The Synergistic Antiwear Performances of Organic Titanium Compounds Containing Sulfur with Borate Ester Additive

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). 3e 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. 3e 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. 3e 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). 3e proposed synergistic antiwear mechanism involves an effective interaction between TiDDC or TiS and BNO additive, respectively.


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][2][3][4][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].
e 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][12][13].So, it is desirable to partially or totally replace ZDDP with other lubricant additives without reducing the performances of formulated engine oils.
e 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][20][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.

Oil Samples and Additives.
e 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.
e model additives in different proportions were weighed, mixed, and dissolved in 150 SN mineral base oil with viscosity 5.1 mm 2 /s at 100 °C.

Characterization of TiDDC and TiS.
e structures of TiDDC and TiS were confirmed through Fourier-transform infrared spectroscopy (FTIR) with PerkinElmer Spectrum Two Infrared Instrument.e stretching vibration absorption band of N-H at 3400 cm −1 was observed in TiDDC.e bending vibration absorption band of C-N at 1513 cm −1 was observed in TiDDC, which confirmed the formation of the secondary amine compound.e 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.e 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.
e same C-H bands were observed at 2929 and 2872 cm −1 in TiS, which were the characteristic peak of methene.e 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.

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.e 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.
e 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.en, the average of the three wear scar diameters was calculated and cited as the wear scar diameter reported in this paper.
e load-carrying capacities of oils were characterized as maximum nonseizure load (P B value) and weld load (P D value), which was evaluated by a short extreme pressure test (10 s) according to ASTM D2783-88.

Surface Analysis.
e 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.e 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.

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.
e P B and P D 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 P B and P D 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 P B and P D 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. e binding energy of B 1s , N 1s , O 1s , Fe 2p , S 2p , and Ti 2p 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.
e 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 2 Journal of Spectroscopy 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.erefore, the SEM analysis of the worn surfaces also confirms the synergistic effects of TiDDC with BNO additives on the antiwear properties.e 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.ese 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.ough 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. ese observations indicate that the titanium oxides formed on the worn surface play an important role in improving antiwear properties of lubricants.

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   6; the results shows that the addition of TiS and BNO to base oils could all improve the load-carrying properties (P B and P D values), and TiS is better than BNO.

Journal of Spectroscopy
When BNO is combined with TiS, at constant total concentration of the additive packages, the P B and P D 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.

Conclusion
(1) Titanium dibutyldithiocarbamates (TiDDC) as lubricant additive possess better antiwear properties than ZDDP.ough 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.ese 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.

Figure 4 :
Figure 4: SEM photographs of worn scar lubricated with additive containing oil under different loads: (a) BNO with TiS at 392 N; (b) BNO with TiS at 490 N; (c) BNO at 490 N; (d) TiS at 490 N.

Table 2 :
Load-carrying properties of TiDDC and BNO.

Table 3 :
Binding energy of elements on the worn surface and the main compounds.

Table 4 :
Atomic concentration (%) of elements on the wear scar by EDX.

Table 5 :
Antiwear properties of TiS and BNO.

Table 6 :
Load-carrying capacities of BNO mixed with TiS.