Abstract
Two oil-soluble organic titanium compounds (OTCs) such as titanium dialkyldithiocarbamate (TiDDC) and sulfurized titanate (TiS) were synthesized and identified by Fourier-transform infrared spectroscopy (FTIR). The antiwear and extreme pressure properties of TiDDC or TiS with borate ester containing nitrogen (BNO) additive in mineral base oils were evaluated by four ball tester. The results show that TiDDC and TiS not only possess good antiwear and load-carrying properties, respectively, but also exhibit good antiwear synergism with BNO additive without impairing extreme pressure performances. Moreover, the synergistic antiwear properties of the said additives are improved significantly under the optimum additives ratios. The topography of wear scar and the composition and chemical states of typical elements on the rubbing surfaces were analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) and X-ray photoelectron spectrometer (XPS). The proposed synergistic antiwear mechanism involves an effective interaction between TiDDC or TiS and BNO additive, respectively.
1. Introduction
In the field of lubrication, zinc dialkyldithiophosphate (ZDDP) has been widely utilized as a multifunctional lubricant additive exhibiting good antiwear and antioxidative properties in engine oils [1–5]. However, the existence of phosphorus in ZDDP would cause the catalyst to be poisoned, thus shortening the useful life of the catalytic converter [6, 7]. In addition, the existence of zinc element contributes to the emission of particulates in the exhaust [8]. The demand for reduction of phosphorous content in engine oils has forced the oil suppliers to improve the formulation of lubricant additives [9, 10]. Moreover, the International Lubricant Standardization and Approval Committee (ILSAC) provided the GF-5 performance standards that place limits of 0.08% max. on phosphorus in the finished engine oils, so as to improve fuel economy, environmental protection, and emissions system compatibility [11–13]. So, it is desirable to partially or totally replace ZDDP with other lubricant additives without reducing the performances of formulated engine oils.
Organic titanium compounds (PFTCs) as lubricant additives are capable of enhancing antiwear, antioxidation, and fiction reducing performances in the boundary lubrication regime and improving fuel economy in engine oils [14–17].
The required function of lubricants is achieved by appropriate balance of different lubricating additives [18]. Investigations of synergistic effects that can optimize the composition and expand the areas of application of additive packages are of considerable scientific and practical interests [19–21]. In the present study, the synergistic antiwear and load-carrying properties of borate ester containing nitrogen (BNO) with titanium dialkyldithiocarbamate (TiDDC) or sulfurized titanate (TiS), respectively, were investigated, and subsequent exploitation of the results for the development of a synergistic antiwear additives composition were offered.
2. Experimental
2.1. Oil Samples and Additives
The organic borate ester additive containing nitrogen (BNO) was purchased from Vanderbilt Company; and titanium dialkyldithiocarbamate (TiDDC) and sulfurized titanate (TiS) were synthesized in the lab. All concentrations of additives used in the investigation are expressed in percentages by weight if not stated otherwise. The model additives in different proportions were weighed, mixed, and dissolved in 150 SN mineral base oil with viscosity 5.1 mm2/s at 100°C.
2.2. Characterization of TiDDC and TiS
The structures of TiDDC and TiS were confirmed through Fourier-transform infrared spectroscopy (FTIR) with PerkinElmer Spectrum Two Infrared Instrument. The stretching vibration absorption band of N-H at 3400 cm−1 was observed in TiDDC. The bending vibration absorption band of C-N at 1513 cm−1 was observed in TiDDC, which confirmed the formation of the secondary amine compound. The asymmetric and symmetric stretching vibration absorption bands of C-H were observed at 2965 and 2902 cm−1, respectively, which indicated the existence of methyl in the product. The stretching vibration absorption bands of C=S and C-S were detected at 1125 cm−1 and 967 cm−1, respectively, and the stretching vibration absorption bands of Ti-S was detected at 521 cm−1, which confirmed main functional groups of TiDDC.
The same C-H bands were observed at 2929 and 2872 cm−1 in TiS, which were the characteristic peak of methene. The stretching vibration absorption bands of C=O and C-O were detected at 1740 cm−1 and 1068 cm−1, respectively. In addition, the absorption band of C-C bonds was detected at at 826 cm−1, and the stretching vibration absorption bands of Ti-S was detected at 603 cm−1, which confirmed main functional groups of TiS.
2.3. Tribological Tests
Tribological properties of 150 SN oils containing additives were evaluated with four ball tester at a rotating speed of 1450 rpm and room temperature about 20°C. The balls used in the tests were made of GCr15 bearing steel (AISI 52100) at a diameter of 12.7 mm with HRC of 59 to 61. All balls were ultrasonically rinsed with petroleum ether for 10 min before the experiment.
The antiwear properties of oils were evaluated under loads of 392, 490, and 588 N, respectively, for 60 min according to ASTM D4172-82, and they were characterized by average wear scar diameters (WSD). An optical microscope was used to determine the wear scar diameters of the three lower balls with an accurate reading to 0.01 mm. Then, the average of the three wear scar diameters was calculated and cited as the wear scar diameter reported in this paper.
The load-carrying capacities of oils were characterized as maximum nonseizure load ( value) and weld load ( value), which was evaluated by a short extreme pressure test (10 s) according to ASTM D2783-88.
2.4. Surface Analysis
The chemical states of rubbing surface on the worn scar were investigated using an X-ray photoelectron spectrometer (XPS), which was conducted using a PHI-6100 electrometer. The radiation source was Mg Kα line with pass energy of 29.35 eV. All binding energies were compared with a reference standard of 284.6 eV for carbon. Profiles and elemental distributions of the worn scar were obtained using the CSM-950 scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis. Particular attention was paid to the atomic concentration of elements on the worn scars of the steel balls. Before XPS and SEM analysis, all samples were ultrasonically rinsed with hexane and petroleum ether for 10 min.
3. Results and Discussion
3.1. Antiwear and Load-Carrying Properties of BNO with TiDDC in Mineral Oil
In order to limit phosphorous content in engine oils, the authors select titanium dibutyldithiocarbamates (TiDDC) to replace zinc dialkyldithiophosphate (ZDDP). TiDDC and organic borate ester (BNO) were added to 150 SN base oil in different proportions; the wear scar diameters (WSD) of balls lubricated by different oil formulations under different loads are reported in Table 1. It shows that the TiDDC and BNO additives all exhibit better antiwear properties than base oil at each experimental load, and the antiwear properties of TiDDC are better than ZDDP under the same condition. When TiDDC was combined with BNO, at constant total concentration of the package, they possess good antiwear properties in oils. Especially, at the concentration of BNO lower than or equal to TiDDC in base oils, they exhibit good antiwear synergism. For instance, the mixtures of 0.25% BNO with 0.75% TiDDC exhibit the best antiwear synergism, and the mixtures 0.5% BNO with 1.0% are better.
The and values of oils containing different additives are summarized in Table 2; the results show that TiDDC and BNO could improve the load-carrying properties (enhance and values) of 150 SN base oils, respectively, and extreme pressure properties of TiDDC is better than BNO. At constant total concentration of the additives, the combination of BNO with TiDDC also could improve and values of base oils comparing with the same dosage of TiDDC.
In order to investigate antiwear synergism between TiDDC and BNO, the worn surface of the ball lubricated by 150 SN oils containing 1.0% TiDDC and 0.5% BNO was analyzed by XPS, SEM, and EDX. The binding energy of B1s, N1s, O1s, Fe2p, S2p, and Ti2p XPS spectra on the worn surface under 490 N and the main compounds is summarized in Table 3, from which we can conclude that titanium oxides, sulfide, N-containing compound, and iron oxides were formed on the worn surfaces under the tribostressed process.
The profiles and elemental distribution on the wear scars at 392 N, 490, and 588 N are shown in Figures 1(a)–1(e) and Figures 2(a)–2(c), and the atomic concentration of the elements on the wear scars is listed in Table 4. As displayed in Figure 1, the worn surface of the balls lubricated by 150 SN oil containing 1.0% TiDDC and 0.5% BNO is smoother than the worn surface of oils containing only 2.0% TiDDC or 2.0% BNO under the load of 588 N, and the area of worn scar at each load is smaller than TiDDC or BNO. Moreover, the enlarged micrographs (×1500) confirm the scratched surface of wear scar lubricated by specific oils. Especially, the rubbing surfaces of wear scar lubricated by BNO or TiDDC only had already been broken to different degrees at the same load. However, the rubbing surfaces of wear scar lubricated by oil containing BNO with TiDDC are uniform and smooth at each load, which contributed to the development of thick lubricating films caused by faster deposition rate than the wear rate of films. While for the individual components, the wear rate is faster than the deposition rate, so as the lubricating films are thinner. Therefore, the SEM analysis of the worn surfaces also confirms the synergistic effects of TiDDC with BNO additives on the antiwear properties.
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The data in Table 4 were obtained from Figures 2(a)–2(c) using a computer program and normalized to 100 for all given compositions. It was discovered that the contents of Ti and S atoms on the worn surface were decreased with the increase of the loads. These changes suggest that the increase of loads would result in greater wear rate than the forming rate of the lubricating film, thus leading to the decrease of S and Ti contents, which is consistent with the increase of WSD. Though TiDDC additive contains higher contents of S and Ti than the complex of TiDDC and BNO additive, the contents of Ti on the wear scars from oils containing 1.0% TiDDC under 490 N are less than that of the complex; the atomic concentration of Ti is 0.756 in Figure 3. These observations indicate that the titanium oxides formed on the worn surface play an important role in improving antiwear properties of lubricants.
3.2. Antiwear and Load-Carrying Properties of TiS with BNO in Mineral Oil
Sulfurized titanate (TiS) and organic borate ester (BNO) were added to 150 SN base oil, and the wear scar diameters (WSD) of tested balls are listed in Table 5. We can see clearly that TiS or BNO additives possess better antiwear properties than base oils at each experimental load, and the antiwear properties of BNO is better than TiS at the same condition. Moreover, there are no obvious changes that occurred with the increase of additive concentration. When TiS is combined with BNO, the antiwear properties of lubricants are enhanced at different degrees compared with each alone. For instance, at constant total concentration of additive package, the WSD of the oils containing 0.75% TiS and 0.25% BNO are lower than that of oils containing 1.0% TiS or 1.0% BNO alone. The WSD of oils containing 1.0% BNO with 1.0% TiS are lower than that of oils containing 2.0% BNO or 2.0% TiS alone, which are confirmed by SEM photographs of worn scar displayed in Figures 4(a)–4(d).
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The and values of oil containing different additives are listed in Table 6; the results shows that the addition of TiS and BNO to base oils could all improve the load-carrying properties ( and values), and TiS is better than BNO. When BNO is combined with TiS, at constant total concentration of the additive packages, the and values of the base oils containing TiS and BNO were enhanced at different degrees and similar to the same dosage of TiS. For instance, the mixtures of oil containing 0.75% TiS and 0.25% BNO are almost equal to 1.0% TiS alone, so does the oil containing 1.0% TiS and 1.0% BNO.
4. Conclusion
(1)Titanium dibutyldithiocarbamates (TiDDC) as lubricant additive possess better antiwear properties than ZDDP. Though the combination of TiDDC with BNO might influence their performance negatively, the concentration of BNO is lower than TiDDC under the constant sum of dosage, and the good antiwear synergism was obtained.(2)Sulfurized titanate (TiS) as lubricant additive also possesses better antiwear properties than ZDDP; when TiS was combined with BNO in base oils, they exhibit good synergistic antiwear properties.(3)Even with the small additions of described above, nearly half of the phosphorus from ZDDP can be beneficially replaced by smaller amounts of boron in the lubricating oils, and the phosphorus in the lubricants may be removed with addition of TiDDC with BNO. These open up an opportunity for the formulation of friendly additive packages, which deserves to be the object of more detailed studies related to particular lubricant specifications.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was subsidized by funds from the National Natural Science Foundation of China (Grant 51575525) and the Natural Science Foundation of Jiangsu Province (Grant BK20151137, BK20161188). The project was also supported by the Tribology Science Fund of the State Key Laboratory of Tribology (Grant SKLTKF17B11).