reduced graphene oxide composites

Tribology International 141 (2020) 105951

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Tribology International journal homepage: http://www.elsevier.com/locate/triboint

Enhanced tribological properties of diesel engine oil with Nano-Lanthanum hydroxide/reduced graphene oxide composites Bo Wu a, Hui Song a, Chuan Li a, Ruhong Song a, Tianming Zhang b, Xianguo Hu a, * a b

School of Mechanical Engineering, Hefei University of Technology, Hefei, 230009, PR China Anhui Runpu Nano-Technology Co., Ltd, Anqing, 246003, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Nano-La(OH)3 Graphene oxide Composites Tribological properties

Nano-lanthanum hydroxide/reduced graphene oxide (nano-La(OH)3/RGO) composites were successfully pre­ pared as anti-wear additives for diesel engine oil. Response surface methodology was adopted to explore the influences of temperature, contact pressure, and composite concentration on the tribological properties of diesel engine oil. The anti-wear mechanism of nano-La(OH)3/RGO composites was simultaneously studied by comparing with the tribological behavior of graphene oxide and nano-La(OH)3 pure particles. Results indicate that nano-La(OH)3/RGO composites can remarkably improve the anti-wear performance of diesel engine oil under boundary lubrication conditions. In particular, the anti-wear performance of diesel engine oil increased by 44% after adding 0.1 wt% composites at a temperature of 80 � C and a contact pressure of 1.62 GPa. The syn­ ergistic anti-wear mechanism of graphene and lanthanum oxide was proposed on the basis of worn surface characterization.

1. Introduction Friction and wear are widely known to result in mechanical equip­ ment failure and huge energy resource dissipation [1]. Among the ma­ chinery losing of diesel engine, the friction and wear between engine internal transmission components are approximately 75% [2]. Diesel engine oil must effectively control friction and reduce wear of engine mechanical components to ensure the efficient operation of mechanical parts and extend the service life of the diesel engine [3]. The anti-wear additive plays a key role in the tribological properties of diesel engine oil. However, with the rapid development of high-efficiency mechanical equipment, the load per unit mass of a heavy-duty vehicle engine be­ comes heavy, causing traditional lubricant additives to hardly meet the new diesel engine equipment demands of high speed, high temperature and heavy load operating conditions [4,5]. Therefore, developing novel effective anti-wear lubricant additives with good wear resistance and high bearing capacity is urgently needed to meet the demands of heavy-duty equipment under severe service conditions. Graphene, as a 2D honeycomb lattice arranged by sp2 carbon atoms, has elicited considerable attention in the field of lubrication materials due to its ultrathin layer structure and excellent physicochemical properties [6,7]. However, graphene is prone to forming aggregates due

to the strong π–π bonding interaction between nanosheets, and the chemical inertia leads to poor compatibility with other compounds [8]. Consequently, graphene experiences difficulty in achieving stable dispersion in oil-based systems; thus, its application as lubricant additive is limited. Graphene oxide (GO), as a derivative of graphene, has several functional groups such as hydroxyl, carboxyl and carbonyl on the surface [9]. GO has been found to exhibit certain dispersion stability in lubricants after surface modification. A number of studies have reported the good tribological properties of GO in lubricants. Senatore et al. [10] investigated the tribological behavior of GO nano­ sheets in mineral oil under different conditions and proved that GO easily forms protective films that prevent direct contact between steel surfaces and improve the tribological behavior of the base oil. Chen et al. [11] systematically investigated the tribological properties of a few layered GO sheet in hydrocarbon base oil and found that the coefficient of friction and wear are decreased with the addition of GO sheets. Nevertheless, similar to graphene, GO is easily corrugated during sliding, which increases friction between interfaces and results in the loss of its excellent tribological properties [12]. Surface functionaliza­ tion can be used to effectively improve the tribological properties of GO, and it has attracted much attention in the present decade. Numerous methods have been utilized to develop functional GO nanomaterials,

* Corresponding author. E-mail address: [email protected] (X. Hu). https://doi.org/10.1016/j.triboint.2019.105951 Received 15 June 2019; Received in revised form 23 August 2019; Accepted 3 September 2019 Available online 4 September 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.

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including noncovalent grafting, covalent bonding, functionalization with nanoparticles, and substitutional doping [13]. Functionalization with nanoparticles can be realized by depositing nanoparticles onto GO. This method provides a research direction for the development of novel graphene-based lubricant additives. Recently, rare earth elements have attracted considerable attention in the field of lubrication because of their special physical and chemical properties, such as strong chemical activity, large atomic radius and low electronegativity [14–16]. Nano-La(OH)3, as an important light rare earth compound, has been found to have unique tribological properties that differ from those of other metal oxides due to the strong chemical activity at the friction interface of La with a special 4f electronic struc­ ture. Zhao et al. [17] found that the La element in nano-La(OH)3 can function as a catalyst to accelerate the ion exchange reaction of O, Si and Fe tribofilm formation, which make serpentine/La(OH)3 composites to exhibit better tribological properties than those of a single serpentine or La(OH)3 particle in lubricating oil. Zhang et al. [18] found that succi­ nimide modified La(OH)3 nanoparticles and ZDDP exert a synergistic effect on improving the tribological performance of liquid paraffin. The preceding references demonstrate that nano-La(OH)3 has good compatibility with and tribological synergistic effect on other com­ pounds in lubricants; thus, it is more promising as a lubricant additive than other metal oxides. Accordingly, nano-La(OH)3 with special tribological properties was selected in the current study to functionalize GO and obtain a novel nano-La(OH)3/RGO composite additive for diesel engine oil. We expect to significantly improve the tribological properties of diesel engine oil through the synergistic effect of nano-La(OH)3 and GO, so as to meet the harsh conditions of a diesel engine at high speed, temperature and load. Temperature and contact pressure are known to exert certain in­ fluences on the anti-wear and anti-friction properties of a lubricant. Exploring the applicability of nano-La(OH)3/RGO composites at different temperatures and contact pressures is necessary to provide data reference for their application as diesel engine oil additives. In the current study, response surface methodology (RSM) was used to sys­ tematically analyze the effects of different temperatures, contact pres­ sures and composite concentrations on the anti-wear and anti-friction properties of diesel engine oil because it is a useful experimental design method to study the effects of multivariate variables, including their interaction on response value [19–21]. The anti-wear mechanism of nano-La(OH)3/RGO composites in diesel engine oil was also investi­ gated by comparing with the tribological behavior of GO and nano-La (OH)3 pure particles. This study provides an important theoretical basis for the application of nano-La(OH)3/RGO composites as highly effective anti-wear additives for diesel engine oil.

Corporation. Ammonia solution (25%) was purchased from Yangzhou Hubao Chemical Reagent Co. Ltd. LaCl3⋅7H2O was purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd. Ethyl alcohol was purchased from Jiangsu Qiangsheng Chemical Co. Ltd. All chemical reagents are analytically pure.

2. Experimental

2 WR 13 a ¼ 2ð Þ 3 E’

(1)

4W

(2)

2.2. Synthesis of nano-La(OH)3/RGO composite LaCl3⋅7H2O (0.45 g) was dissolved in deionized water (50 ml), GO (0.023 g) was dispersed in the solution using ultrasonic 1 h. Oleyl amine (0.65 g) and ammonia solution (0.95 g) were mixed evenly in ethyl alcohol (100 ml). Thereafter, the ethyl alcohol mixture solution was poured into the GO mixed dispersion, which was heated to 70 � C and stirred for 3 h. After the reaction, the mixture was centrifuged and filtered, and the solid was washed to a pH of 7–8 and dried at 105 � C for 12 h. Nano- La(OH)3/RGO composites were obtained and kept in a hermetic bottle for subsequent uses. 2.3. Tribological tests and models 2.3.1. Tribological tests High frequency reciprocating rig (HFRR), as a controlled recipro­ cating friction and wear testing equipment, is frequently used to assess the performance of fuels and lubricants [22,23]. In this study, an HFRR produced by Jinan YiHua Tribology Testing Technology Co., Ltd., China was utilized to conduct tribological tests. The schematic of this HFRR is presented in Fig. 1. The physical indexes of the standard ball-on-disk friction pairs used are listed as follows: Balls—AISI 52100 bearing steel, φ6 mm, hardness 58–66 HRC (647–861 HV), and Ra 0.016 μm; Disk—AISI 52100 bearing steel, φ10 mm, thickness 3 mm, hardness 190–210 HV30, and Ra 0.016 μm. The tribological tests were done at different temperatures and loads for 60min under a high sliding velocity of 0.10 m/s (1000 μm stroke and 50 Hz frequency). Different concen­ trations of nano-La(OH)3/RGO composites were evenly and stably dispersed in diesel engine oil with 60 min of ultrasonic oscillation. As shown in Fig. 1, no clear stratification of the obtained suspensions in the centrifuge tube occurred and only a minimal amount of precipitation appeared at the tube bottom after 28 days of standing. This result demonstrated the excellent dispersion stability of nano-La(OH)3/RGO composites in diesel engine oil. The average Hertz pressure of ball-on-disk contact was calculated with the Eq. (1) and Eq. (2). To elucidate the lubrication regime and wear mechanism, the minimum oil film thickness and Lambda ratio were calculated according to the Dowson and Hamrock minimum film thickness formula (Eq. (3) and Eq. (4)) [24–26].

2.1. Materials

P¼

GO was purchased from Shandong OBO New Material Co. Ltd. Diesel engine oil (CF-4 20W-50) was produced by Anhui Runpu NanoTechnology Co. Ltd (Physical and chemical properties are shown in Table 1). Oleyl amine was purchased from Shanghai Aladdin Industrial

0:49

hmin ¼ 3:63RðαE’ Þ λ¼

Table 1 Physical and chemical properties of the diesel engine oil (CF-4 20W-50). Parameters

Specification

Standard

Kinematical viscosity (40 � C)/(mm2⋅s 1) Kinematical viscosity (100 � C)/(mm2⋅s 1) Density (20 � C)/(kg⋅m 3) Open cup flash point/� C Pour point/� C Water content/% TBN/(mg KOH/g) Sulfated ash content (m/m)/%

140.3 18.5 849 223 22 trace 9.14 0.023

ASTM D445 ASTM D445 ASTM D1298 ASTM D92 ASTM D97 ASTM D95 ASTM D2896 ASTM D874

π a2

hmin 1=2

ðR2a1 þ R2a2 Þ

η0 U

ð

E’ R

W Þ0:68 ð ’ 2 Þ ER

0:073

ð1

e

0:68k

Þ

(3) (4)

where a is the Hertz contact diameter, E’ is the effective modulus of the elasticity (E’ ¼ 233 Gpa), W is normal load, R is the equivalent curva­ ture radius (R ¼ 3mm), P is the average Hertz pressure, α is the visco­ sity–pressure coefficient (α ¼ 2:2 � 10 8 m2 =N), η0 is the dynamic viscosity (40� C; η0 ¼ 0:119Ns=m2 ; 60� C; η0 ¼ 0:0477Ns=m2 ; 80� C; η0 ¼ 0:0231Ns=m2 ), U is the sliding speed (U ¼ 0:10m=s), k is the elliptical parameter (k � 1:03), Ra1 and Ra2 are the surface roughness of ball and disk. The tribological tests were performed at different temperatures ranging from 40 � C to 80 � C and different loads ranging from 1.96 N to 2

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Tribology International 141 (2020) 105951

Fig. 1. The schematic diagram of HFRR, dispersion stability photographs of lubricants and characterization of wear zone.

9.80 N. These values corresponded to Hertz contact pressures ranging from 0.95 GPa to 1.62 GPa and λ ratio ranging from 2.09 to 0.60. The aforementioned data indicated that the initial lubrication regime ranged from the mixed lubrication regime to the boundary lubrication regime [27].

COF or Wear ¼ β0 þ β1 A þ β2 B þ β3 C þ β11 A2 þ β22 B2 þ β33 C2 þ β12 AB þ β13 AC þ β23 BC

2.4. Characterization

2.3.2. Response surface methodology RSM is a statistical method to establish and verify the mathematical model between parameters and responses with the least number of trials [28]. Box–Behnken design (BBD) as a kind of RSM, is usually adequate for conducting process analysis and obtaining quadratic surface model. This method was employed to study the relationship between the vari­ ables (temperature (A), load (B), and composite concentration (C)) and response (friction coefficient (COF) and wear volume (Wear)). Table 2 shows the high levels (þ1) and low levels ( 1) of the BBD experiment variables. According to the BBD for three variables, 15 experiments including 12 axial points and 3 center replicate points were required. Eqs. (5)–(7) shows an empirical polynomial [19], which was usually utilized to describe the BBD model. For the three variables in this paper, the empirical polynomial can be expressed as Eq. (8). The statistical software of Design-Expert 8.0.6 (Stat-Ease Inc., USA) was used to perform regression analysis on the experimental data, fit out and eval­ uate the significance of the model equation. Friction coefficient (COF) ¼ f (A,B,C),

(5)

Wear volume (Wear) ¼ f (A,B,C),

(6)

j 1 X n X

n X

Y ¼ β0 þ

β i Xi þ i¼1

n X

βij Xi Xj þ i¼1

j¼2

High-resolution transmission electron microscope (HRTEM, JEM–2100F, Japan), Raman spectroscopy (HR Evolution, HORIBA Jobin Yvon) and Powder X-ray diffraction (XRD, X’Pert PRO MPD, Holland) were used to characterize the microstructure and chemical composition of the nano-La(OH)3/GO composites. Field emission scanning electron microscopy (FESEM, Hitachi model SU8010, Japan) and 3D laser scanning microscopy (Keyence model VK-X100, Japan) were employed to observe the wear zones on steel disks. Raman spectroscopy (HR Evolution, HORIBA Jobin Yvon), energy dispersive spectroscopy (EDS, Hitachi model SU8010, Japan) and X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, America) were employed to investigate the chemical composition of the tribo-film on the wear zones. 3. Results and discussion 3.1. Characterization of nano-La(OH)3/RGO composites The XRD patterns of GO and nano-La(OH)3/RGO composite are presented in Fig. 2a. The positions and relative intensities of diffraction peaks could be found as the hexagonal phase (P63/m (176), Joint Committee on Powder Diffraction Standards file number 36-1481) of La (OH)3 particles, which demonstrates that the La(OH)3 nanoparticles were successfully synthesized [17,29–31]. The diffraction hump appearing at 26� is attributed to the stacking of graphene sheets [32,33]. The disappearance of diffraction peaks at 11� and 42� indicates that GO was reduced to graphene after the precipitation reaction [34]. This result illustrates that the La(OH)3 nanoparticles were successfully anchored on the RGO nanosheets. Fig. 2b shows the Raman spectra of GO and nano-La(OH)3/RGO composites. The D band (1350 cm 1) attributed to the disordered carbon and G band (1580 cm 1) attributed to the sp2 carbon of graphene can be observed, respectively [35,36]. The graphitization degree of carbon materials can be defined using the ratio of intensities ID/IG. However, the ID/IG value of nano-La(OH)3/RGO composites (1.54) is slightly higher than that of the GO (1.23), which can be ascribed to the introduction of La atoms during precipitation reaction. Fig. 3 shows the micrographs of GO sheets, nano-La(OH)3 and nanoLa(OH)3/RGO composites. As presented in Fig. 3a, the GO sheets are transparent and have a corrugated shape. Evident lamellar aggregation

(7)

βii X 2i ;

i¼1

where Y represents the predicted response (COF, Wear), β0 represents a constant, βi represents the linear coefficient, βij represents the interac­ tion coefficient, and βii represents the quadratic coefficients.

Table 2 Independent variables and their coded levels for the BBD. Factor

Code

Units

Coded variable levels

Temperature Load (contact pressure) Concentration

A B

�

C N (Gpa) wt%

40 1.96 (0.95) 0

1

C

0

þ1

60 5.88 (1.37) 0.05

80 9.80 (1.62) 0.1

(8)

3

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Tribology International 141 (2020) 105951

Fig. 2. XRD (a) and Raman (b) analysis of the GO and nano-La(OH)3/RGO composites.

can be observed in the lattice-resolution HRTEM image of the GO sheets (Fig. 3b). Fig. 3c shows a stick-shaped structure of nano-La(OH)3 with a length of approximately 150 nm and a width of approximately 40 nm. The measured interplanar spacing is 0.319 nm and 0.327 nm (Fig. 3d). These values correspond to the lattice plane distances of (101) planes and (110) planes of hexagonal La(OH)3, respectively [31]. Fig. 3e shows the HRTEM image of nano-La(OH)3/RGO composites. Several stick-shaped nanoparticles similar to nano-La(OH)3 were attached to the GO sheets. The clear lattice fringes similar to the hexagonal La(OH)3 lattice further demonstrated that the stick-shaped nanoparticles on the GO sheets are highly crystalline nano-La(OH)3, as observed in Fig. 3f. Interestingly, nano-La(OH)3 was mostly attached uniformly at the edges and folds of the GO sheets. This observation may be attributed to the fact that more activity sites can be provided at these regions for La(OH)3 crystal growth [12]. EDS spectrum of the nano-La(OH)3/RGO compos­ ites from its HRTEM image is shown in Fig. 3g. High intensity peaks of the La appeared, further indicating that nano-La(OH)3 were successfully anchored on the GO surfaces.

adequacy of the COF model. The model high F-value of 56.49 and low pvalue of 0.0002 indicate that the quadratic model is significant. The lack-of-fit term is not significant as the high P-value of 0.0541 and small F-value of 17.64, which further verifies that the COF model is statisti­ cally valid. Correlation coefficient R2 value can be used to evaluate the quality of the model. The developed model is well when the model R2 value near to 1, and the predicted value of the response is close to the corresponding actual value [38]. The high values of R2 (0.9903) and adjusted R2 (0.9727) of COF model illustrate the actual and predicted COF values are very closely. All these findings illustrate that the model can be further used to analyze the effect of variables on COF [39]. ANOVA result of Wear is shown in Table 5. The model high F-value of 8.81 and low p-value of 0.0137 indicate that the quadratic model is significant. The lack-of-fit term is not significant as the high P-value of 0.6953 and small F-value of 0.55, which further justify that the Wear model is statistically valid. The high values of R2 (0.9407) and adjusted R2 (0.8338) of Wear model illustrate the actual and predicted Wear values are very closely. According to the aforementioned values, the model can be further employed to analyze the effect of variables on Wear.

3.2. Box-Behnken design

3.2.2. Effects of variables on COF In ANOVA, the model terms with P-value less than 0.05 are signifi­ cant, and the model terms with P-value greater than 0.10 are not sig­ nificant [40]. According to each variable P-value shown in Table 4, temperature and load have significant effects (P < 0.05) on COF, whereas composite concentration has the least significant effect (P > 0.10). Fig. 5 shows the 3D response surface plots and 2D interaction plots, which were constructed to illustrate the effects of factors on COF. The effect of temperature and load on COF is shown in Fig. 5. Fig. 5a and a* show that when diesel engine oil without nano-La(OH)3/RGO composites and the load is fixed at a certain value, the COF generally increases with temperature increasing from 40 � C to 80 � C. Besides, COF increases more under high load than under low load as temperature increases. This phenomenon may be due to the high temperatures cause to the diesel engine oil viscosity decrease, which leads to the less stable molecular films between the contact surfaces and the lower or breakdown of the boundary lubrication effect [41]. Haseeb et al. [42] also found that the friction coefficient of the three biodiesels increased with increasing temperature from 30 � C to 75 � C. Fig. 5a and a* as well show that when diesel engine oil without nano-La(OH)3/RGO compos­ ites and the temperature is fixed at a certain value, COF first increased with the load increasing from 1.96 N (0.95Gpa) to 7.84 N (1.51Gpa), and then exhibits a slightly decreasing tendency above 7.84 N (1.51Gpa). Since the initial lubrication state is from mixed lubrication to boundary lubrication (λ from 2.09 to 0.60), the change of COF may be attributed to

3.2.1. Development of regression model equation and statistical analysis The BBD experimental matrixes and corresponding response values are shown in Table 3. Runs 12, 13, and 14 were the center point runs, which used to test the variable parameters. Based on the BBD experi­ mental results, the quadratic model relationship between the response (COF and Wear) and the coded independent variables (A, B, and C) were achieved as follows: COF

¼

0:17680 þ 0:020403A þ 0:074707B 0:00076C þ 0:000132A2 0:08970B2 0:00279C2 þ 0:006034AB 0:01133AC þ 0:001835BC

Wear

¼

496:9502 þ 103:1921A þ 189:3539B 73:3178C þ 13:88396A2 3:65946B2 þ 35:41196C2 14:938AB 50:7618AC 49:7834BC

(9)

(10)

The coefficients of one factor terms and quadratic terms represent the influence of single factor on the response value, while the co­ efficients of two factor terms represent the influence of interaction on the response value. The positive sign and negative sign before the terms indicate a synergistic effect and an antagonistic effect on the response, respectively [37]. The normal probability plots of residuals for COF and Wear were used to test the normality of the data, which are shown in Fig. 4. As can be seen from the plots that the residuals are close to the straight line, revealing the errors are normally distributed [28]. ANOVA of the COF is shown in Table 4, which was used to justify the 4

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Fig. 3. HRTEM images of GO sheets (a) and (b), nano-La(OH)3 (c) and (d), nano-La(OH)3/RGO composites (e) and (f), EDS results of nano-La(OH)3/RGO com­ posites (g).

5

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high friction coefficient appears under high temperatures and loads in particular. However, with the addition of nano-La(OH)3/RGO compos­ ites into diesel engine oil, the increasing trend of COF with the tem­ perature and load increase slowed down, as shown in Fig. 5b and c. Fig. 5b* and c* indicate that when the temperature is above 60 � C and the load is above 5.88 N (λ <1.03, into boundary lubrication region), the COF of diesel engine oil containing nano-La(OH)3/RGO composites is lower than that of pure diesel engine oil. This phenomenon indicates that nano-La(OH)3/RGO composites can increase the adaptability of diesel engine oil to high temperature and load, and can also improve the anti-friction performance of diesel engine oil under boundary lubrica­ tion state.

Table 3 Box-Behnken Design matrix and results. Run

Factors

Response 1 COF

Response 2 Wear ( � 103μm3)

A: Temperature (� C)

B: Load (N)

C: Concentration (wt%)

1

60(0)

0.1(þ1)

0.1643

616.143

2

60(0)

0(-1)

0.1600

788.979

3

40(-1)

0.1(þ1)

0.1514

363.699

4

60(0)

0(-1)

0.0103

332.609

5

40(-1)

0.05(0)

0.1522

622.768

6

60(0)

0.1(þ1)

0.0075

354.924

7

40(-1)

0.05(0)

0.0033

209.156

8

80(þ1)

0.05(0)

0.0129

409.782

9

80(þ1)

0.05(0)

0.1854

764.837

10

80(þ1)

0(-1)

0.2149

818.949

11

40(-1)

0(-1)

0.1328

474.212

12

60(0)

0.05(0)

0.1736

580.910

13

60(0)

0.05(0)

0.1712

409.248

14

60(0)

0.05(0)

0.1786

483.641

15

80(þ1)

9.80 (þ1) 9.80 (þ1) 5.88 (0) 1.96 (-1) 9.80 (þ1) 1.96 (-1) 1.96 (-1) 1.96 (-1) 9.80 (þ1) 5.88 (0) 5.88 (0) 5.88 (0) 5.88 (0) 5.88 (0) 5.88 (0)

0.1(þ1)

0.1882

505.389

3.2.3. Effects of variables on wear According to each P-value of variable shown in Table 5, all factors of the temperature, load, and composite concentration are found to have significant effects (P < 0.05) on Wear. Fig. 6 shows the 3D response surface plots and 2D interaction plots, which were constructed to illus­ trate the effects of factors on Wear. Effects of temperature and load on Wear are shown in Fig. 6. Fig. 6a Table 4 ANOVA for response of dependent variable COF for BBD.

the lubrication regime changes from hydrodynamic region to the boundary layer lubrication region during sliding, which directly leads to the change of friction force between friction pairs [43]. Anthony et al. [27] found that for the 15W-50 diesel engine oil, a change from a low COF at 1 N to a higher COF at 10 N occurred, which remained stable to the higher loads and roughly in agreement with the variation trend of COF obtained in this study. The COF of the diesel engine oil without composites presents a sig­ nificant increase trend with the temperature and load increase, and the

Source

Sum of Squares

Degree of freedom

Mean Square

F Value

p-valuea Prob> F

Model A B C AB AC BC A2 B2 C2 Residual Lack of Fit Pure Error Cor Total

0.080838 0.003271 0.049282 4.65E-06 0.00014 0.000513 1.29E-05 6.47E-08 0.027402 2.88E-05 0.000795 0.000766

9 1 1 1 1 1 1 1 1 1 5 3

0.008982 0.003271 0.049282 4.65E-06 0.00014 0.000513 1.29E-05 6.47E-08 0.027402 2.88E-05 0.000159 0.000255

56.48964 20.57447 309.943 0.033678 0.879552 3.227813 0.081386 0.000407 172.3359 0.181052

0.0002 0.0060 <0.0001 0.8709 0.3914 0.1323 0.7869 0.9847 <0.0001 0.6882

17.63968

0.0541

2.9E-05

2

1.45E-05

0.081633

14

R-square ¼ 0.9903, Adj R-Squared ¼ 0.9727. a Significant at the 5% level.

Fig. 4. Normal plot of residuals for COF (a) and Wear (b). 6

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mathematical theoretical reference for the application of nano-La(OH)3/ RGO composites as anti-wear additive for diesel engine oil.

Table 5 ANOVA for response of dependent variable Wear for BBD. Source

Sum of Squares

Degree of freedom

Mean Square

F Value

p-valuea Prob> F

Model A B C AB AC BC A2 B2 C2 Residual Lack of Fit Pure Error Cor Total

429282.6 85029.53 273261 42923.49 857.2306 10307.02 9520.978 711.7451 45.6074 4630.179 27083.46 12262.32

9 1 1 1 1 1 1 1 1 1 5 3

47698.07 85029.53 273261 42923.49 857.2306 10307.02 9520.978 711.7451 45.6074 4630.179 5416.692 4087.439

8.805757 15.69769 50.44796 7.924301 0.158257 1.902826 1.757711 0.131398 0.00842 0.854798

0.0137 0.0107 0.0009 0.0373 0.7072 0.2263 0.2422 0.7318 0.9305 0.3976

0.551569

0.6953

14821.14

2

7410.57

456366.1

14

3.3. Wear mechanism analysis 3.3.1. Tribological performance comparison amongst GO, nano-La(OH)3 and nano-La(OH)3/RGO composites To explore the anti-wear mechanism of nano-La(OH)3/RGO com­ posites in diesel engine oil, anti-wear performance comparison was conducted amongst GO, nano-La(OH)3 and nano-La(OH)3/RGO com­ posites at a temperature of 80 � C and a load of 9.8 N. The results are presented in Fig. 8. As shown in the figure, the anti-wear performance of diesel engine oil demonstrated evident differences after adding three types of nanoparticles at 0.1 wt%. Wear volume increased when 0.1 wt% GO was added to diesel engine oil, indicating that the anti-wear per­ formance of diesel engine oil decreased. This finding may be attributed to the corrugation effect of GO under stress during the friction process that results in energy loss and wear increase [12]. When 0.1 wt% nano-La(OH)3 was added to diesel engine oil, wear volume decreased, indicating that nano-La(OH)3 improved the anti-wear performance of diesel engine oil to a certain extent. Zhao et al. also found that La(OH)3 improves the anti-wear performance of lubricating oil [17]. However, when diesel engine oil was added with 0.1 wt% nano-La(OH)3/RGO composites, wear volume significantly decreased by 44%. The result indicates that the composites can remarkably improve the anti-wear performance of diesel engine oil compared with GO and nano-La (OH)3. This finding demonstrates that GO and nano-La(OH)3 exert a synergistic anti-wear effect after compound. The details of the anti-wear mechanism analysis are discussed in the following sections.

R-square ¼ 0.9407, Adj R-Squared ¼ 0.8338. a Significant at the 5% level.

and a* show that when diesel engine oil without nano-La(OH)3/RGO composites, Wear increased sharply with increasing temperature and load. Wear increased from 474212 μm3 to 818949 μm3 with the tem­ perature increasing from 40 � C to 80 � C when the load is fixed at 5.88 N (1.37 GPa). Wear also increased from 332609 μm3 to 788979 μm3 with the load increasing from 1.96 N (0.95 GPa) to 9.8 N (1.62 GPa) when the temperature is fixed at 60 � C. Thus, temperature and load have a strong influence on the anti-wear performance of diesel engine oil, which may be due to the variation of temperature and load that can change the lubrication regime of diesel engine oil. When the temperature increases, the diesel engine oil viscosity decreases and the molecular films between the contact surfaces becomes less stable. The hydrodynamic lubrication effect becomes low or breaks down, which leads to the high wear volume [41]. When the load increases, the hydrodynamic lubricant regime gradually weakens and the metal asperities gradually in contact, which also result in high wear volume [43]. However, as shown in Fig. 6b and c, with the addition of nano-La (OH)3/RGO composites into diesel engine oil, the increasing trend of Wear with the temperature and load increase obviously slowed down. Fig. 6c and c* show that Wear increased from 363699 μm3 to 505389 μm3 with the temperature increasing from 40 � C to 80 � C when diesel engine oil with 0.1 wt% nano-La(OH)3/RGO composites and load is fixed at 5.88 N (1.37 GPa). Wear increased from 354924 μm3 to 616143 μm3 with the load increasing from 1.96 N (0.95 GPa) to 9.8 N (1.62 GPa) when the diesel engine oil with 0.1 wt% nano-La(OH)3/RGO composites and temperature is fixed at 60 � C. In addition, the Wear of diesel engine oil containing nano-La(OH)3/RGO composites is dramat­ ically lower than that of pure diesel engine oil when the temperature is above 60 � C and the load is above 5.88 N (λ <1.03, into boundary lubrication region), as shown in Fig. 6. This phenomenon indicates that nano-La(OH)3/RGO composites can fully guarantee the anti-wear per­ formance of diesel engine oil at different temperatures and loads, and significantly improve anti-wear performance especially under boundary lubrication state. The anti-wear mechanism analysis is discussed in the following sections.

3.3.2. Morphological characterization of worn surface Fig. 9 shows the optical micrographs, 3D micrographs, SEM images and profile curves of the worn surfaces on steel disks lubricated with diesel engine oil and diesel engine oil containing 0.1 wt% of different nanoparticles, namely, GO, nano-La(OH)3 and nano-La(OH)3/RGO composites at a load of 9.8 N and a temperature of 80 � C. As shown in Fig. 9, the steel balls and disks exhibited various degrees of wear when different lubricants were used. The largest wear scar was observed on the steel ball and disk lubricated with diesel engine oil containing 0.1 wt % GO (Fig. 9d). Remarkable symbols of pits and deep furrows on the steel disk were observed from the optical and SEM images (Fig. 9e and f). These symbols are indicative of the abrasive wear caused by the pro­ longed asperity movement on the surface of friction pairs [36]. Accordingly, the wear scar on this disk is the broadest and the deepest as seen from the profile curve (Fig. 9e). When nano-La(OH)3 was used as additive for diesel engine oil, the wear scar on the steel ball and disk decreased to a certain degree compared with that with pure diesel en­ gine oil (Fig. 9g and h). The profile curves of the worn surface also showed that the width and depth of the wear scar decreased compared with that with pure diesel engine oil (Fig. 9h). Several pits and furrows can still be observed in the SEM image (Fig. 9i). This finding is also ascribed to the abrasive wear caused by the prolonged asperity move­ ment on the friction surface [36]. However, when the friction surface was lubricated with diesel engine oil containing 0.1 wt% nano-La (OH)3/RGO composites, the wear scar on the steel ball and disk decreased significantly compared with that with pure diesel engine oil (Fig. 9j and k). The profile curve of the worn surface presented the narrowest and shallowest wear scar compared with those of the other lubricants (Fig. 9k). As shown in the SEM image (Fig. 9l), the wear scar is even and smooth, and the pits on the surface are dramatically reduced in size and number, indicating that the nano-La(OH)3/RGO composites can evidently improve the anti-wear performance of diesel engine oil. This observation may be attributed to several protective films formed on the surface of friction pairs during sliding. These films effectively prevent direct contact between steel-material friction pairs and eventually reduce the wear extent of the friction pair surface [44].

3.2.4. Validation of the model In all run groups of the model, confirmation experiments were con­ ducted to validate the mathematical model and repeated three times for each run group. The response actual value of each group was calculated by the average value of three experiment results. Fig. 7 shows the cor­ relation between the actual and predicted values of COF and Wear, indicating that the developed models are suitable and the predicted values from the models are in good agreement with the actual values from the experimental data. The model can provide a certain 7

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Fig. 5. 3D response surface plots (a, b, c) and 2D interaction plots (a*, b*, c*) for the effects of A and B at different C levels on COF: C: 0 wt% (a, a*), C: 0.05 wt% (b, b*), C: 0.1 wt% (c, c*).

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Fig. 6. 3D response surface plots (a, b, c) and 2D interaction plots (a*, b*, c*) for the effects of A and B at different C levels on Wear: C: 0 wt% (a, a*), C: 0.05 wt% (b, b*), C: 0.1 wt% (c, c*).

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Fig. 7. The correlation of actual and predicted values of COF (a) and Wear (b).

(OH)3/RGO composites before and after friction is shown in Fig. 10c and c*, respectively. As clearly seen from Fig. 10c*, nano-La(OH)3/RGO composites decomposed into smaller lamellar composite structures and evenly dispersed during friction testing. The enlarged image inserted into Fig. 10c* shows that these composites are composed of small sheets stacked in parallel, with several monodispersed nanorods between the lamellar layers. No apparent corrugation was found on the surface of the lamellar layers. This finding may be due to nano-La(OH)3 occupying the active sites at the edges and folds of the GO sheets, and thus effectively inhibiting the pleating effect of GO whilst preventing the agglomeration of La(OH)3 nanorods during the sliding process. This small lamellar composite structure may more easily enter friction pairs to form a pro­ tective film, and thus effectively prevent direct contact between metal asperities and significantly reduce wear. This finding is consistent with the friction testing results. Further worn surface composition analysis was conducted to investigate the anti-wear mechanism of nano-La (OH)3/RGO composites as follows. Fig. 8. Tribological performance comparison amongst GO, nano-La(OH)3 and nano-La(OH)3/RGO composites.

3.3.4. Composition characterization of worn surface The EDS results of the worn surfaces lubricated with different lu­ bricants are provided in Table 6. As seen from Table 6, the main ele­ ments of C, O, Fe, S, P and Zn were found on the worn areas lubricated with pure diesel engine oil. Fe and C are from the steel matrix, and O may be from the oxygenated compounds in diesel engine oil [46]. S, P and Zn may come from the ZDDP additive in the diesel engine oil. When the worn areas are lubricated with diesel engine oil containing 0.1 wt% of three types of nanoparticles, an increase in O content and a decrease in Fe content were observed possibly because more Fe substrates were oxidized or formed into other compounds compared with that in pure diesel engine oil lubrication. When the worn surface was lubricated with diesel engine oil containing 0.1 wt% nano-La(OH)3, the S, P and Zn contents increased to a certain extent, indicating that nano-La(OH)3 can promote S, P and Zn to form the tribofilms on the friction surface and consequently improve the anti-wear performance of diesel engine oil. This finding may be due to the unique electronic structure and high chemical activity of La in La(OH)3, which plays a catalytic role in pro­ moting the formation of tribofilms at the friction interface [17,47]. This result is consistent with the previous reports of Zhang et al. [18], Zhao et al. [17] and Wang et al. [48], who also found that lanthanum oxide exhibits special tribological properties. However, when the worn areas were lubricated with diesel engine oil containing 0.1 wt% nano-La (OH)3/RGO composites, the S, P and Zn contents were more signifi­ cantly increased, indicating that the nano-La(OH)3/RGO composites can

3.3.3. Micrographs of nanoparticles after friction testing To explore the microstructural changes of nanoparticles, GO, nanoLa(OH)3 and nano-La(OH)3/RGO composites were cleaned several times with ethanol after the friction experiment and characterized via TEM (Fig. 10). As shown in Fig. 10a and a*, GO broke into unevenly sized pieces and became thicker. More evident corrugation was found on the pieces after friction. This phenomenon is the same as that observed by Hu et al. [45], who also found that GO undergoes such structural changes during the rubbing process. Hu et al. indicated that this finding may be attributed to the fact that after the GO nanosheets broke into unevenly sized pieces, more defect structures are generated on these pieces. These defect structures cause the agglomeration of GO pieces in all directions to form thick abrasive particles during the friction process, resulting in the severe abrasive wear of the friction pairs [45]. This result is consistent with the micrographs of the worn surface (remarkable symbols of pits and deep furrows were observed). Fig. 10b and b* show the microstructure of nano-La(OH)3 before and after friction, respec­ tively. The La(OH)3 nanorods agglomerated into larger block particles during the friction process. These larger block La(OH)3 particles caused three-body abrasion in the friction pairs, and such abrasion increased wear volume. This finding corresponded to the evident furrow morphology on the surface of a wear scar. The microstructure of nano-La 10

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Fig. 9. Morphological characterization of worn surfaces lubricated with different lubricants: (a) (b) (c) lubricated with diesel engine oil, (d) (e) (f) lubricated with diesel engine oil containing 0.1 wt% GO, (g) (h) (i) lubricated with diesel engine oil containing 0.1 wt% nano-La(OH)3 and (j) (k) (l) lubricated with diesel engine oil containing 0.1 wt% nano-La(OH)3/RGO composites. (a) (d) (g) (j) Optical micrographs of steel ball, (b) (e) (h) (k) optical micrographs, 3D micrographs and profile curves of steel disk and (c) (f) (i) (l) SEM images of steel disk.

further promote the formation of S, P and Zn tribofilms during sliding compared with the worn surface lubricated with diesel engine oil con­ taining 0.1 wt% nano-La(OH)3. This finding corresponds to the friction testing results that nano-La(OH)3/RGO composites are more beneficial to improve the anti-wear performance of diesel engine oil. It is may be due to the fact that nano-La(OH)3 cannot easily enter the friction interface because it agglomerates into large particles during the friction when it is added to diesel engine oil alone. This condition limits its role in promoting the formation of tribofilms at the friction interface. How­ ever, the preceding micrograph characterization of nanoparticles after friction indicated that nano-La(OH)3/RGO composites decomposed into

smaller lamellar composites upon friction when they were used as ad­ ditive for diesel engine oil. Then, they easily entered into the friction interface and fully demonstrated the role of nano-La(OH)3 in promoting the formation of tribofilms. A higher La content on the worn surface further indicates that more nano-La(OH)3 has entered into the friction interface. To explore the formation of graphene film in friction pairs, Raman mappings of the worn surfaces lubricated with diesel engine oil con­ taining 0.1 wt% GO and diesel engine oil containing 0.1 wt% nano-La (OH)3/RGO composites are shown in Fig. 11. When nano-La(OH)3/RGO composites were used as additive for diesel engine oil, a large number of 11

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Fig. 10. TEM images of different nanoparticles before (a–c) and after (a*-c*) the friction testing: a and a* to GO, b and b* to nano-La(OH)3, c and c* to nano-La (OH)3/RGO composites. Table 6 EDS results (Atomic%) of worn surface lubricated with different lubricants. Lubricant CF-4 CF-4 containing 0.1 wt% GO CF-4 containing 0.1 wt% nano-La(OH)3 CF-4 containing 0.1 wt% La(OH)3/RGO

Atomic percentage/% C

O

Fe

S

P

Zn

La

Others

21.95 21.58 21.14 21.64

20.27 24.57 21.59 25.28

53.12 48.69 50.07 42.51

1.04 1.13 1.22 2.14

0.57 0.71 1.15 2.06

0.67 0.71 1.66 3.03

0 0 0.15 0.36

2.38 2.61 3.02 2.98

strong G (1580 cm 1) characteristic peak distributions of graphene were evidently observed on the worn surface (Fig. 11b). By contrast, when GO was used as additive, the G characteristic peak distribution was extremely small and weak (Fig. 11a). This finding indicates that compared with GO, nano-La(OH)3/RGO composites can more effec­ tively enter into the friction interface to form graphene protective films on worn surface during sliding. Thus, the anti-wear performance of diesel engine oil is significantly improved. This finding also verified the morphological analysis results that nano-La(OH)3/RGO composites can more easily enter into the metal asperities during sliding because they are decomposed into small lamellar composite structures. To further investigate the composition of tribofilms on the worn surfaces, XPS spectra of the worn surfaces lubricated with diesel engine oil and diesel engine oil containing 0.1 wt% nano-La(OH)3/RGO com­ posites are presented in Fig. 12. The survey scan XPS spectra of the worn surfaces are shown in Fig. 12a. As illustrated in the figure, the charac­ teristic peaks of C1s, O1s, Fe2p, S2p, P2p and Zn2p were detected on the worn surfaces lubricated with pure diesel engine oil and diesel engine oil containing 0.1 wt% nano-La(OH)3/RGO composites. The new charac­ teristic peaks of La3d were detected when nano-La(OH)3/RGO com­ posites were added to diesel engine oil to lubricate the worn surface. Fig. 12b shows the XPS spectra of C1s on the worn surface. The peaks at 284.7 (peak 1), 285.4 (peak 2) and 288.9 (peak 3) eV belong to C–C or – C, C–O and C– – O compounds, respectively [49]. This finding in­ C– dicates that several organic compounds from diesel engine oil are adsorbed onto the worn surface to form the tribofilms. In general, the area ratio of the characteristic peak can reflect the relative contents of

the chemical components under the same conditions. When nano-La (OH)3/RGO composites were added to diesel engine oil, the relative – C increased to a certain extent. This phenomenon peak area of C–C or C– may be caused by graphene entering the friction interface to form tribofilms. Fig. 12c and d shows the XPS spectra of P2p and S2p on the worn surface, respectively. The characteristic peak areas of P 2p3/2 (peak at 133.2 eV) and S 2p3/2 (peak at 161.7 eV) were significantly enhanced when nano-La(OH)3/RGO composites were added to diesel engine oil, indicating that more P and S tribofilms were generated on the worn surface [50]. Fig. 12e presents the XPS spectra of O1s on the worn surface. As seen in Fig. 12e, the peak at 530.1eV (peak 1) belongs to metal oxide (Fe–O, Zn–O or La–O) [51]. The peak at 531.2eV (peak 2) belongs to C–O and – O [52]. The peak at 532.0eV (peak 3) belongs to P–O [52–54]. The C– relative peak area of the O compound showed that more P–O compounds were generated on the worn surface after adding nano-La(OH)3/RGO composites to diesel engine oil. This finding may be attributed to the fact that nano-La(OH)3/RGO composites can promote the formation of metal phosphate tribofilms on a worn surface. Cui et al. [53] and Cen et al. [55] reported that metal phosphate can act as an excellent anti-wear agent on friction pairs. This information is consistent with our friction testing results. Fig. 12f shows the XPS spectra of Zn2p on the worn surface. The peaks at 1021.7 (peak 1) and 1022. 4 (peak 2) eV belong to ZnO and zinc phosphate, respectively [52,56]. The relative content of zinc phosphate increased evidently when nano-La(OH)3/RGO compos­ ites were added to diesel engine oil, further proving that the composites 12

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Fig. 11. Raman mapping of worn surfaces lubricated with diesel engine oil containing 0.1 wt% GO (a) and diesel engine oil containing 0.1 wt% nano-La(OH)3/RGO composites (b).

can effectively promote the formation of phosphate compounds with good anti-wear performance on friction pairs. Fig. 12g shows the XPS spectra of Fe2p on the worn surface. The peaks at 709.8 (peak 1), 710.8 (peak 2) and 711.5 (peak 3) eV are belong to FeO, Fe2O3 and FeOOH, respectively [57], and the peaks at 712.1 (peak 4) and 712.8 (peak 5) eV are belong to FeS and FePO4, respec­ tively [57,58]. When nano-La(OH)3/RGO composites were added to diesel engine oil, the relative content of Fe–O decreased and that of FeS increased remarkably, indicating that more FeS tribofilms were formed on the worn surface and played a certain anti-wear role. Fig. 12h shows the XPS spectra of La3d on the worn surface. The new peaks at 835.1 (peak 1) and 836.1 (peak 2) eV after adding nano-La(OH)3/RGO com­ posites to the diesel engine oil belong to La2O3 and LaPO4 [15,18,59], respectively. This finding indicated that La entered the friction in­ terfaces and promoted the formation of tribofilms on the worn surface. Zhao et al. and Wang et al. also found that La plays a catalytic role in lubricants by promoting the formation of tribofilms in the friction pro­ cess and thus reducing wear and friction [17,47,48].

certain abrasive wear at the friction interface and inhibiting its good anti-wear performance (Fig. 13b). However, when nano-La(OH)3/RGO composites were added to diesel engine oil, the composites were first decomposed into smaller lamellar composites and effectively entered the friction interface during friction. Subsequently, the smaller lamellar composites formed the tribofilms of graphene and lanthanum oxide at the friction interface. These films effectively prevented direct contact between metal asperities and reduced the wear of the worn surface. Simultaneously, lanthanum oxide played a catalytic role at the friction interface, promoting the generation of more metal phosphate and sul­ phide anti-wear tribofilms at the friction interface. Lastly, anti-wear performance was further improved under the synergistic effect of gra­ phene and lanthanum oxide (Fig. 13c). 4. Conclusions Nano-La(OH)3/RGO composites were prepared and used to improve the tribological properties of diesel engine oil. RSM was adopted to explore the influences of temperature, load, and composite concentra­ tion on the tribological properties of diesel engine oil. A series of ana­ lyses was conducted to investigate the wear mechanisms of nano-La (OH)3/RGO composites in diesel engine oil. The findings led to the following conclusions.

3.4. Wear mechanism Nano-La(OH)3/RGO composites, as diesel engine oil additives, exhibit excellent anti-wear property. From the preceding characteriza­ tion analysis of the worn surfaces lubricated with diesel engine oil and diesel engine oil containing 0.1 wt% of three types of nanoparticles, the anti-wear mechanism model of nano-La(OH)3/RGO composites was hypothesised. When friction pairs were lubricated with pure diesel en­ gine oil, numerous pits and furrows were generated on the steel surface due to the direct contact between metal asperities under high loads. When GO was added to diesel engine oil, agglomeration and corrugation phenomena were prone to occur during sliding, leading to the worn aggravation of friction pairs (Fig. 13a). When nano-La(OH)3 was added to diesel engine oil, nano-La(OH)3 promoted the formation of S, P and Zn tribofilms on the friction surface and improved the anti-wear perfor­ mance of diesel engine oil to a certain extent. However, nano-La(OH)3 also tends to agglomerate into large particles during sliding, resulting in

(1) Nano-La(OH)3/RGO composites were successfully prepared via the uniform synthesis of La(OH)3 nanoparticles on GO surface. (2) The effects of temperature, load and composite concentration on COF and Wear were determined via ANOVA. Their numerical relations with COF and Wear were successfully established, providing a certain mathematical theoretical reference for the application of nano-La(OH)3/RGO composites as anti-wear ad­ ditives for diesel engine oil. (3) Nano-La(OH)3/RGO composites can fully guarantee the tribo­ logical performance of diesel engine oil at different temperatures and contact pressures and significantly improve anti-wear per­ formance under boundary lubrication conditions. In particular, 13

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Fig. 12. XPS results of the worn surfaces lubricated with diesel engine oil and diesel engine oil containing 0.1 wt% nano-La(OH)3/RGO composites: (a) survey, (b) C1s, (c) P2p, (d) S2p, (e) O1s, (f) Zn2p, (g) Fe2p and (h) La3d.

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Fig. 13. Schematic of the wear mechanism of steel disks lubricated with (a) diesel engine oil containing GO, (b) diesel engine oil containing nano-La(OH)3 and (c) diesel engine oil containing nano-La(OH)3/RGO composites.

the anti-wear performance of diesel engine oil increased by 44% after adding 0.1 wt% composites at a temperature of 80 � C and a contact pressure of 1.62 GPa. (4) The wear mechanism indicated that the nano-La(OH)3/RGO composites formed tribofilms of graphene and lanthanum oxide at the friction interface. These films effectively prevented direct contact between metal asperities and reduced the wear of the worn surface. Simultaneously, lanthanum oxide played a cata­ lytic role at the friction interface, promoting the generation of more metal phosphate and sulphide anti-wear tribofilms at the friction interface. Lastly, anti-wear performance was further improved under the synergistic effect of graphene and lanthanum oxide.

[5] Johnson DW. Lubricants for turbine engines. In: Agarwal RK, editor. In recent progress in some aircraft technologies. Vienna: ExLi4EvA; 2016. p. 36–53. [6] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666–9. [7] Chronopoulos DD, Bakandritsos A, Pykal M, Zboril R, Otyepka M. Chemistry, properties, and applications of fluorographene. Appl Mater Today 2017;9:60–70. [8] Gan C, Liang T, Li W, Fan X, Zhu M. Amine-terminated ionic liquid modified graphene oxide/copper nanocomposite toward efficient lubrication. Appl Surf Sci 2019;491:105–15. [9] Hontoria-Lucas C, L� opez-Peinado AJ, L� opez-Gonz� alez JdD, Rojas-Cervantes ML, Martín-Aranda RM. Study of oxygen-containing groups in a series of graphite oxides: physical and chemical characterization. Carbon 1995;33:1585–92. [10] Senatore A, D’Agostino V, Petrone V, Ciambelli P, Sarno M. Graphene oxide nanosheets as effective friction modifier for oil lubricant: materials, methods, and tribological results. ISRN Tribol 2013;2013:1–9. [11] Chen Z, Liu YH, Luo JB. Tribological properties of few-layer graphene oxide sheets as oil-based lubricant additives. Chin J Mech Eng 2015;29:439–44. [12] Berman D, Erdemir A, Sumant AV. Approaches for achieving superlubricity in twodimensional materials. ACS Nano 2018;12:2122–37. [13] Wu J, Huang XZ, Kim B, Lu XH, Feng X, Roland L, et al. CuO nanosheets produced in graphene oxide solution: an excellent anti-wear additive for self-lubricating polymer composites. Compos Sci Technol 2018;162:86–92. [14] Sharma SP, Dwivedi DK, Jain PK. Effect of La2O3 addition on the microstructure, hardness and abrasive wear behavior of flame sprayed Ni based coatings. Wear 2009;267:853–9. [15] Xu YF, Peng YB, Zheng XJ, Dearn KD, Xu HM, Hu XG. Synthesis and tribological studies of nanoparticle additives for pyrolysis bio-oil formulated as a diesel fuel. Energy 2015;83:80–8. [16] Pan ZH, Wang TC, Chen L, Stefan I, Huang ZH, Zhao BX. Effects of rare earth oxide additive on surface and tribological properties of polyimide composites. Appl Surf Sci 2017;416:536–46. [17] Zhao FY, Bai ZM, Fu Y, Zhao D, Yan CM. Tribological properties of serpentine, La (OH)3 and their composite particles as lubricant additives. Wear 2012;288:72–7. [18] Zhang ZF, Liu WM, Xue QJ. The tribological behaviors of succinimidemodifiedlanthanum hydroxide nanoparticles blended with zincdialkyldithiophosphate as additives in liquid paraffin. Wear 2001;248:48–54. [19] Radhika N, Raghu R. Dry sliding wear behaviour of aluminium Al–Si12Cu/TiB2 metal matrix composite using response surface methodology. Tribol Lett 2015;59: 2. [20] Selçuk B. Feasibility of utilizing response surface methodology for predicting wear of steels. IMechE 2011;225:277–87.

Acknowledgments This research was supported by the National Natural Science Foun­ dation of China (Grant No. 51675153) and Major Science and Tech­ nology Special Project in Anhui (Grant No.17030901084), which are gratefully acknowledged. The authors thank Professor Kunhong Hu and Dr. Enzhu Hu of Hefei University for their assistance in the experimental analyses and discussion. References [1] Zin V, Agresti F, Barison S, Litti L, Fedele L, Meneghetti M, et al. Effect of external magnetic field on tribological properties of goethite (a-FeOOH) based nanofluids. Tribol Int 2018;127:341–50. [2] Wei HJ. Study on the tribology of the cylinder and piston ring of the vehicle diesel with surface roughness and lubrication oil. Adv Mater Res 2011:589–92. [3] Xu XH, Sun SR, Wang P, Lei AL, Peng GH. Study on tribology performance of diesel engine oil using SRV4 tribometer. Tribol Online 2015;10:172–6. [4] Johnson D, Bachus M, Hils J. Interaction between lubricants containing phosphate ester additives and stainless steels. Lubricants 2013;1:48–60.

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[21] Chang BP, Akil MH, Nasir RB, Khan A. Optimization on wear performance of UHMWPE composites using response surface methodology. Tribol Int 2015;88: 252–62. [22] Konicek AR, Jacobs PW, Webster MN, Schilowitz AM. Role of tribofilms in wear protection. Tribol Int 2016;94:14–9. [23] Kontou A, Southby M, Spikes HA. Effect of steel hardness on soot wear. Wear 2017; 390–391:236–45. [24] Hamrock BJ, Dowson D. Ball bearing lubrication: the elastohydrodynamics of elliptical contacts. New York: Wiley; 1981. [25] Hamrock BJ, Dowson D. Isothermal elastohydrodynamic lubrication of point contacts: Part III-fully flooded results. J Lubr Technol 1977;99:264. [26] Hamrock BJ, Schmid SR, Jacobson BO. Fundamentals of fluid film lubrication. CRC Press, Inc; 2004. [27] Somers AE, Howlett PC, Sun J, MacFarlane DR, Forsyth M. Transition in wear performance for ionic liquid lubricants under increasing load. Tribol Lett 2010;40: 279–84. [28] Vettivel SC, Selvakumar N, Narayanasamy R, Leema N. Numerical modelling, prediction of Cu–W nano powder composite in dry sliding wear condition using response surface methodology. Mater Des 2013;50:977–96. [29] Yin YD, Hong GY. Synthesis and characterization of La(OH)3 nanorods by hydrothermal microemulsion method. Chin Chem Lett 2005;16:1659–62. [30] Hu CG, Liu H, Dong WT, Zhang YY, Bao G, Lao CS, et al. La(OH)3 and La2O3 nanobelts—synthesis and physical properties. Adv Mater 2007;19:470–4. [31] Feng J, Li XH, Wang MZ, Zheng XL, Bai JT, Wang L, et al. One-pot, template-free synthesis of hydrophobic single-crystalline La(OH)3 nanowires with tunable size and their d0 ferromagnetic properties. RSC Adv 2015;5:16093–100. [32] Wei W, Yang S, Zhou HX, Lieberwirth I, Feng XL, Mullen K. 3D graphene foams cross-linked with pre-encapsulated Fe3O4 nanospheres for enhanced lithium storage. Adv Mater 2013;25:2909–14. [33] Jaleh B, Khalilipour A, Habibi S, Niyaifar M, Nasrollahzadeh M. Synthesis, characterization, magnetic and catalytic properties of graphene oxide/Fe3O4. J Mater Sci Mater Electron 2016;28:4974–83. [34] Yang MM, Zhang ZZ, Yuan JY, Guo F, Men XH, Liu WM. Growth of Mo2C nanoparticles on graphene as lubricant filler for high tribological performances of fabric self-lubricating liner composites. RSC Adv 2016;6:110070–6. [35] Pol VG, Shrestha LK, Ariga K. Tunable, functional carbon spheres derived from rapid synthesis of resorcinol-formaldehyde resins. Acs Appl Mater Inter 2014;6: 10649–55. [36] Wang BB, Hu EZ, Tu ZQ, Dearn DK, Hu KH, Hu XG, et al. Characterization and tribological properties of rice husk carbon nanoparticles Co-doped with sulfur and nitrogen. Appl Surf Sci 2018;462:944–54. [37] Ahmad MA, Rasyidah A. Optimization of preparation conditions for mangosteen peel-based activated carbons for the removal of Remazol Brilliant Blue R using response surface methodology. Chem Eng J 2010;165:883–90. [38] Ahmad AA, Hameed BH, Ahmad AL. Removal of disperse dye from aqueous solution using waste-derived activated carbon: optimization study. J Hazard Mater 2009;170:612–9. [39] Remen� arov� a L, Pipí�ska M, Florkov� a E, Horník M, Rozlo�zník M, Augustín J. Zeolites from coal fly ash as efficient sorbents for cadmium ions. Clean Technol Environ 2014;16:1551–64. [40] Rajakumar S, Muralidharan C, Balasubramanian V. Establishing empirical relationships to predict grain size and tensile strength of friction stir welded AA 6061-T6 aluminium alloy joints. Trans Nonferrous Met Soc China 2010;20: 1863–72.

[41] Maleque MA, Masjuki HH, Haseeb ASMA. Effect of mechanical factors on tribological properties of palm oil methyl ester blended lubricant. Wear 2000;239: 117–25. [42] Haseeb ASMA, Sia SY, Fazal MA, Masjuki HH. Effect of temperature on tribological properties of palm biodiesel. Energy 2010;35:1460–4. [43] Booser ER. Handbook of lubrication (theory and practice of tribology). Brighton: CRC Press; 1984. [44] Ahmed A, Mohamed K, Hou XJ, Turkson RF, Peng Z, Chen XD. Enhancing the thermophysical properties and tribological behaviour of engine oils using nanolubricant additives. RSC Adv 2016;6:77913–24. [45] Hu Y, Wang Y, Zeng Z, Zhao H, Li J, Ge X, et al. BLG-RGO: a novel nanoadditive for water-based lubricant. Tribol Int 2019;135:277–86. [46] Du PF, Chen GX, Song SY, Chen HL, Li J, Shao Y. Tribological properties of muscovite, CeO2 and their composite particles as lubricant additives. Tribol Lett 2016;62. [47] Nan F, Zhou K, Liu S, Pu JB, Fang YH, Ding WX. Tribological properties of attapulgite/La2O3 nanocomposite as lubricant additive for a steel/steel contact. RSC Adv 2018;8:16947–56. [48] Wang DJ, Zhu DC, Li HF, Chen GX. Tribological properties of muscovite/La2O3 composite powders as lubricant additives. Tribol Trans 2015;58:577–83. [49] Hou KM, Wang JQ, Yang ZG, Ma LM, Wang ZF, Yang SR. One-pot synthesis of reduced graphene oxide molybdenum disulfide heterostructures with intrinsic incommensurateness for enhanced lubricating propertie. Carbon 2017;115:83–94. [50] Soltanahmadi S, Morina A, van Eijk MCP, Nedelcu I, Neville A. Tribochemical study of micropitting in tribocorrosive lubricated contacts: the influence of water and relative humidity. Tribol Int 2017;107:184–98. [51] Jin F, Yang G, Song S, Zhang S, Yu L, Zhang P. Synthesis of nanostructured lanthanum fluoborate modified by oleylamine and evaluation of its tribological properties as a lubricating additive in synthetic ester. Surf Interface Anal 2016;48: 1033–9. [52] Njiwa P, Minfray C, Le Mogne T, Vacher B, Martin J-M, Matsui S, et al. Zinc dialkyl phosphate (ZP) as an anti-wear additive: comparison with ZDDP. Tribol Lett 2011; 44:19–30. [53] Cui S, Wan S, Zhu Q, Tieu AK, Zhu H, Wang L, et al. Tribochemical behavior of phosphate compounds at an elevated temperature. J Phys Chem C 2016;120: 25742–51. [54] Xu Y, Yu J, Dong Y, You T, Hu X. Boundary lubricating properties of black phosphorus nanosheets in polyalphaolefin oil. J Tribol 2019;141:072101. [55] Cen H, Morina A, Neville A, Pasaribu R, Nedelcu I. Effect of water on ZDDP antiwear performance and related tribochemistry in lubricated steel/steel pure sliding contacts. Tribol Int 2012;56:47–57. [56] Morina A, Neville A, Priest M, Green JH. ZDDP and MoDTC interactions and their effect on tribological performance – tribofilm characteristics and its evolution. Tribol Lett 2006;24:243–56. [57] Jiang Z, Yang G, Zhang Y, Gao C, Ma J, Zhang S, et al. Facile method preparation of oil-soluble tungsten disulfide nanosheets and their tribological properties over a wide temperature range. Tribol Int 2019;135:287–95. [58] Castro L, Dedryv� ere R, El Khalifi M, Lippens PE, Br�eger J, Tessier C, et al. The spinpolarized electronic structure of LiFePO4 and FePO4 evidenced by in-lab XPS. J Phys Chem C 2010;114:17995–8000. [59] Lvanova OP, Naumkin AV, Vasilyev LA. An XPS study of compositional changes induced by argon ion bombardment of the LaPO4, surface. Vacuum 1996;47:67–71.

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