Article pubs.acs.org/IECR
Tribological Properties of Alkylphenyl Diphosphates as HighPerformance Antiwear Additive in Lithium Complex Grease and Polyurea Grease for Steel/Steel Contacts at Elevated Temperature Xinhu Wu,† Qin Zhao,†,‡ Gaiqing Zhao,† Junming Liu,† and Xiaobo Wang*,† †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *
ABSTRACT: The alkylphenyl diphosphates pentaerythritol tetrakis(diphenyl phosphate) (PDP) and trimethylolpropane tris(diphenyl phosphate) (TDP) were evaluated as the antiwear additives in lithium complex grease and polyurea grease at 200 °C. The results indicated that both additives may effectively reduce the sliding friction and wear as compared to the base greases. The tribological performances were generally better than the normally used molybdenum disulfide (MoS2)-based additive package in lithium complex grease and also in polyurea grease. Boundary lubrication films composed of Fe(OH)O, Fe3O4, FePO4, and compounds containing the P−O bonds were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance. phosphorus-based flame retardants and have the advantages of high molecular weight and high thermal and chemical stability as well as very low vapor pressure, indicating that they might be potential candidates as antiwear additives under high temperature in lubricating greases.11 However, the existing studies of PDP and TDP are on the effects of flame retardant,12,13 and much less is known with respect to the tribological performances of PDP and TDP, especially as AW additives under harsh friction conditions. The present paper reports the tribological properties of PDP and TDP as AW additives in lithium complex grease (LCG) and polyurea grease (PG) for steel/steel contacts at elevated temperature, and their tribological behavior were compared with molybdenum disulfide (MoS2), which is one of the traditional AW additives for lubricating greases.14,15
1. INTRODUCTION Lubricating greases are semisolid colloidal dispersions consisting of a thickening agent in a liquid lubricant. They owe their consistency to a gel-forming network where the thickening agent is dispersed in the lubricating base fluid. Grease is a preferred form of lubrication in certain applications because it gives low friction, is easily confined, and has a long lubricating life at low cost.1,2 However, with rapid advances in aircrafts, turbines, automobiles, trucks, farm equipment, railroad equipment, industrial machinery, and so forth, the demand for the grease to operate at high load, high speed, and high temperature conditions is increasing.2−6 Therefore, it is increasingly significant that the grease compositions can provide adequate lubrication at high temperatures, that is, 300−450 °F (149−232 °C)3,7 or higher. To address these problems, various highperformance lubricating greases such as calcium soap thickened greases, aluminum complex soap greases, lithium complex soap greases, polyurea greases, synthetic hydrocarbon greases, silicone greases, and so forth were developed to meet the demand of high temperature lubricants.5,8−10 However, only a minority of high temperature lubricant additives especially the antiwear (AW) agents were commercially available, which limits the formulation of high temperature lubricants. The tribological properties of hydroquinone bis(diphenyl phosphate) (HDP) in poly(alkylene glycol) (PAG) have been studied,11 and the results indicate that HDP exhibited excellent friction-reducing and antiwear performance at elevated temperature. Thus, it is essential to do some research on organophosphate compounds as grease additives under high temperature. Pentaerythritol tetrakis(diphenyl phosphate) (PDP) and trimethylolpropane tris(diphenyl phosphate) (TDP) as flame retardants have been studied in polycarbonates and acrylonitrile−butadiene−styrene (PC/ABS).12,13 They are halogen-free, © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The chemical structures of PDP and TDP are given in Figure 1, which were prepared according to previously reported method.16 Diphenyl chlorophosphate (≥99.0%) was purchased from J&K Chemical. Pentaerythritol (≥98.0%) and trimethylolpropane (≥95.5%) were obtained from Sinopharm Chemical Reagent Co., Ltd. MoS2 (particle size of 0.5 μm) was also commercially obtained from Shanghai Shen Yu Industry & Trade Co., Ltd. All chemicals used were of analytical grade and were used as received without any further purification. Synthesis of pentaerythritol tetrakis(diphenyl phosphate) (PDP) and trimethylolpropane tris(diphenyl phosphate) (TDP): 0.85 g (6.0 mmol) of pentaerythritol, 6.66 g (24.6 Received: Revised: Accepted: Published: 5660
January 8, 2014 March 18, 2014 March 18, 2014 March 18, 2014 dx.doi.org/10.1021/ie500105v | Ind. Eng. Chem. Res. 2014, 53, 5660−5667
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wear of the discs, and the averaged values are reported in this paper. The morphology and chemical composition of the worn surfaces were analyzed by JSM-5600LV scanning electron microscope (SEM) and PHI-5702 multifunctional X-ray photoelectron spectrometry (XPS) using Al Kα radiation as exciting source. The binding energies of the target elements were determined at a pass energy of 29.35 eV, with a resolution of about ±0.3 eV, using the binding energy of contaminated carbon (C 1s, 284.8 eV) as reference.17
3. RESULTS AND DISCUSSION 3.1. Thermal Stability. The TGA curves of PDP and TDP are shown in Figure 2. It is seen that the temperatures for 5 wt %
Figure 1. Molecular structures of PDP and TDP.
mmol) of diphenyl chlorophosphate, 0.2 g of AlCl3, and 50 mL of toluene were introduced into a dry 100 mL four-necked flask with a thermocouple, a dip tube for N2 purge, a product condenser, and a vacuum receiver. The mixture was heated to 100 °C while keeping the flask devoid of moisture. After being stirred at this temperature for 20 h, toluene was removed and the resultant precipitate was washed repeatedly with water and acetonitrile. A white powder, 5.52 g (yield of 86.5%), was obtained after drying under a vacuum at 80 °C until a constant weight. TDP was synthesized using the same method and a white powder product was obtained (yield of 87.0%). The structures of PDP and TDP were characterized by Fourier transformation infrared (FTIR) spectroscopy and nuclear magnetic resonance (1H NMR, 400 MHz; 13C NMR, 100 MHz) spectroscopy. PDP. FTIR (KBr, disc): 3070, 2960, 1590, 1490, 1290, 1211, 1130, 960, 773, 684, 567, 501 cm−1. 1H NMR (DMSO-d6, 400 MHz, δ/ppm): 7.25 to 7.48, 4.47. 13C NMR (DMSO-d6, 100 MHz, δ/ppm): 150.18, 150.12, 130.60, 126.07, 120.37, 120.32, 70.27, 70.20, 36.39, 36.34, 36.29. TDP. FTIR (KBr, disc): 3070, 2970, 1590, 1490, 1290, 1211, 1130, 960, 773, 684, 567, 501 cm−1. 1H NMR (DMSO-d6, 400 MHz, δ/ppm): 7.11 to 7.38, 4.18, 1.30, 0.86. 13C NMR (DMSOd6, 100 MHz, δ/ppm): 152.57, 152.51, 129.80, 129.77, 127.02, 120.41, 120.37, 61.61, 46.06, 21.56, 7.28. Lithium complex grease and polyurea grease were obtained from our laboratory. Their typical properties are listed in Table S1 (Supporting Information). The base greases and additives with different concentrations were mixed thoroughly prior to the tests and finely ground three times in a three-roller mill. 2.2. Thermal Analysis. The thermal properties of the sample were measured on an STA 449 C Jupiter simultaneous thermogravimetric-differential scanning calorimeter (TGDSC). A total of 5 mg of sample was placed in the thermogravimetric analysis (TGA) sample holder. The temperature was set to increase from ambient temperatures to approximately 800 °C at a heating rate of 10 °C/min in air. The weight loss was monitored in the TG-DSC analysis. 2.3. Tribology Test. The friction and wear tests were evaluated on an Optimol SRV-IV oscillating reciprocating friction and wear tester. The contact between the frictional pairs was achieved by pressing the upper running ball (10 mm diameter, AISI 52100 steel, hardness of approximately 58−60 HRC) against the lower stationary disc (ø 24 × 7.9 mm, AISI 52100 steel, hardness of approximately 59−61 HRC). The tribological tests were conducted at an amplitude of 1 mm and a relative humidity of 50−60%. The wear volume of the lower disc was measured by a MicroXAM 3D noncontact surface mapping profiler. Three repetitive measurements were performed for each
Figure 2. TGA thermograms of PDP and TDP in an air atmosphere.
weight loss of PDP and TDP are 281.8 and 211.6 °C, respectively. Moreover, the total decomposition temperature is about 800 °C, revealing its high thermal stability. The decomposition temperatures (Td) are 387 and 382 °C for PDP and TDP, respectively. PDP exhibited better thermal stability than TDP. 3.2. Friction and Wear Behavior. 3.2.1. PDP and TDP as Additives for Lithium Complex Grease (LCG). The tribological properties of PDP and TDP in LCG at 200 °C were first investigated, and MoS2 was used for comparison. The coefficient of friction evolution and wear volume of sliding discs are shown in Figures 3 and 4 and Supporting Information Figure S1. When the concentration of PDP and TDP reaches 3 wt %, the lubricants have very low and stable friction coefficients, more importantly, with the lowest wear volume. By further increasing the PDP and TDP concentration above 3 wt %, the antiwear (AW) property could not be improved any more. It is seen that 3 wt % PDP and TDP can improve the AW properties of the base greases by 15 times and 6 times as compared with pure LCG, respectively (Figure 4). Therefore, 3 wt % PDP and TDP are the optimum concentration to provide significant friction reduction and AW properties. However, MoS2 as a comparison exhibits a relatively high friction coefficient under the same conditions (Figure 4a). From Figure 4b, it is seen that the wear volumes of the steel/steel contacts lubricated by 3% MoS2 is also similar with the base grease. The possible reason might be relevant to the oxidation of MoS2 at high temperature and the products of MoS2 oxidation can cause an increase of friction.15 5661
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Figure 3. (a) Friction coefficient curves lubricated by LCG plus PDP with various concentrations at 200 °C; (b) wear volumes of steel/steel contacts lubricated by LCG plus PDP with various concentrations at 200 °C (load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration, 30 min).
Figure 4. (a) Friction coefficient curves and (b) wear volumes of steel/steel contacts lubricated by LCG with 3 wt % MoS2, 3 wt % PDP, and 3 wt % TDP at 200 °C (load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration, 30 min).
Figure 5. (a) Friction coefficient curves lubricated by PG plus PDP with various concentrations at 200 °C; (b) wear volumes of steel/steel contacts lubricated by PG plus PDP with various concentrations at 200 °C (load, 100 N; stroke, 1 mm; frequency, 25 Hz; duration, 30 min).
wider use temperature range.18−20 It is evident that all greases tested here experience a running-in period at a constant load of 100 N and 200 °C. Further, 3% is the optimum concentration for the two additives to provide significant friction and wear
3.2.2. PDP and TDP As Additives for Polyurea Grease (PG). Polyurea grease is mostly used for lubrication of the various devices at high temperatures with many excellent properties, such as good oxidation stability, high mechanical stability, and 5662
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Figure 6. (a) Friction coefficient curves and (b) wear volumes of steel/steel contacts lubricated by PG with 3 wt % MoS2, 3 wt % PDP, and 3 wt % TDP at 200 °C (load = 100 N; frequency = 25 Hz; stroke = 1 mm; duration = 30 min).
Figure 7. (a) Variations of the friction coefficient with time during a load ramp test from 100 to 500 N for LCG plus 3 wt % MoS2, 3 wt % PDP, and 3 wt % TDP at 200 °C; (b) wear volume losses of the steel discs lubricated by LCG plus 3 wt % MoS2, 3 wt % PDP, and 3 wt % TDP at 200 °C (load = 100− 500 N; frequency = 25 Hz; stroke = 1 mm).
reduction (Figure 5 and Supporting Information Figure S2). Figure 6 shows the changes in friction coefficients and wear volumes of sliding discs under lubrication of polyurea grease plus 3 wt % MoS2, 3 wt % PDP, and 3 wt % TDP. It can be seen that the tribological properties of the two additives in polyurea grease are different. The friction coefficients of the four lubricants increased in the following sequence: 3 wt % PDP < 3 wt % TDP < polyurea grease 0.5) under the same conditions, and so the test had to be stopped after 9 min. Figures 8b and 9b demonstrate that wear volumes increased in the following sequence: 3 wt % PDP < 3 wt % TDP < 3 wt % MoS2 < LCG, and 3 wt % PDP < 3 wt % TDP < 3 wt % MoS2 < polyurea grease, respectively. These are consistent with the results shown in Figures 4, 6, and 7 and Supporting Information Figure S3. Both the friction reduction and AW capability of PDP and TDP are very prominent during the test, and the tribological properties of the two additives are better than those of MoS2 under the same conditions. These results verify that PDP and TDP as additives in the two base greases both in the constant load, variable load, and variable frequency tests have better tribological performance for steel/steel contacts at 200 °C. 5665
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Figure 12. XPS spectra of Fe 2p, O 1s, P 2p, N 1s, and Mo 3d of the worn surfaces lubricated by (a) LCG + 3 wt % PDP, (b) LCG + 3 wt % TDP, (c) LCG + 3 wt % MoS2, (d) PG + 3 wt % PDP, (e) PG + 3 wt % TDP, and (f) PG + 3 wt % MoS2 at 200 °C (SRV load = 100 N; frequency = 25 Hz; stroke = 1 mm; duration = 30 min).
oxidation can cause an increase of friction,15 which may reduce the lubrication capabilities of MoS2. This is consistent with the result in section 3.2.1. Given the above explanations, it is suggested that chemical reactions occurred on the wear surface in the frictional process with the generation of a boundary lubrication film composed of Fe(OH)O, Fe3O4, FePO4, compounds containing the P−O bonds, nitrogen oxide, and so forth, which significantly contribute to the friction-reducing and antiwear properties of PDP and TDP in the two base greases at elevated temperature.
Table 2. Binding Energies of Typical Elements from the XPS Measurements binding energy (eV) additive a b c d e f
LCG + 3 wt % PDP LCG + 3 wt % TDP LCG + 3 wt % MoS2 PG + 3 wt % PDP PG + 3 wt % TDP PG + 3 wt % MoS2
Fe 711.5, 725.3 711.5, 725.1 711.6, 725.2 711.7, 725.3 711.6, 725.0 711.2, 725.1
O
P
532.5
133.7
133.7
533.0
N
532.2
232.9, 235.9
532.2
133.7
401.1
532.6
133.7
401.0
532.4
Mo
401.0
4. CONCLUSIONS Two alkylphenyl diphosphates, pentaerythritol tetrakis(diphenyl phosphate) (PDP) and trimethylolpropane tris(diphenyl phosphate) (TDP), were synthesized and their tribological properties have been studied as high temperature friction-
232.9, 235.9
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reducing and antiwear additives in lithium complex grease and polyurea grease at 200 °C. The friction and wear test results show that the two additives reduce friction significantly and have excellent antiwear properties during lubrication of steel/steel contacts. XPS analysis revealed that the good tribological properties of PDP and TDP are attributed to the formation of a surface-protective film composed of Fe(OH)O, Fe3O4, FePO4, and compounds containing the P−O bonds. The film was generated on the worn surface, which resulted in good frictionreducing and antiwear performance at elevated temperature.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information on performance parameters, effect of concentration, evolution of friction coefficients, effect of loads. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*X. Wang. Tel.: +86-931-496-8285. Fax: +86-931-827-7088. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful for financial support of this work by “973” Program (2013CB632301).
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REFERENCES
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