Application of Alkylphosphate Ionic Liquids as Lubricants for Ceramic

Article pubs.acs.org/IECR

Application of Alkylphosphate Ionic Liquids as Lubricants for Ceramic Material Yuexia Guo,†,‡ Dan Qiao,*,† Yunyan Han,†,‡ Lin Zhang,§ Dapeng Feng,*,† and Lei Shi† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Research Institute of Lanzhou Petrochemical Company, Lanzhou 730060, P. R. China S Supporting Information *

ABSTRACT: Three kinds of alkylphosphate ionic liquids were synthesized and evaluated as lubricants for silicon nitride−silicon nitride and silicon nitride−zirconium oxide contacts under different test conditions. The results show that silicon nitride substrates exhibit better load-bearing capability compared with zirconium oxide because of their special physical properties and chemical activity. Meanwhile, alkylphosphate ionic liquids exhibit better friction-reducing and antiwear properties than the common ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (L-P104) and the base oil poly(α-olefin) (PAO). Scanning electron microscopy and X-ray photoelectron spectroscopy results indicate that tribochemical reactions occur between the alkylphosphate ionic liquids and ceramic substrates without corrosion during the friction process. When this is combined with the test results for contact angles, it can be concluded that the excellent lubricity of alkylphosphate ionic liquids is ascribed to the synergistic effect between the physical adsorption and tribochemical reactions as well as the polar molecular structure and physical properties of the lubricant.

1. INTRODUCTION Engineering ceramics are widely used in industry because of their mature fabrication technologies and unique properties, such as high hardness, high strength, low density, great chemical inertness, and remarkable corrosion and wear resistance. It is interesting that engineering ceramics do not show the disadvantages of traditional metal materials, which are easily oxidized and corroded. Moreover, these special characteristics make engineering ceramics excellent high-temperature-, wear-, and corrosion-resistant materials1 that are used in many applications, such as cylinder liners, turbocharger parts, piston crowns, and valve seats and guides.2,3 Silicon nitride (Si3N4) and zirconium oxide (ZrO2) are two typical ceramic materials with high structural strength, low thermal expansion coefficient, and high thermal conductivity. They are suitable for use under high-temperature conditions, mainly applicable to cutting tools, high-temperature ballbearing rotating components, wire drawing dies, and chemical equipment.4 It is generally known that the meshing points and moving parts of the equipment have some lubrication problems containing sliding friction, faster sliding velocity, and serious wear and tear. Therefore, lubricating oils with high performance should be developed rapidly. At present, water-based lubricants and polyols are the main lubricating materials for ceramics. They react with the substrates during the friction process to form a protective layer and reduce the wear and friction.1,4−6 Nevertheless, both of these lubricants still have some shortcomings.1,7 For instance, the applications of water-based lubricants are limited by the test temperature because of their high volatility and high freezing point. In addition, the tribochemical reaction films formed on the ceramic substrates © 2015 American Chemical Society

are very thin under lubrication with water-based lubricants, resulting in high friction coefficients and severe wear.8 On the other hand, polyols are not ideal lubricants because of their low boiling points and high vapor pressures.9 Thus, water-based lubricants and polyols are unsuited for the stable lubrication of Si3N4 and ZrO2 ceramics in harsh environments. Ionic liquids (ILs) are a class of novel materials that have attracted great attention because they are nonvolatile, thermally stable, nonflammable, and naturally polarized. Hence, they have been used in many fields, such as catalysis, electrochemistry, separation science for extraction of heavy-metal ions, green chemistry as solvents, and materials for optoelectronic applications.10,11 In 2001, Ye et al.12 first reported that ILs are excellent lubricants for a variety of materials. Later, increasing attention was given to ILs as lubricants, and they were widely studied by many researchers.13−16 Meanwhile, many kinds of functionalized ILs with particular performances were designed and synthesized to satisfy the applications of different friction pairs and solve a series of tribological matters.17−21 It is known that the structure and ion pairing of the ILs are the key factors in determining their properties. There are more than one million combinations of anions and cations that show potential applications and different tribological performance.22 In addition, study results have shown that the traditional antiwear lubricants with phosphorus elements exhibit excellent antiwear and friction-reducing performance.23 As a result, Qiao Received: Revised: Accepted: Published: 12813

September 2, 2015 October 28, 2015 December 8, 2015 December 8, 2015 DOI: 10.1021/acs.iecr.5b03260 Ind. Eng. Chem. Res. 2015, 54, 12813−12825

Article

Industrial & Engineering Chemistry Research et al.24 designed a series of phosphate ILs and investigated their friction properties for metal substrates compared with common ILs. It turned out that phosphate ILs indeed exhibited remarkable friction-reducing properties. On the basis of the above discussion, alkylphosphate ILs were selected as lubricants for Si3N4−Si3N4 and Si3N4−ZrO2 contacts in this study. Moreover, the tribological properties of 1-butyl-3-methylimidazolium hexafluorophosphate (L-P104) and poly(α-olefin) (PAO) were also investigated as reference lubricants. More importantly, a discussion of the lubricating mechanism of the alkylphosphate ILs based on contact angle measurements and X-ray photoelectron spectroscopy (XPS) analyses is provided in this paper.

2.3. Physical Properties of the Lubricants. The viscosities of the ILs and PAO were measured using a SYP1003-III oil kinematic viscosity meter, and the viscosity indexes were calculated automatically according to the ASTM D2270-93 standard. In addition, the thermal properties of the lubricants were examined on a PerkinElmer TGA-7 thermogravimetric analyzer from ambient temperature to 800 °C at a heating rate of 10 °C/min in flowing nitrogen. The results of the above tests are listed in Table 2. 2.4. Corrosion Tests. Corrosion tests were conducted to inspect the corrosion properties of the alkylphosphate ILs for Si3N4 and ZrO2 ceramic substrates in an environmental chamber (test conditions: temperature, 100 °C; relative humidity, 80%; duration time, 24 h). Polished Si3N4 and ZrO2 ceramic blocks (10 mm × 10 mm) were used as substrates, and 0.2 mL of alkylphosphate ILs were dropped onto the substrates. After the corrosion tests, the ILs were removed, and then the Si3N4 and ZrO2 ceramic plates were ultrasonically cleaned in acetone. Later, the morphologies of the Si3N4 and ZrO2 surfaces were observed by scanning electron microscopy (SEM) using a JSM-5600LV scanning electron microscope. The results of the corrosion tests are shown in Figure 2. 2.5. Tribological Tests. The tribological behaviors of the lubricants for the ceramic contacts were evaluated on a SRV-IV oscillating reciprocating tribometer with a ball-on-disc configuration. The upper running ball was Si3N4 (diameter 10 mm), and the roughness was about 50 nm. The lower stationary discs were Si3N4 and ZrO2 (ø 24.0 mm × 7.9 mm). In order to obtain the friction properties of the lubricants under different test conditions, variable-frequency and variable-load tests were carried out first. The variable-frequency experiments were performed under a load of 400 N and an amplitude of 1 mm for 30 min with variation of the frequency from 10 to 30 Hz. The variable-load experiments were conducted at frequency of 25 Hz and an amplitude of 1 mm for 30 min with variation of the load from 100 to 500 N. Friction tests were also performed at 20 and 100 °C to evaluate the tribological properties with a relative humidity of 15−50%. The Si3N4 balls and Si3N4 and ZrO2 discs were ultrasonically cleaned in petroleum ether and acetone three times prior to the tests, and then 0.2 mL of lubricant was dropped onto the ball-on-disc contact area. Friction coefficients were continuously recorded with time for each test. The wear volume losses of the ceramic discs after the friction tests were measured with a Micro XAM 3D noncontact surface mapping profiler. Three repetitive measurements were performed for each sample. The average values are reported in this article. 2.6. Analysis of Worn Surfaces. The morphologies of the worn surfaces were observed by SEM and energy-dispersive spectroscopy (EDS) on a scanning electron microscope equipped with an energy-dispersive spectrometer. The elemental composition and chemical states of the typical elements on the worn surface were analyzed using a PHI-5702 multifunctional X-ray photoelectron spectrometer with Al Kα radiation as the excitation source. The binding energies of the target elements were determined at a pass energy of 29.35 eV with a resolution of approximately 0.3 eV. The binding energy of contaminated carbon (C 1s, 284.80 eV) was used as a reference. The Si3N4 and ZrO2 worn discs were ultrasonically cleaned in acetone after the friction tests.

2. EXPERIMENTAL PROCEDURES 2.1. Preparation of the Alkylphosphate ILs. Three kinds of alkylphosphate ILs, butylammonium dibutylphosphate (BADBP), tetrabutylammonium dibutylphosphate (TBA-DBP), and 1-butyl-3-methylimidazolium dibutylphosphate (BMIMDBP), were synthesized according to the literature,24 and their chemical structures are shown in Figure 1. Briefly, the synthetic

Figure 1. Chemical structures of the ILs.

procedure for BMIM-DBP was as follows: the tributylphosphate and methylimidazole were mixed and heated at 150 °C under a nitrogen atmosphere for 10 h. Then the compound was purified by washing with ether. The synthetic procedure for BA-DBP and TBA-DBP was as follows: dibutylphosphate and the alkylammonium were mixed at room temperature for 12 h, and the products were purified by sequential washing with distilled water and chloroform. L-P104 was prepared according to the previous study.25 All of the reagents used in the experiments were of analytic grade. Moreover, the molecular structures of the synthesized ILs were confirmed by NMR spectroscopy and electrospray ionization time of flight mass spectrometry. The results are shown in sections S1 and S2 in the Supporting Information. 2.2. Preparation of the Friction Pair Materials. Silicon nitride ceramics were bought from Shanghai Weijin Ceramic Technology Company, and zirconium oxide ceramics were homemade through hot-pressing sintering at 1350 °C. The mechanical properties of the materials are listed in Table 1. Table 1. Mechanical Properties of the Si3N4 and ZrO2 Ceramics material

density (g/cm3)

hardness (HV)

roughness (nm)

Si3N4

3.2

1521

100

ZrO2

5.86

1225

50

composition (wt %) 1.0 Al2O3, 99.0 Si3N4 100.0 ZrO2 12814

DOI: 10.1021/acs.iecr.5b03260 Ind. Eng. Chem. Res. 2015, 54, 12813−12825

Article

Industrial & Engineering Chemistry Research

3. RESULTS AND DISCUSSION 3.1. Physical Properties of the Lubricants. The kinematic viscosities and thermal decomposition temperatures of the lubricants are shown in Table 2. It can be seen that the

surfaces of the Si3N4 and ZrO2 ceramic blocks have very mild color changes for (a) BA-DBP, (b) TBA-DBP, and (c) BMIMDBP after the corrosion tests. Moreover, there are no obvious corrosion behaviors of the three alkylphosphate ionic liquids on the ceramic substrates from the SEM micrographs. The reason is that alkylphosphate ILs are adsorbed on the substrate surfaces and are difficult to hydrolyze under the test conditions,22 resulting in negligible corrosion of the surfaces. 3.3. Tribological Properties for Two Different Friction Pairs. 3.3.1. Si3N4−Si3N4 Contact. 3.3.1.1. Lubricating Properties under Variable-Frequency and Variable-Load Test Conditions. Figure 3 shows the evolution of the friction coefficients with time and wear volumes of the Si3N4 discs lubricated with the five lubricants under (a, b) variablefrequency (10−30 Hz, 400 N) and (c, d) variable-load (100− 500 N, 25 Hz) test conditions. Each friction coefficient curve is the average value of three tests. Compared with the traditional base oil PAO, the four ILs exhibit stable and exciting frictionreducing properties under all of the test conditions. In addition, for lubrication with ILs a decreasing trend in the friction coefficient with increasing test frequency and load is observed. The wear volumes of the four ILs are also similar and lower than that of PAO. However, L-P104 shows a weaker loadcarrying capacity. Therefore, the lubrication effect of L-P104 for the Si3N4 disc failed when the test load was increased to 500 N. On the contrary, all of the friction coefficients for lubrication with the alkylphosphate ILs maintained relatively stable values under the variable-frequency and variable-load test conditions. It can be concluded that alkylphosphate ILs possess excellent load-carrying capacity for use in high speed running environments. 3.3.1.2. Lubricating Properties at Different Temperatures. The above test results also show that the friction coefficients under lubrication with the ILs are low and stable when the frequency is 25 Hz and the load is 300 N. Therefore, constantload (300 N) and constant-frequency (25 Hz) experiments were conducted at 20 and 100 °C to evaluate the tribological properties at different temperatures. It can be seen from Figure 4 that all of the ILs have lower friction coefficients than PAO at the two test temperatures. In addition, L-P104 still possesses a lower friction coefficient than the alkylphosphate ILs at room temperature, and this is attributed to the lower antishearing

Table 2. Physical Properties of the Lubricants kinematic viscosity (mm2/s) lubricant

40 °C

100 °C

viscosity index

decomposition temperature (°C)

BA-DBP TBA-DBP BMIM-DBP L-P104 PAO

2053.45 902.04 247.69 71.56 17.86

190.21 33.01 17.42 10.38 3.93

218 43 70 131 115

233.5 212.7 272.5 398.1 275.2

viscosities of the alkylphosphate ILs are higher than those of LP104 and PAO. Moreover, the viscosity of L-P104 is lower than that of BMIM-DBP even though the two ILs have the same cationic moiety. This test result can be attributed to the relatively small motion resistance of L-P104, which would decrease the kinematic viscosity. In addition, BA-DBP shows the highest viscosity among the three alkylphosphate ILs, while the viscosity of TBA-DBP is higher than that of BMIM-DBP. This means that alkylammonium ILs possess higher viscosities than imidazolium ILs. This can be explained as follows: when the ILs have the same cationic and anionic skeleton structure, an increase in bond polarity increases the molecular dipole− dipole effect, which can increase the viscosity of the IL.24 According to the thermogravimetric analysis, the decomposition temperatures of the ILs and PAO are all above 200 °C, which means that all of the lubricants are endowed with great thermal stability. Furthermore, the decomposition temperature of the alkylimidazolium alkylphosphate IL is higher than those of alkylammonium dibutylphosphate ILs, which is mainly attributed to the chemical stability of the five-membered-ring cationic structure. 3.2. Corrosion Tests. The corrosion tests were conducted with the three alkylphosphate ILs on the Si3N4 and ZrO2 ceramic substrates. Figure 2 presents the corrosion properties of the three alkylphosphate ILs on the Si3N4 and ZrO2 substrates after the corrosion tests. It can be seen that the

Figure 2. Corrosion tests of (a) BA-DBP, (b) TBA-DBP, and (c) BMIM-DBP on (1) Si3N4 and (2) ZrO2 substrates. Test conditions: temperature, 100 °C; relative humidity, 80%; corrosion time, 24 h. 12815


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Figure 3. Evolution of friction coefficients with time and wear volumes of the Si3N4 discs lubricated with different lubricants under (a, b) variablefrequency (10−30 Hz, 400 N, 1 mm) and (c, d) variable-load (100−500 N, 25 Hz, 1 mm) conditions.

Figure 4. Evolution of friction coefficients with time and wear volumes of the Si3N4 discs lubricated with different lubricants in constant-load and constant-frequency experiments (300 N, 25 Hz, 1 mm) at (a, b) 20 °C and (c, d) 100 °C.

strength of the boundary lubricating films.26 Nevertheless, there are some fluctuations in the curve of the friction coefficient for lubrication with L-P104 at 100 °C. The friction behaviors of all of the alkylphosphate ILs are similar, and all of the friction coefficients are less than 0.10. Moreover, the wear volumes under lubrication with the alkylphosphate ILs are much lower than those with PAO and L-P104, which may be ascribed to the

stronger interaction between the alkylphosphate ILs and the substrates13 as well as the absence of corrosion of the ceramic substrates. In addition, Si3N4 has a negatively charged surface that would offer active sites to attract cations to the specimen surface,7 forming an adsorbed electric double layer on the surface. Consequently, the ILs exhibit excellent load-carrying capacities and friction-reducing properties. 12816


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Figure 5. SEM morphologies of the worn surfaces of the Si3N4 discs lubricated with different lubricants and 3D optical images of the wear scars: (a, b, k) BA-DBP; (c, d, l) TBA-DBP; (e, f, m) BMIM-DBP; (g, h, n) L-P104; (i, j, o) PAO. SRV conditions: load, 300 N; stroke, 1 mm; frequency, 25 Hz; temperature, 100 °C; duration, 30 min.

In addition, 1-methyl-3-decylimidazolium bromide (DMIMBr) and a mixture of PAO40 and PAO4 (mPAO), which have viscosities similar to those of BA-DBP and BMIM-DBP were selected as reference lubricants to investigate the friction coefficients and wear volumes under the same test condition for the Si3N4−Si3N4 contact in order to further explore the friction performance of the alkylphosphate ILs. The results are shown in sections S3 and S4. It can be seen that the lubricating performances of the alkylphosphate ILs are better than those of DMIM-Br and mPAO. Consequently, the alkylphosphate ILs are suitable for use with the ceramic material. 3.3.1.3. Surface Analysis of the Si3N4 Disc by SEM. Figure 5 shows the SEM morphologies and 3D optical images of the worn Si3N4 disc surfaces lubricated with different lubricants at a high temperature of 100 °C. It can be seen that there are no obvious scratches on the worn Si3N4 surfaces lubricated with the alkylphosphate ILs and that the worn surfaces are smooth. On the contrary, some slight worn traces are seen on the Si3N4

surface lubricated with L-P104. Unfortunately, the worn Si3N4 surface lubricated with PAO is rough, and some signs of relatively severe wear are visible on the worn surfaces, which can be confirmed from the 3D optical images. The above results suggest that the alkylphosphate ILs have excellent antiwear properties, which can be attributed to the following three reasons: (a) The special properties of Si3N4 ceramic, such as high hardness and great corrosion resistance, allow them to bear high loads and high temperatures, showing great corrosion- and wear-resistant performance. (b) Si3N4 has a negatively charged surface that would offer active sites to attract the cations of the ILs to the specimen surface.7 As a result, an adsorbed electric double layer is formed on the worn surface during the friction process. Many IL molecules are concentrated on the adsorbed layer, which can improve the loadcarrying capability and decrease the friction coefficient.27 Besides, the anionic moieties of the alkylphosphate ILs are larger than that of L-P104, so a thicker electric double layer is 12817


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Figure 6. SEM morphologies of the worn surface of a Si3N4 disc lubricated with L-P104. Test conditions: 100−500 N, 25 Hz, 1 mm, 30 min.

results demonstrate that the ILs are suitable to use under high sliding speed conditions. It is different that the test load has little influence on the friction coefficients under lubrication with the ILs, and they maintain a steady value as the load increases. Nevertheless, the friction coefficients of PAO and L-P104 have some large fluctuations under the test conditions, confirming that PAO and L-P104 are unfit for lubrication of ZrO2. In addition, the order of the friction coefficients and wear volumes for the five lubricants is PAO > L-P104 > BMIM-DBP > TBADBP > BA-DBP. Consequently, BA-DBP shows the best friction-reducing and antiwear properties among the five lubricants for the Si3N4−ZrO2 contact. 3.3.2.2. Lubricating Properties at Different Temperatures. Figure 8 shows the evolution of the friction coefficients with time and wear volumes of the ZrO2 discs lubricated with the five lubricants at (a, b) 20 and (c, d) 100 °C (experimental conditions: 25 Hz, 300 N) to evaluate the tribological properties at different temperatures. It can be seen that the four ILs possess lower friction coefficients and wear volumes than PAO and that the friction coefficient curves of the ILs are extremely stable at 20 °C. Moreover, the tribological behaviors of the alkylphosphate ILs are better than that of L-P104. For the alkylphosphate ILs, the order of the friction coefficients is BMIM-DBP > TBA-DBP > BA-DBP. However, when the test temperature rises to 100 °C, the order is changed to TBA-DBP > BMIM-DBP > BA-DBP. The reason for this change is that a five-membered ring exists in the chemical structure of BMIMDBP, which can make it more stable than TBA-DBP at the high temperature. In addition, BA-DBP shows the greatest frictionreducing and antiwear properties among the five lubricants for the Si3N4−ZrO2 contact under all of the test conditions. This is due to the effect of hydrogen ions28 and high thermal motion of the molecules,29 which are enough to form an ordered adsorbed boundary layer. From Figure 8b,d it can be observed that the wear volumes of ZrO2 discs lubricated with the alkylphosphate ILs are much lower than that of the common IL L-P104, with a maximum reduction of 65%. All of the above discussions suggest that the alkylphosphate ILs are more suitable for the Si3N4−ZrO2 contact than L-P104 and PAO. 3.3.2.3. Surface Analysis of the ZrO2 Disc by SEM. The SEM morphologies and 3D optical images of the worn ZrO2 disc surfaces lubricated with different lubricants at high temperature are shown in Figure 9. The Si3N4−ZrO2 contact shows the same variation trend as its Si3N4−Si3N4 counterpart: the three alkylphosphate ILs can be used as excellent antiwear lubricants for the Si3N4−ZrO2 contact. However, the worn surface lubricated with PAO is characterized by a wide and deep wear scar with severe exfoliation. Compared with the Si3N4 disc, the ZrO2 plates exhibit more wear under the same test conditions. There are many obvious

formed on the contact surface under lubrication with the alkylphosphate ILs. Accordingly, the wear scars of the Si3N4 lubricated with the alkylphosphate ILs are shallower than those with L-P104. (c) The contact surface would react with ILs to form a boundary lubricating film containing nitrogen, oxygen, and phosphorus compounds, as can be proved by XPS analysis. Thus, the boundary lubricating film would provide a protective effect and prevent the friction surfaces from contacting directly. 3.3.1.4. Failure Mechanism of L-P104 As Determined by SEM and EDS. From Figure 3c it can be seen that lubrication failure occurred under lubrication with L-P104 when the load was increased to 500 N. In order to investigate the failure mechanism, SEM was employed to examine the morphologies of the wear tracks on the surface, as shown in Figure 6. It can be seen that the worn surface has obvious abrasion and exfoliation. What is more, the elemental composition was analyzed by EDS, and the results are shown in Table 3. Some typical active Table 3. Contents of Typical Elements (wt %) on the Worn Si3N4 Surface Lubricated with L-P104 As Determined by EDS Analysisa element wt %

N 18.98

O 4.55

F 0.97

P 0

a

Test conditions: variable-load experiments (100−500 N, 25 Hz, 1 mm, 30 min).

elements existed on the worn surface, such as F and O, which were derived from the decomposition of L-P104. However, it is worth mentioning that there was no P element on the worn surface, which is mainly related to the thin physical adsorption film of L-P104. It can be seen that the anionic moiety of L-P104 is much smaller than those of the alkylphosphate ILs, and thus, L-P104 cannot form a thick adsorption film on the surface during the friction process. Therefore, the lubricating film is easier to break and the wear scar surface is exposed under the high-load test conditions, resulting in the lubrication failure by L-P104. As a result, no P element was detected on the worn surface by EDS. 3.3.2. Si3N4−ZrO2 Contact. 3.3.2.1. Lubricating Properties under Variable-Frequency and Variable-Load Test Conditions. Figure 7 shows the evolution of the friction coefficients with time and wear volumes of the ZrO2 discs lubricated with the five lubricants under different test conditions. It can be seen that the alkylphosphate ILs exhibit better tribological properties with lower steady-state values of friction coefficients and lower wear volumes compared with L-P104 and PAO under the aforementioned test conditions. The friction coefficients of the ILs have a downward trend with increasing frequency, which is similar to the test results for the Si3N4−Si3N4 contact. The 12818


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Figure 7. Evolution of friction coefficients with time and wear volumes of the ZrO2 discs lubricated with different lubricants under (a, b) variablefrequency (10−30 Hz, 400 N, 1 mm) and (c, d) variable-load (100−500 N, 25 Hz, 1 mm) conditions.

Figure 8. Evolution of friction coefficients with time and wear volumes of the ZrO2 discs lubricated with different lubricants in constant-load and constant-frequency experiments (300 N, 25 Hz, 1 mm) at (a, b) 20 and (c, d) 100 °C.

matching parallel grooves on the sliding surfaces, and the wear scars become wider and deeper in the order BA-DBP < TBADBP < BMIM-DBP < L-P104. According to Figure 9h, the worn surface lubricated with L-P104 is desquamated, and the wear scar is harsher. In contrast, the worn surfaces are relatively smooth except for some grooves under lubrication with the alkylphosphate ILs. The reason for this phenomenon is that HF

may be generated on the worn surface because of the hydrolysis of PF6−, corroding the substrates and leading to serious wear.13 Furthermore, the wear debris would be generated at the contact interface. According to related research,30 a wear debris layer at the contact interface bed may reduce contact between the surfaces directly and reduce wear. However, the wear debris is too mobile to gather for L-P104 because of the low viscosity. 12819


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Figure 9. SEM morphologies of the worn surfaces of the ZrO2 discs lubricated with different lubricants and 3D optical images of the wear scars: (a, b, k) BA-DBP; (c, d, l) TBA-DBP; (e, f, m) BMIM-DBP; (g, h, n) L-P104; (i, j, o) PAO. SRV conditions: load, 300 N; stroke, 1 mm; frequency, 25 Hz; temperature, 100 °C; duration, 30 min.

mechanism of the ILs as lubricants for ceramics. The XPS spectra of typical elements (N 1s, P 2p, O 1s, and Si 2p) for the surfaces lubricated with (A) PAO and (B) BA-DBP were measured after the friction tests, and XPS analysis results for (C) a neat Si3N4 disc and (D) the neat IL BA-DBP were obtained before the tests. All of these results are shown in Figure 10. Moreover, deconvolution of the peaks was performed to obtain the binding energies of the different elements, and the curve-fitting procedures are displayed in section S5. It can be found from Figure 10B,D that the P 2p peak of the neat IL BA-DBP at 132.7 eV is assigned to the alkylphosphate anion. However, the P 2p peak is shifted to 133.9 and 135.5 eV after the tribological test. When this is combined with the peaks of O 1s at 530.5 and 532.1 eV for the surface lubricated with BA-DBP, it can be conducted that phosphorus oxide and PO43− emerged during the friction tests.31 In addition, the binding energy of N 1s also has great change compared with that of neat BA-DBP, as the N 1s peak

Thus, it was not easy to form the wear debris layer to reduce wear. On the basis of the above results, it can be concluded that the alkylphosphate ILs have better friction-reducing and antiwear abilities than L-P104 for the Si3N4−ZrO2 friction pair. 3.3.3. Performance Comparison for Si3N4−Si3N4 and Si3N4−ZrO2 Contacts. From the above test results, it can be seen that the ZrO2 discs exhibit more wear than the Si3N4 discs. There are two reasons for this issue The first reason is the difference in their physical properties: the bearing capacity of the ZrO2 ceramic is lower because of its lower hardness and roughness, so the grinding crack on the ZrO2 ceramic surface was deeper than that on Si3N4 disc during the friction process. The other reason is that the Si3N4 ceramic has a higher molecular activity than ZrO2 and more easily reacts with the ILs to form the adsorbed electric double layer and tribochemical lubricating film. 3.4. Surface Analysis by XPS. 3.4.1. Si3N4 Ceramic. An XPS analysis was also carried out to further explore the friction 12820


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Figure 10. XPS spectra of N 1s, Si 2p, O 1s, and P 2p for (A, B) the worn Si3N4 surfaces lubricated with (A) PAO and (B) BA-DBP, (C) a neat Si3N4 disc, and (D) neat BA-DBP IL.

Figure 11. Contact angles of the lubricants (a) BA-DBP and (b) PAO on Si3N4 surfaces.

can be inferred that SiOx is produced during the friction process.1 Furthermore, the binding energy of N 1s under lubrication with BA-DBP (396.9 eV) undergoes some changes compared with that of the neat Si3N4 disc (397.7 eV). Consequently, the above results further demonstrate that the IL reacts with the contact surfaces and a lubricating film is formed. Wettability is also an important property for the formation of a stable adsorption film on a surface. The relevant parameter to characterize the wettability of the lubricant is its contact angle on the contact surface, with a lower contact angle being indicative of better wettability.32 Figure 11 shows the contact angles of (a) BA-DBP and (b) PAO on the Si3N4 surface. It can be seen that these contact angles are similar and are all far less

in the neat IL appears at 400.9 eV and changes to 396.9 eV after friction. On the basis of these findings, it can be concluded that tribochemical reactions between the IL and substrate occurred during the friction process. Observing the Si 2p and N 1s spectra of surfaces lubricated by (A) PAO and (B) BA-DBP as well as that of (C) the neat Si3N4 disc, it can be seen that the Si 2p and N 1s peaks in (A) and (C) exhibit no signficant differences, which means that PAO does not react with the Si3N4 disc during the friction process. On the contrary, there is a peak shift for BA-DBP, as the Si 2p peak is at 102.6 eV for the neat Si3N4 disc but is transferred to 103.2 eV after the lubrication with BA-DBP. From this, in combination with the O 1s peak at 532.1 eV, it 12821


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Figure 12. XPS spectra of Zr 3d, O 1s, N 1s, and P 2p for (A, B) the worn ZrO2 surfaces lubricated with (A) PAO and (B) BA-DBP, (C) a neat ZrO2 disc, and (D) neat BA-DBP IL.

tests. Moreover, deconvolution of the peaks was performed to obtain the binding energies of the different elements, and the curve-fitting procedures are displayed in section S5. It can be seen that the XPS results for the Si3N4−ZrO2 contact are similar to those for Si3N4−Si3N4 contact. To be specific, the N 1s peak of the neat IL BA-DBP at 400.9 eV (D) is assigned to the alkylamine cation, which is shifted to 399.6 eV after the tribological test (B). This means that the chemical state of N changed in the friction process. Moreover, the binding energy of the P 2p peak also changed after the friction tests, from 132.7 eV (D) to 135.7 and 136.7 eV (B). On the basis of the above analysis, it can be concluded that tribochemical reactions between the IL and ZrO2 substrate occurred during the friction process. Furthermore, there seems to be no change for the Zr 3d and O 1s peaks under lubrication with (A) PAO compared with (C) the neat ZrO2 disc, which means that PAO did not react with the ZrO2 disc during the friction process. In contrast, the Zr 3d peaks of the neat ZrO2 disc at 182.3 and 184.6 eV shifted

than 90°, which means that the IL and PAO both have excellent wettability for the Si3N4 disc surfaces and adsorb on the substrates well. On the basis of the above analysis, it can be concluded that the IL and PAO both have favorable adsorption for the Si3N4 disc surfaces. However, tribochemical reactions occurred on the Si3N4 discs lubricated with the polar IL lubricants. Unfortunately, the nonpolar PAO lubricant cannot react with the surface. Consequently, the excellent tribological performance of the ILs is due to the synergistic effect of the physical adsorption and tribochemical reactions. Thus, the ILs show better tribological performance than PAO under the same test conditions. 3.4.2. ZrO2 Ceramic. XPS analysis of the Si3N4−ZrO2 contact was also carried out to explore the friction mechanism. Figure 12 shows the XPS spectra of typical elements (N 1s, P 2p, O 1s, and Zr 3d) for the surfaces lubricated with (A) PAO and (B) BA-DBP. The XPS analysis results for (C) a neat Si3N4 disc and (D) neat IL BA-DBP were also obtained before the 12822


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Figure 13. Contact angles of the lubricants (a) BA-DBP and (b) PAO on ZrO2 surfaces.

to 184.3 and 186.6 eV after lubrication with BA-DBP. This result, combined with the P 2p peaks at 135.7 and 136.7 eV and O 1s peaks at 532.1 and 533.8 eV, can be referred to phosphorus oxide and compounds composed of Zr, O, C, P, and H such as Zr(HPO4)2 and ZrP2O7.33 Consequently, the above results also demonstrate that the IL reacted with the contact surfaces. Figure 13 shows the contact angles of (a) BA-DBP and (b) PAO on the ZrO2 surface. It can be seen that both BA-DBP and PAO possess excellent wetting behavior on the ZrO2 surface and strongly adsorb on the surface. In conclusion, the ILs and PAO both have favorable adsorption on the ZrO2 substrate. Nevertheless, XPS analyses confirmed that tribochemical reactions occurred upon lubrication with ILs during the friction tests, and the nonpolar lubricant PAO does not react with the surface. Thus, ILs could provide outstanding tribological performance. From the above analysis, it can be concluded that the tribochemical reactions occurred under lubrication with ILs on the contact areas for Si3N4 and ZrO2 discs. Moreover, tribofilms composed of typical elements P, O, N, and C are formed. The tribofilms provide a protective effect and reduce the wear greatly by preventing surface contact. However, the PAO has no active elements and cannot react with the substrates. Accordingly, the ILs show better tribological preformance than PAO under the same test conditions.

Figure 14. Lubricating mechanism under lubrication with ILs.

In addition, fewer grooves are present on the substrates lubricated with alkylphosphate ILs, in contrast to the sliding surface under the lubrication with L-P104. There are two reasons that account for this. First, L-P104 is very sensitive to moisture, and HF is generated to erode the substrates35 because of hydrolysis. Thus, more wear is produced both on the Si3N4 and ZrO2 surfaces. In contrast, the antiwear behavior is significantly improved by lubricating with alkylphosphate ILs. There is no obvious corrosion of the ceramic substrates according to the corrosion tests. Besides, the molecular structure and physical properties of the lubricants also play an important role in determining their tribological performances. It is known that the thickness of the adsorbed layer is closely related to the molecular structures of the ILs. L-P104 and BMIM-DBP have the same cation, yet the anionic moiety of BMIM-DBP is larger than that of L-P104, enabling BMIMDBP to form a thicker adsorbed layer on the surfaces and increase the load-carrying capacity. Furthermore, the viscosities of alkylphosphate ILs are higher than that of L-P104, resulting in better antiwear properties. In summary, the polar alkylphosphate IL lubricants have the best tribological performance on the ceramic contacts among the studied lubricants. The order of the lubricating properties of these materials is as follows: alkylphosphate ILs > L-P104 > PAO.

4. DISCUSSION For Si3N4−Si3N4 and Si3N4−ZrO2 contacts lubricated with the same ILs and PAO, the friction coefficients and wear volumes were different. This is mainly attributed to their physical and chemical properties. The hardness and roughness of the ZrO2 ceramic are lower than those of the Si3N4 ceramic, so the loadcarrying capacity of the ZrO2 ceramic is inferior, leading to the higher friction coefficient and wear volume. On the other hand, the ZrO2 ceramic has weaker surface activity than the Si3N4 ceramic, and the tribochemical reaction with ILs on the ZrO2 ceramic surface is slower. As a result, the tribological performance of the ILs with the Si3N4 ceramic is better than that with the ZrO2 ceramic. Interestingly, the alkylphosphate ILs exhibit better tribological properties than PAO and L-P104 for both the Si3N4− Si3N4 and Si3N4−ZrO2 contacts. It is known that the excellent tribological properties of lubricants are ascribed to the formation of a boundary protective film.34 Figure 14 gives the lubricating mechanism under lubrication with ILs. According to the analysis of XPS and contact angle results in this study, it can be concluded that the polar IL lubricants are absorbed on the friction surface, after which the tribochemical reactions occur and the lubricating films are formed on the two kinds of substrates under lubrication with ILs. Nevertheless, the nonpolar PAO lubricant is only adsorbed on the friction surface and cannot react with the ceramic substrates, producing an inferior lubricating film. Consequently, the polar IL lubricants could exhibit better tribological performance than the nonpolar PAO lubricant on the ceramic substrates.

5. CONCLUSION The focus of this paper was to study the tribological properties of alkylphosphate IL lubricants on two substrates, Si3N4 and ZrO2 ceramics. Moreover, the common IL L-P104 and the nonpolar lubricant PAO were selected as reference lubricants. In addition, the corrosion properties of the alkylphosphate ILs were also measured. The results show that the Si3N4 ceramic possesses a lower friction coefficient and wear volume than ZrO2 because of its greater hardness and better surface activity. XPS, SEM, and contact angle analysis results demonstrate that the tribological properties of the lubricants are related to the 12823


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(12) Ye, C. F.; Liu, W. M.; Chen, Y. X.; Yu, L. G. Room-Temperature Ionic Liquids: a Novel Versatile Lubricant. Chem. Commun. 2001, 21, 2244. (13) Zhou, F.; Liang, Y. M.; Liu, W. M. Ionic Liquid Lubricants: Designed Chemistry for Engineering Applications. Chem. Soc. Rev. 2009, 38, 2590. (14) Bermudez, M. D.; Jimenez, A. E.; Sanes, J.; Carrion, F. J. Ionic Liquids as Advanced Lubricant Fluids. Molecules 2009, 14, 2888. (15) Minami, I. Ionic Liquids in Tribology. Molecules 2009, 14, 2286. (16) Palacio, M.; Bhushan, B. A. Review of Ionic Liquids for Green Molecular Lubrication in Nanotechnology. Tribol. Lett. 2010, 40, 247. (17) Shah, F. U.; Glavatskih, S.; MacFarlane, D. R.; Somers, A.; Forsyth, M.; Antzutkin, O. N. Novel Halogen-Free Chelated Orthoborate-Phosphonium Ionic Liquids: Synthesis and Tribophysical Properties. Phys. Chem. Chem. Phys. 2011, 13, 12865. (18) Qu, J.; Bansal, D. G.; Yu, B.; Howe, J. Y.; Luo, H. M.; Dai, S.; Li, H. Q.; Blau, P. J.; Bunting, B. G.; Mordukhovich, G.; Smolenski, D. J. Antiwear Performance and Mechanism of an Oil-Miscible Ionic Liquid as a Lubricant Additive. ACS Appl. Mater. Interfaces 2012, 4, 997. (19) Minami, I.; Inada, T.; Sasaki, R.; Nanao, H. Tribo-Chemistry of Phosphonium-Derived Ionic Liquids. Tribol. Lett. 2010, 40, 225. (20) Khatri, P. K.; Thakre, G. D.; Jain, S. L. Tribological Performance Evaluation of Task-Specific Ionic Liquids Derived From Amino Acids. Ind. Eng. Chem. Res. 2013, 52, 15829. (21) Fan, M. J.; Yang, D. S.; Wang, X. L.; Liu, W. M.; Fu, H. Z. DOSS-Based QAILs: As Both Neat Lubricants and Lubricant Additives with Excellent Tribological Properties and Good Detergency. Ind. Eng. Chem. Res. 2014, 53, 17952. (22) Somers, A. E.; Khemchandani, B.; Howlett, P. C.; Sun, J. Z.; MacFarlane, D. R.; Forsyth, M. Ionic Liquids as Antiwear Additives in Base Oils-Influence of Structure on Miscibility and Antiwear Performance for Steel on Aluminum. ACS Appl. Mater. Interfaces 2013, 5, 11544. (23) Zhang, S. W.; Hu, L. T.; Qiao, D.; Feng, D. P.; Wang, H. Z. Vacuum Tribological Performance of Phosphonium-Based Ionic Liquids as Lubricants and Lubricant Additives of Multialkylated Cyclopentanes. Tribol. Int. 2013, 66, 289. (24) Qiao, D.; Wang, H. Z.; Feng, D. P. Tribological Performance of Phosphate Ionic Liquids as Lubricants for Steel-on-Aluminum Contact. Proc. Inst. Mech. Eng., Part J 2013, 227, 1261. (25) Hagiwara, R.; Ito, Y. Room Temperature Ionic Liquids of Alkylimidazolium Cations and Fluoroanions. J. Fluorine Chem. 2000, 105, 221. (26) Chen, W.; Zhang, Y. Fabrication and Frictional Behavior of MoS2 Composite Lubricating Films. Mater. Mech. Eng. 2008, 32, 47. (27) Wong, P. L.; Huang, P.; Meng, Y. The Effect of the Electric Double Layer on a Very Thin Water Lubricating Film. Tribol. Lett. 2003, 14, 197. (28) Kuerten, D.; Winzer, N.; Kailer, A.; Pfeiffer, W.; Spallek, R.; Scherge, M. In-situ Detection of Hydrogen Evolution During Lubricated Sliding Contact. Tribol. Int. 2016, 93, 324. (29) Jiménez, A. E.; Bermúdez, M. D.; Carrión, F. J.; MartínezNicolás, G. Room Temperature Ionic Liquids as Lubricant Additives in Steel-Aluminium Contacts: Influence of Sliding Velocity, Normal Load and Temperature. Wear 2006, 261, 347. (30) Cai, Z. B.; Meyer, H. M.; Ma, C.; Chi, M. F.; Luo, H. M.; Qu, J. Comparison of the Tribological Behavior of Steel-Steel and Si3N4-Steel Contacts in Lubricants with ZDDP or Ionic Liquid. Wear 2014, 319, 172. (31) Somers, A. E.; Biddulph, S. M.; Howlett, P. C.; Sun, J. Z.; MacFarlane, D. R.; Forsyth, M. A. Comparison of Phosphorus and Fluorine Containing IL Lubricants for Steel on Aluminium. Phys. Chem. Chem. Phys. 2012, 14, 8224. (32) Westerholt, A.; Weschta, M.; Bösmann, A.; Tremmel, S.; Korth, Y.; Wolf, M.; Schlücker, E.; Wehrum, N.; Lennert, A.; Uerdingen, M.; Holweger, W.; Wartzack, S.; Wasserscheid, P. Halide-Free Synthesis and Tribological Performance of Oil-Miscible Ammonium and Phosphonium-Based Ionic Liquids. ACS Sustainable Chem. Eng. 2015, 3, 797.

physical absorption and tribochemical reaction. Besides, the study results also show that the lubricating performances of the lubricants are concerned with their molecular structure and physical properties. Consequently, the polar alkylphosphate IL lubricants exhibit better lubricating properties than L-P104 and PAO for both Si3N4 and ZrO2 ceramic contacts without corrosion of the substrates. In summary, alkylphosphate ILs can be used as high-performance lubricants for Si3N4−Si3N4 and Si3N4−ZrO2 contacts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03260. Structure characterization of the synthesized ILs, tribological properties of DMIM-Br and mPAO, and curve-fitting procedures for the different elements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 0931 4968075. *E-mail: [email protected]. Phone: +86 0931 4968170. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (51175492 and 51405477).

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REFERENCES

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