Electrophoretic deposition of hydroxyapatite fiber reinforced hydroxyapatite matrix nanocomposite coatings

Electrophoretic deposition of hydroxyapatite fiber reinforced hydroxyapatite matrix nanocomposite coatings

Accepted Manuscript Electrophoretic deposition of hydroxyapatite fiber reinforced hydroxyapatite matrix nanocomposite coatings Morteza Farrokhi-Rad P...

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Accepted Manuscript Electrophoretic deposition of hydroxyapatite fiber reinforced hydroxyapatite matrix nanocomposite coatings

Morteza Farrokhi-Rad PII: DOI: Reference:

S0257-8972(17)30982-9 doi:10.1016/j.surfcoat.2017.09.051 SCT 22723

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

19 June 2017 6 September 2017 21 September 2017

Please cite this article as: Morteza Farrokhi-Rad , Electrophoretic deposition of hydroxyapatite fiber reinforced hydroxyapatite matrix nanocomposite coatings. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2017.09.051

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ACCEPTED MANUSCRIPT Electrophoretic deposition of hydroxyapatite fiber reinforced hydroxyapatite matrix nanocomposite coatings Morteza Farrokhi-Rada*

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Department of Materials Engineering, Faculty of Engineering, Azarbaijan Shahid Madani

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University, P.O.Box : 53714-161, Tabriz, Iran

*Corresponding author: Tel.: +984131452559, Fax: +984134327526, E-mail address:

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[email protected], [email protected],

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a

ACCEPTED MANUSCRIPT Abstract Two-component suspensions of hydroxyapatite nanoparticles with spherical (S-HA) and fiber (FHA) morphologies were prepared in isopropanol using triethanolamine (TEA) as the dispersant. In-situ kinetics of deposition and current density were recorded during EPD at 60V from

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suspensions with different wt% of F-HA and TEA. f factor decreased against TEA concentration

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in all suspensions except the one-component suspension of F-HA particles. The detachment of particles from single component F-HA deposits did not occur during EPD due to the mechanical

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interlocking of F-HA particles in their microstructure. F-HA particles reinforced the coatings and prevented from their cracking during drying by bridging mechanism. S-HA particles infiltrated

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and filled the pores formed by stacking of the F-HA particles in deposit. S-HA particles were

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also grafted to F-HA particles in TEA-containing suspensions via hydrogen bonding between TEA molecules adsorbed on the particles. The coating deposited from the suspension with 75

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wt% of F-HA particles had the best corrosion resistance in SBF solution at 37.5˚C.

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Keywords: Electrophoretic deposition; Fiber hydroxyapatite particles (F-HA); Spherical

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hydroxyapatite particles (S-HA); Nanocomposite coatings; Triethanolamine (TEA).

ACCEPTED MANUSCRIPT 1. Introduction Hydroxyapatite (HA) is a calcium phosphate very similar to the inorganic part of the human bone and hard tissues both in morphology and composition [1]. HA has been widely used in biomedical applications due to its high bioactivity, biocompatibility, osteoconductivity and

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biodegradability [2-4]. However, HA has poor mechanical properties such as low hardness and

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fracture toughness restricting its usage in load bearing orthopedic applications [5-10]. To overcome this problem, HA is usually applied as the coating on the metallic implants such as

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titanium and 316L stainless steel to combine the high mechanical strength of the underlay metallic substrate and excellent bioactivity of the upper HA coatings. HA coatings on metallic

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implants can also prevent from their corrosion induced by body fluid [11]. The corrosion of

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implant releases the metal ions into the surrounding tissues leading to their inflammation [12].

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Moreover, in general metals do not have sufficient bioactivity and applying, for example, HA and bioactive glass coatings on their surface can significantly promote their bioactivity. Several

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processes have been used to apply HA coatings on the metallic substrates such as sol-gel [13, 14], thermal [15] and plasma spraying [16], electrolytic deposition [17], electrodeposition [18]

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and so on. Electrophoretic deposition (EPD) is another technique which has been widely used in

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recent years to deposit HA coatings on the metallic substrates [19-30]. EPD is a two-step process: in the first step, charged particles dispersed in a suitable solvent move towards the oppositely charged electrode (substrate) under the influence of an applied electric field; in the second step, they deposit on the substrate and form a relatively dense particulate layer on it [31]. EPD has several advantages like simplicity, need to low-cost equipments, short formation times, forming the even coatings on the substrates with complex shapes and capability to control the

ACCEPTED MANUSCRIPT microstructure and thickness of deposits by simple adjustment of EPD parameters such as voltage and time [32].

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The kinetics of EPD can be stated by the following equation [32, 33]:

Where

is the EPD rate,

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(1)

is the electrophoretic mobility of particles, E is the applied electric

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field and c is the concentration of particles in the suspension. f factor or sticking parameter is a ) implying the efficiency of deposition [33, 34].

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constant (f

0,

r,

and

are permittivity of vacuum, relative dielectric constant of suspension

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Where

(2)

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Electrophoretic mobility of particles can be calculated by the following equation [35]:

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medium, zeta potential of particles and viscosity of suspension, respectively. Another way to enhance the mechanical strength of HA is the addition of reinforcing second

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phase into its microstructure. Several reinforcing phase like Ti-Fe particles [36], rare earth oxides [37], alumina [38], zirconia [39] and carbon [40-42] fibers and carbon nanotubes [30, 4346] have been used to enhance the mechanical strength of HA. The reinforcing phase must also have high biocompatibility and bioactivity. So one-dimensional HA particles seem to be the most-promising reinforcement for HA. W. Suchanek et al [47, 48] fabricated HA/HA whiskers composites and found that HA whiskers increase the fracture toughness of HA without decreasing its biocompatibility and bioactivity. In this work HA particles with fiber morphology

ACCEPTED MANUSCRIPT (F-HA) were used as the reinforcement to enhance the mechanical integrity of HA coatings. The effect of F-HA particles amount in the suspension on the EPD process as well as the characteristics of obtained coatings has been investigated in this work.

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2. Experimental

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Hydroxyapatite nanoparticles with spherical (S-HA) and fiber (F-HA) morphologies were

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synthesized according to the methods described in Ref. [49] and [50], respectively. X-Ray diffraction (XRD) analysis was used to identify phases in the synthesized nanopowders. The

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microstructure of nanopowders was also observed by the scanning electron microscope (SEM).

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Isopropanol (99.8%, Merck Co.) and triethanolamine (TEA, reagent grade, Merck Co.) were used as the solvent and dispersant for suspension preparation, respectively. To prepare the

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suspensions, firstly different concentrations of TEA (0, 0.67, 1.33, 2, 2.67, 3.33 and 4mL/L)

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were dissolved in isopropanol by magnetic stirring for 15mins; then S-HA and F-HA particles with different wt% ratios (F-HA wt%: 0, 15, 25, 50, 75 and 100) but constant total particles

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concentration of 10g/L were added into the prepared isopropanol-TEA solutions. The

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suspensions were then magnetically stirred and ultrasonically dispersed for 24h and 10min, respectively. The zeta potential of particles was measured in suspensions with different

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compositions by Malvern instrument. The samples for zeta potential analysis were prepared by the method described in Ref. [34]. Electrophoretic deposition (EPD) was carried out at 60V using a two-electrode cell with an electrodes distance of 1cm. The plates of 316L stainless steel with the dimensions of 40mm×20mm×1mm were used as the substrate. The substrate area of 20mm×20mm was exposed to suspension during EPD; the reminder area of substrates was insulated by polymeric adhesive tape. The counter electrode was also a same plate as the substrate. In-situ kinetics of deposition and current density during EPD were recorded based on

ACCEPTED MANUSCRIPT the method described in Ref. [34]. During in-situ recording of the EPD kinetics the voltage was applied for 6min and then switched off for 2mins. The effective voltage (Veff) across the suspensions was calculated by the following equation:

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(3)

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Where Vapp is the applied voltage (60V), i0 and it are the current density at starting point (t=0s)

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and moment t of EPD, respectively.

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Sticking parameter (f factor) was calculated using the data obtained by in-situ recording of the

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EPD kinetics according to the Ref. [34].

The optimum concentrations of TEA in suspensions with different wt% of F-HA were

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determined by the results of zeta potential analysis. The concentrations of TEA at which the zeta

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potentials are the highest were selected as its optimum concentrations.

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The coatings deposited at 60V for 30s from the suspensions with different wt% of F-HA and optimum concentration of TEA were selected for further analysis. These coatings were dried at

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room temperature overnight and then sintered at 800˚C for 1h under the flowing argon gas

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atmosphere. The microstructure of the dried coatings was investigated by SEM at low and high magnifications. The corrosion resistance of the sintered coatings was investigated by electrochemical polarization technique in SBF environment at 37.5˚C. SBF was prepared using the method described in Ref. [51]. A three-electrode cell was used for electrochemical corrosion studies. The platinum wire mesh, saturated calomel electrode (SCE) and coated substrates were used as the counter, reference and working electrodes, respectively.

ACCEPTED MANUSCRIPT 3. Results and Discussion The XRD patterns of both synthesized S-HA and F-HA powders are shown in figure 1. Both powders show the characteristic peaks of hydroxyapatite. The XRD pattern of F-HA sample shows more intensive and narrow peaks implying that F-HA powders have more crystallinity

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and/or coarser particles size than S-HA powders. The higher crystallinity of F-HA powder can be

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due to the lower feed rate (10mL/h) of phosphorus precursors ((NH4)2HPO4 solution) into the calcium precursors (Ca(NO3)2 solution) in its synthesizing process providing longer time for the

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crystallization of F-HA particles and so more degree of crystallinity for them.

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SEM images from synthesized powders are shown in figure 2. The SEM images verify the nearly spherical and fiber morphologies of S-HA and F-HA powders, respectively. Also as can be seem

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the F-HA particles are coarser than S-HA ones.

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The zeta potential of particles in the suspensions with different wt% of F-HA particles against

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TEA concentration is shown in figure 3. As can be seen, the zeta potential of particles increases to a maximum value with TEA concentration and then decreases with its further addition. It was

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found in author previous work [28] that H+TEA ions generated via TEA protonation in

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isopropanol are chemically adsorbed on the surface of HA particles via hydrogen bonding with their surface P-OH groups. The adsorption of H+TEA ions on the surface of HA particles enhances their surface charge and so zeta potential. The zeta potential of one-component suspensions of S-HA and F-HA particles (10g/L) is the highest in optimum TEA concentrations of 2 and 0.67mL/L, respectively. It can be said that the surface of particles is saturated by H+TEA ions at optimum concentrations of TEA. F-HA particles are coarser and so have less specific surface area so that their surface is saturated by H+TEA ions at less concentrations of

ACCEPTED MANUSCRIPT TEA so that the optimum concentration of TEA is less in F-HA suspension. The optimum concentrations of TEA in two-component suspensions are 2, 2, 1.33 and 1.33 for those with 15, 25, 50, and 75 wt% of F-HA particles, respectively. The results for in-situ kinetics of EPD at 60V from suspensions with different wt% of F-HA and

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TEA concentrations are shown in figure 4. As can be seen EPD is the fastest from the

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suspensions with 0, 75 and 100 wt% of F-HA particles and optimum concentration of TEA due to the highest zeta potential and so electrophoretic mobility of particles in them.

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The rate of reduction in deposition weight after switching off the voltage increases with TEA

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concentration for all suspensions except the one-component suspensions of F-HA. The reduction in deposition weights after switching off the voltage is due to the particles detachment from

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deposits into the suspensions at their interface. The detailed discussion explaining the

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detachment of particles is reported in Ref. [34]. Briefly, the detachment of particles from deposit into the suspension is because of the force exerted on them as a result of an electrochemical

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potential difference generated at the interface. The electrochemical potential difference at the

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interface and so the force exerted on the particles increase as the TEA concentration in the suspension increases resulting in the faster detachment of particles in suspensions with higher

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amount of TEA. The weights of deposits prepared from one-component suspensions of F-HA remain nearly constant after switching off the voltage. Long F-HA particles are mechanically interlocked to each other in the microstructure of deposits prepared from their one-component suspensions so that the force exerted on them at the interface cannot detach them from deposits into the suspensions. The sticking parameter (f factor) of particles in different suspensions is shown against TEA concentration in figure 5. f factor at any moment is the fraction of particles which sit in the deposit and do not detach from it at that moment [34]. The faster detachment of

ACCEPTED MANUSCRIPT particles from deposit into the suspension results in the less f factor. So, as seen in figure 5, f factor decreases with TEA concentration for all suspensions except the one-component suspensions of F-HA particles. f factor is about 1 for all TEA containing suspensions of F-HA particles since the detachment of particles does not occur in deposits prepared from them.

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Although the zeta potential and so the mobility of particles increase with TEA addition into the

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suspensions with 15, 25 and 50 wt% of F-HA, the EPD rate decreases from them because of the substantial reduction in their f factor.

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The effective voltage across the suspensions during EPD at 60V from different suspensions is

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shown in figure 6. As can be seen the effective voltage decreases with EPD time for all suspensions due to the voltage drop over the deposit formed on the substrate. However, the

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reduction in effective voltage is less in case of suspensions with 0, 15 and 25 wt% of F-HA. The

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addition of TEA into the suspensions with 50, 75 and 100 wt% of F-HA results in the considerable drop in the effective voltage (around 50%). The magnitude of voltage drop (Vdrop)

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over the deposit depends on the resistance of deposit (Rdep) as well as the current density passing

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through the EPD circuit (Vdrop=R.i). The higher the wt% of F-HA in the wet deposits the higher is their resistance due to the coarser size of F-HA particles preventing more effectively from the

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electro-diffusion of free ions toward the substrate electrode during EPD. Moreover, the current density increases with TEA addition into the suspensions due to its ionization enhancing their conductivity. So voltage drop is higher in TEA containing suspensions with high wt% of F-HA particles (≥50) resulting in the lower effective voltage for them. The resistance of wet deposits increases as they thicken with EPD time so that the effective voltage and so deposition rate decreases more rapidly during EPD from TEA containing suspensions with F-HA wt% ≥50 (figure 4 (d, e and f)).

ACCEPTED MANUSCRIPT The SEM images from dried coatings deposited at 60V from suspensions with different wt% of F-HA and optimum concentration of TEA are shown in figure 7. As expected the amount of FHA particles in the coatings increases as their wt% increases in their corresponding suspensions. The coating deposited from the one-component suspension of F-HA particles has more open

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mirostructure. The stacking of long F-HA particles on the substrate during EPD leads to the

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formation of deposit with coarse pores and open microstructure. However, these pores can be

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infiltrated and filled by the finer S-HA particles in the coatings deposited from two-component suspensions. Also it can be seen that the surface of F-HA particles is covered by finer S-HA

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particles in the deposits prepared from two-component suspensions. Figure 8 shows the

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mechanism proposed for the bonding between S-HA and F-HA particles. TEA (as well as H+TEA ions) molecules are chemically adsorbed on the surface of both S-HA and F-HA

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particles via hydrogen bonding with their surface P-OH groups. The TEA molecules adsorbed on

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the surface of S-HA and F-HA particles can bond to each other via hydrogen bonding and graft

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S-HA to F-HA particles.

The SEM image from the coating deposited from TEA-free suspension with 75 wt% of F-HA is

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shown in figure 9. As can be seen the surface of F-HA particles is not covered by S-HA

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nanoparticles in case of this coating due to the absence TEA. As mentioned, the adsorbed TEA molecules act as the link and bond S-HA and F-HA particles to each other. Also compared to the coating deposited from the suspension with same wt% of F-HA but also with 2mL/L TEA (fig. 7e), this coating has more open microstructure. This can be due to the less infiltration efficiency of finer S-HA particles through the pores formed by F-HA particles due to the lower zeta potential of both type of particles in TEA-free suspension. The higher zeta potential results in the stronger electrostatic repulsion forces between the S-HA nanoparticles and pore walls (F-HA

ACCEPTED MANUSCRIPT particles) promoting the infiltration process. E. Stroll et al [52] also found that the infiltration of ceramic fiber mat with alumina nanoparticles is more efficiently carried out when the strong electrostatic repulsion force exist between them. The electro-osmotic flows around the deposited particles cause in their rearrangement and increase the packing density of deposit [53-56]. The

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higher the zeta potential of particles, the more intensive is the electro-osmotic flows in the

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suspension are another reason for its more open microstructure.

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deposit [53]. So the weaker electro-osmotic flows in the deposit prepared from TEA-free

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The low-magnification SEM images for the coatings deposited from the suspensions with different wt% of F-HA and optimum concentration of TEA are shown in figure 10. The cracking

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in EPD coatings is because of the mechanical stresses exerted on them as a result of shrinkages

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induced during their drying. As can be seen the coating deposited from one-component suspension of S-HA nanoparticles (0 wt% of F-HA) is highly cracked. There are also few tiny

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cracks in the microstructure of coating deposited from the suspension with 15 wt% of F-HA.

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However, the coatings deposited from the suspensions with 25 wt% of F-HA or more have thoroughly crack-free microstructures. So cracking decreases and eventually completely

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inhibited as the F-HA amount in the suspension and consequently in the coating increases. The

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SEM image from the crack generated in the coating deposited from the suspension with 15 wt% of F-HA is also shown as the inset in figure 10(b). As can be seen F-HA particles bridge across the crack and tend to prevent from its generation and propagation. F-HA particles are mechanically interlocked in the microstructure of composite coatings and thereby reinforce them efficiently. Also the bonding of the S-HA nanoparticles to the F-HA particles increases the mechanical integrity of the composite coatings. The amount of F-HA particles in the coating deposited from the suspension with 15 wt% of F-HA is not high enough to withstand the

ACCEPTED MANUSCRIPT mechanical stresses induced during its drying and prevent from crack generation in it. However, there are more F-HA particles in the coatings deposited from the suspensions with 25 wt% of FHA or more providing the reinforcement which is enough to prevent from their cracking. The polarization curves for the coatings deposited from the suspensions with different wt% of F-

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HA and optimum concentration of TEA are shown in figure 11. The values of corrosion current

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density (icorr) and potential (Ecorr) extracted from polarization curves are also listed in table 1. The corrosion rate of substrate increases as it is coated using one-component suspension of S-HA

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particles (0 wt% of F-HA). The S-HA coating is highly cracked (figure 10(a)) so that an intensive local corrosion can occur inside its cracks resulting in the higher corrosion rate for the

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substrate coated with it. Although the coating deposited from the suspension with 15 wt% of F-

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HA is also cracked, it decreases the corrosion rate of the substrate. This coating has only few tiny cracks so that it can more efficiently limit the reaching of corrosive fluid to the substrate surface.

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The coatings deposited from the suspensions with 25 wt% of F-HA or more have thoroughly

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crack-free microstructures. Also they have good mechanical integrity because of their relatively high content of F-HA particles acting as the reinforcing phase. So these coatings can act as the

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excellent barrier against the corrosive medium and significantly increase the corrosion resistance

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of the substrate. Among the coatings, the one deposited from the suspension with 75 wt% of FHA has the highest efficiency in preventing from the corrosion of substrate (the minimum icorr). The relatively high content of F-HA particles considerably reinforces the microstructure of this coating. Moreover, the S-HA particles present in the suspension sit in the deposit during EPD and fill the coarse pores formed by the stacking of the F-HA particles in it. Also the grafting of S-HA nanoparticles on the surface of F-HA particles increases the mechanical integrity of the coating. icorr is slightly higher in case of coating deposited from the one-component suspension of

ACCEPTED MANUSCRIPT F-HA particles (100 wt% of F-HA) compared to those deposited from the ones with 25, 50 and 75 wt% of F-HA. This coating is completely built by interlocked F-HA particles and so it has high degree of mechanical integrity. Also it has a more open microstructure since large number of coarse pores is formed by stacking of the F-HA particles on the substrate. On the other hand,

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there are no S-HA particles in the suspension to sit in the deposit and fill these coarse pores

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during EPD. Corrosive fluid can diffuse through these unfilled pores and reach the substrate

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surface to corrode it leading to the slightly higher icorr for the coating deposited from one-

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component suspension of F-HA particles.

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4. Conclusion

H+TEA ions generated by TEA protonation were adsorbed on the surface of both S-HA and F-

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HA particles and promoted their zeta potential and so colloidal stability. The optimum

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concentration of TEA was lower in case of F-HA particles due to their less specific surface area. Sticking parameter (f factor) decreased with TEA addition into all suspensions except the one-

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component suspension of F-HA particles. The detachment of particles from F-HA deposit into

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the suspension did not occur during EPD due to the mechanically interlocked F-HA particles in it. F-HA particles reinforced the coatings and completely prevented from crack generation in the

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coatings deposited from the suspensions with 25 wt% of F-HA particles or more. The relatively coarse pores were formed as a result of F-HA particles stacking on the substrate during EPD. These pores can be infiltrated and filled by finer S-HA particles present in the suspensions. S-HA particles were also bonded to F-HA particles in TEA-containing suspensions through the hydrogen bonding between the TEA molecules adsorbed on their surface. So the reinforcement efficiency of F-HA particles promoted in the presence of S-HA particles. The coating deposited

ACCEPTED MANUSCRIPT from the suspension with 75 wt% of F-HA particles had the best corrosion resistance in SBF solution at 37.5˚C.

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ACCEPTED MANUSCRIPT Figures captions Figure 1. XRD pattern for synthesized S-HA and F-HA powders. Figure 2. SEM image for synthesized (a) S-HA and (b) F-HA nanoparticles.

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Figure 3. Zeta potential of particles in the suspensions with different wt% of F-HA particles

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against TEA concentration.

Figure 4. In-situ kinetics of EPD at 60V from the suspensions with (a) 0, (b) 15, (c) 25, (d) 50,

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(e) 75 and (f) 100 wt% of F-HA and different concentrations of TEA (voltage is switched off at

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t=360s).

Figure 5. Sticking parameter (f factor) at initial times of EPD at 60V from the suspensions with

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different wt% of F-HA particles against TEA concentration.

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Figure 6. Effective voltage (Veff) during EPD at 60V from the suspensions with (a) 0, (b) 15, (c)

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25, (d) 50, (e) 75 and (f) 100 wt% of F-HA and different concentrations of TEA.

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Figure 7. High-magnification SEM images from the dried coatings deposited at 60V for 30s from the suspensions with (a) 0, (b) 15, (c) 25, (d) 50, (e) 75 and (f) 100 wt% of F-HA and optimum

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concentrations of TEA ((a, b, c): 2mL/L, (d, e): 1.33mL/L and (f): 0.67mL/L). Figure 8. Schematic representation of the mechanism proposed for the bonding of S-HA to F-HA particles. Figure 9. SEM image from the dried coating deposited at 60V for 30s from the TEA-free suspension with 75 wt% of F-HA.

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the suspensions with (2) 0, (3) 15, (4) 25, (5) 50, (6) 75 and (7) 100 wt% of FHA and optimum concentrations of TEA ((2, 3, 4): 2mL/L, (5, 6 ): 1.33mL/L and (7): 0.67mL/L)

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environment and 37.5˚C.

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FHA: 0

FHA: 15

FHA: 25

FHA: 50

FHA: 75

FHA: 100

substrate

wt%

wt%

wt%

wt%

wt%

icorr (μA/cm2)

43.68

66.02

16.25

0.93

0.81

Ecorr (mV Vs. SCE)

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wt%

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1.53

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Table 1. icorr and Ecorr for the bare substrate and substrates coated at 60V for 30sec from the

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Highlights:

EPD of Fiber (F-HA)/Sphere (S-HA) hydroxyapatite composite coatings were performed.



S-HA was bonded to F-HA particles in TEA-containing suspensions.



F-HA particles efficiently reinforced the microstructure of composite coatings.



Coarse pores were formed by stacking of long F-HA particles in deposit during EPD.



Finer S-HA particles efficiently filled these coarse pores during EPD.

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