Significant enhancement of anti-friction capability of cationic surfactant by phosphonate functionality as additive in water

Tribology International 112 (2017) 86–93

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Significant enhancement of anti-friction capability of cationic surfactant by phosphonate functionality as additive in water

MARK

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Yurong Wanga,b, Qiangliang Yua, Zhengfeng Maa, Guowei Huanga,b, Meirong Caia, , ⁎ Feng Zhoua, , Weimin Liua a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Surfactant Ionic liquids Water-based lubrication additives Antiwear

A new N-(3-(diethoxyphosphoryl)propyl)-N,N-dimethyloctadecan- 1-ammonium bromide (NP) surfactant was synthesized. The NP together with 1 wt% sodium D-gluconate or 1 wt% triethanolamine displays remarkable lubricating property as the water-based lubricant additive and provides a non-corrosive environment for steel in aqueous solution. The lubricating mechanism was tentatively discussed according to electrical contact resistance, X-ray photoelectron spectroscopy and Quartz Crystal Microbalance measurement. These results show a stable protective film is formed on the contact surface by physical adsorption and tribo-chemical reactions. Surprisingly, the water-based additive has excellent anti-friction properties comparable to oil-based lubricants and superior to cationic surfactant analogue. Therefore, this NP is a potential efficient additive in water-based lubricating fluids.

1. Introduction Machineries and equipments in industries cannot normally operate without the effective lubrication. Human being will eventually run out of energy resources because of the friction and wear, not to mention the serious energy shortage now. Therefore, the lubrication has become an indispensable technology to delay the process. The petroleum-based lubricants are widely used in industries and can make equipment operate smoothly, reduce the energy consumption and extend the life [1]. However, these petroleum-based lubricants have low flash point, flammability and poor thermal conductivity, which make them unable to be applied in some working conditions with fire and explosion danger, such as metal processing, coal mining and etc [2]. Furthermore, the shortage of petroleum resources and advert environmental impact also constrain the use of petroleum-based lubricants. To this end, the real eco-friendly, non-flammable, high heat capacity, availability and cheap water based lubricants have become the choice in these particular areas. Water as a cooling and lubricating fluid has the advantages of incombustibility, excellent cooling ability, environmental compatibility and low cost [3] and its formulations are widely used as the metalworking fluids [4,5]. However, the low viscosity and viscosity-pressure coefficient of water is difficult to form the effective viscoelastic lubricating film in the friction parts like oils and has serious corrosion on metal as a friction material. These drawbacks greatly limit

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their practical applications. Additive as the essence of lubricants can improve the defective lubricants of the stability and functionality, so it obviously enhances the efficiency and durability of industrial equipment. Meanwhile, the effective additive like a mechanical heart device plays an irreplaceable role in the field of water-based lubricants. To solve the abovementioned drawbacks, more and more effective additives including friction modifier, antiwear, antioxidant, viscosity modifier, dispersant, detergent are used in water-based lubricant [3,5–8]. It is always required to develop more efficient, no-corrosive and low cost waterbased additives. Ionic liquids (ILs) have unique physicochemical properties, such as high thermal stability, flexible molecular design, excellent lubricating and AW performances. [9–14]. It was widely used as a potential efficient material in many fields [15–20] and also as high performance synthetic lubricants and additives [10,18,21–31]. For example, previously reported ILs as oil additives have superior extreme-pressure and antiwear properties, noncorrosiveness and friction reduction [18,19,32]. Some ILs have been successfully applied as oil-based additives in industrial equipment. Moreover, it was also reported that ILs could be used for ceramic lubrication as water-based additives [7,8,33]. However, ILs as water-based additives for metal lubrication are very rare because of routine serious corrosion [3]. So it is of great significance to prepare an efficient non corrosive IL as the water-based

Corresponding authors. E-mail addresses: [email protected] (M. Cai), [email protected] (F. Zhou).

http://dx.doi.org/10.1016/j.triboint.2017.03.034 Received 18 January 2017; Received in revised form 3 March 2017; Accepted 30 March 2017 Available online 01 April 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved.

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cants, the kinematic viscosity was examined by a SYP1003-III viscometer at 25 °C. According to GB6144-85 procedure, it was used to detect the corrosion resistance of different lubricant by the cast iron strip corrosion test. Put three pieces of cast iron strips immerse into 16 ml of water, NP consisting of 1 wt% GAS, NP consisting of 1 wt% TEA, respectively. Then, put them into a constant container of 55 °C ± 2 °C. After 24 h, take out the cast iron strips immersed in samples and the corrosion level was evaluated according to the corrosion standards.

additive. The application of ILs as an efficient lubricant additive in waterbased fluids has drawn great interests due to their unique physicochemical properties comparable to other additives. In this paper, a novel kind of ionic liquids analogue, N-(3-(diethoxyphosphoryl) propyl)-N,N-dimethyloctadecan-1-ammonium bromide (denoted as NP) surfactant was synthesized and evaluated as the water-based lubrication additives. The introduction of phosphonate functionality significantly improves the friction reduction, anti-wear and anti-corrosive properties. A synergistic effect of nitrogen and phosphorus element was found. It provides us an energy-efficient non-corrosive alternative to the traditional ILs as water-based lubricant additive for the metal cutting fluids applications.

2.3. Friction and wear test In order to get the different mass concentrations ILs analogue (0, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, and 4%), different mass NP consisting of 1 wt% GAS or 1 wt%TEA was respectively added to the same quality of water. The tribological properties of NP consisting of 1 wt% GAS or 1 wt% TEA as the water-based lubricant additives at different mass concentration were evaluated by an Optimal SRV-IV oscillating reciprocating friction and wear tester. In this experiment, the friction coefficient (COF) and wear volume losses (WV), two parameters for the determination of reducing-friction and anti-wear properties of different lubricants, were carried out applying a load of 100 N, a frequency of 25 Hz, and 1 mm-amplitude at 25 °C for 30 min. Each specimen was measured for 3 times at a relative humidity of 40–50% for 3 times. The hardness of AISI 52100 bearing steel ball was 700–800 HV and its diameter was 10 mm. The lower stationary disc was ø 24 mm×7.9 mm and made up of AISI52100 bearing steel with hardness of 750–850 HV. The mean roughness (Ra) of the lower steel discs and steel ball were polished to about 0.02 µm with CW400 -CW2000 SiC abrasive paper. The best concentration of NP consisting of 1 wt% GAS or 1 wt% TEA was select from different mass concentrations under the above conditions and then measured on the frequency conversion and the variable load experiment. It was used to record automatically the corresponding friction curves and electrical contact resistance by a computer using a data-acquiring system linked to the SRV-IV tester. A MicroXAM-3D noncontact surface mapping profiler was used to examine the wear volumes on steel surfaces. The worn surface morphologies of wear scars on steel surfaces lubricated different lubricants were examined on a JEOL JSM-5600LV scanning electron microscope (SEM) (JEOL, Japan). After SRV-IV tests, the chemical composition of the wear scars on steel surfaces

2. Experimental section 2.1. Chemicals and NMR characterization The N-(3-(diethoxyphosphoryl)propyl)-N,N-dimethyloctadecan-1aminium bromide, NP was synthesized by N,N-Dimethyl-n-octadecylamine and intermediate, which was prepared by 1,3-dibromopropane and triethyl phosphite. The molecular structure of NP is displayed in Fig. 1. These feedstocks including N,N-Dimethyl-n-octadecylamine, 1,3-dibromopropane, triethyl phosphite, sodium D-gluconate (GAS), triethanolamine (TEA) and hexadecyl trimethyl ammonium bromide (CTAB) are obtained from Energy Chemical, Tianjin Heowns Biochem LLC. The deionized water is used in the whole experiment and all the percentage content refers to the mass concentration in this article. The preparation of NP was confirmed by 1H, 13C, 31P NMR and elemental analysis. 1H NMR (400 MHz, D2O) δ (ppm): 4.33 – 4.00 (m, 4 H, OCH2CH3), 3.45 (d, 4 H, NCH2CH2), 3.18 (s, 6 H, NCH3), 2.04 (d, 4 H, NCH2CH2), 1.79 (s, 2 H, PCH2), 1.60 – 1.08 (m, 36 H, N(CH2)2(CH2)15, OCH2CH3), 0.90 (t, 3 H, N(CH2)17CH3). 13C NMR (100 Hz, D2O) δ (ppm): 63.37, 62.93, 62.87, 51.43, 32.02, 30.08, 29.97, 29.87, 29.68, 29.53, 29.2, 26.11, 22.68, 22.35, 21.64, 20.24, 16.06, 16.01, 13.89. 31P NMR (162 MHz, D2O) δ (ppm): 32.48. Elem. Anal. Calcd for C27H59BrNO3P: P, 5.56. Found P, 5.43. 2.2. Viscosity and corrosion test In order to understand the physical properties of different lubri-

Fig. 1. Molecular structures of (a) sodium D-gluconate (GAS), (b) triethanolamine (TEA), (c) hexadecyl trimethyl ammonium bromide (CTAB) and (d) N-(3-(diethoxyphosphoryl) propyl)-N,N-dimethyl octadecan-1-aminium bromide (NP).

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does improve the anti-friction and anti-wear ability of water dramatically. Fig. 3(a, b) exhibits that COF and WV of only 0.5% NP consisting of 1 wt% GAS or 1 wt%TEA reach about 0.075–0.1 and 29– 30×10−5 mm3, whereas those of pure water, 1 wt% GAS, 1 wt%TEA, 0.5% CTAB consisting of 1 wt% GAS (0.5% CTAB+1 wt% GAS) and 0.5% CTAB consisting of 1 wt% TEA (0.5% CTAB+1 wt%TEA) are far bigger than NP. It illustrates that only 0.5% NP can significantly reduce the COF and WV of water. This concludes that NP consisting of 1 wt% GAS or 1 wt%TEA is a highly-efficient additive with excellent lubrication and anti-wear properties in water. Although 0.5% NP consisting of 1 wt% TEA (0.5% NP+1 wt% TEA) reduces the COF and WV evidently, the COF of 0.5% NP consisting of 1 wt% GAS (0.5% NP+1 wt% GAS) is far less than that of 0.5% NP+1 wt% TEA. It insinuates that 0.5% NP +1 wt% GAS has better lubricating performances than 0.5% NP+1 wt% TEA. We hypothesize that GAS molecule containing carboxylate anion has better adsorption capacity on the worn surface than that of TEA during the friction process. So, 0.5% NP+1 wt% GAS formed a more stable protective film, which can prevent the direct contact of the friction pair surface. These results indicate that NP with phosphonate functionality as lubricant additive in aqueous solution has generally better lubricating properties than the others. The friction reducing and extreme-pressure capability of NP consisting of 1 wt% GAS or 1 wt%TEA as water-based lubricant additives was further investigated in the following tests of changing the applied load and frequency. Fig. 4(a) shows the variation of COF of six different lubricants with a load ramp test from 100 to 400N stepped by 100 N intervals with 5-min test duration for each load at 25 °C. As shown in this picture, the COF of 1 wt%TEA, 0.5% CTAB+1 wt% TEA, 1 wt% GAS, and 0.5% CTAB+1 wt% GAS is all extremely large and fluctuant during the whole experiment. When the load reaches 200N, the signals of lubrication failure of 1 wt%TEA occurs within 7 min. The remaining mean COF is about 0.46, 0.40, 0.28, and 0.19, respectively. However, the evolution of COF of 0.5% NP+1 wt% GAS is the smallest and the most stable (about 0.07) in all lubricants during the whole experiment. Remarkably, NP consisting of 1 wt% GAS as the water-based additives improves the friction reducing properties. Moreover, it is found that the maximum load carrying ability of 0.5% NP+1 wt% GAS surpasses 400 N, whereas that of 1 wt%TEA, 1 wt% GAS, 0.5% CTAB+1 wt% TEA and 0.5% CTAB+1 wt% GAS is only 200N, 300N, 200N and 300N, respectively. This further demonstrates that NP as the water-base additive does help to improve the extreme-pressure capability of water. Meanwhile, it is also found that the COF of 0.5% NP+1 wt% TEA has a little fluctuation and bigger than 0.5% NP+1 wt% GAS during the variable load experiment from Fig. 4(a). It suggests that 0.5% NP+1 wt % GAS as the water-based lubricant additive has a superior frictional behavior than 0.5% NP+1 wt% TEA. In addition, Fig. 4(b) shows a frequency ramp experiment of six different additives from 10 to 40 Hz stepped by 5 Hz intervals with 5-min test duration for each frequency at room temperature. Although 0.5% NP+1 wt% TEA shows a smaller and more stable COF(0.118) than other four additives, 0.5% NP+1 wt% GAS exhibits the best constant frictional behavior with an average COF of 0.085 during the whole frequency conversion test. It is suggested that 0.5% NP+1 wt% GAS has a more stable frictional behavior in comparison to other additives in the entire testing process. These remarkable results further indicate NP consisting of 1 wt% GAS or 1 wt %TEA as the lubricant additive has a great lubricating and extremepressure performance in aqueous solution.

lubricated different lubricants were performed on a PHI5702 multifunctional X-ray photoelectron spectroscope (XPS) by using Al-Kα radiation as the exciting source. The binding energies of the target elements were determined by a passing energy of 29.35 eV and the reference was the binding energy of carbon (C1s:284.8 eV). 2.4. QCM test To better evaluate the adsorption properties of water-based lubricant additives, the Quartz crystal microbalance with dissipation (QCMD) measurements were performed at room temperature by a Q-Sense microbalance (Sweden) with a rate of 50 μl/min. And the experiments need to use the commercial gold-coated quartz chips (QSX-301, QSense). 3. Results and discussion 3.1. Viscosity and corrosion test The kinematic viscosity of different lubricants at room temperature and the corrosion test of cast iron strips in different lubricants are summarized in Table 1. The results show that the addition of 0.5 wt% NP consisting of 1 wt% GAS or 1 wt%TEA to water only slightly increases its viscosity, but it can effectively reduce the corrosive attack to metals unlike many other corrosive lubricants. After the corrosion test, the color of the cast iron strips immersed in NP consisting of 1 wt % GAS or 1 wt%TEA showed almost no change compared with that before test. According to the standard shade guide, the corrosion level of NP consisting of 1 wt% GAS or 1 wt%TEA is evaluated as A (no rust and shiny as new), whereas the serious corrosion level is defined as D (heavy rust or severe loss of light). Compared with water, the corrosion level of NP consisting of 1 wt% GAS or 1 wt%TEA was greatly reduced. These results indicate that NP consisting of 1 wt% GAS or 1 wt%TEA has no corrosion to metal in water and qualifies as the water-based lubricant additive. 3.2. Friction and wear test The tribological properties of NP consisting of 1 wt% GAS or 1 wt% TEA as water-based lubricant additives to lubricate steel/steel contacts are summarized in Fig. 2–4. They present the evolution of COF and WV of lower sliding disc lubricated by different lubricants under the different conditions. Every data point in Fig. 2–4 represents the average values from three tests. Fig. 2(a, b) shows the evolution of the COF and WV on steel sliding discs lubricated by different concentration of NP consisting of 1 wt% GAS (NP +1 wt% GAS) and NP consisting of 1 wt%TEA (NP +1 wt% TEA). With increasing the mass concentration of NP, the value of COF and WV of NP reduce a lot and keep no change when the mass concentration is above 0.5%. The data demonstrated that 0.5% is the optimal mass concentration. What's more, NP as the lubricant additive Table 1 The viscosity property and corrosion grade of different lubricants. Lubricants

Kinematic viscosity at 25 °C (mm2/s)

Corrosion gradea

water 1%GAS 1%TEA 0.5% CTAB+1 wt% GAS 0.5% CTAB+1 wt%TEA 0.5%NP+1 wt% GAS 0.5% NP+1 wt%TEA

0.83 0.93 0.96 0.98 1.00 0.97 0.996

D A A A A A A

3.3. Analysis of worn surfaces The SEM and 3D micrograph measurements were performed to better understand the wear situations of the lower steel discs lubricated by different lubricants. As displayed in Fig. 5(a, a1, b, b1), the worn steel surface lubricated by 1 wt% GAS or 1 wt%TEA has a particularly deep and wide wear scar with a lot of deep grooves and serious scratches. Meanwhile, it is also found the worn surface under the lubrication of

a : A, no rust and shiny as new; B, no rust but a slight loss of light; C, light rust and slight loss of light; D, heavy rust or severe loss of light.

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Fig. 2. (a) Friction coefficient; (b) Wear volume of steel discs lubricated by NP plus 1 wt% GAS or 1 wt%TEA with different concentration at room temperature. (SRV load: 100 N, frequency: 25 Hz, stroke: 1 mm, duration: 30 min). 0% concentration refers to the blank of 1 wt% GAS or 1 wt% TEA.

Fig. 3. (a) Friction coefficient; (b) Wear volume of steel discs lubricated by water, 1 wt% GAS, 1 wt%TEA, 0.5% CTAB+1 wt% GAS, 0.5% CTAB+1 wt%TEA, 0.5% NP+1 wt% TEA and 0.5% NP+1 wt% GAS at room temperature (SRV load: 100 N, frequency: 25 Hz,stroke: 1 mm, duration:30 min).

Fig. 4. The variation of friction coefficient with time lubricated by different lubricants during (a) a load ramp test from 100 to 400 N at a frequency of 25 Hz; (b) a frequency ramp test from 10 to 40 Hz at a load of 100 N and at room temperature.

exhibits the 3D optical microscopic images consistent with the data of measured wear volumes and Table 2 shows the approximate values of depth, width and length of different lubricants. The wear scars lubricated with 0.5% NP+1 wt% GAS (Fig. 5e2) and 0.5% NP+1 wt% TEA (Fig. 5f2) are much narrower and shallower than that of 1 wt% GAS (Fig. 5a2), 1 wt% TEA (Fig. 5b2), 0.5% CTAB+1 wt% GAS (Fig. 5c2) and 0.5% CTAB+1 wt% TEA (Fig. 5d2). It further explains NP consisting of 1 wt% GAS or 1 wt% TEA as the water-based lubricant additives has great anti-wear properties. The results obtained are consistent with the above-mentioned data of friction and wear testing.

0.5% CTAB+1 wt% GAS or 0.5% CTAB+1 wt% TEA has a rather wide and deep wear scar with serious scuffing and many narrow grooves in Fig. 5(c, c1, d, d1). However, the wear and tear of the steel surface lubricated by 0.5% NP+1 wt% GAS and 0.5% NP+1 wt% TEA are greatly alleviated with comparatively shallow, narrow and wear scar in Fig. 5(e, e1, f, f1). Remarkably, the worn surfaces of NP consisting of 1 wt% GAS or 1 wt%TEA greatly reduce grooves and scuffing in comparison to other lubricants. This suggests NP consisting of 1 wt% GAS or 1 wt% TEA in aqueous solution expresses the excellent lubricating and antiwear performances. In addition, Fig. 5(a2−f2)

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Fig. 5. SEM and 3D morphologies of worn scars lubricated by different lubricants at 25 °C: 1 wt% GAS(a, a1), 1 wt% TEA(b, b1), 0.5 wt% CTAB+1 wt% GAS (c, c1), 0.5 wt% CTAB+1 wt % TEA (d, d1), 0.5 wt% NP+1 wt% GAS(e, e1), and 0.5 wt% NP+1 wt% TEA(f, f1) (SEM magnification: the above is 60×and the below is 500×, load:100 N, stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

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+1 wt% TEA has two peaks. One peak of N1s at 402.2 eV refers to the N of triethanolamine according to the peak of N1s at 401.9 eV lubricated by 1 wt% TEA in Fig. 7(a, c). The other peak of N1s at about 399.9 eV becomes a little wider and has a clear chemical shift compared with neat IL, possibly corresponding to nitrogenoxide, and / or Fe(NO), and / or Fe(NO2)compounds [36]. This peak of N1s lubricated by 0.5 wt% NP+1 wt% TEA is similar to that of 0.5 wt% NP+1 wt% GAS. Meanwhile, it is also found the binding energies of P, O and Fe lubricated by 0.5 wt% NP+1 wt% TEA are similar to that of 0.5 wt% NP +1 wt% GAS. This reveals they have similar lubrication mechanisms. Moreover, the peak of P2p at 134.6–137.7 eV is possibly ascribed to phosphorus oxide (P2O4, P2O5, PO3, POBr2). Considering the XPS peak of Fe2p (about 710.7 eV and 724.3 eV)and O1s (about 534.3 eV), it may be attributed to Fe2O3, Fe(OH)O, FeOOH, FeO, Fe3O4 [36]. From these results, it indicates the nitrogen and phosphorus do have a synergistic effect, so the NP has excellent lubricating properties as antiwear additive for water. Furthermore, we may speculate NP consisting of 1 wt% GAS or 1 wt% TEA as the water-based additive not only experienced a complicated tribo-chemical reaction forming a protective film containing of Fe3O4, Fe2O3, Fe(OH)O, FeOOH, phosphorus oxide, nitrogen oxide, etc, but also formed a physical adsorption film on the steel worn surface during the friction process. This protective film plays an important role in reducing friction and abrasion resistance by preventing the friction pair surface from friction and wear. To demonstrate the adsorption capacity of NP on the worn substrates, the QCM measurement by changing in frequency of a quartz chip was carried out [34,37,38]. The principle of operation is that the mass added to or removed from the electrode causes the frequency shift, Δƒ, related to the mass change, Δm. Fig. 8 shows the adsorption quantities of 1% GAS, 0.5% NP+1% GAS, 0.5% CTAB +1% GAS by using the water as the solvent to collect the baseline and the gold as the electrode in all experiment. It is found the Δƒ (32 Hz) at 0.5% NP+1% GAS has a much bigger value than the Δƒ (19 Hz) at 0.5% CTAB +1% GAS and the Δƒ (10 Hz) at 1% GAS. It implies NP has stronger adsorption ability on the worn surface than that of CTAB. Perhaps, this is because cationic surfactant (NP) containing polar functional group of phospholipids has better boundary adsorption capability to substrates than CTAB. Furthermore, the frequency can restore its original value after rushing with the solvent of water. This indicates the absorption is reversible and more likely to be a physical absorption. Based on previous report [39–42] and QCM results, we proposed the absorption mode of this cationic surfactant. Fig. 8(b) shows the corresponding adsorption diagram of Fig. 8(a), which indicates that cationic surfactant and part of anion of GAS are likely to be adsorbed on the substrate by physical absorption. These findings are in agreement with the above-mentioned results of friction and wear test.

Table 2. The approximate values of depth, width and length of different lubricants for the 3 D figures in Fig. 5(a2−f2). Lubricants

Depth (μm)

Width (mm)

Length (mm)

1 wt% GAS 1 wt% TEA 0.5 wt% CTAB+ 1 wt% GAS 0.5 wt% CTAB+1 wt% TEA 0.5 wt% NP+1 wt% GAS 0.5 wt% NP+1 wt% TEA

5 10 5 5 2 2

0.7 0.8 0.68 0.8 0.41 0.48

1.6 1.6 1.6 1.6 1.4 1.5

Fig. 6. Change in contact resistance during friction test at room temperature (SRV load: 100 N, frequency: 25 Hz, stroke: 1 mm).

3.4. Mechanism analysis As expected, the above results suggest NP consisting of 1 wt% GAS or 1 wt% TEA has the potential to be a high-efficient water-based lubricant additives. The electrical contact resistance (ECR) measurement, X-ray photoelectron spectroscopy (XPS) and Quartz Crystal Microbalance (QCM) were carried out to understand the lubrication mechanism of NP more clearly. It is an effective proof to illustrate an insulating tribo-film forming on the steel worn surface by ECR measurement [34,35]. The ECR were measured to confirm the lubricating mechanism of NP as the waterbased additive. Fig. 6 presents the evolution of ECR with the friction time under the different lubricants. It is found the value of ECR of NP consisting of 1 wt% GAS or 1 wt% TEA as the water-based additive is far higher than that of 1 wt% GAS, 1 wt% TEA, 0.5% CTAB+1 wt% GAS and 0.5% CTAB+1 wt% TEA. This suggests a comparatively stable insulating tribofilm of NP forms on the steel contact area during the friction process. In addition, 0.5% NP+1 wt% GAS has a larger value of ECR in comparison to 0.5% NP+1 wt% TEA. Compared with 0.5% NP +1 wt% TEA, the carboxylate of 0.5% NP+1 wt% GAS is more easily adsorbed on the worn surface during the friction process. So, 0.5% NP +1 wt% GAS formed a more stable protective film and has a higher ECR. The results show that 0.5% NP+1 wt% GAS has the best tribological properties in all lubricants, which is consistent with the previous tribological data. XPS was measured to clearly understand the elements of compositions and chemical states of a boundary film on the worn surfaces. Fig. 7 presents the XPS spectra of N1s, P2p, Fe2p and O1s on the steel worn surfaces lubricated by 1 wt% GAS, 1 wt% TEA, 0.5 wt% NP+1 wt % GAS, 0.5 wt% NP+1 wt% TEA and neat NP. It is found the peaks of N1s, P2p and O1s on the steel worn surfaces lubricated by 0.5 wt% NP +1 wt% TEA have an apparent chemical shift in comparison to neat NP. This suggests the complex tribo-chemical reaction occurred with forming a relatively stable protective film in the friction process. Fig. 7(c) shows the binding energy of N1 lubricated by 0.5 wt% NP

4. Conclusion A novel water-soluble ionic liquids analogue, NP was prepared and discussed as the lubricant additive in aqueous solution. The NP as the water-based lubricant additive presents a remarkable tribological property with lower COF and WV comparable to oil-based lubricants. NP consisting of 1 wt% GAS or 1 wt% TEA provides a non-corrosive environment for steel in aqueous solution. In addition, the NP also exhibits excellent extreme-pressure and abrasion resistance behaviors. The reason is tentatively discussed by ECR measurement, XPS analysis and adsorption performance test QCM. These results illustrate a stable protective film is formed on the contact surface though a complicated tribo-chemical reaction and a physical adsorption. This protective film effectively prevents the direct contact of sliding pairs and plays a key in lubrication and abrasion resistance. Therefore, so-prepared NP with efficient lubrication properties will definitely improve the field of waterbased technology to produce a cost-effective lubricant additive applied in water-based fluid. 91

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Fig. 7. The XPS spectra of N1s, P2p, Fe2p and O1s of the worn surfaces lubricated by (a) 1 wt% TEA, (b) 0.5 wt% NP+1 wt% GAS, (c) 0.5 wt% NP+1 wt% TEA, (d) neat NP and (e) 1 wt % GAS (SRV: load: 100 N, stroke: 1 mm, frequency: 25 Hz, duration: 30 min, temperature: 25 °C).

Fig. 8. (a) The changes in frequency of QCM chip gold (The solvent of water is the baseline); (b) Schematic of boundary adsorption of NP. [2] Wang J, Wang J, Li C, Zhao G, Wang X. A high-performance multifunctional lubricant additive for water–glycol hydraulic fluid. Tribol Lett 2011;43:235–45. [3] Espinosa T, Jiménez M, Sanes J, Jiménez A-E, Iglesias M, Bermúdez M-D. Ultralow friction with a protic ionic liquid boundary film at the water-lubricated sapphire–stainless steel interface. Tribol Lett 2014;53:1–9. [4] Wang J, Wang J, Li C, Zhao G, Wang X. Tribological performance of poly (sodium 4‐styrenesulphonate) as additive in water–glycol hydraulic fluid. Lubr Sci 2012;24:140–51. [5] Tomala A, Karpinska A, Werner WSM, Olver A, Störi H. Tribological properties of additives for water-based lubricants. Wear 2010;269:804–10. [6] Brinksmeier E, Meyer D, Huesmann-Cordes A, Herrmann C. Metalworking fluids— Mechanisms and performance. CIRP Ann-Manuf Technol 2015;64:605–28. [7] Xie G, Liu S, Guo D, Wang Q, Luo J. Investigation of the running-in process and friction coefficient under the lubrication of ionic liquid/water mixture. Appl Surf Sci

Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51675512 and 51227804), Natural Science Foundation of Gansu Province (Grant Nos.1606RJZA051) and ‘‘973’’ program (2013CB632301). References [1] Kulacki KJ, Lamberti GA. Toxicity of imidazolium ionic liquids to freshwater algae. Green Chem 2008;10:104–10.

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