Characterization and one-step synthesis of Hydroxyapatite-Ti(C,N)-TiO2 composite coating by cathodic plasma electrolytic saturation and accompanying electrochemical deposition on titanium alloy

Accepted Manuscript Characterization and one-step synthesis of HydroxyapatiteTi(C,N)-TiO2 composite coating by cathodic plasma electrolytic saturation and accompanying electrochemical deposition on titanium alloy

Jiewen Huang, Xinmin Fan, Dangsheng Xiong, Jianliang Li, Heguo Zhu, Min Huang PII: DOI: Reference:

S0257-8972(17)30613-8 doi: 10.1016/j.surfcoat.2017.06.010 SCT 22421

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

6 April 2017 2 June 2017 4 June 2017

Please cite this article as: Jiewen Huang, Xinmin Fan, Dangsheng Xiong, Jianliang Li, Heguo Zhu, Min Huang , Characterization and one-step synthesis of HydroxyapatiteTi(C,N)-TiO2 composite coating by cathodic plasma electrolytic saturation and accompanying electrochemical deposition on titanium alloy, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.06.010

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ACCEPTED MANUSCRIPT Characterization and one-step synthesis of Hydroxyapatite-Ti(C,N)-TiO2 composite coating by cathodic plasma electrolytic saturation and accompanying electrochemical deposition on titanium alloy Jiewen Huang*, Xinmin Fan, Dangsheng Xiong, Jianliang Li, Heguo Zhu, Min Huang School of Material Science and Engineering，Nanjing University of Science and Technology, PR. China

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Keywords

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Plasma electrolytic saturation; electrochemical deposition; Hydroxyapatite; Ti(C,N); TiO2 Abstract: Besides oxides ceramic coating, cathodic plasma electrolytic saturation (PES) could

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produce more other types ceramic coatings, including nitrides, carbonitrides, borides, etc., on nearly all kinds of metal substrate. Owing to the accompanying electrochemical deposition(ED)

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effect of PES, special functional compounds could be deposited in mentioned ceramic coatings by PES simultaneously in special designed electrolyte. In this study, the target Hydroxyapatite

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(HA)-Ti(C,N)-TiO2 composite coatings were synthesized in one step by PES and accompanying ED in calcium nitrate and sodium dihydrogen phosphate containing formamide based

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electrolyte. The phase structures, functional groups of molecules, chemical compositions of the

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surfaces and binding energies of atoms in coatings were characterized by X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) respectively. The surfaces morphologies, cross

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sections morphologies, and element maps of the surface were analyzed by Scanning electron microscopy (SEM) and energy dispersive spectrum (EDS). The coatings exhibit porous

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structure with island like bulges or spheroids around the pores. The phases formed in coatings are verified to be constituted of HA, Ti(C,N) and anatase. The suggested formation mechanism of anatase in coatings differs to that obtained on anodic titanium surface in PEO process. The phase of HA distributes homogenously on surfaces and displays as clusters congressed by nanoscale needles or rods with their transverse length about ~10nm. The size of the pores and bulges, the crystalline amount of HA phase, and the thickness of the coatings all increase with increasing duration time. The thickness reaches about 56±7μm when the duration time grew to 30min, 26% higher than that of the HA-free coating produced in formamide electrolyte with the same process parameters.

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1. Introduction Titanium and its alloy are the general candidate materials for dental and orthopedic implant application for their combination of excellent mechanical properties, corrosion resistance and good biocompatibility. However, they are not bioactive and could hardly form a direct chemical bond with bone tissue [1-4]. The titanium implant is apt to be isolated by fibrous tissues from

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the surrounding bone. Synthetic Hydroxyapatite (HA) and HA-type compounds have superior

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biocompatibility and bioactivity. They have composition similar to that of dental and bone mineral, which is important mineral leading chemically bonding to the bone tissue.

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Unfortunately they could not be used as load-bearing medical implant directly because of their brittleness and low fatigue resistance[5, 6]. The idea of coating titanium with HA or HA-type

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compounds to obtain both desirable mechanical properties and bioactivity has been practiced by different approaches, including electrochemical-deposition[7, 8], sol-gel method[9-11], and

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plasma spray[12-15], etc. However, the common problem of these direct coating methods is the weak bonding of the coating to substrate [1, 4, 16-20]. Comparing to these methods, Plasma

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electrolytic oxidation (PEO) is gained more attention in recent years to synthesis HA or HA-

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type compound containing TiO2 coating on titanium and its alloy [1, 16-18, 21]. It is proved that this composite coating has advanced inherence to the substrate [22] and could spontaneously bond with a living bone [21].

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As one subclass of plasma electrolytic technique (PET), PEO has been applied on valve metal to process oxide coatings. The valve metal is treated as anodic electrode. With comparison to

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PEO, another group of PET, plasma electrolytic saturation (PES) treats the metal as cathodic electrode. This cathodic metal could be nearly any kinds of metal, including not only the valve metals, but also other kinds such as iron, steel, copper, and zinc[23, 24]. The metal surface could be saturated with nonmetals like C, N, B, and O, therefor might be covered by the chemical reaction production like carbides, nitrides, borides, oxides or their combinations[2529]. It could also be alloyed by metal element such as W, Mo, and V, when desirable element containing electrolyte is selected[30]. So, rather than PEO, it is PES could provide more categories of composite coating for choice on substrate metals. Among these coatings, titanium nitride or carbonitride coating is the promising coating for implant application because of its

ACCEPTED MANUSCRIPT excellent biocompatibility, corrosion and wear resistance[31-36]. According to the principle of PES, electrochemical deposition (ED) would also take place as an accompanying effect in PES procedure on cathode. Therefore, special functional compound could be deposited synchronously in coatings prepared by PES in special designed electrolyte. PES may offer a new means for choice to produce bioactive HA containing titanium nitride or carbonitride, or other composite coatings on titanium. The feasibility of synthesis HA -Ti(C,N)-TiO2 composite

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coating by cathodic PES and ED on Ti6Al4V was explored in this work.

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2. Experimental details 2.1 Sample preparation

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Ti6Al4V plates with size of 20mm×10mm×3mm were used as cathode and the graphite plate was used as anode. The Ti6Al4V plates were vacuum annealed at 910℃for 5 hours and

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then progressively grounded with abrasive papers. The samples were ultrasonic cleaned with distilled water and acetone. Formamide, calcium nitrate, sodium dihydrogen phosphate,

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potassium chloride, and deionized water with proper fraction were chose to consist the electrolyte. The electrolyte bath was water-cooled and its temperature was maintained lower

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than 40℃. Sample S1, S2 and S3 were prepared in this electrolyte with the applied voltage of

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300V, for 10, 20 and 30min individually. The pulse frequency and the duty cycle was fixed as 1000HZ and 60% respectively. For comparison, formamide electrolyte without the addition of calcium nitrate, sodium dihydrogen phosphate was used to practice plasma electrolytic

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carbonitriding on sample V1. Its other electrical parameters were the same with S3. Table1 listed the detailed parameters and corresponding codes of coatings. After the processing,

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samples were washed with distilled water and tried at room temperature. Table1. Electrical parameters and corresponding coating Duration

Electrolyte

Coating

Electrolyte constitution time/min

S1

10

S2

20

S3

30

V1

30

code calcium nitrate, sodium dihydrogen S

phosphate, formamide ， potassium chloride, and deionized water

V

formamide，potassium chloride, and

ACCEPTED MANUSCRIPT deionized water 2.2 Characterization of the coating In this study, the phases structure of the coatings were detected by Bruker D8 X-ray diffraction(XRD) using Cu-Ka radiation between 20° and 80° angles with a step size 0.01°/sec. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR; Bruker 55FTIR) was used to analyze the chemical functional group. For each sample, 32 scans were recorded

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within the spectral range of 500–4000 cm-1 in the % transmittance mode with a resolution of 4

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cm-1. The chemical composition and binding energies of atoms on the coatings were detected by X-ray photoelectron spectroscopy (XPS; PHI QUANTERA II) with an Al-Kα radiation

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(1486.61 eV) X-ray source. The take off angle was 45°. High-resolution spectra of C, N, O, Ca and P were also performed to furtherly study their chemical state. PHI Multi-peak software

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were applied to calculate the chemical composition and fit the high-resolution spectra with a Shirley background and a Gaussian/Lorentzian line shape. The accuracy in BE determination

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is 0.1 eV, and in atomic ratio ~8~10% rel. Field emission scanning electron microscope (FESEM; FEI Quanta 250F) and the energy-dispersive spectrometry (EDS; Oxford INCA

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energy 300) were applied to investigate the surface morphologies, the element mappings and

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the cross section morphologies. The average thickness of coatings was calculated from 20 measurements on the section FESEM images with standard deviation. 3. Results and discussion

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3.1 X-ray diffraction

The surface XRD patterns of the raw materials, coating S1, S2, S3 and V1 are given in

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Fig.1. As is shown in Fig.1(b), the formation of Ti(C, N) and anatase are verified as the new phases on V1 surface after the sample being treated in electrolysis V for 30min. The formation mechanism of Ti(C, N) phase could be explained principally from the plasma-enhanced diffusion process and chemical reaction on the electrode surface[23, 30]. The heat effect could cause the local boiling and decomposition of the solution electrolyte adjacent to the electrode at the initial stage of the PES process as the flowing[32]: HCONH2  HCN  H2O

(1)

HCONH2  NH3  CO 

(2)

ACCEPTED MANUSCRIPT When the applied voltage rises up and exceeds the critical value, the breakdown of the envelope emerges, followed by further decomposition and ionization of the intermediate products shown in formula (1) and (2). The active radicals and ionic components including C+, N+, etc. accelerated by electric fields, bombard and heat the sample[37, 38]. Cations get electron on the surface of the cathode then transform into active atoms[39], providing enhanced diffusion , saturation and formation of Ti(C, N) phase.

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As for anatase, its characteristic peak is not found in the profile of coating S1, but found

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in that of S2 and S3. When the duration time increases to 30min, the intensity of the peak increases to the level similar to that in V1, as shown in Fig1.(d) and (e). Deduction could be

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obtained from the comparison among Fig.1(b)~(e) that the source of oxygen in anatase relates nothing to the calcium nitrate and sodium dihydrogen phosphate added in electrolysis S. Its

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origination is speculated to be H2O vapor in the envelope, one part of which is the evaporation of water existing in electrolyte, the other is from the decomposition production of HCONH2 as

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formula (1) shown. The theory foundation of PES[23] shows the synthesis of titanium compound coating on cathodic titanium is a process controlled by diffusion and saturation of

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the activated atoms into titanium substrate. The same mechanism might be adopted to explain

formula (3) shows.

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the formation of anatase. The activated atoms of oxygen are speculated to be obtained as

H2O→2H2↑+2[O]

(3)

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This mechanism is different with that of the anatase formed on anodic titanium substrate in PEO process[3]. The increasing tendency of the peak for anatase from S1 to S3 illustrates that

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the saturation of O shows more important role when the duration time extends to and above 20min at the applied voltage of 300V in the used electrolyte system during PES process. It is also inferred that the formation of anatase might be related to the applied voltage which decides the value of the temperature. It needs to be further studied and confirmed whether there is a critical value of the voltage for the phase of anatase to form in the same electrolyte. It could be seen from Fig.1 (c)~(e) that, besides Ti(C, N) and anatase, HA is obtained in coatings S1, S2, and S3 synthetized in the electrolyte S. Sources of HA are attributed to the calcium nitrate and sodium dihydrogen phosphate added in electrolyte V. Its formation is suggested to be the result of the ED effect in PET process[23, 30]. With the consideration of

ACCEPTED MANUSCRIPT the similar formation mechanism of HA prepared by the electrochemical deposition[40, 41], the main reactions to form PO3-4 and OH  take place on the cathode are probably descripted as equation (4)~(7). Driven by the force of electric filed between the electrodes in electrolyte, the cations of Ca2+ migrate to the cathode and react with PO3-4 and OH  to form HA

2H2 PO4 +2e  2HPO42  H2

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H2 PO4 +2e  PO34  H2

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2HPO24 +2e  2PO34  H2

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precipitating on the cathode as equation (8) shown.

2H2O+2e  2H2  2  OH 



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10Ca 2+ +6PO43- +2OH  Ca10  PO4 6  OH 2

(4) (5) (6) (7) (8)

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The increase of relative peak intensity of HA is also found when the duration time grows from 10 min to 30 min, indicating the amount growth of HA crystalline. The explanation of this change might be summed up in two aspects. One is, the longer the duration time is provided,

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the more HA could be produced through the reaction as formula (8) and (9) shown. The other

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one is, the growth of duration time would promote the crystallization of amorphous phase of

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HA[3], if it forms in initial stage of PES.

Fig1. XRD patterns of Ti6Al4V(a), coating V1 (b) , S1(c), S2(d) and S3(e) 3.2. ATR-FTIR analysis The results of the ATR-FTIR spectrum were recorded for each sample processed with different duration time, as Fig. 2 shown. The general band assignment and wavenumber are

ACCEPTED MANUSCRIPT given in Table 2, supported by literature[42-48]. The presence of P-O bond stretching vibration mode in the range of 590-595 cm-1, 1014-1027 cm-1, and 1420-1450 cm-1 indicate the existence of PO43-. The stretching vibration mode of OH- in 1645-1653 cm-1, 3336-3363 cm-1, as well as the libration of OH- at 634-668 cm-1, indicate the existence of hydroxyl in HA structure. These results confirm the formation of HA in coating S1, S2 and S3, being consistent with the peak assignment result of HA phase in XRD analysis.

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It could be seen from Fig. 2, the peaks intensity of PO43- and OH- band for coating S1

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produced in 10min are lower, which reveals the little amount of the HA formation in shorter duration time. These bands are also relatively broader. It is the typical sign of the presence of

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low ordered calcium deficient HA[43]. For coating S2 and S3, the intensity of these peaks increase and all bands grow much sharper. These changes indicate the more and higher ordered

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HA form in coatings when the duration time grows.

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Fig.2 ATR-FTIR spectrum of coating S1 (a), S2 (b) and S3 (c)

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Table 2 General band assignment and wavenumber of ATR-FTIR spectrum analysis for the composite ceramic coatings[42-48]. Wavenumber(cm-

Band

number

1

assignment

1

590-595

ν4（PO43-）

HA; P-O stretching vibration

2

634-668

νs（OH-）

HA;

Peak

)

Phase

O-H

libration

and

stretching vibration 3

1014-1027

ν3（PO43-）

HA ; P-O stretching vibration

4

1420-1450

ν3（PO43-）

HA; P-O stretching vibration

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1645-1653

νs（OH）-

HA; O-H stretching vibration

6

1958-2360

νs（P-H）

HA; P-H stretching vibration

7

3336-3363

νs（OH-）

HA; O-H stretching vibration

3.3 XPS analysis

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The XPS was employed to further understand the composition and structure of the coatings. Fig 3 reveals the existence of element C, N and O on the surface of sample V1. As for sample

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S1, S2 and S3, Ca and P are also found on the surface. The characteristic peak of Ti could be seen obviously in surveys of V1 and S1, but hardly seen in those of S2 and S3, which agrees

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well to the literature[3].

Narrowscan spectra were used to evaluate the chemical state information of the concerned

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element C, N, O, Ca, and P. No obvious peak position shifting of the identical element is found for the coatings. As is shown in Fig.5(a), Among the two peaks resulted from the cleavage of

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C1s peak, the one at 281.3eV correspond to Ti-C bond[33], while 284.6ev is assigned to the CH bond respectively. The peak of N1s is deconvoluted into two peaks. The one at 397.3eV

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agrees well with Ti-N bond[49], while the other one at 398.6ev indicates the bond of C-N. The

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O1s peak at the binding energy 530.1~531.0eV as seen in Fig.5(c) belongs to the O-Ti bond in TiO2[3, 5].Ca2p spectrum reveals a doublet with Ca 2p3/2 at 347.4eV and Ca 2p1/2 at 351.0eV, corresponding to the existence of Ca2+[3, 48, 50]. The P2p peak at the binding energy of

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133.1eV evidences the presence of PO43- in coating, indicating the presence of HA in the coating[3, 48, 50].

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The atomic concentration of the composition above listed in Table 3 shows the increase tendency of Ca and P with the increasing duration time, which grows from 1.5% to 9.1%, 1.1% to 5.5% individually. The calculated ration of Ca/P in coating S1, S2 and S3 is respectively 1.41, 1.53, and 1.65, which is close to that of ideal HA. It could be deduced that the increase of the duration time do contribute to the amount increase of crystalline HA, which is also supported by XRD and ATR-FTIR analysis results and discussions.

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Fig. 3 XPS spectra results of coating V1, S1, S2 and S3

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(a)

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(e)

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Fig. 4 High resolution XPS spectra results of C1s (a), N1s (b), O1s(c)Ca2p (d) and P2p (e)

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Table 3 Chemical compositions of the coatings determined by XPS Calculated ratio

Atomic concentration (%) Coating N

O

Ca

P

Ti

S1

66.3

2.4

25.6

1.5

1.1

3.2

1.41

S2

56.7

3.8

29.5

4.9

3.2

1.9

1.53

S3

47.2

4.6

9.1

5.5

0.2

1.65

V1

62.4

0

0

3.5

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C

33.4

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of Ca/P

27.6

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3.4 Surface morphology and element mapping analysis The surface SEM morphologies of all the samples produced at different duration time shown

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in Fig 5(a)~(c) all exhibit porous structure with island like bulges or spheroids adjacent to the pores. The pores are homogeneous distributed on the coatings produced in 10min(Fig. 5(a)), then distributed randomly when duration time increases to 20min and 30min(Fig. 5 (b) and (c)). The size of these pores increases while the number of them decreases with the growth of the duration time. The pores, also called micro discharge channels, are left by the ejection of the local surface melting and the following implodes of the plasma bubbles[23]. The plasma discharge tends to form at weak sites of the surface or the formed coatings owing to the presence of dielectric breakdown, where the breakdown voltage is relative lower. The location of the formed pores in early stage might be this kind of sites, for it means the relative smaller local

ACCEPTED MANUSCRIPT thickness of the coatings and the relative lower breakdown voltage. So with the increase of the duration time, these pores expand, and eventually connect with each other, leading decrease of the pores’ number, as is shown in Fig. 5(b) and (c). It could be seen more clearly from Fig.5(d) that, for S3, when duration time increases to 30min, contiguous depression regions nearly substitute the pores in surface, being separated by the bulges or spheroids. The origin of these bulges and spheroids is attributed to the solidification of the local molten substrate or early

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formed coatings, which is ejected from the pores and subsequently cooled by the electrolyte. It

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could be seen from Fig. 5 (e) and (f), coating V1 shows morphology characters similar to S3, implying that the addition of calcium nitrate and sodium dihydrogen phosphate does not change

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the morphology under the relative lower magnification observation. The element maps of coating S3 shown in Fig. 6 (b)~(i) of the selected zone in Fig. 6(a) demonstrate the homogenous

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distribution of C, N, O, Ca and P on the surface.

Morphologies under Higher magnification above 50,000 times reveal the distinguish

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difference between coating S1~S3 and V1. Clusters shown in Fig.7 (a)~(c) are found to be congregated by nano-scale needles and shot rods with the transverse length about ~10nm.These

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morphology features are similar with the HA containing coatings processed by other technics

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reported in literatures [8, 17, 44]. It also reveals that more regular shaped rods form in coatings with average length of ~400nm when the duration time grows to 30 min. Promoted nucleation and growth of HA caused by the increase of the duration time might be applied to explain this

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result. However, as is shown in Fig.7 (d), V1 surface is covered with nano-scale globular particles, which are cooled and solidified from the local molten liquid drops ejected from the

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discharge channels. So the difference of these morphologies between S-serial coatings and coating V1 could be attributed to the addition of calcium nitrate and sodium dihydrogen phosphate in formamide electrolyte.

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(e)

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(f)

Fig. 5 Surface morphologies of S1(a), S2(b), S3(c) and its edge (d), and V1(e) and its edge(f)

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(b)

(d)

(e)

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(a)

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(f)

(h)

(i)

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(g)

Fig. 6 FESEM image of the selected zone (a) and its element mappings Ca (b), P (c), C (d), N (e), O (f), Ti (g), Al (h), and V (i) on coating S3

(b)

(c)

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Fig. 7 Surface morphologies of S1(a), S2(b), S3(c) and V1(d)

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3.5 Cross section morphology and thickness of coatings

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The cross section morphology of samples (Fig.8(a)~(c)) shows the thickness increasing of the coating with the increasing duration time. The average thickness of coatings are listed in Table 4. The main characters of the coating synthesized in 10 min could be describe as porous,

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thin and discontinuous. Continuous coating forms on sample S2 in 20 min duration procedure and its average thickness is 28±3μm. Much thicker coating with 56±7μm of its thickness covers

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the substrate when the duration time grows to 30 min. However, both of coating S2 and S3 contain a dense inner layer adjacent to the titanium substrate but a relative loose, porous top layer. It implies that the pores in early formed coating would eventually be filled with the subsequent deposition and covered by the later formed ‘bulges’ or ‘spheroids’ mentioned above. Therefore, accompanying the increase of thickness, the inner layer of the coating grows denser. The cross section morphology of coating V1 presents characters similar to S3. Nevertheless, coating S3 is nearly 26.0% thicker than V1. Whether this discrepancy in thickness is due to the volume expansion caused by the deposition of HA in coating S3, still need to be furtherly verified.

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(b)

(c)

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Fig. 8 Cross section morphologies of S1(a), S2(b), S3(c) and V1(d)

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Table 4 Average thickness of the coatings Average thickness (μm)

S1

16±1

S2

28±3

S3

56±7

V1

44±3

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Coating

4. Conclusion

In this study, HA-Ti(C,N)-TiO2 coatings were successfully synthesized in one step by cathodic PES and accompanying ED in calcium nitrate and sodium dihydrogen phosphate containing formamide based electrolyte. PES was applied to produce the Ti(C, N)-TiO2 coating in formamide electrolyte for comparison. Conclusions could be drawn as below: 1) The addition of calcium nitrate and sodium dihydrogen phosphate in formamide electrolyte contributed to the HA formation in coatings. After 10 min PES and ED treatment, the crystalline phase of HA formed in coating. Its amount increased with the increasing duration time.

ACCEPTED MANUSCRIPT 2) HA-Ti(C,N)-TiO2 coatings and HA-free Ti(C,N)-TiO2 coating all exhibit the porous structure with island like bulges or spheroids around the pores. When the duration time grew, the size of pores increased, while the number of them decreased. 3) The formed HA distribute homogenously on surfaces and display as clusters, which are congregated by nano-scale needles or rods with the transverse length about ~10nm. While for Ti(C, N)-TiO2 coating, the surface is covered with nano-scale globular particles.

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4) The suggested formation mechanism of anatase in coatings is described as the plasma

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enhanced diffusion and saturation of activate oxygen atoms, differing to that of anatase obtained on anodic titanium surface in PEO process.

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5) The thickness of the HA containing coatings increased with the growth of the duration time. It reached 56±7μm when the time grew to 30min, 26% higher than that of the HA-free coating

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produced in the same time. Acknowledgements:

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This work was supported by National Natural Science Foundation of China 51401108. The authors would like to thank the Materials Characterization Facility of Nanjing University of

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Science and Technology for the analysis equipment provision of XRD, SEM and EDS.

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ACCEPTED MANUSCRIPT Highlights

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Hydroxyapatite-Ti(C,N)-TiO2 coatings are prepared by plasma electrolytic saturation. HA is formed by electrochemical deposition, the accompanying effect of PES. Hydroxyapatite shows morphology as clusters congregated by nano-needles or rods. Formation of TiO2 on cathodic titanium surface is first verified in PES process. Formation mechanism of TiO2 differs to that got in plasma electrolytic oxidation.

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