Synthesis, Characterization, and Tribological Evaluation of SDS

Research Article pubs.acs.org/journal/ascecg

Synthesis, Characterization, and Tribological Evaluation of SDSStabilized Magnesium-Doped Zinc Oxide (Zn0.88Mg0.12O) Nanoparticles as Efficient Antiwear Lubricant Additives Kalyani,† Rashmi B. Rastogi,*,† and D. Kumar‡ †

Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi-221005, India Department of Ceramic Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi-221005, India

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S Supporting Information *

ABSTRACT: Pure zinc oxide (ZnO) and magnesium-doped zinc oxide (ZMO) nanoparticles (NPs) with the composition of Zn0.88Mg0.12O have been prepared by an autocombustion method. The as-synthesized ZMO nanoparticles were calcined for 2 h at 800 and 1000 °C to yield different-sized ZMO nanoparticles abbreviated as ZMO-1 and ZMO-2, respectively. These nanoparticles have been characterized by powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. The average size of these nanoparticles is found to be 30, 27, 39, and 44 nm, respectively for ZnO, ZMO, ZMO-1, and ZMO-2. A stable dispersion of ZMOs (ZMO, ZMO-1, and ZMO-2) nanoparticles in paraffin oil has been achieved with an appropriate percentage of surfactant sodium dodecylsulfate (SDS), abbreviated as SZMOs (SZMO, SZMO-1, and SZMO-2). The effect of the particle size of these nanoparticles on the tribological behavior of the paraffin oil has been investigated at an optimized additive concentration (0.25% w/v with 0.10% SDS) using different ASTM D4172 and D5183 standards. In addition, a test has been conducted by varying the loads for a 30 min time duration and by varying the test durations at 392 N load. All tribological testing of SZMOs nanoparticles were conducted on a four-ball lubricant tester. These tribological tests revealed that the SZMOs nanoparticles act as excellent antiwear agents and friction modifiers and also enhance the load-bearing ability. Being the smaller particle size, SZMO nanoparticles (27 nm) exhibited better tribological behavior than SZMO-1 (39 nm) and SZMO-2 (44 nm). The morphology of the worn surfaces lubricated with nanoparticles and without SZMOs at a 392 N applied load for 60 min and at higher loads for a 30 min test duration has been examined by scanning electron microscopy (SEM) and contact mode atomic force microscopy (AFM) analyses. Energy-dispersive X-ray (EDX) analysis of the surface lubricated with SZMO nanoparticles shows the presence of zinc, magnesium, iron, carbon, and oxygen on the steel surface which confirmed the adsorption of the additive on the interacting/rubbing surface. These elements form tribochemical film on the interacting surfaces to prevent the metal−metal contact thereby reducing wear and friction. KEYWORDS: Four-ball tester, Antiwear lubricant additives, Zn0.88Mg0.12O nanoparticles, Material characterization, Surface analysis, AFM, SEM, EDX

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INTRODUCTION Conventionally, antiwear (AW), extreme pressure (EP), and antioxidant additives are incorporated into base oil to reduce friction and wear thus increasing the life of contact interfaces.1−5 In addition, other additives such as detergent,6 rust inhibitor,7 viscosity index improver,8,9 and friction modifier10,11 are also added to the lubricant for respective properties. According to literature surveys, lots of organic compounds containing triboactive elements such as boron,12−15 nitrogen,15,16 oxygen,13,14 sulfur,13,14 phosphorus,13,17,18 and halogens19 are being used as AW and EP lubrication additives. These active elements have strong tendencies to be adsorbed on the interacting metal surfaces to form in situ protective tribochemical film, which enhances the tribological properties of base lube under lubricating conditions.20,21 This thin © XXXX American Chemical Society

tribofilm prevents direct metal-to-metal contact and welding of surface asperities, thus reducing wear. Besides this, tribological performance of metal complexes of various N-, S-, and P-containing ligands has also been explored.22−25 Zinc dialkydithiophosphates (ZDDP) are the most successful lubricant additives ever developed. These are being used as multifunctional, antiscuffing, antioxidant, antiwear, and corrosion-inhibiting additives in lubricants.25−27 However, the excessive use of S, P, halogen, and metal-containing additives has been prohibited due to their high SAPS levels and several negative impacts on the environment as well as engines.28−35 It Received: March 7, 2016 Revised: April 13, 2016

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DOI: 10.1021/acssuschemeng.6b00472 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

named ZMO, were further calcined at 800 and 1000 °C for 2 h and designated as ZMO-1 and ZMO-2, respectively. Tribological Characterization. Sample Preparation. The suspensions of ZMOs NPs having concentrations of 0.00, 0.25, 0.50, and 1.00% (w/v) with 0.10% SDS in paraffin oil were made by stirring for 30 min on a magnetic stirrer to obtain a uniform mixture. In order to form stable suspensions, these blends were further sonicated for 1 h at room temperature. The entire tests were carried out at an optimized concentration, 0.25% w/v ZMOs + 0.10% SDS. Specimen. Balls of 12.7 mm diameter made of AISI 52100 alloy steel having hardness of 59−61 HRc were used for the tests. Before and after each test, balls were cleaned with n-hexane and thoroughly air-dried. The liquid paraffin used in the present paper was of the same quality as used in our previous studies.23 Antiwear Testing. The tribological properties of paraffin oil and the SZMOs nanoparticles dispersed in paraffin oil were evaluated using a Four-Ball Lubricant Tester (Ducom Instruments Pvt. Ltd., Bangalore, India) according to standard test procedures ASTM D4172 and ASTM D5183. Besides this, the mean wear scar diameter (MWD) has also been measured by varying the loads of 294, 392, 490, 588, and 686 N for 30 min test durations and varying times of 15, 30, 45, 60, 75, and 90 min durations at 392 N load. The friction and wear tests were repeated thrice, and their average values are reported in this paper. The details of various tribological parameters are provided in the S1 for the Supporting Information. Suspension Stability of SZMOs. To investigate the suspension stability of SZMOs NPs in paraffin oil, the optimized concentration of SZMO and SZMO-2 NPs were further diluted 10 times in order to record their UV spectra. Suspension stability of SZMOs was studied by means of absorbance measurement using a UV−visible spectrophotometer (Thermo) at different time intervals. Details of instruments used are mentioned in S2 of the Supporting Information.

is necessary to develop new additives which are comparable to or better than ZDDP and have compatibility to be used in catalytic converters that control the pollution in the environment. To achieve the aforesaid objectives, efforts have been made to find a better alternative of ZDDP.23,36 Nanoparticles (NPs) owing to their small size, large surface area, and high surface energy, play important roles in electronics, photonics, magnetism, conductors, field effect transistors, semiconductors, medicines, photodetectors, solar cells, and tribology, too.37−42 Nanoparticles such as CuO, ZrO2, ZnO,43 TiO2,44 CeO2,45 CaCu2.9Zn0.1Ti4O12,46 lanthanum borate,47 etc. have been investigated as oil lubricant additives. The admixture of these particles with base oil enhances the extreme pressure, antiwear, and friction-reducing properties of the lubricating base oil. Being environmentally friendly, ZnO and MgO nanoparticles48 have been used for various applications in tribology. Wang et al. have explored the metal self-repairing behavior of magnesium metal.49 In addition to this, magnesium palmitateand magnesium stearate-based detergents have been used as antioxidants.50 The antiwear and antifriction properties of magnesium borate nanoparticles have been reported.51 Since lubricants also acts as coolants, the thermal conductivity of additive molecules is very important in reducing asperity temperature at the metal−metal interface. The thermal conductivity of MgO NPs is 3.5-fold higher than those of ZnO NPs.52 The potential applications of these ZnO and MgO NPs have prompted us to synthesize the Mg-doped ZnO NPs. The Mg2+ions are readily incorporated into the crystal lattice of ZnO because the ionic radius of Zn2+ is 0.74 Å, which is very close to that of the Mg2+ ion, 0.72 Å.48 The synthesis protocol for the Mg-doped ZnO nanoparticles as reported by Yang et al.48 needs specific reaction conditions; however, the protocol adopted in this investigation is inexpensive and easy to synthesize under normal conditions. Herein, the present communication reports the synthesis and characterization of magnesium-doped zinc oxide nanoparticles (Zn0.88Mg0.12O, ZMO) of sizes 27, 39, and 44 nm, respectively, for ZMO, ZMO-1, and ZMO-2. The effect of particle size of the as-prepared ZMOs nanoparticles along with 0.10% SDS in paraffin oil has been evaluated on a four-ball lubricant tester. It is anticipated that the presence of magnesium as the dopant in ZnO NPs may enhance the tribological properties of base lube. In addition to this, it is expected that as the size of these ZMOs NPs decreases their tribological properties increase and thereby improve the friction and wear properties of paraffin base oil.

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RESULTS AND DISCUSSION Characterization of ZMOs Nanoparticles. The X-ray diffraction patterns (Bruker D8 Advance) of ZnO and ZMOs NPs synthesized by the autocombustion method are shown in Figure 1. Well-defined diffraction peaks for the hexagonal

EXPERIMENTAL SECTION

Chemicals. All materials used in the synthesis were of analytical reagent grade and employed without any further purification. Zinc nitrate hexahydrate (Merck, ≥99.6%), magnesium nitrate hexahydrate (Merck, ≥99%), citric acid anhydrous (Merck, ≥99.5%), and sodium dodecyl sulfate (SRL chemicals, ≥99%) were used as precursors. Synthesis of ZMOs Nanoparticles. The synthesis of Mg-doped zinc oxide (Zn0.88Mg0.12O) NPs has been achieved using the autocombustion method.53 In this method, zinc nitrate hexahydrate (10.72 g) in 20 mL of DD water and magnesium nitrate hexahydrate (1.22 g) in 5 mL of DD water were mixed together. The solution of citric acid (4.69 g) in 10 mL of DD water was added to this mixture. The mixed solution was evaporated with continuous stirring at 200 ± 5 °C on a hot plate until it turns into gel and is finally burnt. Thereafter, within a few seconds of combustion reaction, blackish ash appeared. The obtained ash was calcined at 600 °C in air for 4 h. Thus, prepared magnesium-doped zinc oxide nanoparticles (Zn0.88Mg0.12O),

Figure 1. Powder X-ray diffraction pattern of pure ZnO and Mgdoped ZnO nanoparticles having different particle sizes.

wurtzite phase structure of ZnO are clearly visible in the diffraction pattern of ZnO NPs. Diffraction peaks are located at 31.6°, 34.4°, 36.1°, 47.5°, 56.5°, 62.8°, 66.1°, 67.8°, 68.9°, 72.5°, and 76.8° corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) faces of wurtzite ZnO NPs, respectively (JCPDS No. 36-1451). Absence of any peak due to magnesium oxide in the XRD B


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spectra (Oxford Inst., Netherlands) of ZMO NPs also confirms the formation of Mg-doped ZnO NPs mentioned in Figure S1of the Supporting Information. Stability of SZMOs Nanofluids. UV−visible spectra of SZMOs nanoparticles recorded in the range from 300 to 600 nm exhibit a broad band around 360−370 nm confirming the formation of ZMO NPs.54 The stability of nanofluids is strongly affected by the characteristics of the suspended particles such as particle morphology and their size in base oil. Besides this, the addition of a surfactant improves the stability of the suspension. The relative stability of SZMOs nanofluids has been investigated with the help of UV−visible absorption at different time intervals. The UV−visible spectrum of SZMOs is shown in Figure 3a. With the expense of time, the characteristic absorption of the SZMO NPs displayed hypochromic shift, i.e., a decrease in absorptivity (in inset). The hypochromicity confirms the initiation of aggregation of SZMOs NPs in suspension. The absorbance of SZMOs NPs in oil suspensions decreases with increasing sediment time. After 1 h, the relative concentration of the SZMO NPs remains almost constant until 24 h showing the stability of nanofluids; thereafter, there is a gradual decrease in the relative concentration of SZMO nanofluids with time. On the other hand, SZMO-2 NPs are comparatively less stable than SZMO. After 48 h, relative concentration is maintained over 62% compared to the initial concentration of SZMO NPs. Figure 3b exhibits photographs of SZMOs suspension in paraffin oil at different time durations where it is clearly shown that after 48 h SZMO NPs remain suspended to a greater extent as compared to SZMO-2 NPs. Additive Optimization. The lubrication properties of different SZMOs and SZnO NPs in paraffin oil have been evaluated on a four-ball tester machine. Figure 4 displays the optimization result for the SZMOs and SZnO NPs for different additive concentrations, load 392 N, speed 1200 rpm, and test duration of 60 min. In the absence of additives, the mean wear scar diameter (MWD) was found to be very large, but in the presence of SZMOs and SZnO NPs, it was fairly reduced at each concentration. As the concentration of SZMOs NPs in the base oil increases, the extent of reduction in the value of MWD also increases. On addition of 0.25% w/v SZMOs NPs to the base lube, huge reductions in the MWD values are observed. Thereafter, a further increase in the concentration up to 1.00% w/v had little effect on the tribological behavior. Therefore, 0.25% w/v is an optimum concentration to provide significant

spectra confirmed the formation of a single phase. The average crystallite size of the ZnO, ZMO, ZMO-1, and ZMO-2 NPs was obtained as 30, 27, 39, and 44 nm, respectively, using the Debye−Scherrer formula: D = kλ/β cos θ

where, λ is the wavelength of the X-ray, k is a constant taken as 0.89, θ is the diffraction angle, and β is the full width at halfmaxima (fwhm). The crystallite size derived from the XRD data revealed that upon doping of magnesium into ZnO, the average crystallite size of the parent ZnO NPs has reduced. Besides this, the average crystallite size of ZMOs NPs increased with an increase in temperature. The TEM micrographs (Technai-G2, FEI, Eindhoven, Netherlands) of the synthesized ZMO NPs and the corresponding selected area electron diffraction (SAED) patterns are shown in Figure 2. From the TEM images, particle

Figure 2. TEM image of ZMO nanoparticles along with selected area electron diffraction (SAED) patterns.

sizes of ZMO NPs were measured manually using the ImageJ software, and the corresponding projected areas of the particles were converted to particle diameters. The average particle size of ZMO NPs is about 25 nm, which correlates very well with the size obtained from the powder XRD pattern recorded for the same sample. The SAED pattern corresponding to ZMO NPs gives a set of diffraction rings, which can be clearly assigned to the diffractions of the (100), (002), (101), (102), (110), (103), and (200) planes, respectively, of the wurtzite structure of ZnO. These results are consistent with the XRD characterization. The presence of magnesium in the EDX

Figure 3. (a) Relative absorbance of SZMO and SZMO-2 nanoparticles at different time intervals and UV−visible spectra of SZMO nanoparticles at different settling times (in inset). (b) Optical photographs of the different SZMOs NPs suspended in pure paraffin oil at different settling times. C


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strongly size dependent. This indicates that SZMOs NPs, when used as antiwear additives, play important roles in enhancing the tribological properties of paraffin oil. Figure 6 exhibits the variation of coefficient of friction with sliding time at a 392 N applied load. In general, in the presence

Figure 4. Variation of mean wear scar diameter in absence and presence of different additive concentrations in paraffin oil at a 392 N applied load and 60 min duration.

friction and wear reduction. The reduction of MWD in the presence of SZnO is comparatively much smaller than that in the presence of SZMOs nanoparticles. Figure 4 reveals that SDS-stabilized Mg-doped ZnO NPs performed better than pure zinc oxide. Antiwear Behavior. Figure 5 shows the changes in the average coefficient of friction (COF) and mean wear scar

Figure 6. Variation of COF with sliding time in the presence and absence of different nanoparticles in paraffin oil at 392 N, rotating speed of 1200 rpm, temperature of 75 °C, test duration of 60 min, and concentration of 0.25% w/v of nanoparticles.

of SZMOs NPs, the value of COF gradually decreases with time, and after some time, it becomes stabilized except for SZMO-2. The abnormal trend of SZMO-2 may be due to its larger particle size (44 nm). Having a larger particle size, SZMO-2 NPs tend to settle in the paraffin oil after some time due to gravity; this results in poor dispersion stability and therefore high COF values. Thus, the admixtures of SZMOs NPs in paraffin oil act as friction modifiers except SZMO-2. In order to study the wear rates, the tribological tests also have been conducted for paraffin oil at a 392 N applied load for different time durations of 15, 30, 45, 60, 75, and 90 min in the presence and absence of nanoadditives. The behavior of MWD and mean wear volume (MWV) with time is mentioned in Figures S2 and S3 of the Supporting Information. The values of running-in and steady-state wear rates are mentioned in Table S1 of the Supporting Information, which clearly depicts that these values of wear rates are tremendously reduced in the presence of SZMOs. The running-in wear rate is found to be larger than the steady-state wear rate. The plots of running-in and steady-state wear rate are given in Figures S4 and S5 of th eSupporting Information. The lower value of the steady-state wear rate is desirable as it is a measure of machine life. It has been observed that the reduction in the wear rate is also size dependent, and the value of the steady-state wear rate is successively reduced to 69%, 66%, and 55%, respectively, for SZMO, SZMO-1, and SZMO-2 NPs. The performance of these additives as a function of load has been evaluated at different loads of 294, 392, 490, 588 and 686 N for 30 min of test duration in paraffin oil (Figure 7). The values of MWD at each load are higher in the case of plain paraffin oil than those of its admixtures. It is shown in the figure that the paraffin oil bears load only up to 490 N, whereas SZMO-1 and SZMO-2 NPs bear loads up to 588 N. The smallest SZMO NPs successfully bear loads up to 686 N. Being the smallest, these NPs act as nanobearing and/or undergo

Figure 5. Comparison of MWD and COF of steel balls lubricated with different nanoparticles in paraffin oil at 392 N, rotating speed of 1200 rpm, temperature of 75 °C, test duration of 60 min, and concentration of 0.25% w/v of nanoparticles.

diameter (MWD) in the presence and absence of 0.25% w/v SZnO and SZMOs NPs in paraffin oil at a constant load of 392 N. In general, on addition of nanoparticles to the paraffin oil, both the COF and MWD are significantly reduced. The mean wear scar diameter is larger in the case of paraffin oil alone, but in the presence of different SZMOs and SZnO nanoparticles, it appreciably reduces. The value of MWD is magnificently reduced in the presence of SZMOs NPs compared to SZnO NPs. Among the SZMOs NPs, the value of MWD and average COF successively decreases for SZMO-2, SZMO-1, and SZMO. The values of MWD and COF were found to be lowest for the surface supplemented with a blend of SZMO NPs (27 nm), whereas the largest MWD and COF values are observed in the case of SZMO-2 (44 nm) NPs. The lowering of both MWD and COF values in the presence of SZMOs NPs is D


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Proposed Mechanism. Since the friction and wearreducing properties of SZMOs NPs in paraffin oil increase with a decrease in nanoparticles size, this indicates that the observed tribological behavior is size dependent. Further, the superior tribological behavior of SZMOs over SZnO NPs shows the effect of Mg-doping on ZnO. Thus, both of these characteristics (size and doping) are responsible for the enhanced tribological properties of ZnO. Xie et al.52 have reported that the thermal conductivity of MgO NPs is 3.5-fold higher than that of ZnO NPs. Thus, it is expected that doping of Mg into ZnO might have increased thermal conductivity. As a result, these Mg-doped nanoparticles reduce the asperity temperature, acting as coolant and finally resisting adhesion. The antiwear mechanism of the investigated additives may follow four different processes: (i) The nanoparticles may melt and become welded on the interacting surfaces to form a tribofilm, which may prevent direct asperity−asperity contact. (ii) These may act as a third body on rubbing surfaces, i.e., like nanobearings. (iii) These may get tribosintered on the surfaces under operating conditions. (iv) These nanoparticles may function as coolants, which resist the increase in asperity− asperity temperature preventing welding.43 These SZMOs nanoparticles may follow all the aforesaid pathways except the first one because the melting temperatures of metallic nanoparticles are very high. Under a lower load, these nanoparticles act as nanobearing and coolants. At higher loads, they may follow the coolant, nanobearing, and/or tribosinterization mechanisms. However, with an increase in load, the amount of Zn and Mg on the worn surfaces also increases showing that the tribosinterization mechanism is predominant at higher loads, which is evident from the EDX analysis. Therefore, SZMOs NPs follow either the nanobearing and/or tribosinterization mechanisms and act as coolant at the steel−steel interface under operating conditions, minimizing metal−metal contact, and thus, friction and wear are reduced. Surface Characterization. Surface Morphology. The surface morphology of wear scars on the steel ball surface lubricated with paraffin oil, SZMO (27 nm), and SZMO-2 (44 nm) at 392 N for a 60 min time duration was studied by scanning electron microscopy (ZEISS SUPRA 40) and atomic force microscopy (Nanosurf easyscan2). The SEM images for the surface lubricated with paraffin oil show much wider wear scars due to severe scuffing apparent in Figures 9a,b. However, the width of the wear scar is smaller in size for the steel lubricated with a blend of SZMOs nanoparticles (Figures 9c− f). These NPs filled the surface irregularities resulting in smoothening of the surfaces. As expected from tribological studies, MWD in the case of SZMO is found to be quite smaller than that of SZMO-2, indicating its excellent AW properties. The SEM images of the worn surface lubricated with and without additives at high load (490 N) also support the results obtained in the AW test (Figure S6, Supporting Information). Surface smoothness of the wear track lubricated with an admixture of nanoadditives after the tribological tests was examined by contact mode AFM at a 392 N load for a 60 min test duration. The 2D and 3D AFM images of the wear scar in the presence of base oil and SZMO NPs along with several roughness parameters are shown in Figure 10. From the figure, it is evident that the area as well as the line roughness are found to be extremely large (Sq = 556 nm; Rq = 537 nm) in the case of the paraffin oil, whereas in the presence of SZMO, SZMO-1, and SZMO-2 NPs (Sq = 47−95 nm; Rq = 41−120 nm) these have magnificently reduced. A large difference in the average

Figure 7. Variation of mean wear scar diameter with applied load for paraffin oil containing 0.25% w/v of SZMOs nanoparticles for a 30 min test duration.

tribosinterization in between the steel−steel interface and thus prevent adhesion on the surfaces.43,46 Coefficient of Friction Test. To investigate the COF behavior of these SZMOs NPs at a steady-state zone, the friction test has been performed according to ASTM D5183. Figure 8 displays a load ramp test with the increment of a 98 N

Figure 8. Variation of frictional torque as a function of step loading (with an increment of 98 N load for every 10 min of the test run) and time for different SZMOs nanoparticles: sliding speed, 600 rpm; temperature, 75 °C, and additive concentration, 0.25% w/v.

load for every 10 min of the test run (600 rpm; 75 °C) stepped up from 98 N to 2254 N for paraffin oil with and without additives. From the figure, it is apparent that the COF of paraffin oil increases drastically when the applied load increases from 980 to 1078 N, whereas a very small increase in COF is observed in the case of the tested nanoparticles for the same increment of load. It is evident that all these NPs are better friction modifiers compared to the base oil. The SZMO NPs successfully carry the load up to 2254 N with a lower value of COF, whereas SZMO-1 and SZMO-2 NPs bear the load only up to 2058 and 1960 N, respectively. These results indicate that the doped NPs are excellent friction modifiers, and their behavior follows the same trend as observed in the case of the antiwear test. E


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Figure 9. SEM micrographs of the worn steel surface lubricated with different nanoparticles in paraffin oil for a 60 min test duration at 392 N applied load: (a, b) Paraffin oil, (c,d) SZMO, and (e,f) SZMO-2 nanoparticles.

peak−valley height is obtained for the tribopairs lubricated with paraffin oil alone (2.63 μm), whereas very small surface irregularities are found in the case of SZMOs (300−680 nm). These additives are capable of reducing area surface roughness up to 91%. Thus, the AFM images also proved the potential application of these Mg-doped ZnO NPs toward minimizing friction and wear. The AFM analysis of the steel ball lubricated with SZMOs NPs under a high load was also investigated, corresponding to the 2D and 3D images shown in Figure S7 of the Supporting Information. As the load increases, the values of area roughness, line roughness, and also peak−valley height increases. The AFM studies at high load also support the results obtained from SEM investigation under similar test conditions. Tribochemistry. In order to investigate the tribochemistry of paraffin oil and its blends with SDS-stabilized Mg-doped ZnO nanoparticles, the EDX analysis has been performed. The EDX spectra of a worn steel surface lubricated with 0.25% w/v of

SZMO nanoparticles under a 392 N load have been shown and compared with base lube. Figure 11a shows the prominent additional peaks for Zn and Mg elements on the worn surface lubricated with SZMO, whereas these elements are completely absent in the case of EDX spectra of a surface lubricated with base oil shown in Figure 11b. The elemental mapping (Figure S8,Supporting Information) shows that the distribution of Zn, Mg, and O are throughout the surface of the wear scar. The occurrence of the additives’ constituents on the sliding surface confirmed that the SZMO nanoparticles illustrate the referred tribosinterization mechanism. The atomic % of these elements for the surface supplemented with SZMO (27 nm) and SZMO2 (44 nm) further increases as the load increases to 490 N (Figure S9, Supporting Information), indicating that the tribosinterization phenomenon is predominantly favorable at high load. Under extreme conditions, these nanoparticles may fill the valleys of interacting surfaces, and the rest of the F


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Figure 10. 2D and 3D AFM images of the worn steel surface lubricated with different additives in paraffin oil for a 60 min test duration at a 392 N applied load: (a,b) Paraffin oil, (c,d) SZMO, (e,f) SZMO-1, and (g,h) SZMO-2 nanoparticles.

respectively. The suspension stability of blends of 0.10% SDS with ZMOs nanoparticles in paraffin oil was confirmed from UV−visible spectra, and it remains almost unaffected up to 48 h. The blends of SZMOs nanoparticles show better antiwear properties than SZnO nanoparticles. The SZMOs nanoparticles as additives in paraffin oil exhibited excellent friction reduction and had antiwear and load-carrying properties in order of decreasing particle size. The running-in and steady-state wear rates of SZMOs nanoparticles have been found to be much lower than paraffin base oil. The SEM and AFM studies also support the results obtained from the tribological studies. EDX analysis shows the presence of Zn and Mg elements on the sliding surface emphasizing the role of these additives in enhancing lubrication. Thus, being free from sulfur, phosphorus, and halogens, these novel nanoparticles exhibit excellent tribological properties and have the potential to be developed as low SAPS antiwear additives for boundary lubrication conditions.

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Figure 11. EDX analysis data of a worn steel surface lubricated with paraffin oil in the presence and absence of different additives for a 60 min test duration at a 392 N applied load: (a) SZMO nanoparticles and (b) paraffin oil.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00472. Experimental details of tribological testings, instrumentation details, EDX spectra of ZMO NPs, variation of MWD and MWV with respect to time, wear rates of paraffin oil and additives, SEM and AFM images of surfaces lubricated with paraffin oil and additives at 490N for 30 min, elemental mapping of worn surfaces lubricated with SZMO NPs at 392 N for 60 min of test duration, EDX spectra of worn surfaces lubricated with SZMO and SZMO-2 at 490N for 30 min of test duration. (PDF)

nanoparticles act as nanobearing, thus preventing steel−steel interfaces from being welded.

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CONCLUSIONS In summary, the magnesium-doped zinc oxide (Zn0.88Mg0.12O; ZMO) nanoparticles were successfully prepared by an autocombustion method, and the variation in the particle size of ZMOs NPs was made by further increasing the temperature. Formation of doped nanoparticles was confirmed by XRD, which correlates very well with the SAED pattern of the TEM images. The crystallite size of SZMOs nanoparticles was found to be 27, 39, and 44 nm for SZMO, SZMO-1, and SZMO-2, G


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(19) Somers, A. E.; Biddulph, S. M.; Howlett, P. C.; Sun, J.; MacFarlane, D. R.; Forsyth, M. A comparison of phosphorus and fluorine containing IL lubricants for steel on aluminium. Phys. Chem. Chem. Phys. 2012, 14, 8224−8231. (20) Philippon, D.; De Barros-Bouchet, M. I.; Lerasle, O.; Le Mogne, T.; Martin, J. M. Experimental simulation of tribochemical reaction between borates esters and steel surface. Tribol. Lett. 2011, 41, 73−82. (21) Furlong, O.; Miller, B.; Kotvis, P.; Adams, H.; Tysoe, W. T. Shear and thermal effects in boundary film formation during sliding. RSC Adv. 2014, 4, 24059−24066. (22) Morina, A.; Neville, A.; Priest, M.; Green, J. H. ZDDP and MoDTC interactions and their effects on tribological performancetribofilm characteristics and its evolution. Tribol. Lett. 2006, 24, 243− 256. (23) Jaiswal, V.; Gupta, S. R.; Rastogi, R. B.; Kumar, R.; Singh, V. P. Evaluation of antiwear activity of substituted benzoylhydrazones and their copper(II) complexes in paraffin oil as efficient low SAPS additives and their interactions with the metal surface using density functional theory. J. Mater. Chem. A 2015, 3, 5092−5109. (24) Ratoi, M.; Niste, V. B.; Alghawel, H.; Suen, Y. F.; Nelson, K. The impact of organic friction modifiers on engine oil tribofilms. RSC Adv. 2014, 4, 4278−4285. (25) Martin, J. M.; Onodera, T.; Minfray, C.; Dassenoy, F.; Miyamoto, A. The origin of anti-wear chemistry of ZDDP. Faraday Discuss. 2012, 156, 311−323. (26) Mosey, N. J.; Woo, T. K. A quantum chemical study of the unimolecular decomposition mechanisms of zinc dialkyldithiophosphate antiwear additives. J. Phys. Chem. A 2004, 108, 6001−6016. (27) Spikes, H. The History and mechanism of ZDDP. Tribol. Lett. 2004, 17, 469−489. (28) Spikes, H. Low and zero-sulphated ash, phosphorus and sulphur antiwear additives for engine oils. Lubr. Sci. 2008, 20, 103−136. (29) Korcek, S.; Sorab, J.; Johnson, M. D.; Jensen, R. K. Automotive lubricants for the next millennium. Ind. Lubr. Tribol. 2000, 52, 209− 220. (30) Rokosz, M. J.; Chen, A. E.; Lowe-Ma, C. K.; Kucherov, A. V.; Benson, D.; Paputa Peck, M. C.; McCabe, R. W. Characterization of phosphorus-poisoned automotive exhaust catalysts. Appl. Catal., B 2001, 33, 205−215. (31) Bartz, W. J. Ecotribology: Environmentally acceptable tribological practices. Tribol. Int. 2006, 39, 728−733. (32) Isaksson, M.; Frick, M.; Gruvberger, B.; Ponten, A.; Bruze, M. Occupational allergic contact dermatitis from the extreme pressure (EP) additive zinc bis ((O,O′-di-2-ethylhexyl) dithiophosphate) in neat oils. Contact Dermatitis 2002, 46, 248−249. (33) Cisson, C. M.; Rausina, G. A.; Stonebraker, P. M. Human health and environmental hazard characterization of lubricating oil additives. Lubr. Sci. 1996, 8, 145−177. (34) McFall, D. Automakers push ‘Start’ button on GF-5 oils. Lube Rep. 2004, 4, 31. (35) Canter, N. Special Report: Trends in extreme pressure additives. Tribology and Lubrication Technology 2006, 10, 10−17. (36) Jaiswal, V.; Kalyani; Rastogi, R. B.; Kumar, R. Tribological studies of some SAPS-free Schiff bases derived from 4-aminoantipyrine and aromatic aldehydes and their synergistic interaction with borate ester. J. Mater. Chem. A 2014, 2, 10424−10434. (37) Astruc, D.; Boisselier, E.; Ornelas, C. Dendrimers designed for functions: From physical, photophysical, and supramolecular properties to applications in sensing, catalysis, molecular electronics, photonics, and nanomedicine. Chem. Rev. 2010, 110, 1857−1959. (38) Weintraub, B.; Zhou, Z.; Li, Y.; Deng, Y. Solution synthesis of one-dimensional ZnO nanomaterials and their applications. Nanoscale 2010, 2, 1573−1587. (39) Wu, J. M.; Chen, Y. R.; Lin, Y. H. Rapidly synthesized ZnO nanowires by ultraviolet decomposition process in ambient air for flexible photodetector. Nanoscale 2011, 3, 1053−1058. (40) Mirin, N. A.; Ali, T. A.; Nordlander, P.; Halas, N. J. Perforated semishells: Far-field directional control and optical frequency magnetic response. ACS Nano 2010, 4, 2701−2712.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91 5422368428. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors, Ms. Kalyani is thankful to CSIR-New Delhi, India for financial assistance as Senior Research Fellowship. The authors are thankful to the Incharge, CIFC, IIT (BHU) Varanasi, India for providing SEM with EDX facilities. The authors are also thankful to Head, SAIF Centre, IIT Bombay for carrying out TEM studies.

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REFERENCES

(1) Singh, A.; Gandra, R. T.; Schneider, E. W.; Biswas, S. K. Lubricant degradation and related wear of steel pin in lubricated sliding against a cast steel disc. ACS Appl. Mater. Interfaces 2011, 3, 2512−2521. (2) Unnikrishnan, R.; Jain, M. C.; Harinarayan, A. K.; Mehta, A. K. Additive-additive interaction: an XPS study of the effect of the ZDDP on the AW/EP characteristics of molybdenum based additives. Wear 2002, 252, 240−249. (3) Rastogi, R. B.; Yadav, M.; Bhattacharya, A. Application of molybdenum complexes of 1-aryl-2,5-dithiohydrazodicarbonamides as extreme pressure lubricant additives. Wear 2002, 252, 686−692. (4) Vipper, A. B. Antioxidant properties of engine oil detergent additives. Lubr. Sci. 1996, 9, 61−70. (5) Wiklund, P. The response to antioxidants in base oils of different degrees of refining. Lubr. Sci. 2007, 19, 169−182. (6) O’Connor, S. P.; Crawford, J.; Cane, C. Over based lubricant detergents- a comparative study. Lubr. Sci. 1994, 6, 297−325. (7) Gisser, H.; Messina, J.; Snead, J. Hydroxyaryl stearic acids as oxidation and rust inhibitors in lubricants. Ind. Eng. Chem. 1956, 48, 2001−2004. (8) Smeeth, M.; Spikes, H.; Gunsel, S. Performance of viscosity index improvers in lubricated contacts. Langmuir 1996, 12, 4594−4598. (9) He, G.; Wang, X.; Zhou, M.; Zhang, X.; Qiao, J. Novel thermos thickening lubricant with elastomeric nano-particles. RSC Adv. 2015, 5, 67343−67347. (10) Li, X.; Zhao, Y.; Wu, W.; Chen, J.; Chu, G.; Zou, H. Synthesis and characterizations of graphene-copper nanocomposites and their antifriction application. J. Ind. Eng. Chem. 2014, 20, 2043−2049. (11) Ratoi, M.; Niste, V. B.; Alghawel, H.; Suen, Y. F.; Nelson, K. The impact of organic friction modifiers on engine oil tribofilms. RSC Adv. 2014, 4, 4278−4285. (12) Shah, F. U.; Glavatskih, S.; Antzutkin, O. N. Boron in tribology: from borates to ionic liquids. Tribol. Lett. 2013, 51, 281−301. (13) Shah, F. U.; Glavatskih, S.; Antzutkin, O. N. Synthesis, physicochemical, and tribological characterization of S-di-n-octoxyboron O,O′-di-n-octyldithiophosphate. ACS Appl. Mater. Interfaces 2009, 1, 2835−2842. (14) Karmakar, G.; Ghosh, P. Soybean oil as a biocompatible multifunctional additive for lubricating oil. ACS Sustainable Chem. Eng. 2015, 3, 19−25. (15) He, Z.; Rao, W.; Ren, T.; Liu, W.; Xue, Q. The tribochemical study of some N-containing heterocyclic compounds as lubricating oil additives. Tribol. Lett. 2002, 13, 87−93. (16) Zeng, X.; Li, J.; Wu, X.; Ren, T.; Liu, W. The tribological behaviors of hydroxyl-containing dithiocarbamate-triazine derivatives as additives in rapeseed oil. Tribol. Int. 2007, 40, 560−566. (17) Gao, F.; Furlong, O.; Kotvis, P. V.; Tysoe, W. T. Reaction of tributylphosphite with oxidized iron: Surface and tribological chemistry. Langmuir 2004, 20, 7557−7568. (18) Kim, B.; Mourhatch, R.; Aswath, P. Properties of tribofilms formed with ashless thiophosphate and zinc dialkyldithiophosphateunder extreme pressure conditions. Wear 2010, 268, 579−591. H


Research Article

ACS Sustainable Chemistry & Engineering (41) Huang, T.; Xin, Y.; Li, T.; Nutt, S.; Su, C.; Chen, H.; Liu, P.; Lai, Z. Modified graphene/polyimide nanocomposites: Reinforcing and tribological effects. ACS Appl. Mater. Interfaces 2013, 5, 4878−4891. (42) Huang, H.; Hu, H.; Qiao, S.; Bai, L.; Han, M.; Liu, Y.; Kang, Z. Carbon quantum dot/CuSx nanocomposites towards highly efficient lubrication and metal wear repair. Nanoscale 2015, 7, 11321−11327. (43) Battez, A. H.; Gonzalez, R.; Viesca, J. L.; Fernandez, J. E.; Fernandez, J. M. D.; Machado, A.; Chou, R.; Riba, J. CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants. Wear 2008, 265, 422−428. (44) Zhang, L.; Chen, L.; Wan, H.; Chen, J.; Zhou, H. Synthesis and tribological properties of stearic acid-modified anatase (TiO2) nanoparticles. Tribol. Lett. 2011, 41, 409−416. (45) Bai, G.; Wang, J.; Yang, Z.; Wang, H.; Wang, Z.; Yang, S. Preparation of a highly effective lubricating oil additive-ceria/graphene composite. RSC Adv. 2014, 4, 47096−47105. (46) Jaiswal, V.; Rastogi, R. B.; Kumar, R.; Singh, L.; Mandal, K. D. Tribological studies of stearic acid-modified CaCu2.9Zn0.1Ti4O12 nanoparticles as effective zero SAPS antiwear lubricant additives in paraffin oil. J. Mater. Chem. A 2014, 2, 375−386. (47) Hu, Z. S.; Dong, J. X.; Chen, G. X.; He, J. Z. Preparation and tribological properties of nanoparticles lanthanum borate. Wear 2000, 243, 43−47. (48) Yang, Y.; Jin, Y.; He, H.; Wang, Q.; Tu, Y.; Lu, H.; Ye, Z. Dopant-induced shape evolution of colloidal nanoparticles: The case of zinc oxide. J. Am. Chem. Soc. 2010, 132, 13381−13394. (49) Wang, L.; Zhou, X. C.; Li, Q. Q.; Yuan, C. Q.; Yan, X. P.; Chen, Y. H. Application of metal self-repairing additives on cylinder-piston ring rubbing pairs. Proceedings of CIST and ITS-IFToMM 2008, Beijing, China. (50) Mohammed, A. H. A.-K.; Ahmad, M. R.; Al-Messri, Z. A. K. Synthesis, characterization and evaluation of overbased magnesium fatty acids detergent for medium lubricating oil. Iraqi J. Chem. Petrol. Engg. 2013, 14, 1−9. (51) Hu, Z. S.; Lai, R.; Lou, F.; Wang, L. G.; Chen, Z. L.; Chen, G. X.; Dong, J. X. Preparation and tribological properties of nanometer magnesium borate as lubricating oil additive. Wear 2002, 252, 370− 374. (52) Xie, H.; Yu, W.; Chen, W. MgO nanofluids: higher thermal conductivity and lower viscosity among ethylene glycol-based nanofluids containing oxide nanoparticles. J. Exp. Nanosci. 2010, 5, 463−472. (53) Kalyani; Jaiswal, V.; Rastogi, R. B.; Kumar, D. The investigation of different particle size magnesium-doped zinc oxide (Zn0.92Mg0.08O) nanoparticles on the lubrication behavior of paraffin oil. Appl. Nanosci. 2015, DOI: 10.1007/s13204-015-0471-1. (54) Xiong, H. M.; Shchukin, D. G.; Mohwald, H.; Xu, Y.; Xia, Y. Y. Sonochemical synthesis of highly luminescent zinc oxide nanoparticles doped with magnesium (II). Angew. Chem., Int. Ed. 2009, 48, 2727− 2731.

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