Preparation, Dispersion and Tribological Properties of Oleophilic Lanthanum Hydr

Wu Bo; Zhang Qiangqiang; Song Hui; Yang Bingxun; Tian Ming; Hu Xianguo

(School of Mechanical Engineering, Hefei Uniνersity of Technology, Hefei 230009)

Abstract: Lanthanum hydroxide/graphene oxide nanocomposites (La(OH)3/GO) modified by octadecylamine (OCA),oleylamine (OLA), and polyvinylpyrrolidone (PVP) were prepared, respectively, as the base oil additives. The dispersion stability of different modi fied La(OH)3/GO in base oil was studied by means of centrifugation. The tribological properties of oleophilic La(OH)3/GO in base oil were investigated using an UMT ball-disc tribometer. The micro-morphology and chemical composition of the worn surface were characterized by 3D laser microscope, SEM, EDS, XPS, and Raman spectroscopy, respectively. The wettability performance of the worn surface was also studied based on the contact angle measurements. The test results showed that the OLA-La(OH)3/GO nanocomposites had good dispersion stability in base oil.The anti-wear performance of base oil was improved signi ficantly by the addition of OLA-La(OH)3/GO nanocomposites.The characterizations of worn surface showed that the OLA-La(OH)3/GO nanocomposites could form the metal oxide and graphene protective films effectively on the friction interface and thus increased the oil wettability of the worn surface,thereby resulting in an improved wear resistance.

Key words: oleophilic modi fication; La(OH)3/GO nanocomposites; dispersion stability; tribological property

1 Introduction

Graphene oxide (GO), as a derivative of two-dimensional graphene, has gained much attention as lubricating materials recently. Studies have shown the excellent tribological properties of GO with a good potential as a lubricant additive[1]. However, GO is prone to agglomeration and stacking with graphite blocks because of its ultra-thin layer structure, thereby losing its excellent tribological properties. In addition, the hydroxyl and carboxyl groups of GO are hydrophilic, which cannot make GO disperse stably in the lubricating oil[2]. These phenomena can limit its use as a lubricant additive greatly. The methods of surface functionalization with inorganic nanoparticles[3]and organic compounds[4]have been used to solve the problem of agglomeration and unstable dispersion of GO in the lubricating oil, which makes GO a novel lubricant additive.

Nano-lanthanum hydroxide (La(OH)3), as a rare earth compound, exhibits good tribological performance due to its special electronic structure and the high chemical activity of La[5]. Zhang, et al. found that nano-La(OH)3and ZDDP have a synergistic anti-wear effect in liquid paraffin[6]. This allows nano-La(OH)3to be added to lubricant as anti-wear additive to reduce the use of nonenvironmentally friendly organic compounds additives.However, similar to other nanoparticles, aggregation and dispersion problems are roadblocks to the synthesis and application of nano-La(OH)3. This problem also limits the wide application of nano-La(OH)3as a lubricant additive.Thus, La(OH)3nanoparticles were in situ synthesized on GO nanosheets by chemical precipitation to obtain the La(OH)3/GO nanocomposites in this paper. On the one hand, the functional groups and defects on the GO surface act as the sites for nucleation and growth of the La(OH)3nanoparticles, making the La(OH)3evenly dispersed on the surface of GO without agglomeration[7].On the other hand, the insertion of La(OH)3nanoparticles into the adjacent interlaminar space of GO reduces the agglomeration tendency of nanosheets and prevents the stacking of nanosheets[8]. This approach can solve the agglomeration problem of GO and La(OH)3nanoparticles in lubricating oil effectively. Moreover, in order to further improve the dispersion stability of La(OH)3/GO in lubricating oil, three compounds (octadeclamine(OCA), oleylamine (OLA), and polyvinylpyrrolidone(PVP)) were selected to modify the nanocomposites.The tribological properties under boundary lubrication conditions and wear mechanisms of oleophilic La(OH)3/GO nanocomposites in base oil were investigated. This study can provide a theoretical basis for the application of La(OH)3/GO nanocomposites as new high-performance anti-wear additives of lubricating oil in the future.

2 Experimental

2.1 Materials

Lanthanum chloride heptahydrate was purchased from the Tianjin Zhiyuan Chemical Reagent Co., Ltd. OCA,OLA, and PVP were purchased from the Aladdin Reagent(Shanghai) Co., Ltd. Ethyl alcohol and ammonia water were purchased from the Sinopharm Chemical Reagent Co., Ltd. The GO and base oil (150SN) were purchased from the Anhui Runpu Nano-Technology Co., Ltd. All the above chemical reagents were analytically pure reagents.

2.2 Preparation of modi fied La(OH)3/GO nanocomposites

0.46 g of lanthanum chloride heptahydrate and 0.023 g of GO were added into 50 ml of deionized water, and then ultrasonic dispersion was conducted for 1 h to obtain a mixed solution A. A certain amount of OCA, OLA, or PVP and 0.92 g of ammonia water were added into 100 ml of ethyl alcohol and were mixed evenly to obtain a solution B. Then, the solution A was heated to 70 ℃, and then the solution B was added to the solution A gradually.After being subject to reaction for 1 hour, the precipitate was filtered, and then washed with ethyl alcohol and deionized water until the pH value of the filtrate was equal to aroud 7. Finally, the precipitate was dried at 105 ℃ in a drying oven for 12 hours and then was cooled down to room temperature to obtain different kinds of modified La(OH)3/GO nanocomposites (OCA-La(OH)3/GO, OLA-La(OH)3/GO, and PVP-La(OH)3/GO).

2.3 Dispersion stability test

0.1 g of different La(OH)3/GO nanocomposites was added to 10 ml of ethyl alcohol and the mixture was dispersed for 30 min by ultrasonic process. The nanocomposites dispersion liquid was added into 10 g of base oil, which was stirred at a high speed for 30 min. Then the mixed liquid was heated to 110 ℃ and stirred simultaneously until the ethyl alcohol was completely evaporated.The base oil containing 0.1% of different La(OH)3/GO nanocomposites was obtained and then centrifuged at various speeds for 5 min to observe the precipitation at the bottom of the centrifuge tube to evaluate the dispersion stability.

2.4 Tribological test

The tribological properties of GO and different La(OH)3/GO nanocomposites in base oil were investigated with a UMT-2 ball-disk sliding tribometer (Bruker, Germany).The schematic diagram of the tribometer is shown in Figure 1. The tribological tests were conducted under a load of 40 N and a sliding velocity of 5 mm/s for 30 min at room temperature. The λ value calculated by the Dowson and Hamrock minimum film thickness formula was about 0.115,denoting that the lubrication was in a state of boundary lubrication regime[9]. All tests were repeated three times under the same experimental conditions.

Figure 1 Schematic diagram of tribometer working principle

2.5 Characterization

High-resolution transmission electron microscopy(HRTEM, JEM-2100F), 3D laser scanning microscopy(Keyence model VK-X100), field emission scanning electron microscopy (FESEM, Hitachi model SU8010),energy dispersive spectroscopy (EDS, Hitachi model SU8010), X-ray diffractometry (XRD, X’Pert PRO MPD), Raman spectroscopy (HORIBA Jobin Yvon),Fourier transform infrared spectrometry (FTIR, Nicolet),and X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi) were employed to characterize the micromorphology and chemical composition of the La(OH)3/GO nanocomposites and the worn surfaces. A contactangle system (CA100C) with a computer-controlled charge-coupled device (CCD) camera was used to characterize the wettability of the worn surfaces.

Figure 2 Micrographs and EDS analysis of La(OH)3/GO nanocomposites: (a) HRTEM image, (b) Lattice-resolution HRTEM image, (c) EDS

3 Results and Discussion

3.1 Characterization

The HRTEM images of La(OH)3/GO nanocomposites are shown in Figure 2. As presented in Figure 2(a), the GO sheets are transparent and appear as silky waves, except for some wrinkles on the surface. Some stick-shaped nanoparticles (within the red dashed frame), about 120 nm in length and 40 nm in width, were clearly deposited on the GO nanosheets. Figure 2(b) shows the lattice-resolution HRTEM image of the La(OH)3/GO nanocomposites.The measured interplanar spacing of the stick-shaped nanoparticles was 0.327 nm and 0.319 nm, corresponding to the lattice plane distance value of (110) planes and (101)planes of hexagonal La(OH)3, respectively. The clear lattice fringes suggested the high crystalline nature of La(OH)3nanoparticles grown on the GO nanosheets. The EDS spectrum of La(OH)3/GO nanocomposites from HRTEM image is shown in Figure 2(c). The high intensity peaks of the element La also indicated that La(OH)3nanoparticles were deposited on the GO surfaces successfully.

Figure 3 shows the SEM images and particle size distribution of GO and different La(OH)3/GO nanocomposites. Figure 3(a) indicates that GO presented the obviously folded shape with an aggregation state of 2―5 μm particles. However, after the composing process of GO and La(OH)3, the surface folded morphology of GO disappeared and the La(OH)3/GO nanocomposites presented an agglomerated lamellar structure with a size of 0.5―1.6 μm, as depicted in Figure 3(b). Figure 3(c)and Figure 3(e) show that the size distribution of OCALa(OH)3/GO and PVP-La(OH)3/GO nanocomposites was uneven and some obviously large agglomerated particles exist, indicating that OCA and PVP were not conducive to the dispersion of La(OH)3/GO nanocomposites during the preparation process. However, as shown in Figure 3(d), the OLA-La(OH)3/GO nanocomposites were in a good lamellar dispersion state, with a relatively uniform particle size distribution of 0.3 μm―0.8 μm. This fact indicates that during the preparation of nanocomposites,OLA is conducive to the dispersion of La(OH)3/GO so as to form the relatively uniform nanocomposites.Figure 4(a) shows the Raman spectra of GO and different La(OH)3/GO nanocomposites. The D band of the disordered carbon at 1 350 cm−1and G band of the sp2carbon at 1 590 cm−1could be observed from all La(OH)3/GO nanocomposites[10]. The intensity ratio ofID/IGcould be used to define the graphitization degree of carbon materials[11]. However, theID/IGvalues of all La(OH)3/GO nanocomposites were slightly higher than that of the GO, which could be ascribed to the increase in the disorder of the graphitic structure due to the binding of La3+with the free oxygen functional groups on the GO surface to form La(OH)3nanoparticles[12]. Figure 4(b)shows the XRD patterns of GO and different La(OH)3/GO nanocomposites. The characteristic diffraction peaks of La(OH)3particles with the hexagonal phase could be observed in the patterns of all La(OH)3/GO nanocomposites, which demonstrated that La(OH)3was synthesized successfully. The diffraction peak strength of La(OH)3/GO nanocomposites was weakened after modification by three modifiers, which might be caused by the introduction of modifier molecules.The disappearance of diffraction peaks at 11° and 42°indicates that GO was reduced to some extent after the precipitation reaction. These results illustrated that the La(OH)3nanoparticles were successfully anchored on the GO nanosheets in all La(OH)3/GO nanocomposites.

Figure 3 SEM images and particle size distribution of different nanoparticles

Figure 5 shows the FTIR spectra of GO and different La(OH)3/GO nanocomposites. As for GO, the wide absorption peak at 3 393 cm-1was attributed to the stretching vibration of -OH groups and the adsorbed water molecules. The absorption peaks at 1 724 cm-1,1 042 cm-1, 1 410 cm-1, and 1 630 cm-1were attributed to the C=O stretching vibration peak of -COOH groups,the stretching vibration peak of C-O (epoxy) groups, the stretching vibration peak of C-OH groups, and the skeletal vibration peak of C=C, respectively. As shown in the spectra of La(OH)3/GO nanocomposites, it is found that the vibration absorption peak of the oxygen-containing functional groups of GO weakened or disappeared. The wide absorption peak of GO at 3 393 cm-1became a sharp vibration peak of O-H bond in hydroxide at 3 590 cm-1,while a new stretching vibration peak of La-O appeared at 646 cm-1. It indicated that La(OH)3was successfully synthesized and chemically bonded to the GO nanosheets.

Figure 4 Raman spectra (a) and XRD patterns (b) of GO and different La(OH)3/GO nanocomposites

As for the PVP-La(OH)3/GO nanocomposites, the absorption peaks at 1 656 cm-1, 1 200 cm-1, 1 480 cm-1,and 1 394 cm-1were attributed to the stretching vibration peak of C=O, the stretching vibration peak of C-N, and the bending vibration peak of C-H, respectively, indicating that PVP was successfully adsorbed onto the surface of La(OH)3/GO nanocomposites. As for the OCA-La(OH)3/GO and OLA-La(OH)3/GO nanocomposites, the apparent peaks at 2 920 cm-1and 2 850 cm-1were attributed to -CH3and -CH2- stretch vibration peaks, indicating that the two nanocomposites were modi fied with long-chain lipophilic alkane groups of OCA and OLA successfully. In addition,the C=O stretching vibration peak of -COOH and C-O(epoxy) groups became weaker, and the new stretching vibration peak of C-N groups appeared in OLA-La(OH)3/GO nanocomposites as compared with the GO, indicating that some new C-N bonds were formed between the amino groups of OLA and carboxyl groups of GO[13].

To further investigate the combination form between OLA and La(OH)3/GO nanocomposites, the XPS spectra of OLA-La(OH)3/GO nanocomposites are shown in Figure 6. In the C1s XPS spectrum, the C-C (284.69 eV),C-OH (285.50 eV) and C=O (289.51 eV) groups peaks of GO could be clearly observed. However, no remarkable peak of C-O epoxy groups (at about 286.69 eV) of GO could be found. Besides, the peaks of C-N (285.95 eV)and C(O)-N (287.79 eV) groups appeared in the C1s spectrum, indicating that the -NH2groups of OLA reacted with the C-O and COOH groups of GO to form new covalent bonds on the surface of GO[14]. In the N1s XPS spectrum, two distinct peaks of N-C (399.62 eV) and N-C(O) (400.50 eV) groups further proved that OLA was covalently modi fied on the GO nanosheets. Moreover, a peak at 401.50 eV, which could be assigned togroups, appeared in the N1s XPS spectrum, indicating that some OLA molecules might be bound to GO nanosheets in the form of ionic bonds[15]. In addition, the La 3d XPS spectrum showed that both La 3d5/2and La 3d3/2had strong companion peaks (at 839.45 eV and 856.45 eV), reflecting the strong La-O covalent bond in the nanocomposites. This may be caused by the oxygen atoms of GO gaining electrons from the conjugated system on the lamellas, which can increase the coordination capacity of oxygen atoms[15]. It also indicates that La(OH)3and GO were chemically bonded, which was consistent with the results of FTIR analysis.

Figure 5 FTIR spectra of GO and different La(OH)3/GO nanocomposites

3.2 Dispersion property

Table 1 shows the bottom images of the centrifuge tubes with different oil samples after centrifugation at different speeds for 5 minutes. It can be seen that the base oil containing different La(OH)3/GO nanocomposites had less precipitates at the bottom of centrifugal tube than that of the base oil containing GO after centrifugation at different speeds, indicating that anchoring La(OH)3on the GO sheet could improve its dispersion stability in base oil. Moreover, the base oil containing OLALa(OH)3/GO nanocomposites had the least precipitates at the bottom of centrifugal tube after centrifugation at all speeds, indicating the best dispersion stability of OLALa(OH)3/GO nanocomposites in the base oil. This may be due to the presence of long-chain alkanes on the surface of La(OH)3/GO modified by OLA, which inhibited the agglomeration between nanocomposites and improved the oil-wet performance of nanocomposites. Therefore,OLA was selected for the modification of La(OH)3/GO nanocomposites and the tribological performance of OLA-La(OH)3/GO nanocomposites was further studied.

Figure 6 The C1s, N1s, O1s and La3d XPS spectra of OLA-La(OH)3/GO nanocomposites

Table 1 The bottom images of the centrifuge tubes with different oil samples after centrifugation process at different speeds for 5 minutes

3.3 Tribological performance

Figure 7 shows the comparison of tribological performance between GO, La(OH)3/GO, and OLALa(OH)3/GO nanocomposites. As shown in Figure 7, the wear volume of the lower disk decreased by 38%, 31%,and 60%, respectively, after the addition of GO, La(OH)3/GO, and OLA-La(OH)3/GO nanocomposites to the base oil. It indicated that GO and La(OH)3/GO nanocomposites could improve the anti-wear performance of the base oil,while La(OH)3/GO nanocomposites could further improve the anti-wear performance of the base oil after the process of OLA modi fication.

Figure 7 Comparison of tribological performance among GO, La(OH)3/GO and OLA-La(OH)3/GO nanocomposites

3.4 Worn surface analysis

Figure 8 shows the 3D wear profiles and worn areas lubricated by the base oil and the base oil containing GO, La(OH)3/GO nanocomposites, and OLA-La(OH)3/GO nanocomposites, respectively. It can be seen that the wear scar lubricated by the base oil had a deepest depth,while the wear scar became shallow after adding GO or La(OH)3/GO into the base oil. However, the wear scar lubricated by the base oil containing OLA-La(OH)3/GO nanocomposites had a shallowest depth. Figure 9 shows the SEM images of the worn surfaces lubricated by different oil samples. As shown in Figure 9(a), the obvious pitting and deep furrows could be observed on the wear scar lubricated by base oil. When GO or La(OH)3/GO nanocomposites were added to the base oil, the furrows on the wear scar became shallower and the pitting became much smaller, as shown in Figure 9(b) and Figure 9(c). However, the furrows on the wear scar lubricated by base oil containing OLA-La(OH)3/GO nanocomposites were the shallowest and no obvious pitting could be found on the wear scar, as shown in Figure 9(d). This phenomenon agreed well with the tribological experimental results.

The EDS results of the worn surfaces lubricated by different oil samples are illustrated at the bottom insets of Figure 9. When GO, La(OH)3/GO and OLA-La(OH)3/GO nanocomposites were added to the base oil, the O content increased signi ficantly, indicating that some metal oxide protective films were effectively formed at the friction interface. Moreover, La could be detected on the wear scars lubricated by the base oil containing OLALa(OH)3/GO nanocomposites, indicating that La entered the friction interface to form protective films.

Figure 10 shows the Raman spectra of the worn surfaces lubricated by different oil samples. When GO, La(OH)3/GO, and OLA-La(OH)3/GO nanoparticles, respectively,were added to the base oil, theID/IGvalues of carbon were signi ficantly reduced, indicating that the graphene protective films were formed at the friction interface under the sliding action. In addition, theID/IGvalue of carbon at the friction interface lubricated by the base oil containing OLALa(OH)3/GO nanocomposites was the smallest, indicating that more graphene protective film originating from OLALa(OH)3/GO nanocomposites was formed at the interface.

Figure 11 shows the O 1s XPS spectra of the worn surfaces lubricated by different oil samples. When GO was added to the base oil, a weak peak appeared at 532.20 eV in the O1s XPS spectrum of the worn surface (Figure 11(b)),which could be attributed to the C=O/C-O characteristic peak of GO. This meant that some GO entered the friction interface to form a GO protective film during the sliding process. However, when the La(OH)3/GO nanocomposites were added to the base oil, the O1s XPS spectrum of the worn surface (Figure 11(c)) showed two new peaks at 532.20 eV and 531.12 eV, which could be attributed to the characteristic peaks of C=O/C-O in GO and LaOOH,respectively. This fact indicated that the nanocomposites entered the friction interface to form the lanthanum oxide and GO protective film during the sliding process.However, when the OLA-La(OH)3/GO nanocomposites were added to the base oil, the characteristic peaks of C=O/C-O and LaOOH in the O1s XPS spectrum (Figure 11(d)) were signi ficantly stronger than those of La(OH)3/GO nanocomposites. This fact indicated that more nanocomposites entered the friction interface to form more lanthanum oxide and GO protective films during the sliding process. This result was consistent with the above EDS and Raman spectra analysis.

Figure 8 The 3D wear pro files and worn areas lubricated by the base oil (a and a＊) and the base oil containing:GO (b and b＊), La(OH)3/GO (c and c＊), and OLA-La(OH)3/GO nanocomposites (d and d＊)

Figure 12 shows the contact angle between water and worn surface lubricated by different oil samples. It was found that the contact angle between water and the worn surface lubricated by base oil containing OLALa(OH)3/GO nanocomposites was the highest among the four oil samples, indicating that the tribofilms formed by OLA-La(OH)3/GO nanocomposites at the interface could improve the oil wettability of the worn surface significantly. This might be caused by the oil-wet long carbon chain functional groups in the OLA-La(OH)3/GO nanocomposites adsorbed on the worn surface, which reduced the surface tension of the lubricating oil and the metal interface, thus making the lubricating oil more prone to spreading on the friction interface to form the lubrication film. It was in agreement with its excellent anti-wear performance.

Figure 10 Raman spectra of the worn surfaces lubricated by base oil (a), base oil containing: GO (b), La(OH)3/GO (c),and OLA-La(OH)3/GO nanocomposites (d)

3.5 Wear mechanism

According to the above characterization analysis, the antiwear mechanism by inference can be state as follows.Compared to GO and La(OH)3/GO nanocomposites, the OLA-La(OH)3/GO nanocomposites had good dispersion stability in base oil, which were immune to aggregation and could enter the friction interface to form the metal oxide and graphene protective films more effectively.Moreover, the oil-wet long carbon chain functional groups of the OLA were adsorbed on the worn surface effectively, which could reduce the surface tension of the lubricating oil and the metal interface, thus making the lubricating oil more prone to spreading on the friction interface to form a lubricating film. Finally, the anti-wear performance was significantly improved based on the synergistic effect of OLA, metal oxide, and graphene.

Figure 9 SEM images and EDS results of the worn surfaces lubricated by base oil (a) and base oil containing:GO (b), La(OH)3/GO (c), and OLA-La(OH)3/GO nanocomposites (d)

Figure 11 The O 1s XPS spectra of the worn surfaces lubricated by base oil (a) and base oil containing:GO (b), La(OH)3/GO (c), and OLA-La(OH)3/GO nanocomposites (d)

Figure 12 Contact angle between water and worn surface lubricated by different oil samples

4 Conclusions

Three kinds of modi fied La(OH)3/GO nanocomposites were prepared by the one-step method successfully. The modi fier can influence the size and morphology of La(OH)3/GO nanocomposites. The OLA-La(OH)3/GO nanocomposites in a state of relatively uniform lamellae showed good dispersion stability and excellent anti-wear performance in the base oil. Under the boundary lubrication condition, the anti-wear performance of the base oil increased by 60% after adding 0.1% of OLA-La(OH)3/GO nanocomposites, because the OLA-La(OH)3/GO nanocomposites could enter the friction interface to form the metal oxide and graphene protective films effectively. The oil-wet long carbon chain functional groups of OLA were adsorbed on the worn surface, making the oil more prone to spreading on the rubbing interfaces to form the lubricating film. Finally, the anti-wear performance was signi ficantly improved with the help of the synergistic effect of OLA, metal oxide, and graphene.

Acknowledgment:This research was supported by the National Natural Science Foundation of China (Grant No. 51675153).