Growth and characterization of hydroxyapatite nanorice on TiO2 nanofibers

Materials Chemistry and Physics 144 (2014) 301e309

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Growth and characterization of hydroxyapatite nanorice on TiO2 nanofibers Loubna Chetibi a, Djamel Hamana a, Slimane Achour b, * a b

Unit of Materials Science and Applications, University of Constantine, 25000 Constantine, Algeria Ceramic Laboratory, University of Constantine, 25000 Constantine, Algeria

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Hydroxyapatite nanoparticles in the form of nanorices were grown on TiO2 nanofibers.
The structure and corrosion in simulated body fluid were studied.
Heating induces Ti and O diffusion in opposite direction through hydroxyapatite.
Hydroxyapatite coating exhibits good corrosion resistance in simulated body fluid.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 January 2013 Received in revised form 13 October 2013 Accepted 29 December 2013

Hydroxyapatite (HA) coating with nanoparticles like nanorice is fabricated on chemically pretreated titanium (Ti) surface, through an electrochemical deposition approach, for biomaterial applications. The Ti surface was chemically patterned with anatase TiO2 nanofibers. These nanofibers were prepared by in situ oxidation of Ti foils in a concentrated solution of H2O2 and NaOH, followed by proton exchange and calcinations. Afterward, TiO2 nanofibers on Ti substrate were coated with HA nanoparticles like nanorice. The obtained samples were annealed at high temperature to produce inter diffusion between TiO2 and HA layers. The resultant layers were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Infrared Spectroscopy (FTIR), corrosion tests in SBF solution, and Electron Probe Micro Analysis (EPMA). It was found that only Ti from the titanium substrate diffuses into the HA coating and a good corrosion resistance in simulated body fluid was obtained. Ó 2014 Elsevier B.V. All rights reserved.

Keywords: Biomaterial XPS SEM TEM Electron probe Corrosion

1. Introduction Owing to their excellent mechanical properties, good biocompatibility and chemical stability, titanium and its alloys are well recognized as one of the most attractive biomaterials that have been extensively used for dental and orthopedic implants over the past few decades [1e3]. Nevertheless, when Ti or Ti alloy is

* Corresponding author. E-mail addresses: [email protected] (L. Chetibi), [email protected] (D. Hamana), [email protected] (S. Achour). 0254-0584/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.12.037

implanted in bone, the bonding of the implant with living bone often does not develop [4] or a long time of several months is required to achieve the integration of the implant with bone tissue. Calcium phosphate (CaP) based system has great potential for biomedical application such as implantation [5]. CaP is ubiquitously present in the body in the form of amorphous calcium phosphate (ACP) as well as crystalline hydroxyapatite (HA, Ca10(PO4)6(OH)2), and constitutes the major component of bone and tooth enamel [5]. A current way to achieve suitable bone-bonding is to deposit a hydroxyapatite (HA) layer, with composition similar to the mineral part of the bone, at the implant surface [6e14]. Plasma spraying is the most used coating technique. However, since this technique

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provides poor adhesion, alternative solutions have been developed. The most attractive solution is to deposit a HA layer on Ti substrate by electrodeposition method because it is simple and can be operated at low temperature. Additionally, the thickness and chemical composition of the coatings can be well controlled through the regulation of the processing parameters [15,16]. Substrate surface roughness is particularly important, not only because a rough surface can provide increased wettability of the HA precursor solution on the substrate, but also because mechanical interlocking between the HA layer and the substrate may be enhanced to avoid the failure of the HA coating under shear stress [17]. Recent reports indicate that modifying Ti surface with high surface area of 1D TiO2 nanostructure films [18] brought about by the small particle size is beneficial to show better mechanical interlocking between HA and Ti than conventional microroughened Ti surface. On the other hand, there is much attention paid to the corrosion behavior of metallic Ti and (HA þ TiO2) composites in simulated body fluid (SBF) solution [19,20]. Crystalline anatase TiO2 nanostructures such as nanotubes [21] have a beneficial role for easiest HA nucleation, owing to their high surface area and possible good epitaxial orientation relationship between HA phase and anatase crystalline phase. Hydrothermal treatment of TiO2 in an alkaline solution is a developed way to prepare titanate nanotubes [22,23] and nanowires [24]. Compared with nanotubes, nanowires are more stable at high temperature and in acidic or alkaline solution [25]. A lot of micrographs of the HA deposit obtained using conventional electrodeposition, usually, showed micro size flake type structure having hexagonal crystal structures [26,27], dense layer of hydroxyapatite-carbon nanotubes coatings were developed on Ti substrate coated TiO2 nanotubes [28] using sol gel method. In this work, a dense and homogeneous HA layer with novel nanoparticles like nanorice have been prepared by conventional electrodeposition on nano structured TiO2 layers that were deposited on Ti in the form of anatase nanofibers. To our knowledge, this combination has never been studied before. The HA coated samples were further heat treated at high temperature in order to increase bonding strength of the deposited layer. Also, corrosion tests in SBF of the prepared samples were conducted on both uncoated and HA coated TiO2 nanofibers.

2.1.2. Hydroxyapatite deposition Electrodeposition of HA [15,16] was conducted at room temperature in an electrolyte containing 0.04 mol l1 CaCl2, 0.027 mol l1 (NH4)2HPO4 and 3% H2O2. Thus, this solution is in Ca to P molar ratio of 1.67 with a pH value adjusted to 6.5, and is close to physiologic conditions. The deposition was carried out by applying a constant potential of 3 V. The solution was stirred during electro-deposition to obtain a uniform electrolyte concentration. Some of the obtained coatings were further treated in 0.1 mol l1 NaOH solution at 80  C for 2 h. Then, the coated samples were rinsed with distilled water and dried at 80  C for 1 h. 2.2. Characterization methods

2. Experimental

The surface morphology of TiO2 nanofibers and the HA coatings were observed by means of scanning electron microscopy (FESEM, Nova nano SEM 630) and transmission electron microscopy (TEM, Titan). The cristallinity and the structure of the coatings were examined using X-ray diffractometry (Bruker D-8 Advance powder diffractometer). All XRD patterns were recorded over 2q range of 10e90 with a step of 0.02 . Data were collected using CuKa X-rays at 40 kV and 30 mA. XPS study was carried out in a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Ka X-ray source (hn ¼ 1486.6 eV) operating at 150 W and a multi-channel plate and delay line detector under 1.0  109 mbar vacuum. The survey and high-resolution spectra were collected at fixed analyzer pass energies of 160 and 20 eV, respectively. The instrument work function was calibrated to give an Au 4f7/2 metallic gold binding energy of 83.95 eV and the spectrometer dispersion was adjusted to give a binding energy of 932.63 eV for metallic Cu 2p3/2. To avoid differential charging, the samples were mounted in floating mode. Cyclic polarization data were carried out using an EG&G Princeton Applied Research model 273 potentiostat (263A). The obtained data are given in the form of potential vs. current density curves. The coatings were also characterized by FTIR using a Thermo Scientific Nicolet spectrometer (Nicolet 6700 FT-IR) in the wave number range of 400e4000 cm1. Finally, the inter diffusion zone between the substrate and the coating, upon annealing, was evaluated using CAMECA (SX 100 model) electron probe micro analyzer (EPMA). The scratch tests were performed by using a Zwick testing machine of materials equipped with micro indenter.

2.1. Materials

3. Results and discussion

The coating process involves two steps: 2.1.1. Anatase TiO2 nanofiber film formation Titanium foils (99.6% of purity, from Goodfellow) were cut into specimens with the size 15  20  0.3 mm3 and mechanically polished using varying grit abrasive paper (800, 1000, and 1200). The final roughness of the surface is in the order of the conventional polishing roughness (0.32  0.06 mm). Then the specimens were treated by sonicating in ethanol and pickling in a 5 wt% oxalic acid solution at 100  C for 2 h, followed by rinsing with distilled water and drying. The polished Ti metal foils were soaked in a mixed solution of 16 ml H2O2 (30 wt%) and 16 ml NaOH (10 M) in a Teflonlined autoclave. The autoclave was maintained at 80  C for 24 h. After the autoclave was cooled to room temperature, the Ti foils were rinsed gently with distilled water. Thereafter, the as prepared samples were protonated through two cycles of ion exchange in 50 ml 0.1 M HCl for 2 h and subsequently taken out, rinsed to neutral with distilled water, dried at 80  C for 1 h and finally calcined at 400  C for 1 h to obtain anatase nanofiber films on the Ti surface [23].

The microstructure and elemental composition of the treated Ti surface, after being soaked in an autoclave in a mixture solution of concentrated H2O2 and NaOH for 24 h and calcined at 400  C for 1 h, is shown in the SEM micrographs of Fig. 1. These micrographs reveal that the treated Ti surface is well covered with an obvious network of TiO2 nanofibers forming ropes like with an average diameter of about 50 nm. The ropes interweave to form a hierarchical nano porous structure on the Ti surface. The TiO2 nanofibers were further characterized by transmission electron microscopy. Fig. 2 shows TEM and HRTEM images of the powders scraped off from the treated Ti surface. In this figure, the nanofiber structure shows large free surface, which is necessary to promote faster nucleation and growth of HA on the Ti surface. The electron diffraction pattern presented in Fig. 2b was taken from the nanofibres shown in Fig. 2a. It exhibits ring diameters corresponding to the crystalline anatase phase (Table 1). In fact, the stable TiO2 phase bellow 400  C is anatase (According to the equilibrium diagram) and the rutile phase can exist from 600 to high temperatures. Following Uchida et al. [29], crystalline anatase phase has a beneficial role for easiest HA nucleation, owing to possible good

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Fig. 1. SEM surface morphology of the treated Ti surface in a mixed solution of H2O2 (30 wt%) and NaOH (10 M) at 80  C for 24 h, thereafter protonated through two cycles of ion exchange in HCl solution and finally calcined at 400  C for 1 h to obtain anatase nanofiber film on the Ti substrate.

Fig. 2. TEM image (a), electron diffraction pattern (b), and HRTEM image (c) of the powders scraped off from the treated Ti surface covered with TiO2 nanofibers.

epitaxial orientation relationship between HA phase and crystalline anatase phase. Moreover, high resolution TEM imaging (Fig. 2c) shows that the individual nanofibers which constitute the ropes are very thin (about 5 nm in diameter) and well crystallized as can be Table 1 Ring diameters of the TEM electron diffraction pattern with their corresponding dhkl taken from the TiO2 nanofiber film prepared on the Ti substrate.

R1 R2 R3 R4 R5

Ring diameter (nm)

dhkl

0.347 0.296 0.250 0.188 0.196

d101 d102 d111 d200 d210

seen from the lattice plan interference fringes. However, the surface of the individual nanofiber seems rather disordered. XPS and XRD measurements were carried out to further confirm the chemical state of the different components. The XPS surveys of the TiO2 nanofiber layer (Fig. 3a) and the HA coating (Fig. 3b) show that the TiO2 layer is composed of Ti, O, C and the HA coating is mainly composed of Ca, P, O and C. Fig. 3ceh show the high resolution XPS and curve fittings of C 1s and O 1s spectra of the as prepared uncoated and HA coated TiO2 nanofibers heat treated at 900  C. The formation of TiO2 on the Ti surface before HA deposition is clearly shown by the high resolution Ti 2p XPS spectrum in Fig. 3c, with the Ti 2p 3/2 peaked at 458.3 eV and a Ti 2p splitting of 5.7 eV. The spectrum obtained after HA deposition and heat treatment (Fig. 3d) presents a considerable relative decrease in

400

200

Ti 2p 3/2

Ti 2p (TiO2)

Ti 2p 1/2

5,7 eV

10 0 445 50

450

455

(e)

460

TiO2

465

16

C=O C-O

2,5 C 1s (TiO2)

530

532

C-C/C=C

534

25

460

C-O C=O C-H

0,0 278 280 282 284 286 288 290 292

Ca 3s

P 2s P 2p

465

Ca10(PO4)6(OH)2

470

O 1s (HA/TiO2)

TiO2

10

C=O C-O CaCO3 H2 O

5

526 528 530 35 C 1s (HA/TiO2) 30 Intensity (Kcps)

Intensity (Kcps)

(f)

15

2,0

0,5

455

20

(g)

1,0

2p 1/2

5,7 eV

6 450

536

1,5

0

Ti 2p (HA/TiO2)

0 528

200

8

0 526

400

2p 3/2

10

Intensity (Kcps)

Intensity (Kcps)

OH

(d)

600

12

40

10

(b)

14

O 1s (TiO2)

20

O 1s

5

470

30

O KLL

10

Ca LMM Na 1s

15

0 1200 1000 800

(c)

30 20

20

0

Intensity (Kcps)

600

HA/TiO2

Na KLL Ca 2s Ca 2p1/2 Ca 2p3/2 C 1s

10 0 1200 1000 800

Intensity (Kcps)

Ti 3s Ti 3p

C 1s

20

40

25

Ti 2s Ti 2p1/2

O KLL

30

Ti LMM Ti LMM

50 40

(a) Ti 2p 3/2

60 TiO2

4 Intensity x10 (cps)

O 1s

L. Chetibi et al. / Materials Chemistry and Physics 144 (2014) 301e309

Intensity x 104 (cps)

304

532

534

536

538

(h)

C-C/C=C

25 20 15 10 5 0 280

C-O

C-H 282

C=O 284

286

288

CaCO3 290

292

Fig. 3. XPS survey of Ti (a), HA (b), XPS spectra of Ti 2p, O 1s and C 1s core level spectra before HA deposition (c, e and g) and after HA deposition (d, f and h) on the TiO2 nanofibers/ Ti substrate.

intensity with slight chemical shift of 0.1 eV indicating that TiO2 remains stoichiometric, but could transform to rutile phase after heat treatment [30]. The O 1s spectrum of the as prepared sample (Fig. 3e) can be fitted to five peaks located 529.6, 531, 531.9 and 532.5 eV, and attributed to TiO2 nanofibers [31], OH [32e34], C]O and CeO [33,35], respectively. After HA deposition and heating, the O 1s band (Fig. 3f) can be fitted to six peaks. The four peaks located at about 529.6, 531.9, 532.6 and 534.4 eV are attributed to TiO2 nanofibers [31], C]O, CeO and H2O [34], respectively. The other

two peaks located at 531.1 and 533.5 eV are attributed to the phosphate group of hydroxyapatite (Ca10(PO4)6(OH)2) [36,37] and carbonate group CO3 [38], indicating that the top layer formed on the sample surface is partly composed of carbonate (probably CaCO3 in very low concentration) containing apatite. The O 1s peak fitting is further supported by the suggested work of Kunze et al., beside others that are now added to the list of references. In fact, Kunze et al. have found a peak at 531.39 eV and assigned it to either carbonate or phosphate-containing components. Nevertheless, this

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Table 2 Atomic concentration deduced from the main XPS peaks of the TiO2 nanofiber surface. Elements

O

C

Ti

Atomic concentration (%)

55.7

17.7

26.6

Ti

Intensity (a.u.)

500 400

Ti

300

Ti

Ti Ti

200 TiO2

100 20

Ti

Ti

Fig. 6. TEM-EDS analysis of the HA powder scraped off from the HA coated surface.

Ti

40

60

80

2θ (°) Fig. 4. XRD diagram of the anatase TiO2 nanofiber film prepared on the Ti substrate.

peak cannot be attributed to carbonate (CaCO3), since the carbonate related peak should be situated at much higher binding energy at about 5333.5 eV as it is found here and in Voigts et al. [38]. Usually, authors dealing with material containing calcium carbonate study, wrongly attributed the O1s band situated at about 531 eV (which belongs to the oxygen BE in CaO) to the CaCO3. Because of the presence of carbon (more electronegative), the binding energy of oxygen in CaCO3 should be much higher at about 533.5 eV as suggested in the present work and by Voigts. In addition, Kunze et al. did not show any de-convolution of the O 1s spectra to precisely determine CeO, C]O and carbonate bands in details. Fig. 3g and h show the C 1s spectrum before and after HA deposition and heating. De-convolution of this spectrum reveals four peaks; an intense one at 284.6 eV and three others located at about 283.5 eV, 286.1 eV and 288 eV. These peaks can be attributed to CeC/C]C, CeH, CeO and C]O bonds [39,40], respectively. The additional peak of the coated HA spectrum at 289 eV is attributed to

the carbonate group CO3 [32,41] in carbonate containing apatite. Moreover, the O/Ti atomic ratio, before heat treatment, is calculated to be nearly 2 as shown in Table 2, which corresponds to stoichiometric anatase TiO2 atomic composition. The X-ray diffraction spectrum of the treated Ti plate is presented in Fig. 4 where the diffraction peak at 25.3 can be assigned to the anatase phase structure and the strong peaks at 38.4 , 40.2 , 53 and 70.6 to the background of the Ti substrate. The SEM images of the HA layer shown in Fig. 5 reveal nanoparticles in the form of nanorice immerging on the TiO2 nanofibers network. According to EDS analysis in TEM (Fig. 6), this new layer is made up of Ca, P, O, C and Cu elements (the C and Cu elements come from the coating of the samples with carbon before TEM imaging and copper grid holder, respectively). To study the crystal structure of the electrodeposited nanoparticles, we analyzed the XRD patterns of the electrodeposited coatings on the TiO2 nanofibers before and after alkaline treatment. The XRD pattern (Fig. 7a) of the as electrodeposited coating mostly consists of calcium hydrogen phosphate (CaHPO4e2H2O, CHP), which transformed to HA coating in alkaline solution (Fig. 7b). In this figure, the diffraction peaks at 25.84 , 32 , 28.09 and 49.32 belong to the HA phase. No diffraction peaks of other phases can be seen in the XRD diagram. The same figure, also, shows the presence

Fig. 5. SEM images of the crystalline HA coating with nano rice grain-shaped fabricated by electrodeposition in an electrolyte containing 0.04 mol l1 CaCl2, 0.027 mol l1 (NH4)2HPO4, and 3% H2O2 at a constant potential of 3V, then treated in an alkaline solution (0.1 M NaOH) at 80  C for 2 h.

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600

CHP

(a)

500

HPO43PO43CO32OH-

Absorbance (a.u)

Intensity (a.u)

5 400 300 200

CHP CHP

100

3

Ti

Ti

1 20

40

60

1000

80

2000

3000

4000

Wave number (cm-1)

250

Intensity (a.u)

4

2

CHP

0

(b)

Ti Ti

200

Ti

150 HA

100

HA

Ti

Ti

Ti

TiO2

50

HAHA

0 20

40

60

80

2 Fig. 7. XRD patterns of the as prepared HA coating on TiO2 nanofibers, which mostly consisted of calcium hydrogen phosphate (CHP) as a precursor of HA (a). After electrodeposition the samples were treated in 0.1 M NaOH at 80  C for 2 h, this treatment transforms the CHP to HA (b).

of anatase nanofibers beside the diffraction peaks of the titanium substrate which still appear strong in comparison to other compounds. In order to improve the crystallization and the inter diffusion between TiO2 nanofibers and HA nanoparticles and relax the electrodeposition stresses [15], the samples were annealed at 900  C for 1 h. The XRD pattern of the annealed samples (Fig. 8) obviously shows better crystallization of HA coating with sharp diffraction peaks and anatase transformation to rutile with disappearance of the Ti characteristic peaks. The rutile related peaks are very intense. Therefore, it is unlikely that all the rutile phase originates from the anatase transformation only. The increase of the TiO2 thickness is certainly due to the titanium substrate oxidation as a result of oxygen diffusion during annealing. Fig. 9 shows the FTIR spectrum of the HA coating after annealing where the bands 1000 HA: Hydroxyapatite

R

Ti: Titanium

800

R: Rutile

Intensity (a.u)

6

600 400 HA

200

HA

HA HA

HA

R R HA R HA R R R HAHA R

0 10

20

30

40 2

50

60

(°)

Fig. 8. XRD pattern of the HA coated TiO2 nanofibers sample annealed at 900  C for 1h.

Fig. 9. FTIR spectrum of the HA coated TiO2 nanofibers sample annealed at 900  C for 1h.

situated at 973 cm1 and in the region 1065e1160 cm1 characterize phosphate n1 and phosphate (PO3 4 ) n3 vibration modes, respectively [5]. The bands in the region of 1377e1458 cm1 are due to n3 vibration mode of carbonate CO2 3 in accordance with the result of XPS analysis, while the band related to OH- is observed at about 1631 cm1. There are faint absorptions at about 870 cm1 and 1226 cm1, which belong to HPO2 4 [10]. Significant band that may be due to water adsorption is, also, discerned between 2700 and 3050 cm1 and the characteristics of the whole spectrum are similar to those of bone mineral phase [42,43]. Any metal intended for use as a biomaterial should exhibit excellent pitting and crevice corrosion resistance in body fluid. This can be determined by carrying out cyclic polarization experiments in SBF solution at 37  C. The SBF solution has the following concentrations: NaCl: 7.934 g l1, NaHCO3: 0.350 g l1, KCl: 0.222 g l1, K2HPO4: 0.174 g l1, MgCl2e6H2O: 0.303 g l1, CaCl2: 0.545 g l1, Na2SO4e10H2O: 0.161 g l1 [44,45]. It was buffered at pH ¼ 6.8 with trishydroxyaminomethane and hydrochloric acid (HCl) at 37  C. Corrosion tests were carried out on uncoated and HA coated TiO2 nanofiber films. A saturated calomel electrode (SCE) was used as reference and graphite as counter electrode. Cyclic polarization data was obtained in the form of current density vs. potential curves and the potential was increased at a rate of 5 mV min1. The cyclic voltammetry curves for both uncoated and HA coated TiO2 nanofibers are presented in Fig. 10. It is interesting to note that the current density remains almost constant at very low values during the cathodic and anodic cycles. The corrosion parameters extracted from these curves (using 352 soft Corr III software) are shown in Table 3. It can be seen that both the uncoated and coated TiO2 nanofibers have lower anodic current density (Icorr) compared to pure Ti foil (34.66 mA/cm2) and nano HA films (100 nA/cm2) prepared using electrophoretic method [5]. In the present work, Icorr values of the TiO2 nanofibers and the HA/TiO2 coatings are evaluated to be 0.044 and 0.025 nA/cm2, respectively. Compared to other studies [19,20], these anodic corrosion currents are almost negligible and present the lowest corrosion rate. Also, one can observe the considerable improvement of corrosion resistance as a result of coating TiO2 nanofibers by HA nanostructure. Moreover, the anodic and cathodic potentiodynamic curves of the HA coated sample have the same corrosion potential (with a potential of almost 0 mV), which is a further evidence of higher stability. However, the anodic and cathodic curves corresponding to the TiO2 nanofibers alone do not have the same potential. They are shifted towards noble metallic potential direction and reach 260 mV. Moreover, no one of the tested sample exhibits appreciable hysteresis, indicating the absence of passivity breakdown that was usually observed by other

L. Chetibi et al. / Materials Chemistry and Physics 144 (2014) 301e309

2

Current density (A/cm )

-9

4,0x10

-9

3,0x10

-9

2,0x10

-9

1,0x10

0,0

(a)

-2

TiO2 nanofibers -1

0

E(V)

1

2

3

-9

2

Current density (A/cm )

5,0x10

-9

4,0x10

-9

3,0x10

-9

2,0x10

-9

1,0x10

0,0

(b)

-2

HA coating -1

0

1

2

3

E(V) Fig. 10. Cyclic voltammograms in simulated body fluid of uncoated (a) and HA coated (b) TiO2 nanofibers. Table 3 Results of electrochemical corrosion tests in SBF solution for both uncoated and HA coated TiO2 nanofibers. TiO2 nanofibers

Ti foil [5] Icorr (nA cm Rp (MU) Ecorr (mV)

2

)

34.66 mA/cm 246.7

2

0.044 nA/cm 495 268

2

HA coating 0.025 nA/cm2 873.5 0

studies [20]. This phenomenon may be caused by the presence of the chloride ions in the SBF solution that transfer through micro cracks and pores which can form in the coating film and act as direct paths between the coating and the Ti substrate [46].

307

The corrosion currents are very close because the deposited films are chemically inert and do not interact with the corrosive medium. Consequently, the origin of the corrosion current here may stem from direct paths between the coating and the substrate. The coating cannot be a perfect insulating barrier. It is worth mentioning that the potential corrosion of the HA coating is slightly decreased in comparison to TiO2 nanofibers alone. This decrease may be due to the porous structure of the HA coating, as it has been suggested by some authors [47,48]. It has been proposed [49] that the corrosion mechanism of HA coating on Ti with porous structure involved two steps. Firstly, hydrogen ions are produced at the interface area where corrosion of titanium occurs. It is then followed by the dissolution of HA in the high hydrogen ions concentration area. The local PH of the interface is very low because the hydrogen ions cannot be well circulated out of the interface and the dissolution of HA catalytically speeds up. Since the HA/TiO2 layer with specific nanostructure can act as a well barrier between the solution and the Ti substrate. Consequently, dense coatings, effectively, act as a well barrier to the transport of electrons and ions between the substrate and the SBF solution. The corrosion process here may depend on the porous degree and the pores size. In other works [19,50,51], the above mentioned process involves the apparition of big pores on the surface or dissolution of HA after corrosion test. So, the corrosion resistance of the HA coating in this work is really improved, since the obtained micrographs (Fig. 11a) after corrosion test (Fig. 11b), show that the corrosion current is very low and the corrosion resistance is very high (873.5 MU), in comparison with the literature. In addition, there is no shift between the anodic and the cathodic curves which is a further evidence of the stability of the hydroxyapatite film. Lastly, the adhesion of the HA layer to the TiO2 nanofibers can be improved by solid state diffusion. It is well known that solid state diffusion between couples can produce strong metallurgical bonds with less defects and better strength. Annealing at high temperature (900  C) of the HA/TiO2/Ti couples can produce diffusion of Ti, O, Ca and P species through the interfaces, which may improve the interfacial adhesion between HA coating and the substrate. In this context, EPMA was used to analyze the inter diffusion between Ti, TiO2 and HA coating. Fig. 12a shows a backscattered electron image of the HA/TiO2/Ti layers annealed during 1 h at 900  C. It is clear from the EPMA elemental cartography (depicted by bright zones) shown in Fig. 12 (b, c and d) that Ti (whose concentration is

Fig. 11. HA coating on Ti before (a) and after (b) corrosion test.

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Fig. 12. EPMA cross section observation of the Ti/TiO2/HA multilayer after annealing at 900  C.

proportional to the density of the bright pixels in Fig. 12b) diffused much deeper into the HA coating since the HA coating becomes brighter, indicating the existence of Ti species in the HA layer. However, the Ca and P species diffusion toward the substrate (left of the figure) is much slower (absence of bright pixels related to Ca or P). This diffusion asymmetry is due to the difference in the diffusivity rates [52] and the intrinsic diffusion coefficients of Ti and HA species [53]. It can be concluded that the diffusivity rate and the intrinsic diffusion coefficient of Ti are much higher than that of Ca and P ones in the present temperature range. Also, this asymmetric diffusion may be due to the presence of high diffusivity paths in the coating that does not exist in bulk Ti substrate. It is difficult to follow the oxygen diffusion by EPMA. Nevertheless, XRD results show that oxygen has diffused deeply into the substrate since thicker rutile was detected below the HA coating. In order to confirm the effect of the heat treatment at high temperature, we used scratch test which consists in moving an indenter (with pyramidal shape) on the surface of the specimens under an initial constant force (20 N). The applied force produces a damage of the film whose extent depends on the adherence. This test makes it

possible to compare the adherence of different coatings on the same substrate. Clearly, it appears that heat treatment at 900  C under air can further enhance the adhesion strength as shown in Fig. 13a and b. Improving adherence and corrosion resistance of the HA coated Ti substrates is still a subject of numerous studies. In a future work, we will report on the effect of introducing carbon nanotubes between TiO2 and HA on the resulting coating properties. 4. Conclusion A simple electrochemical technique was used to form HA layer with nanorice particles morphology on TiO2 nanofibres. These nanofibers acted as template and anchorage for growing hydroxyapatite nanoparticles-like nanorice during subsequent electrodeposition process. Inter diffusion between HA coating and the substrate was achieved by annealing the obtained samples at 900  C. The analysis of the HA/TiO2/Ti structure shows that Ti diffused in the HA film while O diffused into the substrate, which may improve the interfacial adhesion between the HA coating and

Fig. 13. Optical micrographs of micro scratching test of the samples surfaces (a) HA/TiO2/Ti and (b) HA/TiO2/Ti annealed at 900  C.

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