Investigation on microstructure and properties of electrodeposited Ni-Ti-CeO2 composite coating

Accepted Manuscript Investigation on microstructure and properties of electrodeposited Ni-Ti-CeO2 composite coating Lianbo Wang, Yuantao Zhao, Chuanhai Jiang, Vincent Ji, Ming Chen, Ke Zhan, Florent Moreira PII:

S0925-8388(18)31622-0

DOI:

10.1016/j.jallcom.2018.04.288

Reference:

JALCOM 45926

To appear in:

Journal of Alloys and Compounds

Received Date: 24 January 2018 Revised Date:

25 April 2018

Accepted Date: 26 April 2018

Please cite this article as: L. Wang, Y. Zhao, C. Jiang, V. Ji, M. Chen, K. Zhan, F. Moreira, Investigation on microstructure and properties of electrodeposited Ni-Ti-CeO2 composite coating, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.04.288. 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.

ACCEPTED MANUSCRIPT Investigation on microstructure and properties of electrodeposited Ni-Ti-CeO2 composite coating

Ke Zhanc, Florent Moreirab a

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Lianbo Wanga, Yuantao Zhaoa, Chuanhai Jiang a∗, Vincent Jib, Ming Chena,

School of Materials Science and Engineering, Shanghai Jiao Tong University, No.800

b

ICMMO/LEMHE, UMR 8182, Université Paris-Sud 11, Orsay Cedex, 91405 France

School of Materials Science and Engineering, University of Shanghai for Science and

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c

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Dongchuan Road, Shanghai 200240, P.R. China

Technology, 516 Jungong Road, 200093, Shanghai，PR China

Abstract

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Ternary Ni-Ti-CeO2 composite coatings were synthesized from Watts bath containing various concentrations of mixed particles including Ti microparticles and CeO2 nanoparticles (fixed ration 10 : 1 by weight) by electrodeposition.

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Microstructures of the composite coatings were comprehensively investigated by

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XRD, SEM, AFM and TEM. The incorporation of mixed particles gave birth to grain refinement, attenuation of soft mode [200] texture and random-orientated grain growth of nickel deposits. Typically, as for the composite coating electrodeposited form Watts bath containing relatively low concentrations (less than 22 g/L) of mixed particles, the presence of pores inside the coatings, asperities on the surface and cluster-like distribution of Ti microparticles in composite coating, restricted the

∗

Corresponding author, E-mails: [email protected] (Chuanhai Jiang) 1

ACCEPTED MANUSCRIPT improvement of properties. By increasing concentration of mixed particles to 88 g/L, high-quality Ni-Ti-CeO2

composite

coating

with

uniformly distributed

Ti

micropariticles was achieved. The incorporation of Ti microparticles should take great

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responsibility for the presence of pores and asperities, while the incorporation CeO2 nanoparticles played very important role in the formation of dense and smooth composite coatings. Microhardness and wear resistance were also improved by

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increasing contents of mixed particles in composite coatings. The incorporation of

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mixed particles leaded to increasement of friction coefficient and the friction coefficient presented increasing tendency by increasing the concentrations of mixed particles in Watts bath.

Keywords: Ni-Ti-CeO2 composite coating; Ti microparticles; CeO2 nanoparticle;

1. Introduction:

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incorporation behavior; microstructure; wear resistance.

Electroplated Ni matrix composite coatings by co-deposition of second phase

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solid particles have been intensively investigated in recent years [1-3]. Various kinds

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of particles (e.g. Al2O3, SiC, CeO2, Al and Ti) have been applied as reinforcement to fabricate Ni matrix composite coatings [4-8]. The incorporated particles can bring in evolution of microstructure and endow the composite coatings with desired properties like satisfied corrosion resistance, excellent oxidation resistance and good wear resistance [1, 2, 8]. Typically, among these particles reinforced Ni matrix composite coatings, Ni-Ti composite coatings have received considerable attention for several decades. Zhao et al. produced Ni-Ti composite coatings by co-electrodeposition of Ti 2

ACCEPTED MANUSCRIPT nanoparticles and the Ni-Ti composite coatings possessed improved hardness and corrosion resistance compared with pure Ni coating [8]. However, as an active metal, the synthesis process and storage of Ti nanoparticles is critical and complicated,

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which greatly limit its wide application. Therefore, the application of Ti microparticles is more promising and meaningful. Considerable efforts have already been devoted to improve the properties of Ni matrix composite coatings by

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incorporating Ti microparticles [9-14]. Napyoszek-Bilnik et al. electroplated Ni-Ti

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composite coatings on carbon steel by addition of Ti microparticles [12]. S. Srikomol et al. synthesized Ti micorparticle reinforced Ni-Ti composite coatings with improved hardness [11]. Despite the incorporation of Ti microparticles bring in enhanced properties at some degree, there are always some unsatisfied defects like porosity,

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huge asperities on surface and non-uniformly distributed Ti microparticles found [11, 12, 14], which still confine the further application of Ti microparticles reinforced Ni-Ti composite coatings. To compensate these unsatisfied defects, some measures

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have been done by researchers. Ni-Ti composite coatings were annealed in vacuum

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with a pre-vacuum of about 10-5 Torr, which produced a dense and homogenous layer [15]. A. Serek et al [13] modified the structure and properties of Ti microparticles reinforced Ni-Ti composite coatings by a subsequent step of heat treatment in nitrogen atmosphere. Compact and smooth Ni-Ti composite coatings were obtained and they attributed the enhanced hardness to the presence of new phase TiN. Chufeng Sun et al [16] also developed the hardness and wear resistance of Ti microparticles reinforced Ni-Ti composite coating by heat treatment in nitrogen atmosphere. Although the 3

ACCEPTED MANUSCRIPT additional step of heat treatment ameliorates the structures and properties of the Ti microparticle reinforced Ni composite coatings, this inevitably increases the costs and makes the synthesis process more complex. Further efforts are required to be done to

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compensate the weakness of Ti microparticle reinforced composite coatings in a convenient way.

As one of the powerful rare earth oxides, CeO2 nanoparticles have been

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extensively employed in Ni matrix composite coatings [6, 17-20]. Yujun Xue [20]

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reported that Ni-CeO2 composite coatings possessed better wear resistance than that of pure Ni coating. Meenu Sirvastava et al. proposed enhanced corrosion resistance and improved hardness were obtained by the addition of CeO2 nanoparticles [21]. Moreover, co-electrodeposition of nanoparticles like Al2O3 and CeO2 into metal

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matrix might give birth to compact and smooth composite coating excepting evolution of microstructure like grain refinement [3, 22]. Co-electrodeposition of various kinds of particles together into a deposit is an effective and convenient way to combine the

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characteristics of different metallic or nonmetallic material in a desirable fashion, e.g.

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Ni-TiO2-SiC, Ni-Al-CeO2 and Ni-Cr-CeO2 composite coatings [19, 23, 24]. As a result, co-electrodeposition of Ti microparticles and CeO2 nanoparticles has the potential to overcome the disadvantages of Ti microparticle reinforced Ni matrix composite coatings.

In present work, ternary Ni-Ti-CeO2 composite coatings were fabricated on 304 stainless steel substrate by electrodeposition from Watts bath containing various concentrations of Ti microparticles and CeO2 nanoparticles (fixed ration 10 : 1 by 4

ACCEPTED MANUSCRIPT weight). Microstructures of the composite coatings were comprehensively investigated in this work. Wear measurement under different loads were performed to explore the mechanical properties of the composite coatings. The aim of this work

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was to investigate the evolution of microstructure and wear properties of Ni-Ti-CeO2 composite coatings and finally to produce the ternary Ni-Ti-CeO2 composite coating with satisfied microstructure and enhanced wear properties by adjusting the

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concentrations of Ti microparticles and CeO2 nanoparticles.

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2. Experimental

2.1. Electrodeposition of Ni-Ti-CeO2 composite coating

The electroplated Ni-Ti-CeO2 composite coatings were synthesized from traditional Watts bath containing various concentrations of Ti microparticles (99.9 %

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in purity, ST-Nano Science and Technology CO., LTD) and CeO2 nanoparticles (99.9 % in purity, Aladdin Reagent). As shown in Fig. 1, the average sizes of Ti microparticles

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and CeO2 nanoparticles were about 2 µm and 100 nm respectively. Both shapes of Ti microparticles and CeO2 nanoparticles were irregular and polyhedral. The ration of Ti

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microparticle concentration and CeO2 nanoparticle concentration in Watts bath were fixed to 10 : 1. The concentrations of mixed particle including Ti microparticles and CeO2 nanoparticles in Watts bath were 0 g/L, 11 g/L, 22 g/L, 44 g/L and 88 g/L. Prior to electrodeposition, the Watts bath were pretreated as below to adequately disperse the particles: (a) magnetically stirred for 3 h; (b) ultrasonic dispersion for 0.5 h in a cuboid ultrasonic instrument (DL-120A, Shanghai Zhixin Instrument Co. Ltd.).

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ACCEPTED MANUSCRIPT The 304 stainless steels and pure nickel plates (99.5 % in purity, Aladdin Reagent) were used as cathode and anode, respectively. The use of 304 stainless steel is low cost and easy preparation but on the basis of the accuracy of the experiments.

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The vertical cathode (1.5 × 1.5 cm2) and anode (3.0 × 3.0 cm2) were parallelly placed in electrolyte at a constant distance 30 mm. The temperature was kept at 45 ± 2 ℃ by placing the plating tank in the water bath kettle during electrodeposition. Before

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electrodeposition, the substrates were degreased in 10 vol. % HCl bath for 15 s and

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then washed using deionized water. The electrodeposition was performed with an applied direct current density of 5 A/cm2 under a continuous magnetic stirring of 300 r/min for 80 mins. After electrodeposition, the resultant samples were cleaned using deionized water with the help of ultrasound. Table 1 shows the detail

2.2. Characterization

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electrodeposition conditions and compositions of Watts bath.

The surface morphology and cross-section of the samples were examined using

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field-emission scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and the

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affiliated dispersive X-ray spectroscopy (EDS) was utilized to evaluate the composition of the composite coatings. Typically, the CeO2 contents in the composite coatings were semi-quantitatively expressed in the form of Ce mass ratio evaluated from the EDS results. Atomic force microscope (AFM, Nanoscope V, Veeco USA) was applied here to explore the surface topography. The surface roughness of the composite coating was quantitatively investigated from the result of AFM data with the help of software “Gwyddion”. The crystal structure and crystallographic 6

ACCEPTED MANUSCRIPT orientation were achieved using X-ray diffraction (XRD, Ultima IV and SmartLab, Rigaku, Janpan). The Rietveld XRD refinement [25] was employed to explore the microstructure (e.g. grain size and microstrain) of the ternary Ni-Ti-CeO2 composite

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coatings with the help of software MAUD. The simulated XRD patterns were obtained considering the presence of Ni phase, Ti phase and CeO2 phase. Simultaneously, the effects of diffraction optics and instrumental factors are taken in

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to account. The peak-shape function is assumed to be pseudo-Voigt (pV) function and

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can be expressed as below:

Pv(2θ) = ∑  (1 − )(1 +   ) + exp (−2 ×   ) Where S = (2θ − 2θ )/ ,



(1)

is the Bragg angle corresponding to !

radiation, A is the asymmetric part of the instrumental function. ,  and  are

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the full width at half maximum. Gaussian component and the scale parameter of pV function, respectively. The crystallite size and microstrain of the ternary Ni-Ti-CeO2 composite coatings were estimated by using Popa medel of anisotropic “size-strain”

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broading. The shift, broadening and asymmetry in the profile induced by deformation

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or twin faults were refined by using Warren model. The texture of the electrodeposited coatings was refined by using Harmonic texture model. Transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) was applied to authenticate the distributions of Ni grain sizes and investigate the electrodeposition behavior of the Ti microparticles and CeO2 nanoparticles in Ni deposits.

Wear properties of different coatings were characterized by a reciprocating tribometer (UMT-Tribolab, BRUKER, USA). GCr15 steel balls (Ø 10 mm) were used 7

ACCEPTED MANUSCRIPT as counterparts during wear properties measurements. The wear measurements were performed at different loads of 2 N, 4 N and 6 N under frequency of 5 Hz over a stroke of 10 mm. Accordingly the line velocity was 0.1 m/s and the total sliding

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distance was 120 m. The duration of each wear test was 20 mins. The weight loss of each sample after wear measurements was measured using a precise electronic analytical balance with an accuracy of 0.01 mg (Discovery DV215CD, Ohaus

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Corporation, USA). To avoid data scattering, three measurements were performed and

3. Results and Discussion

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the values of weight loss were expressed by the average value of this three results.

3.1 Composition and microstructure of the coatings

As shown in Fig 2, obviously, both of the contents of Ti and CeO2 increased by

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increasing the concentrations of mixed particles in Watts bath. The contents of CeO2 increased from ~0.75 wt % to ~6.43 wt % and Ti increased from ~4.23 wt % to ~9.04 wt % in composite coatings as the concentration of mixed particles in Watts bath

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raised from 11 g/L to 88 g/L. Interestingly, the slope of the curve of CeO2 contents is

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larger than that of Ti contents (see in in Fig. 2), illustrating that the incorporation rate of CeO2 nanoparticles was larger than that of Ti microparticles, as the mixed particles concentration increased from 22 g/L to 88 g/L. Particle size is one of the significant effects on incorporation rate into metal deposit [2]. That is, CeO2 nanoparticles with smaller size might possess a higher probability to be captured by the growing Ni deposits than that of the larger Ti microparticles during electrodeposition [26, 27]. In addition, the surface conditions played very important roles in the co-deposition 8

ACCEPTED MANUSCRIPT behavior of particles due to the difference of surface physical or chemical properties, which also had great influence on incorporation rate of particles. Rietveld XRD refinement was applied to explore microstructure of Ni-Ti-CeO2

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composite coatings with the help of software MAUD [25]. As shown in Fig. 3, the black point plot and red solid line were real experimental data and simulated data, respectively. The main diffraction peaks of each phase were marked in Fig. 3. Apart

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from the peaks of nickel deposits, the existence of CeO2 nanoparticles were ensured

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by the peaks located at 2θ = 28.5 o, 33.0 o and 47.4 o, which could be indexed to the diffractions of (111), (200) and (220) planes of CeO2 (JCPDS Card No.43-1002), respectively. The peaks centered at 2θ= 35.0 o, 38.4 o, and 40.1 o, could be assigned to the diffractions of (100), (002) and (101) planes of Ti (JCPDS Card No.89-2959),

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respectively. The result of XRD verified the co-existence of Ti microparticles and CeO2 nanoparticles in Ni deposits.

The obtained grain size and microstrain of Ni deposit of different coatings were

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shown in Fig. 4. Pure Ni coating possessed the largest grain size. Grain sizes reduced

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distinctly from ~199 nm to ~146 nm by the addition of mixed particles (11 g/L). The decrease of Ni grain size might result from the influence of incorporated particle on the growth behavior of Ni grains. During electrodeposition, the captured particles by the growing Ni deposits could provide enormous new nucleation sites due to the effect of heterogeneous nucleation and suppressed the grow rate of Ni grains, which leaded to grain refinement [2]. Besides, the captured particles could refine the grain sizes by disturbing further growth of Ni grains on the cathode during electrodeposition [28]. 9

ACCEPTED MANUSCRIPT Typically, the incorporated CeO2 nanoparticles should play a primary role in grain refinement because of its larger specific surface [29] and small size, which could provide more nucleation sites and more efficiently disturb Ni grain growth compared

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with Ti microparticles. Keep increasing the concentrations of particles from 22 g/L to 88 g/L, grain sizse of Ni deposits further decreased from ~146 nm to ~39 nm. It has to be pointed out although the grain size kept decreasing by increasing the mixed

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particles contents in this work, high contents of incorporated particles would not

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always result in grain refinement. Aggregation would appear at very high contents of incorporated particles in coatings, which could deteriorate the microstructure and bring in lots of defects. The microstrain of the ternary Ni-Ti-CeO2 composite coatings increased with the increase of embedded particle contents as shown in Fig. 4, which

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was in accordance with the result of previous works [28]. The incorporation of mixed particles brought in grain refinement and simultaneously led to the increase of various defects like grain boundaries, dislocations, point defects and phase interface (Ni

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deposits/mixed particles). When the Ni atoms were locally deviated from the

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equilibrium position by these defects, lattice distortion occurs. Therefore, the Ni-Ti-CeO2 composite coating was more heavily strained when high contents of mixed particles were incorporated into the coatings. TEM images under bright field and dark field were performed to examine the

distribution of Ni deposits and to authenticate the certainty of the Ni grain sizes estimated by the Rietveld refinement method. The bright field images illustrated the decreasing tendency of Ni grain sizes by increasing the concentration of mixed 10

ACCEPTED MANUSCRIPT particles in Watts bath. As shown in Fig. 5, this phenomenon could be more clearly discovered in dark field images. Pure Ni coatings presented the largest the grain size of about 226 nm. The grain size of Ni deposits decreased from about 226 nm to about

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156 nm by the addition of mix particles (11 g L-1) in Watts bath. With the mixed particles concentration increasing from 22 g/L to 88 g/L, the grain size further diminished to about 50 nm. The variation tendency of grain sizes obtained by TEM

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was similar with the result estimated by Rietveld XRD refinement. However, the grain

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sizes estimated by Rietveld XRD refinement were smaller than that of the ones estimated from TEM, which was likely due to fact that the XRD calculated grain size was sub-grain size substantially.

To explore the state of the incoporated Ti microparticles in the Ni deposits, TEM

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examination were done and shown in Fig. 6. The result of SEAD at the position B proofed the “polygonal dark phase” was Ti microparticle (see in Fig. 6(b)). It had to been clearfied that the whole Ti microparticle was not a single crystal, which resulted

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from the synthetic process of the Ti microparticle. As depicted in Fig. 6(a), the

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polygonal Ti microparticle was closely embedded in Ni deposits and there was no pores or intergap found between the Ti microparticle and Ni deposits, which could be more clearly observed in the HR-TEM (see in Fig. 6(d)). Interestingly, some Ni grains around the Ti microparticle were in the form of radial growth (see in Fig. 6(a), marked by white dotted lines with arrows). The circumfluent electric field lines around the conductive Ti microparticle might take great resposibility for the radial growth Ni grains on the surface of Ti microparticles during electrodeposition [30, 31]. The 11

ACCEPTED MANUSCRIPT diffraction rings belonged to Ni phase, shown in Fig. 6(c), proving the existence of polycrystalline Ni deposits. The different lattice parameters and coefficients of thermal expansion between Ti particles and nickel deposits might induce a tangential

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tensile hoop stress into the surrounding nickel deposits [32, 33]. This kind of tensile hoop stresses due to occluded Ti particles likely benefited to the gowth of twin crystals [33], which could be observed in Fig. 6(a).

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TEM examination of incoporated CeO2 nanoparticle in the composite have also

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been done to investigate the incoporation behaviour. The position of CeO2 nanopartiles were confirmed by EDS. As shown in Fig. 7(a), the CeO2 nanoparticle was closely embedded in Ni deposits. The HR-TEM clearly demonstrated the interface between the CeO2 nanoparticle and Ni deposits was pore-free (see in Fig.

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7(b)). The growth form of deposited Ni grains around CeO2 nanoparticle was differet from that of Ti microparticle. There was no obvious radial growth Ni grains found around the un-conductive CeO2 nanoparticle, due to the different conductivity with Ti

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microparticles. The tensile hoop stresses due to occluded CeO2 nanoparticles also

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conributed to the gowth of twin crystals [33]. Pole figures were obtained to characterize the microstructure of texture of

different coatings, which was very essential for wear, corrosion and oxidation resistance [34, 35]. As depicted in Fig. 8, pure Ni coating possessed a typical [200] fiber texture, while the [200] texture were depressed by the addition of mixed particles (11 g/L). Keep increasing the concentration of mixed particles to 88 g/L, the [200] texture vanished (see in Fig. 8(c)). Therefore, we could conclude that the 12

ACCEPTED MANUSCRIPT incorporation of mixed particles lead to the change of Ni grains from (200) preferential grain growth to random-orientated grain growth. This could be explained by the effect of incorporated mixed on grain refinement and disturbing the preferred

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oriention growth [28]. 3.2 Surface morphology of coatings

Fig. 9 shows the surface morphology of different coatings. Pure Ni coating

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demonstrated typical “pyramidal” structure. The “pyramidal” structure changed to

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“spherical” structure by introducing mixed particles (see in Fig. 9(b)). It could be likely due to grain refinement of the Ni deposits resulted from the incorporated particles [8, 28]. There were no obvious variations on the surface morphologies between Ni-Ti-CeO2 composite coatings containing various contents of mixed

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particles, excepting the average sizes of the protuberances on surface decreased by increasing the concentrations of mixed particles from 11 g/L to 88 g/L in Watts bath. The composite coating, electrodeposited from Watts bath containing 88 g/L

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mixed particles, was fractured in liquid-nitrogen (about -197 ℃) to originally discover

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the distribution state of embedded particles. As presented in Fig. 9(e), intact polygonal Ti microparticles could be easily found. Obviously, the polygonal Ti particles (~2 µm) uniformly dispersed in the Ni deposits and there are no big fluctuations on the surface. It has to be pointed out that the CeO2 nanoparticles were not observed in the cross-section due to the small diameter size (~100 nm). Cross-section views of polished samples were shown in Fig. 10. Obviously, the contents of Ti microparticles increased with increasing the concentration of mixed 13

ACCEPTED MANUSCRIPT particles from 0 g/L to 88 g/L, which was in accordance with the results of EDS. The pure Ni coating was smooth and compact (see in Fig. 10(a)). Compared with the pure Ni coating, the Ni-Ti-CeO2 composite coating electrodeposited from Watts bath

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containing 22 g/L mixed particles was rough and porosity. Introducing a second solid phase into Ni deposits would inevitably change the surface microstructure. However, the embedded “polygon” Ti micro-particles should predominate in the formation of

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coarse surface and porosity rather than CeO2 nanoparticles. This could be explained

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and proved in the cross-section view. As shown in Fig. 10(b) and 10(e), the protuberances on the surface of the Ni-Ti-CeO2 composite coating always located at the positions where Ti microparticles were buried. During electrodeposition, as the conductive Ti microparticles were captured by the growing Ni deposits, nickel

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deposits kept growing on the surface of established Ni deposits and Ti microparticles simultaneously. However, because the edges of captured Ti particles usually possessed lower resistance, nickel ions were preferentially electrodeposited on the surface of

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conductive Ti particles [9]. This could be clearly observed in the cross-section view.

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As depicted in Fig. 10(b), at the last stage of electrodeposition, althoug the captured Ti microparticles have been covered by Ni deposits, the near areas without absorbed Ti microparticles have not been sufficiently replenished by nickel deposits, which greatly contribute to the formation of coarse and porous surface. Typically, some pores inside the coating could be discovered (see in Fig. 10(e)). The pores might be formed when two protuberant parts grew together before the interspace was fully filled by Ni deposits during electrodeposition (see in Fig. 10(e)). The formed pores (see in Fig. 14

ACCEPTED MANUSCRIPT 10(b)) could significantly deteriorate the mechanical properties of the composite coating. In addition, the incorporated Ti microparticles occurred in the form of clusters, illustrating the inhomogeneous distribution of Ti microparticles inside the

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Ni-Ti-CeO2 composite coating electrodeposited from Watts bath containing 22 g/L mixed particles. It has also to be pointed out that an “initial layer (~2 µm) with no or very few Ti microparticles” could be observed [36].

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By increasing the concentration of mixed particles from 22 g/L to 88 g/L, a

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smooth and pore-free high-quality composite coating was achieved despite some small fluctuations on the surface (see in Fig. 10(c) and 10(f)). Although the incorporated Ti microparticles in the composite coating increased, the composite coating became smooth and compact. The increased contents of CeO2 nanoparticles in

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the composite coating should be in charge of these modifications. A. Lozano-Morales et al. also found that the surface microstructure of Ni deposit changed from rough to smooth by the addition of γ-Al2O3 nanoparticles [22]. In addition, the inhomogeneous

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distribution of Ti microparticles became uniform through the whole composite coating

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by increasing the contents of mixed particles from 22 g/L to 88 g/L in the Watts bath (see in Fig. 10(c)). The uniform dispersion of Ti microparticles could embow the composite coating with satisfied mechanical properties. The “initial layer” also disappeared at the interface between substrate and electrodeposited coating. Surface topography and 3D view were obtained by AFM to quantitatively analyze the state of surface roughness, which had important influence on properties like fatigue property and wear resistance. The Ra (the arithmetic average deviation of 15

ACCEPTED MANUSCRIPT outline) was applied here to characterize surface roughness. The values of Ra were 0.247 µm, 0. 586 µm, 0.433 µm 0.279 µm, corresponding to pure Ni coating and Ni-Ti-CeO2 composite coatings electrodeposited from Watts bath containing 11 g/L,

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22 g/L, 88 g/L, respectively. The minimum value of Ra appeared on pure Ni coating and the values of Ra substantially increase to 0.586 µm by addition of mixed particles (11 g/L). The values of Ra progressively decreased from 0.586 µm to 0.279 µm with

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increasing the mixed particles concentration from 11 g/L to 88 g/L in Watts bath. As

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discussed above, the incorporation of Ti microparticles should predominate in the increase of Ra values, while the increasing contents of CeO2 nanoparticles played a very important role in the formation of smooth surface. Simultaneously, the sizes of surface peaks and valleys decreased obviously (see in Fig. 11), which was in

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accordance with the results of the surface morphology (see in Fig. 9). 3.3 Microhardness of the coatings

The measured microhardness was shown in Fig. 12. The values of

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microhardness substantially increased from 243.2 Hv to 360.4 Hv by the addition of

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mixed particles (11 g/L) and progressively increased to 464.6 Hv with increasing the concentrations of mixed particles in Watts bath to 88 g/L. The increased microhardness of Ni-Ti-CeO2 composite coatings could be ascribed to the evolution of microstructures. As shown in Fig. 4 and illustrated in the TEM evaluation of the composite coatings, Ni grain sizes decreased obviously by increasing the contents of mixed particles in the composite coatings. According to the well-known Hall-Petch equation, the microhardness was inversely proportional to grain sizes [37]. Therefore, 16

ACCEPTED MANUSCRIPT the grain refinement of Ni-Ti-CeO2 composite coatings leaded to the increase of microhardness. Furthermore, the transformation of texture was equally important for the improved microhardness. Generally for FCC crystal structure, the presence of low

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atom density (200) plane usually possessed minimum hardness and maximum ductility [38]. As discussed above, the results of pole figure (see in Fig. 8) suggested the incorporation of mixed particle brought in the texture evolution of Ni deposits

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from (200) preferred orientation to the random-orientated grain growth, which also

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contributed to the improved microhardness. In addition, the improved microhardness was related to the dispersion hardening effect induced by the mixed particles inside the composite coating, which could inhibit the movement of dislocation [39]. 3.4 wear behavior of the coatings

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The friction of coefficient (cof) of different composite coatings under different loads were performed and shown in Fig. 13. Obviously, all the cof of the pure Ni coatings under different loads present two distinct regions (see in Fig. 13 (a)),

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including running-in stage and steady stage. The running-in stage was the process to

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remove surface asperities and planish the uneven surface of the coatings [40]. Initially, the contact area between the coatings and the counterpart of steel ball was small. Stress concentration occurred at the positions of the protruding asperities. Severe plastic deformation emerged and the protruding asperities were gradually planished. With wear proceeding, the roughly initial surface changed into smooth and the contact area increased. Then the wear behavior got into the steady stage and the cof became stable. It had to be pointed out that some sharp peaks could be easily found in the 17

ACCEPTED MANUSCRIPT steady stage of pure Ni coating cof curves. This could be associated with the severe plastic deformation and adhesive wear due to the low microhardness. The process of adhesion, delamination, debonding and re-stabilization leaded to these sharp peaks.

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As for the Ni-Ti-CeO2 composite coating, there was no obvious running-in stage found under low load of 2 N (see in Fig. 13(b), (c) and (d)). The cof gradually increased and reached to an relatively stable stage. The disappearance of

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running-stage could be ascribed to the increase of microhardness and yield strength of

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the composite coatings. When the wear behavior began, under the loads of 2 N, although the stress concentrations still existed on the positions of the protruding asperities, plastic deformation level of protruding asperities reduced significantly due to the increased microhardness and yield strength. With wear proceeding, the wear

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damages of protruding asperities were insignificant (see Fig. 15(b) and 15(c)). As the wear tests finished after 20 mins, the wear surfaces were sitll not effectively planished. That was, under low load of 2 N, the whole process of reciprocating sliding tests had

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always been in the stage of “running-in stage” to planished the rough surface of the

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Ni-Ti-CeO2 composite coating. The low load of 2 N was under the bearing capability of the composite coating. Interestingly, keep increasing the loads from 2 N to 4 N and 6 N (see in Fig. 13(b) and 13(c)), the running-stage re-appeared again in the Ni-Ti-CeO2 composite coating, illustrating the protruding asperities could be efficiently removed again. Compared with pure Ni coatings, the durations of running-in stage for Ni-Ti-CeO2 composite coatings were larger under the loads of 4 N and 6 N, which suggested that more time should be taken to remove the surface 18

ACCEPTED MANUSCRIPT asperities due to the improved microhardness and the increased surface roughness. Simultaneously, the amounts of sharp peaks decreased evidently by the addition of mixed particles revealing the degeneration of adhesive wear. The amounts of sharp

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peaks demonstrated decreasing tendency by increasing the contents of mixed particles in the composite coatings.

As shown in Fig. 13(b) and 13(c), the cof curves of the coating electrodeposited

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from Watts bath containing 22 g/L mixed particles was tempestuous, proving the

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metastable wear process, which was adverse to the application of composite coating. The metastable wear process might result from the porosity inside the coating and the jagged surface (see in Fig. 10(b) and 10(e)). By increasing mixed particles concentration to 88 g/L, high-quality composite coatings with no pores were obtained,

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which leaded to the smooth and stable cof curves.

The average cof values were estimated from the steady stage of sliding process and listed in Table 2. Pure Ni coatings possessed the lowest cof values compared with

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Ni-Ti-CeO2 composite coatings, no matter under the loads of 2 N, 4 N and 6 N

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respectively. It was likely due to the different wear behavior between pure Ni coating and Ni-Ti-CeO2 composite coatings. As for the Ni-Ti-CeO2 composite coating, after short periods of sliding wear, stress concentrations emerged between Ni deposits and incorporated particles. With sliding wear proceeding, the embedded particles might be pulled out from the Ni deposits and the moved along with the steel ball. This boosted the abrasion wear and promoted the increase of the cof due to the abrasive effect [41]. Keep increasing the particles concentration from 0 g/L to 88 g/L in Watts bath, the 19

ACCEPTED MANUSCRIPT friction coefficient displayed an increasing tendency under the same load. It was reasonable that the increasing particles concentration was bound to increase the probability of the pulled out particles from Ni deposits during sliding and

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consequently enhanced the friction coefficient. Additionally, the cof presented a decreasing trend by increasing the loads for each sample. Ni oxides usually formed during sliding wear and the formed Ni oxides on the wear surface were favorable for

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reducing the cof [39]. The amounts of the formed Ni oxides increased with increasing

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the loads during sliding wear, because of the higher energy under relatively higher loads.

Weight loss was obtained to examine the wear resistance of different composite coatings and shown in Fig. 14. Obviously, weight loss was inversely proportional to

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concentrations of mixed particles in Watts bath. For example, under the load of 2 N, the weight loss decreased from 2.13 mg to 1.84 mg by the addition of mixed particles (22 g/L). Keep increasing the concentrations of mixed particles from 22 g/L to 88 g/L,

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the weight loss further decreased to 1.37 mg, revealing the enhanced wear resistance.

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The enhanced wear resistance could be correlated with the microstructure evolution of the coatings induced by the incorporation of mixed particles. As discussed above, the incorporation of particles gave birth to grain refinement and the extinction of the [200] texture with the minimum hardness and maximum ductility of Ni deposits, which greatly benefited to the improved microhardness and avoid severe plastic deformation during wear tests. This was in accordance with Archard’s proposal that the wear rate was inversely to its microhardness [39]. Furthermore, the dispersed particles in the 20

ACCEPTED MANUSCRIPT composite coating could restrain severe plastic deformation by obstructing the motion of dislocations. Typically, the reduction of direct contact area between the Ni deposits and sliding ball also played very important roles in decreasing the weight loss during

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wear process [40]. The variation tendency of weight loss under loads of 4 N and 6 N was similar with that under load of 2 N.

The improvement of wear resistance of the Ni-Ti-CeO2 composite coating

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electrodeposited at 22 g/L mixed particles was unsatisfied, which was likely due to the

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deteriorated structure like pores and jagged surface (see in Fig. 10(b) and 10(e)). The protuberant bulges could be easily fractured under tangential stress loaded by the friction coupling and detached from the coating surface during wear, which contributed to the weight loss. In addition, the presence pf pores and inhomogeneous

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distribution of Ti microparticles in the composite coating also bring bad influence for the improvement of wear resistance. By increasing the concentrations of mixed particles to 88 g/L in Watts bath, compact, pore-free and Ti microparticles uniformly

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distributed high-quality Ni-Ti-CeO2 composite coatings were obtained and satisfied

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improvement of wear resistance was achieved. Fig. 15 shows the SEM images of wear tracks under different loads of different

coatings. As for the pure Ni coating under the load of 2 N, adhesion wear was the main wear mechanism and severe plastic deformation could be obviously observed on the wear track (see in Fig. 15(a)), suggesting the poor wear resistance of the pure Ni coating. The level of adhesive wear reduced and wear grooves on the wear surface became more obvious (see in Fig. 15(b)) by incorporation of mixed particles into Ni 21

ACCEPTED MANUSCRIPT deposits. The presence of wear grooves was typical feature of abrasive wear, suggesting the main wear mechanism began transmitting from adhesive wear to abrasive wear by incorporation of mixed particles. The transformation of wear

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mechanism proved the improvement of wear resistance by the addition of mixed particles [41], which was likely due to the modification of microstructure and enhanced microhardness. Keep increasing mixed particles concentration from 22 g/L

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to 88 g/L, wear damage of Ni-Ti-CeO2 composite coating was negligible under the

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low load of 2 N (see in Fig. 15(c)) and no obvious scratches were found on the wear surface. The surface asperities were not entirely removed and the surface was not efficiently planished after 20 mins reciprocating friction. This might be the reason why no running-in stage were found in the cof curves of Ni-Ti-CeO2 composite

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coatings under the low load of 2 N (see in Fig. 14). The width and depth of wear tracks progressively became small by increasing the mixed particles concentrations in Watts bath, revealing the improved wear resistance of the composite coatings as well.

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Fig. 15(d), (e) and (f) show wear tracks of different coatings under the load of 4

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N. Pure Ni coatings still demonstrated the worst wear resistance with severer plastic deformation and adhesive wear. By the addition of mixed particles (22 g/L), the presence of more wear grooves suggested the transformation of wear mechanism from adhesive wear to abrasive wear (see in Fig.15(e)). With the concentrations of mixed particles increasing from 22 g/L to 88 g/L, the level of adhesive wear further decreased and the main wear mechanism was abrasive wear (see in Fig. 15(f)), signifying the improvement of wear resistance [41]. There were no substantial 22

ACCEPTED MANUSCRIPT variation of the wear tracks under the load of 4 N and 6 N, excepting the increasing depth and width of the wear track. 4. Conclusion

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In conclusion, Ni-Ti-CeO2 composite coatings were fabricated on 304 stainless steel substrates by electrodeposition from traditional Watts bath containing different concentrations of mixed particles including Ti microparticles and CeO2 nanoparticles.

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The contents of Ti and CeO2 in composite coatings increased with the increasing

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concentrations of the mixed particles in Watts bath from 0 g/L to 88 g/L. The incorporations of mixed particles gave birth to microstructure evolution of Ni deposits, including grain refinement, attenuation of soft mode [200] texture and random-orientated growth of Ni grain. Interestingly, some radial growth Ni grains

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around Ti microparticles were observed, which was different from the Ni grains around CeO2 nanoparticles. It was likely due to the different electric-field distribution between the conductive Ti microparticles and non-conductive CeO2 nanoparticles

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during electrodeposition.

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The evolution of microstructure greatly contributed to the improvement of microhardness and accordingly to the enhancement of wear resistance. The weight loss decreased with increasing contents of mixed particles in coatings, proving the enhanced wear resistance of the composite coating. By increasing the mixed particles concentration in Watts bath, the main wear mechanism transmitting from adhesive wear to abrasive wear also benefited the enhancement of wear resistance. Typically, the frustratingly enhanced wear resistance of Ni-Ti-CeO2 composite coatings 23

ACCEPTED MANUSCRIPT electrodeposited from Watts bath containing low concentrations of mixed particles (less 22 g/L) might arise from the deteriorated structures like presence of pores inside the coating, protuberances on surface and cluster-like distribution of Ti microparticles.

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Compact, pore-free and smooth composite coatings with uniformly dispersed Ti micraoparticles were obtained by increasing the concentration of mixed particles to 88 g/L, which played very important role in improving wear resistance. The

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incorporation of mixed particles leaded to increasement of friction coefficient and the

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friction coefficient further increased with increasing mixed concentrations in Watts bath. Acknowledgments

This work was financial supported by the National Natural Science Foundation

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of China (51605293). The authors are also greatly thankful to Vicent Morales, Rui Wang and Zhenbiao Dong for the help of synthesizing samples and Materials Testing

Reference:

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and Analysis Center for the help of TEM and SEM.

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Fig caption:

Fig. 1 (a) SEM images of particles Ti microparticles (b) TEM images of CeO2 nanoparticles. Fig. 2 CeO2 and Ti contents in Ni deposits as a function of mixed particles concentrations in Watts bath. 29

ACCEPTED MANUSCRIPT Fig. 3 Results of Rietveld refinement method of the composite coatings electrodeposited in Watts bath containing 88 g/L mixed particles. Fig. 4 Variation of grain sizes and microstrain of different composite coatings.

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Fig. 5 TEM images of different coatings under bright and dark field (a) and (d) pure Ni coatings; Ni-Ti-CeO2 composite coatings electrodeposited from Watts bath containing (b) and (e) 11 g L-1, (c) and (f) 22 g L-1, (d) and (h) 88 g L-1 mixed particles

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Fig. 6 TEM examination of composite coating electrodeposited from Watts bath

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containing 88 g/L mixed particles (a) TEM image of the composite coating (b) SEAD performed at the position B (c) SEAD performed at the position C (d) HR-TEM of interface between Ti particle and Ni deposit.

Fig. 7 TEM examination of incoporated CeO2 nanoparticle in nickel deposits (88 g/L

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mixed particles).

Fig. 8 XRD and pole figures of different coatings (a) pure Ni coating; Ni-Ti-CeO2 composite coatings electrodeposited from Watts bath containing (b) 22 g/L and (c) 88

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g/L mixed particles.

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Fig. 9 Typical SEM images of (a) pure Ni coating; Ni-Ti-CeO2 composite coatings electrodeposited in Watts bath containing (b) 11 g/L (c) 22 g/L (d) 44 g/L (f) 88 g/L mixed particles; (e) cross-section at 88 g/L. Fig. 10 cross-section views of different coatings (a) pure Ni coating; Ni-Ti-CeO2 composite coating electrodeposited from Watts bath containing (b) 22 g/L and (c) 88 g/L mixed particles; (d), (e) and (f) corresponding enlarged pictures at white dotted line marked places. 30

ACCEPTED MANUSCRIPT Fig. 11 AFM pictures of (a) pure Ni coatings; Ni-Ti-CeO2 composite coatings electrodeposited in Watts bath containing (b) 11g/L (c) 22 g/L (d) 88 g/L mixed particles.

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Fig. 12 Hardness of different composite coatings. Fig. 13 Variations of the friction coefficient of different coatings as a function of time with Watts bath containing different particle concentrations (a) pure Ni coating (b) 22

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g/L (c) 88 g/L.

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Fig. 14 weight loss of different composite coatings after wear measurements. Fig. 15 SEM images of the wear tracks (a), (d) and (g) pure Ni coatings under 2 N, 4 N and 6 N; (b), (e) and (h) Ni-Ti-CeO2 composite coating prepared at 22 g/L mixed particles under 2 N, 4 N and 6 N; (c), (f) and (i) Ni-Ti-CeO2 composite coating

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prepared at 88 g/L mixed particles under 2 N, 4 N and 6 N.

31

ACCEPTED MANUSCRIPT Table. 1 Electrodeposition conditions and electrolyte compositions. Electrodeposition condition

240 (±0.01)

current density: 5 (±0.1) A/dm2

40 (±0.01)

pH: 3.6 (±0.2)

30 (±0.01)

magnetic agitation: 300 (±10) rpm

0.2 (±0.01) 0, 11, 22, 44, 88 (±0.01)

temperature: 45 (±1) duration: 80 mins

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nickel sulfate hexahydrate (NiSO4·6H2O) nickel chloride hexahydrate (NiCl2·6H2O) boric acid (H3BO3) dodecyl sodium sulfate (C12H25NaO4S) mixed particles: Ti and CeO2 particles (10:1)

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Concentration (g/L)

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Electrolyte composition

ACCEPTED MANUSCRIPT Table 2 the average friction coefficient of the different composite coatings under different loads Samples (g/L)

cof (2 N)

0

0.4311 (± 0.0281)

0.4188 (± 0.0279) 0.4046 (± 0.0326)

22

0.4420 (± 0.0481)

0.4239 (± 0.0338)

88

0.4519 (± 0.0657)

0.4362 (± 0.0309) 0.4661 (± 0.0315)

cof (6 N)

0.4151 (±0.0269)

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cof (4 N)

D

M A

TE D

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ED

M AN

ED

M AN

TE

D

M AN

ED

PT

M AN US

D

M A

EP TE D

M AN US

C

ED

PT

M AN US

ED

PT

M AN US

ED

PT

M AN US

ED

M AN

ED

M AN

ED

M AN

CE ED

PT

M AN US

CR

I

ACCEPTED MANUSCRIPT 1. Fixed ratio of Ti and CeO2 particles were incorporated into Ni coatings. 2. The addition of particles led to grain refinement and random-orientated growth. 3. Incorporated Ti particles promoted the formation of pores and surface asperities.

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4. CeO2 nanoparticles promoted the formation of high-quality composite coating. 5. Enhanced hardness and wear property were achieved by changing particle

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contents.