Article pubs.acs.org/IECR
Preparation, Characterization, and Tribological Evaluation of a Calcium Borate Embedded in an Oleic Acid Matrix Sheng Han,*,† Sizhou Liu,‡ Yuhong Wang,† Xiaoli Zhou,† and Lifeng Hao† †
New Energy Material Lab, ShangHai Institute of Technology, 120 Caobao Road, Shanghai, 200235, P.R. China Shanxi Datong University, Datong, Shanxi, P.R. China
‡
ABSTRACT: Calcium borate nanoparticles embedded in an oleic acid liquid matrix (CBNL) with high stability in base oil were synthesized as nanoparticle lubricant additives designed as potential solution to problems of insolubility and stable dispersity using the conventional preparation method. The microstructures of the as-synthesized samples were characterized by laser particle analysis, infrared spectra, and thermogravimetric analysis. The results indicated that, in CBNL, the composition of the nanosized calcium borate was 2CaO·B2O3·xH2O, and the average size of the calcium borate nanoparticles was about 18 nm. The tribological properties of CBNL as a lubricating oil additive were evaluated on a four-ball tribometer. CBNL exhibited excellent tribological capacities including load-carrying, friction-reducing, and antiwear properties. The worn surface of the steel ball was investigated by a three-dimensional noncontact surface profile meter and X-ray photoelectron spectroscopy. The tribological mechanism of the nanoparticles was discussed. The excellent tribological properties exhibited by the as-synthesized sample resulted from a wear-resistant film containing both depositions and the tribochemical reaction products that comprise B2O3, FeB, and Fe2O3.
1. INTRODUCTION Conventional lubricant additives are compounds that contain several tribologically active elements, such as sulfur, phosphorus, or chlorine.1−3 However, they are commercially unsatisfactorily used because of their pungent odor, extreme corrosive properties, and poor thermal stability.4,5 Thus, these factors significantly affect their practical application and prompted their total or partial replacement. Borate nanoparticles have increasingly attracted attention because of their antioxidative characteristics, relatively low toxic level, pleasant odor, and nonvolatility.6,7 Borate nanoparticles are insoluble in base oil. Thus, compatibility measures with all kinds of base oils need to be conducted prior to their use. Recently, more intense studies on borates as lubricant additives have been performed. Hu et al.8 prepared nanosized zinc borate, lanthanum borate, and magnesium borate and, then, studied their tribological behaviors in 500 SN base oil using sorbitol monostearate as dispersing agent. They found that the base oil containing these materials provided better antiwear (AW) and friction-reducing properties than base oil alone. Tian et al.9 successfully prepared a hydrophobic zinc borate via a wet method using oleic acid as modifying agent. They concluded that the friction reduction of the hydrophobic zinc borate in base oil was better than that of pure zinc borate. These conventional methods required the addition of nanoparticles in solid form into base oil, and then, dispersing them by heating and stirring, inducing the reagglomeration of nanoparticles.10 Therefore, the solubility of nanoparticles in liquid form is more superior to those in solid form in terms of being used as a lubricant additive. As a result, several studies have provided new preparation methods with aim of using nanoparticles in liquid form.11−13 In the current study, calcium borate nanoparticles embedded in an oleic acid liquid matrix (CBNL) with high stability in base © 2012 American Chemical Society
oil were successfully synthesized. Their tribological properties in base oil as lubricating additives were evaluated using a fourball tribometer. The worn surface morphology was observed using an interferometric surface profilometer, and the tribological mechanism was preliminarily investigated using Xray photoelectron spectroscopy (XPS).
2. EXPERIMENTAL SECTION 2.1. Materials. Oleic acid and calcium hydroxide were of chemical grade. Xylene, boric acid, methanol, calcium oleate, and n-heptane were of analytical grade. All the reagents were used without any further purification and treatment. Distilled water was applied for all synthesis and procedures. MVIS 250 base oil was obtained from the Petrachina Lanzhou Lubricating Oil R&D Institute. Its typical properties are listed in Table 1. A commercial zinc butyloctyldithiophosphate (T202), obtained from the Lanzhou Lubrizol Additive Co., Ltd., was used for Table 1. Typical Characteristics of MVIS 250 as Base Oil kinematic viscosity (mm2/s) 40 °C 100 °C viscosity index pour point (°C) flash point (°C) acid value (mg KOH/g) color
Received: Revised: Accepted: Published: 13869
42.85 6.037 78 −9 199 0.05 yellowish
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based on the American Iron and Steel Institute Standard Steel No. E-52100. 2.4. Analysis of the Rubbed Surface. After the four-ball test, the ball was cleaned in a ligroin ultrasonic bath. A MicroXAM interferometric surface profilemeter (ADE Co.) was used to analyze the worn surface morphology and the wear extent. Compared with the conventional methods used in tribological analysis, the interferometric surface profilemeter is more advantageous because it can provide three-dimensional (3D) surface profiling measurements, as well as more intuitionistic and in-depth topographic information than scanning electron microscopy (SEM). Moreover, it does not require the use of cantilevers compared with atomic force microscopy, thus it can receive more direct topographic information of the scar.14 The X-ray photoelectron spectra (XPS) analyses of the elements on the wear scar were conducted on a PHI-5702 electron spectrometer using pass energy of 188 eV and Mg Kα line excitation source with the reference C 1s at 284.6 eV.
comparison with CBNL. Its typical properties are listed in Table 2. Table 2. Physical Properties of T202 density (20 °C, g/cm3) flash point (open, °C) S content (wt %) P content (wt %) Zn content (wt %) PH value color
1.13 >180 17.05 7.32 9.17 >5.5 amber
2.2. Preparation of CBNL. Oleic acid, xylene, calcium hydroxide, and boric acid were placed in a 250 mL threebottom flask equipped with a thermometer, reflux condenser, and mechanical stirrer. The flask was heated at 70 °C in water bath. After the temperature was stable, the mixture containing distilled water and methanol was added dropwise to the prepared solution with continuous stirring. The dropping time lasted for 36 h. Afterward, the solvent in the resulting mixture was removed using a rotary evaporation apparatus. A light yellow viscous mixture was obtained. The product mixture was diluted with n-heptane in a volume ratio of 1:1 and, then, filtered. Finally, the filtrate was evaporated via rotary evaporation, and the resulting product was obtained. The main properties of the calcium borate are listed in Table 3.
3. RESULTS AND DISCUSSION 3.1. Composition of the Nanosized Calcium Borate in CBNL. The synthesized product appeared as a yellow viscous liquid. According to the atom ratio of boron and calcium from the atomic absorption spectroscopy (AAS), it can be concluded that the nanosized calcium borates in CBNL have an average composition of 2CaO·1.15B2O3·xH2O,15 where x represents a positive integer from 0 to 10. The value of x can be varied by changing the temperature and time used in the dewatering step. In addition, the atom ratio of boron and calcium (∼2:1) added in the reaction was different from that tested by AAS. The acquired composition mainly resulted from the unused boric acid at the studied reaction condition. However, this unused boric acid can be removed to avoid affecting the appearance of the product. The composition of the synthesized product was further confirmed by subsequent Fourier transform-infrared spectroscopy (FT-IR) and TG experiments. The difference in acid value between the synthesized product and oleic acid, corresponding to 65.3 and 202 mg KOH/g, respectively, are attributed to the oleic acid loss during the preparation process, adsorption of oleic acid on the calcium borate surface, and possible reaction between oleic acid and calcium hydroxide. This result elucidates the reason why the atom ratio of boron and calcium added in the reaction was different from that tested by AAS. 3.2. Structural Characterization of CBNL. Following the unique form of CBNL, the size of the nanosized calcium borates in CBNL were characterized using a laser particle analyzer (LPA), in which CBNL was diluted in oleic acid. The in situ tested method avoids the reagglomeration of nanoparticles during sample preparation in powder form for transmission electron microscopy or SEM, which can also be conducted during the addition of nanoparticles in oil to form larger particles.10 Figure. 1 shows the distribution of the particle sizes of nanosized calcium borate in CBNL. The nanosized calcium borate has an average size of about 18 nm, and with the size distribution measured using a half-bandwidth of 20 nm. This result indicated that the novel method is effective in obtaining surface modified inorganic nanoparticles. The FT-IR spectra of pure oleic acid, pure calcium oleate, and CBNL are shown in Figure 2 to confirm the CBNL structure. For pure oleic acid, the bands at about 2923 and 2852 cm−1 are assigned as −CH2 asymmetric and symmetric
Table 3. Properties of the CBNL property
value
appearance dynamic viscosity (100 °C, mm2/s) acid value (mg KOH/g) B% Ca% B/Ca (molar ratio)
yellow liquid 198.7 65.3 2.15 6.96 1.15
2.3. Characterization. The structure and size of the samples were observed using a Nano ZS 3600 laser particle analyzer (LPA). The samples were diluted in oleic acid and, then, centrifuged to eliminate the effect of impurity and dust on the tested results. The infrared (IR) spectra of the samples were characterized on a Bio-hRad FTS-165 spectrometer, with wave numbers ranging from 4000 to 400 cm−1 at 4 cm−1 resolution. Thermogravimetric analysis (TG) was carried out using a 500 analyzer. Tests were performed at heating rates of 20 °C/ min from 50 to 900 °C in atmospheric air, using tabular αAl2O3 as reference weight losses. The friction-reducing and AW properties were evaluated on an MRS-1J four-ball tribometer at a rotating speed of 1450 rpm, which was controlled by a chip in the four-ball tester, at an error rate of ±5 rpm, with testing duration of 30 min at room temperature (25 °C). Friction coefficients were recorded throughout each test using a strain gauge equipped with the four-ball tester, which was cleaned in a litrasonic bath using acetone for 30 min. The wear-scar diameters (WSD) were measured with an optical microscope. The balls used in the test were 12.7 mm with a hardened high speed carbon of 59−61 hardness, surface roughness (Ra) of 0.00762−0.0152 μm, and compositions of Fe 95.14 wt %, Cr 1.60 wt %, and C3.26 wt % 13870
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Figure 1. Distribution of particle size of CBNL. Figure 3. TG of (a) pure oleic acid, (b) pure calcium oleate, and (c) CBNL.
weight loss occurred below 130 °C, which can be attributed to trace amounts of solvent. The weight loss from 205.60 to 550 °C can be attributed to several reasons, including the decomposition of pure oleic acid, organic groups on the surface of calcium borate, and crystal water contained by the compound. The weight change was minimal beyond 550 °C. In addition, the decomposition temperature of CBNL was the highest among the three samples. This excellent thermal stability helps increase the quality of base oil. Therefore, CBNL possesses good thermal stability (initial decomposition temperature are above 200 °C). Furthermore, the results confirm that CBNL is different from pure calcium oleate from the plot. A more compact multilayer of oleic acid was formed on the surface of calcium borate in the CBNL because of the use of oleic acid as a carrier. As a result, the longchain hydrocarbons on the surface of calcium borate have stronger interaction with the carrier, which provides CBNL with better solubility in lubricating oil compared with other surface-modified nanoparticles in powder form. 3.3. Load-Carrying Capacity. Oleic acid, as a surface modification agent, can form a good chemical adsorption film within the tribological system. This property of oleic acid affects the tribological property of the system. Similarly, calcium oleate, which may be generated in CBNL, also exhibits tribological properties. Therefore, aside from the base oil alone and base oil with CBNL, the base oils with oleic acid and calcium oleate were also investigated. The main purpose of the investigation is to illustrate the tribological properties of CBNL and explore the origin of the tribological properties of CBNL, calcium borate nanoparticles, or oleic acid. The extreme pressure performance of the additives in base oil was evaluated as the maximum nonseizure load (pB) and the weld load (pD). The results of the evaluation are shown in Table 4. All these additives in the study can improve the pB value of base oil in the order of T202 > CBNL > oleic acid. In addition, the AW capacity of base oil was greatly improved after adding T202 and CBNL. Conversely, oleic acid impaired the AW capacity of base oil. Therefore, CBNL can improve the load-carrying capacity, which is slightly worse than that of T202. The load-carrying capacity of CBNL is different from
Figure 2. FT-IR spectra of (a) pure oleic acid, (b) pure calcium oleate, and (c) CBNL.
stretching, respectively. The band at 1711 cm−1 is assigned to the stretching vibration of CO. The band at 1467 cm−1 is assigned as −CH3 bending vibration. For pure calcium oleate, all the bands at about 2923, 2852, and 1467 cm−1 are visible. Moreover, the absorption that occurred at about 3475 cm−1 are characteristic of the stretching vibrations of O−H, which resulted from trace amounts of H2O contained in pure calcium oleate. For the CBNL, the band at 1698 cm−1 (Figure 2c) is a characteristic of the stretching vibrations of H−O−H. The presence of this band proves that crystal water exists in CBNL. This result is similar to the above-mentioned composition result. In addition, the crystal water can be varied by changing the temperature and time used in the dewatering step. Both of the bands at 1100 and 970 cm−1 are assigned as asymmetric stretching of B(4)−O.9,16 No absorption bands of other valence bond of boron were observed (Figure 2c). This result shows that the structure of calcium borate in the CBNL is unitary, 2CaO·1.15B2O3·xH2O. The differences of FT-IR spectra between pure calcium oleate and CBNL confirm their structural difference, which shows that the existing calcium oleate is extremely weak in CBNL. The samples were analyzed by TGA to investigate the nature of CBNL. Figure 3 shows the TGA plots of pure oleic acid, pure calcium oleate, and CBNL. For pure oleic acid (Figure 3a), a total weight loss of nearly 100 wt % was exhibited after heating from 130 to 300 °C. This phenomenon was also consistent with the boiling point of pure oleic acid, 286 °C. For pure calcium oleate, a total weight loss of above 90 wt % was observed. About 30 wt % weight loss occurred below 200 °C, which may be attributed to the existence of a solvent. For CBNL (Figure 3c), a total weight loss of 72.132 wt % from 50 to 900 °C was observed. The weight loss increased rapidly from 200 to 500 °C. Meanwhile, about 20 wt % calcium borate
Table 4. Load-Carrying Capacity of Additives
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lubricants
base oil
5 wt % oleic acid/ base oil
5 wt % T202/ base oil
5 wt % CBNL/ base oil
pB(N) pD(N)
372.6 1569.1
539.4 1235.6
882.6 1961.3
509.9 1689.1
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that of oleic acid, illustrating the difference between the properties of CBNL and oleic acid. 3.4. Antiwear Properties of Calcium Borate Nanoparticles. The WSD is an indication of the wear extent after sliding contact. The relationship between WSD and the differently applied load is listed in Figure 4. The WSD of
Figure 6. Effect of applied load on wear-scar diameter.
to that of T202, as shown in Figure 6. Both of the two additives improved the AW property of the base oil. Compared with the T202 added-oil under all the observed loads, the WSD of CBNL added-oil was lesser, confirming that CBNL in base oil possesses excellent AW capacity. 3.5. Friction-Reducing Properties of Calcium Borate Nanoparticles. The friction coefficient under different loads and at the same concentration of 2 wt % is presented in Figure 7. The friction coefficient of base oil was unstable with the
Figure 4. Effect of applied load on wear-scar diameter.
base oil decreased significantly after adding CBNL or calcium oleate. Furthermore, the AW property of base oil with CBNL was better than that with calcium oleate, except for the applied load of 392 N. This phenomenon indicates that the calcium borate nanoparticles, and not the calcium oleate, are associated with the AW property of base oil with CBNL. Moreover, oleic acid also has a negative effect on the WSD of the base oil alone, which also shows that the calcium borate nanoparticles are important in the AW property of CBNL. Compared with the AW properties of oleic acid and calcium oleate, the unique AW property of CBNL is acquired from its calcium borate nanoparticle content. The relationship between the WSD and the concentration of additives under 392 N is listed in Figure 5. The curves in Figure
Figure 7. Effect of applied load on friction coefficient.
increase of applied load. The friction coefficient of calcium oleate was the smallest in all the observed samples, followed by oleic acid, CBNL, and base oil. The friction coefficient of CBNL added-oil was slightly worse than that of oleic acid added-oil, which may be caused by an effective alkyl layer absorbed on the friction zone for oleic acid. On the basis of the friction coefficient of all observed samples, it could be deduced that calcium borate in the oleic acid matrix was prepared successfully. The ability of reducing friction came from calcium borate nanoparticles in CBNL. Figure 8 clearly reveals the influence of the concentration of additives on the friction coefficient under 392 N. Both T202 and CBNL can significantly improve the friction-reducing ability of base oil in the concentration range of 1−5 wt %. Furthermore, an optimum concentration for T202 and CBNL, corresponding to 4 and 5 wt %, respectively, was observed. The optimum concentration of CBNL was similar to that of T202. Thus, CBNL can minimally cause metal corrosion without affecting the reducing-friction ability. The friction coefficient of CBNL in base oil under different loads and at the same concentration of 5 wt % is presented in Figure 9. The T202
Figure 5. Effect of additive concentration on wear-scar diameter under 392 N.
6 depict the AW properties of all the additives used in the study under different loadings at the same concentration of 5 wt %. The WSD of base oil decreased significantly after adding CBNL or T202, especially at a relatively high concentration. Although no content of S was found in CBNL, the AW property of CBNL was better than of that of T202 at the same concentration, except within the concentration of 1 wt %. This result can be ascribed to the addition and even distribution of boron. The curvilinear obtained in WSD of CBNL is similar 13872
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mechanistic actions of calcium borate nanoparticles as base oil additives. Boron, calcium, oxygen, and iron were detected, and the results are shown in Figure 11. The binding energy of
Figure 8. Effect of the concentration of additives on friction coefficient under 392 N.
added-oil shows much higher friction coefficient than that of CBNL added-oil.
Figure 9. Effect of applied load on friction coefficient.
Therefore, CBNL possesses better friction-reducing and AW abilities compared with T202. These complicated relations may be attributed to the addition of boron in the molecular structure and the even distribution of boron on the friction zone, derived from the unique preparation method of CBNL. 3.6. Worn Surface Analysis. Figure 10 shows the 3D plots of the worn surface lubricated with base oil alone and with
Figure 10. 3D plot of the worn surface lubricated with base oil alone (a) and CBNL (b) under 196 N.
CBNL added-oil under 196 N. More scratches and deeper furrows were observed on the wear scar obtained from the test lubricated with base oil alone than that with base oil with 5 wt % CBNL. This result further verifies that CBNL possesses good AW ability. The XPS analysis of the worn surface determines the chemical structure of the tribofilm and examines the
Figure 11. XPS of the worn surface (CBNL, 5 wt % and 392 N). 13873
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(2) Gong, D.; Zhang, P.; Xue, Q. Studies on the relationship between structure of chlorine-containing compounds and their wear and extreme-pressure behavior. Lubr. Eng. 1990, 46, 566−572. (3) Zhang, J; Liu, W. M.; Xue, Q. J.; Ren, T. H. A study of N and S heterocyclic compounds as a potential lubricating oil additive Wear. Wear 1999, 224, 160−164. (4) Milner, J. L.; Phillips, R. L.; Ozbalik, N. Odor reduction of lubricant additives packages. U.S. Patent 6,133,207, 2000. (5) Zhang, J; Zhang, Z. J.; Liu, W. M.; Xue, Q. J. The tribological behaviors of 2-mercaptobenzoxazole derivatives as additives in liquid paraffin. Wear 1998, 219, 184−187. (6) Choudhary., R. B.; Pande, P. P. Lubrication potential of boron compounds:an overview. Lubr. Sci. 2002, 14 (2), 211−222. (7) Bakunin, V. N.; Yu.Suslov, A; Kuzmina, G. N.; Parenago, O. P. Recent achienement in the synthesis and application of inorganic nanoparticles as lubricant components. Lubr. Sci. 2005, 17 (2), 127− 145. (8) Hu, Z. S.; Dong, J. X.; Chen, G. X. Preparation and tribological properties of nanoparticle lanthanum borate. Wear 2000, 243, 43−47. (9) Tian, Y. M.; Guo, Y. P.; Jiang, M Synthesis of Hydrophobic Zinc Borate Nanodiscs for Lubrication. Mater. Lett. 2006, 60, 2511−2515. (10) Mohsen, M; Neway, D; John, H. Modification of sheet metal forming fluids with dispersed nanoparticles for improved lubrication. Wear 2009, 267, 1220−1225. (11) Bakunin, V. N.; Kuzmina, G. N.; Kasrai, O. P.; Parenago, O. P.; Bancroft, G. M. Tribological behavior and tribofilm composition in lubricated systems containing surface-capped molybdenum sulfide nanoparticles. Tribol. Lett. 2006, 22, 289−296. (12) Gao, Z. H.; Hao, L. F.; Huang, W; Xie, K. C. A novel liquidphase technology for the preparation of slurry catalysts. Catal. Lett. 2005, 102, 139−141. (13) Zhou, J. F.; Wu, Z. S; Zhang, Z. J.; Liu, W. M.; Xue, Q. J. Tribological behavior and lubricating mechanism of Cu nanoparticles in oil. Tribol. Lett. 2000, 8, 213−218. (14) Wang, Y. G.; Li, J. S.; Ren, T. H. A potential approach to replace sulfurized olefins with borate ester containing xanthate group in lubricating oil. Chin. Sci. Bull. 2008, 992−667. (15) Liu, Z. H.; Zuo, C. F.; Li, S. Y. Standard molar enthalpy of formation of Ca2[B4O7(OH)2]. Thermochim. Acta 2005, 433, 196− 199. (16) Ding, X; Zhao, J.; Liu, Y.; Zhang, H; Wang, Z. Silica nanoparticles encapsulated by polystyrene via surface grafting and in situ emulsion polymerization. Mater. Lett. 2004, 58, 3126−3130. (17) Hu, Z. S.; Dong, J. X.; Chen, G. X. Preparation and tribological properties of nanoparticle lanthanum borate. Wear 2000, 243, 43−47. (18) Wagner, C. D.; Riggs, W. M.; Davis, L. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin−Elmer Corporation: Eden Prairie, MN, 1979.
B1S located at 192.0 and 187.9 eV indicates that B2O3 and FeB are present on the worn surface, suggesting that calcium borate nanoparticles were degraded to B2O3, and then reacted with the metal surface. The unidentified peak located at 196.5 eV was similar to that found by Hu.17 No clear calcium signal was detected, suggesting that calcium was not involved in the tribological reaction. The binding energy of iron located at 707.1 and 711.2 eV corresponds to FeB and Fe2O3, respectively.18 Combined with the binding energy of 530.8 eV in the O2P spectra, it can be further ascertained that Fe2O3 must be produced. Therefore, tribological reaction occurred between the synthetic nanoparticles and the metal surface during the sliding process. The tribological mechanism can be determined as follows: First, the nanoparticles contained in base oil were deposited or adsorbed on the worn surface because of the chemical and/or physical action. For CBNL, the chemical action was greater than the physical action because the nanoparticles were coated with oleic acid. CBNL was evenly dispersed and formed a continuous film with increasing shearing effect. This film can facilitate the changes in friction. Second, the film reacted with itself and with the substrate because of the extreme pressure effect and high temperature. The reaction products include B2O3, FeB, and Fe2O3, which were proven by XPS. Both the depositions and the reaction products formed a wear resistance film and provided the base oil with excellent tribological properties.
4. CONCLUSIONS (1) Calcium borate nanoparticles embedded in an oleic acid liquid matrix (CBNL) were successfully synthesized. The average size of calcium borate nanoparticles in CBNL was about 18 nm, and the structure of CBNL was different from that of calcium oleate. (2) The load-carrying property of CBNL as additive was slightly worse than that of T202. However, CBNL possessed better AW and friction-reducing abilities than T202 under the studied loads, which was derived from the unique structure of CBNL. (3) Both the depositions and the tribochemical reaction products comprised of B2O3, FeB, and Fe2O3 exhibited excellent tribological properties.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the Shanghai Leading Academic Discipline Project (Project Number J51503), National Natural Science Foundation of China (Project Number 20976105), Science and Technology Commission of Shanghai Municipality (Project Number 09QT1400600), Innovation Program of Shanghai Municipal Education Commission (Project Number 11ZZ179), and Innovation Program of Shanghai Municipal Education Commission (Project Number 09YZ387).
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REFERENCES
(1) Herdan., J. M. Trends in gear oil additive. Lubr. Sci. 1997, 9 (2), 195−206. 13874
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