Fabrication and characterization of hydroxyapatite/Al₂O₃ biocomposite coating on titanium

WU Zhen-jun(吴振军)¹, HE Li-ping(何莉萍)², CHEN Zong-zhang(陈宗璋)¹

1. College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China;

2. College of Mechanical and Automotive Engineering, Hunan University, Changsha 410082, China

Received 2 June 2005; accepted 2 August 2005

Abstract:

A novel biocomposite coating of hydroxyapatite/Al₂O₃ was fabricated on titanium using a multi-step technique including physical vapor deposition(PVD), anodization, electrodeposition and hydrothermal treatment. Anodic Al₂O₃ layer with micrometric pore diameter was formed by anodization of the PVD-deposited aluminum film on titanium and subsequent removal of part barrier Al₂O₃ layer. Hydroxyapatite coating was then electrodeposited onto the as-synthesized anodic Al₂O₃ on titanium. A hydrothermal process was finally applied to the fabricated biocomposite coating on titanium in alkaline medium. Scanning electron microscopy(SEM), energy dispersive spectrometry(EDS) and X-ray diffractometry(XRD) were employed to investigate the morphologies and compositions of the pre- and post-hydrothermally treated hydroxyapatite/Al₂O₃ biocomposite coatings. The results show that micrometric plate-like Ca-deficient hydroxyapatite (CDHA) coatings are directly electrodeposited onto anodic Al₂O₃ at constant current densities ranging from 1.2 to 2.0 mA/cm² using NaH₂PO₄ as the phosphorous source. After hydrothermal treatment, the micrometric plate-like CDHA coating electrodeposited at 2.0 mA/cm² is converted into nano-network Ca-rich hydroxyapatite (CRHA) one and the adhesion strength is improved from 9.5 MPa to 21.3 MPa. A mechanism of dissolution-recrystallization was also proposed for the formation of CRHA.

Key words:

anodic Al2O3; hydroxyapatite; biocomposite coating; medical metals; PVD; electrodeposition; hydrothermal treatment;

1 Introduction

Currently, many synthetic materials, such as medical metals (titanium and its alloys), Bioglass^?, CaP bioceramics, alumina, and some biodegradable polymers and so on, have been developed for hard tissue repair and replacement[1,2]. Among these available systems, titanium and its alloys coated with hydroxyapatite(HA), a kind of CaP bioceramic with formula of Ca₁₀(PO₄)₆(OH)₂ which serves as the main inorganic composition of human bone[3], have attracted much attention due to the simultaneous achievements of good mechanical properties and superior biocompatibility[4, 5]. So far, some methods, including plasma spray[6], laser ablation[7], sol-gel process[8], biomimetic growth [9], electrocrystallization[10], and electrophoretic deposition[11], have been applied to the fabrication of HA coatings on medical metals. However, the HA-coated medical metals fabricated by these methods still have some disadvantages including the release of toxic elements from metal substrate, thermal expansion mismatch of HA coating and metal substrate, and compositional diversity of the outer coating because of the non-total covering and high temperature contained in the fabrication procedure inherently or subsequently [12, 13].

To solve the problems mentioned above, several recent studies have focused on the introduction of intermediate layers, as examples of microarc TiO₂ and anodic Al₂O₃, between bioactive HA coating and metal substrate and obtained some helpful results[14-17]. Especially, an in situ CaP(aggregate)/Al₂O₃ composite coating with as-designed “T” shape effect has been successfully fabricated on titanium in our previous work [16,17]. But to achieve long-term reliable biocompatibility, HA outer layer is preferable. Hereinafter, a modified multi-step technique combining physical vapor deposition(PVD), anodization, electrodeposition, and hydrothermal treatment is employed to fabricate a novel HA/Al₂O₃ biocomposite coating on titanium. The morphologies and compositions of as-fabricated biocomposite coatings were investigated by scanning electron microscopy(SEM), energy dispersive spectrometry(EDS), and X-ray diffractometry (XRD). A mechanism for the formation of Ca-rich HA (CRHA) layer during hydrothermal treatment was also proposed.

2 Experimental

2.1 Fabrication process

The raw materials and chemicals used in the present work are as follows.

Commercially pure titanium discs (Ti, d 18 mm×1 mm, ＞95%, China), PVD Al target (＞99.99%, UK), Na₃PO₄·12H₂O (analytic reagent, ≥97%, China), Ca(NO₃)₂·4H₂O (analytic reagent, ≥99%, China), NaH₂PO₄·2H₂O (analytic reagent, ≥99%, China), NaOH (analytic reagent, ≥96%, China).

The procedure for fabricating hydroxyapatite/Al₂O₃ biocomposite coating on titanium is schematically shown in Fig.1. The processes including PVD, anodization, electrodeposition, and hydrothermal treatment are detailed as below.

Fig.1 Fabrication procedure of HA/Al₂O₃ biocomposite coating on titanium

1) Deposition of Al film on titanium by PVD

Commercially pure titanium was cut into small discs with a dimension of 18 mm diameter and 1 mm thickness. The titanium discs were mechanically ground and polished, ultrasonically washed with acetone and deionized water. After pre-treatment, titanium discs were then used as substrates subjected to PVD. The deposition of Al film was carried out by a magnetron sputtering ion plating system (TEER, UK) using highly pure Al     (＞99.99%) as target under the condition of -100 V bias voltage, 5.0 A aluminum target current, and 60 min duration at a pressure of 2.66×10^－4 Pa. Pure aluminum film with a thickness of approximately 2.5 μm finally formed on titanium disc.

2) Anodization of Al film on titanium

The PVD-deposited Al film on titanium was ultrasonic-washed in acetone for 20 min to clean its surface. And then the pre-treated Al film was anodized potentiostatically in 100 g/L Na₃PO₄ solution at 80 V and 15 ℃ for 30 min using a two-electrode electrolytic system, in which the titanium disc deposited with Al film acted as anode (working electrode) and a stainless steel plate as cathode (counter electrode). The distance between anode and cathode was fixed at 3.5 cm. To reduce the resistance of anodic Al₂O₃ and prompt the soakage of electrolyte during electrodeposition, the anodized specimens were immersed in a 5% phosphoric acid solution at 30 ℃ for 40 min. After this process, the barrier layer of anodic Al₂O₃ became thinner, and accordingly the pores were widened.

3) Electrodeposition

The electrolyte for electrodeposition was an aqueous solution containing 0.08 mol/L Ca(NO₃)₂ and 0.04 mol/L NaH₂PO₄. The pH value of electrolyte was adjusted to 5-6 by 0.1 mol/L NaOH solution. Electro- deposition was performed at constant current densities ranging from 0.25 to 2.0 mA/cm² and 23 ℃ for 15 min with mild magnetic stirring. The cathode (working electrode) was titanium disc covered with anodic Al₂O₃ and the anode (counter electrode) was a platinum plate. The distance between anode and cathode was 3.0 cm.

4) Hydrothermal treatment

The composite coatings formed on titanium were hydrothermally treated at 150, 170, 190 ℃ with a pressure of 1.25 MPa for 4 h in an autoclave. The hydrothermal medium was an alkaline solution of ammonia (25%-28% in water)-water (1?80 in volume ratio). The hydrothermally treated specimens were finally dried at 100 ℃ for 1 h in a resistance furnace.

2.2 Characterization and analysis

The morphologies of anodic Al₂O₃ with widened pores, the electrodeposited coatings, and the hydrothermally treated composite coatings were characterized by scanning electron microscopy (SEM, JEOL/JSM-5600) at an acceleration voltage of 20 kV. The element types and their atomic percentages of the pre- and post-hydrothermally treated composite coatings were determined by energy dispersive spectrometry (EDS) attached to SEM. X-ray diffractometry(XRD, Siemens D5000) was conducted to analyze phases of the obtained biocomposite coatings on titanium using Cu K_α radiation at a voltage of 35 kV with a current of 30 mA and a scanning rate of 0.04 (?)/s.

The composite coating electrodeposited at 2.0 mA/cm² was used as the sample subjected to tensile test and its adhesive strength was determined before and after hydrothermal treatment at a motion rate of 0.02 mm/s by an electronic versatile materials testing machine (CMT-7105, China). The binder was a mixture of epoxy resin and polyamide resin (1?1).

3 Results and discussion

3.1 Formation of anodic Al₂O₃ with big pores on titanium

Fig.2 shows the surface morphology of PVD- deposited Al film on titanium. Compared with the one obtained at 4.5 A target current in Ref.[16], no obvious protrudent aggregation is observed in Fig.2, which indicates a good amalgamation of aluminum crystal during PVD at 5 A. It is thought that even surface of Al film is beneficial for the formation of anodic Al₂O₃ with uniform pores[18]. Therefore, a PVD target current higher than 5 A is preferable for the uniform deposition of HA coating.

Fig.2 Surface morphology of PVD-deposited Al film on titanium

Fig.3 exhibits anodic behavior of PVD-deposited Al film on titanium. It is a typical curve of aluminum anodization at constant cell voltage, which consists of three continuous phases, namely, a quick drop of current density, then the increase of current density, and finally the stabilization of current density, indicating the fast formation of barrier Al₂O₃ layer, the initial growth of porous Al₂O₃ layer, and the further development of porous Al₂O₃ layer, respectively[19]. There is no obvious decrease of current density once again in the curve as shown in Fig.3 after the formation stage of porous Al₂O₃ layer, suggesting that the anodization of PVD-deposited Al film on titanium is incomplete. This case also could be elucidated from the fact that compact anodic TiO₂ layer with higher resistance would lead to an obvious decrease of current density if anodization reaches the titanium substrate[20].

It has been proved that there were only fine pores with diameter less than 300 nm formed on pure aluminum using Na₃PO₄ electrolyte at tens of cell voltages[21]. To prompt a good soakage of solution and obtain uniform HA coating during electrodeposition, anodic Al₂O₃ layer on titanium with uniform big pores and better conductivity is needed. For this purpose, it is considered that chemical dissolution of anodic Al₂O₃ in diluted H₃PO₄ is feasible. When immersed in diluted H₃PO₄, the bottom and the wall of pores can be chemically dissolved and thus the barrier Al₂O₃ layer becomes thinner, and meanwhile, the pores of anodic Al₂O₃ layer were widened[22]. This process was applied to the anodic Al₂O₃ on titanium in the present work. Uniform pores with diameter approximately 1 μm, can be clearly observed from the SEM micrograph of the prepared anodic Al₂O₃ on titanium as shown in Fig.4.

Fig.3 Anodic behavior curve of PVD-deposited Al film on titanium at constant voltage of 80 V in Na₃PO₄ solution

Fig.4 Surface morphology of porous anodic Al₂O₃ after immersed in 5% phosphoric acid solution at 30 ℃ for 40 min

3.2 Effect of current density on morphologies and compositions of electrodeposits

The electrodeposition of the ions containing calcium and phosphorous onto porous surface of anodic Al₂O₃ was conducted at constant current mode. Ca(NO₃)₂ and NaH₂PO₄ served as the calcium source and the phosphorous source, respectively. Fig.5 shows the electrodeposition rate, which is expressed as the electrodeposited mass of one milliampere within one minute and denoted as v_C, at different current densities as the anodic Al₂O₃ area on titanium for electrodeposition is kept at 2.0 cm². v_C increases firstly with the current density and reaches a maximum value at approximately 1.2 mA/cm², and then decreases at higher current density accompanied by the obvious release of H₂. Therefore, the appropriate current density should be set at 1-2 mA/cm² to obtain enough electrodeposits and to avoid too much H₂ release during electrodeposition.

Fig.5 Variation of electrodeposition rate with current density at identical substrate area (2.0 cm²)

Fig.6 shows the surface morphologies of electro- deposits formed at different current densities. The electrodeposited coatings are mainly constructed with plate-like crystals with 10-25 μm length and 2-5 μm width growing away from the substrates. The images suggest that the higher the current density is, the bigger and denser the electrodeposited crystals are.

According to the XRD patterns shown in Fig.7, the electrodeposits prepared at different current densities are hydroxyapatite(HA) coatings. Besides the diffraction peaks of titanium substrate and anodic Al₂O₃, some characteristic peaks of HA can also be observed at the angles of 26? and 30?-34?, which corresponding to the (002), (211), (112), and (300) diffractions. It is clear that there is an orientation growth of (002) plane in HA, which consists with the observation by others when electrodepositing bioactive coatings[23]. It is also found from Fig.7 that the diffraction peaks of (211), (112), and (300) planes are intensified while the diffraction peak of (002) is weakened with the increase of current density. This indicates that the structure of electrodeposited crystal is more similar to that of the standard HA at higher current density, which can also be identified according to mole ratios of Ca to P closer to 1.67 as shown in Table 1.

Table 1 also shows that the mole ratios of Ca to P of the direct electrodeposits on anodic Al₂O₃ are less than 1.67 (the mole ratios of Ca to P of stoichiometric HA). This may be due to the incorporation of HPO₄ groups into the lattice of apatite crystal and a resulting layer of Ca-deficient HA (CDHA) with molecular formula of Ca_10－x(HPO₄)_x(PO₄)_6－x(OH)_2－x (0≤x≤1) formed[24]. The content of HPO₄ groups, denoted by the value of x, can be calculated from the formula as

                            (1)

where  R_Ca/P is the mole ratio of Ca to P. It has been reported that the structural characterization of CDHA is similar to that of standard HA[25], therefore, the diffraction peaks of the obtained CDHA coating, as shown in Fig.7, are in a good agreement with those of stoichiometric HA. In addition, no titanium element is detected by EDS due to the shielding effect of aluminum, which consists with the EDS result obtained by our previous work[17] and also identifies the incomplete anodization of PVD-deposited Al film on titanium substrate just as concluded from Fig.3.

Fig.6 Surface morphologies of coatings electrodepsited at different current densities: (a) 1.2 mA/cm²; (b) 1.7 mA/cm²;  (c) 2.0 mA/cm²

Table 1 Mole ratios of Ca to P of coatings electrodeposited on anodic Al₂O₃ at different current densities

Fig.7 XRD patterns of coatings electrodeposited on anodic Al₂O₃-Ti at different current densities

It is interesting that the direct deposition of HA, although not stoichiometric one, is realized in the present work using NaH₂PO₄ as phosphorus source and NaOH as buffering reagent. Differently, NH₄H₂PO₄ is often used as the phosphorus source to electrodeposit bioceramic coatings on medical metals and non-HA products, such as brushite and other CaP phases, have been obtained[26]. It is generally accepted that the formation of non-HA deposits should be due to the lack of OH^－ ions around the cathode[27]. Although high current density can increase the concentration of OH^－ ions surrounding cathode by consuming more H^＋ ions or producing OH^－ ions directly[28] through the cathodic reactions as

2H⁺+2e=H₂↑                                (2)

H₂O+2e=2OH^－+H₂↑                          (3)

O₂(dissolved)+2H₂O+4e=4OH^－                  (4)

However, there exist unavoidable disadvantages, such as the formation of many spiracles in the coatings and the adhesive strength decrease of deposits due to the release of too much H₂ gas. Thus, high current density is improper for the fabrication of bioceramic coatings with good adhesion.

When using NH₄H₂PO₄ as phosphorus source, another cathodic reaction occurs

+OH^－=NH₃↑+H₂O                     (5)

Reaction(5) would be accelerated because of the NH₃ gas evolution during the continuous motion of  ions to cathode under the driving of potential difference. Furthermore, the continuous releasing H₂ also can act as carrier to take the generated NH₃ away from cathode. Hence, additional OH^－ ions consumption caused by reaction(5) could be avoided through the substitution of Na⁺ for  in the present work. Therefore, the produced OH^－ ions may be enough for the formation of HA. This indicates that the electrolyte type could be one of key factors for the direct electrodeposition of HA.

3.3 Effect of hydrothermal temperature on composi- tions  and morphologies of composite coatings

Fig.8 shows the XRD patterns of the electro- deposited CDHA with 2.0 mA/cm² after hydrothermal treatment at different temperatures. Compared with the directly electrodeposited coating(EC), the hydro- thermally treated ones are still composed of HA according to their XRD patterns. With the increase of hydrothermal temperature, the diffraction peaks corresponding to (211), (112), and (300) become higher than the one to (002), indicating that the hydrothermal treatment leads to higher crystallinity of the electrodeposited coatings.

Fig.8 XRD patterns of electrodeposited coating (EC, 2.0 mA/cm²) on anodic Al₂O₃-Ti after hydrothermal treatment at different temperatures

The SEM images of the hydrothermally treated coatings at different temperatures are given in Fig.9, showing that the coatings become more uniform with the increase of hydrothermal temperature. The micrometric outer plate-like HA coating (Fig.6(c)) is transformed into nanometric network-like (Fig.9(d)) one at 190 ℃. It is expected that this network-like structure textured with nanowires could stimulate the in-growth of new-born bone to achieve a “T” shape effect as designed by our previous work[16], which can also enhance the adhesive strength between bioceramic coating and substrate

Fig.9 SEM images of electrodeposited coating (EC, 2.0 mA/cm²) after hydrothermal treatment at different temperatures: (a) 150 ℃, (b) 170 ℃, (c) 190 ℃; (d) Magnified photo of framed area in (c)

The EDS results in Table 2 show that the electrodeposited CDHA is converted into Ca-rich hydroxyapatite(CRHA) with mole ratio of Ca to P higher than 1.67 after being hydrothermally treated at 190 ℃ for 4 h.

Table 2 Experimental and theoretical Ca/P atomic ratios of the coatings electrodeposited on anodic Al₂O₃ with different current densities after hydrothermal treatment at 190 ℃

It has been proposed that a mechanism, including the chemical dissolution of CaP phases, then static adsorption of Ca²⁺, and final induction of HA, may contribute to the formation of CRHA in synthetic fluid [29]. Thereinafter, a mechanism similar to the one in synthetic fluid for the formation of CRHA outer layer in the present biocomposite coating is outlined.

During hydrothermal treatment, the alkaline ammonia could be ionized to form OH^－ as

NH₃?H₂O=+OH^－                                      (6)

The electrodeposited CDHA would react with OH^－ ions and then be converted to stoichiometric HA based on the following reactions:

Ca_10－x(HPO₄)_x(PO₄)_6－x(OH)_2－x+6OH^－=(4–x)Ca(OH)₂+

2Ca₃(PO₄)₂+2+xH₂O                     (7)

Ca(OH)₂+3Ca₃(PO₄)₂=Ca₁₀(PO₄)₆(OH)₂          (8)

Because reaction(8) could not consume all Ca(OH)₂, which is produced more than Ca₃(PO₄)₂ according to the chemical equilibrium of reaction(7), the remaining Ca(OH)₂ would be contained in the stoichiometric HA newly formed through reaction(8) since Ca(OH)₂ is a kind of relatively stable calcium precipitate in alkaline medium. While the resultant  from reaction(7) may form ammonium phosphate salt and dissolve in the hydrothermal medium. According to reaction(7), the theoretical mole ratios of Ca to P of CRHA, shown as , can be calculated from the formula as below:

                         (9)

Based on Eqn.(1), x value of the electrodeposited CDHA with different current densities can be obtained. The corresponding theoretical mole ratios of Ca to P of CRHA are then calculated from the obtained x values according to formula (9) and displayed in Table 2. Although the solubility is very low, there is still some dissolved Ca(OH)₂ entering into hydrothermal medium since these experimental mole ratios of Ca to P are less than the theoretical ones as shown in Table 2.

The high dispersity of the resulting Ca(OH)₂ precipitate contained in stoichiometric HA or the overlap of diffraction peaks may lead to no discovery of Ca(OH)₂ by XRD. On the basis of the details above, the formation of the present CRHA outer layer could be mainly dependent on a procedure of dissolution (reaction (7)) and recrystallization (reaction (8)), accordingly, the elicited mechanism could be denominated as dissolution-recrystallization one.

In addition, the diffraction peak of Al₂O₃·H₂O can also be found in Fig.8, which indicates the sealing of anodic Al₂O₃ pores caused by hydrothermal environment in autoclave[30].

3.4 Adhesive strength of composite coating

Using a mixture of epoxy resin and polyamide resin (1?1) as binder, it is tested that the adhesive strength of the HA/Al₂O₃ biocomposite coating formed at 2.0 mA/cm² on titanium is 9.5 MPa and then increases to 21.3 MPa, a value which could be compared to that of plasma-sprayed HA coatings on titanium[31], after hydrothermal treatment at 190 ℃ in autoclave.

It is reported that the adhesive strength between bioactive coatings and medical metal substrates could be improved by mechanically interlocking interface[32]. In this work, the mechanical interlocking may also be established due to the introduction of porous anodic Al₂O₃ intermediate layer which in-situ formed on PVD-deposited Al film on titanium. However, both the micrometric size of anodic Al₂O₃ pores and the interspace among electrodeposited CDHA crystals could weaken the enhancing effect for adhesion from mechanical interlocking. So, the hydrothermal treatment subsequent to electrodeposition is necessary for the HA/Al₂O₃ biocomposite coating on titanium to improve its adhesive strength and to enhance its biocompatibility. In other words, the narrowed pores and the nanometric network structure, which derive from the sealing of anodic Al₂O₃ pores and dissolution- recrystallization of HA coating as described in section 3.3, respectively, are beneficial for the adhesive strength of as-obtained biocomposite coating.

4 Conclusions

1) After anodization and chemical dissolution, anodic Al₂O₃ layer with uniform pores of approximately 1μm diameter forms on titanium. The XRD and EDS results show that Ca-deficient HA (CDHA) coatings are electrodeposited onto anodic Al₂O₃ on titanium due to the incorporation of HPO₄ group into the lattice of HA crystal. It is proposed that a potential dissolution- recrystallization procedure may lead to the conversion of CDHA coatings into Ca-rich HA(CRHA) ones during hydrothermal treatment. Tensile test results suggest that adhesive strength of the fabricated hydroxyapatite/Al₂O₃ biocomposite coating on titanium could be enhanced through a tighter interlocking between HA and anodic Al₂O₃ caused by hydrothermal treatment.

   2) It is expected that the sandwich-like structure of hydroxyapatite/Al₂O₃/Ti obtained in the present work would result in a combination of superior biocompatibility with good mechanical properties and thus would provide a new choice for the surface biological modification of clinical hard tissue implants.

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Foundation item: Project(50472031) supported by the National Natural Science Foundation of China

Corresponding author: CHEN Zong-zhang; Tel: +86-731-8822276; E-mail: wooawt@yahoo.com.cn

(Edited by LONG Huai-zhong)

Abstract: A novel biocomposite coating of hydroxyapatite/Al₂O₃ was fabricated on titanium using a multi-step technique including physical vapor deposition(PVD), anodization, electrodeposition and hydrothermal treatment. Anodic Al₂O₃ layer with micrometric pore diameter was formed by anodization of the PVD-deposited aluminum film on titanium and subsequent removal of part barrier Al₂O₃ layer. Hydroxyapatite coating was then electrodeposited onto the as-synthesized anodic Al₂O₃ on titanium. A hydrothermal process was finally applied to the fabricated biocomposite coating on titanium in alkaline medium. Scanning electron microscopy(SEM), energy dispersive spectrometry(EDS) and X-ray diffractometry(XRD) were employed to investigate the morphologies and compositions of the pre- and post-hydrothermally treated hydroxyapatite/Al₂O₃ biocomposite coatings. The results show that micrometric plate-like Ca-deficient hydroxyapatite (CDHA) coatings are directly electrodeposited onto anodic Al₂O₃ at constant current densities ranging from 1.2 to 2.0 mA/cm² using NaH₂PO₄ as the phosphorous source. After hydrothermal treatment, the micrometric plate-like CDHA coating electrodeposited at 2.0 mA/cm² is converted into nano-network Ca-rich hydroxyapatite (CRHA) one and the adhesion strength is improved from 9.5 MPa to 21.3 MPa. A mechanism of dissolution-recrystallization was also proposed for the formation of CRHA.