)

where t is the time for test duration in second, D is the pitch circle diameter (PCD) in mm and N is the rotational speed of the top ball in RPM.

3.0. RESULTS AND DISCUSSION 3.1. Characterization of Functionalized Cu Nanoparticles. Functionalization is the process by which oleic acid is coated on the surface of Cu nanoparticles. In Cu nanoparticles, F


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1066.2 cm−1 due to C−O stretching, 1416.1 and 1315.5 cm−1 due to CC stretching and bending, respectively. Thus, from both spectra it is observed that the peaks are repeated at similar frequencies, this confirms that the Cu nanoparticles are functionalized with oleic acid. Figure 7 shows the XRD pattern of surface-modified Cu nanoparticles. It is observed that a large protrusion emerges at angles of 42−50°, which can be tentatively assigned to Cu metal, but it seems unreasonable to estimate the particle size based on the Scherrer equation, since the width of diffraction lines is too large. The line broadening of X-ray diffraction is due to the small particle size and presence of organic modifier.19 The TG/DTG spectrum for functionalized Cu nanoparticles is shown in Figure 8. The TGA analysis was carried out in the temperature range of 40 to 900 °C under nitrogen flow with 10 °C/min heating rate. The figure shows the weight loss of Cu oleate. Very strong endothermic peaks were obtained at 200 and 425 °C. The DTG curve of functionalized Cu nanoparticles shows significant weight loss at 190 and 335 °C. The weight loss occurs due to the decomposition of the outer oleate layer. This suggests that the functionalized Cu nanoparticles have thermal stability over the operating temperature range and do not degrade under the controlled temperature atmosphere. Therefore, the functionalized Cu nanoparticles can be efficiently used as additive for lubrication. The surface morphology of functionalized Cu nanoparticles obtained from the SEM images is shown in Figure 9a. The SEM micrographs reveal that the functionalized Cu nanoparticles are smooth and have an elongated shape. The EDX pattern is given in Figure 9b. The characteristic peak of Cu suggests that the Cu nanoparticle is bonded with oleic acid by the O−CO (ester) bond. 3.2. Dispersion Stability of Copper Nanofluids. The virgin Cu nanoparticles have a tendency to agglomerate and rebuild. The hydrophilic nature of Cu nanoparticles due to the presence of a polar moiety such as OH is responsible for such behavior. In a face centered cubic structure, Cu has strong lone pair−lone pair interactions which avoid dispersion. On the contrary when functionalized with oleic acid, Cu nanoparticles show good dispersibility in base fluids. The stable dispersion is achieved with the help of a steric repulsive force and van der Waals interaction that originate from long alkyl carboxylic acid chains present on the surface of Cu nanoparticles and the alkyl chains of lubricant base fluids. In the case of an unstable dispersion the functionalized Cu nanoparticles would settle over time resulting into two distinct layers within the lubricant blend. Because of the separation of Cu nanoparticles clear oil would be distinctly visible in the upper portion of the sample. Similarly while using the UV visible technique, the absorbance of UV−visible decreases over time due to the separation of Cu nanoparticles. If the undisturbed lubricant blends are inspected for UV−visible absorbance a good dispersion shall reveal negligible or small change in the absorbance over a desired time period. Figure 10a−b shows the absorbance of UV light by the Cu nanofluids over a span of 30 days. Figure 10a shows the difference in the absorbance of UV light for the different concentrations of Cu nanoparticles. It is clearly observed that the absorbency decreases with a decrease in the concentration of the particles. Further it is observed that the maximum absorbency of Cu nanofluid occurs at the wavelength of 326 nm (λmax). Figure 10b shows the variation in maximum UV absorbency of Cu nanofluids over a span of 30 days. It is observed that the

Figure 15. Variation in wear rate with concentration of Cu nanoparticles.

Figure 16. SEM images of used ball specimens lubricated with (a) base oil; (b) 0.20% Cu base oil nanofluid; (c) commercial oil; (d) 0.2% Cu commercial oil nanofluid.

and their corresponding functional groups are listed in Table A (Appendix-A). Thus, when tested the virgin oleic acid shows a strong characteristic stretching frequency at 1708.8 cm−1 due to CO stretching, 2923.9 and 2853.7 cm−1 due to C−H stretching, 1411.8 cm−1 CH2 bond deformation vibration, 1284.4 cm−1 due to C−O stretching, and 722.6 cm−1 rocking vibration of CH2 bond. This represents the signature spectra of oleic acid. Further, when the Cu nanoparticles are functionalized with oleic acid, IR frequency peaks present in signature spectra of oleic acid should also be repeated in the IR spectra of functionalized Cu nanoparticles. The IR spectra of functionalized Cu nanoparticles shows stretching frequencies at 2923.9 and 2853.7 cm−1 due to C−H stretching, 1709.8 cm−1 due to CO stretching, 1587.4 cm−1 due to C−H bending, G


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Figure 17. EDX pattern of used ball specimens lubricated with (a) base oil (b) 0.20% Cu nanofluid.

Figure 13a,b shows the repeatability of the results obtained from five different experiments. The values of COF and WSD are plotted with error bars calculated using the standard deviation technique. It is clearly observed from the figure that the results for COF and WSD are highly repeatable and the percentage error for the nanofluids prepared using base oil and commercial oil, is of the order of 1.5% and 2% for the COF and WSD, respectively. A comparative assessment of tribo-performance is shown in Table 3. The COF decreases with increase in concentration of functionalized Cu nanoparticles. It is observed that there is almost 16% reduction in friction and 51% reduction in wear in the case of 0.2% Cu nanofluid when compared with that of the base oil. The base oil reported the COF and WSD to the order 0.133 and 0.919 mm, respectively. The lowest values of COF and WSD that is, 0.112 and 0.454 mm, respectively, have been observed for 0.20% Cu nanofluid. Similar observations have been reported for the nanofluids prepared using commercial oil. The commercial oil which is a blended product of additives and base oil has a COF and WSD of the order 0.090 and 0.391, respectively. When blended with Cu nanoparticles, the COF and WSD decreased to 0.085 and 0.379, respectively, at 0.2% concentration of nanoparticles. Figure 14 shows the variation in COF and WSD with change in concentration of Cu nanoparticles in base oil. The figure clearly reveals that both the COF and WSD reduce simultaneously with increase in concentration of Cu nanoparticles in base oil until the 0.2% concentration. Thereafter it is observed that the COF as well as WSD both report an increasing trend. Similar behavior has been observed for the nanofluids prepared using commercial oil. Thus, 0.2% Cu nanoparticles in base fluids is the optimum concentration of nanoparticles for effective lubrication. The variation in wear rate with concentration of Cu nanoparticles is shown in Figure 15. The results show that the wear rate is high in the case of base fluid lubricated samples while it is lower in the case of Cu nanofluid lubricated specimens. The percentage change in wear volume for Cu nanofluid lubricated samples when compared to the base oil is in the range of 85−94%. The blending of Cu nanoparticles at an optimum dosage of 0.2 wt % concentration reduces the wear volume significantly by 94%. Similarly for the commercial oil nanofluid lubricated samples the percentage change in wear volume is in the range of 24−48%. The blending of Cu nanoparticles at an

maximum UV absorbency decreases nominally over the time. The decrease in absorbency after 30 days for the 0.1% and 0.2% Cu nanofluids are found to be 4.91% and 5.82%, respectively. Thus, revealing a stable dispersion of Cu nanofluids over the period of 30 days. 3.3. Physico-chemical Properties of Copper Nanofluids. The physicochemical properties of base oil, commercial oil, and nanofluids (blended with the 0.20% Cu nanoparticles) are tabulated in Table 2. The blending of functionalized Cu particles into the base oil very negligibly affects the physicochemical properties. The density and viscosity have almost a negligible change while the viscosity index has improved. Similarly the flash and pour point temperatures have very minor changes. 3.4. Tribo-performance of Cu Nanofluids. The triboperformance behavior of Cu nanofluids and base fluids for the experiment performed as per the ASTM D 4172B procedure is shown in Figure 11. The tribo-performance of the lubricant blends reveals that the COF decreases with increase in concentration of functionalized Cu nanoparticles. The kinetic friction represented by the friction at the end of the test reveals that there is significant reduction in friction for the 0.2% Cu nanofluid. The friction is higher in the case of base fluids and decreases with increase in concentration of Cu nanoparticles. Similar trends have been observed for the nanofluids prepared using base oil and the commercial oil. However, reduced values of COF are observed in the case of commercial oil nanofluids as the commercial oil is a blended product and already contains the antifriction additives in it. It is also observed that the decrease in kinetic friction (i.e., the friction at the end of the test) with increase in concentration of Cu nanoparticles is more significant for the base oil nanofluids than the commercial oil nanofluids. However, no aberrant behavior is observed; this suggests that the functionalized Cu nanoparticles are compatible with the existing additives of the commercial lubricant. Figure 12 shows the wear scars observed on the ball test specimens lubricated with base oil and the base oil nanofluids. It is observed from the figure that the specimens lubricated with Cu nanofluids have relatively smaller wear scar as compared to the base oil lubricated specimens. The WSD decreases with an increase in the concentration of Cu nanoparticles until 0.2% concentration. However, beyond 0.2% the WSD have reported an increasing trend. H


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Figure 18. Influence of load on friction behavior of lubricant blends: (a) 10 kg; (b) 20 kg; and (c) 40 kg.

Table 4. Comparative Assessment of COF and WSD for Lubricant Blends at Different Loads COF (μ)

a

WSD (mm)

sample no.

lubricant blendsa

10 kg

20 kg

40 kg

10 kg

20 kg

40 kg

1 2 3 4 5 6

BO BO + 0.1% Cu BO + 0.2% Cu CO CO + 0.1% Cu CO + 0.2% Cu

0.092 0.090 0.089 0.107 0.096 0.095

0.116 0.103 0.095 0.098 0.094 0.091

0.133 0.118 0.112 0.090 0.087 0.085

0.479 0.371 0.361 0.328 0.324 0.319

0.616 0.425 0.414 0.351 0.349 0.344

0.919 0.572 0.459 0.391 0.385 0.380

Notation: BO, base oil; CO, commercial oil. I


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Figure 19. Influence of speed on the friction behavior of lubricant blends: (a) 600 rpm; (b) 1200 rpm; and (c) 1800 rpm.

optimum dosage of 0.2 wt % concentration reduces the wear volume by 48%. The enhancement in tribo-performance with increase in concentration of Cu nanoparticles can be attributed to the ability of Cu nanofluids to form stable boundary films which separate the contacting surfaces. These boundary films become strong with increase in the concentration of Cu nanoparticles thus reducing

the friction and wear. The Cu nanoparticles present in the nanofluid get adsorbed on the steel surfaces under pressure. Thus, they form a boundary film that keeps the surfaces from contacting each other and helps in reducing friction. The strength of the adsorbed film increases with an increase in the concentration of the Cu nanoparticles. However, their ability to reduce friction increases only up to the optimum concentration. J


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Industrial & Engineering Chemistry Research Table 5. Comparative Assessment of COF and WSD for Lubricant Blends at Different Speeds COF (μ)

a

WSD (mm)

sample no.

lubricant blendsa

600 rpm

1200 rpm

1800 rpm

600 rpm

1200 rpm

1800 rpm

1 2 3 4 5 6


0.137 0.119 0.115 0.110 0.108 0.104

0.133 0.118 0.112 0.090 0.087 0.085

0.130 0.113 0.110 0.115 0.099 0.076

0.851 0.546 0.437 0.384 0.369 0.357

0.919 0.572 0.459 0.391 0.385 0.380

2.412 2.309 0.766 0.475 0.464 0.456


figure that at any given load lower values of COF are observed for the contacts lubricated with Cu nanofluids. Further it is also observed that the COF values decrease monotonically with an increase in the concentration of Cu nanoparticles. The COF values however increase with an increase in contact load. A similar trend is also observed in the case of WSD. The WSD increases with an increase in contact load; however, at a given load the Cu nanofluid lubricated contact reports lower values of WSD as compared to the base oil lubricated contacts. The comparative assessment of COF and WSD is given in Table 4. As the contact load increases from 98.1 N to 392.4 N the Hertzian contact pressure increases from 2.16 to 3.42 GPa. At such a high contact pressure the Cu nanofluid is able to sustain a lubricating film and reduce the metal-to-metal contact thereby resulting in lower values of friction and wear. The results reveal that the addition of Cu nanoparticles in base fluid enhances the load bearing capacity of the lubricant blend. The load bearing capacity also increases with increase in concentration of Cu nanoparticles. The percentage reduction in COF and WSD for 0.2% Cu nanofluid with respect to the base oil at lower loads (98.1 N) is 3% and 25%, respectively. This behavior gets amplified at higher loads (392.4 N) where the COF and WSD reduced by 16% and 50%, respectively. However, for the commercial oil nanofluids the friction and wear gets reduced marginally by 6% and 3%, respectively, at higher loads. 3.5.2. Effect of Speed. The influence of speed on the triboperformance behavior of Cu nanofluid was investigated by varying the speed from 600 to 1800 rpm. Figure 19 shows the variation in COF over the entire test duration. It is observed from the figure that the COF decreases with increase in contact speed. Further at a given rotational speed the COF decreases with an increase in the concentration of Cu nanoparticles. However, in the case of wear behavior it is observed that the WSD increases with increase in contact speed and decreases with increase in concentration of Cu nanoparticles. A comparative assessment of COF and WSD at different rotational speeds is shown in Table 5. With an increase in rotational speed the centrifugal force increases as a result of which the contacting bodies get separated and thicker lubricating film is formed. Because of this, a reduced value of COF is observed. However, with an increase in rotational speed, the sliding (linear) speed at the contact increases which signifies that for a given test duration the contact traverses longer distance. Hence, the WSD is observed to increase with increase in speed. However, at any given speed the WSD and COF decreases with increase in concentration of Cu nanoparticles in nanofluids prepared using base oil. This decrease in COF and WSD is 16% and 49%, respectively, at lower speeds (600 rpm) and 15% and 68% respectively at higher speed (1800 rpm). It is observed that there is no

At doses above the optimum concentrations, the friction starts to increase. This rise in friction can be attributed to the increase in interlayer shearing due to the increase in concentration of the Cu nanoparticles.34 The base oil (containing no additive of any type) is inefficient to form boundary films that can protect the surfaces from damage. However, the commercially formulated oil contains various additives that help in protecting the surfaces from damage. When the functionalized Cu nanoparticles are blended in these base fluids, the antifriction and antiwear performance behavior of both of the base fluids becomes enhanced. In the case of mineral base oil the decrease in wear rate after the addition of Cu nanoparticles is of the order of 94%. This decrease in wear rate is only due to the contribution of Cu nanoparticles. However, in the case of commercial oil the decrease in wear rate is of the order of 48%. This decrease in wear rate is due to the combined influence of additives (already present in commercial oil) and the Cu nanoparticles added into it. The additives present in the commercial oil interact with the Cu nanoparticles, a result of which is slightly lower values of wear rate reduction being observed with commercial oil. The surface morphology of used test specimens observed through SEM is shown in Figure 16. The wear scars on the base oil lubricated ball test specimens reveal scratching marks along the sliding direction with relatively deep scratches at the center. The surfaces are observed to be ploughed by asperities resulting in cutting wear. On the contrary the Cu nanofluid lubricated surfaces are smooth with small shallow pits and minor scratches in the sliding directions. Similar to the case of commercial oil lubricated specimens, a scoring type surface damage is observed. The scoring marks are parallel to the sliding direction. However, in the case of commercial oil nanofluid lubricated specimens, the surface is smooth with shallow pits. Minor rubbing marks in the sliding direction are also observed. The wear of a rubbing type with micropits is observed. Such a behavior in the case of commercial oil nanofluid lubricated specimens may be due to the coexistence of an initial additive package and the Cu nanoparticles. The EDX analysis of used ball test specimens shown in Figure 17 reveals the presence of Cu, C, and O, which signifies the formation of boundary film of Cu oleate. These elements from the lubricant have been adsorbed on the steel surface and therefore show their presence in the used test specimens. Due to the formation of a Cu oleate boundary film, wear of the test specimens is observed to be less along with reduced contact friction. 3.5. Influence of Operating Parameter. 3.5.1. Effect of Load. The influence of load on the tribo-performance of Cu nanofluid was investigated by varying the load from 98.1 N (10 kgf) to 392.4 N (40 kgf). The variation in COF over the entire test duration is shown in Figure 18. It is clear from the K


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Figure 20. Influence of temperature on friction behavior of lubricant blends: (a) 25 °C; (b) 50 °C; and (c) 75 °C.

3.5.3. Effect of Temperature. The influence of temperature on the tribo-performance of Cu nanofluid was investigated by varying the lubricant temperature from 25 to 75 °C. Figure 20 shows the variation in COF over the entire test duration for lubricated contacts at different temperatures. It is observed from the figure that the COF increases marginally with increase in temperature. However, at a given temperature the COF decreases substantially with increase in concentration of Cu

appreciable change in friction at lower and higher speeds as it is predominantly governed by the centrifugal force, but a significant change in WSD is observed at lower and higher speeds. The reduction in WSD at higher speed is due to the presence of Cu nanoparticles. Similar observations are made for the nanofluids prepared using commercial oil. The friction and the wear reduction are of the order of 6% and 7%, respectively, at lower speed and 34% and 9%, respectively, at higher speed. L


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Industrial & Engineering Chemistry Research Table 6. Comparative Assessment of COF and WSD for Lubricant Blends at Different Temperature COF (μ)

a

WSD (mm)

sample no.

lubricant blendsa

25 °C

50 °C

75 °C

25 °C

50 °C

75 °C

1 2 3 4 5 6


0.123 0.112 0.109 0.096 0.094 0.087

0.119 0.111 0.107 0.099 0.098 0.093

0.133 0.118 0.112 0.090 0.087 0.085

0.867 0.502 0.457 0.421 0.418 0.411

0.898 0.519 0.457 0.399 0.387 0.385

0.919 0.572 0.459 0.391 0.385 0.380


nanofluids as compared to the base fluids. On the basis of the experimental results obtained, it can be emphasized that the blending of functionalized Cu nanoparticles enhances the triboperformance of lubricant blends. The oil-soluble functionalized Cu nanoparticles when added with the lubricant results into a stable colloid yet homogeneous solution. Under severe operating conditions the nominal lubricant film tends to thin down resulting in metal-to-metal contact, thereby yielding higher COF and WSD values. However, in the case of Cu nanofluids the Cu nanoparticles form a thin protective layer on the contacting surfaces thereby separating them. The Cu nanoparticles get deposited on the crusts and troughs of the surfaces and present bearing-like behavior. The mechanism of lubrication under such conditions is depicted in Figure 21. The formation of boundary films has been confirmed from EDX analysis which reveals the presence of Cu on the used specimen surfaces. Thus, the Cu nanofluids offer efficient lubrication in the mixed/partial lubrication regime where the lubricant film is relatively thin. The oil-soluble functionalized Cu nanoparticles thus suffice to be used as additives for improved tribological performance of lubricants.

Figure 21. Lubrication mechanism of Cu nanofluids.

Table A. IR Absorptions Frequencies and Their Functional Groups characteristic absorptions (cm−1)

functional groups

2845−2950 1350−1480 1735−1750 1700−1725 1000−1300 991−910 720−730

C−H stretching of saturated hydrocarbon C−H bending of saturated hydrocarbon CO stretching of Ester CO stretching of carboxylic acid C−O stretching of Ester CC−H stretching C−C bending of saturated carbon atom

4.0. CONCLUSIONS The present study has investigated the influence of operating conditions on the tribological performance of Cu nanofluids prepared by blending functionalized oil soluble Cu nanoparticles in base oil and commercial multigrade oil in varying concentrations. The Cu nanoparticles were functionalized using oleic acid to obtain a stable colloidal yet homogeneous dispersion. The Cu nanoparticles were characterized by analytical techniques such as FTIR, XRD, TGA, SEM, and EDX. The dispersion stability of Cu nanofluids was assessed by the UV− visible technique. The tribological performance of Cu nanofluids was investigated using four-ball tribo-tester. The optimum concentration of Cu nanoparticles and the influence of operating parameters on tribo-performance of Cu nanofluids have been investigated. The results reveal the following: • Functionalization of Cu nanoparticles with oleic acid results in stable dispersion of Cu nanoparticles in the nanofluids. • The 0.20% Cu nanofluid results in minimal COF and WSD at 3.4 GPa contact pressure and 0.46 m/s sliding velocity. • The Cu nanofluids help in enhancing the tribo-performance under severe operating conditions of load, speed, and temperature. • The Cu nanoparticles enhance the load bearing capacity and film forming capability of lubricating oil due to formation of boundary films as revealed from the EDX analysis.

nanoparticles. Similarly in the case of wear behavior, the WSD increases marginally with increase in temperature but it decreases substantially with increase in concentration of Cu nanoparticles. The comparative assessment of COF and WSD for Cu nanofluid lubricated contact at different temperatures is given in Table 6. The results reveal that the decrease in COF for 0.2% Cu nanofluid with respect to base oil is 11% and 16%, respectively, at 25 and 75 °C temperature. However, the WSD reveals that there is substantial 47% reduction in wear at lower temperature (25 °C) which increases to 50% at higher temperature (75 °C). A similar trend is observed for the Cu nanofluids prepared using commercial oil. The friction and wear decreases marginally by 9% and 3%, respectively, at lower temperature and 8% and 3%, respectively, at higher temperature. The presence of Cu nanoparticles shows marginal influence on the performance due to the presence of additives in the commercial oil. An improved tribological performance with increase in temperature can be attributed to thermal conductivity of the Cu nanofluids. The use of Cu nanofluids as heat transfer fluids has been extensively studied, and it has been reported to be effective in dissipating the heat to the surroundings.35,36 As a result of this, improved tribo-performance is observed for Cu M


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• The functionalized Cu nanoparticles can be used as an antifriction and antiwear additive enhancing the triboperformance under mixed/partial lubrication regime. • The influence of Cu nanoparticles on tribo-performance is well pronounced in the case of base oil nanofluids as the entire functionality is dependent on the concentration of nanoparticles. However, in the case of commercial nanofluids, due to the presence of an existing additive package the enhancement in tribo-performance is not that substantial but still an improved performance is observed. This suggests that the Cu nanoparticles can be used as additive for the new formulations as well as existing commercial oils for improved tribo-performance.

(11) Huang, H. D.; Tu, J. P.; Gan, L. P.; Li, C. Z. An investigation on tribological properties of graphite nano-sheets as oil additive. Wear 2006, 261, 140. (12) Sunqing, Q.; Junxiu, D.; Guoxu, C. Wear and friction behavior of CaCO3 nano-particles used as additives in lubricating oil. Lubr. Sci. 2000, 12 (2), 205−212. (13) Lee, K.; Hwang, Y.; Cheong, S.; Choi, Y.; Kwon, L.; Lee, J.; Kim, H. S. Understanding the role of nano-particles in nano oil lubrication. Tribol. Lett. 2009, 35, 127−131. (14) An, V.; Irtegov, Y.; de Izarra, C. Study of tribological properties of Nano-lamellar WS2 and MoS2 as Additives to Lubricants. J. Nanomater. 2014, 865839, 1−8. (15) Hwang, Y.; Lee, J. K.; Jung, Y. M. Stability and thermal conductivity characteristics of nano-fluids. Thermochim. Acta 2006, 2007, 70−74. (16) Li, D.; Fang, W.; Hong, B.; Guo, Y.; Lin, R. Preparation of welldispersed silver nano-particles for oil-based nano-fluids. Ind. Eng. Chem. Res. 2010, 49, 1697−1702. (17) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Andreeva, D.; Tabakova, T.; Ilieva, L.; Iadakiev, V. Gold, Silver and Copper Catalysts Supported on TiO2 for Pure Hydrogen Production. Catal. Today 2002, 75 (1−4), 169. (18) Li, X.; Wang, X.; Zhu, D. Evaluation on dispersion behavior of the aqueous copper nano-suspensions. J. Colloid Interface Sci. 2007, 310, 456−463. (19) Zhang, C.; Zhang, S.; Song, S.; Yang, G. Preparation and tribological properties of surface capped Cu nano-particles as a water based lubricant additive. Tribol. Lett. 2014, 54, 25. (20) Yu, H. L.; Xu, Y.; Shi, P. J.; Xu, B. S.; Wang, X. L.; Liu, Q. Tribological properties and lubricating mechanisms of Cu nanoparticles in lubricant. Trans. Nonferrous Met. Soc. China 2008, 18 (3), 636−641. (21) Sahu, R. K.; Somashekhar, S. H. Investigation on Copper nanofluifd obtained through micro electrical discharge machining for dispersion stability and thermal conductivity. Procedia Eng. 2013, 64, 946−955. (22) Zin, V.; Agresti, F.; Barison, S.; Colla, L. The synthesis and effect of copper nano-particles on the tribological properties of lubricant oils. IEEE Trans. Nanotechnol. 2013, 12, 5. (23) Yu, W.; Xie, H. A review on Nano-fluids: Preparation, stability mechanisms, and application. J. Nanomater. 2012, 2012, 1. (24) Mukherjee, S.; Paria, S. Preparation and stability of Nano-fluidsA review. IOSR- JMCE 2013, 9 (2), 63−69. (25) Drzaga, M.; Lemanowicz, M.; Dzido, G.; Gierczycki, A. Preparation of metal oxide-water nano-fluids by two step method. Appl. Chem. 2012, 51 (5), 213−215. (26) Morioka, T.; Takesue, M.; Hayashi, H.; Watanabe, M.; Smith, R. L. Anti-oxidation properties and surface interaction of polyvinylpyrrolidone-capped zerovalent copper nano-particles synthesized in supercritical water. ACS Appl. Mater. Interfaces 2016, 8, 1627−1634. (27) Lee, G.; Kim, C. K.; Lee, M. K.; Rhee, C. K. Facile synthesis of surface oxide free copper nano-particles by in-situ coating with oleic acid. Powder Technol. 2014, 261, 143. (28) Deng, D.; Jin, Y.; Cheng, Y.; Qi, T.; Xiao, F. Copper nanoparticles: Aqueous phase synthesis and conductive film fabrication at low sintering temperature. ACS Appl. Mater. Interfaces 2013, 5, 3839− 3846. (29) Zhou, J.; Yang, J.; Zhang, Z.; Liu, W. Study on the structure and tribological properties of surface-modified Cu nano-particles. Mater. Res. Bull. 1999, 34 (9), 1361−1367. (30) Li, D.; Xie, W. Preparation and properties of copper oil based nano-fluids. Nanoscale Res. Lett. 2011, 6, 373−379. (31) Viesca, J. L.; Battez, A. H.; Gonzalez, R. Antiwar properties of carbon coated Cu nano-particles used as an additive to a polyalphaolefin. Tribol. Int. 2011, 44, 829−833. (32) Zhou, J.; Wu, Z.; Zhang, Z.; Liu, W.; Xue, Q. Tribological behavior and lubricating mechanism of Cu nano-particles in oil. Tribol. Lett. 2000, 8, 213−219.

APPENDIX-A The functional groups present in the chemical compound being tested, resonate at particular IR stretching frequencies. These main IR stretching frequencies and their corresponding functional groups are listed in Table A.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel +91-135-2525929. Fax: +91-135-2660202. ORCID

Ajay Kumar: 0000-0001-6850-9389 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to director I.I.P. for his kind permission to publish these results. And we acknowledge the great help extended by CSC-0118/08 for funding the project.

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Influence of Operating Parameters on the Tribological Performance of

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