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Rare Metals2019年第7期

Preparation and tribological behavior of electrodeposited Ni-W-GO composite coatings

Xue-Hui Zhang Xiao-Xian Li Wei-Jiang Liu Yu-Qi Fan Hao Chen Tong-Xiang Liang

School of Materials Science and Engineering, Jiangxi University of Science and Technology

School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology

作者简介：*Tong-Xiang Liang,e-mail: liang_tx@126.com;

收稿日期：18 April 2017

基金：financially supported by the Natural Science Foundation of Jiangxi Province (Nos. 20161BAB216121,20161BAB206136 and GJJ150638);the National Natural Science Foundation of China (No. 91326203);

Preparation and tribological behavior of electrodeposited Ni-W-GO composite coatings

Xue-Hui Zhang Xiao-Xian Li Wei-Jiang Liu Yu-Qi Fan Hao Chen Tong-Xiang Liang

School of Materials Science and Engineering, Jiangxi University of Science and Technology

School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology

Abstract：

Ni-W-GO composite coatings were successfully plated on 45# steel substrate by co-electrodeposition technique in a Ni-W electrolyte solution,with different contents of graphene oxide(GO)nanoparticles in suspension.The structure,phase composition and surface morphology of as-plated composite coatings were characterized by Raman,X-ray diffraction(XRD),scanning electron microscopy(SEM)attached with energy disperse spectroscopy(EDS),respectively.The hardness and tribological behavior of the present coatings were also evaluated by Vickers Hardness tester and high-speed reciprocating friction and wear tester,and the wear mechanism was discussed as well.The results show that layer-structured GO nanoparticles significantly affect the microstructure and grain size of the Ni-W-GO composite coatings.Meanwhile,GO nanoparticles embedded in NiW-GO coatings can obviously improve the hardness and wear resistance in comparison with the corresponding NiW coatings.The highest microhardness and wear resistance of Ni-W-GO composite coatings are obtained with 0.15 g·L^-1GO employing.

Keyword：

Ni-W-GO composite coatings; Graphene oxide; Electrodeposition; Microstructure; Tribological behavior; Microhardness;

Received： 18 April 2017

1 Introduction

Composite electrodeposition is a desirable new surface intensification technique for novel metal matrix composites,due to the remarkable advantages of low cost,fast deposition and the high performance of the composite coatings with.As a good environment-friendly material for promising substituting the hard chromium film,Ni-W coatings have been widely used in many aggressive environments such as aerospace,electronics,machinery,energy and marine,because of their wonderful wear resistance,stability,high hardness,low coefficient of thermal expansion,high corrosion resistance,antifriction and magnetic properties [ 1, 2, 3, 4] .

Although these Ni-W coatings have excellent properties,the need for coatings with higher performance has made researchers and scientists to further study.It was reported that the co1ntent of tungsten could strongly affect the hardness,grain size and structure of the Ni-W coating,and finally affect the coating’s performances [ 5] .Panagopoulos et al. [ 6] revealed that better mechanical properties of electrodeposited NiW alloys were obtained in coatings with grain sizes within 10-15 nm range and the tungsten content was above 30 at%.But in reality,it has many technical limitations to obtain higher tungsten content,because the solution stability and the cathode current efficiency decrease with W content of the Ni-W solid solution increasing [ 7] .Moreover,Ni-W coating also increases undesired residual tensile stress with the increase of hardness and wear resistance.Recently,it was found that the incorporation of various micro/nano-particles (such as ZrO₂ [ 8] ,A1₂O₃ [ 9] ,SiC [ 10] ,TiO₂ [ 11] ,Si₃N₄ [ 7] ,BN [ 12] ,diamond [ 13] ,B [ 14] and MWCNTs [ 15] ) into the Ni-W alloy matrix can obviously reduce residual tensile stress,and futher improve the mechanical and friction properties as well,by using co-deposition method.Additionally,researchers indicate that the influence of coating performance depends on the intrinsic characteristic of particles and their content in the matrix.

In this regard,it is well known that graphene,a new carbon material and a single 2D sp²-hybridized carbon sheet [ 16, 17] ,has attracted a lot of attention from researchers due to its excellent mechanical,antifriction and wear resistance properties.Indeed,quite a few literatures were also reported that Ni-graphene composite coatings have been successfully fabricated by electrodeposition and the embedded graphene nanoparticles have resulted in enhancing the wear,mechanical,corrosion resistance and other properties of the Ni matrix [ 18, 19] .In order to obtain graphene-based composite coatings with excellent performance,maintaining the uniform distribution in the plating solution is a major requirement.Regrettably,graphene presents difficulty in dispersion and simultaneously produces aggregation effect to influence the properties in a certain degree due to its high chemical stability and huge specific surface energy [ 20] .So,graphene is not suitable for adding into the solution easily and directly.In order to avoid these shortcomings,graphene oxide (GO),the oxidizing form of graphene including plenty of inherent oxygen-containing hydrophilic functional groups,such as hydroxyl,carboxyl and epoxy groups,has drew considerable attention.The special structure of GO can make a homogenous and stable dispersion in the plating bath,accordingly forming a uniform coating [ 21, 22] .Not only that,the same to graphene,GO-based composite coatings also have excellent mechanical and wear resistance [ 23, 24] .Hence,it would be expected that the incorporation of GO reinforcing particles into Ni-W-based alloy allows the composite coatings with outstanding performances to be obtained.In the view of above.Fan et al. [ 25] reported a Ni-W-GO composite coating with high amount of GO addition,prepared by pulsed current deposition.The results indicated that the incorporation of GO particles enables a slight increase in the mechanical and corrosion resistance of coatings.

In this paperwork,it is aimed to obtain a novel Ni-W-GO composite coating embedded with low content of GOs addition,by using direct current co-deposition technique in a Ni-W plating bath.In addition,the surface morphology,microstructure,microhardness and wear resistance of the resulted Ni-W-GO composite coatings were investigated and discussed as well.

2 Experimental

2.1 Materials

GOs were supplied by Foshan Yuhong Nano Science and Technology Co Ltd (YHNST,China),which obtained by the modified Hummer’s method [ 26] .The morphology of GOs can be seen from Fig.1.It reveals that these nanoparticles with fakes like layer structure have diameters of 1-2μm and thickness of only several nanometers.Other chemical reagents were all of analytical pure grades and prepared by deionized water.

2.2 Deposition process

Ni-W-GO composite coatings were co-deposited on plain45#steel substrates from a Ni-W bath solution embedding GOs by a direct current electroplating technique.The concentration of GOs was obtained from 0.05 to0.20 g·L^-1.Compositions of the plating bath and operational parameters of the process are presented in Table 1.Co-deposition was carried out at a current density of5 A.dm^-2 for 60 min with magnetic stirring speed of300 r·min^-1.The plating bath,using ammonia water and hydrochloric acid solution to adjust the pH of 9,was maintained at a temperature of (60±5)℃by an automatic heat control unit.The 45#steel sheet with dimensions of 30 mm×30 mm×2 mm was used as a cathode,and the high purity graphite of size 50 mm×50 mm×5 mm was applied as an anode,respectively.Prior to codeposition,the substrate was mechanically polished with different grades of waterproof abrasive papers (600-2000).Edges and corners of the substrate should also be rounded to eliminate the edge effect.The substrate was degreased with the mixtures of sodium-salt and OP-emulsifier to remove the oil impurities at a temperature of about 85℃,cleaned with heated distill water,and immersed in mixed acid to wipe off rust and dust.After being activated in 10%HCl solution for 10 s,the substrate was finally washed with deionized water and transferred into the plating bath immediately.Prior to plating,GOs was continuously stirred using a magnetic stirrer at 300 r·min^-1 for about 25 h.During the last 1 h,the suspension was additionally treated by ultrasound at room temperature to prevent agglomeration and descending.

2.3 Deposit characterization

The phase structures of the composite coatings were performed by an X-ray diffractometer (XRD,X’Pert PRO,PANalytical,Holland) with Cu Kα1 radiation (40 kV,40 mA),and the crystallite size was calculated by Scherrer equation.The surface and cross-section morphology of the composite coatings were investigated and analyzed by means of field emission scanning electron microscope(FESEM,JSM-7001F) attached with energy dispersive X-ray spectrometer (EDS).Meanwhile,a load of 0.49 N with a dwell time of 10 s was applied to measure the microhardness of the composite coatings by using a FM-ARS9000 Vickers hardness tester.The mean values of the hardness were taken as an average of five measurements.The roughness of the surface of the composite coatings was tested by a roughness measuring station (SRM-1-D type).The average roughness value (R_a) was obtained.The probe movement and traveling distance were 0.5 mm·s^-1 and5 mm,respectively.The tribological properties were evaluated using a high-speed reciprocating friction and wear tester (HSR-2 M) with a load of 3 N for 1 h as Si₃N₄ball (Φ4 mm) to counterpart.The reciprocating rubbing speed was 300 r·min^-1,and the reciprocating rubbing distance was 5 mm.After wear testing,the surface morphologies of the specimens were conducted by FESEM and three-dimension al scanning pro filometer (NanoMap-500LS).The wear track was used to calculate the wear rate.The specific wear rate can be determined by normalizing the wear volume with the distance traveled in friction and the applied load.

Fig.1 SEM images of typical surface morphologies of GOs:a lower magnification and b higher magnification

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Table 1 Compositions of bath solution

Fig.2 XRD patterns of Ni-W-GO coatings with different concen-trations of GOs

3 Results and discussion

3.1 Microstructure and morphology of composite coatings

Figure 2 shows XRD patterns of as-plated Ni-W as well as Ni-W-GO composite coatings.According to the curves,all XRD patterns of the coatings demonstrate the characteristic peaks corresponding to crystallographic planes (111),(200)and (220) of nickel 2θat 44.33°,51.52°and 75.26°,respectively.Besides,all the coatings reveal the crystalline fcc crystal structure with the predominant plane (111).However,no diffraction peak for GOs is observed,suggesting that GOs content in the coating is extremely low and falls behind the XRD detection limit.Further observation can be found that increasing GOs content in plating bath leads to widening of Ni(111) diffraction peak.The crystallite size of the composite coatings can be calculated by Debye Sherrer’s equation [ 27] and the values are given in Table 2.

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Table 2 Crystallite size,composition and surface roughness of composite coatings

Fig.3 SEM images of surface morphology:a Ni-W coating,b Ni-W-GO composite coating with 0.15 g·L^-1 GOs in plating bath,and c higher magnification of b;d EDS results of selected area in c

where d is the crystallite size,k is the Scherer constant(typical value is 0.89),λis the incident radiation of Cu(0.15418 nm),βis the full width half maxima (FWHM)andθis the diffraction angle.As Table 2 shows,calculated from (111) plane of the coatings,NiW-GO coating of which GOs content in plating bath is 0.15 g·L^-1 has the smallest crystallite.It is chiefly because the embedded GOs adsorb metal ions and complexes shown in electrolyte such as[(Ni)(HWO₄)(Cit)]^2- [ 2] .When GOs come across the Ni-W growth centers,they inhibit the growth of nuclei by blocking the surface of growing metal and increase the rate of nucleating over grain growth during the electrodeposition process,thus resulting in the decrease of crystallite size [ 28] .Additionally,Table 2 gives the chemical composition of the composite coatings measured by EDS.A further increase of GOs content is observed in the composite coatings with GOs addition increasing in the plating bath.Moreover,the content of tungsten and nickel in the composite coatings slightly decreases.

Fig.4 a Typical cross-section SEM image and b linear EDS analysis of Ni-W-GO composite coating with 0.15 g·L^-1 GOs in plating bath

Fig.5 Microhardness measurements of composite coatings

The surface roughness of the NiW coating and NiW-GO composite coating were evaluated,and the values are also shown in Table 2.It can be seen that the average surface roughness (HR_a) value of Ni-W coating is 0.82μm which is larger than those of Ni-W-GO composite coatings with different GO additions.This phenomenon indicates that the nature characteristic of GOs incorporated in the composite coatings has acted as lubricants successfully,which influence the tribological performance [ 29] .

Typical SEM images of Ni-W and NiW-GO composite coating with 0.15 g·L^-1 GOs in the plating bath are clearly shown in Fig.3.The surface morphologies of the composite coatings display a fine,smooth,compact and cauliflower-like structure with very small nodules.Meanwhile,the average nodular size of Ni-W-GO composite coating is smaller than that of corresponding Ni-W coatings (Fig.3a,b).The higher magnification of as-plated NiW-GO coating and corresponding EDS analysis of the selected area are displayed in Fig.3c,d.It can be seen that there are many layered sheets which are typically GOs obviously embed in the coating.Additionally,EDS results can also prove the existence of GOs in as-plated Ni-W-GO coatings.

Fig.6 Friction coefficient of composite coatings with different concentrations of GOs

The cross-sectional microscopic morphology and the linear EDS analysis of NiW-GO composite coating with0.15 g·L^-1 GOs addition are presented in Fig.4.It demonstrates that there are a clear interface and a close bond between the coating and the matrix,while the thickness of the coating is～10μm.At the same time,the linear EDS analysis (Fig.4b) indicates that the major components of the coatings are nickel,tungsten,carbon and oxygen,where nickel and tungsten are the dominant elements in the composite coatings.Moreover,this detection of carbon and oxygen can further demonstrate that the presence of GOs embedded in the as-plated coating.

3.2 Microhardness of composite coatings

The Vickers hardness measurements of the composite coatings are shown in Fig.5.It can be seen that the microhardness of Ni-W-GO composite coatings is found to increase in comparison with that of single NiWcoatings without GOs employing.The observed increase in the hardness with GOs content increasing is mainly accompanied by the dispersion strengthening and particle enhancement effects due to the incorporation of GOs in the plating bath.GOs itself has higher hardness,which can cause the particle strengthening to the composite coatings [ 30] .It clearly proposes the role of GOs to enhance the asplated coatings due to that the embedded particles distribute mainly in the grain boundaries and act as a barrier to the propagation of the slip planes [ 31] .Additionally,other strengthening mechanism is mainly due to fine grain strengthening and dispersion strengthening effects [ 5] .Further observation can be found that the optimum concentration of GOs addition is 0.15 g·L^-1 and the hardness value is HV 688.Excess GOs employing in the plating bath,can facilitate the particle collisions but cause the particle agglomeration phenomenon,resulting in the slight decrease in hardness.

Fig.7 Wear rate of composite coatings

Fig.9 Two-dimensional profiles of wear track of composite coatings with different concentrations of GOs

3.3 Tribological behavior of composite coatings

Figure 6 illustrates the variation curves of the coefficient of friction (COF) of NiW and Ni-W-GO composite deposits in dry friction environment.The wear rates of as-plated coatings are displayed in Fig.7.As can be observed,all Ni-W-GO coatings have lower COF as compared to the Ni-W coating without GOs employing.Moreover,the COF rapidly increases to a certain value,fluctuates and then stabilizes at a constant value during the whole examination,while the friction behaviors of all as-presented Ni-W-GO composite coatings are similar.In addition,it is clearly found that COF decreases with the amount of GO increasing and reaches a minimum value of about 0.55when the bath contains 0.15 g·L^-1 GOs.However,when the amount is higher than 0.15 g·L^-1,the excess GOs reunite,resulting in the increase of COF.Moreover,without employing GOs,the wear rate of the NiW coating is2.52×10^-6 mm³·N^-1·m^-1.The NiW-GO composite coatings with 0.15 g·L^-1 GOs addition has the lowest wear rate (1.66×10^-6 mm³·N^-1·m^-1),which is only 34.13%that of W-Ni alloy.As known,the COF of the coating depends on both the microstructure of the surface and the number of embedded nanoparticles [ 28] .Indeed,the suitable amount of GOs dispersion in the plating bath can improve the ability of the coating to resist local deformation,provide a large amount of nuclei and increase the number of grains,thereby forming double hard particle and fine grain strengthening effects to the NiW-GO coating [ 24] .On the other hand,GOs with high wear resistance and good self-lubrication effect,separated from matrix,act as solid lubricants,and also reduce the direct contact between two wear surfaces,thus reduce the overall wear rate of the composite coatings [ 32] .As a result,in this work,the GOs embedded in the coating considerably improve the wear resistance of the Ni-W-GO composite coating.The COF and microhardness are two parameters that affect the wear performance.

Fig.8 SEM images of worn surface morphologies of composite coatings with different concentrations of GOs:a 0 g·L^-1,b 0.05 g·L^-1,c 0.10 g·L^-1,d 0.15 g·L^-1,and e 0.20 g·L^-1

Fig.10 Three-dimensional profiles of frictional surface of composite coatings with different concentrations of GOs:a 0 g·L^-1,b 0.05 g·L^-1,c 0.10 g·L^-1,d 0.15 g·L^-1,and e 0.20 g·L^-1

Fig.11 SEM images of worn surface:a Ni-W coating and b Ni-W-GO coating (0.15 g·L^-1 GO addition)

In order to further evaluate the wear properties of the composite coatings,the worn surface morphology,the twodimensional (2D) and the three-dimensional (3D) profiles of wear track are presented in Figs.8,9 and 10,respectively.As also can be seen,the wear tracks of Ni-W coatings are much larger,longer and deeper in dimension compared with those of NiW-GO composite coatings with GOs addition under the same testing conditions.Moreover,when the amount of GOs addition is 0.15 g·L^-1,all results reach minimum.These conclusions agree with the wear rate and the COF values.Additionally,obvious abrasive grooves,clear scuffing and some large wear debris can be observed in the wear tracks of Ni-W coating,while only shallow grooves and smooth worn surface exist in NiW-GO coatings (Fig.11).

Coupling with wear analysis above,extensive abras ve wear is the main wear mechanism throughout the whole wear process,and mild adhesive wear also can be found [ 25] .It is well known that GO is a single atomic layer and the carbon mmaterial has a planar structure,the feature of this layer structure can easily form solid lubricants and play an important role in antifriction.At the same time,the GOs embedded in the composite coatings can transfer the friction load.It also can play a pinning role and the second phase enhancing,and so effectively improve the hardness of the coatings.According to the Archard theory,wear performance is proportional to the hardness of the composite [ 33] .When Si₃N₄ ball wear against the coatings,these nanoparticles can impede the penetration of abrasive grains into the coating and improve the plastic deformation resistance of the composite coating.So,corresponding to the micro hardness values of the Ni-W-GO composite coatings,better wear resistance of the as-plated coatings can be ascribed to their higher Vickers hardness in comparison with that of NiW matrix.All above analyses clearly prove that GOs can apparently improve the wear resistance of Ni-W composite coatings.

4 Conclusion

In this present study,Ni-W-GO composite coatings,with low content of GOs addition,were successfully prepared by co-deposition technique.The microstructure,hardness and wear characteristics of the composite coatings were investigated,and their wear mechanism was discussed as well.Experimental results show that layer-structured GOs positively affect the microstructure and grain size of the coatings.Some cauliflower-like structure,with smaller nodules,is obtained in the Ni-W-GO composite coatings in comparison with the corresponding Ni-W coatings.GOs embedded in the composite coatings can significantly improve the mechanical and wear properties of the coatings.However,excess GO addition results in the decrease in the performances of the composite coatings.When the amount of GO addition is 0.15 g·L^-1,the comprehensive performances are the best.The microhardness value is HV688 and the wear rate is 1.66×10^-6 mm³·N^-1·m^-1(34.13%that of the Ni-W coating).Moreover,extensive abrasi ve wear is the mainly wear mechanism of the present coatings.

Acknowledgements This work was financially supported by the Natural Science Foundation of Jiangxi Province (Nos.20161BAB216121,20161BAB206136 and GJJ150638) and the National Natural Science Foundation of China (No.91326203).