Li Yi; Liu Xinyang; Liu Dajun; Sun Hongwei

(1. China Petroleum & Chemical Corporation, Beijing 100728; 2. SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Surface Modified TiO2Nanoparticles—an Effective Anti-wear and Anti-friction Additive for Lubricating Grease

Li Yi1; Liu Xinyang2; Liu Dajun2; Sun Hongwei2

(1. China Petroleum & Chemical Corporation, Beijing 100728; 2. SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

The surface modifed TiO2nanoparticles were prepared by using 12-hydroxystearic acid chemically modifed on the TiO2surface. The average size of the TiO2particles is about 30 nm. The optimum ratio of tetrabutyl titanate to 12-hydroxystearic acid was 1/0.5. The bonding form between 12-hydroxystearic acid and TiO2nucleus was investigated by FTIR, DSC, TGA and XRD techniques. The lubricating grease containing the surface modifed TiO2nanoparticles possesses excellent anti-wear and anti-friction properties. Compared with the grease without TiO2, the PBvalue can be increased by 52% as the best performance of the grease containing surface modifed TiO2nanoparticles, while the friction coeffcient can be reduced by 33% with the addition of a small amount of TiO2nanoparticles, and meanwhile the wear scar diameter decreases by 25%.

TiO2nanoparticles; lubricating grease; anti-wear; anti-friction

1 Introduction

Nanomaterials have been garnering significant attention over the past few years due to their special properties. The large ratio of surface atomic number to the total atomic number brings about the volume effect, the quantum effect, and the surface and interfacial effect. In recent years, nanomaterials have been widely applied in the research of catalysis, luminescent materials, magnetic materials, semiconductor materials, biology, medicine, and many other felds.

The application of nanomaterials in the lubricating oil has been studied. Many researchers have studied the tribological properties of different kinds of nanomaterials as additives of lubricating oil[1-2], including nano-layered inorganic materials[3-6], nano soft metal[7-8], nano rare earth compounds[9-10], nano oxide, nano hydroxide[11], and others. It is considered that the addition of nanomaterials signifcantly improve the antiwear and anti-friction properties of lubricating oils. At present, there are three different theories on the antiwear mechanism of nanomaterials (microrolling theory, surface deposited film theory and infiltration lattice strengthening surface theory). Although the theories need to be further improved, researchers are optimistic to the application of anti-wear and anti-friction nanomaterials, which is considered as a candidate to take the place of the traditional EP/AW agent, so as to solve the environmental problems brought about by S, P, and Cl[4]. It has been proved that nanomaterials have good tribological properties in lubricating oil, but it is rarely reported in lubricating grease. In view of the correlation between lubricating oil and grease, nanomaterials may be also an effective anti-wear and anti-friction additives for lubricating grease.

In this paper, the surface modified TiO2nanoparticles were prepared by using 12-hydroxystearic acid and tetrabutyl titanate. The microstructure of the nanoparticles was investigated. Furthermore, the surface modified TiO2nanoparticles were applied in the preparation of lubricating grease, and the anti-wear and anti-friction properties were studied.

2 Experimental

2.1 Materials

Reagents were purchased from Alfa Aesar, Aldrich or Acros and used without further purification unlessotherwise noted. Ethanol was dried with K2CO3and distilled from Na/benzophenone under a nitrogen atmosphere.

2.2 Instrumentation

The IR spectra were obtained from a Thermo Nicolet Avatar 360 FT-IR spectrometer. The transmission electron microscopy (TEM) images were obtained using a JEM-2000FXII microscope. Thermogravimetric analysis were performed on a TA5000-TGA2950 analyzer. Differential scanning calorimetry measurements were performed on a TA5000-DSC2910 instrument. X-ray diffraction spectra were obtained from an X’ Pert powder X-ray diffractometer.

2.3 Preparation of surface modified TiO2nanoparticles

3.4 g (10 mmol) of tetrabutyl titanate were dissolved in 100 mL of ethanol under stirring at 60oC, and then an appropriate amount of 12-hydroxystearic acid was added. After 30 minutes, 0.5 g of distilled water was slowly added to the mixture. A yellow sol was formed after several hours, then the solvent was evaporated by a rotary evaporator and the remaining substance was dried under vacuum to yield the surface modifed TiO2nanoparticles.

2.4 Preparation of lubricating grease

The 500SN base oil, lithium 12-hydroxystearate and surface modified TiO2nanoparticles were employed as the base oil, the thickener and the anti-wear additive, respectively, to prepare the lubricating grease.

3 Results and Discussion

3.1 Characteristics of the surface modified TiO2nanoparticles

The morphology of the surface modified TiO2nanoparticles was investigated by the transmission electron microscope (TEM). The TEM image in Figure 1 indicates that the average diameter of the nanoparticles is around 30 nm featuring good monodispersity which is free of aggregation. The microstructure of the surface modified TiO2nanoparticles can be inferred as 12-hydroxystearic acid coated on the surface of TiO2nanoparticles, which is shown in Figure 2.

Figure 1 TEM image of the organic acid modified titanium dioxide

Figure 2 The model of surface modified TiO2nanoparticles

The surface modification of TiO2nanoparticles was validated by the studies using the Fourier transform infrared spectrometry (FTIR). Figure 3(a) is the FTIR spectra of 12-hydroxystearic acid, the bands at 2 915 cm-1and 2 848 cm-1are assigned to the stretching vibration of –CH2– and –CH3, respectively, while the band at 719 cm-1is assigned to the in-plane bending vibration of –CH2–. The band at 1698 cm-1is assigned to the carbonyl stretching vibration. Figure 3(b), (c), (d) are the spectra of TiO2nanoparticles modified by 12-hydroxystearic acid, the mole ratio of tetrabutyl titanate to 12-hydroxystearic acid is 1/1.5, 1/1, 1/0.5, respectively. Compared to Figure 3(a), the bands assigned to non-polar groups of 12-hydroxystearic acid still exist, nevertheless, the band of carbonyl stretching vibration gradually declines and two bands at 1 545 cm-1and 1 448 cm-1emerge, which can be attributed to the asymmetric and symmetric stretching vibrationbands of the carboxylate groups, respectively, indicative of the formation of carboxylate groups on the surface of TiO2nanoparticles. It can be further inferred that the modifcation of 12-hydroxystearic acid on the surface of TiO2could contribute to the formation of nanosized particles. When the ratio of tetrabutyl titanate to 12-hydroxystearic acid reaches 1/0.5, the band at 1 698 cm-1nearly disappears, which is indicative of the completely consumption of 12-hydroxystearic acid.

Figure 3 FTIR spectra of (a) 12-hydroxystearic acid and surface modified TiO2nanoparticles with tetrabutyl titanate/12-hydroxystearic acid mole ratio of (b) 1/1.5, (c) 1/1, and (d) 1/0.5

The differential scanning calorimetry (DSC) curves of 12-hydroxystearic acid and surface modified TiO2nanopaticles are shown in Figure 4. Figure 4(a) represents the DSC curve of 12-hydroxystearic acid, wherein an absorption band at 80oC stems from the molten 12-hydroxystearic acid. As the temperature increases to over 210oC, oxidative combustion of 12-hydroxystearic acid emerges and a large amount of carbon dioxide and water is released. Figure 4(b) represents the DSC curve of the 12-hydroxystearic acid modified TiO2nanoparticles, which indicates that the band of oxidative combustion has not emerged until 258oC, which is about by 50oC higher than that of 12-hydroxystearic acid. This may occur because the linkage of 12-hydroxystearic acid and TiO2enhances the stability of the acid, resulting in an increased oxidative combustion temperature.

Figure 4 DSC curves of (a) 12-hydroxystearic acid, and (b) surface modified TiO2nanoparticles

The curve for thermogravimetric analysis (TGA) of the surface modified TiO2nanoparticles is shown in Figure 5. The weight loss between 70-100oC is attributed to the decreasing amount of liquid water. The modifcation layer of TiO2nanoparticles decomposes as the temperature increases, and the weight reduces to 21% when the temperature reaches 365oC, which is in agreement with the result of DSC measurements. The TGA curve further indicates that there is no any weight loss after 450oC, we can infer that the appearance of strong exothermic peak is associated with the transition of the crystalline phase in TiO2nanoparticles.

Figure 5 TGA curve of surface modified TiO2nanoparticles

In order to validate that the transition of crystalline phase in TiO2nanoparticles is the main reason for the appearance of strong exothermic peak in DSC curve, the X-ray diffraction analysis was employed to investigate the crystalline phase of nanoparticles calcined at different heating temperature (Figure 6). There was no peak in the XRD spectrum of nanoparticles calcined at 60oC, which was indicative of non-crystalline structure of the TiO2nanoparticles. There was a wide peak in the XRD spectrum of TiO2calcined at 400oC, indicating to the formation of anatase-TiO2. For the case of TiO2nanoparticles calcined at 550oC, the peak became sharper, which revealed the formation of a large amount of crystals. When thecalcination temperature reached 700oC, the peak became stronger and sharper, denoting the transformation of TiO2crystals from the anatase type to the rutile type. The results of XRD measurements provided strong evidence for the crystalline phase transition of TiO2nanoparticles at temperatures exceeding 450oC.

Figure 6 XRD spectra of TiO2nanoparticles treated at different heating temperatures

3.2 Tribological performance of the lubricating grease containing surface modified TiO2nanoparticles

The 500SN base oil, lithium 12-hydroxystearate and surface modified TiO2nanoparticles were employed as the base oil, the thickener and the anti-wear additive, respectively, to prepare the lubricating grease. ThePBvalues of the greases with different mass fractions of TiO2were measured. Figure 7 reveals that the addition of TiO2nanoparticles to the grease resulted in a dramatically enhancedPBvalue with a maximum value at 1.5%, which was by 52% higher than the grease without additive. Further addition of TiO2nanoparticles led to a decrease of thePBvalue. The excess nano-sized additive might form the aggregates and made the friction process unstable.

Figure 7PBas a function of additive concentration of surface modified TiO2nanoparticles

The friction coefficient of the greases with different amount of the surface modified TiO2nanoparticles is shown in Figure 8. The test results revealed that the addition of a small amount of TiO2nanoparticles led to a dramatical decrease of the friction coeffcient, with the minimum value identifed at 0.3% of TiO2nanoparticles added, which was by 67% lower than the grease without TiO2additive. The further additions of TiO2nanoparticles caused an increase of the friction coeffcient. The reason for this phenomenon is that the nanoparticles additive has two opposite effects on the anti-friction performance of the lubricant system. On the one hand, the nanoparticles can effectively reduce the friction coefficient by providing the effective bearing effect on the surface of the tribomates, and on the other hand, the nanoparticles in the lubricating oil flm may damage the integrity of the lubricating oil flm, causing local disturbance of the oil flm and the increase in the friction coeffcient. When the bearing effect is dominant, the friction coeffcient decreases, and otherwise the friction coefficient increases.

Figure 8 Friction coefficient as a function of concentration of surface modified TiO2nanoparticles added

Figure 9 shows the change in the wear scar diameter of the grease containing different concentrations of surfacemodified TiO2nanoparticles. It can be seen that the grease containing the surface-modifed TiO2nanoparticles could dramatically reduce the wear scar diameter of the steel ball. The wear scar diameter reached a minimum value when the TiO2nanoparticles amount was 0.3% and the diameter was by 25% lower than that of the grease without TiO2nanoparticles, while a further addition of the TiO2nanoparticles would not change the abrasion resistance property of the grease.

Figure 9 Wear scar diameter as a function of concentration of surface modified TiO2nanoparticles

The grease containing 1% of TiO2nanoparticles was selected to investigate the wear scar diameter formed under different applied load. Figure 10 reveals that the wear scar diameter gradually increased with an increasing load. In comparison with the grease without additive, the grease containing TiO2nanoparticles exhibited lower wear scar diameter formed under the full load range, which was indicative of a better abrasion resistance property of the grease.

Figure 10 Wear scar diameter as a function of applied load with lubrication of grease alone and grease containing 1% of TiO2nanoparticles

3.3 Studies on anti-wear and anti-friction mechanism of surface modified TiO2nanoparticles

The scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used to analyze the surface of the steel ball after tribology test. It can be seen from Figure 11 that the surface of the steel ball remained smooth after the tribology test.

The quantitative analysis result of EDX experiment is shown in Figure 12 and the relative content of the elements calculated by the software is shown in Table 1. The EDX element distribution analysis revealed that after the tribology test, the steel ball surface contained not only Fe, Cr, and Mn elements, but also a small amount of Ti element, which could be the contribution of additive. The surface of TiO2nanoparticles contained a large amount of hydroxyl radicals and unsaturated residues, which had high surface energy. When the frictional force was exerted on the tribomates, the fash temperature caused by the friction could make the surface modified TiO2nanoparticles adsorbed on the metal surface through the hydroxylated surface to form a layer of deposited film. In the process of friction, the torn off TiO2deposition film could be quickly displaced by the subsequent adsorption to bring into full play its good lubrication effect. Ti element was also detected on the surface of the steel ball in the XPS full scan spectrum, which could further validate the above hypothesis.

Figure 11 SEM photograph and the distribution of elements by EDX analysis of the worn surface after tribological test

Figure 12 Analysis of the concentration of elements on the worn surface by EDX

Table 1 Concentration of elements on the worn surface after tribological test as determined by EDX.

Figure 13 XPS analysis of element concentration on the worn surface

4 Conclusions

The surface modified TiO2nanoparticles were prepared by using 12-hydroxystearic acid chemically modifed on the TiO2surface. The average size of the particles was about 30 nm. There was a strong interaction between the terminal carboxyl group of surface modification agent 12-hydroxystearic acid and the TiO2nanometersized nuclei. The optimum ratio of tetrabutyl titanate to 12-hydroxystearic acid was 1/0.5, and the oxidative decomposition temperature of the surface modification layer was increased due to the presence of nano-particles. The bonding form between 12-hydroxystearic acid and TiO2nuclei was also validated by FTIR, DSC, TGA and XRD experiments.

The lubricating grease containing the surface modified TiO2nanoparticles has excellent anti-friction and antiwear properties. Addition of nanoparticles to the grease resulted in a dramatically enhanced PBvalue, and when a 1.5 m% of the surface modified TiO2nanoparticles were added to the grease, PBvalue was by 52% higher than the grease without TiO2. However, the further additions of TiO2nanoparticles caused the decrease of the PBvalue. Addition of a small amount of TiO2nanoparticles also resulted in a dramatical decrease of the friction coeffcient, which reduced by 33% as compared to the grease without TiO2additive. Further additions of TiO2nanoparticles caused an increase of the friction coefficient. The wear scar diameter reached a minimum when 0.3% of the TiO2nanoparticles were added to the grease, while the wear scar diameter was by 25% lower than that of the grease without TiO2nanoparticles. The wear scar diameter remained stable with further addition of the TiO2nanoparticles to the grease.

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date: 2017-05-03; Accepted date: 2017-05-20.

Dr. Liu Xinyang, Telephone: +86-10-82368858; E-mail: liuxinyang.ripp@sinopec.com.