WC hybrid nanocomposites produced by electroless co-deposition

Journal of Alloys and Compounds 654 (2016) 185e195

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Structural and sliding wear properties of Ag/Graphene/WC hybrid nanocomposites produced by electroless co-deposition rul Çetinkaya Mehmet Uysal*, Hatem Akbulut, Mahmud Tokur, Hasan Algül, Tug Sakarya University Engineering Faculty, Department of Metallurgical & Materials Engineering, Esentepe Campus, 54187, Sakarya, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 May 2015 Received in revised form 17 July 2015 Accepted 30 August 2015 Available online 4 September 2015

The main objective of this work has been the deposition of hybrid silver/WC/Graphene nanocomposites and characterization of their tribological behaviors. Graphene as a conductive solid lubricant additive was introduced into Ag matrix from the electrolytes in which submicron WC particles and Graphene nanosheets were suspended. The main purpose for two different reinforcements is to improve both wear and friction properties. The friction and wear behaviors of Ag/WC/Graphene coatings on the metal substrates against M50 steel ball were tested under dry sliding wear conditions. Comprehensive characterizations were performed using Scanning Electron Microscopy, X-Ray Diffraction analysis, Raman spectroscopy and 3D profilometry facilities. Tribological test results have revealed that even small amounts of Graphene addition are able to drastically improve the antifriction and antiwear properties of hybrid nano Ag matrix composites. A possible explanation for these results is that the co-deposition of Graphene not only provides an enhanced effect for nanocomposites to produce better wear resistance, but also forms a local protective layer on the contact surfaces to reduce the friction. The investigation shown that hybrid reinforcements of sub-micron WC and Graphene hold great potential applications as effective load bearing and solid lubrication for Ag matrix composites and possibly for similar alloys. © 2015 Elsevier B.V. All rights reserved.

Keywords: Sliding wear Metal-matrix composite Hardness Electrical contacts Wear testing

1. Introduction Electrical contacts provide electrical connection and often perform other functions. The primary purpose of an electrical connection is to allow the uninterrupted passage of electrical current across the contact interface [1]. Electrical contact materials are widely used in different low voltage switch devices, such as relays, contactors, circuit breakers and switches, and their properties are of importance to the switching capacity, reliability, stability and service life of integral electrical systems. The materials used for electrical contacts in these applications have to be considered to keep the efficiency of the contact in term of electrical conductivity, thermal conductivity and high wear resistance [1e3]. Many kinds of contact materials have been used in severe condition. In most cases, the development of contact materials is accomplished by experimental selection of the composition of metal matrix composites that can resist to wear and have the required electrical

* Corresponding author. Tel.: þ90 5554223435, þ90 2642955795; fax: þ90 2642952601. E-mail address: [email protected] (M. Uysal). http://dx.doi.org/10.1016/j.jallcom.2015.08.264 0925-8388/© 2015 Elsevier B.V. All rights reserved.

characteristics. The development of novel materials with excellent mechanical and electrical as well as corrosion resistance properties is of great importance for high efficiency and reliability of future electrical contacts [4]. Recently, among novel materials, silver and silver alloys are often chosen because of significant electric conductivity and oxidative stability. Silver based materials suggested in using electrical contacts must have a good combination of electrical conductivity, wearing qualities, and resistance to erosion and welding. Otherwise, the contacts will erode, causing poor contact and arcing [5e7]. Hence, it is a challenge for material researchers to develop a new material instead of Ag and Ag alloys contact materials. So far, many silver based composite contact materials, such as Ag/ Graphite, Ag/WC and Ag/CNT have been developed. For example, Wang et al. [8] reported on an Ag/Graphite/CNT electrical contact composite material that was synthesized by powder metallurgical method. They suggested that Ag/Graphite/CNT electrical contact material, with the increase of graphite content, exhibits good selflubricating property and wear-resistance but electrical conductivity, thermal conductivity declined. Grand in et el [9]. investigated the production of silver-graphite composite materials for sliding electrical contact applications with the aim to optimize tribological

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and electrical properties. Such composites exhibited improved mechanical, physical and chemical properties such as lower density, good mechanical strength, oxidation resistance, increased high-temperature performance limits and improved wear-abrasion resistance, depending on the properties of the metallic matrix and those of the reinforcing phase as well [10]. The use of tungsten as a reinforcement material for silver matrix has been studied by several researchers [11e13]. Tungsten, being a refractory metal, provides some degree of wear and arcing resistance when used with silver as an electrical contact material. However, for electrical contacts and similar applications, materials having good electrical conductivity and wear resistance along with lower density are desirable. Due to its lower density, tungsten carbide in place of tungsten has been used as reinforcement for silver based composites for electrical contact applications [12,14]. There are a lot of advantages in using WC as the reinforcement in silver based composites. It has a lower density (15.63 g/cm3) as compared to tungsten (19.3 g/cm3) and WC retains its room temperature hardness up to 1400  C. Its wear resistance is better than that of wear-resisting tool steels. WC undergoes no phase changes during heating and cooling and retains its stability for very long service times at high temperatures [12,15]. Compared with carbon nanotubes, Graphene with a plate shape is easier to handle and disperse in solvents or all kinds of matrices [16]. Therefore, we are confident that Ag/WC/Graphene has a good potential to replace conventional Ag material as a candidate for the next-generation MMCs for contact materials, specifically for MEMs. Graphene has received considerable attention in recent years due to its superior properties, if compared to conventional materials. Graphene, as the perfect two-dimensional (2-D) lattice of sp2bonded carbon atoms, has recently attracted tremendous attention because of its unique properties such as high Young's modulus, high fracture strength, and thermal conductivity, unique thermal, mechanical, and electrical properties and it is expected to be one of the emerging self-lubricating materials [16e18]. In comparison with polymers and ceramics, Graphene-based metal matrix composites have been little researched. Most of the existing reports focus on the deposition of nanoparticles of noble metals and oxides on the surface of Graphene to impart new functionalities, such as catalytic, energy storage, photocatalytic, sensory and optoelectronic [18]. However, to best of author's knowledge, there has been no report on the fabrication of Ag/WC/Graphene composite materials. In this paper, Ag/WC/Graphene composites were prepared for the first time by electroless silver coating on copper substrates. Structural, mechanical properties such as the wear properties and hardness of the Ag/WC/Graphene composites were tested for developing new generation contact materials for possible future MEMs/NEMs applications.

Graphene oxide, 30 mg of the graphite oxide particles was added to 100 ml distilled water, and the Graphene oxide sheets were separated from the graphite oxide structure using an Ultrasonic Processor (UP400S) at 20 kHz, with a power of 60 W for 2 h. Finally, the reducing agent hydrazine hydrate (1 ml) was added and the solution was heated on water bath to 85  C for 24 h. Graphene was gradually precipitated out as a black solid, which was filtered and washed repeatedly with distilled water.

2. Experimental

The morphology and microstructures of the composite coating were characterized by a scanning electron microscope (SEM, model JEOL e JSM 6060 LV). XRD analysis was performed with X-ray

2.1. Synthesis of graphene Natural graphite flakes were exfoliated according to the procedure given in the paper by Çetinkaya et al. [16], in which the details were described. In brief, Graphite oxide (GO) was obtained from graphite flakes (Alfa Aesar, 100 mesh in size) using the method described by Hummers. The graphite particles were pretreated to activate their surface 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  C for 120 s in an open air atmosphere. After the pretreatment process, the Hummers method was used to synthesize graphite oxide. To prepare the

2.2. Electroless deposition process The Ag, Ag/WC, Ag/Graphene and Ag/WC/Graphene composite coatings were deposited on 5 cm  5 cm  0.5 cm copper plates by using electroless coating technique. The composition and deposition parameters of the bath are listed in Table 1. All the chemicals used in the pretreatment and electroless plating procedures are of analytical purity (>99%). The copper plates were mechanically polished with different abrasive papers in order to obtain a smooth, bright and uniform surface and then the copper substrates were activated in 25% H2SO4 solution for 2 min. After preparation of the solution, specimens were put into the electroless plating bath. The specimens were immersed in the electroless plating bath, which contained silver salts solution (A), glucose-based 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 just before the bath was applied for electroless coating. AgNO3 (99.8%) and ammonia were used for preparation of Ag solution. Transparent Ag solution was first prepared by dissolving the AgNO3 in the water with addition of the aqueous ammonia, then NaOH was added to the solution, finally several drops of aqueous ammonia were added until the Ag (NH3)þ solution became transparent again. After preparation of the solution, copper plates were immersed into the electroless plating bath. Plating time kept constant at 2 h for each electroless coating run. The Ag/WC, Ag/Graphene and Ag/WC/Graphene composite coatings were obtained from the bath solution containing the WC particles (10 g L1)and Graphene (100 mg L1), respectively. The range of WC particle size used in the experiment was 0.1e1 mm. The amount of surfactant, CTAB (cetyltrimethylammonium bromide) was fixed at 300 mg L1. Prior to composite coating the bath solution was stirred using a magnetic stirrer at 600 rpm for about 12 h, and subsequently ultrasonic agitation was conducted using an Ultrasonic Processor (UP400S) at 20 kHz, with a power of 60 W for 30 min in order to prevent agglomeration of WC particles and Graphene and also providing suspension of reinforcements in the electrolyte. 2.3. Characterizations

Table 1 Composition and deposition parameters of the plating bath for Ag, Ag/WC,Ag/Graphene and Ag/WC/Graphene composite coating. Silver solution (A) AgNO3 NH3$H2O NaOH pH Plating time (h) Temperature ( C) WC Graphene

Reduction solution (B) 5 (g/100 ml) 3.5 ml/100 ml 4 (g/100 ml) 11e12 2 25e30 10 g L1 100 mg L1

Glucose C2H5OH H2O

3g 10 ml 100 ml

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diffractometer (Rigaku D/MAX/2200/PC model device) using CuKa radiation (l ¼ 1.54050 Å) with 1 /min scanning speed using a grazing angle of 5 in the 2q range between 10 and 90 . Microhardness tests of the samples were measured from the crosssections of the composite coatings with a Vickers microhardness indenter (Leica VMHT) with a load of 10 g for 15 s. At least five measurements were performed for each sample and the average values from the five replica tests were reported. The reciprocating tribological behaviors of the coatings sliding against a M50 steel ball with 10 mm in diameter was examined on a Tribometer (CSM Instruments) designed 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 and 60% at a constant applied load of 1.0 N. All the samples were cleaned by ultrasonically washing in acetone before and after each test. The experimental parameters were: frequency 10 Hz and stroke 2 mm. At least, three replicate friction and wear tests were carried out for each specimen and the average was reported with their errors. The sliding speed and the sliding distance in reciprocating 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 profilometer, KLA Tencor P6. 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 ¼ V=ðF  SÞ




.  mm3 Nm

These measurements were also compared with the vertical transducer depth profiles, and thus, the wear rate of the pure Ag and its composites were determined. Raman spectroscopy tests were also performed to show the existence of the different components on the wear surfaces. 3. Results and discussion 3.1. Microstructural results Fig. 1a shows the XRD patterns of the pure silver, Ag/WC, Ag/ Graphene and Ag/WC/Graphene electroless composite coatings.

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The XRD pattern of Ag shows a strong peak at 2q ¼ 38.4 and 2q ¼ 44.6 , 2q ¼ 64.7, 2q ¼ 77.7, 2q ¼ 81.8 , corresponding to (111), and (200), (220), (311) and (222) reflection peaks, respectively. The XRD pattern of the Ag/WC composite coating shows strong peaks for WC at 2q ¼ 31.8 , 35.6 and 48.5 , corresponding to (001), (100) and (101) planes, respectively. In the Ag/Graphene XRD patterns, the reflection peaks do not show significant difference from those of the pure Ag deposited layer but some peak broadening was detected since Graphene causes grain refinement in the Ag matrix. There is no sign referring the Graphene peak since the deposited amount of Graphene is lower than 5. wt %. By comparing the XRD patterns in Fig. 1a, it can be observed that dispersing the WC particles has a profound effect on the preferred orientation of silver matrix growth whereas graphene has negligible silver growth orientation difference. Contrary to our results, Kuang et al. [18] reported that Ni could grow on the graphene sheets after reduction during the electrodeposition process. Therefore, it can be concluded that Ag cannot grown on the graphene sheets rather, graphene penetrated between the Ag grains during electrodeposition. Thus, in the case of pure Ag and Ag/Graphene coatings the peak intensity ratios of the (111) and (200) diffractions are correspondingly higher than the intensities belonging to the Ag/WC and Ag/WC/Graphene composites, which is an evidence that introducing WC resulted in growing the Ag films in polycrystalline manner. The stress build-up in Ag lattices owing to WC and Graphene dispersion was quantified by determining the broadening and shifting in the Ag(111) peak obtained by grazing incidenceX-ray diffraction and shown in Fig. 1b. It is reported that the formation of fine grain and a high density of defects caused by a large strain in the particles for the mechanically alloyed powders [21,22] and nanograins [23] deposited as thin films. As the Ag layers were deposited with the introduction of WC and Graphene, peak broadening, represented by the full width at half maximum (FWHM) of the Ag (111) peak in Fig. 1b, provided a clear evidence of the build-up of microstrains in Ag lattices. There are also indication for macrostresses because there is noticeable shift in the Ag (111) peak. The lattice microstrain was calculated from the FWHM of the Ag (111) peak and presented in Fig. 2. The lattice strain and the mean grain size of silver for pure silver, Ag/WC, Ag/Graphene and Ag/WC/Graphene coatings were calculated from XRD data taking into account CuKa1 radiation after Ka2 stripping using the WilliamsoneHall method [24] and the results are presented in Fig. 2. The instrumental broadening was

Fig. 1. a) XRD patterns of the electroless composite coatings and b) detailed (111) diffraction of silver.

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Fig. 2. Variation of lattice strain and grain size in Ag Ag/WC, Ag/Graphene and Ag/WC/ Graphene coatings.

determined using a Si standard (provided with the diffractometer), and subtracted from the experimental breadth to obtain the “physical” broadening of each diffraction peak which was then used for the WilliamsoneHall calculations. The lattice strains and grain size were obtained by least squares fitting using the Unit Cell program (Jade7.5 Search-Match-Smooth). Dispersing WC, Graphene and WC/Graphene into the silver matrix, significant reduction in grain size and increase in lattice strain were observed and this may be attributed to a severe lattice distortion and grain size refinement. For pure Ag and Ag/WC, their strain lattices are 3.1  103 and 3.6  103, respectively. By dispersing graphene into the silver matrix, lattice strain increases to 4.21  103. Ag/WC/ Graphene composite coating has higher strain lattice value (4.84  103) compared with other nanocomposite coatings. Since the XRD analysis did not show any reflection referring to the Graphene in the Ag/Graphene and Ag/WC/Graphene coatings, Raman spectroscopy analysis was performed and the results are shown in Fig. 3. As can be seen from Fig. 3, the peaks observed at 215 cm1, 340 cm1, 815 cm1 and 887 cm1Raman shift values show the structure of nano WC powders [25] and when the Graphene nano sheets were dispersed in the composite, new two

Fig. 3. Raman spectra of Ag/WC and Ag/WC/Graphene coatings.

peaks are observed at 1325 cm1 and 1569 cm1as Raman shift values corresponding to D and G bands of Graphene, respectively [19,26]. The morphological features of all Ag and Ag composite coatings were characterized by SEM. The surface morphology of the electroless pure silver coating, Ag/WC, Ag/Graphene and Ag/WC/Graphene composite coated samples are shown in Fig. 4. The typical smooth surface morphology of electroless silver coating can be clearly observed in Fig. 4a. The deposition with nodular morphology is the characteristic feature of electroless deposition of silver, which happens by silver nucleation at isolated points and then growth in lateral directions [22]. It is observed from the Fig. 4b that the surface morphology of the Ag/WC composite shows uniform, bright, smaller sized grains with less hillocks structures. Moreover, the Ag/WC composite microstructure revealed that the WC particles incorporated on to the coating surface during coating, hinders the crystal growth and increase nucleation sites for reduction of Ag ions, which results in a fine grained and intact arrangement of Ag crystals in the composite coating [27,28]. However, in Fig. 4c and d the surfaces of the Ag/Graphene and Ag/ WC/Graphene composites change significantly with high roughness. There is a considerable amount of porosity in the as deposited Ag/Graphene and Ag/WC/Graphene composite structures, which is an indication of partial permeability of Graphene for ions to deposit on substrate. It has been shown that the Graphene layers interact with each other to generate an open pore structure, which provides an easy path for the insertion and extraction of electrolyte ions through the Graphene surfaces [20]. It can be found some endwise sheets penetrated into the coated layer shown from the Fig. 4c and d on the surface of the composites. These endwise dispersions are signs of homogeneous dispersion of reduced Graphene sheets in the silver matrix [20]. As shown in Fig. 4c and d, the Ag/Graphene and Ag/WC/Graphene composite coatings, the layers of the Graphene sheets dispersed between the silver depositions resulted in decreasing the grain size of the silver matrix structure. The cross-sectional micrographs of the deposited materials are shown in Fig. 5. As can be seen from the Fig. 5, the coating thicknesses are between 4 and 6 mm in the deposited materials. The cross-section of the electroless pure silver coating exhibit a smooth surface with a dense deposition structure and good continuity between substrate and coating (Fig. 5a). As can be seen from the Fig. 5b, Ag/WC composite coating reveals higher surface roughness compared with pure silver coating and homogenous distribution of WC particles. With an excellent interface continuity. However, dispersing only Graphene nanosheets into the Ag matrix yields rough surface and some voids between coating and the substrate (Fig. 5c). Since the soft Ag smeared into the porosities during polishing of the sample, it is not possible to observe porosities. In the Ag/WC/Graphene composite cross-sectional micrographs, surface roughness and some porosities are visible with a good interfacial continuity (Fig. 5d). The higher magnification SEM image presented in Fig. 6a shows the more obvious curled sheets which are typically Graphene sheets. EDS analysis from Fig. 6b also proves the existence of Graphene in the composites. The presence of Ag in the EDS spectrum indicates that Ag ions and WC nanoparticles could be reduced on the conductive Graphene surfaces. Using sonication during composite coating deposition was found to be vital in the production of homogenous Ag/WC/Graphene coating. When using a magnetic stirrer to disperse the electrolyte without using sonication resulted in the formation of coatings with microstructures containing large, poorly dispersed Graphene nanosheet agglomerates. The coatings also featured very inconsistent Graphene volume fractions, with large regions of the silver matrix devoid of Graphene, in addition to very high surface roughness and porosity throughout the

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Fig. 4. Surface SEM images of: a) pure Ag b) Ag/WC c) Ag/Graphene d) Ag/WC/Graphene composite coatings.

composite coating [29]. Composite coating without sonication led to the formation of Graphene agglomerates within the electrolyte, which were then transported to the deposition surface. SEM analysis of coatings fabricated in this condition revealed that the Graphene nanosheet agglomerates adsorbed at the deposition surface acted as seeding locations for silver growth (Fig. 5c). In the present study, Graphene contained electrolytes were firstly proceed by using only magnetic stirrer and coating were carried out without sonication. Porosity and coating surface roughness were also found to be reduced by using sonication. However, a notable reduction in the volume fraction of deposited Graphene with using sonication was also observed (Fig. 5a). Using sonication of the electrolyte during composite coating reduced the issue of deposition of agglomerated

Graphene by breaking up any suspended bundles and preventing the Graphene re-agglomeration [29,30].

3.2. Hardness of the coatings The Vickers method was used to investigate the hardness of pure silver and silver matrix composite coatings. Fig. 7 shows the hardness of the Ag, Ag/Graphene, Ag/WC and Ag/WC/Graphene coatings after electroless composite coating. From Fig. 7, it can be seen that the microhardness of the Ag/WC and Ag/WC/Graphene composite coatings is higher than that of the pure Ag coating. For a pure silver coating, the hardness is approximately 79 Hv. Then, it slightly increases to 88 Hv by introducing the WC particles into the matrix. The improvement in the hardness of the composite coatings

Fig. 5. Cross-sectional SEM images of: a) pure Ag b) Ag/WC c) Ag/Graphene d) Ag/WC/Graphene composite coatings.

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Fig. 6. SEM image of a) high magnification image of Ag/WC/Graphene, b) EDS spectrum of the Ag/WC/Graphene.

is related to the dispersion hardening effect caused by WC particles in the composite matrix, which obstructs the shift of dislocation in silver matrix according to the Orowan strengthening mechanism [31]. It is clear from the data that the silver matrix composite reinforced with WC displays the highest hardness among the materials tested. Another reason in the increase of hardness is likely due to the formation of a more refined and compacted microstructure. The hardness of the Ag/Graphene nanocomposite coating (77 Hv) is lower than that of pure Ag film (79 Hv). Additionally, dispersing WC and also WC/Graphene into Ag matrix is expected to create more hardness. As also stated by Pavithra and co-workers [32], uniform dispersion and distribution of Graphene throughout the soft matrix arrests the grain growth during deposition and subsequently blocks the dislocation motion. The decrease and/or small increment in hardness can be caused by the remarkably high porosity since Graphene sheets deposited in endwise geometry and to the changed nucleation and growth mechanisms of the silver coating. The shifting nucleation and growth mechanism leads to produce significant amount of porosity which plays important role to decrease the load bearing mechanism of the silver matrix. Depending on the porous structure of the Graphene reinforced silver depositions, the load cannot be transferred on the graphene nanosheets and this limits the contribution of high strength sheet reinforcement. As indicated in the microstructural analysis Graphene nanosheets are generally located between the adjacent grains associated with localized porosity in the Graphene dispersed regions. Since the amount of the dispersed Graphene nanosheets is low, the load transferring mechanisms from the silver matrix to

Fig. 7. The hardness of composite coatings.

reinforcing Graphene cannot work sufficiently to prevent plastic deformation. Therefore, Graphene dispersion cannot contribute to increase the hardness of the composite. Similar problems were reported in different metal matrix composites and these results are in good agreement with other studies in this field [33,34]. 3.3. Wear and friction of the coatings The amount of wear for each coating was determined from the wear track measurement with 3D profilometry. Since no assessable wear and wear tracks were obtained from the ball surfaces, the wear data about the steel balls were not presented. Wear rates of the Ag, Ag/Graphene, Ag/WC and Ag/WC/Graphene coatings are presented in Fig. 8a. As can be seen from Fig. 8a, the wear resistance is the lowest in the Ag matrix coating and the Ag/WC/Graphene coated material yields highest wear resistance. By introducing both WC and Graphene into the Ag matrix material, the hybrid reinforcements with WC and Graphene coatings resulted in remarkably high wear resistance in spite of the porosity increment in the samples where Graphene was introduced. For example, the wear rate of the silver matrix is 5.5  105 mm3/Nm at 30 mm/s and reduced to 4.5  105 mm3/Nm when both WC submicron particles and Graphene nanosheets were co-deposited with Ag matrix. This refers approximately to 18% increase in the wear resistance. The improvement in the wear resistance is also obtained in the Ag/WC and Ag/Graphene nanocomposite coatings. Although very high surface roughness and porosity occurred in the Ag/Graphene and Ag/WC/Graphene composite coatings, the increment in the wear resistances to some extent, assessed as significant for the deposited nano coatings. If the surface roughness values would be equalized or smooth multilayered Ag/WC/Graphene would be produced higher wear resistance could be obtained. It is may be worthy of note that the friction coefficient of the Ag film decreased with introducing WC and Graphene. For the Ag nanocomposite coatings reinforced with WC and graphene, the friction coefficient shows a decreasing tendency. As Fig. 8b shows, the friction coefficient is decreased almost continuously with introducing WC, Graphene and WC/Graphene. Experiments showed no significant difference in friction coefficient while varying the reciprocating cycles thus, showing a steady state feature for all the deposited films. As can be seen from the Fig. 8c, introducing WC caused slightly higher friction coefficient scattering depending on the sliding cycles. Pure Ag and Ag/Graphene film showed lower scattering in the coefficient of friction compared with WC reinforced depositions. In spite of increasing surface roughness of the deposited layers reinforced with Graphene, the decrease in the friction coefficient is significant, because, normally, increased surface roughness of the materials in sliding wear results in increasing shear forces and

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Fig. 8. Wear behavior of pure Ag and Ag composite coatings: a) the wear rate, b) friction coefficient and c) variation of the friction coefficients with sliding cycles.

therefore, high friction coefficients. Hence, Liang et al. [35]. demonstrated that the friction coefficient of the Graphene Oxide film formed with electrophoretic deposition on silicon substrates increases with the increase in surface roughness and they concluded that surface roughness should be controlled. Agglomeration of the reinforcing phase and increasing surface roughness can result in formation of abrasive wear on the counterface materials during sliding wear [36,37]. However, the significant decrease in the friction coefficient of the coatings reinforced with WC and Graphene is attributed to good load bearing and self-lubrication effect of Graphene. It can be suggested that Graphene particles smear out over the asperities to prevent cold welding, which greatly reduces the friction coefficient as also obtained in the Ag/ carbon black composite coatings [38]. Graphene based coatings and nanocomposites were not only reported for the nano-scale friction of graphene. Attempts to apply graphene coatings as a solid lubricant at the macro-scale have also been performed [39]. In this current work, the effect of the Graphene addition to Ag depositions seems also significant for macro scale coatings to control and reduce friction coefficient. 3.4. Wear mechanisms Fig. 9 shows SEM micrographs of worn surfaces of the Ag and Ag/WC, Ag/Graphene and AG/WC/Graphene nanocomposite coatings tested at constant sliding conditions. The worn area of the pure Ag coating is relatively high compared to that of the WC and Graphene reinforced composite coatings (Fig. 9a). Almost the entire surface of the worn area of the pure Ag coating is damaged, while the surfaces of the nanocomposites coating show damage as partly

deformed contact points (Fig. 9bec). This means that the Ag nanocomposite coatings are damaged only on the surface of their asperities, which are the convex sections. As shown in Fig. 9a, the wear mechanism of the pure Ag coating exhibits a predominant abrasive wear. This is because of lower hardness of the Ag coating compared with steel counterbody. Since the Ag coating is a solid lubricant material which provides limited adhesion to the counterbody and this causes material removal from the Ag coating most probably by micro-cutting and/or micro-plowing mechanisms. Therefore, there is no evidence for smearing of the wear debris on the pure Ag surface because of the solid lubricant ability of Ag, which possesses excellent oxidation protection [40]. The abrasive wear character is also evidenced in 3D profilometry scanning results of the worn surfaces that are presented in Fig. 10. From Fig. 10a, it is apparent that worn surface of the pure Ag exhibits large and deep worn track compared with the nanocomposite coatings showing good consistency with SEM worn surface micrographs (Fig. 9a). The repeated reciprocating sliding resulted in increasing the contact temperature and thus increasing lubricability of Ag film. As shown from Fig. 9, Ag/WC and Ag/Graphene composite coatings exhibit a mixed mode of adhesive and abrasive wear mechanisms. When the pure Ag film worn surface is compared with the Ag/WC, Ag/Graphene, and Ag/WC/Graphene worn surfaces, shown in Fig. 9b, c and d respectively, the wear feature seems to shift from abrasive to mixed mode wear mechanism of adhesion and abrasion. Abbott and co-workers [41] have shown a similar change in wear mechanism when they introduced SiC, micro and Al2O3 nano particles into Ag coatings using electrolytic deposition. It is seen from the Fig. 9 that silver surface is smooth whilst WC and Graphene

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Fig. 9. SEM worn surface micrographs of the coatings: a) pure Ag, b) Ag/WC, c)Ag/Graphene and d) Ag/WC/Graphene.

reinforced films have worn very fine scratches and micron sized particles have gouged larger sized tracks although the amount of material is less than that removed from the pure silver film. The worn surface of the Ag/WC nanocomposite coating shows plastic deformation, fine abrasive grooves and also very fine wear debris (Fig. 9b). Small amount of needle-like wear debris is visible in the wear track of Ag/WC nanocomposite film. When the film Ag/WC slides against the stainless steel ball, micro-joints ruptured because of surface roughness induced shearing and therefore adhesion effect as also stated by Yang et al. [42] for the n-octanethiol film containing doped silver nanoparticles on silicon wafer.3D surface profilometry scanning also reveals high surface roughness and shows narrow and shallow wear track with increasing the surface roughness because of WC addition (Fig. 10b).

In the case of Ag/Graphene composite, the worn surface exhibits smoother nature compared with the other nanocomposite surfaces. There is not enough evidence on the fine wear debris and abrasive groove formation (Fig. 9c). This implies that Graphene nano sheets behave as solid lubricants. In spite of high surface roughness of Ag/ Graphene nanocomposite coating, the worn surface 3D scanning shown in Fig. 9c exhibits lower abrasive tracks. However, the Ag/ Graphene composite 3D surface structure after wear testing shows larger and deeper wear damage compared with Ag/WC composite surface. It is believed and stated in the Raman analysis that Graphene nano sheets behaves as solid lubricant and prevent to increase friction coefficient in the Ag/WC composite [43]. In the case of Ag/WC/Graphene composite worn surface investigation, the surface of this composite reveals plastic deformation

Fig. 10. 3D profilometry results of the coatings a) pure Ag, b) Ag/WC, c) Ag/Graphene and d) Ag/WC/Graphene.

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traces, groove formation and some wear product smearing on the surface (Fig. 9d). Formation of smearing is believed to occur because of high surface roughness in the Graphene reinforced coatings. In the Ag/WC/Graphene composite the wear tracks are characterized by the presence of very fine grooves along the sliding direction, which become more intense with WC reinforcement addition. It could thus be deduced that there is also third body abrasion caused mainly by the detachment of WC submicron particles, which interpose themselves between the deposit and the counter material, causing third body abrasion. In the 3D profilometry analysis belonging to the Ag/WC/Graphene composite (Fig. 10d), it is obvious that the wear damage and surface roughness were decreased. This results in providing both better load bearing and self-lubrication effect when hybrid type of reinforcement is used. Graphene based films were also reported as good protective coatings and sensing for MEMs applications [44,45]. The experimental findings presented in this work have shown that Ag/Graphene and Ag/WC/Graphene coatings are good candidates in order to decrease stiction and increase wear resistivity and improve tribological performances of the MEMs devices. EDS analysis from the worn surfaces of Ag and Ag/Graphene films shown that the O content was 8 at % and only 0.8 at%, respectively suggesting that Graphene could reduce oxidation and provide lubrication in the Graphene reinforced nanocomposite films. Indeed, it is difficult to detect the Graphene on the worn surfaces since the Graphene sheets mechanically alloyed with Ag matrix and/or embedded into the surface. For this reason, the Ag/ WC/Graphene nanocomposite coating was analyzed with Raman spectroscopy after the wear testing and the results are presented in Fig. 11. If the Raman analysis results after the wear testing are compared to the analysis before wear testing, presented in Fig. 3, it is obviously seen that the intensity of the WC peaks observed at 229 cm1, 345 cm1 and 887 cm1 decrease whereas the Graphene peaks do not show any significant difference after sliding. The new peaks detected at 498 cm1, 572 cm1 and 658 cm1 on the surface of Ag/WC/Graphene nanocomposite coating after sliding test attributed to the formation of iron oxide. The formation of the iron oxide was also reported in previous studies [46e48]. They have reported that while the peaks between 290 cm1 and 607 cm1 indicated the presence of hematite (aFe2O3), the peaks at about 660 cm1 exhibited existence of the magnetite (Fe3O4). Moreover, the reason why the WC peaks

Fig. 11. Raman spectra of Ag/WC/Graphene coating after sliding.

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intensities decrease on the Raman spectra is that separation of WC sub-micron powders after sliding. Maintaining the Graphene structure after wear testing is probably decisive to get low friction coefficient values. There are similar results reported by different authors that discuss how Graphene provides solid lubrication effect. Borkar and Harimkar [49] and Wu et al. [50] reported that even in the case of CNT reinforcing of the metallic materials, the structure of CNTs transform into the graphite or Graphene structures because of the high temperature or mechanical effect and provide self-lubrication. In this study, the typical characteristics of Graphene nano structure and microscopic studies of the worn surface both for pure Ag and Agnanocomposites reveal that the Graphene particles are indeed present on the worn surface and spread out along the sliding direction. The same experimental findings and related discussion were reported by Zhai et al. [51]. for the Ni3Al matrix Graphene nanocomposites. In order to further explore the effect of Graphene and WC on the wear mechanisms of the silver coatings, the morphologies of worn surfaces are investigated at high magnification and the results are presented in Fig. 12. We have also presented a schematic model in Fig. 13 to explain the details of the materials removal, lubrication effect of the reinforcements in the nanocomposites. As shown in Figs. 12a and 13a, the worn surface of pure silver coating exhibits only very fine grooves, which were formed by abrasive wear, emanated from the micro-cutting and/or micro plowing of the Ag matrix. Scratches mostly characterize the worn surface of the pure silver coating that shown both with the SEM worn surface (Fig. 12a) and presented model (Fig. 13a). It can be concluded that pure silver coating reveals several grooves with severe wear tracks. In the Ag/WC composite coatings, the worn surface exhibit some large grooves associated with wear debris spallation (Figs. 12b and 13b). The worn surface of the Ag/Graphene nanocomposite shown in Fig. 12c and schematized in Fig. 13c reveal nanograins of silver deposition covered by a flat layer. EDS analysis was performed for the worn surface and the result is show as an inset graphic in the Fig. 12c. As can be seen from the EDS analysis that the smooth region on the worn surface is characterized by a carbon rich layer, which is graphene layer. As mentioned before, in spite of highly porous deposition layer in the Ag/Graphene nanocomposite the wear rate and friction coefficient properties were improved remarkably because of the both dispersion strengthening effect between the silver grains and lubrication effect between the sliding surfaces. We have shown these two graphene contribution effect in Fig. 13c. The worn surface of the hybrid Ag/WC/Graphene composite shown in Figs. 12d and 13d, on the other hand, exhibits a mixed mode of wear because of the lubrication effect on some regions and abrasive wear characteristics of WC. There are some micro plowing starches, believed to be caused by detachments of the WC particles. Although the porosity of the Graphene codeposited composites layer was increased there are a number of reasons to make graphene nano plates as good lubricant additive [51]. The possible reasons are: i) Graphene nanoplates do not easily conform to surface asperities but rather spread or cover them. (ii) Graphene has a stratified structure with intrinsic strength, mobile flake shape and thus protects surfaces from becoming damaged and results in reduced wear and friction. (iii) Graphene has no transient effect in friction, that is, the friction reaches its steady state. As also stated by Nieto et al. [52,53], graphene nano sheets undergo specific folds and form a corrugated like structure. Such folding provides a source of the stress dissipation mechanisms during load bearing and results in low wear rates during sliding process. Therefore, graphene provides also good lubricating effect. Since we have no significant ball wear in the experiments,

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Fig. 12. High magnification SEM worn surface micrographs of the coatings: a) pure Ag, b) Ag/WC, c)Ag/Graphene and d) Ag/WC/Graphene.

Fig. 13. Schematic representation of worn surface: a) adhesive wear in pure Ag, b) abrasive grooves in Ag/WC, c) graphene lubrication in Ag/Graphene and d) mixed mode of wear in Ag/WC/Graphene.

graphene nano sheets have no or little adhesion to the counterface steel ball and provide a lubricating effect, resulting in the reduction of friction coefficient [54]. 4. Conclusions Pure films Ag and the corresponding composites Ag/WC, Ag/ Graphene and Ag/WC/Graphene were produced by electroless coating. Results indicated that introducing WC, Graphene and both WC and Graphene resulted in decreasing the grain size of the Ag matrix. Introducing WC and Graphene as hard and flexible reinforcements resulted in changing the nucleation and growth mechanisms of the Ag matrix. Thus, introducing reinforcement of

WC and Graphene caused more porosity and higher surface roughness in the nanocomposite coatings. Introducing WC resulted in increasing the microhardness of the deposited layer but Graphene caused lower hardness values since it increased the surface roughness and also caused associated high porosity. Wear and friction results showed that introducing WC and WC/Graphene yielded remarkably high wear resistances in spite of increased surface roughness. In spite of highly porous structure, single graphene reinforcement also caused increased wear resistance and this as attributed to graphene surface lubrication effect. Because of the improved load bearing and self-lubricating effect, all the nanocomposites exhibited lower friction coefficient compared with pure Ag deposited layers. The wear mechanisms of the pure Ag

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showed predominantly abrasive wear with fine adhesive grooves, whereas the nanocomposites exhibited mixed mode of abrasive þ adhesive type of wear. The present study showed that WC/Graphene nanocomposites are good candidates as contact materials for MEMs devices. In addition, in the present study it has been experienced that if the Ag/Graphene nanocomposites are produced in the form of multilayer coatings by controlling the Ag decoration on the Graphene sheets, this can lead to develop efficient future nano and micro contact materials for applications.

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