Microstructure and properties of sol-enhanced Ni-Co-TiO2 nano-composite coatings on mild steel

Accepted Manuscript Microstructure and properties of sol-enhanced Ni-Co-TiO2 nano-composite coatings on mild steel Yuxin Wang, See Leng Tay, Shanghai Wei, Chao Xiong, Wei Gao, R.A. Shakoor, Ramazan Kahraman PII:

S0925-8388(15)30544-2

DOI:

10.1016/j.jallcom.2015.07.147

Reference:

JALCOM 34840

To appear in:

Journal of Alloys and Compounds

Received Date: 28 April 2015 Revised Date:

10 July 2015

Accepted Date: 17 July 2015

Please cite this article as: Y. Wang, S.L. Tay, S. Wei, C. Xiong, W. Gao, R.A. Shakoor, R. Kahraman, Microstructure and properties of sol-enhanced Ni-Co-TiO2 nano-composite coatings on mild steel, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.147. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Microstructure and properties of sol-enhanced Ni-Co-TiO2 nano-composite coatings on mild steel

R. A. Shakoor3, Ramazan Kahraman3 1

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Yuxin Wang1, See Leng Tay1, Shanghai Wei1, Chao Xiong1, 2, Wei Gao1,∗

Department of Chemical & Materials Engineering, the University of Auckland, PB 92019, Auckland 1142, New Zealand

School of Photoelectric Engineering, Changzhou Institute of Technology,

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2

Changzhou 213002, China

Department of Chemical Engineering, College of Engineering, Qatar University, PB

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3

2713, Doha, Qatar

Abstract

Ni-Co-TiO2 nano-composite coatings were electroplated on mild steel by adding

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transparent TiO2 sol (0-50 mL/L) into the Ni-Co plating solution. The microstructure, mechanical property and corrosion resistance of the composite coatings were systematically investigated. It was found that after adding an optimum sol

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concentration into the plating bath, a good dispersion of TiO2 nanoparticles can be achieved in the Ni-Co coating matrix, resulting in a significant improvement in coating mechanical properties. Ni-Co-12.5 mL/L TiO2 coating possessed the best

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microhardness, wear resistance and corrosion resistance. However, adding excessive quantities of sol (more than 12.5 mL/L) into the electrolyte caused nanoparticle agglomeration and created a porous structure, deteriorating the properties of coatings.

Key words: Sol-enhanced technique, Ni-Co coating, nano-composite coatings, electroplating.

∗

Corresponding author. Department of Chemical & Materials Engineering, the University of Auckland, PB 92019, Auckland 1142, New Zealand. Tel.: +64 9 3737599 Ext. 88175, Fax: +64 9 3737463. E-mail addresses: [email protected] (Wei Gao), [email protected] (Yuxin Wang).

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1. Introduction Ni-Co coatings have attracted great attention due to their unique properties, such as high strength, good corrosion resistance and favorable magnetic properties [1-5]. Many methods including electroplating, electroless deposition, physical and chemical

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vapor deposition, and plasma spraying can be used for manufacturing Ni-Co coatings [6-8]. Among these methods, electroplating is one of the simple ways to realize industrial applications due to its advantages of simple process, low cost, working at ambient atmosphere and good reproducibility [9]. Additionally, the weight percentage

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of Co, which determines the microstructure and properties of Ni-Co coating, could be easily adjusted and controlled by altering the parameters of electroplating process [5,

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10].

Recently, much effort has been devoted to fabricating Ni-Co composite coatings [11-15]. A certain amount of different solid nano/micro particles or nanotubes are mixed with bath solution, and then co-deposited onto the substrate with Ni-Co matrix.

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These Ni-Co composite coatings possess improved mechanical and corrosion properties compared to pure Ni-Co coatings [16-18].

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For metal-matrix composite coatings, it is well-known that closely dispersed second phase particles can provide strong dispersion strengthening effect. However, the

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agglomeration of fine particles cannot be completely avoided due to the large surface energy of small particles. Consequently, it has been a long term challenge to develop a simple feasible way to prepare highly dispersed nanoparticles reinforced metal-matrix composite coatings. Recently, we have solved this problem by developing a novel technique: the sol-enhanced coating method [19]. It is known that sol-gel method can be used to produce metal oxides, especially the oxides of silicon and titanium [20]. Our sol-enhanced coating method combines the sol-gel process and electroplating to in-situ form nanoparticle reinforced composite coatings. Fine nano-particles can be generated and in-situ deposited into the coating matrix when proper amount of sol

ACCEPTED MANUSCRIPT was added into the electrolyte. It can form highly dispersed nanoparticles in the coating matrix, resulting in significantly improved mechanical properties [19].

A wide range of composite coatings have been prepared and studied by using this

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method, including Ni-TiO2 [21], Ni-B-TiO2 [22], Ni-P-TiO2 [23] and Ni-P-ZrO2 [24]. In the present work, the sol-enhanced method has been applied for electro-deposition of Ni-Co-TiO2 nano-composite coatings in consideration of their promising properties and wide applications. The microstructure and properties of sol-enhanced Ni-Co-TiO2

2.1 Sample preparation

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2. Experimental details

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nano-composite coatings were systematically investigated.

Both Ni-Co coatings and sol-enhanced Ni-Co-TiO2 nano-composite coatings were electroplated using a modified watt’s bath. The basic watt’s bath contains 250 g/L NiSO4-6H2O, 40 g/L NiCl2-6H2O and 35 g/L H3BO3. 30 g/L CoSO4-7H2O and 0-50

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mL/L TiO2 sol were added into the basic watt’s bath in order to conduct sol-enhanced Ni-Co-TiO2 plating. This modified watt’s bath was prepared using Sigma analytical grade reagents. The pH value of bath was adjusted by H2SO4 and NaOH to be around

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3.5. TiO2 sol was prepared as reported in the previous research [19, 23]: 8.68 mL of tetrabutylorthotitanate [Ti (OBu) 4] was dissolved into the mixture solution of 35 mL

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ethanol and 2.82 mL diethanolamine. After magnetic stirring for 2 h, it was hydrolyzed by adding a mixture of 0.45 mL deionized water and 4.5 mL ethanol dropwise under magnetic stirring.

The electroplating system consists of a mild steel sample as the cathode and a Ni plate as the anode. The steel substrates were mechanically polished using SiC paper to a grit of #1200, then degreased ultrasonically in ethanol and pre-treated in 1 mol/L HCl solution for 2 min in order to remove the surface oxide scale. The specimens were then rinsed thoroughly with distilled water, and electroplated by using a TENMA

ACCEPTED MANUSCRIPT 72-8355 DC power supply. The electrolyte was magnetically stirred at a rate of 400 rpm and maintained at 55°C. The current density was set to 2 A/dm2, and the coating was conducted for 30 min.

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2.2 Sample Characterization The coating surface morphologies and composition were analyzed using a FEI Quanta 200 field emission environmental scanning electron microscope (ESEM). For the cross section analysis, the samples were cold mounted by using EpoxySet resin

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(ration of resin: hardener, 25:3). The cross-section surfaces were ground with SiC paper to 1200 grit followed by cloth polish with diamond up to 1 micron. The

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backscattered electrons were analyzed with a solid state detector to examine the interface between the substrate and coatings. The phase structure, average grain size and preferred orientation of coatings were determined by XRD using (D2 Bruker X-ray diffractometer) operated at 30 kV and 10 mA with the Cu-Kα radiation. TEM

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analysis was conducted by using a FEI Tecnai T12 transmission electron microscope.

Vickers microhardness was conducted using a load of 100 g with a holding time of 15 s. The results for the hardness were the average of 5 measurements. The wear

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property of coatings was tested using a micro-tribometer (Nanovea, USA) in air at 25°C, relative humidity of ~50% and under dry, non-lubricated conditions. All wear tests were performed under a load of 5 N, a sliding speed of 2 m/min and a contact

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radius of 6 mm for a total sliding distance of 20 m.

The corrosion resistance of the coatings were investigated by potentio-dynamic polarization measurements in 3.5 wt.% NaCl electrolyte using an electrochemical workstation (CHI604D). All corrosion tests were carried out at room temperature (25°C) using a three-electrode system with platinum mesh as auxiliary, saturated calomel electrode (SCE) as reference and coated specimen as working electrode. The exposed surface area of all samples was 1 cm2. The working electrode was immersed in the electrolyte and left until the steady-state open potential was attained. Thereafter,

ACCEPTED MANUSCRIPT the potentiodynamic polarization tests were conducted at a constant scan speed of 1 mVs-1. The corrosion current density and corrosion potential were determined based on Tafel's extrapolation.

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3. Results and discussion 3.1 XRD spectrum of coatings

The X-ray diffraction (XRD) patterns of Ni-Co and sol-enhanced Ni-Co-TiO2 nano-composite coatings are shown in Fig. 1. According to the equilibrium Ni-Co

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phase diagram, there is a single phase (α) solid solution extending from pure Ni to 65 at.% Co, with two phase (α+ε) region extending from 65 at.% to 75 at.% Co, and a

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single phase (ε) region continuing to pure Co at ambient condition. The Co content of the sample is around 40 at.% according to our preliminary XRF results. Therefore, the XRD patterns are presented as face-centered cubic solid solution phase structure with a slight shift of the pure Ni peaks. The shift should be attributed to the Co incorporation in the crystalline lattice of Ni [25]. No TiO2 peaks could be seen from

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the sol-enhanced Ni-Co-TiO2 nano-composite coatings, probably due to the low quantity and highly dispersive form of the nano-particles.

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3.2 Cross-section of coatings

The cross-section morphology of Ni-Co coating and sol-enhanced Ni-Co-TiO2 nano-composite coatings was analyzed by ESEM as shown in Fig. 2. All coatings

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present a similar thickness of approximately 9 µm. The deposition rate of coatings remains substantially constant with an increasing sol content of the plating solution. No defects or cracks were observed at the interfaces of either the Ni-Co or the sol-enhanced Ni-Co-12.5 mL/L TiO2 coatings (Figs. 2a and 2b), providing evidence of good adhesion between the steel substrate and coating. No obvious TiO2 particles were observed in the cross-section of Ni-Co-12.5 mL/L TiO2 coating, which is probably due to their small size and the relatively low TiO2 content of the coating. However, with increasing TiO2 sol concentration in the bath, detachment and cracking lines can be clearly seen in Fig. 2c. There are small Ti enriched areas along the

ACCEPTED MANUSCRIPT cracking lines, which were confirmed by the EDS probe. The addition of excessive quantities of TiO2 sol tends to cause the agglomeration of TiO2 nanoparticles, and create a porous structure at the grain boundaries, leading to a deterioration of coating

3.3 Mechanical property of coatings 3.3.1 Microhardness

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adhesion.

Fig. 3 presents the microhardness value of the as-deposited Ni-Co-TiO2

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nano-composite coatings as a function of TiO2 sol concentration in the bath. The microhardness of Ni-Co coating was ~651 HV. At a low sol concentration,

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microhardness increases significantly with increasing sol addition. The microhardness was increased to the peak value of ~834 HV when the TiO2 sol concentration is 12.5 ml/L. However, further increases of the concentrations of TiO2 sol result in a decrease of coating microhardness. The microhardness of sol-enhanced Ni-Co-15 mL/L TiO2 and Ni-Co-20 mL/L TiO2 coatings were decreased to ~736 HV and ~657 HV

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respectively. When the sol concentration is equal to or greater than 20 mL/L, the microhardness of coatings decreased to the same level as the pure Ni-Co coating.

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The improvement of microhardness should be attributed to the highly dispersed TiO2 nanoparticles in the coating matrix. After adding the sol into the electrolyte, TiO2 nanoparticles were formed in-situ, and then absorbed and embedded into the

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deposited surface. The strengthening mechanism resulting from particles dispersion hardening is known as Orowan mechanism: closely spaced nano-particles in a coating matrix can result in good improvement of mechanical property. The Orowan mechanism also indicates that the smaller size of dispersed particles and shorter distance between them, the better strengthening effect to the alloy [22].

Fig. 4 shows the TEM images and electron diffraction patterns of Ni-Co and sol-enhanced Ni-Co-12.5 mL/L TiO2 coatings. The Ni-Co coating shows a mixed crystal structure which confirmed by the inserted electron diffraction pattern in Fig.

ACCEPTED MANUSCRIPT 4a. The black points highlighted in the round frame were pitting caused by the ion beam thinning during the TEM sample preparation. As for the sol-enhanced coating with the optimal mechanical properties, highly dispersed TiO2 nano-particles with an average size below 15 nm were distributed uniformly throughout the coating as shown

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in Fig. 4b. This result indicates that the proper addition of sol (12.5 mL/L) can effectively avoid particle agglomeration and produce a highly dispersed distribution of TiO2 nano-particles in the coating matrix, significantly improving the mechanical properties of the coating. However, further addition of TiO2 sol beyond the optimum

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resulted in a significant agglomeration of TiO2 nanoparticles in the plating solution. These agglomerated nano-particles were embedded into the growing Ni-Co coating

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matrix, formed a loose cluster and a porous structure nearby [26]. The larger particles reduce the effect of dispersion strengthening and lead to a deterioration of the mechanical property.

3.3.2 Wear resistance

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Tribological and wear property of Ni-Co-TiO2 coatings were studied. Fig. 5 shows the average friction coefficient of the coatings. The Ni-Co coating possesses the highest friction coefficient (~0.619) compared with the sol treated coatings. The coating

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produced with an optimum concentration of sol (12.5 mL/L) showed an average friction coefficient of ~0.547.

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The decrease in friction coefficient is probably because that the incorporation of a proper amount of TiO2 nanoparticles in the coating matrix can play the role of solid lubricant during the wear process. However, when excessive sol were added into the electrolyte, large amount of nanoparticles in-situ formed and tend to agglomerate as clusters. These large clusters were co-deposited into the Ni-Co coating matrix with other nano-particles of different sizes, forming a rough coating surface and porous structure during the plating process. The mechanical interaction plays a more active role during the wear process. The average friction coefficient of Ni-Co-50 mL/L TiO2 increases to ~0.579.

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The wear track images, wear track width and wear volume loss of Ni-Co coating and sol-enhanced nano-composite Ni-Co-TiO2 coating were shown in Figs. 6 and 7. It can be observed that the Ni-Co coating has a relatively wide wear track with a width,

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depth and volume loss of ~409 µm, ~7.0 µm and ~4.59×10-13 m3, respectively. There are obvious plowing lines and some large wear debris on the worn surface. The main wear mechanism of Ni-Co coating is adhesive wear, which is the most common wear

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mechanism for electroplated Ni-Co, Ni-P and Ni-B coatings.

After adding TiO2 sol into the plating solution, the worn area decreases and the plough

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lines became shallower and more uniform. This is probably caused by the incorporation of nanoparticle reinforcement. These embedded nanoparticles may have the polishing effect on the frictional surface, and disperse the contact pressure between the frictional counter bodies during the wear process.

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On the coating produced with 12.5 mL/L sol, the corresponding wear track width and wear volume loss decreased to ~308 µm, ~3.96 µm and ~1.47×10-13, respectively. The significant improvement of wear resistance with hardness increase and friction

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coefficient decrease could be interpreted by using Archard’s law. According to the Archard’s law, under the same wear test conditions, the wear rate is inversely proportional to the material hardness and proportional to the friction coefficient [10,

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13]. Although the hardness of Ni-Co-50 mL/L TiO2 coating is at the same level as Ni-Co coating, the wear resistance (~340 µm, ~4.82 µm and ~2.19×10-13) is still much better than that of pure Ni-Co coating due to its lower friction coefficient.

3.4 Corrosion property of coatings Fig. 8 presents the polarization curves of Ni, Ni-Co and sol-enhanced Ni-Co nano-composite

coatings.

The

electrochemical

properties

obtained

from

potentiodynamic curves are listed in Table 1. The corrosion potential and corrosion current density of mild steel substrate were ~ -0.522 V and 11.402 µA/cm2 Comparing

ACCEPTED MANUSCRIPT with the mild steel with a corrosion potential Ecorr= and corrosion current density Icorr The corrosion potential Ecorr and corrosion current density Icorr of Ni coating were ~ -0.385 V and 3.112 µA/cm2, while the Ecorr and Icorr value obtained for Ni-Co coating were ~ -0.488 V and 7.064 µA/cm2, respectively. In principle, higher corrosion

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potential and lower corrosion current density indicate a better corrosion property. Ni deposit showed better corrosion resistance because of its dense nature and free from local defects compared to Ni-Co deposits.

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The corrosion potential Ecorr and corrosion current density Icorr of sol-enhanced Ni-Co-12.5 mL/L TiO2 coating was observed to be ~ -0.403 V and ~5.244 µA/cm2,

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respectively, indicating an improved corrosion resistance but still below the traditional Ni coating. Excessive sol addition caused a deterioration of the corrosion resistance of the composite coating, evidenced by the Ecorr of -0.454 V and increased Icorr of ~5.903 µA/cm2 for sol-enhanced Ni-Co-50 mL/L TiO2 coating.

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When proper amount of TiO2 sol (12.5 mL/L) was added, fine TiO2 nano-particles were incorporated uniformly into the coating matrix as evidenced by Figs. 2b and 4b. The TiO2 nano-particles were embedded in the Ni-Co matrix and filled in gaps, cracks

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and micro holes, resulting in a denser microstructure. Additionally, these nano-sized TiO2 particles have high corrosion resistance themselves and can act as inert physical barriers to the initiation and development of defect corrosion, substantially improving

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the corrosion resistance of the coating [27].

When excessive sol (50 mL/L) was added into the electrolyte, the corrosion resistance decreased due to nanoparticles agglomeration. The large quantity of TiO2 nano-particles in the plating bath commence to agglomerate prior to depositing onto the coating surface due to their small size and high surface energy. Then these agglomerates and nano-particles were deposited under the combined drive of Van der Waals forces, gravity and mechanical movement, driven by continuous agitation. A porous structure was formed due to the heterogeneous distribution as confirmed by

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4. Conclusion A novel Ni-Co-TiO2 nano-composite coating was electrodeposited by utilizing an

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innovative sol-enhanced method. The microhardness has been significantly increased to ~834 HV comparing with ~651 HV of Ni-Co coating. The wear resistance and corrosion resistance of this sol-enhanced coating have also been improved significantly. By adding proper amount of sol solution into the electrolyte, TiO2

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nano-particles with a size of ~15 nm were highly dispersed within the coating matrix, resulting in a significant dispersion strengthening effect. Excessive of sol addition

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caused a porous structure, resulting in a deterioration of dispersion strengthening effect and decreasing corrosion resistance. Further investigations are being conducted to study the magnetic property of this novel Ni-Co-TiO2 nano-composite coating in order to broaden its application.

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Acknowledgments

The project is funded by NPRP Grant #NPRP-4-662-2-249 from the Qatar National Research Fund (a member of Qatar Foundation). It is also supported by a New Marsden

Grant,

China

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Zealand

Postdoctoral

Science

Foundation

project

(2013M531849) and Auckland UniServices project. The authors would like to thank

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the technical stuff in the Department of Chemical and Materials Engineering and the Research Centre of Surface and Materials Science for various assistances. We also want to express our gratitude to Mr Glen Slater, Chris Goode and technical stuff in Rigg Electroplating Ltd.

References [1] Z. X. She, Q. Li, Z. W. Wang, C. Tan, J. C. Zhou, L. Q. Li. Highly anticorrosion, self-cleaning superhydrophic Ni-Co surface fabricated on AZ91 magnesium alloy.

ACCEPTED MANUSCRIPT Surf. Coat. Technol. 251(2014) 7-14. [2] B. Tury, G. Z. Radnóczi, G. Radnóczi, M. L. Varsányi. Microstructure properties of pulsed plated Ni-Co alloy. Surf. Coat. Technol. 202 (2007) 331-335. [3] M. Srivastava, V. Ezhil Selvi, V. K. William Grips, K. S. Rajam. Corrosion

Coat. Technol. 201 (2006) 3051-3060.

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resistance and microstructure of electrodeposited nickel-cobalt alloy coatings. Surf.

[4] Y. H. You, C. D. Gu, X. L. Wang, J. P. Tu. Electrodeposition of Ni-Co alloys from a deep eutectic solvent. Surf. Coat. Technol. 206 (2012) 3632-3638.

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[5] Y. D. Li, H. Jiang, D. Wang, H. Y. Ge. Effects of saccharin and cobalt concentration in electrolytic solution on microhardness of nanocrystalline Ni-Co

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alloys. Surf. Coat. Technol. 202 (2008) 4952-4956.

[6] A. Masoero, B. Morten, G.L. Olcese, M. Prudenziati, F. Tango, F. Vinai. Magnetic properties of Ni-Co thick-film magnetoresistors. Thin. Solid. Films. 350 (1999) 214-218.

[7] Arthur A. Tracton. Coatings materials and surface coatings. 2007, CRC Press,

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Boca Raton.

[8] Arthur A. Tracton. Coatings Technology Handbook. 2006, CRC Press, Boca Raton.

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[9] Nasser Kanani, Electroplating, Elsevier Advanced Technology, Oxford, 2004. [10] L. P. Wang, Y. Gao, Q. J. Xue, H. W. Liu, T. Xu. Microstructure and tribological properties of electrodeposited Ni-Co alloy deposits. Appl. Surf. Sci. 242 (2005)

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326-332.

[11] L. Shi, C. F. Sun, P. Gao, F. Zhou, W. M. Liu. Electrodeposition and characterization of Ni-Co-carbon nanotubes composite coatings. Surf. Coat. Technol. 200 (2006) 4870-4875.

[12] S. T. Oh, M. Sando, K. Niihara. Mechanical and magnetic properties of Ni-Co dispersed Al2O3 nanocomposites. J. Mater. Sci. 36 (2001) 1817-1821. [13] Meenu Srivastava, V. K. William Grips, Anjana Jain, K. S. Rajam. Influence of SiC particle size on the structure and tribological properties of Ni-Co composites. Surf. Coat. Technol. 202 (2007) 310-318.

ACCEPTED MANUSCRIPT [14] B. Bahadormanesh, A. Dolati. The kinetics of Ni-Co/SiC composite coatings electrodeposition. J. Alloys Comp. 504 (2010) 514-518. [15] Meenu Srivastava, V.K. William Grips, K.S. Rajam. Electrodeposition of Ni-Co composites containing nano-CeO2 and their structure, properties. Appl. Surf. Sci. 257

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(2010) 717-722. [16] M. Srivastava, V.K. William Grips, K.S. Rajam, Structure and properties of electrodeposited Ni-Co-YZA composite coatings. J. Appl. Electrochem. 38 (2008) 669-677.

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[17] M. Srivastava, V.K. William Grips, K.S. Rajam, Influence of Co on Si3N4 incorporation in electrodeposited Ni. J. Alloys Compd. 469 (2009) 362-365.

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[18] B. Ranjith, G. Paruthimal Kalaignan. Ni-Co-TiO2 nanocomposite coating prepared by pulse and pulse reversal methods using acetate bath. Appl. Surf. Sci. 257 (2010) 42-47.

[19] W. W. Chen, W. Gao. Plating or coating method for producing metal-ceramic coating on a substrate. Patent NZ578038. New Zealand; 2009.

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[20] Z. G. Zang, X. S. Tang. Enhanced fluorescence imaging performance of hydrophobic colloidal ZnO nanoparticles by a facile method. J. Alloys Compd. 619(2015) 98-101.

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[21] W. W. Chen, Y. D. He, W. Gao. Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings. Surf. Coat. Technol. 204 (2010) 2487-2492. [22] Y. X. Wang, S. J. Wang, X. Shu, W. Gao, W. Lu, B. Yan. Preparation and property

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of sol-enhanced Ni-B-TiO2 nano-composite coatings. J. Alloys Comp. 617 (2014) 472-478.

[23] W. W. Chen, W. Gao, Y. D. He. A novel electroless plating of Ni-P-TiO2 nano-composite coatings. Surf. Coat. Technol. 204 (2010) 2493-2498. [24] Y. X. Wang, X. Shu, S. H. Wei, C. M. Liu, W. Gao, R. A. Shakoor, Ramazan Kahraman. Duplex Ni-P-ZrO2/Ni-P electroless coating on stainless steel. J. Alloys Comp. 630 (2015) 189-194. [25] Y. D. Li, H. Jiang, D. Wang, H. Y. Ge. Effects of saccharin and cobalt concentration in electrolytic solution on microhardness of nanocrystalline Ni-Co

ACCEPTED MANUSCRIPT alloys. Surf. Coat. Technol. 202 (2008) 4952-4956. [26] Y. X. Wang, S. H. Wei, Y. Ju, W. Lu, B. Yan. Mechanical properties and microstructure of Au-Ni-TiO2 nano-composite coatings. Mater. Charact. 102(2015): 189-194.

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[27] P. Q. Dai, Y. H. Zhong, X. Zhou. Corrosion characteristic of pulsed electrodeposition Ni-Co/SiC nanocomposite coating. Surf. Eng. 2011, 27(1): 71-76.

[28] B. R. Tian, Y. F. Cheng. Electrolytic deposition of Ni-Co-Al2O3 composite coating on pipe steel for corrosion/erosion resistance in oil sand slurry. Electrochimica

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Acta, 2007, 53: 511-517.

ACCEPTED MANUSCRIPT Figure and Table Captions: Fig. 1 XRD spectra of Ni-Co and sol-enhanced Ni-Co-TiO2 nano-composite coatings Fig. 2 Cross-section morphology of: (a) Ni-Co and sol-enhanced nano-composite coating (b) Ni-Co-12.5 mL/L TiO2, (c) Ni-Co-50 mL/L TiO2

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Fig. 3 Microhardness of sol-enhanced Ni-Co-TiO2 nano-composite coating Fig. 4 TEM images of coatings: (a) Ni-Co coating and (b) sol-enhanced Ni-Co-12.5 mL/L TiO2 coating. (The arrows indicate the TiO2 nano-particles)

Fig. 5 Average friction coefficient of (a) Ni-Co and sol-enhanced nano-composite

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coating (b) Ni-Co-12.5 mL/L TiO2, (c) Ni-Co-50 mL/L TiO2

Fig. 6 Wear track images: (a) Ni-Co and sol-enhanced nano-composite coating (b)

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Ni-Co-12.5 mL/L TiO2, (c) Ni-Co-50 mL/L TiO2

Fig. 7 Wear depth and wear volume of (a) Ni-Co and sol-enhanced nano-composite coating (b) Ni-Co-12.5 mL/L TiO2, (c) Ni-Co-50 mL/L TiO2

Fig. 8 Potentiodynamic polarization curves: (a) mild steel, (b) Ni, (c) Ni-Co, (d) Ni-Co-12.5 mL/L TiO2, and (e) Ni-B-50 mL/L TiO2 composite coatings in 3.5% NaCl

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aqueous solution.

Table 1 Electrochemical parameters of potentiodynamic polarization curves calculated

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from the Tafel extrapolation method.

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Fig. 1 XRD spectra of Ni-Co and sol-enhanced Ni-Co-TiO2 nano-composite coatings

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Fig. 2 Cross-section morphology: (a) Ni-Co and sol-enhanced nano-composite coating, (b) Ni-Co-12.5 mL/L TiO2, and (c) Ni-Co-50 mL/L TiO2

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Fig. 3 Microhardness of sol-enhanced Ni-Co-TiO2 nano-composite coating

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Fig. 4 TEM images of coatings: (a) Ni-Co coating, and (b) sol-enhanced Ni-Co-12.5 mL/L TiO2 coating. (The arrows indicate the TiO2 nano-particles)

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Fig. 5 Average friction coefficients: (a) Ni-Co and sol-enhanced nano-composite

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coating, (b) Ni-Co-12.5 mL/L TiO2, and (c) Ni-Co-50 mL/L TiO2

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Fig. 6 Wear track images: (a) Ni-Co and sol-enhanced nano-composite coating, (b) Ni-Co-12.5 mL/L TiO2, and (c) Ni-Co-50 mL/L TiO2

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Fig. 7 Wear depth and wear volume: (a) Ni-Co and sol-enhanced nano-composite

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coating, (b) Ni-Co-12.5 mL/L TiO2, and (c) Ni-Co-50 mL/L TiO2

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Fig. 8 Potentiodynamic polarization curves: (a) mild steel, (b) Ni, (c) Ni-Co, (d)

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Ni-Co-12.5 mL/L TiO2, and (e) Ni-B-50 mL/L TiO2 composite coatings in 3.5% NaCl

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aqueous solution.

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Table 1 Electrochemical parameters of potentiodynamic polarization curves calculated from the Tafel extrapolation method. Ecorr (V vs. SCE)

Icorr (×10−6 A/cm2)

Mild steel

-0.522±0.003

11.402±0.008

Ni

-0.385±0.002

3.112±0.004

Ni-Co

-0.488±0.003

7.064±0.006

Ni-Co-12.5 mL/L TiO2 -0.403±0.002

5.244±0.003

Ni-Co-50 mL/L TiO2

5.930±0.006

AC C

EP

TE D

SC

M AN U

-0.454±0.004

RI PT

Sample

ACCEPTED MANUSCRIPT

Highlights

A novel Ni-Co-TiO2 nano-composite coating was developed based on the watts

RI PT

solution.

Highly dispersed Ni-Co-TiO2 nano-composite coating was in-situ electrodeposited.

AC C

EP

TE D

M AN U

SC

This novel coating may find broad applications after further modification.