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Rare Metals2019年第11期

Tribological behavior of a Ni-WS₂ composite coating across wide temperature ranges

Shuai Cui Wen-Sheng Li Ling He Li Feng Guo-Sheng An Wei Hu Chun-Xia Hu

School of Materials Science and Engineering,Lanzhou University of Technology

State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals,Lanzhou University of Technology

作者简介：*Wen-Sheng Li e-mail:wensheng-li@sohu.com;

收稿日期：8 March 2017

基金：financially supported by International Science and Technology Cooperation Program of China(No.2015DFR51090);the Supporting Program of Gansu Province(No.1604WKCA008);

Tribological behavior of a Ni-WS₂ composite coating across wide temperature ranges

Shuai Cui Wen-Sheng Li Ling He Li Feng Guo-Sheng An Wei Hu Chun-Xia Hu

School of Materials Science and Engineering,Lanzhou University of Technology

State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals,Lanzhou University of Technology

Abstract：

In order to reduce the friction coefficient of a pure nickel coating and extend the lifetime of metal parts under extreme friction conditions,a series of Ni-based WS₂-composite solid lubrication coating containing different WS₂ concentrations were prepared on a 45#mild carbon steel substrate by electroplating.The cyclic voltammetry method was used to investigate the electroplating regulation of the Ni-WS₂ composite coatings.X-ray diffraction(XRD) and scanning electron microscopy(SEM) were used to analyze the microstructures and wear surfaces of the composite coatings,the tribological properties and wear mechanisms of the composite coatings with different WS₂ concentrations.The results show that the addition of WS₂ can promote the cathode polarization of the electroplating process,and the polarization degree goes up with the increase in WS₂ concentrations.The friction coefficient of Ni-composite coatings significantly decreases by the addition of WS₂ particles.The lowest friction coefficient at room temperature is obtained at a value around 0.01-0.03 from the coating deposited in the electrolyte solution with a 30 g·L^-1 WS₂ concentration.The friction coefficient of the Ni-WS₂ composite coating remains in 0.01-0.03 with the increase in temperature from room temperature to 300℃.When the temperature goes up to 500℃,the friction coefficient manifests a continuous increase to 0.12,because WS₂ is gradually oxidized into WO₃ and therefore loses its lubrication ability.

Keyword：

WS₂; Self-lubricating composites coating; Frictional wear; Temperature;

Received： 8 March 2017

1 Introduction

Surface coatings play an important role in prolonging the lifetime of components by enhancing the surface mechanical performance.Owing to varying environments,it is unavoidable that areas with intermittent contact are worn faster than areas with less contact,which may result in prefailure,catastrophic damage or loss [ 1] .Thus,it is important to find solid lubricant materials that exhibit and maintain a reduced friction coefficient under harsh conditions including vacuums,high speeds and even high temperatures.Examples of applications requiring such materials in extreme environments include bearings and bushings for space satellites and vehicles[2-].A solid lubricant can solve these problems for sliding parts with low friction under dry conditions mostly in the aerospace industry through incorporating coatings with some transition metal disulfides with a low friction such as MoS₂ [ 5, 6, 7] ,WS₂ [ 8, 9, 10] ,graphite and boron nitride [ 11, 12, 13] .To overcome deficiencies such as low hardness of single solid lubricant coating,a strategy of combining the functions of pure nickel has been proposed.Electroless pure nickel coatings have been widely used in the chemical,mechanical and electronic industries because of their high hardness and abrasion resistance [ 14] .These composite coatings include both soft and hard particles simultaneously to achieve the wear resistance and lubrication characteristics of a single layer.

Tungsten disulfide (WS₂) possesses a layered structure with good lubrication property,and it provides strong adhesion to the substrate.The present research focused on using sputtering method to develop a Ni coating embedded with WS₂ to be used to reduce the wear condition of coatings.Sun et al. [ 15] used sputtering method to prepare MoS₂/WS₂ multilayer thin films and found that the film exhibited a much lower friction coefficient and longer lifetime compared to a single film.WS₂/MoS₂/C composite films were prepared in an Ar/C₂H₂ atmosphere by magnetron reaction sputtering by Zhou et al. [ 16] ,which expanded the application of WS₂/MoS₂ films under humid conditions.Sputtering method is restricted by the target material and unsuited for large area deposition.Electroplating is one of the effective ways to improve the performance of lubrication,coating wear resistance and corrosion resistance,while electroplating has the advantages of high density,low porosity and flexible methodologies.This work aimed at electroplating a widetemperature,self-lubricating Ni-WS₂ composite coating that will possess high hardness combined with good lubrication properties.

2 Experimental

All chemicals were analytical grade to minimize the effect of impurities.The electrolyte solution for nickel plating contained NiSO₄·6H₂O (250 g·L^-3),NiCl₂·4H₂O(45 g·L^-3) and H₃BO₃ (40 g·L^-3) in distilled water.The pH for the plating bath was adjusted by H₂SO₄ or NaOH to4.0±0.5,because this value can induce the lowest internal stress during Ni electroplating [ 17, 18] .The electrolyte solution was stirred in a multi-position magnetic stirrer.The solid WS₂ lubricant particles were 3-5μm in diameter.Cetyl trimethyl ammonium bromide (CTAB) was added to the electrolyte solution with 0,10,20,30 and40 g·L^-1 WS₂ particles,and the corresponding mass ratio of CTAB/WS₂ is 0.02.

45#mild carbon steel plate was chosen for the electroplating substrate.Prior to deposition,the substrate was sand-blasted followed by alkaline solution flushing using6 g NaOH,2 g Na₂CO₃,3 g Na₃PO₄ and 1 g Na₂SiO₃.The substrate was immersed into 50%HCl solution for30 s and then rinsed with distilled water.A pure nickel plate was used as the anode.The electroplating was performed with a DF-101S magnetic stirrer at 50℃and a current density of 4 A·L^-2 for 90 min to produce NiWS₂ composite coating.

Cyclic voltammetry (CV) method was used in a CS electrochemical workstation to determine a suitable potential range for the reduction reaction.The working electrode was anΦ3-mm glassy carbon electrode.The counter electrode was a platinum electrode.A saturated calomel electrode (SCE) was used for the reference electrode.The morphology of the composite coatings was observed using a Quanta 450 FEG scanning electron microscope (SEM).The phase compositions of the wear debris were detected by a Renishaw's inVia micro-Raman spectrometer with a laser wavelength of 514 nm.The three-dimensional contours of the composite coating were characterized by laser scanning confocal microscopy (LSCM,C2 Plus,Nikon,Japan).The macro-hardness of the composite coating was measured using HRS-150 hardness tester with a load of0.98 N and a dwell time of 10 s.The microstructures of the coatings were characterized by X-ray diffractometer (XRD,Bruker D8).The reported macro-hardness values were the average of at least five measurements on different locations around the center section of each composite coating.

A reciprocating HT-1000 high-temperature wear tester(Fig.1) was used to evaluate the friction behaviors at 25,100,200,300,400 and 500℃,and 6-mm-diameter GCr 15steel balls with hardness of HRC (62-66) were used as the counter body.A constant load of 20 N was applied in all tests,and the running time was 10 min for the Ni-WS₂composite coating.The sliding frequency was 5 Hz,and the sliding stroke was 5 mm.The wear volume was calculated using the integral method.

3 Results and discussion

3.1 Deposition rate curve for composite coating

Figure 2 shows WS₂ and Ni concentrations in the composite coatings as a function of WS₂ bath concentration.The increase of WS₂ concentrations with the increase in WS₂ bath concentrations indicates an increasing co-deposition of WS₂ into the coating.The co-deposition of WS₂particles depends on the number of collisions between the particles and cathode.Then,the inert WS₂ particle is attracted to the cathode due to the applied voltage in the electroplating process.In the high WS₂ bath concentration region,the increments of WS₂ bath concentration alleviate the collision effect between the particles and the cathode.Thus,WS₂ content of the composite coating increases slightly [ 19] .Figure 2 also shows that with the increase in WS₂ bath concentration,Ni concentration decreases.The deposition rate curve further explains that with the increase in WS₂ bath concentration,an increasing number of WS₂particles preferentially occupy the position of Ni crystal to inhibit the crystallization of nickel in the process of electrochemical deposition.

Fig.1 A schematic representation of reciprocating wear tester apparatus

Fig.2 WS₂ and Ni concentrations of coatings versus WS₂ bath concentration

3.2 Morphology and phase of composite coating

3.2.1 Morphology of composite coating

Figure 3 shows SEM images of surfaces and cross section,and LSCM images of surface profile of the composite coatings with different WS₂ concentrations.It can be seen that after deposition,many WS₂ particles are incorporated into the deposition matrix with Ni.When CTAB keeps within 20 mg·g^-1 WS₂ specified proportion,with the increase in WS₂ concentration,the coating surface becomes smooth.The porosity and particle reunion phenomenon are reduced.However,when WS₂ concentration is 30 g·L^-1,the coating surface is the best (Fig.3d).Because WS₂ particles have a negative surface charge,cationic surfactants like CTAB will be easily adsorbed on the suspended particle [ 20] .Therefore,a net positive charge is formed through the adsorption of cationic surfactants that inhibit the formation of particle clusters and lead to more stable particle suspension in the bath [ 21] .With the addition of more surfactant,the electric double layer formatting on the particle is compressed and the dispersion stability is poor [ 22] .As shown in Fig.3e,when WS₂ concentration is 40 g·L^-1,the smoothness of Ni-WS₂composite coating surface gradually becomes worse.

3.2.2 Phase of composite coating

XRD patterns of Ni and Ni-WS₂ composite coating are shown in Fig.4.It can be seen that pure Ni coating is a crystalline structure and does not contain another phase.The main crystal planes of the pure Ni coating are (111),(200) and (220).With the addition of WS₂ particles to the coating,XRD pattern appears WS₂ diffraction peaks,such as (002),(004),(103) and (006).With the increase in WS₂concentration,(002) diffraction peak intensity gradually increases and demonstrates the changing trend in WS₂content in the coating.However,(111) diffraction peak intensity gradually weakens because WS₂ particles have a pinning effect on the nickel crystal boundary and can inhibit the crystallization of nickel.

3.3 CV curves of composite coating

CV curves for Ni-WS₂ composite in electrodeposition baths are plotted and are shown in Fig.5,in which the electrochemical performance of the electrodes in the plating baths was characterized.It is evident that there is a reduction peak as the potential becomes more negative,and this peak means that Ni²⁺is reduced to Ni⁺or Ni [ 23] .The reduction potential of nickel is about-1.6 V through the experiments.With the addition of WS₂,the reduction potential becomes increasingly negative,because the adsorption of WS₂ particles is in competition with Ni crystal growth.An increasing number of WS₂ particles adsorbing CTAB preferentially occupy the position of Ni crystal with the increase in WS₂ concentration,which increases the real area of the electrode surface,and the local current density of the electrode surface decreases,and the cathode diffusion length increases [ 23] .Thus,the deposition of nickel ions in the cathode surface requires more polarization,as the cathode electrode potential becomes increasingly negative.

Fig.3 SEM images of surface,cross section and LSCM images of surface profile of Ni electrodeposition at different WS₂ concentrations:a 0 g·L^-1,b 10 g·L^-1,c 20 g·L^-1,d 30 g·L^-1 and e 40 g·L^-1

3.4 Mechanical and tribological properties of composite coating

Figure 6 illustrates the friction coefficient of Ni and NiWS₂ composite coatings at different WS₂ bath concentrations at 25℃.It is observed that the friction coefficient of the pure Ni coating is nearly 12 times that of Ni-WS₂ composite coating,and with the increase in WS₂concentration,the friction coefficient gradually decreases.The lowest friction coefficient at room temperature is obtained from the coating deposited in the electrolyte solution with a WS₂ concentration of 30 g·L^-1,because WS₂ concentrations in the composite coating rise with increase in particle concentrations in the bath.The lamellar structure existing in the WS₂ particles crystallites is helpful for sliding.Thus,WS₂ grinding produced by wear can easily form transfer film during the process of friction to avoid direct contact between the metals,thereby reducing the friction coefficient [ 24, 25, 26, 27] .However,if WS₂ concentrations are excessive,the thickness of the transfer film will increase and lead to molecular accumulation of the lubricating agent to produce viscoelastic resistance,and thus,the friction coefficient increases [ 28] .

Fig.4 XRD patterns of Ni and Ni-WS₂ composite coatings at different WS₂ bath concentrations

Fig.5 CV curves of Ni-WS₂ coating

Fig.6 Friction coefficient of Ni and Ni-WS₂ composite coatings at different WS₂ bath concentrations at 25℃

Fig.7 Wear rates and macro-hardness for Ni-WS₂ composite coatings as a function of WS₂ bath concentration

Fig.8 Friction curves for Ni-30 g·L^-1 WS₂ composite coatings at different temperatures

Wear rates and the macro-hardness of Ni-WS2 composite coatings at different WS₂ bath concentrations are shown in Fig.7.The results show that the wear rate of the composite coating is the inverse of the macro-hardness.It is clear that WS₂-co-deposited particles reduce the wear resistance.As the amount of WS₂ in the coating increases,the coating wears out more easily because of the soft nature of WS₂,resulting in a low macro-hardness.

Fig.9 SEM images of worn surface of Ni-30 g·L^-1 WS₂ composite coatings at different temperatures:a 25℃,b 100℃,c 200℃,d 300℃,e 400℃and f 500℃

Fig.10 SEM images of different wear debris at 500℃

The typical friction curves for Ni-30 g·L^-1 WS₂ composite coatings at different temperatures are shown in Fig.8.Within 25-300℃,the friction coefficient value slightly decreases and then rises within 400-500℃.Figure 9 shows SEM images from worn surfaces of Ni-30 g·L^-1 WS₂ composite coating at different testing temperatures.Small grooves,small flaking pits and microcracks can be found on the worn surfaces of the composite coating,and the worn surfaces of the composite coating get smooth gradually with temperature changing from 25 to300℃;but for the big grooves,the flaking off is more severe at 400 and 500℃,as shown in Fig.9e,f.To understand the wear mechanism of Ni-WS₂ composite coating,a series of SEM observation are obtained.Figure 10a,b show SEM images of the wear debris at 500℃.As can be seen,there are pieces of coating coming off,and the presence of large particles can be seen.Consequently,the main wear mechanism of the composite is fatigue delaminating.

Fig.11 Raman spectrum of worn Ni-WS₂ surfaces at 500℃

The reason that the friction coefficient slightly drops within 25-300℃is that with the increase in temperature,the molecules are encouraged to move;therefore,the transfer film is speeded up to form and prevent direct contact between the metal materials to reduce the friction coefficient [ 29] .In addition,WS₂ particle is very sensitive to the atmospheric environment,and the friction coefficient of WS₂ significantly increases in a damp environment [ 30] .Therefore,when there is an increase in temperature,the air humidity reduces,and then the friction coefficient drops significantly.

The friction coefficient goes up at 400-500℃,because WS₂ will gradually be oxidized into WO₃ at 425℃ [ 31] ,and there is degradation in lubrication performance.To further demonstrate the presence of WO₃ at 500℃,the phase constituents of wear debris were examined through Raman spectrum,as shown in Fig.11.The main phase is WO₃,and it is documented that a composite coating with this phase is harmful to the lubrication performance [ 31] .Thus,the worn surfaces of the composite coating can be found in the grievous grooves,as shown in Fig.9f.

4 Conclusion

In this study,electrochemical deposition of a Ni-WS₂ selflubricating composite coating results in a remarkable selflubricity and lower friction coefficient at a value around0.01-0.03,compared with a pure Ni coating.With a30 g·L^-1 WS₂ concentration,the Ni-WS₂ composite coating has the lowest friction coefficient.The Ni-WS₂ composite coating presents remarkable self-lubricity over a wide temperature range from 25 to 300℃.At over400℃,the WS₂ will gradually be oxidized into WO₃ and lose its lubricating ability.The reduction potential for Ni ions becomes increasingly negative with the increase in WS₂ concentration.This is explained by that WS₂ particles adsorb CTAB preferentially to occupy the position of the Ni crystal,promoting the cathode polarization.

Acknowledgements This study was financially supported by International Science and Technology Cooperation Program of China (No.2015DFR51090) and the Supporting Program of Gansu Province (No.1604WKCA008).