A tribological study of DIPE esters containing ionic liquids as high temperature and heavy load lubricants

Tribology International 142 (2020) 105971

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A tribological study of DIPE esters containing ionic liquids as high temperature and heavy load lubricants Qin Zhao a, Yuanyuan Li b, Xinhu Wu a, Rui Ma a, Gaiqing Zhao a, Xiaobo Wang a, * a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Gansu Provincial Hospital of Traditional Chinese Medicine, Lanzhou, 730050, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Statistical analysis Ionic liquids High temperature and heavy load lubricant

The novel DIPE esters containing ILs can be obtained by stirring DIPE esters with LiTFSI in different molar ratios. It can be seen from the tribological results that these ILs in DIPE esters showed more excellent friction-reduction and AW performance under high temperature and heavy load. A statistical regression using variance analysis was developed to examine the determining factor of anti-wear performance. Among all the experimental parameters, temperature and load were the main parameters affecting wear loss. Based on the observation of worn surfaces by SEM and XPS, the formation of the physical adsorption films of ILs in DIPE and the boundary tribochemical films were believed to be responsible for the excellent tribological performances at elevated temperature.

1. Introduction In the industrial lubrication field, including industrial oils and greases, greater attention have being placed to the synthetic ester based oils because of its high thermal stability and excellent lubrication per­ formance in recent years [1–5]. Especially in the most advanced modern lubrication system, the lubricants are obliged to satisfy the extreme working conditions such as the continuous flat press hot press machine of wood-based panel equipment with high temperature over 200 � C which only the high performance synthetic ester-based lubricants could be enough to better service. Based on the principle of fluid lubrication, the proper viscosity is the key to optimizing the reliable operation performance of liquid lubrica­ tion system. Although many synthetic ester oils have been synthesized and researched in order to satisfy the requirements of high temperature lubricants, the viscosities of the synthetic ester oils commonly used at high temperatures did not necessarily meet the heavy-duty requirements under harsh conditions at the same time. In order to solve the above problems, some macromolecule compounds called tackifiers or viscosity index improvers need to be added into synthetic ester oils, such as ethylene-propylene copolymer (OCP), polyisobutylene (PIB), poly­ methacrylate (PMAs), etc. These substances could improve the viscosity and viscosity-temperature properties of lubricating oil to a certain extent, but the insurmountable defect is that the chain molecular structure of polymer compounds is easy to oxidation and decomposition

at high temperatures [6]. Therefore, synthetic ester oils designed for high temperature and heavy load applications should be treated simul­ taneously with high viscosity and good tribological performance in order to provide adequate lubrication protection for metal surfaces to prevent vibration, shocks and wear. Vitally important, lubrication fail­ ures of mechanical equipments at high temperatures and high loads should be efficaciously avoided [7]. In recent years, ILs as lubrication oils exhibited generally good tribological performance at elevated temperature [8–13]. The mecha­ nism studies showed that ILs could be easily adsorbed on the sliding surface of frictional pairs to form strongly ordered adsorbed films, and subsequently effective boundary tribo-chemistry reactive films would form so as to reduce friction and wear [14–16]. Furthermore, some ionic liquids containing specific functional groups exhibited excellent high temperature tribological properties [17–19]. Therefore, combining the basic structure of ionic liquids and ester oils through molecular structure design and synthesis, it is hopeful to develop a new high temperature lubricating ester base oil with excellent performance at high tempera­ ture. Fan and his colleagues firstly reported in-situ synthesis method for IL additives in synthetic esters [20–24]. This method greatly simplified the preparation of the ILs and enhanced the tribological prosperities of base ester oils. Furthermore, Wang and his co-worker reported that ionic liquids formed in polyol esters and vegetable oils were high performance lubricants at temperatures of 200–300 � C [25–27]. From the results of previous work, the high content functional group in polyol esters could

* Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.triboint.2019.105971 Received 31 May 2019; Received in revised form 21 August 2019; Accepted 18 September 2019 Available online 19 September 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.

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not only increase the viscosity of the synthetic esters to some degree, but also possess friction-reduction and wear resistance performance at elevated temperature. Although many synthetic esters have been stud­ ied, including saturated TMP ester, saturated PE ester and saturated TMA ester, theoretically could form IL (Li[polyol ester])TFSI functional groups at a certain concentration, the relatively low concentrations ILs – O groups in branch containing saturated DIPE ester which have six C– chains could better meet the demands of high temperature and heavy load conditions since the more branched chains with the greater the viscosity. However, no studies have been done yet to find out whether ILs containing saturated DIPE ester could enhance the high temperature tribological properties. Therefore, this work focuses the tribological performance of ILs containing saturated DIPE ester in details. To further illustrate the above structural performance design and boundary lubri­ cation effects, the DIPE ester with viscosity enhancers should be required for comparison. However, on accounted of the special molec­ ular structure of the selected DIPE ester oil, most traditional viscosity enhancers are unable to dissolve in it to form stable base oils. Therefore, the commercially available high viscosity synthetic ester was chosen as viscosity index improvers of DIPE ester to compare the tribological properties at elevated temperature. Simultaneous, given the importance of tribochemical processes in affecting lubrication in the boundary regime, it was clear that environ­ mental conditions can affect wear and friction by altering the compo­ sition and mechanical properties of the tribolayer. A variety of different behaviors depended on the particular substrate/environment combina­ tion and experimental conditions. Many research for tribological ap­ plications had attempted to relate lubrication efficiency and wear resistance to the surrounding external conditions on solid surfaces [28–33]. Therefore, in order to further confirm the parameters affecting tribological performance, the statistical analysis was put to use to determine the influence factors of temperature, load and mole ratio of ILs on the tribological properties of the ILs-containing DIPE ester. A mathematical regression equation was established and verified by a large number of examples. The aim of this paper was to confirm the ILs-containing DIPE ester with enhanced friction-reducing and wear-resistant performance at high temperature and to make an analysis of factors affecting wear volumes under high temperature and heavy load, and to compare the experimental results with the statistical results under the limitation of research factors.

Table 1 Physical properties of DIPE ester. ILs

DIPE DIPE/ LiTFSI ¼ 1:0.5 DIPE/ LiTFSI ¼ 1:0.75 DIPE/LiTFSI ¼ 1:1 DIPE/ 3986 ¼ 1:25%wt

Kinematic viscosity (mm2/ s)

Viscosity Indexb

copper strip test/ corrosion gradec

40 � C

100 � C

a

318.2 1465

23.0 60.2

3.46 6.01

90 90

1a 1a

3455

97.5

7.58

88

1a

6217 1230

138.2 84.6

9.13 9.89

89 145

1a 1a

200 � C

a

Kinematic viscosity is calculated by the viscosity temperature formula. The viscosity index was determined according to the ASTM D2270-93 method. c The copper strip corrosion test was performed according to the ASTM D130-83 method (temperature, 120 � C; time, 3 h). b

Jupiter simultaneous TG-DSC instrument. A total of 5 mg of sample was placed in the TGA sample holder. The temperature was programmed to increase from 25 � C to approximately 800 � C at a heat rate of 10 � C/min in air. The oxidation stability of the forming IL lubricants was also tested by using NETZSCH DSC204 HP instrument (PDSC, O2 flow rate 40 ml min 1). Rheological analysis was carried out on an Anton Paar instrument (model MCR 302) in oscillation mode. The values of storage modulus, G0 , and loss modulus, G00 , as a function of a variety of variables: shear strain, γ, oscillation frequency, ω, and temperature, were recorded for each fluid. Moduli measurements were performed at fixed oscillation frequency and temperature but variable shear strain in order to establish quickly the universal class(e.g., Newtonian liquids, linearly elastic, nonlinear viscoelastic, gel) to which the complex belong. 2.3. Tribology test and surface analysis Tribological experiments were carried out on an Optimol SRV-V oscillating reciprocating friction and wear tester with an upper ball sliding the lower disc. Table 2 exhibited a summary of the test param­ eters chosen for the experiments carried out on each tribometer. In order to verify whether the experiments were conducted entirely within the boundary lubrication regime, the dimensionless film thickness param­ eter, Λ ¼ hmin/Sq, was calculated, where hmin is the minimum oil film thickness and Sq the composite standard deviation of the surface roughness [34] In this case, hmin was estimated through the well-known formula [35]. The result of Λ of the conditions chosen for the wear tests was also listed in Table 2. The pressure–viscosity coefficients (α) of the DIPE ester referred to relevant literature [36]. The hmin of DIPE com­ pounds yielded 3.83 nm (100 N), 3.71 nm (200 N), 3.63 nm (300 N) under 200 � C. Based on a surface roughness of about 25 nm, all the tribological tests under these testing conditions were in the boundary lubrication regime(Λ < 1) [37]. The wear of the lower disc was measured by a microxam threedimensional non-contact surface surveyor. The wear surface morphology was analyzed by JSM-5600LV scanning electron micro­ scope (SEM). The chemical composition of the films was determined by phi-5702 multifunctional X-ray photoelectron spectroscopy (XPS) with Al Ka as excitation source. The binding energy of contaminated carbon (C1s ¼ 284.8ev) was taken as a reference. Three repetitive measure­ ments were performed for each disc.

2. Experimental method 2.1. Materials The lithium salts (LiTFSI) was purchased by J&K Scientific LTD (No.69, Beichen west road, Chaoyang district, Beijing, China). Saturated DIPE ester was obtained from the Qingdao Lubemater corporation (No.621, Jiushui east road, Laoshan district, Beijing, China). Highviscosity synthetic ester (Priolube 3986) was purchased from CRODA corporation(No.45, Nanchang road, Huangpu district, Shanghai, China). The IL lubricants were formed by agitating the DIPE ester/LiTFSI with molar ratios of 1/0.5, 1/0.75 and 1/1.1 at 60–80 � C until it was totally dissolved. The compared DIPE-3986 complex was prepared by adding 25 wt % 3986 in DIPE ester. All the other chemicals were used without further treatment. 2.2. Physico-chemical analysis The FTIR spectra of IL lubricants were tested on a Bruker Tensor 27 FT-IR spectrometer between 4000 and 560 cm 1 with a resolution of 2 cm 1. The kinematic viscosity of these ILs compounds at 40 � C and 100 � C were tested by ASTM D445-2017 method and shown in Table 1. In addition, the copper strip test (120 � C,3 h) was conducted by the ASTM D130-2012 method. Thermogravimetric analysis (TGA) was studied on a STA 449 F3

2.4. Statistical analysis According to the selected parameters of tribological test, the response coefficient (wear volumes) was obtained. A mathematical model and regression equation were established with Minitab and 2

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influences of the Liþ-O¼C coordination bond formation. In the mean time, Fig. 2b demonstrates the carbonyl band was boarded and shifted to lower frequency with increasing LiTFSI ratios due to the more conver­ sion of the free carbonyl groups in this system. These results were in agreement with those in previous literature [20].

Table 2 Configuration and experimental parameters of the SRV. Test parameters

Values

Tribometer Configuration Sliding Movement

SRV-5 Ball-on-disc Reciprocating (linear stroke) ø 10 mm ø 24 � 7.88 mm ASI 52100 ASI 52100 61-65 HRC 61-65 HRC 25 nm 10 nm 115/1.232 GPa 115/1.232 GPa 25–200 � C 30min 1 mm 25 Hz 100–300 N 2.155/1.437 GPa 2.467/1.645 GPa 2.715/1.810 GPa 2.925/1.950 GPa 3.108/2.072 GPa 3.83 nm 3.75 nm 3.71 nm 3.67 nm 3.63 nm

Tribo-pair geometry Material of tribo-pair Hardness Roughness(Ra) Elastic modulus/Yield strength(σ0.2) Temperature Duration Amplitude Speed Load The peak/average Hertz pressures

hmin DIPE/LiTFSI(1:1) at 200 � C as a typical calculation

Ball Disc Ball Disc Ball Disc Ball Disc Ball Disc

100 N 150 N 200 N 250 N 300 N 100 N 150 N 200 N 250 N 300 N

3.2. Rheological analysis Oscillating shear tests were carried out at stable angular frequency (ω) ¼ 10 rad/s with the shear strain increased from 0.01 to 100. Fig. 3a shows the loss (G00 ) as function of strain had continues plateau region of dynamic moduli. At the same time, The loss (G00 ) moduli exhibiting viscous response was larger than storage (G0 ) in the whole range, which means that the viscous property was larger than elastic property in the whole frequency range. To further determine fluid behavior of the forming Li[DIPE)TFSI compounds, the typical shear stress–shear rate data are showed in Fig. 3b. The typical of Newtonian fluids was verified according to the linear relation between shear stress and shear rate. Moreover, all dynamic viscosity did not change when increasing the shear rate, displaying a typical Newtonian fluids behavior, suggesting the forming Li[DIPE)TFSI compounds had a good anti-shear ability. 3.3. Thermal analysis Fig. 4a shows the TGA curves of DIPE, DIPE-3986 and DIPE-LiTFSI at different ratios. It is clearly seen that the decomposition temperature (Td) of the DIPE containing ILs increased compared with the DIPE ester (266 � C). The decomposition temperature (Td) of DIPE-LiTFSI at molar ratios of 1/0.5, 1/0.75 and 1/1.1 were 355, 363, 373 � C, respectively. DIPE-3986 with macromolecular compound has a good decomposition temperature of 399 � C. Fig. 4b depicts the curves gotten from PDSC experiments for pure DIPE and DIPE-LiTFSI compounds from the point of view of oxidation stability. In the case of pure DIPE, the exothermic peak at 246.2 � C was associated with the special molecule structure. Compared with pure DIPE, all exothermic peaks for DIPE-LiTFSI com­ pounds exceeded the 210 � C, which may be assigned to the compounds with the Liþ-O function groups could not influence the IOT greatly. Furthermore, the DIPE-LiTFSI compounds still had better thermal sta­ bility than DIPE-3986 complex with the peak at 197.2 � C. The above results stated clearly that the DIPE-LiTFSI compounds could adapt to high temperature environment and has certain thermal stability.

verified by several test cases. Verification tests (Table 5) were carried out under multiple loads and temperatures within a given experimental condition. 3. Results and discussion 3.1. Physical properties and characterization Possible chemical reactions of DIPE ester containing ILs are shown in Fig. 1. The kinematic viscosities of these compounds are shown in Table 1. As can be seen from the Table 1 that the ratios of DIPE/LiTFSI have a significant influence on their kinematic viscosity, an increase in the forming of DIPE/LiTFSI groups could resulted in an increase of viscosity which may due to the larger molecular space structure. Espe­ cially, the (Li[DIPE])TFSI compounds shows higher viscosity at 200 � C when compared DIPE, though DIPE added with 25% 3986 could also increase the high temperature viscosity. It also can be observed in Table 1 that almost no corrosion on copper strips tested with the (Li [DIPE])TFSI compounds and their corrosion grade can be defined as 1a. Fig. 2a shows the FT-IR results of DIPE/LiTFSI compounds with different molar ratios (1:0.5, 1:0.75, 1:1). The characteristic absorption peaks of pure DIPE esters at 1737 cm 1 was attributed to its carbonyl stretching in the IR spectrum. After the chemical transformation, the carbonyl stretching band was shifted from 1737 to 1711 cm 1 owing the

3.4. Tribological performance The tribological behaviors of pure DIPE and DIPE-LiTFSI compounds at temperature programmed conditions were first investigated by using DIPE-3986 complex for comparison from 25 � C to 200 � C. The evolution trend of friction coefficient curves for pure DIPE, DIPE-3986 and DIPELiTFSI compounds are shown in Fig. 5. From the test results, DIPE-LiTFSI compounds with different ratios could not effective reduce the friction coefficient compared with DIPE when the temperature below 100 � C, which showed no difference with the previous results [20]. The dependence of the friction coefficient on the Sommerfeld number and

Fig. 1. Schematic structure formation of [Li(Synthetic Ester)]TFSI. 3

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Fig. 2. FTIR spectra of pure DIPE and DIPE LiTFSI compounds with various molar ratios. (a, Entire spectrum; b, Amplified spectrum of the 1500-2000 cm-1 range).

Fig. 3. Rheological data of forming (Li[DIPE)TFSI compounds. (a, Measurement of the G0 and G00 varying with applied shear strain; b, the typical shear stress–shear rate data).

Fig. 4. Thermal analysis data of forming (Li[DIPE)TFSI compounds. (a, the TGA results; b, The PDSC results).

judged the lubrication regime contrasted with stricbeck curve as shown in small figure in Fig. 5. Contrasted with typical Stribeck curve, we can deduce that the tribology test at the temperature below 100 � C may be in the mixed regime or the hydrodynamic lubrication regime. With the increase of temperature from 25 � C to 50 � C, the Sommerfeld number decreased and the friction coefficient decreased. In the mean while, with

the increase of temperature over 100 � C, the viscosity of the oils de­ creases greatly with temperature, resulting in the Summerfield number decreased and the friction coefficient increased. Contrasted with typical Stribeck curve, we can deduce that the tribology test at the temperature over 100 � C may be in the boundary regime. The results were consistent with the above calculation of the Λ values over 100 � C in Table 2. 4

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Fig. 5. Temperature-ramp friction testing results for DIPE and DIPE-lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) compounds (temperature ¼ 25–200 � C; load ¼ 100 N; stroke ¼ 1 mm; frequency ¼ 25 Hz).

In terms of the comparison between different oils, though the DIPE3986 complex with higher viscosity exhibited a relative lower friction coefficient compared with the pure DIPE between 100 � C and 200 � C under boundary condition, the DIPE-LiTFSI compounds with similar viscosity compared with the DIPE-3986 complex presented much lower friction coefficient than both the pure DIPE and the DIPE-3986 complex at the same test conditions. This phenomenon further demonstrated that though higher viscosity could decrease the friction coefficient at some content, the boundary tribo-chemical lubrication films formation on the wore surface of steel contacts lubricated by the DIPE-LiTFSI compounds began to play a very important role to further reduce the friction coef­ ficient at high temperature. The friction reduction and wear-resistant properties of the pure DIPE, the DIPE-3986 complex and the DIPE-LiTFSI compounds with different ratios at 200 � C were further tested by different loads (100–300 N). As shown in Fig. 6, the average friction coefficient pure DIPE oil was all around 0.15 by different loads at 200 � C. In the mean­ time, the DIPE-3986 complex showed relatively better friction

coefficient around 0.14 by different loads, which may be due to the higher viscosity at elevated temperature. However, the DIPE-LiTFSI compounds with different ratios indeed behaved relatively small fric­ tion coefficient about 0.12 compared with the pure DIPE and the DIPE3986 complex oils. For comparison of concentration ratios results, there was no obvious difference between three ratios because the results were within the range of error. Further increasing the rations of DIPE/LiTFSI had no positive effect on tribological performance of the DIPE-LiTFSI compounds. This trend can also be verified in Fig. 7. The friction coef­ ficient curves of pure DIPE oil ascended to above 0.14 in about few minute later and then waved between 0.14 and 0.16 under 300 N load at 200 � C. After about 400s running-in duration, the DIPE-LiTFSI com­ pounds with different ratios entered a relatively stable friction process with relatively low friction coefficient. All test samples had an unstable initial stage in the beginning of the test process in Fig. 7. The possible reason for the nosier of initial 400s data was that the absorbed layer generated by molecular structure of DIPE may be easily damaged at high

Fig. 6. Friction coefficient of the discs lubricated by DIPE, DIPE-3986, DIPELiTFSI with molar ratios of 1/0.5, 1/0.75 and 1/1 at 200 � C(load ¼ 100–300 N, stroke ¼ 1 mm; frequency ¼ 25 Hz).

Fig. 7. Evolution of the friction coefficient by DIPE, DIPE-3986, DIPE-LiTFSI with molar ratios of 1/0.5, 1/0.75 and 1/1 at 200 � C(load ¼ 300 N, stroke ¼ 1 mm; frequency ¼ 25 Hz). 5

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temperature, which causing the unstable friction coefficient. In the mean time, although the wear volumes compared with the pure DIPE oils in Fig. 8 was obvious decreased by at least 3 times as many for the sliding disc under lubrication of DIPE-LiTFSI compounds with different ratios at 100 N and 200 N loads, the DIPE-LiTFSI compounds with different ratios still behaved similar wear resistance. When considering the solubility and economy of LiTFSI in DIPE, the DIPE-LiTFSI com­ pounds with 1:0.5 would be optimum proportion to provide excellent friction reduction under high temperature and heavy load. Moreover, regardless of the particular consideration, the higher ratios of com­ pounds will not be recommended. In addition, all the DIPE-LiTFSI compounds with different ratios still had a good anti-wear perfor­ mance than the DIPE-3986 complex oils, which was consistent with the trend of friction-reducing function. The results indicated that DIPELiTFSI compounds had outstanding tribological property at 200 � C and can be used as high temperature and heavy load lubricants.

Table 3 Design date and wear results.

3.5. Statistical analysis To further analyze the factors affecting wear volumes under high temperature and heavy load, a liner regression model was established and the experimental results obtained from the design data (Table 3). At the same time, the forms of variance analysis are listed in Table 4. It should be noted that in this paper, the effects of ratios, pressure and temperature of ionic liquid in DIPE on tribological properties were mainly studied due to the adsorption and tribochemical reactions of the ionic liquid were mainly affected by these factors. Therefore, other pa­ rameters, such as time and distance which were proportional to wear volumes to some extent according the theoretical Archard model, had been fixed in experiments and statistical analysis. It can be seen that the interaction between temperature and load had a remarkable effect on the wear volumes of disc lubricated by DIPELiTFSI compounds, which the factor P ¼ 0.01 causes. The P value of other factors acting alone or interacting was greater than 0.05, which further showed that the correlation between these factors and wear rate was weak. These results further confirmed the conclusion of tribological experiment that the different concentration had little effect on wear volumes within the range of observation. When the regression coeffi­ cient R (R ¼ 0.905) was closer to 1, the wear volumes changed signifi­ cantly with the change of the interaction and the individual action of the influencing factors. In addition, the closer R2 (R2 ¼ 81.95%) was to 100%, the higher the fitting degree of regression equation was. There­ fore, the regression equation represented 81.95% fitness and predicted wear under limited conditions. At the same time, R2 was very close to R2

Exp. No.

Temp. (� C)

Load (N)

Ratio

Wear Volume(μm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

100 100 100 100 100 100 100 100 100 150 150 150 150 150 150 150 150 150 200 200 200 200 200 200 200 200 200

100 100 100 200 200 200 300 300 300 100 100 100 200 200 200 300 300 300 100 100 100 200 200 200 300 300 300

0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1

268904 171742 148300 548622 303187 284226 832781 617185 312497 223666 363096 391089 447368 621951 643774 1061512 1451587 1051142 671437 733868 865929 858205 938035 1076696 2868993 3463251 2126368

(adjusted), which indicated that the regression equation was very reliable. A linear regression model was established to research the correlation between the wear parameters (load, temperature and ratio) and results (wear volumes). The regression equation by Minitab was as follows: Average wear volume(m3) ¼ 4.018 � 10 4-8.352 � 10 6 � tempera­ ture-3.222 � 10 6 � loadþ2.382 � 10 4 � ratioþ8.356 � 10 8 � temperature � loadþ3.838 � 10 6 � temperature � ratio5.048.9 � 10 6 � load � ratio. The equation can be used to predict the wear value in the limit range of the factors studied in this paper, and the regression model well fits the actual value when ratios of DIPE/LiTFSI ranged from 1:0.5 to 1:1, the temperature ranged from 100 to 200 � C and the load ranged from 100N–300 N. Fig. 9a and b shows residual normal diagram and predic­ tion of actual wear and wear value diagram. It can be seen from Fig. 9a that the distribution of residual was concentrated, which showed that residual was normal distribution. Fig. 9b shows that the predicted value was positively correlated with the actual value, which showed that the regression equation can better predict the change of wear volumes under the limitation of research factors. The accuracy of the regression equation for predicting wear was verified by a series of statistical tests. The predicted values obtained from the regression equation were in contrast to the experimental data, and the results can be seen in Table 5. The results show that the error between the predicted value and the actual value of the regression equation was less than 10%. This further indicated that the wear rate under limited conditions can be predicted by the linear regression equation established in this experiment. 3.6. Wear surface analysis Fig. 10 depicts the SEM micrographs of the worn steel surfaces lubricated by the pure DIPE, the DIPE-3986 complex and the DIPELiTFSI compounds with different ratios at 200 � C. It is obvious that the worn surfaces of the steel lubricated by pure DIPE and the DIPE-3986 complex showed considerably wider, longer and deeper wear scars. However, the worn surfaces lubricated by the DIPE-LiTFSI compounds with different ratios were smaller and shallower. Simultaneous, it can be

Fig. 8. Wear volumes of the discs lubricated by DIPE, DIPE-3986, DIPE-LiTFSI with molar ratios of 1/0.5, 1/0.75 and 1/1 at 200 � C(load ¼ 100–300 N, stroke ¼ 1 mm; frequency ¼ 25 Hz). 6

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Table 4 Variance analysis for the wear volumes. Source

DF

Temperature ( C) Load (N) Ratio Temperature � Load Temperature � Ratio Load � Ratio Error Total �

1 1 1 1 1 1 20 26

Seq.SS

Adj.SS 12

5.684 � 10 5.497 � 1012 4.317 � 1010 2.095 � 1012 2.761 � 1010 1.912 � 1011 2.982 � 1012 1.652 � 1013

Adj.MS 11

1.531 � 10 6.673 � 1010 3.114 � 109 2.095 � 1012 2.761 � 1010 1.912 � 1011 2.982 � 1012

11

1.531 � 10 6.673 � 1010 3.114 � 109 2.095 � 1012 2.761 � 1010 1.912 � 1011 1.491 � 1011

F

P

1.03 0.45 0.02 14.05 0.19 1.28

0.887 0.785 0.667 0.001 0.672 0.271

R ¼ 0.905(P < 0.05), R2 ¼ 81.95%, R2(adjusted) ¼ 76.53%. Notes: R, Regression; S, Standard deviation; DF, Degrees of freedom; Seq SS, Sequential sum of squares; Adj SS, Adjusted sum of squares; Adj MS, Adjusted mean squares.

Fig. 9. (a) Residual normal diagram and (b) Prediction of actual wear and wear value diagram.

clearly seen from SEM that both DIPE and DIPE-3986 caused serious abrasive wear and adhesive wear scar although the DIPE-3986 was slightly smaller. Nevertheless, the DIPE-LiTFSI compounds with different ratios produces a relative smoother wear trace with narrow furrows and micro cracks caused by adhesive wear. These observations

show that the boundary lubrication film of DIPE-LiTFSI compound can prevent the friction surface from contacting directly at high tempera­ ture, while the boundary oil film of pure DIPE, even the complex of DIPE-3986, may be fragile and easily wrecked by pressure and me­ chanical force. The three-dimensional optical microscopic image of the 7

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were obvious oxygen (O), iron (Fe), fluorine (F) and sulfur (S) signals on the worn surface, which indicated that the chemical reaction occurred between DIPE-LiTFSI compounds and steel surface during the sliding course. The peak values of Fe2P appeared at 710.3 and 724.3 eV, indi­ cating that tribochemistry process was involved in the formation of FeOOH and Fe3O4 [18,38]. The existence of Fe3O4 and FeOOH can be confirmed in comparison with the wide peak of O 1s in the range of 530.1–531.8eV [38–40]. The peaks appeared at 684.5–685.6 eV in the XPS spectra of F 1s, which were related to the formation of FeF2 and FeF3 [18,41]. The S 2p peaks appear at around 168.9 eV and could belong to FeSO4 [41]. From the above XPS analysis, the conclusion can be made that a stable boundary lubrication film had been formed between the DIPE-LiTFSI compounds and steel surface. The film composed of Fe3O4, FeOOH, FeSO4, FeF2 and FeF3 and C–O bonding, which was helpful for formed ILs to have the good friction-reducing and AW performance at high temperature and heavy load. So in these cases, the excellent tribological properties are attributed to the polarity induced physical adsorption films of formed ILs on the surfaces and further tribochemical reaction films of TFSI with the sliding metallic surfaces.

Table 5 The predicted values obtained from the regression equation were compared with the actual experimental values. No

Temp. (� C)

Load (N)

Ratio

Actual volumes (μm3)

Predicted volumes(μm3)

Error(%)

1 2 3 4 5 6 7 8 9 10 11 12

150 150 150 150 150 100 100 100 200 200 200 200

100 100 100 200 300 150 150 150 150 150 150 150

0.5 0.75 1 0.5 0.75 0.5 0.75 1 0.5 0.75 1 0.75

240478 308151 371045 875558 1403512 249515 237763 207973 903826 1003776 1101873 974125

234772.46 312017.91 389263.35 913615.24 1417271.8 269111.67 235279.03 201446.39 879276.03 941383.73 1003491.4 941383.73

2.37259 1.25488 4.910011 4.346634 0.980383 7.853807 1.04467 3.13804 2.71623 6.21576 8.92858 3.3611

worn surface of lubricant lubrication were also shown in Fig. 11, which further confirmed the surveyed wear volumes and SEM morphology. It is undoubtedly shown that the DIPE-LiTFSI compounds formed had good wear resistance capacity. XPS curves analysis was bring to further investigate the lubricating mechanism of the DIPE-LiTFSI compounds during the fiction condition at elevated temperature. The binding energies of representative ele­ ments on the surface of original wear marks are shown in Fig. 12. There

4. Conclusions In this paper, ionic liquids in DIPE esters using as high temperature and heavy loads lubricants can be obtained by dissolving and reacting DIPE esters with LiTFSI with various concentrations. Tribological results

Fig. 10. SEM micrographs of worn steel discs lubricating by different lubricants at 200 � C: Pure DIPE(a1, a2); DIPE-LiTFSI with molar ratios of 1/0.5(b1, b2); 1/0.75 (c1, c2); 1/1 (d1, d2); DIPE-3986 complex(e1, e2).

Fig. 11. 3D optical microscopic images of worn steel discs lubricating by different lubricants at 200 � C: Pure DIPE(a1, a2); DIPE-LiTFSI with molar ratios of 1/0.5(b1, b2); 1/0.75 (c1, c2); 1/1 (d1, d2); DIPE-3986 complex(e1, e2). 8

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Fig. 12. Fe2p, O1s, F1s, and S2p XPS regions scan for wear scars lubricating by DIPE-LiTFSI with molar ratios of 1/0.5, 1/0.75, 1/1 at 200 � C(load ¼ 200 N, stroke ¼ 1 mm; frequency ¼ 25 Hz).

showed that DIPE-LiTFSI compounds has good friction reduction and wear resistance, and can be used for lubrication of steel/steel contact parts under high temperature and heavy load. A statistical regression model for the influence of sliding wear parameters on wear behavior was established. Among all the experimental parameters, temperature and load were the main parameters affecting wear loss. In addition, the DIPE-LiTFSI compounds formed were also better than the usual hightemperature heavy-duty lubricants, i.e. DIPE-3986 complex. X-ray photoelectron spectroscopy (XPS) analysis showed that the wear surface formed a boundary lubrication film consisting of Fe3O4, FeF3, FeF2, FeOOH, and C–O bonds. Thin films formed were considered to be the reason for the superior tribological properties of ILs at high temperatures.

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