Preparation and properties of Ni-W-P-TiO2 nanocomposite coatings developed by a sol-enhanced electroplating method

Zhen He,Yu Zhou,Yuxin Wang,,Pingyi Guo,,Wensen Jiang,Caizhen Yao,3,Xin Shu

1 School of Materials Science and Engineering,Jiangsu University of Science and Technology,Zhenjiang 212100,China

2 Cedars-Sinai Medical Center,Los Angeles,CA,USA

3 Laser Fusion Research Center,China Academy of Engineering Physics,Mianyang 621900,China

4 Hunan Aerospace TianLu Advanced Materials Testing Co.Ltd,Changsha 410299,China

Keywords:Ni-W-P TiO2 Electroplating Mechanical property

ABSTRACT Several Ni-W-P-TiO2 nanocomposite coatings were developed by the sol-enhanced electroplating method.The phase and elemental compositions of coatings were determined,and the surface and cross-section morphology were characterized.The mechanical and corrosion performance were systematically tested.The results revealed the addition of 5 ml?L-1 TiO2 sol caused a compact coating surface,while higher concentrations of TiO2 reduced the coating thickness and led to the inferior surface microstructure.The comparison in physiochemical properties of prepared coatings confirmed the superior performance of the Ni-W-P-TiO2 nanocomposite coating at 5 ml?L-1 TiO2 sol addition.Under this condition,the best mechanical properties were achieved when abrasive wear was the dominating wearresistance mechanism,and the best corrosion resistance was obtained due to its smooth and compact surface microstructure.

1.Introduction

Protective coatings based on surface technologies could provide viable solutions to improve the service life for most engineering parts in modern industry.As an extensively used coating material,nickel–phosphorous (Ni–P) coating is garnering growing research attention due to its unique physicochemical advantages in coating uniformity,hardness,corrosion and wear resistance [1–6].In recent studies,alloying a third coating element into the Ni-P matrix has been extensively accepted to enhance the coating performance.Versatile ternary Ni-P based coating materials have been developed with promising properties,such as Ni–Cu–P coating,Ni–Fe–P coating,Ni–Re–P coating,coating and Ni–W–P coating [2,7–11].Among these,Ni–W–P system serves as an excellent protective coating material with its huge enhancement in corrosion resistance at the proper tungsten incorporation.The electroplated Ni–W–P coatings are widely applied in the field of metal corrosion protection and decoration due to the excellent performance and mature fabrication process [12–14].

Recent research focuses on the modification techniques for electroplated Ni–W–P coatings to further optimize the coating properties.For example,Zhouet al.investigated the pulse electroplating technique for Ni–W–P coatings,which improved the anticorrosion performance[15].Embedding inert particles is an extensively employed modification method for coating optimization[16–21].For instance,reports have incorporated different inert particles in Ni–W–P coating system.Luet al.and Huet al.prepared the optimized Ni–W–P-PTFE and Ni–W–P-SiO2coatings and stated the considerable enhancement in antifouling and anti-corrosion performance [22,23]

We have recently demonstrated the preparation of nanoparticle-reinforced composite coatings using a novel solenhanced electroplating technique.This technique avoids the nanoparticle agglomeration due to in-situ nanoparticle formation in electroplating electrolytes,which improves the physicochemical properties of resulting composite coatings.For instance,the solenhanced Co-P-TiO2and Ni-Co-TiO2coatings have been comprehensively investigated in our earlier studies [24,25].This solenhanced method brings forth huge improvement in coating hardness,wear and corrosion resistance.However,the TiO2nanoparticle modification has never been reported in Ni-W-P coating to date,and the sol-enhanced Ni-W-P-TiO2coating system is worthy of a thorough study.

Fig.1.The XRD patterns of Ni-W-P and Ni-W-P-TiO2 composite coatings.

In this regard,this work studied the Ni-W-P-TiO2composite coating made by the sol-enhanced electroplating method.Comprehensive discussions have been made regarding the physicochemical properties of composite coatings.This research of sol-enhanced Ni-W-P-TiO2system will help to improve the Ni-W-P coatings further and expand their industry applications.

2.Experimental Method

2.1.Sample preparation

The experiments used deionized water and analytical grade chemicals.The deionized water was prepared by UPR-Ⅱpurifying machine(Ulupure,China),and all the chemicals were bought from Aladdin Reagent,China.The brass coupons were pre-treated before being used as the substrate in the next electrodeposition process.The first pretreatment step was ultrasonic cleaning brass coupons in acetone for 2 min,followed by the chemical etching step which soaked the brass coupons in the alkaline solution of 50 g?L-1sodium hydyoxide+10 g?L-1sodium dihydrogen phosphate at the temperature of 65 °C for 5 min.Then,the brass coupons were electrochemically activated in the acid solution of 20 g?L-1citric acid+60 g?L-1ammonium citrate under a current density of 50 mA?cm-2for 60 s,with a stainless-steel sheet being the anode.These brass coupons were cleaned with deionized water and then dried after each of the above processes.

The electroplating of Ni-W-P and Ni-W-P-TiO2layers was carried in a beaker cell.The prepared brass substrate acted as the cathode with the exposed dimension of 30 mm × 20 mm,while the pure nickel sheet acted as the anode.The TiO2sol was prepared as described in our previous work [26].Before the electroplating,the pre-synthesized TiO2sol was slowly added into the electrolytic bath.Table 1 describes the experimental details employed in the electroplating process.The pH of the electrolytic bath was adjusted by adding sulfuric acid solution.

Table 1 The electroplating conditions employed in the experiments

Table 2 The calculated corrosion parameters of Ni-W-P and Ni-W-P-TiO2 coatings

Fig.2.The XPS spectrums and fitting results of Ni-W-P-TiO2 composite coatings at 50 ml?L-1 TiO2 sol addition for different elements of (a) Ni,(b) P,(c) W,(d) Ti.

2.2.Sample characterization

The crystal structural information was recorded by X-ray Diffraction (XRD) for the Ni-W-P and Ni-W-P-TiO2coatings using an XRD analyzer(XRD-6000X,Shimazu,Japan).The tests were conducted using Cu Kα radiation at the step size of 0.02°,and the 2θ scanning range was chosen from 20°to 80°.The surface morphologies of the prepared coatings were identified by a scanning electron microscope (SEM,Pro X,Phenom,the Netherlands).The detailed compositional information of the prepared coatings was characterized by X-ray photoelectron spectroscopy (XPS,ESCALAB 250Xi,Thermo Fisher,USA).The Al Kα radiation at 1486.6 eV was applied in XPS tests.

The mechanical performance was determined by Vickers hardness tests and tribological tests.In Vickers hardness tests,a 50 g load was employed for 10 s for each indentation,and five random indentations were performed in each sample at room temperature.The tribological tests were conducted by a micro-tribometer(UMT-2,CETR,USA) at room temperature,with a steel ball with a diameter of 9.58 mm as the counter surface.In each test,the applied load was 7 N at the sliding speed of 20 mm?s-1for 10 min.The morphologic information of the resulting wear tracks was observed by SEM,and their compositional information was determined by the equipped energy dispersive spectrometer(EDS) detector.

Fig.3.The surface morphology of (a) Ni-W-P coating,and Ni-W-P-TiO2 coatings with different addition of TiO2 sol:(b) 5 ml?L-1,(c) 10 ml?L-1,and (d) 50 ml?L-1.

Fig.4.The cross-sectional SEM images of (a) Ni-W-P coating,and Ni-W-P-TiO2 coatings with different addition of TiO2 sol:(b) 5 ml?L-1,and (c) 50 ml?L-1.

The Tafel tests and electrochemical impedance spectroscopy(EIS) tests were conducted in a typical three-electrode beaker cell,with a platinum counter electrode and a saturated Ag/AgCl/KCl reference electrode.These electrochemical tests were performed at room temperature by an electrochemical workstation (CS2350,CorrTest,China).Before each measurement,the sample was immersed in the electrolyte solution (3.5%(mass) NaCl) for up to 30 min to stabilize the open circuit potential.The sweep rate in the tests was set as 1 mV?s-1.The EIS tests were conducted at an AC amplitude of 5 mV at the frequency range from 0.01 Hz to 100,000 Hz.

3.Results and Discussion

3.1.Structural characterization

The phase compositions were presented in Fig.1.All patterns displayed a broad peak originating from the amorphous nickel structure in the range between 40°and 50°,which is in good agreement with previous reports [11,22].There existed no diffraction peaks belonging to the tungsten-or phosphors compounds,indicating that W and P atoms were solid dissolved in the nickel lattice in the electroplating process.Additionally,the addition of TiO2hardly changed the phase constituents of composite coatings.Several intense diffraction peaks of brass substrate appeared in the XRD profiles.The penetration depth of X-ray in the XRD scanning depends on many factors such as incident angle,compositions and structure of detected material,and the presence of substrate diffraction peak was also reported in other nickel-based coatings[26–28].No TiO2diffraction peak was obtained in the XRD pattern,which could result from the small-scale incorporation of TiO2particles.

Fig.7.The morphology of center area for wear tracks of (a) Ni-W-P coating,and Ni-W-P-TiO2 coatings with different addition of TiO2 sol:(b) 5 ml?L-1,and (c) 50 ml?L-1.

Fig.8.EDS results for the wear tracks of Ni-W-P and Ni-W-P-TiO2 coatings;the line scanning(as indicated by the blue line)went from the untreated area to the wear track.

To further identify the composition information of prepared coatings,Fig.2 presents the XPS results of Ni-W-P-TiO2coating at 50 ml?L-1sol addition.The fitting results of Ni revealed the majority of Ni-Ni bond in the prepared coating,while a certain extent of oxide was confirmed due to the oxidation of coating surface.The surface oxidation was also observed in the XPS pattern of W (Fig.2(c)),where small proportion oxide peaks were determined.These fitting results of Ni,W,P are in good agreement with previous literature [12,29,30].Also,we should note the fitting results of Ni,W,P are consistent in both Ni-W-P and Ni-W-PTiO2coatings,showing the same fitting peaks for these detected elements.Meanwhile,the presence of TiO2was confirmed for Ni-W-P-TiO2coating.The addition of 50 ml?L-1TiO2sol gave rise to the double peak in the range of 456–468 eV,which clearly showed the existence of incorporated TiO2in the Ni-W-P-TiO2coating.This well matches our previous findings in earlier studies [24,31].

The surface morphology of prepared coatings is depicted in Fig.3.All coatings displayed a nodular surface morphology.Compared to Ni-W-P coating,a smoother coating surface with smaller nodular crystals was revealed for the Ni-W-P-TiO2at 5 ml?L-1TiO2sol.However,a further increase in TiO2sol caused an inferior coating quality.As shown in Fig.3(c) and Fig.3(d),holes and cracks began to appear in the coating surface at 10 ml?L-1TiO2sol,and the severe crack was observed in the surface at a high TiO2sol concentration of 50 ml?L-1.

By adding TiO2sol into the electrolytes,TiO2nanoparticles were in-situ generated and then co-deposited to form Ni-W-P-TiO2composite coatings.At the proper TiO2sol concentration at 5 ml?L-1,the refined and compact surface was obtained due to the appropriate polarization throughout the electrodeposition process.However,when adding excessive TiO2sol,the generated TiO2nanoparticles tended to agglomerate and caused severe inhibition effects on the electrodeposition process.Under this condition,critical hydrogen evolution could occur as the side-reaction and caused the deteriorated coating quality

Fig.4 presents the cross-sectional images of developed coatings.It can be seen that the substrates were adequately covered by the electroplated coatings,indicating the excellent adhesion between the inner substrate and outside coating.The 50 ml?L-1TiO2sol addition resulted in a coating thickness of 10.1 μm,which is much smaller than the thickness of other samples at 14.6 μm.Under 50 ml?L-1TiO2sol addition,a large extent of applied current was consumed in the side-reaction of hydrogen generation,thereby causing the decreased coating thickness.

3.2.Hardness and wear behavior

The tested hardness of Ni-W-P and Ni-W-P-TiO2coatings is illustrated in Fig.5.The Ni-W-P coating presented a hardness of～520 HV,while the addition of 5 ml?L-1TiO2sol increased the hardness to～580 HV.However,the coating hardness decreased when further raising the TiO2sol concentration.It can be seen the excessive TiO2sol addition at 50 ml?L-1caused a much lower hardness of～330 HV.At the proper addition of 5 ml?L-1TiO2sol,the uniformly distributed TiO2nanoparticles in the coating provided dispersion strengthening effect and grain size refinement strengthening effect,therefore improving the coating hardness[32].However,the excessive addition of 50 ml?L-1TiO2sol caused agglomeration of TiO2nanoparticles,which is correlated to its deteriorated coating hardness [31].

Fig.5.The hardness of(a)Ni-W-P coating,and Ni-W-P-TiO2 coatings with different addition of TiO2 sol:(b) 5 ml?L-1,(c) 10 ml?L-1,and (d) 50 ml?L-1.

The wear track images after the tribological tests are presented in Fig.6.The Ni-W-P coating had a wear track width of 342.2 μm.Meanwhile,the significantly decreased track width of 118.9 μm was observed at the addition of 5 ml?L-1TiO2sol,beyond which the increased concentration of TiO2sol caused a larger wear track width.The excessively added 50 ml?L-1TiO2sol led to a wide wear track of 400.4 um.

Fig.6.The wear track images of (a) Ni-W-P coating,and Ni-W-P-TiO2 coatings with different addition of TiO2 sol:(b) 5 ml?L-1,(c) 10 ml?L-1,and (d) 50 ml?L-1.

The wear behaviors were also determined for these Ni-W-P and Ni-W-P-TiO2coatings.Fig.7 illustrates the high magnification images of the wear track center sections for Ni-W-P and Ni-W-PTiO2coatings.In Fig.7(a),the Ni-W-P coating displayed paralleled grooves after the tribological tests,while some peeled-off coating debris was observed on its surface.On the contrary,the Ni-W-PTiO2composite coating at 5 ml?L-1TiO2sol showed a distinct topologic morphology,displaying shallow marks with little wear debris.In Fig.7(c),the tribological tests caused severed coating exfoliation and deep cracks in addition to the paralleled grooves,indicating its weak wear resistance.

To further understand the wear mechanisms,the elemental composition analysis was conducted as shown in Fig.8.In addition to nickel and phosphorus,the EDS line scanning revealed oxygen in Ni-W-P coating as shown in Fig.8(a),indicating the formation of metal oxide.When adding 5 ml?L-1TiO2sol,the element composition of the wear track was consistent with the untreated areas in the coating surface,Fig.8(b).Additionally,a certain extent of titanium was identified in the coating surface as a result of wellembedded TiO2nanoparticles in the coating matrix.Under the addition of 50 ml?L-1TiO2sol,copper and oxygen were identified in the wear track.In the wear tests,the metal oxide was constituted,and the brass substrate was exposed due to the exfoliation of the deposited coating.

Fig.9.The(a)Tafel plots,and(b)Nyquist plots recorded on the Ni-W-P and Ni-WP-TiO2 coatings.

The combined wear analysis shows a set of abrasive wear,adhesive wear and oxidation wear occurred in Ni-W-P coating,as revealed by the deep wear grooves,noticeable surface debris and formation of the oxide.During the pin-disc wear tests,the metal oxide and surface debris were continuously generated on the coating,causing the large and deep wear track.Meanwhile,the best wear resistance was achieved for the Ni-W-P-TiO2composite coating at 5 ml?L-1TiO2sol.Under this case,its strong hardness could decrease the contact area during the pin-disc wear tests,thereby leading to the increased wear resistance [1].The shallow wear grooves indicate the abrasive mechanism was dominant in the wear tests.However,the excessive addition of TiO2sol caused the widest wear track among the examined coatings,which could be explained by its open surface and large inner stress[33].Under this condition,oxidation and adhesive wear occurred in the pindisc wear tests,and the severe adhesive wear caused the coating exfoliation and cracks on the coating.

3.3.Corrosion resistance

The polarization tests were conducted for Ni-W-P and Ni-W-PTiO2coatings,as displayed in Fig.9.The electrochemical parameters calculated from the Tafel plots were summarized in Table 2.The superior corrosion resistance was achieved for the Ni-W-PTiO2coating at the addition of 5 ml?L-1TiO2sol,which showed the smallest corrosion current density of 1.97 μA?cm-2and the minimum corrosion rate of 0.023 mm?a-1.Further adding TiO2sol caused a decreased corrosion resistance,which could result from the inferior surface microstructure with surface holes and cracks.These findings well-matched the Nyquist plots as presented in Fig.9(b).All the samples revealed semicircle Nyquist plots.The largest radius of capacitive loops occurred in the case of Ni-W-PTiO2coatings at 10 ml?L-1TiO2sol,indicating its best corrosion resistance among tested samples [34].

In order to further characterized the corrosion behavior,the corrosion tests were conducted for prepared samples in 3.5%NaCl solutions for 12 h.Fig.10 presents the surface morphology of samples after the corrosion tests.As shown in Fig.10(b),the Ni-W-P-TiO2coating at 5 ml?L-1TiO2sol possessed a good surface condition with few pits after the corrosion tests.Meanwhile,severe pitting corrosion occurred on other samples after the immersion steps,where many pits and even holes appeared on the coating surface.

To conclude,the proper addition of 5 ml?L-1TiO2sol led to the best corrosion resistance of prepared coatings.Under this condition,a smooth and compact surface was prepared with fewer surface defects,which prevents the corrosion attack.However,the holes and cracks on the coating surface caused decreased corrosion resistance when further adding TiO2sol in electrolytes.It is noted that the 10 ml?L-1TiO2sol addition caused a worse corrosion behavior compared to the higher 50 ml?L-1TiO2sol addition,which could be explained by its relatively porous surface structure observed in Fig.3c.

Fig.10.The surface morphology of (a) Ni-W-P coating,and Ni-W-P-TiO2 coatings with different addition of TiO2 sol:(b) 5 ml?L-1,(c) 10 ml?L-1 and (d) 50 ml?L-1,after immersing in 3.5% NaCl for 12 hours.

4.Conclusions

In this study,a series of Ni-W-P-TiO2nanocomposite coatings were developed using sol-enhanced electroplating technique.The addition of TiO2sol caused a more compact coating surface at the proper concentration of 5 ml?L-1,where the best mechanical properties,microstructures,and corrosion resistance were achieved.A further increase in TiO2sol concentration causes an inferior surface microstructure.The strengthening effects for the Ni-W-P-TiO2coating associated with the well-dispersion of TiO2nanoparticles.Also,this study demonstrated the superiority of our Ni-W-P-TiO2coatings prepared at optimal conditions to Ni-W-P coatings.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research was funded by Natural Science Foundation of Jiangsu Province (BK20201008),Key Research and Development Project of Zhenjiang (GJ2020014),National Natural Science Foundation of China (51701087).