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.
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 [
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 [
The required function of lubricants is achieved by appropriate balance of different lubricating 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.
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.
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 (
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.
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
Antiwear properties of TiDDC and BNO.
Samples | Wear scar diameter, mm | ||
---|---|---|---|
392 N | 490 N | 588 N | |
150 SN | 0.63 | 1.12 | Failure |
+1.0% BNO | 0.49 | 0.69 | Failure |
+2.0% BNO | 0.47 | 0.72 | 1.18 |
+1.0% TiDDC | 0.52 | 0.62 | 1.09 |
+2.0% TiDDC | 0.51 | 0.64 | 1.04 |
+1.0% ZDDP | 0.51 | 0.71 | 1.21 |
+1.0% TiDDC + 1.0% BNO | 0.39 | 0.46 | 0.97 |
+0.25% TiDDC + 0.75% BNO | 0.42 | 0.52 | 0.78 |
+0.5% TiDDC + 0.5% BNO | 0.40 | 0.49 | 0.75 |
+0.75% TiDDC + 0.25% BNO | 0.39 | 0.44 | 0.51 |
+1.0% TiDDC + 0.5% BNO | 0.38 | 0.46 | 0.50 |
The
Load-carrying properties of TiDDC and BNO.
Samples | Maximum nonseizure load |
Weld load |
---|---|---|
150 SN | 392 | 1236 |
+1.0% BNO | 647 | 1569 |
+1.0% TiDDC | 745 | 1961 |
+1.0% TiDDC + 1.0% BNO | 862 | 1961 |
+0.25% TiDDC + 0.75% BNO | 696 | 1569 |
+0.5% TiDDC + 0.5% BNO | 745 | 1569 |
+0.75% TiDDC + 0.25% BNO | 745 | 1569 |
+1.0% TiDDC + 0.5% BNO | 804 | 1961 |
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
Binding energy of elements on the worn surface and the main compounds.
Compounds | Binding energy, eV | |||||
---|---|---|---|---|---|---|
B1s | N1s | O1s | Fe2p | S2p | Ti2p | |
|
||||||
+0.5% BNO + 1.0% TiDDC | 190.3 | 400.1 | 529.9 | 710.8 | 160.7 | 458.5 |
|
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|
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N-containing compounds | 399.8 | |||||
FeS | 710.8 | 161.6 | ||||
Fe2O3 | 529.6 | 710.9 | ||||
TiO2 | 529.7 | 458.3 |
The profiles and elemental distribution on the wear scars at 392 N, 490, and 588 N are shown in Figures
SEM photographs of worn scar lubricated by different oils under different loads: (a) oil containing TiDDC and BNO at 392 N; (b) oil containing TiDDC and BNO at 490 N; (c) oil containing TiDDC and BNO at 588 N; (d) oil containing TiDDC at 588 N; (e) oil containing BNO at 588 N.
EDX spectra of wear scars under different loads: (a) 392 N; (b) 490 N; (c) 588 N.
Atomic concentration (%) of elements on the wear scar by EDX.
Analytic area | C | S | Cr | Fe | Ti |
---|---|---|---|---|---|
Wear scar at 392 N | 2.624 | 4.342 | 1.321 | 90.078 | 1.635 |
Wear scar at 490 N | 0.558 | 2.590 | 1.740 | 93.150 | 1.418 |
Wear scar at 588 N | 0.793 | 1.521 | 1.420 | 95.289 | 0.977 |
The data in Table
EDX spectra of wear scars with 1.0% TiDDC under 490 N.
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
Antiwear properties of TiS and BNO.
Samples | WSD, mm | ||
---|---|---|---|
392 N | 490 N | 588 N | |
150 SN | 0.63 | 1.12 | Failure |
+1.0% BNO | 0.49 | 0.69 | Failure |
+2.0% BNO | 0.47 | 0.72 | 1.18 |
+1.0% TiS | 0.62 | 0.85 | 1.06 |
+2.0% TiS | 0.61 | 0.86 | 0.98 |
+1.0% TiS + 1.0% BNO | 0.33 | 0.71 | 0.92 |
+0.25% TiS + 0.75% BNO | 0.44 | 0.94 | 1.56 |
+0.5% TiS + 0.5% BNO | 0.47 | 0.68 | 1.67 |
+0.75% TiS + 0.25% BNO | 0.43 | 0.51 | 0.83 |
+0.5% TiS + 1.0% BNO | 0.37 | 0.86 | 1.12 |
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.
The
Load-carrying capacities of BNO mixed with TiS.
Samples | Maximum nonseizure load |
Weld load |
---|---|---|
150 SN | 392 | 1236 |
+1.0% BNO | 647 | 1569 |
+1.0% TiS | 745 | 1569 |
+1.0% TiS + 1.0% BNO | 921 | 1961 |
+0.25% TiS + 0.75% BNO | 647 | 1569 |
+0.5% TiS + 0.5% BNO | 647 | 1569 |
+0.75% TiS + 0.25% BNO | 745 | 1961 |
+0.5% TiS + 1.0% BNO | 862 | 1569 |
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. 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. 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.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
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).