Well Dispersive TiO2 Nanoparticles as Additives for Improving the

Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 10379−10390

pubs.acs.org/IECR

Well Dispersive TiO2 Nanoparticles as Additives for Improving the Tribological Performance of Polyalphaolefin Gel Lubricant Ruochong Zhang,†,‡ Dan Qiao,† Xuqing Liu,§ Zhiguang Guo,†,∥ Litian Hu,*,† and Lei Shi*,† †

Ind. Eng. Chem. Res. 2018.57:10379-10390. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/25/19. For personal use only.

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Materials, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. ∥ Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430062, China ABSTRACT: The dispersibility of TiO2 nanoparticles in polyalphaolefin (PAO) are improved by the interaction between the chemically modified TiO2 (denoted as TTO) and 1-methyl-2,4-bis (N-octadecylurea) benzene (MOB) gelator. Tribological tests show the outstanding frictionreducing and antiwear performances of PAO10−MOB−TTO under the common friction condition and higher temperature friction condition, respectively. The lubricating property of PAO10−1% MOB−0.5% TTO is further enhanced after UV irradiation for 40 min, which might be attributed to the photocatalytic effect of TTO under UV. The physical adsorption film is enhanced by the immobilization function of MOB. The chemical reaction film including various organic oxides and nitrides are produced for the active element in TTO and MOB. The antiwear performance is further improved by the “mending effect” of TTO with the good dispersibility. The combined effects of various factors promote the excellent lubricating performances of PAO10−1% MOB−0.5% TTO. immobilization function.20,21 Our previous work presented that the sedimentation of WS2 in polyalphaolefin (PAO)8 is prevented by the immobilization function of the gelator.22 However, the dispersibility of the nanosheets is hardly guaranteed just by the interaction of the gelator molecules. The interaction between certain nanoparticles and the gelators in the solvent, which improve the mechanical property of the solvent and the stability of nanoparticles, has been investigated in previous researches.23 Utilization of the interaction between the nanoparticles and LMWG to improve the dispersibility of nanoparticles in base oil has not been investigated up to now, which might be a novel method to abate the agglomeration of nanoparticles. In the present work, gelatinization and chemical modification were conducted to obtain the excellent stability and dispersibility of TiO2 nanoparticles in base oil. 1-Methyl-2,4bis (N-octadecylurea) benzene (MOB) containing a urea group was used as the LMWG with outstanding gel property. The urea group was also designed into the structure of the modification agent of the TiO2 nanoparticles to promote interaction with MOB and improve the dispersibility of TTO

1. INTRODUCTION Some oxide nanoparticles, such as SiO2,1,2 TiO2,3−5 Fe3O4,6 ZnO,7 SnO2,8 and ZrO2,9 have been used as lubricating additives during the past few years because of their good friction-reducing and antiwear performances. Several tens percent decrease of friction coefficient and wear volume are obtained when a little amount of nanomaterials is added into the base oil.10,11 Many reports indicate that nanomaterials, for their small size, possess a better lubricating performance than the corresponding micromaterials, and easily exert the “mending function” on the friction pairs and the “rolling effect” in the lubrication media.12 However, the higher surface energy promotes the agglomeration and sedimentation tendency of nanomaterials in the base oil, which leads to the loss of the nanoparticles’ character and has a negative effect on the lubricating performance.13 Chemical modification of the nanoparticles can effectively alleviate agglomeration and improve their dispersibility in the lubricating oil.4 However, leakage of the lubricating oil in practical application is also a matter worthy of attention. Low molecular weight organic gelators (LMWG) with different structures have been applied to lubricating oils14−16 to trap the oils into a network and change the state of lubricants from liquid to semisolid.17−19 Besides the improvement of the tribological performance, the addition of the LMWG could effectively restrict the fluidity of the base oils through an © 2018 American Chemical Society

Received: Revised: Accepted: Published: 10379

April 18, 2018 July 4, 2018 July 13, 2018 July 13, 2018 DOI: 10.1021/acs.iecr.8b01694 Ind. Eng. Chem. Res. 2018, 57, 10379−10390

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic diagrams of preparation processes of (a) MOB and (b) TTO.

as PAO10−MOB−TiO2, PAO10−MOB−TTO, respectively. The percentage content mentioned in this paper refers to the mass concentration. The preparations of the mixed gel lubricants are according to our previous research.22 2.4. Characterization. The molecular structures of TiO2, TiO2−TDI and TTO were detected by a Nicolet iS10 Fourier Transform Infrared Spectrometer (FTIR) after every step of the chemical reaction. The morphologies of TiO2 and TTO are confirmed by FEI Tecnai F300 high-resolution transmission electron microscope (HRTEM). The comparison of thermal analysis between TiO2 and TTO is used to deduce the organic content in TTO by STA 449 C Jupiter simultaneous TG-DSC from room temperature to 800 °C in air. The interactions between TTO and MOB are corroborated by the shear stress (0.01−1000 Pa, 1 Hz) of RS6000 Rheometer (Germany). The digital pictures and transmission of Agilent Technologies Cary 60 UV−visible spectroscopy are used to investigate the sedimentation properties of PAO10−0.5% TiO2, PAO10−1% MOB−0.5% TiO2, and PAO10−1% MOB−0.5% TTO over time. The dispersion properties of TiO2 and TTO nanoparticles are recorded by JEOL JSM-6701F field emission scanning electron microscopy (FESEM) and HRTEM through the preparation of a MOB xerogel containing the same mass content of the two nanoparticles. The HRTEM samples are prepared according to the following process:23 the sample was prepared through dispersion of 0.1 mg·mL−1 nanoparticles (TiO2 or TTO) in chlorobenzene by stirring 10 min and ultrasound 15 min, then 1 mg·mL−1 MOB was added and heated to dissolve the gelator. The sample was dropped on the copper grid covered with the carbon film when it it was hot and reamined undisturbed until the total volatilization of chlorobenzene.

in the base oil. Better tribological performance and long-term stability of mixed lubricants are expected gains and the lubricating mechanisms of the mixed lubricants for steel are revealed.

2. EXPERIMENTAL SECTION 2.1. Materials. The reagents, including toluene-2,4diisocyanate (TDI), octadecylamine (ODA), chlorobenzene, toluene, and absolute ethanol, were purchased from Aladdin. PAO10 was synthesized according to the previous report.24 TiO2 nanoparticles with the diameter of 25 nm were purchased from Guangzhou HOCH Trading Co., Ltd. The deionized water was obtained by the water purification system. All the reagents used in the present work are of AR grade. 2.2. Preparations of MOB and TTO. MOB was synthesized according to the previous research.25 The preparation of chemical-modified TiO2 was as follows: TiO2−TDI was obtained by mixing TiO2 and sufficient TDI in toluene and stirring the system under reflux for 6 h. TiO2 -TDI was then washed with toluene for five times (5 × 50 mL) to remove the unreacted TDI and dried at 70 °C in the vacuum oven.26 A certain amount of ODA with toluene was added into the toluene with a given mass of TiO2-TDI at the room temperature. After the overnight reaction, the toluene was removed by a reduced pressure distillation. TiO2-TDIODA (TTO) was finally obtained by filtrating with hot toluene for four times (4 × 100 mL) and then being dried in the vacuum oven.25 The preparation processes of MOB and TTO are shown in Figure 1. 2.3. Preparations of Gel Lubricants. The lubricating systems of PAO10 with MOB, TiO2, or TTO are denoted as PAO10−MOB, PAO10−TiO2, or PAO10−TTO respectively, and PAO10 with MOB and TiO2, MOB, and TTO are written 10380

DOI: 10.1021/acs.iecr.8b01694 Ind. Eng. Chem. Res. 2018, 57, 10379−10390

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Figure 2. (a) FTIR spectra of TiO2, TiO2-TDI, and TTO; (b) XRD spectra of TiO2 and TTO.

Figure 3. HRTEM micrographs and EDX elemental counts images of (a and b) TiO2 and (c and d) TTO.

2.5. Tribological Tests. The tribological performances of lubricants are investigated by Optimol SRV-IV oscillating reciprocating friction and wear tester with a ball-on-disk configuration. Both the upper running ball (ø = 10 mm) and the lower stationary disk (ø = 24.0 mm × 7.9 mm) are AISI 52100 steel. The hardness of the friction pairs is 700−800 HV, and the elastic modulus is 206 GPa. The maximum contact pressure is calculated according to Hertz analysis: The maximum contact pressure is related to the load and the contact radius:

P0 =

where R is the radii of the ball and E is the effective elastic modulus. 1 − ν12 1 − ν2 2 1 = + E E1 E2

ν1 and ν2 are Poisson ratio of the ball and the disk and E1 and E2 are the elastic modulus of the corresponding friction pairs. In the present work, ν1 = ν2 = 0.3, E1 = E2 = 206 GPa. Then the maximum contact pressure is 2.70 GPa according to the above formula. The low stationary disk is polished by CW400, CW800, and CW1200 abrasive papers successively under the distilled water and then finally polished with cloth by a metallographic sample polishing machine to obtain the friction pairs with the mean roughness (Ra) about 0.02 μm before the tribological tests. The tribological tests are performed under the following common conditions: load = 200 N; frequency = 25 Hz; amplitude = 1 mm; temperature = room temperature; time = 30 min. The temperature ramp tests of lubricants are

3W 2πa 2

W is an applied normal load; a is the contact radius; which could be obtained by following formula: i 3WR zy zz a = jjj k 4E {

1/3

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Industrial & Engineering Chemistry Research performed from room temperature to 120 °C with the change rate of 20 °C per 5 min. The high temperature tests are recorded under the same condition with the common condition except for the tribological temperature up to 120 °C. All the tribological tests are repeated for five times. The same steel blocks and balls in the tribological tests are used for the convenience of comparison. The mean friction coefficient, real-time friction coefficient, and wear volume are used to measure the tribological performances of different lubricants. What is more, the mean friction coefficient is obtained by the following method: first, the mean friction coefficient value (A1) of the lubricant A during the 30 min is obtained by one test. Then the five mean friction coefficient values (A1, A2, A3, A4, and A5) of lubricant A are obtained for the five repeated tests. Finally, the mean friction coefficient of lubricant A is calculated by the following formula: COF(A) =

Scheme 1. Structure of Organic Material on the Surface of TiO2

A1 + A 2 + A 3 + A 4 + A 5 5

The tribological performances of PAO10 and PAO10−1% MOB−0.5% TTO after the UV treatment are also investigated. The power of the UV lamp is 30 W. Both PAO10 and PAO10−1% MOB−0.5% TTO with the same amount are put into 5 mL beakers. All of the beakers are arrayed 10 cm below the UV lamp and parallel to the direction of the UV lamp to ensure irradiation under the same conditions. The tribological performances of UV-treatment lubricants are studied under the common condition mentioned above. The corresponding three-dimensional morphologies and wear volumes of the wear scars are evaluated by MicroXAM3D noncontact surface mapping profiler. The two-dimensional morphologies are further observed by JSM-5600LV scanning electron microscope (SEM) and OLYMPUSBX51 optical microscope. KEVEX element distribution analysis (EDS) is utilized to investigate chemical composition on the wear scar. The chemical changes of elements of the worn scars after the tribological process were investigated by ESCALAB 250Xi Xray photoelectron spectroscopy (XPS).

Figure 5. Time-lapse images of sedimentation processes of (a) PAO10−0.5% TiO2, (b) PAO10−1% MOB−0.5% TiO2, and (c) PAO10−1% MOB−0.5% TTO.

The peaks of 1654 and 1550 cm−1 are respectively attributed to the stretching vibration of CO and bending vibration of N−H in amide group.28 The peak of 1227 cm−1 is assigned to the C−O vibration.29 What is more, the peak of 2266 cm−1 demonstrates that the other isocyanate in TDI is retained for the relatively weaker reactive activity.30,31 The peak of isoyanate disappears and the peaks of alkyl groups at 2923 and 2851 cm−1 appear after the reaction between TiO2-TDI and ODA, which demonstrates the final product of TTO. Figure 2b demonstrates XRD spectra of TiO2 and TTO.32 The black font of the diffraction peaks in the XRD spectra of TiO2 demonstrates TiO2 in the anatase phase and red font indicates the rutile phase, which shows that the main crystal form of TiO2 is the anatase phase.33 The XRD pattern of TTO shows that there is no crystal transfer between TTO and TiO2 after the series of reactions. 3.2. Morphologies of TiO2 and TTO. Figure 3 shows the morphologies of nonmodified TiO2 and TTO by HRTEM. The morphology of TTO in Figure 3c exhibits the obvious shell−core structure labeled in the red box compared with that of TiO2 nanoparticles in Figure 3a, demonstrating the effective reactions of organics with TiO2. Compared with EDX spectra in Figure 3b,d, the appearance of N element in the TTO further corroborates the existence of a modification layer on the surface of TiO2 nanoparticles. 3.3. TGA Curves. The thermal analysis of TiO2 in Figure 4 shows the 4.13% weight loss during the whole test range, which might be the water content of the material for preservation in the open air.13 It also indicates that TiO2 nanoparticle should be stored in the desiccator or be used after drying. The weight−temperature curve of TTO is almost completely the same as that of TiO2 below 200 °C, which means a similar water content in TiO2 and TTO. A 73.80% residue in TTO after the thermal test indicates the content of the modifier on the surface of the TTO is approximately

3. RESULTS AND DISCUSSION 3.1. Structure of TTO Nanoparticles. The structures of TiO2 and TTO are demonstrated by IR and XRD in Figure 2. The disappearance of the peak at 3433 cm−1 and the appearance of the peak at 3288 cm−1 on the TiO2-TDI curve in Figure 2a indicate the reaction between the hydroxyl groups on the surface of TiO2 and the isocyanate of TDI.27

Figure 4. TGA curves of TiO2 and TTO. 10382


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Figure 6. Transmittances of (a) PAO10−0.5% TiO2, (b) PAO10−1% MOB−0.5% TiO2, and (c) PAO10−1% MOB−0.5% TTO with the evolution of time.

Figure 9. Rheological performances of PAO10−MOB−TiO2, PAO10−1% MOB, and PAO10−MOB−TTO.

mol. The TTO product contained 73.08% TiO2, 22.07% A, and 4.13% water as estimated from the result of TGA. So combined with the mass percentage and the relative molecular weight of A and TiO2−OH, and the ratio of TTO and unmodified TiO2, the coverage is roughly calculated as follows: 73.08 22.07 : ≅ 15: 1 96.87 446

Figure 7. FESEM images of MOB xerogel with the same mass concentration of (a and b) TTO and (c and d) TiO2 nanoparticles.

Then the coverage of TTO is approximately 1/15, which might be the result of steric effect and reaction activity. 3.4. Sedimentation of PAO10−TiO2, PAO10−MOB− TiO2, and PAO10−MOB−TTO. The sedimentation processes of PAO10−0.5% TiO2, PAO10−1% MOB−0.5% TiO2, and PAO10−1% MOB−0.5% TTO are recorded by the digital camera with time elapse photographs. It shows the apparent sedimentation layer at the bottom of the sample bottle in Figure 5a after merely 2 days and that the upper layer becomes gradually clear during the next 45 days, indicating the unstable property of TiO2 in the PAO10. However, TiO2 and TTO nanoparticles remain stable during the whole test period in the PAO10 gel in Figure 5 panels b and c because the immobilization function of MOB prevents the sedimentation of TiO2 and TTO in the base oil. To provide the quantitative analysis of sedimentation, the transmittance of PAO10−0.5% TiO2, PAO8−1% MOB−0.5% TiO2, and PAO10−1% MOB−0.5% TTO were investigated by UV−visible spectroscopy to further record the stability of lubricants. Figure 6 shows that the transmittance of PAO10− 0.5% TiO2, PAO10−1% MOB−0.5% TiO2, and PAO10−1% MOB−0.5% TTO is zero at 0 d, which means both systems do not show obvious sedimentation at the beginning of the test. However, PAO10−0.5% TiO2 presents sedimentation after 4 d because the transmittance value at the wavelength of 800 nm

Figure 8. HRTEM images of MOB xerogel with the (a and b) TiO2 and (c and d) TTO nanoparticles.

22.07% except for the water content. The coverage of TTO is deduced according to the TGA result. It can be deduced from the IR spectra that there is one kind of hydroxyl existing on the surface of the purchased TiO2 molecule because there is only one hydroxyl peak in Figure 2. Then it could be inferred that no more than one organic molecule A in Scheme 1 (the red frame in the following figure) was grafted onto the surface of TiO2. The relative molecular weight of A is MA = 446 g/mol. Every TiO2 molecule possesses one hydroxyl on the surface (TiO2−OH); the relative molecular weight is M1 = 96.87 g/ 10383


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Figure 10. (a) Mean friction coefficients and mean wear volumes (b) real-time friction coefficients of PAO10−1% MOB with different contents of TTO.

Figure 11. (a) Mean friction coefficients and mean wear volumes; (b) real-time friction coefficients of PAO10, PAO10−1% MOB, PAO10−0.5% TiO2, PAO10−1% MOB−0.5% TiO2, PAO10−0.5% TTO, and PAO10−1% MOB−0.5% TTO.

Figure 12. (a) Temperature ramp tests from room temperature to 120 °C with the change rate of 20 °C per 5 min; (b) high temperature lubricating tests of PAO10 and PAO10−1% MOB−0.5% TTO.

structure, which is the reason for immobilization function of MOB gelator. Figure 7c and 7d show many big clusters of TiO2 in the MOB xerogel, demonstrating that the agglomeration of TiO2 nanoparticles would not be prevented although the sedimentation could be alleviated or avoided by the immobilization function of the network structure formed by MOB molecules. From the independent morphologies of the two components, it could be deduced that there is no strong molecular interaction between TiO2 nanoparticles and MOB molecules and TiO2 nanoparticles could agglomerate before being immobilized by MOB. The sizes of TTO clusters in the MOB xerogel are approximately 100 nm in Figure 7a,b, which is much smaller than that of the unmodified TiO2 clusters. Figure 7b also shows that the small TTO clusters are dispersed

increases to 2%. The transmittance reaches more than 50% at 16 d, corroborating the apparent agglomeration and sedimentation of TiO2 nanoparticles in PAO10. The transmittance of PAO10−1% MOB−0.5% TiO2 and PAO10−1% MOB−0.5% TTO remain at zero during the whole test process, indicating that the immobilization function of MOB prevents the sedimentation of TiO2 and TTO. However, the dispersibility of TiO2 or TTO nanoparticles in the gel lubricant needs further investigation although their sedimentation is prevented. 3.5. Dispersibility of TiO2 and TTO in Xerogel. The morphologies of MOB xerogel with the same mass percentage of TTO and TiO2 are recorded by FESEM in Figure 7. It could be seen that the MOB xerogel forms the fibrous network 10384


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Figure 13. (a) The mean friction coefficients of PAO10−1% MOB−0.5% TTO and PAO10 under different UV irradiation time; (b) the real-time friction coefficients of PAO10−1% MOB−0.5% TTO and PAO10 under 40 min UV irradiation.

by 68.45% when the content of TTO in PAO10−1% MOB reached to 0.5%, which shows that PAO10−1% MOB−0.5% TTO possesses the best antiwear performances under the lubricating conditions. The results of Figure 10 demonstrate that PAO10−1% MOB−0.5% TTO exhibits better antiwear performances compared with friction-reduced properties under the common friction conditions. Figure 11 shows the tribological performances of PAO10, PAO10−1% MOB, PAO10−0.5% TiO2, PAO10−1% MOB− 0.5% TiO2, PAO10−0.5% TTO, and PAO10−1% MOB−0.5% TTO to investigate the lubricating function of MOB and TTO in PAO10. The friction curve of PAO10 exhibits a serious running-in period. The mean friction coefficients decrease by 6.15% or 18.97% after the addition of 1% MOB or 0.5% TiO2 into PAO10, which might be due to the formation of the corresponding lubricating films. However, no apparent change of mean friction coefficient is shown among PAO10−0.5% TiO2, PAO10−1% MOB−0.5% TiO2, PAO10−0.5% TTO, and PAO10−1% MOB−0.5% TTO. The mean wear volume in Figure 11a shows that PAO10−1% MOB−0.5% TTO exhibits the lowest wear volume compared with the other lubricants, which further corroborates the excellent antiwear performance of the lubricant. The reasons might be that MOB improves the dispersibility of nanoparticles and promotes the “mending effect” of nanoparticles on the friction surfaces. The tribological performances of PAO10 and PAO10−1% MOB−0.5% TTO under the different temperatures are investigated in Figure 12. It is seen that the huge fluctuation of the running-in period occurs lubricated by PAO10 and the value of friction coefficient gradually reaches 0.5 during the tribological process. The friction coefficient further soars to 0.60 when the temperature changes from 40 to 60 °C, meaning the complete damage of the boundary lubrication films. The phenomenon indicates that PAO10 possesses poor lubricating performance under the higher temperature. However, the friction curve of PAO10−1% MOB−0.5% TTO shows a very stable and low friction coefficient after the running-in period during the whole temperature variation process, which demonstrates that the tribological properties of PAO10−1% MOB−0.5% TTO are dramatically improved compared with that of PAO10 under the temperature ramp tests. Figure 12b shows the friction curves of PAO10 and PAO10−1% MOB− 0.5% TTO lubricated at 120 °C to further investigate the tribological performances under the relatively higher temperature. The friction curve indicates that it is difficult for PAO10 to form the effective boundary lubrication films under 120 °C,

in the internal network structure, further corroborating the interaction between the TTO and MOB molecules. The results show that the modification of TiO2 could efficiently promote the interaction between the nanoparticles and MOB, thereby abating the aggregation of TiO2 nanoparticles. The dispersibility of TiO2 and TTO with the same mass concentration in the xerogel is also investigated by HRTEM in Figure 8. Figure 8panels a and b show the unbroken expanse of TiO2 groups among the fibrous structure of MOB, which implies the aggregation of TiO2 nanoparticles in the base solvent. The aggregation of TTO nanoparticles dramatically decreases in Figure 8 panels c and d, showing the relatively homogeneous dispersion among the MOB fibrous structure. The comparison might demonstrate that the interactions between TTO and MOB abate the aggregation of nanoparticle. 3.6. Interaction between TTO and MOB. The interactions between TTO nanoparticles and MOB molecules are further corroborated by the rheological properties of PAO10−MOB−TiO2, PAO10−1% MOB, and PAO10− MOB−TTO in Figure 9. All of the three systems have the same mass of PAO10 and MOB and the mass of added TiO2 is as same as that of TTO. Figure 9 shows that the storage moduli (G′) decreases from more than 1200 Pa to nearly 400 Pa after adding TiO2 nanoparticles into PAO10−1% MOB and the critical strain also decreases from 46.69 to 27.05 Pa at the same time, which indicates the addition of TiO2 would weaken the immobilization function of MOB. However, the storage module (G′) increases from approximately 1200 Pa to nearly 2000 Pa and the critical strain increases from 46.69 to 84.04 Pa after adding TTO into PAO10−1% MOB, which means that TTO would participate in the gelatinization of MOB molecules and enhance the immobilization function of MOB. 3.7. Tribological Performances. The tribological performances of PAO10−1% MOB with different concentrations of TTO are investigated in Figure 10. The mean friction coefficient in Figure 10a decreases by 6.56% after the addition of 0.1% TTO into PAO10−1% MOB and presents the lowest value (a decrease of 16.39%) when the content of TTO reaches 0.5%. Integrating with the friction curve in Figure 10b, PAO10−1% MOB−0.5% TTO shows the attenuated fluctuation of the running-in period and the lowest friction coefficient after the apparent fluctuation. The antiwear performances of PAO10−1% MOB with different contents of TTO are similar to the friction-reduced performances. The wear volume of PAO10−1% MOB−0.1% TTO decrease by 25.49% compared with that of PAO10−1% MOB and the value further decreases 10385


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Figure 14. SEM morphologies with different magnifications, 3D optical microscopic images, and 2D optical microscopic images of (a1−a4) PAO10, (b1-b4) PAO10−1% MOB, (c1−c4) PAO10−0.5% TiO2, (d1−d4) PAO10−1% MOB−0.5% TiO2, (e1−e4) PAO10−0.5% TTO, and (f1−f4) PAO10−1% MOB−0.5% TTO.

TTO shows the best lubricating performance when the UV irradiation time lasts for 40 min, indicating that the modification could not affect the photocatalysis of TiO2, and the intermediates produced by UV radiation are beneficial to the tribological process. However, the mean friction coefficient of the mixed lubricating system gradually increases with further prolongation of irradiation time, which might be due to the excessive degradation of PAO10. It indicates that the proper UV irradiation is suitable for the lubricating performances of PAO10−1% MOB−0.5% TTO. 3.8. Surface Analysis of Worn Scars. Figure 14 shows SEM morphologies with different magnifications, 3D optical microscopic images, and 2D optical microscopic images of

which is consistent with the results of temperature ramp tests. However, the friction curve of PAO10−1% MOB−0.5% TTO is able to remain stable after the running-in period under the same conditions, which further demonstrates the better lubricating performance of PAO10−1% MOB−0.5% TTO under the higher temperature. TiO2 possesses photocatalysis in organic media under UV irradiation, which could produce intermediates including ketone, alcohol, aldehyde, and organic acid.35,36 Figure 13 shows the friction coefficients of PAO10−1% MOB−0.5% TTO and PAO10 under the different UV irradiation time. The mean friction coefficients of PAO10 have little change with different irradiation times. However, PAO10−1% MOB−0.5% 10386


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Figure 15. SEM micrographs and the corresponding element mappings on the worn scars lubricated by PAO10, PAO10−1% MOB, PAO10−0.5% TiO2, PAO10−1% MOB−0.5% TiO2, PAO10−0.5% TTO, and PAO10−1% MOB−0.5% TTO.

the worn scar slightly decrease under all the lubrication conditions, showing the generation of tribochemical reactions during the lubricating process. The boundary lubrication film might be easily formed after adding additives into PAO10 for the obvious distribution of C element on the worn scar. The distributions of O and Ti become apparent on the surface of the worn scar after the addition of TiO2 or TTO, which might indicate the existence of physical adsorption and tribochemical reaction about TiO2 and TTO on the scar surfaces. The mending effect of TTO on the rough surface might be one of the reasons for the excellent antiwear performance of PAO10− 1% MOB−0.5% TTO.33,34 No signal of N is investigated by EDS because of too little content. XPS could be used to investigate the chemical changes of elements on the wear scar. The XPS spectra of O 1s, C 1s, N 1s, and Ti 2p of TTO, MOB and the worn scars lubricated by different lubricants are investigated in Figure 16 to deduce the lubricating mechanisms of PAO10−1% MOB−0.5% TTO. The C−OH and O−CO are deduced to be produced on the worn scar combining the C 1s of 286.2 eV, 288.6 eV and the O 1s of 530.7 eV, 532.2 eV, 533.4 eV.37,38 The O 1s spectra at 529.9 eV might be ascribed to the iron oxides.39 Some new organic oxide groups are inferred on the worn scar lubricated by PAO10−1% MOB−0.5% TTO for the peak of 288.1 eV, 283.8 eV in C 1s and 531.4 eV in O 1s,40−43 which might be the formation of an effective boundary lubricating film. The C

PAO10, PAO10−1% MOB, PAO10−0.5% TiO2, PAO10−1% MOB−0.5% TiO2, PAO10−0.5% TTO, and PAO10−1% MOB−0.5% TTO, respectively. The worn surfaces of PAO10−1% MOB and PAO10 show similar morphologies for the apparent adhesion effect and furrow phenomenon in Figure 14a2 and 14b2. The change of worn scar is similar after the addition of MOB into PAO10 or PAO10−0.5% TiO2, which both exhibit the abated adhesion and the aggravated furrow on the worn surfaces. In Figure 14e2, there are more furrows rather than adhesions on the worn scar lubricated by PAO10−0.5% TTO compared with the worn scar lubricated by PAO10, and the furrow phenomenon in Figure 14f2 is alleviated efficiently after the addition of MOB into the PAO10−0.5% TTO, showing the smoothest worn surface compared with the surface lubricated by the other lubricants. The obvious clusters are shown on the worn scars surfaces of PAO10−0.5% TiO2 and PAO10−1% MOB−0.5% TiO2 in Figure 14c4 and 14d4, which might be the result of the inhomogeneous distribution of TiO2 in the base oil. And the clusters on the surfaces of PAO10−0.5% TTO and PAO10− 1% MOB−0.5% TTO worn scars in Figure 14e4 and 14f4 are obviously reduced, which further manifests the good dispersibility of TTO in the PAO10. EDS is an effective tool to evaluate the distribution of characteristic elements on the worn scars lubricated with different lubricants. Figure 15 shows that the contents of Fe on 10387


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Figure 16. XPS spectra of C 1s, O 1s, Ti 2p, and N 1s of TTO, MOB, and the worn scars lubricated by different lubricants.

Figure 17. Schematic of lubricating mechanism of PAO10−1% MOB−0.5% TTO.

the lubricating process is deduced together with the results of XPS and EDS. The peaks of Ti 2p at 463.1 and 453.6 eV demonstrate the existence of Ti element with different valence.51−54 The results shows that a complicate chemical reaction film are formed on the worn surface lubricated by PAO10−1% MOB−0.5% TTO for the existence of both additives. The physical adsorption film of the base oil is also enhanced by the immobilization of MOB. Integrating the “mending effect” of TTO, the outstanding tribological performance of PAO10−1% MOB−0.5% TTO is presented as the lubricant of steel and steel friction pair. 3.9. Lubricating Mechanism. Figure 17 shows the lubricating mechanisms of PAO10−1% MOB−0.5% TTO. The excellent lubricating property of PAO10−1% MOB−0.5% TTO might be attributed to the mending effect of TTO nanoparticles and the effective formations of physical

1s spectra of PAO10, PAO10−0.5% TTO, and PAO10−1% MOB−0.5% TTO show the strong peak at 284.8 eV, which might be ascribed to the existence of C−C by the physical adsorption or the tribochemical reaction products of the base oil.44 C−N is speculated to be generated during the friction process combining the new peak of C 1s at 287.2 eV and N 1s at 398.7 eV after the addition of the TTO to PAO10.45,46 The groups of amide and CN exist on the worn scar of PAO10− 1% MOB−0.5% TTO for the peaks of 399.9 and 402.1 eV in N 1s spectra.47,48 The activity of N in the molecules of MOB and TTO promotes the formation of the effective tribochemical reaction film and improves the lubricating performance of PAO10−1% MOB−0.5% TTO. The O 1s spectra at 529.9 eV and the Ti 2p spectra at 458.3 eV on the worn scars of PAO10−0.5% TTO and PAO10−1% MOB−0.5% TTO is ascribed to the TiO2.49,50 The mending effect of TiO2 during 10388


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Industrial & Engineering Chemistry Research adsorption film and tribochemical reaction film. The interaction between the TTO and MOB could improve the dispersion of the nanoparticles in PAO10, which is conducive for TTO to exert the “mending effect” on the worn surface. The immobilization function of MOB promotes the effective formation of physical adsorption film by the base oil. Integrating with the result of EDS and XPS, the strong tribochemical reaction film, containing organic oxides, iron oxides and nitride, is formed on the worn scar lubricated by PAO10−1% MOB−0.5% TTO. These factors work together to promote the excellent lubricating performance of PAO10− 1% MOB−0.5% TTO under different tribological conditions.

acid-modified TiO2 nanoparticles as lubricant additives. Mater. Sci. Eng., A 2003, 359, 82. (4) Zhang, L.; Chen, L.; Wan, H. Q.; Chen, J. M.; Zhou, H. D. Synthesis and Tribological Properties of Stearic Acid-Modified Anatase (TiO2) Nanoparticles. Tribol. Lett. 2011, 41, 409. (5) Ingole, S.; Charanpahari, A.; Kakade, A.; Umare, S. S.; Bhatt, D. V.; Menghani, J. Tribological behavior of nano TiO2 as an additive in base oil. Wear 2013, 301, 776. (6) Zhou, G. H.; Zhu, Y. F.; Wang, X. M.; Xia, M. J.; Zhang, Y.; Ding, H. Y. Sliding tribological properties of 0.45% carbon steel lubricated with Fe3O4 magnetic nano-particle additives in baseoil. Wear 2013, 301, 753. (7) Hernandez Battez, A.; Fernandez Rico, J. E.; Navas Arias, A.; Viesca Rodriguez, J. L.; Chou Rodriguez, R.; Diaz Fernandez, J. M. The tribological behaviour of ZnO nanoparticles as an additive to PAO6. Wear 2006, 261, 256. (8) Bonu, V.; Kumar, N.; Das, A.; Dash, S.; Tyagi, A. K. Enhanced Lubricity of SnO2 Nanoparticles Dispersed Polyolester Nanofluid. Ind. Eng. Chem. Res. 2016, 55, 2696. (9) Hernández Battez, A.; González, R.; Viesca, J. L.; Fernández, J. E.; Díaz Fernández, J. M.; Machado, A.; Chou, R.; Riba, J. CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants. Wear 2008, 265, 422. (10) Wu, Y. Y.; Tsui, W. C.; Liu, T. C. Experimental analysis of tribological properties of lubricating oils with nanoparticle additives. Wear 2007, 262, 819. (11) Padgurskas, J.; Rukuiza, R.; Prosyčevas, I.; Kreivaitis, R. Tribological properties of lubricant additives of Fe, Cu and Co nanoparticles. Tribol. Int. 2013, 60, 224. (12) Peng, D. X.; Chen, C. H.; Kang, Y.; Chang, Y. P.; Chang, S. Y. Size effects of SiO2 nanoparticles as oil additives on tribology of lubricant. Ind. Lubr. Tribol. 2010, 62, 111. (13) Li, Z.; Hou, X.; Yu, L.; Zhang, Z.; Zhang, P. Preparation of lanthanum trifluoride nanoparticles surface-capped by tributyl phosphate and evaluation of their tribological properties as lubricant additive in liquid paraffin. Appl. Surf. Sci. 2014, 292, 971. (14) Mallia, V. A.; Blair, D. L.; Weiss, R. G. Oscillatory Rheology and Surface Water Wave Effects on Crude Oil and Corn Oil Gels with (R)-12-Hydroxystearic Acid as Gelator. Ind. Eng. Chem. Res. 2016, 55, 954. (15) Yu, Q. L.; Fan, M. J.; Li, D. M.; Song, Z. H.; Cai, M. R.; Zhou, F.; Liu, W. M. Thermoreversible gel lubricants through universal supramolecular assembly of a nonionic surfactant in a variety of base lubricating liquids. ACS Appl. Mater. Interfaces 2014, 6, 15783. (16) Huang, G. W.; Yu, Q. L.; Cai, M. R.; Zhou, F.; Liu, W. M. Highlighting the Effect of Interfacial Interaction on Tribological Properties of Supramolecular Gel Lubricants. Adv. Mater. Interfaces 2016, 3, 1500489. (17) Colquhoun, C.; Draper, E. R.; Schweins, R.; Marcello, M.; Vadukul, D.; Serpell, L. C.; Adams, D. J. Controlling the network type in self-assembled dipeptide hydrogels. Soft Matter 2017, 13, 1914. (18) Draper, E. R.; Su, H.; Brasnett, C.; Poole, R. J.; Rogers, S.; Cui, H.; Seddon, A.; Adams, D. J. Opening a Can of Worm(-like Micelle)s: The Effect of Temperature of Solutions of Functionalized Dipeptides. Angew. Chem., Int. Ed. 2017, 56, 10467. (19) Chaudhari, A. K.; Tan, J. C. A mechano-responsive supramolecular metal-organic framework (supraMOF) gel material rich in ZIF-8 nanoplates. Chem. Commun. (Cambridge, U. K.) 2017, 53, 8502. (20) Yu, Q. L.; Wu, Y.; Li, D. M.; Cai, M. R.; Zhou, F.; Liu, W. M. Supramolecular ionogel lubricants with imidazolium-based ionic liquids bearing the urea group as gelator. J. Colloid Interface Sci. 2017, 487, 130. (21) Yu, Q. L.; Huang, G. W.; Cai, M. R.; Zhou, F.; Liu, W. M. In situ zwitterionic supramolecular gel lubricants for significantly improved tribological properties. Tribol. Int. 2016, 95, 55. (22) Zhang, R. C.; Qiao, D.; Liu, X. Q.; Guo, Z. G.; Cai, M. R.; Shi, L. A facile and effective method to improve the dispersibility of WS2 nanosheets in PAO8 for the tribological performances. Tribol. Int. 2018, 118, 60.

4. CONCLUSIONS A novel method is proposed in this work to improve the dispersibility of TiO2 nanoparticles by designing the structures of the modification agent of TiO2 and gelator to achieve the interaction of both additives in the base oil. The results show that the tribological performance under the different lubricating condition is significantly improved when lubricated by PAO10 mixed lubricant and the lubricating performance is further enhanced by proper irradiation of UV. Several reasons for the lubricating performances are put forward based on the results of EDS, XPS spectra, and other characterization. The dispersibility of nanoparticles is improved through the interaction between MOB and TTO, which promotes nanoparticles to exert the mending effect and other lubricating functions. The immobilization function of MOB enhanced the formation of a physical adsorption film. More kinds of organic oxides and some kinds of nitrides are produced by PAO10−1% MOB−0.5% TTO during the friction process, which forms the effective tribochemical reaction film during the lubricating process. The factors work together to achieve the excellent lubricating performances of PAO10−MOB−TTO under different tribological conditions. This work opens up a new possibility to improve the dispersibility of nanoparticles in base oil.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ruochong Zhang: 0000-0001-8431-6202 Zhiguang Guo: 0000-0002-4438-3344 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 51405477, 51575506, 51705506).

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REFERENCES

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Well Dispersive TiO2 Nanoparticles as Additives for Improving the

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