Laser-induced improvement in tribological performances of surface coatings with MoS2 nanosheets and graphene

Accepted Manuscript Laser-induced improvement in tribological performances of surface coatings with MoS2 nanosheets and graphene

Yufu Xu, Quan Zheng, Tao You, LuLu Yao, Xianguo Hu PII: DOI: Reference:

S0257-8972(18)31273-8 https://doi.org/10.1016/j.surfcoat.2018.11.063 SCT 24029

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

16 October 2018 19 November 2018 20 November 2018

Please cite this article as: Yufu Xu, Quan Zheng, Tao You, LuLu Yao, Xianguo Hu , Laser-induced improvement in tribological performances of surface coatings with MoS2 nanosheets and graphene. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.11.063

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ACCEPTED MANUSCRIPT Laser-induced improvement in tribological performances of surface coatings with MoS2 nanosheets and graphene

Yufu Xua, Quan Zhenga, Tao Youa, LuLu Yaob, Xianguo Hua

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a. Institute of Tribology, School of Mechanical Engineering, Hefei University of Technology, Hefei 230009, China

b. School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China

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Abstract: To deal with the corrosive property of bio-oil, laser-induced coatings were synthesized

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on the typical cylinder liner rubbing surfaces. Two nano-additives including MoS2 nanosheets and

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graphene were introduced into the coatings. The tribological performances of the coated surfaces were tested on a multifunctional cylinder liner-piston ring tribometer. The surfaces before and after

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sliding were characterized with modern surface analysis technology including Raman spectra, X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy

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(EDS) and etc. The results show that the nickel-based coated surfaces can effectively prevent the corrosion of the bio-oil. In addition, the laser-induced coatings had better antifriction and antiwear

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performances than the corresponding traditional electroless plating coatings. With the introduction of MoS2 nanosheets or graphene, the tribological performances of the coatings were further

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enhanced. Laser irradiation increased the concentration of the nano-additives in the coatings and the thickness of the tribo-layers, which accounted for their outstanding tribological performances. Keywords: Laser-induced coating; Tribological performance; MoS2 nanosheets; Graphene; Bio-oil

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Corresponding author. E-mail address: [email protected] (Yufu Xu)

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ACCEPTED MANUSCRIPT 1. Introduction The demands for the development of the modern society economy stimulate a rapid energy exploration in the past fifty years. Due to the non-renewability and emission of the greenhouse gases of the traditional fossil energy, seeking alternative energies has become a hot issue in recent

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two decades [1]. Bio-oil as a novel liquid energy has attracted much attention owing to its renewable, carbon-neutral and clean characteristics [2-4]. Unfortunately, the acidic components in

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the bio-oil make it corrosive which limits its application in internal combustion engines.

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Considerable efforts have been made on upgrading the crude bio-oil in order to remove its corrosive

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components [5-7]. However, slow process and high cost in separation and chemical modification of the crude bio-oil makes another method should be chosen to accelerate the application of bio-oil.

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Developing electroless coatings on the key friction pairs in the internal combustion engines such as cylinder liner-piston ring is an effective method [8-11] to reduce wear and corrosion. Many

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electroless coatings with excellent performances have been developed in recent years [12-15]. Xu et al [8] investigated the tribological behavior of the nickel-based coatings on the engine cylinder liner

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under bio-oil lubricated conditions. It was found that the nickel-based based coatings can effectively protect the friction pairs from corrosive wear of the crude bio-oil. Furthermore, the friction

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coefficient of the tribosystem was in accord with the classical Stribeck curve, and the Ni-Cu-P coatings had a better protective effect than the Ni-P coatings. Based on these work, molybdenum disulfide (MoS2) and graphene oxide were embedded in the nickel-based coatings, and much better tribological performances were achieved due to the formation of the tribofilm from the lubricating and synergetic effects of the nano-additives [16]. However, the introduction of the nano-additives in the nickel-based coatings resulted in porous and uneven coated surfaces which were harmful to the

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ACCEPTED MANUSCRIPT surface hardness and cohesion of the coatings. Thus, enhancement of the compatibility of the nanoparticles in the coatings is necessary. Laser irradiation is an effective method to precisely supply energy to upgrade the compatibility of the nanoparticles in coatings [17, 18]. However, to the best of the authors’ knowledge, there is no

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literature covering the tribological behavior of the laser-induced coatings under bio-oil lubrication conditions. In this work, two kinds of typical 2D materials, MoS2 nanosheets and graphene, were

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selected as nano-additives in the coatings owing to their good reported lubrication performances

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[19-21], and the laser-induced coating technic was used. The friction and wear behavior of the

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coatings before and after laser radiation under bio-oil lubrication conditions were compared with

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each other, and finally the corresponding mechanisms were illustrated.

2. Experimental

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2.1 Materials

The cylinder liner specimens were boron cast iron and bought from the Kaishan Cylinder Co.

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Ltd (China). The top piston ring specimens were made from ductile iron and purchased from the Nanjing Feiyan Piston Ring Co., Ltd (China). Before sliding, the specimens were cut into the

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special dimensions for adapting the sample holders of the tribometer. Nickel sulfate (NiSO4·6H2O) was the product of the Shanghai Liangren Chemical Co., Ltd. Sodium hypophosphite (NaH2PO2·H2O), copper sulphate (CuSO4) and citric acid (C6H8O7) were purchased from Sinopharm Chemical Reagent Co., Ltd. The cationic surfactant cetyl trimethyl ammonium bromide (C19H42BrN) and thiourea (NH2CSNH2) were bought from the Shanghai Chemical Reagent Co., Ltd. All of the chemicals used in this work were of analytical grade and used as received without further purification. Two kinds of 2D materials including MoS2 nanosheets 3

ACCEPTED MANUSCRIPT and graphene as nano-additives were received from Hefei Vigon Material Technology Co., Ltd. The transmission electron microscopy (TEM) images and X-ray diffraction (XRD) patterns of the two nano-additives are shown in Fig. 1. The typical thin flake structures can be seen in both of the MoS2 nanosheets and graphene, and the flake area of the graphene is larger than that of MoS2 nanosheets.

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The detected peaks at 2=14.1, 33.5, 39.6, 58.7 in Fig.1c can be assigned to (002), (100), (103), (110) planes in MoS2 nanosheets [22]. The characteristic peaks at 2=24.1 and 43.9 in Fig.1d

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attributed to the (002) and (101) planes in graphene [23]. These XRD patterns confirmed the

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crystalline structures of the MoS2 nanosheets and graphene.

(c)

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Intensity (a.u.)

(a)

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100 nm

103

0

10

20

30

40

110

50

60

70

80

60

70

80

2(degree)

Intensity (a.u.)

002

101

100 nm

0

10

20

30

40

50

2(degree)

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100

(d)

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(b)

002

Fig. 1 TEM images and XRD patterns of the MoS2 nanosheets (a, c) and graphene (b, d)

The bio-oil was supplied by the Anhui Province Key Laboratory of Biomass & Clean Energy at the University of Science and Technology of China with the pH value of 2.4-2.6 and the kinematic viscosity of 13.2 cSt at 40 °C. The main components of the bio-oil are composed of a variety of oxygen containing organics including acids, alcohols, ketones, aldehydes, phenols, esters, 4

ACCEPTED MANUSCRIPT sugars, furans, guaiacols and other compounds, and the main physiochemical properties of the bio-oil can be can be found in the previous work [24].

2.2 Surface coating preparation

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Before electroless plating, the cylinder liner specimens were degreased and activated with alkaline and acid solution respectively. After being cleaned in deionized water, the cylinder liner

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specimens were immersed in the plating solution for 30 min at 88-90 ℃ with a stirring speed of 300

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rpm. The cationic surfactant cetyl trimethyl ammonium bromide was chosen to disperse the MoS2

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nanosheets and graphene in the solution. The composition and operating conditions for the electroless coatings are listed in Table 1. The cylinder liner coating surface was 2 mm below the

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solution surface. An YLP-F10 fiber laser machine (Han's Laser) was used for laser irradiation. A power of 4W and a laser beam scanning velocity of 100 mm/s were applied during the coating

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process. The focus length was 114 mm, and the diameter of the radiation spot was 5 m. The line scanning and pulsed laser mode with laser frequency of 20 kHz was used. The shift distance was 5

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m after scanning one line to ensure that the laser irradiation region was a rectangle with the dimension of 10 mm100 mm. To obtain the uniform coating on the surface, the laser kept working

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during the whole plating process. The schematic of the laser-induced coating setup can be seen in the Fig. 2. The control groups were carried out under the same conditions without the laser irradiation.

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ACCEPTED MANUSCRIPT Table 1 Composition and operating conditions for the electroless coatings Composition and operating

Ni-Cu-P

Ni-Cu-P-MoS2

Ni-Cu-P-Gr

NiSO4·6H2O (g·L−1)

20

20

20

20

NaH2PO2·H2O (g·L−1)

25

25

25

25

CuSO4 (g·L−1)

-

25

25

25

8

8

8

8

0.5

0.5

0.5

0.5

Graphene (g·L )

-

-

MoS2 (g·L−1)

-

-

C19H42BrN (mg·L−1)

-

-

pH

4.6

4.6

88-90

88-90

−1

NH2CSNH2 (mg·L ) −1

o

Temperature ( C)

1.2

-

45

-

4.6

4.6

88-90

88-90

Cylinder liner

10mm

Laser scanning area

X-Y moving stage

100 mm

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PC controller

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Plating solution

Laser beam

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Mirror

Laser

1.2

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Scanner

-

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C6H8O7 (g·L )

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−1

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Ni-P

conditions

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Fig. 2 Schematic of the laser-induced coating setup

2.3 Coating characterization The micro-morphologies and the chemical compositions of the coatings were analyzed by an optical microscope and an energy dispersive X-ray spectroscopy (EDS) respectively. The crystal structures of the coatings were measured by an X-ray Diffraction (XRD, Rigaku D/MAX2500V) instrument with Cu Kα radiation, 2θ varying from 5° to 80° and a scanning velocity of 10°·min-1. 6

ACCEPTED MANUSCRIPT The microhardness of the coatings was carried out by using an MH-3 micro-vickers hardness tester with a 0.98 N load lasting 10 s. The surface roughness of the coatings was detected by a Taylor Hobson-6 surface profile instrument within the area of 4 mm  4 mm. The measurements were

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repeated for three times.

2.4 Tribological tests

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The tribo-tests were performed on a multifunctional cylinder liner-piston ring tribometer. The

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schematic of the test rig is presented in Fig. 3. As can be seen, a reciprocating sliding of the piston

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ring on the cylinder liner and bio-oil lubrication mode was used. Based on the real working operating state and in order to observe the wear in short time, the intensified sliding testing

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conditions were chosen and shown in Table 2. During the sliding, the friction coefficient was calculated by the instrument automatically according to the ratio of friction force to normal load.

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The wear loss of the cylinder liner and piston ring was calculated by the difference of the weight of the samples before and after sliding with an accuracy of 0.1 mg. To reduce accidental errors, each

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friction test was repeated thrice.

After sliding, the worn surfaces of the specimens were analyzed by the scanning electron

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microscope (SEM, JEOL JSM-5600LV), Raman spectrophotometer (Horiba Jobin Yvon Raman, HR Evolution) and the surface profile instrument, respectively. The worn surfaces of the piston rings were observed by an optical microscope. The lateral surfaces of the coatings were analyzed by the EDS with line scanning mode.

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ACCEPTED MANUSCRIPT Load

Piston ring

Coating Bio-oil

Rreciprocating sliding

R=55mm

Coating

Cylinder liner side view Side view

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front view Front view

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Fig. 3 Schematic of the cylinder liner-piston ring tribometer

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Table 2 Tribological testing conditions

Reciprocating frequency Stroke

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Testing conditions

80 mm 100 mL·h-1

Normal load

210 N

Temperature

823 oC

Duration

120 min

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Oil feed rate

Analysis of the initial coatings

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3.1

5 Hz

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3. Results and discussion

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Fig. 4 shows the micro-morphology images of the cylinder liner with different coatings without or with laser irradiation during deposition process. As can be seen, there were many horning lines on the pristine cylinder surface. After coating, the horning lines cannot be clearly observed, which confirms the existence of the coating films on the surfaces. In addition, the laser irradiated surfaces were more even than those without laser irradiation except for the Ni-P coatings. This might be because the laser irradiation not only can enhance the deposition speed [25] of the 8

ACCEPTED MANUSCRIPT autocatalytic Ni-P coatings but also can destroy the integrity of the thin film due to the intensive energy, while the introduction of other components such as copper sulfate in the plating solution can refine grains [26] and thus reduce the destructive effects. Moreover, the nano-additives including both of the MoS2 and graphene were well embedded in the coatings since no clear grains or

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nanosheets can be obviously observed on the coating surfaces, especially with the introduction of copper sulfate in the plating solution. (a)

(b)

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100m

100m

(e)

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100m

(f)

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(d)

(c)

D

100m

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100m

(g)

(i)

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(h)

100m

100m

100m

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100m

Fig. 4 Micro-morphology images of the cylinder liner with different coatings without (b-e) or with (f-i) laser irradiation: (a) pristine surface, (b, f) Ni-P, (c, g) Ni-Cu-P, (d, h) Ni-Cu-P-MoS2 (e, i) Ni-Cu-P-Gr

Fig. 5 shows the surface roughness and microhardness of the cylinder coatings. It can be found that the surface roughness Ra obviously decreased after coating, and the laser-induced coatings had lower surface roughness versus traditional coatings except for the Ni-P coatings. These results 9

ACCEPTED MANUSCRIPT agreed well with those in Fig. 4. In addition, the cylinder surfaces had a higher microhardness after coating, and the laser-induced coatings had higher microhardness than the traditional coatings. Among them, Ni-Cu-P-Gr shows the highest microhardness, which might be come from the high hardness of graphene [27]. 900

5 4

Substrate

3 2 1

(b)

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800 700 600 500 300 200

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6

Traditional Coating Laser-induced Coating Substrate

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Traditional Coating Laser-induced Coating Substrate

(a)

Microhardness (V)

Surface roughness Ra (m)

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100 0

Ni-P

Ni-Cu-P

Ni-Cu-P-MoS2 Ni-Cu-P-Gr

Coatings

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0

Ni-P

Ni-Cu-P

Ni-Cu-P-MoS2 Ni-Cu-P-Gr

Coatings

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Fig. 5 Surface roughness (a) and microhardness (b) of the cylinder liner with different coatings with or

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without laser irradiation

EDS results can effectively show elemental distribution on the surfaces [28, 29]. The chemical

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composition of the coatings detected by EDS is presented in Table 3. The typical elements Ni, P and Cu were detected in the coatings, which came from the autocatalysis deposition reactions in the

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solution and were in consistent with the previous researches [8, 30-32]. With the introduction of MoS2 nanosheets and graphene in the solution, the chemical components including Mo, S and C were found in the coatings, confirming the nano-additives have been successfully embedded in the coatings during the deposition of the Ni-containing coatings. Furthermore, the content of the Mo, S and C element in the laser-induced electroless coatings is almost twice to thrice of that in the traditional coatings, suggesting the laser irradiation is helpful for the deposition of the nano-additives. 10

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Table 3 Chemical composition of coatings Traditional electroless coatings

Laser-induced electroless coatings

Ni-P

Ni-Cu-P

Ni-Cu-P-MoS2

Ni-Cu-P-Gr

Ni-P

Ni-Cu-P

Ni-Cu-P-MoS2

Ni-Cu-P-Gr

Ni

91.6

83.2

81.3

85

93.1

82

79.7

78.9

P

8.4

7.6

7.9

8.6

6.9

5.1

4.8

8.9

Cu

-

9.2

8.9

5.8

-

12.9

11.3

10.4

Mo

-

-

1.1

-

-

-

2.3

-

S

-

-

0.8

-

-

-

1.9

-

C

-

-

-

0.6

-

-

-

1.8

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(wt%)

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Composition

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Fig. 6 presents the XRD patterns of the coatings. For the traditional electroless Ni-P coating,

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there was a typical characteristic peak at ~45o belonging to (011) plane [8], and it shifted to ~43o

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with the addition of the CuSO4 in the solutions. The characteristic (002) peak of MoS2 occurred at ~14.1o in the coatings with the introduction of MoS2 nanosheets in the plating solution, again

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confirming the successful deposition of the nanosheets in the coatings. However, no clear diffraction peak of graphene can be detected in the traditional coatings, which might be caused by

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the low concentration of the graphene in the coatings. For the laser-induced coatings, the typical characteristic peak at ~45o became narrower than those of the traditional coatings, indicating that the laser irradiation can help improvement of the crystallinity or refine the crystal grains [33, 34]. The characteristic diffraction peaks at 2=14.1o, 24.1o can be obviously observed respectively, confirming the existence of MoS2 and graphene in the Ni-Cu-P-MoS2 and Ni-Cu-P-Gr coatings; no other diffraction peaks occurred, indicating the pure components of the coatings.

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ACCEPTED MANUSCRIPT (b)

Intensity (a.u.)

Intensity (a.u.)

(a) Ni-Cu-P-Gr MoS2

Ni-Cu-P-MoS2 Ni-Cu-P

Graphene

Ni-Cu-P-Gr

MoS2

Ni-Cu-P-MoS2 Ni-Cu-P Ni-P

Ni-P

20

30

40

50

60

70

10

80

20

30

40

50

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10

60

70

80

2 (degree)

2 (degree)

Friction and wear behavior

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3.2

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Fig. 6 XRD patterns of the traditional electroless coatings (a) and laser-induced coatings (b)

Fig. 7 shows the friction and wear results of the coated cylinder liner and piston ring tribopairs.

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As can be seen, there was a significant decrease of the friction coefficient after coating, comparing with the pristine tribopairs. With the introduction of copper and nano-additives, the tribological

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performances improved for the nickel-based coatings, matching well with the previous researches [8]. In addition, the laser-induced coatings had the lower friction coefficient than those of the same

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type of the traditional coatings. The Ni-Cu-P-MoS2 coating showed the best antifriction and antiwear effect for the traditional specimens, and its friction coefficient and wear loss decreased by

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41.3% and 53.2% respectively, comparing with the substrate without coatings. As for the laser-induced coatings, the Ni-Cu-P-Gr coating had the lowest friction coefficient and wear loss, and they reduced by 65% and 92.9% respectively, comparing with the pristine specimen. This can be explained by the fact that the laser irradiation had better deposition of the graphene than that of MoS2 nanosheets due to the larger size of the graphene [35]. It also should be noted that the coatings did not increase the wear loss of the counterparts, though it had a higher microhardness than the substrate, which should be resulted from the lubricating roles of copper, MoS2 and 12

ACCEPTED MANUSCRIPT graphene.

0.15

0.10

0.05 20

40

60

80

Time (min)

100

0.05 40

60

80

350

150

50 0

Ni-P

0.15

0.10

0.05

120

Ni-P

Ni-Cu-P

Ni-Cu-P

Ni-Cu-P-MoS2 Ni-Cu-P-Gr

Coatings 350

Left column: Cylinder liner Right column: Piston ring

(f) 300

CE

250

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Wear loss (mg)

300

200

0.20

Left column: Cylinder liner Right column: Piston ring

(e)

100

100

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Time (min)

(d)

Wear loss (mg)

20

0.25

D

0

Ni-Cu-P Ni-Cu-P-MoSNi-Cu-P-Gr Substrate 2

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0.15

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Friction coefficient

Ni-Cu-P-Gr

0.10

0.05

Coatings

Ni-P Ni-Cu-P Ni-Cu-P-MoS2

0.20

0.10

Ni-P

0.25

(c)

0.15

120

Average friction coefficent

0

0.20

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Ni-Cu-P-Gr Substrate

(b)

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0.20

0.25

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Ni-P Ni-Cu-P Ni-Cu-P-MoS2

(a) Friction coeficient

Average friction coefficent

0.25

250 200 150 100 50 0

Ni-Cu-P-MoS2 Ni-Cu-P-Gr

Ni-P

Substrate

Ni-Cu-P

Ni-Cu-P-MoS2

Ni-Cu-P-Gr

Coatings

Coatings

Fig. 7 Friction coefficient (a, c), average friction coefficient (b, d), and wear loss (e, f) of the traditional electroless coatings (a, b, e) and laser-induced coatings (c, d, f)

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The SEM images of the worn surfaces of the cylinder liners and optical microscope photographs (insets) of the piston rings are shown in Fig. 8. For the uncoated specimens (Fig. 8a), many severe furrows appeared on the worn cylinder surfaces and some corrosive pits can be found

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on the piston ring, suggesting a severe corrosion wear [36]. A wrinkled Ni-P coating and pleated piston ring worn surface (Fig. 8b) can be observed for traditional plating. With the introduction of

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copper in the coating (Fig. 8c), there are obvious spalling area on the rubbed surfaces, indicating the

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main wear type can be ascribed to spalling wear for the coating [37], and the colorful surface of the

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countpart might be resulted from the transfer film composed of pure copper and copper oxide. The relative smooth worn surfaces (Fig. 8d) can be found with the addition of MoS2 in the coatings,

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indicating a lower wear loss of the tribopairs. Lots of delamination lines on the coatings (Fig. 8e) and clear furrows on the piston ring occurred, suggesting a typical delamination wear on the coating.

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With the laser irradiation technology being used, nodular structures of the coatings (Fig. 8f) can be observed, and the smoother worn surfaces (Fig. 8g-i) can be obtained, indicating the laser induced

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coatings has better antiwear performances due to the strengthening effect of the laser irradiation.

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Thus, these SEM images agreed well with the tribological results in Fig. 7.

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ACCEPTED MANUSCRIPT (a) Uncoated

(c) Ni-Cu-P

(b) Ni-P

100 m

100 m

100 m

Spalling Wrinkled surface

Severe furrows 20 m

(f) Laser-induced Ni-P

(e) Ni-Cu-P-Gr

20 m

20 m

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(h) Laser-induced Ni-Cu-P-MoS2

(g) Laser-induced Ni-Cu-P

Nodular structures

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Delamination

Smooth surface

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100 m

Smooth surface

(i) Laser-induced Ni-Cu-P-Gr

100 m

Light wear 20 m

20 m

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20 m

100 m

Smooth surface

20 m

100 m

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100 m

100 m

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(d) Ni-Cu-P-MoS2

20 m

20 m

Fig. 8 SEM images of the worn surfaces of the coated cylinder liners and optical microscope

laser-induced coatings (f-i).

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photographs (insets) of the piston rings: uncoated surfaces (a), traditional coatings (b-e) and

To know the effects of the two nano-additives MoS2 and graphene on the tribological behavior, Raman spectra of the two coatings after sliding are shown in Fig. 9. Two characteristic peaks at 383 cm-1 and 408 cm-1 were detected for the Ni-Cu-P-MoS2 coatings (Fig. 9a), which attributed to the E1g1 and A1g mode of the MoS2, respectively. Under the same condition, higher intensity of these two peaks of the laser-induced coating means the higher content of the MoS2 existing on the rubbing surfaces, which accounts for the better tribological performance of the laser-induced 15

ACCEPTED MANUSCRIPT Ni-Cu-P-MoS2 coating. The two most intense features in the Ni-Cu-P-Gr coatings (Fig. 9b) are ascribed to the D band (1355 cm-1) and G band (1584 cm-1) respectively. The Raman G band is caused by the in-plane vibrational E2g phonon of graphene [38]. The D peak originates from the breathing mode of the sp2 carbon atoms, which is caused by structural disorder defects [39, 40] .

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The intensity ratio ID/IG can reflect the degree of disorder of the graphene. As can be seen, the ID/IG ratio (0.297) of the laser-induced Ni-Cu-P-Gr coating was lower than that (0.585) of traditional

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coating, suggesting that the laser-induced coating was beneficial to protect the graphene from

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destruction by comparing with the traditional coating. This should be resulted from the

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Traditional Ni-Cu-P-MoS2 coating

(b)

Traditional Ni-Cu-P-Gr coating Laser-induced Ni-Cu-P-Gr coating

400

410

420

D

G ID/IG=0.585

1355

D

408 390

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380

CE

370

A1g

383

Intensity (a.u.)

1

E2g

Intensity (a.u.)

Laser-induced Ni-Cu-P-MoS2coating

1584

(a)

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enhancement of binding ability of the graphene in the coatings due to the laser irradiation.

ID/IG=0.297

430

-1

Raman shift (cm )

800

1000

1200

1400

1600

1800

2000

-1

Raman shift (cm )

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Fig. 9 Raman spectra of the worn surfaces of the traditional and laser-induced coatings after sliding: Ni-Cu-P-MoS2 (a), and Ni-Cu-P-Gr (b).

3.3 Characterization of the surfaces after sliding Fig. 10 shows the SEM images and EDS line scanning results of the worn and lateral surfaces. For the traditional Ni-Cu-P-MoS2 coating (Fig. 10a), a relative even lateral surface can be observed and the main lubricating elements including Ni, Cu, P, Mo, S significantly decreased and the 16

ACCEPTED MANUSCRIPT substrate Fe element increased accordingly with the increase in depth within ~25 nm; indicating the tribofilm composed of these antiwear components was formed on the rubbing surface. For the traditional Ni-Cu-P-Gr coating (Fig. 10b), a clear crack in the subsurface can be seen, which is consistent with the delamination wear of the worn surface in Fig. 8e. In addition, from the EDS

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results it can be found that the thickness of the tribofilm composed of Ni, Cu, P, and C was ~40 nm. The high concentration of the Ni in the tribo-layer presents its dominated antifriction and antiwear

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roles. For the laser-induced Ni-Cu-P-MoS2 coating (Fig. 10c), there are no significant difference

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with that of the traditional Ni-Cu-P-MoS2 coating from the SEM image of the lateral surface, but

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the EDS results considerably changed. The thickness of the tribofilm being composed of Ni, Cu, P, Mo and S elements amounted to ~50 nm, and there was another ~50 nm transition layer. This might

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explain the better tribological performance of the laser-induced Ni-Cu-P-MoS2 coating. For the laser-induced Ni-Cu-P-Gr coating (Fig. 10d), a hybrid and obscure lateral surface was detected, and

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the concentration of carbon is much higher than that of the traditional coating and that of the pristine laser-induced Ni-Cu-P-Gr coating, which should be attributed to the good combining effects

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in the coating and accumulation roles of the graphene on the rubbing surface with the laser irradiation. The formation of the tribofilm containing high concentration of Ni, P and intact

performances.

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graphene with large size dominated the sliding process and made for the excellent tribological

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Fig. 10 SEM images (left) and EDS line scanning results (right) of the worn and lateral surfaces of the traditional (a, b) and laser-induced (c, d) coatings: Ni-Cu-P-MoS2 (a, c), and Ni-Cu-P-Gr (b, d). 18

ACCEPTED MANUSCRIPT 4. Conclusions In summary, to accelerate the application of bio-oil in industry and prevent the corrosive wear of the tribopairs in bio-oil, novel laser-induced coatings were synthesized on the cylinder liners in the current work. Their friction and wear behaviors were investigated on a multifunctional cylinder

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liner-piston ring tribometer. The corresponding tribological mechanism was disclosed by surface and sub-surface analysis. The major findings based on the research were as follows: Nickel-based coatings including Ni-P, Ni-Cu-P, Ni-Cu-P-MoS2 and Ni-Cu-P-Gr with or

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(1)

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without laser irradiation were successfully prepared with electroless plating technology.

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The laser-induced coatings except for the Ni-P coating showed lower surface roughness and higher microhardness than the corresponding traditional coatings. All these coated specimens show better tribological performances than the uncoated

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tribopairs, and the coatings can prevent the specimens from the corrosion of bio-oil,

(3)

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showing a promising potential in application. For the traditional electroless plating technic, the Ni-Cu-P-MoS2 coating presented the

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optimal antifriction and antiwear roles. Comparing with those of the substrate, the friction coefficient and wear loss of the Ni-Cu-P-MoS2 coating decreased by 41.3% and 53.2%

(4)

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respectively.

For the laser-induced electroless plating technic, the Ni-Cu-P-Gr coating had the lowest friction coefficient and wear loss, and they reduced by 65% and 92.9% respectively, comparing with the pristine specimen.

(5)

The laser-induced Ni-Cu-P-MoS2 coating had a higher concentration of MoS2 and thicker tribo-layer on the rubbing surfaces, which contributed to its good antifriction and antiwear roles. A hybrid and robust tribo-layer with high concentration and complete graphene of 19

ACCEPTED MANUSCRIPT the laser-induced Ni-Cu-P-Gr coating accounted for its excellent tribological performances.

Acknowledgement:

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We thank Mr Tian Xie for his help in the coating preparation. This work is supported by the

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National Natural Science Foundation of China (Grant No. 51875155).

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ACCEPTED MANUSCRIPT Highlights  Novel laser-induced surface coatings were successfully prepared.  All the coated surfaces show better tribological performances.  The laser-induced surface coatings formed thicker tribo-layer during sliding.

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 A hybrid and robust tribo-layer accounted for its excellent performances.

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