graphene hybrid nanocomposites produced by electrophoretic deposition

    Co-deposition of Cu/WC/Graphene Hybrid Nanocomposites Produced by Electrophoretic Deposition Hatem Akbulut, Gizem Hatipoglu, Hasan Algul, Mahmud Tokur, Muhammet Kartal, Mehmet Uysal, Tu˘grul Cetinkaya PII: DOI: Reference:

S0257-8972(15)00499-5 doi: 10.1016/j.surfcoat.2015.07.080 SCT 20506

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

1 May 2015 5 July 2015 6 July 2015

Please cite this article as: Hatem Akbulut, Gizem Hatipoglu, Hasan Algul, Mahmud Tokur, Muhammet Kartal, Mehmet Uysal, Tu˘ grul Cetinkaya, Co-deposition of Cu/WC/Graphene Hybrid Nanocomposites Produced by Electrophoretic Deposition, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.07.080

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ACCEPTED MANUSCRIPT Co-deposition of Cu/WC/Graphene Hybrid Nanocomposites Produced by Electrophoretic Deposition

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Hatem AKBULUT, GizemHATIPOGLU, HasanALGUL, Mahmud TOKUR, MuhammetKARTAL, Mehmet UYSAL, TuğrulCETINKAYA,

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Sakarya University, Engineering Faculty, Deparment of Metallurgical & Materials Engineering, Esentepe Campus, 54187, Sakarya/Turkey Tel:0090 264 295 57 62 Fax:90 0264 295 56 01 E mail: [email protected]

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Abstract

The effect of two different reinforcements of WC and Graphene on the structural and

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tribological properties of copper matrix were investigated. Cu/WC, Cu/Graphene and hybrid Cu/WC/Graphene nanocomposites were co-deposited by electrophoretic deposition to

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improve both electrical and tribological behaviors. The friction and wear behaviors of WC

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and graphene reinforced nanocomposites were investigated against Al2O3 ball under dry

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sliding wear conditions. Comprehensive characterizations were performed using Scanning Electron Microscopy (SEM), X-Ray Diffraction analysis (XRD), Raman Spectroscopy and 3Dprofilometry facilities. Tribological test results revealed that small amount of graphene

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addition was able to drastically improve the antifriction and antiwear properties of hybrid Cu matrix nanocomposites because of its excellent solid lubrication effect. Tribological analysis was shown that hybrid nanocomposite with sub-micron WC and graphene reinforcements provided good load-bearing and tribological properties for possible future MEMs applications. Wear mechanisms investigation shown that co-deposition of WC and graphene resulted in altering wear mechanisms of the Cu matrix. Keywords: Electrophoretic co-deposition, sub-micron WC, graphene nanosheets, sliding friction, solid lubrication, wear mechanisms

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ACCEPTED MANUSCRIPT 1. Introduction The electrical contact materials are widely used in the different low voltage switch devices,

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such as relays, contactors, circuit breakers and switches, and their properties are of

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importance to the switching capacity, reliability, stability and service life of integral electrical

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systems [1, 2]. For high efficiency and reliability of materials used electrical contacts in different applications have to be considered to exhibit excellent mechanical and electrical as

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well as corrosion resistance properties. Traditionally, materials used for electrical contact are mostly, silver and copper as well as their alloys and composites [3-5].Especially, copper and

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its alloys is one of the most widely used materials due to their excellent thermal and electrical conductivities apart from mechanical workability. Although pure metals possess high

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electrical conductivity, it has low wear resistance and may also corrode easily during

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exposure to moisture [6]. Moreover, the applications of metals have been limited because of

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their chemically reactive nature.

The lubricants such as graphite, molybdenum disulfide incorporated into these metal matrixes in order to improve the tribological properties of the metals such as copper, silver but concede

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the electrical conductivity and corrosion resistance. Recently, numerous studies have been made in seeking for better solutions [7]. So far, many metal-based composite contact materials, such as Cu/Graphene, Ag/Graphite/CNT, Cu/CNT have been developed. For example, Wang et al. [8] reported Ag/Graphite/CNT electrical contact composite material that synthesized by powder metallurgical method. They suggested that Ag/Graphite/CNT electrical contact material, with the increase of graphite content, tribological properties were found to increase, but electrical conductivity, thermal conductivity declined. Lauridsen et al. [9] investigated that silver iodide (AgI) coatings have been electrodeposited on Ag-plated Cu coupons as the solid lubricant for developing the wear properties while preservation good electrical conductivity. Copper/Graphene composites have been studied for use as anti-wear 2

ACCEPTED MANUSCRIPT materials. The composites for these applications should possess high electrical and thermal conductivity. Copper matrix has high electrical conductivity and graphite provides good

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lubrication. However, addition of graphite to the copper matrix reduces the electrical

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conductivity of the composite [10-12].

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Graphene, which is composed of sp2 hybridized carbon networks, has been investigated recently as a very attractive material that possesses excellent chemical inactivity, high

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electrical and thermal conductivities, high optical transparency, good thermal stability and good mechanical and tribological as well as corrosion resistance properties[13]. Graphene is

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thermally and chemically stable in ambient air up to 400 °C and exceptionally transparent; more than 90% transmittance is observed for four-layer graphene. Graphene sheets have been

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found many promising potential applications in various energy storage, electrocatalytic

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purposes, photo catalytic, sensor, electrical contact/electrode materials, and optoelectronics, antibacterial property applications due to the increase of the film conductivity or specific

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surface area after incorporating graphene [14-16]. Colloidal deposition methods include electrophoretic deposition (EPD), which can be used to

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deposit functional materials with controlled properties on a metal substrate. In the EPD, charged particles are dispersed in a suitable solution and migrate towards the electrode with opposite charge under the application of electric field [17, 18]. Particles in an EPD suspension are forced to migrate towards an oppositely charged electrode by electric field, and deposit on the substrate forming a relatively dense and homogeneous coating. EPD has recently been drawn attention as important technique for the production of nanocoating structures and thin ceramic composite films on conductive substrates. An EPD process is one of the promising candidates for forming coating due to the advantages of simplicity, low cost equipment and feasible design of complex shapes. EPD offers easy control of the thickness and morphology of the deposited films through a simple adjustment of the deposition time and applied 3

ACCEPTED MANUSCRIPT potential. Therefore, EPD has been used for fabricating many functional materials such as oxides, carbides and polymers [18-21].

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Copper/WC/Graphene composite materials are expected to possess excellent electrical and

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thermal conductivities as well as high wear resistance suitable for contact materials with low

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coefficient of friction. To the best of our knowledge, there is still no report on the production and tribological properties of these hybrid coatings. The purpose of the present study is to

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investigate the improvement in the tribological properties with high conductivity that can be brought by the incorporation of graphene and WC particles on the copper-coated substrate. In

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our work, Copper/WC/Graphene composite films, as potential electrical contact materials, have been synthesized on copper substrate with electrophoretic deposition method. Structural,

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mechanical properties such as the wear properties, surface morphology of the

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Cu/WC/Graphene composites were tested for developing new generation contact materials for

2. Experimental

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possible future MEMs/NEMs applications.

2.1. Synthesis of Graphene

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The graphene oxide (GO) was prepared by an improved Hummers’ method [22]. The graphite particles were pretreated to activate the surfaces of the graphite flakes, facilitating the exfoliation of the van der Waals bonds between the graphene layers. The graphite flakes were dispersed in a 3:1 nitric acid (HNO3): sulfuric acid (H2SO4) 50 ml solution for 2 h with magnetic stirring. After the acid treatment, the graphite particles were heated to 800 oC for 120 s in an open air atmosphere. After the pretreatment process, the Hummers’ method was used to synthesize graphite oxide. To prepare the GO, 30 mg of the graphite oxide particles was added to 100 ml distilled water, and the GO sheets were separated from the graphite oxide structure using an Ultrasonic Processor (UP400S) at 20 kHz, with a power of 60W for 2 h. Finally, the reducing agent hydrazine hydrate (1 mL) was added and the solution was 4

ACCEPTED MANUSCRIPT heated in a water bath to 85 oC for 24 h. Graphene was gradually precipitated out as a black

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solid, which was filtered and washed repeatedly with distilled water.

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2.2. Electrophoretic Deposition Process

Before EPD, the substrates were deposited by electroless copper coating. The bath composition and operation conditions used for preparing the electroless copper coatings are

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given in Table 1. Each chemical constituent was purchased from the Merck Company. Copper

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samples with dimensions of 50 mm × 30 mm × 2 mm were used as substrates. The copper substrates were subjected to a prior mechanical grinding with SiC paper of grades 600and

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1000. The next step of the process was activation etching in sulphric acid for 2 min, directly

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followed by rinsing in flowing distilled water and drying in hot air. The specimens were immersed into the electroless plating bath, which contained copper salts solution (A),

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formaldehyde reducing agent (B), and deionized water; at room temperature for 30 min. The two solutions A and B were mixed in a 1:1 ratio. After preparation of the solution, copper

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plates were immersed into the electroless plating bath. Plating time kept constant at 2 h for each electroless coating run. EPD was achieved via the motion of charged particles, dispersed in a suitable solvent, towards an electrode under an applied electric field. Deposition on the electrode occurs via particle coagulation. Preparation of a stable dispersion of graphene sheet in a suitable solution is a necessary prerequisite for successful EPD. The most common strategy is the production of an electrostatically stabilized dispersion, which, in general terms, requires the preparation of a solvent medium in which the particles have a high zeta potential, while keeping the ionic conductivity of the suspensions low. The Cu, Cu/WC, Cu/Graphene and Cu/WC/Graphene

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ACCEPTED MANUSCRIPT composite coatings were deposited on 5 cm x 5 cm x 0.5 cm copper coated substrates by using electrophoretic coating technique.

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EPD was performed using a two-electrode cell as shown in Fig. 1. The working and counter

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electrodes were then used as the anode and cathode, respectively. The copper coated

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substrates were used as the working electrode, and stainless steel was used as a counter electrode for the electrophoretic coating. The counter electrode stainless steel with dimensions

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30 mm × 20 mm was used as cathode. The stainless steel was washed with acetone and ethanol before usage. The two electrodes were immersed in the prepared solution and

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connected to a DC power supply. Prior to EPD the solution was stirred using a magnetic stirrer and subsequently ultrasonic agitation was conducted using an Ultrasonic Processor

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(UP400S) at 20 kHz, with a power of 60 W for 30 min in order to prevent agglomeration of

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WC particles and Graphene. The working distance between the cathode and anode was fixed at 15mm. The deposition time was kept constant for graphene, WC and WC/Graphene

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depositions and the EPD continued for 30 min. The applied voltage was varied for different reinforcements between 100 and 300 V and shown in Table 2 with all the deposition

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parameters. The coatings were gently rinsed with ethanol and dried in 60 °C oven at overnight. The specimens were deposited with WC powder in an acetone solution containing 10 g/l WC particles. The WC particles have grain size distribution between 0.1-1 μm. WC particles were positively charged by using an anionic surfactant material. The used surfactant was Sodium Dodecyl Sulfate (SDS). The SDS adsorbed onto WC particles to form an electrochemical double layer on the WC particle surfaces. The amount of surfactant, SDS was fixed at 300 mgL-1. To attain a stable suspension of the graphene nano sheets for the EPD on the copper substrates, the obtained graphene nano sheets were first dispersed in acetone under ultrasonic agitation for 1 h. It was aimed to obtain stable suspension for achieving EPD by creating oxygen containing groups on the graphene sheets. Thus, 30 mg of graphene sheets 6

ACCEPTED MANUSCRIPT were added to 100 ml of acetone. 10 mg of Ni(NO3)3 was added as charger material to render the graphene sheets positively charged. Under the applied voltage, the positively charged

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graphene sheets migrated toward the negative electrode, and were subsequently orderly

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deposited onto the surface of the negative electrode.

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2.3. Characterizations

The morphology and microstructures of the composite coating were characterized by a

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scanning electron microscope (SEM, model JEOL – JSM 6060 LV). XRD analysis was carried out with a Rigaku D/MAX/2200/PC model X-ray diffractometer scanning at a speed

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of 1°/min in the 2θ range between 10o and 90o. 3D surface profilometry apparatus (KLATencorP6) was used to determine the surface roughness and the coating thickness on the

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EPD samples. The reciprocating tribological behaviors of the coatings sliding against a

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Al2O3with 10 mm in diameter were examined on a Tribometer (CSM Instruments) designed

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according to DIN 50 324 and ASTM G 99-95a in a ball-on-disk configuration. Dry sliding wear tests were performed at room temperature in the relative humidity between 50-60 % at a constant applied load of 1.0 N. The sliding speed and the sliding distance in reciprocating

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wear were chosen as 30 mm/s and 250 m, respectively. The system measures the friction coefficient and time-dependent depth profiles using sensitive transducers. The depth transducer was located vertically on top of the sample. The SEM analysis was used to study the morphologies of the wear tracks. The wear volumes (V) of the discs were measured accurately by using a 3D profilometer. The obtained values served together with the value of the normal load (F) as well as of the sliding distance (S) to the calculation of the specific wear rate using the following formula: Specific wear rate = V/ (F X S) (mm3 / Nm)

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ACCEPTED MANUSCRIPT After each test, the wear resistance of the studied materials was calculated by measuring the wear width and depth using a 3D surface profilometer and low magnification optical

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micrographs. These measurements were also compared with the vertical transducer depth

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profiles, and thus, the wear rate of the pure Cu and its composites were determined. At least

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three measurements were performed and the averages valued were then calculated for wear rate and the friction coefficient values. Raman spectroscopy tests were also performed to

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show the existence of the different components on the wear surfaces. 3. Results and Discussion

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EPD is one of the colloidal processes in deposition technique. The charged particles in a solution are deposited on substrates under an electrical field [23, 24]. The zeta potential of

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particles plays an important role in the characteristics of electrophoretic coating because it

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determines the stability of the suspension and the direction and migration velocity of the

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particles [19, 25]. Therefore the zeta potentials of the WC particles and graphene nano sheets in the solution were investigated by Zeta sizer Nano ZS90 and the results are presented in Fig. 2.For the coating of the Cu/WC, WC particles were suspended in the acetone solution and the

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measured zeta potential yielded approximately - 40 mV showing stable dispersion of the WC particles for the subsequent EPD. Similarly, as shown in Fig. 2, the graphene nanosheets suspended in the acetone was negatively charged, and the average zeta-potential of the graphene dispersion in acetone could increase to - 48 mV. Moreover, in this study anodic electrophoretic deposition was performed as graphene sheet was negatively charged because of its oxygen functional groups because of used SDS surfactant. Thus, the working electrode, made from copper coated substrate, was assembled to the anode, and stainless steel was used as a counter electrode. The results suggested that the Cu/WC and Cu/Graphene colloidal dispersions displayed good stability and the particle groups were negatively charged due to the electrosteric stabilization. 8

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Fig. 3 shows the results of XRD analysis of electroless coated pure copper, Graphene, WC

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and WC/Graphene composite coatings produced by EPD. The peaks at 2 = 43.22o, 51.1 o and

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74.5 ocorrespond to the diffractions of the (1 1 1), (2 0 0), and (2 2 0) lattice planes of the pure

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face-centered cubic structured copper, suggesting that copper ions have also been completely reduced into metal Cu. The XRD pattern of graphene shows an intense and broad peak, seen at 2θ=25.5°, corresponding to (002) reflection peak which confirms the formation of reduced

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graphene oxide [26]. For the WC coatings, three major diffraction peaks can be seen at 31.44 , 35.61 o and 48.26°, respectively; which correspond to diffraction from the (0 0 1), (1 0 0),

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and (1 0 1) planes of hexagonal WC, respectively [27]. The XRD results indicate that the WC

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graphene and WC/Graphene was deposited on the copper coated substrate by electrophoretic

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deposition. These results have shown that both graphene nanosheests and WC submicron particles were successfully penetrated and adhered on the electroless coating copper substrate.

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The total thickness of the electroless coating plus EPD layers were optimized to get 10 – 12 m to get a tribological tests only at the Cu, Cu/WC, Cu/Graphene and Cu/WC/Graphene

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layers. The high intensity reflections belonging to the reinforcements and small intensity reflections of the copper matrix evidenced that the surface properties are controlled by reinforcements and copper matrix behaves only as a binder. This is significant because one of the main purposes of the present study is to develop flexible and protective coatings for possible future MEMs surface device coating applications to protect their surfaces against to the wear or may be corrosion. The Raman spectra of Cu/Graphene, Cu/WC and Cu/WC/Graphene coatings are shown in Fig. 4. The Raman spectrum presented for the nanocomposite coatings produced by electrophoretic depositions have been used to indicate the presence of graphene and WC that deposited on the copper substrate. In the EPD sample that produced by deposition of graphene 9

ACCEPTED MANUSCRIPT sheets on the copper coated substrate, three reflections are observed at 1325 cm-1 and 1569 cm-1 and 2640 cm-1 as Raman shift values corresponding to D band, G band and 2D bands of

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graphene, respectively [12,28]. The intensities of G and 2D band are related to stacking of

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graphene sheets and number of layers. Bimal et al. have been reported that the single layer

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graphene films show G/2D intensity ratio, having very low value. When the G/2D intensity ratio is high, in this condition it is suggested that there is multilayer graphene and very weak and broadened 2D band shape with increasing number of graphene layer. As shows in Fig.4a,

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G/2D intensity ratio was found to be 5.4, which indicating multilayer graphene sheets. As can

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be seen from Fig.4b-c, the peaks observed at 215 cm-1, 340 cm-1, 815 cm-1 and 887 cm-1, Raman shifts represents the existence of the WC phase and evidenced dispersion of the WC particles [29].

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Fig. 5 shows the SEM images of the electroless copper, Cu/WC, Cu/Graphene and Cu/WC/Graphene coating, which show the morphological differences of composites.

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Fig. 5a presents the SEM image of the electroless copper coated material, which was formed on the substrate before electrophoretic deposition. Electroless copper coatings are

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characterized by nodular morphology. Moreover, the electroless copper deposits became compact and the topography became smoother. Fig. 5b shows the morphology of WC particles deposited on the copper-coated at 300 V for 30 min by electrophoretic deposition The surface morphology of WC coating indicates that WC particles have been deposited successfully on the copper-coated substrate and it can be seen in SEM that WC particles has an irregular shape. Fig. 5c shows SEM image of the graphene sheets obtained after deposition on copper-coated substrate at the optimal applied potential of 100 V for the deposition time of 30 min. It can be seen that a uniform distribution of graphene sheet and high graphene density was obtained throughout the copper-coated substrate. SEM image indicates that the graphene sheets are randomly oriented, with some flakes protruding higher above others and some

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ACCEPTED MANUSCRIPT vertical to the copper-coated substrate. Some of these sheets are folded on the edges while the remaining graphene sheets appear smooth. Moreover, as show in Fig 5c and 5d, the crumples

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and wrinkles are found in graphene sheets because the van der Waals forces between layers of

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graphene would increase after removing the functional groups [12, 30]. Therefore, the

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graphene sheets need more wrinkles to maintain the stability in the 2D structure [30]. Fig.5d shows the morphology of Cu/WC/Graphene composite coated on copper coated substrate at 150 V for 30 min. As seen in SEM, the morphology of graphene shows a wrinkled and folded

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structure and graphene sheets are homogeneously dispersed on the copper-coated

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substrate. Moreover, the WC particles are densely and uniformly distributed on the surface of graphene without agglomeration. The homogenous distribution of WC particles act as spacers and reduce the van der Waals forces between layers of graphene sheets in order to prevent the

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graphene nanosheets from restacking and aggregating[30]. Therefore, graphene provides a good carrier for the WC particles to avoid their aggregation effectively, and WC particles act

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as spacers to increase the stability of exfoliated graphene sheets. The Cu/WC/Graphene composite coating presents the existence of WC particles stacked into graphene sheets. The

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uniform dispersion of the Graphene in the copper-coated matrix is largely because of the decoration of WC particles on the surface of graphene sheets. As stated by Yanxia et al., [13] the Ni nanoparticles that anchored on the laminate-structured graphene nanosheets (GNS) can serve as spacers to effectively prevent the GNS aggregating and restacking. Therefore, they suggested that decoration of the second phase can result in the good dispersion of GNSs in the metal matrix. Since wrinkling of the graphene nanosheets and some of the graphene nanosheets are vertically aligned on the electroless copper coated substrate, this causes to increase the surface roughness of the EPD layers. The surface roughness values for the Cu, Cu/WC. Cu/Graphene and Cu/WC/Graphene were measured 1.28 m, 2.33 m, 6.23 m and 4.75m, respectively.

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ACCEPTED MANUSCRIPT Wear rate of the electroless Cu coating, Cu/Graphene, Cu/WC and Cu/WC/Graphene composite coatings are presented in Fig. 6a. The amount of wear for each coating was

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determined from the wear track measurement with 3D profilometry. As can be seen from the

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Fig. 6a, The Cu/WC/Graphene coating exhibited the lowest wear rate compared with pure Cu

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coating, Cu/Graphene and Cu/WC composite coating. Moreover, we can observe that the graphene sheet reinforced composites generally exhibit more wear resistant than pure copper and Cu/WC composite coatings. In the composite coating with Cu/WC/Graphene the results

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are close to these of the Cu/Graphene composite. The results showed that the graphene and

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WC particle addition improved the wear resistance of copper coating under dry sliding conditions. Introducing both WC and graphene deposited on the copper-coated matrix materials, the Cu/Graphene and Cu/Graphene/WC hybrid composite coatings resulted in

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getting remarkably high wear resistance in spite of porosity increment in the graphene introduced samples[31,32]. In spite of increasing surface roughness of the deposited layers

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reinforced with graphene, the decrease in the friction coefficient is significant. Because, increasing surface roughness of the materials in sliding wear results in increasing shear forces

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and therefore, high friction coefficients [31, 33]. Fig. 6b shows the comparison of average coefficient of friction for the electroless Cu coating, Cu/Graphene, Cu/WC and Cu/WC/Graphene coatings materials. Regarding the Cu/Graphene composite coating, the average friction coefficient is 0.21, which is much lower than that of Cu coating, Cu/WC and Cu/WC/ Graphene composite coatings. The addition of graphene in copper matrix is responsible for the decrease of friction coefficient. The Cu/WC/Graphene coating showed a relatively higher coefficient of friction than the Cu/Graphene composite coating during wear test. We suggested that the graphene sheets and WC particles acted as nanoscale asperities and because of increased surface roughness and existence of submicron WC combined affect higher coefficient of friction was obtained. Since the 12

ACCEPTED MANUSCRIPT graphene dispersion has highest effect to increase surface roughness, using additional WC particles with graphene as hybrid reinforcement, it expected to reveal predominantly abrasive

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wear associated with increased shear forces and three-body abrasion. This causes to yield

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higher interfacial mechanical stress concentration at the sliding interfaces [34]. For the

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Cu/Graphene coating, the reduction in coefficient of friction could be ascribed to the lubrication effect of graphene sheets. Moreover, the nano sheets are multilayered reinforcements and it is easy to bend and tilt the graphene sheets toward the surface of

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composite coating, thus we suggested that the decrease in friction coefficient could be

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attributed to the flattening effect of graphene sheet asperities by sliding multilayered graphene nano sheets leading solid lubrication [35, 36].

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Fig. 7 presents the worn surfaces of the tested coatings at low magnification. It can be seen

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that due to low hardness of the pure copper the worn surface exhibits heavy plastic deformation associated with scuffing (Fig. 7a).Therefore, it produces high friction coefficient.

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Although the high surface roughness (6.23 m), introducing graphene by EPD deposition resulted in decreasing the surface damage and smoother worn surface is seen (Fig. 7b) which

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evidenced that graphene nanosheets both help to increase load bearing and self-lubrication. In the Cu/WC composite worn surface, the wear debris are evidenced that abrasive wear took place and most probably wear tests can result to create a mechanically mixed layer composed with copper matrix, WC particles and wear products eroded from the counterface ball (Fig. 7c). EPD of graphene together with WC particles resulted in decreasing the amount and also size of the wear debris which implies that graphene again behaves a good self-lubrication additive (Fig.7d). However, small grooves along with the sliding direction evidence that the wear mechanism is predominantly abrasive. In order to investigate wear track wear debris and the wear mechanisms of Cu coating, Cu/Graphene, Cu/WC and Cu/WC/Graphene composite coating tested at constant sliding 13

ACCEPTED MANUSCRIPT conditions, were characterized by SEM at higher magnifications and analyzed with EDS (Fig.8).As seen in Fig.8a, the worn surface of pure copper coating exhibits grooves, which

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were formed by plastic deformation, stem from the adhesion between the copper coating and

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counterface. The Cu/Graphene coating exhibited a relatively smooth worn surface (Fig.8b).

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Moreover, the wear produced a lot of flake debris, which further confirmed the existence of surface exfoliation. The EDS analysis for the wear track of Cu/Graphene coating showed existence of C, O elements. Meanwhile, the wear debris collected from the worn surface of

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Cu/Graphene coating analyzed with EDs and shown both C and O together with copper.

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Based on the morphology and EDS analysis (Fig. 8b), the debris produced during the wear of Cu/Graphene composite be formed mixtures of Cu and Graphene, which indicated the formation of a transfer layer. As seen in Fig8c, high magnification worn surface analysis

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showed that WC particles are well penetrated to the surface and behave as good load bearing components. However, due to hard nature and good dispersing effect of WC and well

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dispersion effect, the wear mechanisms seem to be occurred firstly plastic flow of the copper matrix followed by crack formation after deformation hardening and most probably following

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fatigue crack that will cause delamination. Because of the low deformation ability of the Cu/WC and the absence of the self-lubrication additive, several micro cracks were produced perpendicular to the sliding direction. It is normally expected that these crack will cause delamination type surface damage. In the Fig. 8d, high magnification worn SEM surface is indicative that WC addition with WC particles resulted in rough surfaces. These rough surfaces are caused to from because of the graphene sheets that resulted in increasing the surface roughness due to wrinkled nanosheets during EPD. WC particle still remained on the worn surface and the SEM investigation at further high magnification showed that graphene nanosheets covered on the worn surface with graphene islands and protect the surface very well which was also proved by different authors [36-38]. In Fig. 8e, a representative high

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ACCEPTED MANUSCRIPT magnification worn surface micrograph is presented showing not only the lubrication effect of the graphene sheet but also WC decoration on the or between the graphene nanosheets.

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Fig. 9 shows the Raman spectra obtained from the worn surface of Cu/WC/Graphene coating

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after wear test. Comparison with the Cu/WC/Graphene composite coating before wear test

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shows the worn surface exhibited lower Raman characteristic peaks (D, G and 2G peak) of graphene suggesting that graphene is converted to a disordered graphitic structure due to

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modifications during sliding. Besides, Raman spectrum showed a broadening of D, G peak on the worn surface, which was associated with the stress and structural change of Graphene

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sheets during wear test [39] Moreover, the peaks intensities of the WC particles decreased. It can be due to separation of WC sub-micron powders after sliding.

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Fig.10 shows the worn out section of the counterfaceAl2O3 ball. A qualitative assessment can

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be made for the effect of nanocomposites on the wear of the balls for the Cu/Graphene, b)

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Cu/WC and c) Cu/WC/Graphene deposited on the copper coated substrate by electrophoretic deposition after wear test. From Fig. 10a it is clear that the wear damage is minimum on the ball tested against Cu/Graphene nanocomposite and shows fine grooves which believed

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formed by very fine debris from the Cu/Graphene interface. Introducing only WC particles into Cu matrix resulted in producing large and deep grooves which is believed to form by WC particles that create ploughing and wedging on the ball surfaces (Fig. 10b). Since a single load bearing ability is not enough for the nanocomposites like Cu/WC composites for possible electrical contact purposes we have introduced additionally Graphene nanosheets to protect the tribological couples from the wear. In Fig. 10c the surface of the counterface ball is shown tested against the hybrid nanocomposite of Cu/Graphene/WC. As it was illustrated in the wear mechanisms analysis, the introducing Graphene nanosheets resulted in self-lubrication together with load bearing capability since the WC both attached well on the surface of copper and also anchored on the and between the graphene nanosheets. These causes to 15

ACCEPTED MANUSCRIPT decrease the wear rate both the copper matrix and also counterface ball [40, 41]. Therefore, the ball surface exhibit less amount of abrasive grooves and smooth surfaces.

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In order to reveal the effect of the reinforcement addition to the surface and wear damage, we

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have used 3Dsurface profilometry scanning of the worn surfaces after sliding tests. From the

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Fig. 11a, it is indicated that worn surface of the pure copper exhibits large and deep worn tracks compared with the composite coatings. Smearing the wear products and/or scuffing is

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also evident inside the wear track that is consistent with the SEM worn surface micrographs. EPD Graphene nano sheets on the copper coated substrate resulted in revealing very high

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surface roughness associated with high porosity (Fig. 11b). Graphene sheets can easily moves into the interface of the worn surface creating a solid lubricating layer and forms a transfer

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film during the wear test with a good dispersibility and prevents the direct contact between

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asperities [42]. On the other hand, the presenting of WC particles increases the microhardness of which makes copper matrix graphene more resisting against deformation. Therefore, the

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lubricating layer containing graphene can bear higher shear forces and cannot be split easily, resulting in the reduction of friction and wear. The 3Dprofilometreyanalysisfor the Cu/WC

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composite is presented in Fig. 11c and presents deep grooves and heavy surface damage. In the 3Dprofilometry analysis belonging to the Cu/WC/Graphene composite (Fig 11d), it is obvious that the abrasive wear damage is decreased. We suggested that WC particles do not easily form transfer layer during wear test, but the addition of graphene sheets on the coppercoated substrate creates smoother worn surfaces. Since the WC particles decorated between the graphene nanosheets and it is believed that graphene wraps the WC particles and this is attributed to less amount of wear and decreased abrasive groove formation. Our experimental results have shown thatCu/Graphene and Cu/WC/Graphene coatings prepared by EPD are good candidates with excellent tribological properties if applied on the conventional electrical contact materials or may be some MEMs devices to improve tribological performances. 16

ACCEPTED MANUSCRIPT 4. Conclusions The pure copper and Cu/WC, Cu/Graphene and Cu/WC/Graphene composites have been

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successfully prepared on the copper coated substrate by the combination of electroless coating

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and electrophoretic deposition. Morphological studies showed that the produced graphene

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sheets and WC particles composite coating showed uniformly distributed reinforcements in the copper matrix and WC particles strongly anchored on the surface of graphene sheets. The

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result of wear tests have shown that both the Cu/Graphene and Cu/WC/Graphene coatings exhibited excellent wear performance under dry sliding conditions. Cu/Graphene

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nanocomposite exhibited very low friction coefficient because of excellent self-lubrication effect of the graphene. Compared with the Cu matrix, Cu/graphene nanocomposite has shown

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approximately two fold decrease in the wear rate and almost 4 fold decrease in the friction

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coefficient. It was because graphene transfer layer formed on the worn surface of the Cu/Graphene and Cu/WC/Graphene composite coatings during wear test and provided self-

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lubrication. In spite of high porous structure, the increase wear resistance of the Cu/Graphene composite was attributed this self-lubrication and also excellent mechanical strength of the

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graphene. Because of the improved load bearing and self-lubricating effect, all the nanocomposites exhibited lower friction coefficient compared with pure copper deposited layers. The Cu/WC/Graphene composite coatings with excellent tribological properties were assessed as good candidates for possible contact materials for MEMs devices References [1] C.P. Wu , D.Q. Yi , J. Li , L.R. Xiao , B. Wang , F. Zheng, Investigation on microstructure and performance of Ag/ZnO contact material, J. Alloys. Compd. 457 (2008) 565–570. [2] L. Yang, L.Ran, M. Yi, Carbon fiber knitted fabric reinforced copper composite for sliding contact material, Mater, Design 32 (2011) 2365–2369 17

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TablesCaptions

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ACCEPTED MANUSCRIPT Table 1Composition and deposition parameters of the plating bath for electroless copper coating

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Table 2Deposition parameters of Cu/WC, Cu/Graphene and Cu/WC/Graphene for

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ACCEPTED MANUSCRIPT Table 1Composition and deposition parameters of the plating bath for electroless copper coating

Temperature(°C)

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Reduction Solution (B) Formaldehyte 3g C2H5OH 10 ml H2O 100 ml

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Copper Solution (A) CuSO4 5H2O 5 (g/100ml) NaOH 3.5 ml/100ml pH 11-12 Plating time (h) 1 25-30

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Table 2Deposition parameters of Cu/WC, Cu/Graphene and Cu/WC/Graphene for

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Voltage (V) 300 100 150

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Electrophoretic deposition

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Time (min) 30 30 30

Solution Acetone Acetone Acetone

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Fig. 4 Raman spectra of a) Cu/Graphene b) Cu/WC c) Cu/WC/Graphene Fig. 5Surface SEM images of: a) pure copper, b) Cu/ Graphene, c) Cu/WC and d)

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Cu/WC/Graphene

Fig. 6. Wear behavior of pure Cu and Cu composite coatings: a) the wear rate, b) friction

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Fig. 7 SEM worn surface micrographs of the coatings: a) pure copper, b) Cu/Graphene, c)

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Cu/WC and d) Cu/WC/Graphene

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Fig. 8 High magnification SEM worn surface micrographs of the coatings: a) pure copper, b)

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Cu/ Graphene, c) Cu/WC, d) Cu/WC/Graphene and e) higher magnification worn surface of

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Fig.9. Raman spectra of Cu/WC/Graphene coating after wear test Fig.10 SEM worn surfaces of the counterfaceballs of a) Cu/ Graphene, b) Cu/WC and c) Cu/WC/Graphene

Fig.11. 3Dprofilometry results of the coatings a) pure copper, b) Cu/ Graphene, c) Cu/WC and d) Cu/WC/Graphene

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Fig 1 Set of electrophoretic deposition of graphene sheet and WC particles solution

Fig.2Zeta potential of a) Graphene sheet b) WC particles in acetone

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Fig 3 XRD patterns of composite coating produced by electrophoretic deposition.

Fig. 4 Raman spectra of a) Cu/Graphene b) Cu/WC c) Cu/WC/Graphene

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Fig. 5Surface SEM images of: a) pure copper, b) Cu/ Graphene, c) Cu/WC and d)

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Fig. 6. Wear behavior of pure Cu and Cu composite coatings: a) the wear rate, b) friction coefficient

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Fig. 7 SEM worn surface micrographs of the coatings: a) pure copper, b) Cu/Graphene, c)

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Fig.9. Raman spectra of Cu/WC/Graphene coating after wear test

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Fig.10 SEM worn surfaces of the counterfaceballs of a) Cu/ Graphene, b) Cu/WC and c) Cu/WC/Graphene

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Fig.11. 3Dprofilometry results of the coatings a) pure copper, b) Cu/ Graphene, c) Cu/WC

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Highlights Successful electrophoretic deposition of Cu/WC/Graphene nanocomposites

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The wear properties and surface morphology of the Cu/WC/Graphene were tested

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Cu/WC/Graphene composites are good candidates as contact materials for MEMs

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devices.

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