MWCNT nanocomposite coatings

Tribology International 98 (2016) 59–73

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

The effect of sliding speed on the wear behavior of pulse electro Co-deposited Ni/MWCNT nanocomposite coatings Gizem Hatipoglu n, Muhammet Kartal, Mehmet Uysal, Tuğrul Cetinkaya, Hatem Akbulut Sakarya University, Engineering Faculty, Metallurgical & Materials Engineering Department, Esentepe Campus, 54187 Sakarya, Turkey

art ic l e i nf o

a b s t r a c t

Article history: Received 2 October 2015 Received in revised form 31 January 2016 Accepted 1 February 2016 Available online 11 February 2016

In this study, Ni/MWCNT nanocomposite coatings were prepared from a modified Watt's type electrolyte by using pulse current (PC) plating technique. The influence of multi walled carbon nanotube (MWCNT) content in the electrolyte on the tribological properties of nanocomposites were tested by using a reciprocating ball-on disk apparatus, sliding against to M50 steel ball (Ø 10 mm). Coatings were characterized using Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis before and after wear testing. Nanocomposite coatings showed wear rate decrease between 1.31 and 1.40 times compared with pure Ni coatings. Friction coefficient decreased from 0.883 for pure Ni coating to 0.419 for the Ni/MWCNT nanocomposite because of graphitization and self lubrication effect of the MWCNTs. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Electro co-deposition Ni/MWCNT nanocomposite Sliding speed Wear rate

1. Introduction Carbon nanotubes are graphitic sheets rolled into excellent tubes and have diameters ranging from about a nanometer to tens of nanometers [1]. MWCNTs are order of 2–50 coaxial tubes of graphene sheets and they form a tube like structure [2]. Carbon nanotubes (CNTs) show excellent mechanical characteristics, such as high tensile strength and high elastic modulus [3]. Therefore, they have considered as ideal reinforcing fibers for composites [4]. With the discovery of CNTs by Iijima in 1991, many excellent properties of these materials have been reported [5]. Treacy et al. [6] and Chen and co-workers [7] have measured the Young's modulus of isolated nanotubes by measuring the amplitude of their intrinsic thermal vibrations by using TEM analysis which was 1.8 TPa and a bend strength as high as 14.2 GPa. Composite coatings by electrodeposition have been universally studied because of having excellent wear resistance [8–11]. Electrodeposition method has advantages such as being low cost and finishing in short time according to other coating methods [12]. This method has been classified as suitable method which proposes process control and effective use of CNTs for the production of CNT– metal matrix nanocomposites in the form of coatings. The most researched methods for the deposition of Ni–CNT coatings are electro- and electroless deposition [13]. Ni/MWCNT nanocomposite depositions are promising for micro electromechanical systems because of having excellent properties such as wear resistance, n

Corresponding author.

http://dx.doi.org/10.1016/j.triboint.2016.02.003 0301-679X/& 2016 Elsevier Ltd. All rights reserved.

ductility and ferromagnetism [14–16]. Nickel electrodeposition is a method which is universally used for protective and electroforming applications [17]. Chen et al. showed that Ni/MWCNTs coatings, fabricated by electrodeposition, have more wear resistance than pure nickel [18,19]. Pulse electrodeposition is a promising technique in electrodeposition of metals, alloys and the metal matrix composites with different reinforcing particles [20–22]. Yang et al. [23] showed that electrodeposition of nickel using pulse plating can provide coatings with better corrosion resistance and lower porosity. The coatings which have been produced by pulse electrodeposition mostly have a better property than those produced by a direct current electrodeposition method [24]. Although CNT based surface coatings have demonstrated the potential to improve the antifriction and wear-proof properties of certain substrates, one fundamental issue of this technique is yet not fully addressed [25]. In this study, Ni/MWCNT nanocomposite coatings were deposited on copper plates by using pulse current (PC) electro co-deposition technique. The main aim of this study is to investigate the effect of MWCNT content in the deposited coatings and reveal the effect sliding speed on the wear of co-deposited Ni/ MWCNT nanocomposite coatings. In spite of numerous studies on the tribological properties of composite coatings based on MWNCT, there is no systematic investigation on the relationship between lubrication ability and sliding speed in the Ni/MWCNT nanocomposite coatings produced with PC electrodeposition. We believe this work will contribute the relevant literature by analyzing the effect of MWCNT content in the electrolyte thus, MWCNT content in the deposited layer and graphitization effect of the sliding speed

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together with oxidation of the worn Ni matrix surfaces during sliding wear. Therefore, studies were not only concentrated on the structural changes but also investigating the solid self-lubrication and wear mechanisms. In spite of several experimental studies on the Ni/MWCNT electrodeposited coatings have been reported in the literature, to the best of authors' knowledge, there is no such comprehensive work to determine the effect MWCNT content and sliding speed on tribological properties.

2. Experimental methods Because of having hydrophobic surface, in the production of nano phase reinforced nanocomposite with co-electrodeposition Table 1 The composition of the electrolyte and the plating conditions. Nickel sulfate (Ni2SO4  6H2O) (g/L) Nickel chloride (NiCl2  6H2O) (g/L) Boric acid (H3BO3) (g/L) Sodium dodecyl sulfate (SDS) (g/L) Multi-walled carbon nanotube (MWCNT) (g/L) (D:50–60 nm, L: o10 mm) pH Temperature (°C) Current density (mA cm  2) Plating time (min) Pulse on–off time (ms)

300 50 40 0.1 1, 2, 4 5 25 0.30 120 0.5–0.5

process, MWCNTs should be dispersed uniformly in the plating solution. The MWCNTs should also be homogenously suspended in the electrolyte during co-deposition to provide stable and homogenous dispersion in the metallic matrix phase. The surface of carbon nanotubes needs to be functionalized to deposit high amount and homogeneous MWCNTs with matrix materials [26]. Therefore, to eliminate the agglomeration of MWCNTs, firstly the MWCNTs (diameter 50–60 nm, length
10 mm, supplied from Arry Nano) were purified by washing with hydrochloric and nitric acid to remove amorphous carbon particles. At the same time, this surface treatment of MWCNTs was performed to get functional groups on the MWCNT surface and also contribute co-dispersion of MWCNTs uniformly as well [20]. On the other hand, to get a colloidal suspension, surfactant was used in the Watt’s bath. In the present investigation, MWCNTs were added into the solution with the ratio of 2:1 nitric acid to sulfuric acid in a glass bottle. To get the MWCNTs suspension, solution was mixed subsequently with a magnetic stirrer at 100 °C for 1 h. Later, MWCNTs were collected on 0.2 μm filter, rinsed with distilled water and then dried at 100 °C for 4 h. The electrodeposition of the MWCNT reinforced NiMMCs was produced in a Watt's-type bath using pulse electrodeposition method. The composition of the electrolyte and the plating conditions are illustrated in Table 1. As it is noted in the literature that increasing MWCNT content to high concentrations in the bath (higher than 5 g/L) results in decreasing co-deposited MWCNT content in the nanocomposite. Possibly, this can cause to crate agglomeration of MWCNTs in the plating bath containing MWCNTs more than its the saturation

Fig. 1. High magnification SEM surface images of the deposited coatings on the copper substrates: (a) pure Nickel coating and Ni/MWCNT nanocomposite coatings with different amount of MWCNT in the electrolyte, (b) 1 g/L MWCNT, (c) 2 g/L MWCNT and (d) 4 g/L MWCNT.

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Fig. 2. EDS analysis obtained from the surfaces of the coatings: (a) EDS analysis from pure nickel coating surface and (b) EDS analysis from Ni/MWCNT coating surface with 4 g/L MWCNT concentration in the bath.

concentration [15,16,19]. Because of this behavior, we decided to limit the MWCNT concentration to the 4 g/L in the plating bath [9,10]. The concentration of MWCNTs in the plating bath was 1, 2 and 4 g/L, dispersed by ultrasonic agitation for 60 min. During deposition process, the plating electrolyte was stirred with a paddle agitator.

Copper substrate was used as cathode for Ni/MWCNT nanocomposite coatings. Copper substrates have reported in several studies to be used in pulse current (PC) plating technique of Ni/ MWCNT [27–29]. Okamoto et. al. [30] showed that copper have good adhesion properties in electrodeposition processes. Before starting to the electrodeposition, the surface of copper substrate

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was mechanically polished to a 0.05–0.10 mm surface finish with using 400, 600, and 800 grit waterproof sandpapers. Then copper substrate was cleaned and activated in acidic solution. Ni was also plated onto the copper substrate with the similar conditions of MWCNTs nanocomposites to make clear comparison. The duty cycle is described as ton/(ton þ toff), where ton is the working period and toff is the relaxation period. The current waveform of a pulse electrodeposition system can be found in our previous study [31]. Pulse method conditions were as follows: a bath at room temperature, a peak current density 0.3 mA cm  2, a frequency of 100 Hz, a pulse of the duty ratio of 0.50, and a deposition time of 120 min. Microstructural investigations were performed by SEM (JEOLJSM 6060LV instrument) equipped with an energy dispersive spectroscopy (EDS). Rigaku D/MAX/2200/PC model device was used for XRD analysis at a speed of 1 °/min and range between 20° and 90°. From the XRD pattern results, crystallite size of the matrix material was calculated. The hardness of the coatings was measured from the cross-sections of coatings by using a Vicker's micro hardness indenter (Leica VMHT) with a load of 50 g.

Wear and friction tests were performed with a reciprocating ball-on disk CSM tribometer in accordance with DIN 50324 and ASTM G99-95a standards at room temperature and at 50% relative humidity under dry sliding conditions. The counterpart was a M50 steel ball (Ø 10 mm) with a hardness of 62 Rc. 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 tests were performed at a constant applied load of 1.0 N at sliding speeds of 100, 200 and 300 mm/s. After each test, the amount of wear on the composite was calculated by measuring the wear width and depth using a 3D surface profiler (KLA Tencor P6). Each of the tests was repeated at least three times to ensure the repeatability of the obtained wear and friction values. On the other hand, to analyze the morphology of the worn surfaces Raman Spectroscopy and SEM techniques were used. Raman spectroscopy analyze of the products was performed by Kaiser RAMANRXN1 for further investigation of phase composition of products. The Raman scattering spectra of products were recorded using 785 nm invictus laser light source, a low excitation power of 5 mW.

3. Results and discussions 3.1. Microstructure of Ni/MWCNT nanocomposite coatings

Fig. 3. Raman analysis from the surfaces of the coatings for the nanocomposites deposited with various amount of MWCNT concentrations in the bath.

Typical SEM images from the surface of the pure nickel coating and Ni/MWCNT nanocomposite coatings are presented in Fig. 1. The pure nickel coating shows dense structure with pyramidal Ni grains (Fig. 1a). On the other hand, Ni/MWCNT nanocomposite coatings show modification of the Ni grains from pyramidal to spherical particles with addition of the MWCNTs into the electrolyte (Fig. 1b). Introducing the MWCNTs into the deposition bath reveals to degenerated Ni grains from spherical to the complex morphology which includes both pyramidal and spherical Ni grains (Fig. 1c and d). MWCNTs do not only modify the Ni deposition morphology at the deposited layers but also modifies the Ni grains during the MWCNT suspension into the electrolyte. This is believed that this modification emanated from the heterogeneous nucleation and growth of Ni particles by MWCNTs. Because of the functionalization of MWCNTs prior the coating process Ni grains deposit on the defect areas of the MWNCTs. This

Fig. 4. (a) XRD patterns of pure Nickel coating and Ni/MWCNT composite coatings with 1 g/L, 2 g/L and 4 g/L MWCNTs in the electrolyte. (b) High resolution drawing of the (111) peaks for the deposited materials showing shifting and peak broadening for the peak (111).

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Fig. 5. Cross-sectional image of Ni/MWCNT nanocomposite coating with 4 g/L in the bath.

Fig. 6. Microhardness (GPa) variation of pure Nickel coating and Ni/MWCNT nanocomposite coatings with 1, 2 and 4 g/L MWCNT concentration in the bath.

causes to get Ni decorated grains during the suspension period of the MWCNTs and then the Ni decorated MWCNTs subsequently tended to deposit on the copper substrate as cathode surfaces. Because of this particle decoration of Ni on the MWCNTs, the porous structure has been formed on the copper substrate as cathode electrode surfaces. In spite of some regions, show individual CNTs, the microstructure exhibits well distributed CNTs producing a network, which yields covering of the Ni grains by MWCNTs (Fig. 1c). SEM micrographs imply a well-attached MWCNT structure on the individual Ni grains which will be covered by Ni grains with initiating the deposition. It is especially

more visible that increasing the MWCNT concentration leads to get a well-defined network structure (Fig. 1d). The samples which are pure nickel coating and Ni/MWCNT nanocomposite coating with 4 g/L MWCNT concentration in the bath were placed to the EDS specimen holder and EDS analyzes were performed. The EDS analyzes of the pure nickel coating and Ni/MWCNT nanocomposite coatings, deposited with varying content of CNT in the electrolytic bath (4 g/L) are presented in Fig. 2. It can be seen from the EDS analyzes, pure nickel coating contains nickel and some rare amount of oxygen which is believed to form after removing from deposition bath. Ni/MWCNT

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Fig. 7. The relationships between the sliding speed and the friction coefficient in the unreinforced pure nickel coating and Ni/MWCNT nanocomposite coatings with the constant applied load of 1 N.

electrodeposition was a practical method for preparing a Ni/ MWCNT nanocomposite coating. Fig. 4 shows the XRD patterns of the Ni coating and Ni/MWCNT nanocomposite coatings obtained from varying amounts of MWCNT content in the plating bath. The pure nickel deposition is clearly observed to be grown preferentially in the (111) and (200) planes. Copper substrate peaks are observed in the XRD analysis as well. Increasing the concentration of the MWCNTs into the electrolyte and thus increasing co-deposited MWCNTs in the deposited layers leads to increase the preferential growth of Ni matrix in the (111) plane. As can be seen from the Fig. 4 that increasing MWCNT concentration in the electrolyte resulted in a gradual increase in the intensity of the (111) plane. Another feature seen from the Fig. 4 is that introducing higher amount of MWCNTs also caused shifting 2θ angle to the right associated with peak broadening which is an evidence of grain refinement. This is also good evidence that MWCNTs cause to get heterogeneous nucleation, preferentially in the defect regions [34]. The detailed calculation of crystallite size of composite coatings was calculated from XRD patterns. The crystallite size was calculated for pure nickel and different amounts of MWCNT reinforced nickel composite coatings by using Scherrer’s equation [22]: D¼

Fig. 8. The relationships between the sliding speed and the wear rate in the unreinforced pure nickel coating and Ni/MWCNT nanocomposite coatings with the constant applied load of 1.0 N.

nanocomposite coating similarly contains nickel and excessively carbon peak. According to the EDS results, no any impurity was detected and this show there was no reaction product between the reinforcement and Ni matrix. Raman spectroscopy analyses of Ni/MWCNT nanocomposite coatings (1, 2 and 4 g/L) are given in Fig. 3. Raman spectroscopy is used to show the presence of MWCNTs in the deposited composite coatings. The spectrum shows a typical MWCNT characterized by one band with a D peak at around 1345 cm  1, a G peak at around 1585 cm  1, a peak G’ at around 2621 cm  1 and a D þG peak at around 2930 cm  1 [32]. The peak at about 1345 cm  1 is identified as D band which is a double-resonance Raman mode and can be understood as a measurement of structural disorder coming from amorphous carbon and defects. The peak at about 1585 cm  1 can be assigned as the G band originates from the tangential in-plane stretching vibrations of the carbon–carbon bonds within the graphene sheets [33]. The purity of carbon nanotubes is related to the intensities of G and D peak. These results proved that composite

0:9λ B cos θ

ð1Þ

where D is the main crystallite size of the produced films, λ is the wavelength of Cu Kα radiation (0.154 nm), θ is the Bragg diffraction angle and B is the full width at half maximum (FWHM) of the fabricated coatings in radian. The MWCNTs provides both more nucleation sites and retarded crystal growth. This behavior is resulted in a smaller crystal size for the metal matrix in the composite coatings [35]. As also stated by Pavlatou et al. [36], studies on the 2-butyne-1,4-diol addition into the electrolytes, organic additives are effective agents to increase the grain refinement and preferred crystal orientation in the Ni depositions. In the relevant the literature, it has been reported that that the addition of MWCNT particles into the Ni matrix changes the shape of Ni crystallites from pyramidal to spherical and also reduces their grain size [2,37]. In this current work, as mentioned in the XRD results, a gradual decrease in the crystallite size of Ni matrix was observed with increasing MWCNT concentration in plating bath. The finest crystallite size was observed in the electrolyte including 4 g/L MWCNT content which is about 31.5 nm whereas pure Ni matrix yielded 41 nm average crystallite size. The cross-sectional image of Ni/MWCNT (4 g/L) nanocomposite coating is presented in Fig. 5. As shown in Fig. 5, the thickness of Ni/MWCNT nanocomposite layer is 102.69 mm. In addition, it can be seen that the penetration depth is lower than
20% of the nanocomposite coating thickness from the cross-sectional image. Since a constant deposition time was used for all samples, the coating thickness of nanocomposite coatings were experienced as similar. Because of this situation, it is expected that the thickness of coatings are not effective on the tribological properties of nanocomposite coatings. The variation of microhardness of pure nickel and Ni/MWCNT nanocomposite coatings with varying MWCNT is presented in Fig. 6. It can be seen that the addition of MWCNTs to electrodeposited nickel substantially increases the coating hardness. Similar results has been found in the Cu/SWCNT nanocomposite coatings produced by Yang et al. [38]. The hardness increment by carbon nanotubes is attributed to the smaller crystallite size and higher lattice micro-strain compared with a pure Cu coating if a good adhesion was achieved between the metal and carbon nanotubes. Therefore, we have concluded that the Ni/MWCNT nanocomposite

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Fig. 9. Raman Spectroscopy analyzes carried out on the of worn surfaces of (a) Pure Ni, b) Ni/MWCNT nanocomposite coating with 1 g/L, (c) 2 g/L and (d) 4 g/L CNT concentration in the bath at sliding speed of 100 mm/s, 200 mm/s and 300 mm/s.

coatings have good adhesion between the MWCNTs and the Ni matrix [38]. Borkar and Harimkar [39] also reported the effect of MWCNTs on the grain size of the Ni matrix deposited on the stainless steel substrates. They have measured as high as 580 7 15 HV for the Ni/CNT composite compared to 320 715 HV for pure nickel coatings. 3.2. Friction and Wear behavior of Ni/MWCNT metal matrix composites The friction coefficients of pure nickel and Ni/MWCNT nanocomposite coatings with different sliding speeds at a constant load of 1N are shown in Fig. 7. The coefficient of friction is observed to decrease with increasing MWCNT content in the Ni matrix nanocomposites. This behavior can be attributed to formation of selflubricating layers between the steel counterface ball and Ni matrix composite surfaces by increased carbon nanotubes [40]. Rajkumar and Aravindan [41] who investigated tribological behavior of copper–CNT composites, reported that a carbon lubricating film can cover the wear surface and acts as a solid lubricant that decreases the friction coefficient. In addition, they concluded that carbon lubricating film thickness increased with increasing CNT ratio in the

composites. Another significant result given in Fig. 7 is that sliding speed also has a profound effect in decreasing the friction coefficient for both pure Ni coatings and the nanocomposites. The reason of decreasing friction coefficient in the Ni matrix composites with increasing the sliding speed is well documented [42]. One of the main reasons for decreasing friction coefficient is formation NiO layer because of the sliding surface temperature increment, which causes to form stable thin NiO film. The effect of MWCNTs and their content in the electrolyte bath on the wear rate of pure nickel and nickel/MWCNT nanocomposite coating is shown in Fig. 8. It can be clearly concluded that, MWCNTs markedly reduces the wear rate of composites. For instance, while the wear rate of pure nickel coating is 10.33  10  5 mm3/Nm, the wear rate of nickel/MWCNT nanocomposite coating, which deposited with 4 g/L MWCNT is 7.56  10  5 at 100 mm/s sliding speed. This result can be attributed to reinforcing effect of CNTs, which addressed in the several published studies in the microhardness data [43–45]. Bastwros et al. [46] who investigated the friction and wear behavior of Al– CNT composites, reported that the decrease in the wear rate with the increase in CNT content caused from the strengthening of the samples by the CNT reinforcements. Fig. 8 shows the wear rate of

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Delamination

Micro crack Micro crack

Wear debris Micro crack

Fig. 10. SEM images of the worn surface of pure nickel coating at sliding speeds of (a) 100 mm/s (b) 200 mm/s, (c) 300 mm/s.

pure nickel and nickel/MWCNT nanocomposite coatings under different sliding speeds as well. The wear rate is observed to decrease with the increase in the sliding speed. A similar result of decrease in wear rate with increasing sliding speed was reported by Rajkumar and Aravindan [41] and Choi et al. [47]. They have attributed this to the temperature increment at contact surfaces. As stated in the friction coefficient discussion, increasing the sliding speed resulted in increasing surface temperature and thus formation of tribo oxide, which reduced adhesion between the nanocomposite surfaces and the steel ball. Increasing the MWCNT content in the electrolyte thus the increase in the MWCNT content in the deposited layer has a profound effect in the wear rate decrease. As can be seen from the Fig. 8, for the sample that deposited with 4 g/L MWCNT in the electrolyte bath yields lowest wear rate especially with increasing sliding speed. This sharp increase in this composites can be resulted from two possible mechanisms: i) MWCNTs are excellent strengthening agents provide unique load bearing and dispersion effect, ii) increasing sliding speed provides better self-lubrication and protect the Ni surfaces to form graphene layers on the worn surfaces at high temperatures. Both of these mechanisms are ideal in these composites for MEMs applications. As can be concluded from the Fig. 8 that the wear rate decrease is between 1.33 and 1.42 by reinforcing MWCNTs. The wear rate decrease, indeed, is low than is expected. This insufficient

increment of the wear resistance is believed caused from high surface roughness with increasing MWCNT content in the deposited layer. We believe, because of the high surface roughness, the shear forces contribute material removal from the nanocomposite surfaces and the load bearing capacity cannot be activated well enough. Therefore, as shown from the work published by Carpenter et al. [48], the decrease of wear resistance is specifically enhanced when the applied normal load is increased. The variation of surface roughness parameters (Ra) in the different deposited layers were measured with 3D profilometry. MWCNT reinforced nanocomposite coatings exhibit higher surface roughness compared to pure Ni deposition. The values of surface roughness parameters for the pure Ni coating is approximately 0.218 μm. As previously shown in Fig. 1, the surface microstructures of the Ni/ MWCNT nanocomposite coatings show surface inhomogeneities due to either agglomeration of the MWCNTs or leading more porous structure. Similar to the Ni–Co/MWCNT nanocomposite coatings, therefore, higher surface roughness values are obtained in the nanocomposite depositions as 9.225, 1.665 and 2.255 μm for the nanocomposites deposited with 1 g/L, 2 g/L and 4 g/L MWCNT in the electrolyte, respectively [49]. Raman spectroscopy analysis shown in Fig. 9 was performed from the wear tracks in the Ni/MWCNT nanocomposite coatings to clarify the mechanisms responsible for the wear processes. As shown in Fig. 9a, Raman spectroscopy analysis of pure Ni coating

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Sliding direction

Sliding direction

Micro crack

Abrasive groove

Abrasive groove

Sliding direction

Micro crack

Delamination

Fig. 11. SEM images of worn surfaces for Ni/MWCNT composite coating with 1 g/L MWCNT concentration in the bath at the sliding speeds of (a) 100 mm/s (b) 200 mm/s, (c) 300 mm/s.

shows only a band at 556 cm  1 that is characteristic band for NiO. This suggests that the pure Ni coating material exhibited oxidative wear, or tribo-oxidation, since NiO was present throughout the wear track and there is no band except for NiO band. It can be said that the heat, which was generated during dry sliding wear, resulted in an increase in the interfacial temperature between the M50 steel ball counterface and pure Ni coating material. The increase in the temperature would result in the formation of a NiO film, a thermo-oxidation process [50]. The Raman spectroscopy analysis of Ni/MWCNT composite coating with 1 g/L MWCNT in the electrolyte reveals four main bands which are about at 556 cm  1, 676 cm  1, 1345 cm  1 and 1590 cm  1(Fig. 9b). While the band of 556 cm  1could be attributed to the longitudinal optical (1LO) phonon modes of NiO [51], the band of 676 cm  1 is the characteristic band of magnetite [52]. The presence of Fe3O4 at the worn surface is due to adhesive wear of the M50 steel ball counterface [50]. The peak at about 1345 cm  1 is identified as D band and the peak at about 1590 cm  1 can be assigned as the G band of MWCNT material. The Raman spectroscopy analysis of Ni/MWCNT composite coating with 2 g/L is shown in Fig. 9c and shows six main bands which are about 376 cm  1, 493 cm  1, 556 cm  1, 678 cm  1, 1345 cm  1 and 1602 cm  1. The bands of 376 cm  1 and 556 cm  1 could be attributed to the first-order transverse optical (1TO) phonon mode of

NiO. Because of this, from the analysis it can be said that there is NiO layer at the surface. While the band of 493 cm  1 is ascribed to the A1g mode, the band of 678 cm  1corresponds to the Eg mode of magnetite. In addition while the position of the D peak at 1345 cm  1 has not significantly shifted but the G peak has increased, which indicates that the structurally modified layer exhibits a more disordered structure [52]. The Raman spectroscopy analysis of Ni/MWCNT coating with 4 g/L shows the bands of 556 cm  1 that is the band of NiO. On the other hand, Ni/MWCNT nanocomposite coating shows the peak at about 1345 cm  1, which is identified D band and the peak at about 1607 cm  1 can be assigned as the G band of MWCNT material (Fig. 9d). Furthermore, it can be seen that the G peak of Ni/MWCNT coating with 4 g/L shifted as much as 27 cm  1 toward higher wavenumber. As distinct from Ni/MWCNT nanocomposite coatings with 1 g/L and 2 g/L is absence of the peak of Fe3O4 which is about 676 cm  1 and presence of the peak of 2621 cm  1 at Raman spectroscopy. This shows that there was still some M50 steel ball wear but not nearly as much as in the pure Ni composite material [50]. On the other hand, disappearing of the peak of 2621 cm  1 was observed with increasing of sliding speed. This can be attributed that the forming of graphene layer during sliding process [53].

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Sliding direction

Sliding direction

Delamination Delamination

Sliding direction

Delamination

Micro crack

Fig. 12. SEM images of worn surfaces for Ni/MWCNT composite coating with 2 g/L MWCNT concentration in the bath at sliding speeds of (a) 100 mm/s (b) 200 mm/s, (c) 300 mm/s.

When the Raman spectroscopy analysis of all samples was compared, the shifting of the G peak toward higher wave number was observed. This shifting shows increased graphitization because of the sliding wear process. This result is supported by the increase in ratio of intensities (ID/IG) which a measure of the degree of structural disorder/modification as well [54]. In addition it significates increased graphitization [53]. We think that this increase in the graphitization reduces the friction coefficient of Ni/ MWCNT nanocomposite coatings. The position of the D peak at 1345 cm  1 has not shifted for analyzes. In addition the G peak and the full width at half maximum (FWHM) of samples have increased and this indicates that the structurally modified layer presents more disordered structure. The degree of structural disorder of the MWCNT bundles and their defect density increased during laser deposition, because of that the unworn Ni/MWCNT composite ID/IG ratio increased [50]. The SEM images from the wear scars of pure nickel coating at a normal load of 1 N for a distance of 200 m are presented in Fig. 10. The worn surface of pure nickel at sliding speed of 100 shows many cracks and delamination because of coalescence of lateral cracks (Fig. 10a). The formations of crack propagation is indicative of severe wear situation, leading to poor wear resistance of pure nickel coatings. On the other hand, with increasing sliding speed, decreasing of size of crack and delamination are observed (Fig. 10b

and c). This is believed arising from surface temperature increment and thicker NiO formation, which behaves not only an adhesion decrease component but also causes to increase load bearing performance. The morphologies of wear tracks from Ni/MWCNT nanocomposite coatings are shown in Figs. 11, 12 and 13. In Fig. 11, SEM images for worn surfaces of Ni/MWCNT (1 g/L) sample represent the effect of increasing sliding speed on the characteristics of the worn surfaces. From Fig. 11a, analysis of the worn surface of the Ni/ MWCNT (1 g/L) sample indicates plastic deformation and smearing. In addition, ploughing and scratching which indicates abrasive wear were seen at the worn surface. With increasing sliding speed, Fig. 11b shows NiO film and formation of delamination results in crack propagation. Also it can be seen that the adhesion of NiO wear debris. The SEM image of Ni/MWCNT (1 g/L) at sliding speed of 300 mm/s shows more wear debris because of severe delamination. Formation of higher amount of cracks causes fine wear debris because of small sized delamination layers formed by crack propagation, breakage and adhesion of the NiO wear debris. In Fig. 12, SEM images for worn surfaces of Ni/MWCNT (2 g/L) sample demonstrate the effect of increasing sliding speed on the characteristics of the worn surfaces. Decreasing surface damage and delamination are observed with increasing of sliding speed. Fig. 12a shows wear debris smearing on the worn surface at

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Sliding direction

Sliding direction

Micro crack

Micro crack

Sliding direction

Micro crack Abrasive groove

Fig. 13. SEM images of worn surfaces for Ni/MWCNT nanocomposite coating with 4 g/L MWCNT concentration in the bath at sliding speeds of (a) 100 mm/s (b) 200 mm/s, (c) 300 mm/s.

100 mm/s sliding speed. Fig. 12b and c shows less surface damage and relatively smooth surfaces with increasing the sliding speed. When the worn surface of Ni/MWCNT (4 g/L) SEM images was compared to the worn surfaces of Ni/MWCNT (1 g/L) SEM images, the decreasing of surface damage and crack size can be observed. This is evidenced that increasing MWCNT content in the deposited layers resulted in protecting the Ni surfaces from severe wear. In Fig. 13, SEM images for worn surfaces of Ni/MWCNT (4 g/L) sample are presented. From the SEM images of pure nickel and Ni/ MWCNT nanocomposite coatings, it can be concluded that increasing the CNTs which are dispersed in the nickel matrix seems to prevent the plastic deformation during wear. Because of that the absence of severe plastic deformation, the wear resistance of Ni/MWCNT nanocomposite coatings are significantly better than that of pure nickel coatings. The uniform dispersion of MWCNTs into nickel matrix leads to significant increase in microhardness as well as wear resistance of Ni/MWCNT composite coatings [18]. In addition, decreasing of surface damage, delamination and crack size in the SEM images of samples with increasing sliding speed from 100 mm/s to 200 mm/s and 300 mm/s can be attributed to increasing graphitization effect of MWCNT in the composites which causes more self-lubrication capacity and reducing contact between counterpart and sample [45,55–58]. However, another feature seems from the previously presented Fig. 8 and the wear mechanisms is that, there is no

significant difference in the Ni/MWCNT nanocomposites deposited with different MWCNT concentrations in the electrolyte for 200 m/ s sliding speed. This similarity is believed to be caused from the wear mechanisms that are very similar in the 200 mm/s since the wear mechanism is controlled with plastic deformation in the 100 mm/s. In the sliding speed condition of 200 mm/s, the wear controlled by oxidation of Ni matrix and further increase in the sliding sped to 300 mm/s caused to get both oxidation and MWCNT lubrication controlled wear mechanisms. The SEM figure and EDS analysis of wear track at the tip of the M50 steel ball after sliding test of Ni/MWCNT nanocomposite coating with 4 g/L nanotubes in the electrolyte is given in Fig. 14 and shows two different regions have different materials transfer. It can be seen from the EDS analysis of region 1 that the tip surface involves O atoms, which can be attributed to forming of oxide layer on the surface. Besides the SEM figure of region 1 shows abrasive wear. On the other hand, the EDS analysis from region 2 shows that the surface of tip exhibits Ni, which is transferred from the nanocomposite. This result can be attributed to occurring adhesion, plastic deformation between the nanocomposite and steel ball and consequently delamination wear debris causing three-body abrasion of the ball tip. Therefore, higher amount of Ni, oxygen and carbon was detected at the region 2 compared with region 1.

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Fig. 14. (a) The SEM figures and (b-and c) EDS analyses of wear scar at the tip of the M50 steel ball after sliding test at different regions for the Ni/MWCNT (4 g/L) nanocomposite coating.

Table 2 Surface roughness values of coatings. Coatings

Surface roughness (Ra)

Pure nickel coating Ni/MWCNT nanocomposite coating (1 g/L MWCNT) Ni/MWCNT nanocomposite coating (2 g/L MWCNT) Ni/MWCNT nanocomposite coating (4 g/L MWCNT)

0.218 9.225 1.665 2.255

To make a better comparison between the wear mechanisms of the pure nickel and nickel/MWCNT nanocomposite coatings at sliding speed of 100 and 300 mm/s, the worn surfaces were scanned with 3D profilometry. The surface roughness values of samples are illustrated in Table 2 and the results of 3D profilometry analysis are presented in Figs. 15 and 16. The pure Ni coating exhibits very smooth surfaces compared with Ni/MWCNT nanocomposite coatings. When the coatings are compared, Ni/MWCNT nanocomposite coatings exhibited higher surface roughness than pure Ni coating. The value of surface roughness for pure Ni coating is 0.218 mm. On the other hand Ni/ MWCNT nanocomposite coatings higher surface roughness values. The higher surface roughness of Ni/MWCNT nanocomposites are attributed to cohering and formation secondary particles of MWCNTs in the electrolyte [59]. Fig. 15 represents the worn surfaces of the all composite coatings at sliding speed of 100 mm/s. It can be seen that there is a decreasing depth and width of the wear tracks with increasing of MWCNT ratio in the samples. It is also clear from the wear surfaces that, the wear products are produced by delamination. As also

evidenced by EDS analysis these wear debris were smeared on the surface after delamination and yielded NiO. Fig. 16 shows the worn surfaces of the all composite coatings at the highest sliding speed of 300 mm/s. As it is clear from the Fig. 16 that increasing sliding speed resulted in decreasing surface damage and plastic deformation on the wear surfaces. The significant result can be concluded that increasing MWCNT content in the deposited layer resulted in remarkably low wear tracks and also caused to obtain smooth surfaces. Since the co-deposited MWCNTs contributed the load bearing capacity of the Ni alloy. As also mentioned by the Raman spectroscopy analysis, increasing the sliding speed resulted in increasing self-lubrication emanated from both MWCNT graphitization and NiO formation. The effect of MWCNT reinforcement on the lubrication contribution was also stated in the published relevant literature [58,60]. The deposited Ni/MWCNT nanocomposites can be evaluated as good candidates for soft magnetic applications. Due to excellent anti-wear property of Ni–CNT composite coatings can be find application in the aerospace, automobile, and other industries as frictional components [40]. Because of the environmental problems of the chromium coating the studied Ni/MWCNTs can also be good alternative for high pressure valves, drill fitting, car accessories, small aircraft, and electrotechnical parts. The most promising applications for these composite coatings can be considered as MEMs parts for the micro devices [61].

4. Conclusions Pure nickel and Ni/MWCNT nanocomposite coatings with different MWCNTs concentration in the Watts bath were fabricated

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Fig. 15. 3D profilometry results of (a) pure nickel coating and (b) Ni/MWCNT nanocomposite coating with 1 g/L, (c) 2 and (d) 4 g/L MWCNT concentration in the bath at sliding speed of 100 mm/s.

and their tribological properties were investigated. Co-deposition of MWCNTs with Ni matrix caused to modify deposited Ni grains morphology from pyramidal to spherical because of altering nucleation and growth of Ni. Homogenous distribution of MWCNTs was successfully achieved in all the deposited nanocomposite coatings produced with different MWCNT concentrations in the electrolyte bath. Ni/MWCNT nanocomposite coatings exhibited higher microhardness than that of pure nickel coatings and hardness was gradually increased with increasing MWCNTs concentration in the bath. Providing the dispersion of the MWCNTs results in decreasing both both friction coefficient and wear resistance of the Ni matrix. Results showed that increasing sliding speed caused to decrease in friction coefficient and wear rate due to graphitization of MWCNTs leading graphene layers and NiO formation on the sliding surfaces. Pure Ni coating yielded a friction coefficient of 0.755 whereas the nanocomposite coating

produced with 4 g/L exhibited a value of friction coefficient as 0.419 at 300 mm/s sliding speed. Nanocomposite coating showed wear rate decrease between 1.31 and 1.40 times compared with unreinforced pure Ni coating. The improved tribological properties in the nanocomposites was attributed to the self-lubrication and excellent load bearing capability of CNTs. Increasing sliding speed caused to decrease friction coefficient and amount of wear because of MWCNT graphitization on the sliding surfaces. Graphitization was found to be significantly effective to change the wear mechanisms, specifically at the high sliding speed conditions. Increasing MWCNT content in the deposited layers because of the increasing MWCNT concentration in the electrolyte resulted in decreasing plastic deformation and severe delamination and showed smooth surfaces with low surface damage. We believe that the Ni/MWCNT nanocomposite coatings can be used for softmagnetic materials and wear resistant coatings.

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Fig. 16. 3D profilometry results of (a) pure nickel coating and (b) Ni/MWCNT nanocomposite coating with 1 g/L, (c) 2 and (d) 4 g/L MWCNT concentration in the bath at sliding speed of 300 mm/s.

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