Electrodeposition of HAp coatings on Ti6Al4V alloy and its electrochemical behavior in simulated body fluid solution

The HAp coatings were electrodeposited on Ti6Al4V bio-metallic alloy with the elemental compositions given in table 1. A coupon of Ti6Al4V (1.5 × 1 × 0.2 cm³) was used as a cathode (working electrode) for the experiments. Prior to electrodeposition, the cathode was polished with SiC papers (ranging from P320 to P1200 grit), followed by ultrasonic rinsing in distilled water for 15 min and then dried at room temperature. The electrode working area was 1 cm².

The chemicals were used for the experiments: calcium nitrate tetrahydrate (Ca(NO₃)₂ · 4H₂O, M = 236.15 g mol⁻¹, 99% pure), ammonium dihydrogen phosphate (NH₄H₂PO₄, M = 115.03 g mol⁻¹, pure 99%) and sodium nitrate (NaNO₃, M = 84.99 g mol⁻¹, 99% pure) were imported from China.

The electrolyte solution contained 30 mM Ca(NO₃)₂, 18 mM NH₄H₂PO₄ and 60 mM NaNO₃ with the ratio of Ca/P being 1.67 dissolved in distilled water. The presence of NaNO₃ in the electrolyte solution increases the ionic strength of the electrolyte and for potentially exploiting the electrochemical reduction of ${{{\rm{NO}}}_{3}}^{-}$ ions which contributes to generate OH⁻ [17, 18]. The pH of the electrolyte solution was 4.4.

HAp coatings were synthesized in a cell containing 80 ml of the above electrolyte solution with three electrodes: the working electrode was a Ti6Al4V sheet; a Pt foil was used as counter electrode (anode) and a Hg/Hg₂Cl₂/KCl (SCE) electrode was used as reference. The deposition temperature was adjusted at 50 °C by a thermostat (VELP, Italia).

The electrodeposition was carried out using an Autolab with different synthesis conditions: scanning potential ranges: 0 to −1.8; 0 to −1.9; 0 to −2.0; 0 to −2.2 and 0 to −2.5 V/SCE; scan numbers: 1, 3, 5, 7 and 10 scans. HAp coatings were lightly rinsed by distilled water. Then they were dried at the room temperature.

Molecular structure of HAp coating was determined by Fourier transform infrared spectroscopy (FTIR 6700, Nicolet) using KBr pellet technique at room temperature, in the range from 400 to 4000 cm⁻¹ with 16 cm⁻¹ resolution and 16 scans signal average. HAp coatings were lightly separated from the substrate and the phase structure of HAp coatings were characterized by x-ray diffraction (XRD) (Siemen D5005 Bruker-Germany, Cu-Kα radiation (λ = 1.5406 Å)), operated at 40 kV and 30 mA, with step angle of 0.030° s⁻¹ and in a 2θ degree range of 20°–50°. The average crystallite size along c-direction of electrodeposited HAp was calculated from (002) reflection in XRD pattern, using Scherrer's equation:

$$\begin{eqnarray}&&D=\displaystyle \frac{0.9\lambda }{B\;{\rm{cos}}\;\theta },\end{eqnarray} \tag{ 1 }$$

where D (nm) is crystallite size, λ (nm) is the wavelength of the radiation, θ (rad) is the diffraction angle and B is the full-width at half-maximum of the peak along (002) direction

Surface morphology of HAp coatings before and after immersion in the SBF solution were examined using a Hitachi S-4800 scanning electron microscope (SEM). HAp coating thickness was determined following ISO 4288-1998 standard by the Alpha-Step IQ equipment (KLA-Tencor-USA). The coating thickness is the average value of five measurements.

A liter of the SBF solution was prepared by dissolution of chemicals in distilled water (table 2). The pH of the SBF solution was adjusted to 7.4 by 1 M HCl solution and the temperature was maintained at 37 °C by water bath [19–21].

Ti6Al4V and HAp/Ti6Al4V materials were limited to 1 cm² of active area and they were immersed in Falcon tubes containing 40 ml of the SBF solution at 37 °C in a water bath for 1, 5, 7, 11, 14 and 21 days. The electrochemical behavior of the materials was tested by Autolab equipment with a three electrodes cell. The above materials were used as working electrode, a saturated calomel electrode (SCE) as a reference electrode and Pt foil as a counter electrode.

The working electrode was polarized in the potential range of ±10 mV around its open circuit potential (OCP) with a scan rate of 1 mV s⁻¹. From polarization curves ΔE, ΔI values were determined. The polarization resistance R_p and the corrosion current density i_corr were calculated from equations

$$\begin{eqnarray}&&{R}_{{\rm{p}}}=\displaystyle \frac{{\rm{\Delta }}E}{{\rm{\Delta }}i},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{i}_{{\rm{corr}}}=\displaystyle \frac{B}{{R}_{{\rm{p}}}},\end{eqnarray} \tag{ 3 }$$

the B values were determined from the Tafel slopes of i–E curves in the potential range OCP ± 100 mV with a scan rate of 1 mV s⁻¹ by using equation

$$\begin{eqnarray}&&B=\displaystyle \frac{{b}_{{\rm{a}}}|{b}_{{\rm{c}}}|}{2.3({b}_{{\rm{a}}}+|{b}_{{\rm{c}}}|)},\end{eqnarray} \tag{ 4 }$$

where b_a is the value of the anodic Tafel slope and |b_c| is the absolute value of the cathodic Tafel slope. We obtain B = 0.014 26 and 0.0119 corresponding to Ti6Al4V and HAp/Ti6Al4V.

Figure 1 shows the cathodic polarization curve of Ti6Al4V substrate in the above electrolyte at the scanning potential range from 0 to −2.5 V/SCE with a scan rate of 5 mV s⁻¹ at 50 °C. The cathodic polarization curve has similar shape and consistent with the reported results [16, 21]. In this scanning range there is reduction of O₂, ${{\rm{H}}}_{2}{{{\rm{PO}}}_{4}}^{-},$ ${{{\rm{HPO}}}_{4}}^{2-}$ and H₂O to create ${{{\rm{PO}}}_{4}}^{3-}$ and OH⁻. The OH⁻ and ${{{\rm{PO}}}_{4}}^{3-}$ react with Ca²⁺ ions to form HAp coatings on the cathode according to the chemical reaction

$$\begin{eqnarray}&&10{{\rm{Ca}}}^{2+}+6{{\rm{PO}}}_{4}^{3-}+2{{\rm{OH}}}^{-}\to {{\rm{Ca}}}_{10}{({{\rm{PO}}}_{4})}_{6}{({\rm{OH}})}_{2}.\end{eqnarray} \tag{ 5 }$$

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Beside, different compounds of Ca and P are results of following reactions:

$$\begin{eqnarray}&&4{{\rm{Ca}}}^{2+}+{{\rm{HPO}}}_{4}^{2-}+2{{\rm{PO}}}_{4}^{3-}+2.5{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{Ca}}}_{4}{\rm{H}}{({{\rm{PO}}}_{4})}_{3}\\ &&\quad \cdot \;2.5{{\rm{H}}}_{2}{\rm{O}},({\rm{octacalcium}}\;{\rm{phosphate}},\;{\rm{OCP}}),\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{{\rm{Ca}}}^{2+}+{{\rm{HPO}}}_{4}^{2-}+2.5{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{CaHPO}}}_{4}\cdot 2.5{{\rm{H}}}_{2}{\rm{O}},\\ &&\quad ({\rm{dicalcium}}\;{\rm{dihydrogen}}\;{\rm{phosphate}},\;{\rm{DCPD}}).\end{eqnarray} \tag{ 7 }$$

Based on the analysis of the cathodic polarization curve, HAp coating was electrodeposited in the electrolyte solution (pH = 4.4) with different scanning potential ranges: from 0 to −1.8; 0 to −1.9; 0 to −2.0; 0 to −2.2 and 0 to −2.5 V/SCE during 5 scans with a scan rate of 5 mV s⁻¹, at 50 °C.

FTIR spectra of HAp synthesized on the surface of Ti6Al4V with different scanning potential ranges are presented in figure 2. All of the spectra show the characteristic bands of ${{{\rm{PO}}}_{4}}^{3-}$ group (asymmetric stretching vibration of P–O bond at 1030 cm⁻¹; asymmetric O–P–O bending mode at 560 cm⁻¹). Absorption peaks at 3460 and 610 cm⁻¹ usually assigned to the O–H stretching vibration in hydroxyapatite were not clearly distinguished on these spectra. This can be explained by the non-stoichiometry of the apatite phase, with a deficiency in calcium and hydroxide groups. Water bands were visible by a large band in the range 3460 cm⁻¹ (O–H stretching from water molecules) and by the H–O–H bending band at 1630 cm⁻¹. Beside, the presence of peak in the region around 1380 cm⁻¹ can be assigned to the vibration modes of ${{{\rm{CO}}}_{3}}^{2-}.$ The occurrence of ${{{\rm{CO}}}_{3}}^{2-}$ may be due to the reaction of atmospheric CO₂ with OH⁻ ions, hence the content of ${{{\rm{CO}}}_{3}}^{2-}$ ions is rather low [16].

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Figure 3 presents the XRD diffraction patterns of HAp synthesized on the surface of Ti6Al4V in 2θ of 20°–50°. The XRD patterns only exhibit the hydroxyapatite phase (denoted * in figure 3). The typical peaks are found at 2θ ≈ 26° and 32° corresponding to (002) and (211) planes.

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The HAp crystal diameter calculated from the Scherrer equation shows that the crystals have nano-size (89–106 nm). The crystal diameter changes not much when the potential range changes from 0 to −1.8 V/SCE to 0 to −2.0 V/SCE. However, with the larger potential range the crystal diameter increases significantly (table 3). It can be explained as following: with the large potential range, much OH⁻, ${{{\rm{PO}}}_{4}}^{3-}$ ions were generated leading to the large formation of HAp; the HAp particles concentrated to form larger particles.

The SEM images of HAp coatings on Ti6Al4V were shown in figure 4. The results show that with the different potential ranges, the surface morphology does not change. All of the samples, the substrates were covered completely by HAp coatings. At low magnification the crystals of HAp have coral-like shape. At high magnification, SEM images clearly show that HAp crystals have flake-like shape to form coarse granular agglomerates.

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Table 4 showed the thickness of HAp coatings synthesized with different potential ranges. The coating thickness increases and reaches a maximum value at 7.91 μm (the potential range 0 to −2.0 V/SCE, respectively). When the potential range was larger, the thickness of HAp coatings decreases. The coating thickness is 4.68 μm with the potential range from 0 to −2.5 V/SCE. The decrease of the coating thickness can be explained as following: with the large scanning potential range, the further increase in the amount of OH⁻ and ${{{\rm{PO}}}_{4}}^{3-}$ ions on the electrode surface, leading to their diffusion from the electrode surface into the solution and these ions combined with Ca²⁺ to form HAp in the solution. Simultaneously, with the large cathode potential, the reduction of water is promoted, thus the hydrogen bubbles are created on the electrode surface that may reduce adhesion ability of HAp coatings with Ti6Al4V substrate. So the suitable potential range is 0 to −2.0 V/SCE.

Deposition time affects the thickness and surface morphology of HAp coatings. We electrodeposited HAp coatings in the potential range 0 to −2.0 V/SCE with the change of scan number: 1, 3, 5, 7 and 10 scans with constant scan rate (5 mV s⁻¹). SEM images of the HAp coatings on the surface of Ti6Al4V are shown in figure 5. With different scan number, the morphology of HAp coatings is various. It is explained as following: with change of scan number, OH⁻, ${{{\rm{PO}}}_{4}}^{3-}$ ions are differently formed and they affect the formation and crystallization of HAp.

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Table 5 presents the HAp coating thickness synthesized with the different scan numbers. The coating thickness increases and reaches the maximum value after 5 scans. The scan number increases continually, the thickness of HAp coatings decreases due to peeling of HAp coatings. Therefore, 5 scans was chosen to synthesize HAp coatings on Ti6Al4V by electrodeposition method. In the next parts, we synthesized HAp/Ti6Al4V at condition: the potential range from 0 to −2.0 V/SCE; 5 scans; 5 mV s⁻¹ at 50 °C to investigate in vitro test.

The electrochemical behaviors of Ti6Al4V and HAp/Ti6Al4V materials in the SBF solution were also investigated.

3.4.1. The variation of pH solution

Figure 6 shows the pH values of the SBF solutions containing Ti6Al4V and HAp/Ti6Al4V materials with different immersion times at 37 °C. The pH value of the SBF solution before soaking is 7.4. The pH of the SBF solutions decreased during the immersion time. Moreover, the pH of SBF solution containing HAp/Ti6Al4V decreases more strongly in comparison with Ti6Al4V substrate. It can be hypothesized that HAp as nucleation promotes the formation of apatite. The formation of apatite consumed OH^- leading to the reduction of pH solution, the pH solution containing HAp/Ti6Al4V decreases strongly during immersion time. This result shows that the formation of apatite was significant. After 21 immersion days in the SBF solution, the pH value of the solution immersed HAp/Ti6Al4V was 6.16.

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3.4.2. Variation of mass

The mass variation of Ti6Al4V and HAp/Ti6Al4V materials with different immersion times is displayed in figure 7. The mass of Ti6Al4V substrate during immersion time changed not much. After 21 immersion days in the SBF solution, the mass of Ti6Al4V is 0.08 mg cm⁻². With HAp/Ti6Al4V, the mass of material increased during the immersion process. After 21 immersion days the mass variation of HAp/Ti6Al4V is 1.99 mg cm⁻². It is clear that the formation of apatite crystals on the surface of material increased significantly. These results are confirmed by the SEM images (figures 8 and 9).

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3.4.3. The SEM images

3.4.4. Polarization measurements