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Electrochemistry Communications 9 (2007) 1886–1891

One-step approach for nano-crystalline hydroxyapatite coatingon titanium via micro-arc oxidation

Min-Seok Kim a, Jae-Jun Ryu b, Yun-Mo Sung a,*

a Department of Materials Science and Engineering, Korea University, Seoul 136-713, South Koreab Department of Prosthodontics, Medical School, Korea University, Seoul 136-701, South Korea

Received 21 April 2007; received in revised form 25 April 2007; accepted 26 April 2007Available online 10 May 2007

Abstract

Nano-crystalline hydroxyapatite (HAp) films were formed at the surface of Ti by a single-step micro-arc oxidation (MAO) using Ca2+

and P5+ ion-containing electrolytes. The HAp films were 10–25 lm thick, showing strong crystallinity dependence on the CaCl2 concen-tration in the electrolytes. Also, the formation of an amorphous CaTiO3 interlayer was identified to exist between the HAp and Ti sub-strates. In contrast to the previous researches using K2HPO4 for the electrolytes, KH2PO4 was used in this study, and this could allow theformation of the crystalline HAp layer. It is suggested as the most probable mechanism for the HAp formation that the high-densityhydroxyl groups of TiO(OH)2, formed by the reactions between the amorphous CaTiO3 interlayer and the H+ ions from the dissolutionof the KH2PO4, can play a key role in the nucleation and crystal growth of HAp by attracting Ca2+ and P5+ ions in the electrolytes.� 2007 Elsevier B.V. All rights reserved.

Keywords: Hydroxyapatite (HAp); Micro-arc oxidation (MAO); Electrolytes; Coating; Nanocrystals

1. Introduction

Ti and Ti alloys have been widely used for the orthope-dic and dental prosthesis applications due to their superiorload bearing capability and corrosion resistance [1–4].However, they show poor osteointegration properties dueto the bioinertness of the native oxide film. The coatingof hydroxyapatite (HAp: Ca10(PO4)6(OH)2) on the metalsurface has been suggested as the most effective way to pro-vide biocompatibility and osseoconductivity [5–11]. Up tonow, several techniques of HAp coating on titanium suchas plasma spray coating, sputtering, electron beam deposi-tion, chemical vapor deposition, electrophoresis, electro-chemical deposition, and dip coating, have been explored,and among them, plasma spray coating has mostly beenhighlighted for the applications [11–22]. However, usingplasma spray coating, it is very difficult to obtain uniform

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.04.023

* Corresponding author. Tel.: +82 2 3290 3286; fax: +82 2 928 3584.E-mail address: ymsung@korea.ac.kr (Y.-M. Sung).

coating and high crystallinity of the HAp phase. Due to thehigh-temperature melting and the subsequent rapid coolingduring plasma spray coating, amorphous and/or secondaryphases such as tetra calcium phosphate (TCP) and tetracalcium phosphate (TTCP) appear, and these phases arehighly bioresorbable.

Micro-arc oxidation (MAO) has been developed as aroom-temperature electrochemical process suitable for theformation of native ceramic films on the value metals suchas Al, Mg, and Ti to improve their wear and corrosionresistance [23,24]. People also have tried to simply overcoatHAp at the surface of Ti using Ca and P containing electro-lytes by MAO [25–34]. However, the only MAO-treatedsamples showed formation of amorphous films with lowcontent of Ca and P ions, and could not form crystallineHAp. They proposed that the concentration of hydroxylgroups (Ti–OH), which can play a key role in inducingbone-like apatite including HAp, were insufficient in thesesamples [32,33]. Thus, the hydrothermal treatment after theMAO was proposed as a method to increase the crystallin-ity of the films [27–30]. There exist two main effects of the

30 40 50 60 70 80

(a)

T(1

03)

T(0

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H: hydroxyapatite

T: Titanium

H(3

23)

H(1

24)

H(4

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H(3

21)

H(2

13)

H(1

32)

H(2

22)

H(1

13)

H(1

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H(3

00)H

(112

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11)

T(1

01)

Rel

ativ

e In

tens

ity (

A.U

)

(b)

(e)

(c)

(d)

Two-theta

Fig. 1. X-ray diffraction (XRD) patterns of micro-arc oxidized (MAO) Tisamples using different CaCl2 concentrations of (a) 0.05 M, (b) 0.075 M,(c) 0.100 M, (d) 0.125 M, and (e) 0.150 M, respectively.

0.04 0.06 0.08 0.10 0.12 0.14 0.160.0

0.5

1.0

1.5

2.0

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3.0

3.5

4.0

HA

p cr

ysta

llini

ty (

%)

I HA

p (2

11)/I

Ti(0

02)

Molarity of CaCl2

10

20

30

40

50

60

70

80

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100

Fig. 2. The variation in the XRD peak intensity ratio of HAp (211) to Ti(002) and that in the HAp crystallinity with different CaCl2concentrations.

M.-S. Kim et al. / Electrochemistry Communications 9 (2007) 1886–1891 1887

hydrothermal treatments using a high-temperature andhigh-pressure autoclave. One is that the hydrothermallytreated samples can form sufficient Ti–OH groups at thesurface, which serve as the sites for the nucleation ofHAp crystals, and the other is that the Ca and P ions com-bined with hydroxyl groups can transform into the crystal-line HAp at the applied temperature and pressure. Thus, todate, no successful results have been reported on the pro-ducing the highly crystalline HAp films through the sin-gle-step MAO process using electrolytes containing Caand P.

In this study, we for the first time report the success inthe single-step crystalline HAp film formation by only theMAO. We believe we could obtain plenty of Ti–OH bondsby modifying the electrolytes, and the hydroxyl bondscould contribute to the HAp formation by reacting withCa and P ions. The HAp films with a thickness of 10–25 lm were successfully coated on the Ti by the single-stepMAO process, and the crystallinity of the HAp films waseven higher than that of the films hydrothermally treatedafter the MAO.

2. Experimental

Commercially pure Ti (Grade 2, Hyundai Titanium Co.,Incheon, South Korea) plates (10 · 40 · 1 mm) were usedas a substrate material for the MAO treatments. The Tiplates and a stainless steel palate were used as an anodeand a cathode, respectively. For the electrolytes, calciumchloride (CaCl2, 96.0%, Aldrich, Milwaukee, WI) andpotassium phosphate monobasic (KH2PO4, 99.0%,Aldrich, Milwaukee, WI) were dissolved in de-ionizedwater at 50 �C. The applied voltage and current were320–340 V and 34–35 A, respectively. The concentrationof CaCl2 was varied to 0.05, 0.075, 0.10, 0.125, and0.15 M, while the concentration of KH2PO4 was fixed to0.05 M. The MAO treatments were conducted using theelectrolytes at 50 �C for 4 min.

The variation in the crystallinity of the HAp film withthe CaCl2 concentration was investigated using low inci-dence angle (3�) X-ray diffraction (XRD: Rigaku D/MAX-2500/PC, Tokyo, Japan) with a thin film attach-ment. The quantitative analyses on the crystallinity of thefilms were performed using a multipeak separation pro-gram (MDI Jade 5.0, Materials Data Inc., Livermore,CA). The field emission scanning electron microscopy(FESEM: Hitachi S-4300, Tokyo, Japan) and energy dis-persive X-ray spectroscopy (EDS: Hitachi EX-200, Tokyo,Japan) were performed to investigate microstructures ofthe specimens and to analyze the chemical composition,respectively according to the CaCl2 concentration.

3. Results and discussion

The HAp films prepared by MAO using different con-centrations of CaCl2 were investigated for their crystallinityusing XRD as shown in Fig. 1. As the concentration of

CaCl2 increases, the relative intensity of the HAp diffrac-tion peaks existing around 30� increases compared to thatof the Ti. In addition, the diffraction peaks such as HAp(222) and (213), not detectable in the samples with 0.05M of CaCl2, show the apparent intensity increase withthe CaCl2 concentration. On the other hand, the sharp dif-fraction peaks of Ti, for examples Ti (101) and (002),show the rapid intensity decrease with the CaCl2 concen-tration. The intensity ratio of the HAp (211) to Ti (002)peaks is presented according to the CaCl2 concentrationin Fig. 2. The increase in the intensity ratio implies theincrease in the crystallinity of the HAp films. The quantita-tive analysis results on the crystallinity of the HAp filmswere obtained using the multipeak separation softwarefor the XRD data, and also plotted in Fig. 2, and theincrease in the HAp crystallinity of the films is obvious.

Fig. 3. Field emission scanning electron microscopy (FESEM) plan-view images of the micro-oxidized (MAO) samples using CaCl2 concentrations of (a)0.05 M, (b) 0.075 M, (c) 0.100 M, (d) 0.125 M, and (e) 0.150 M, respectively. Low (1000·) and high-magnification (30,000·) images are presented at the leftand right columns, respectively.

1888 M.-S. Kim et al. / Electrochemistry Communications 9 (2007) 1886–1891

Fig. 4. Field emission scanning electron microscopy (FESEM) cross-sectional view image of the micro-arc oxidized (MAO) specimen preparedusing 0.05 M KH2PO4 and 0.10 M CaCl2, and energy dispersive X-rayspectroscopy (EDS) depth profiles of titanium, calcium, and oxygen.

M.-S. Kim et al. / Electrochemistry Communications 9 (2007) 1886–1891 1889

The HAp films prepared using 0.15 M CaCl2 showed thehighest crystallinity of �92%, which is even higher thanthat of the samples hydrothermally treated after theMAO. Of interest is that the secondary phases, a- or b-tri-calcium phosphate (a- or b-Ca3(PO4)2: TCP) and tetracal-cium phosphate (Ca4P2O9: TTCP), which have beenreported to frequently occur during the MAO processand known as highly bioresorbable phases, were notdetected in our samples. This reveals that the films containCa and P ions with the Ca/P ratio very close the stoichiom-etric composition of 1.67. The formation of anatase orrutile phase of titania (TiO2) was not identified in theXRD patterns, which implies that a different mechanismdominates for the formation of the crystalline HAp.

Fig. 3 shows FESEM low (1000·) and high (30,000·)magnification plan view images of the HAp films preparedusing different CaCl2 concentrations, presenting the varia-tion in the morphological features. The specimens pre-pared by MAO treatments using electrolytes of 0.050and 0.075 M CaCl2 did not show any specific morpholog-ical features. These films just consist of irregular micro-structures and less number of pores (Fig. 3a and b).When the concentration of CaCl2 reached to 0.100 and0.125 M, some unique features like cellular structures withthe size of 20–40 lm emerged. The number of poresincreased, and their size was �2–5 lm (Fig. 3c and d).Increasing CaCl2 concentration to 0.125 and 0.150 M gaverosette-like morphologies with some porosity as shown inFig. 3d and e. The coarse structure of the films at theincreased CaCl2 concentration could come from the rapidgrowth of HAp crystals due to the high Ca content anddecreased number of micro-arcs. The microstructure ofthe HAp films is determined by the number of micro-arcs,and the less number of micro-arcs can give coarse micro-structure. It has been found that as the CaCl2 concentra-tion increases, the number of micro-arcs decreases duringthe MAO process. The coarse microstructure can give ben-efit to the HAp films, since it can promote the intergrowthof natural bones and accelerate interfacial bondingbetween the implants and the natural bones. Anotherinteresting characteristic is that numerous nanocrystals,believed to be HAp, were formed at the surface of thefilms, and the number of these nanocrystals graduallyincreased with the concentration of CaCl2. It appears thatespecially, the surface of the samples prepared using thehighest CdCl2 concentration (Fig. 3e) was entirely coveredwith nanocrystals, and thus only a small number of poreswere found. However, noticeable variation in the morpho-logical features was not observed in high magnificationimages (the right column in Fig. 3). All the samplesshowed formation of spherical-shape nanoparticles withthe size distribution of 50–100 nm, and the density of thenanocrystals showed steady increase. The increase in thenanocrystal density with the CaCl2 concentration couldoriginate from the increase in the number of Ti–OHhydroxyl groups. For the samples prepared using electro-lytes containing 0.150 M of CaCl2 show the Ca/P ratio

of �1.66, which is almost identical to the stoichiometricHAp.

The cross-sectional view image of the specimens pre-pared using 0.100 M CaCl2 is shown in Fig. 4. No definitedifference was found in the thickness and morphologicalfeature of the films prepared using various concentrationsof CaCl2. The coating layer was approximately 10–25 lmthick, and there was no distinct discontinuity of the HAplayer in the SEM image. The interfacial bonding of theHAp layer to the Ti substrate seems very tight and interfa-cial cracks or void was not observed. It was also observed

1890 M.-S. Kim et al. / Electrochemistry Communications 9 (2007) 1886–1891

that there exists a recognizable interlayer with a �3–5 lmthickness between the HAp layer and the Ti substrate.EDS line scanning was performed to identify the composi-tion of the interfacial region of the cross-sections. Fig. 4also shows the depth profiles of titanium, calcium and oxy-gen, respectively. Through the contrast in the image, wecould perceive the existence of the interlayer between theTi and the HAp layer. Based upon the profile results ofEDS line scanning, this interlayer seems to be a specificcompound of CaTiO3, and this compound could be amor-phous, since no diffraction peaks were identified from theXRD analyses in Fig. 1. With increase of CaCl2 concentra-tion, the formation of CaTiO3 is promoted, however, theCaTiO3 can be consumed to produce more HAp. Thus,the thickness of the CaTiO3 layer was almost identical inthe samples with different CaCl2 concentrations, while thatof the HAp showed increase in the average thickness from10 to 25 lm with the CaCl2 concentration.

Based upon the XRD, SEM, and EDS analysis results,the possible mechanism of the HAp formation was specu-lated for the MAO-treated samples. The main differencebetween the previous MAO researches and this one liesin the use of a different precursor for the electrolytes. Inthis study, we used KH2PO4 as a precursor for the electro-lyte, and also used K2HPO4 as a reference for the purposeof verifying our mechanism. The specimens prepared usingKH2PO4 for electrolytes revealed high crystallinity of HApin XRD patterns (Fig. 1). However, the specimens pre-pared using K2HPO4 showed no diffraction peaks relatedto the crystalline HAp. Despite the use of the same numberof moles of KH2PO4 and K2HPO4 at a fixed CaCl2 concen-tration, the atomic % of Ca in the samples prepared usingthe electrolyte of KH2PO4 was 7.8 times higher than that ofK2HPO4.

H2PO�4 and HPO2�4 have been known as amphoteric

compounds which can act either as an acid or a base. Basedon Brønsted–Lowry theory, H2PO�4 can make the follow-ing chemical formulae in water:

H2PO�4ðaqÞ þH2OðlÞ !H3OþðaqÞ þHPO2�4ðaqÞ Ka¼ 5:9�10�8 ð1Þ

H2PO�4ðaqÞ þH2OðlÞ !H3PO4ðaqÞ þOH�ðaqÞ Kb¼ 2:9�10�12 ð2Þ

Fig. 5. Schematic diagram showing the HAp nanocrystal formation in the mi0.05 M KH2PO4 and 0.10 M CaCl2.

Here, Ka and Kb are approximate acid and base ionizationconstants, respectively at 50 �C [34]. When the KH2PO4

dissolves in water, there exists competition between the ten-dency of donating (1) and accepting (2) protons in H2PO�4 .Since Ka is much larger than Kb, KH2PO4 prefers to pro-duce H3O+ ions. In contrast, K2HPO4 (Ka� Kb) is mostlikely to produce OH� ions.

Previous researches revealed that the abundant Ti–OHgroups could form via the exchange of the Ca2+ ions com-ing from CaTiO3 with H3O+ ions [26,32,33]. From theSEM and EDS line scanning, we confirmed the existenceof a specific compound containing Ca, Ti and O and form-ing between the Ti and the HAp layer. Although no diffrac-tion peaks were detected for the compound in the XRD, wepredict this interlayer consists of the CaTiO3 compoundswhich could be in an amorphous state, because we couldnot find any other elements except Ca, Ti and O in thisregion.

Therefore, the following mechanism is suggested in thisstudy. As the first step, the electrolytes containing CaCl2and KH2PO4 produce Ca2+, Cl�, K+, and H2PO�4 ions.The Ca2+ ions incorporate with the Ti substrate to formthe amorphous CaTiO3 compounds. Because the Ti plateis used as an anode, it has a tendency to lose electrons toproduce Ti4+ ions, which will react with Ca2+ ions to formthe CaTiO3;

Ti! Ti4þ þ 4e� ð3ÞCa2þ þ Ti4þ þ 3O2� ! CaTiO3 ð4Þ

Here, O2� ions come from the dissociation of water mole-cules in the electrolytes, which occurs at the surface of Tianode during the MAO. As the second step, the H2PO�4ions make H3O+ (H+) ions, and these are combined withthe CaTiO3. As a result, TiO(OH)2 is produced via ex-change of the Ca2+ ions coming from CaTiO3 with H3O+

ions.

CaTiO3 þ 2Hþ ! TiOðOHÞ2 þ Ca2þ ð5ÞAs the third step, the TiO(OH)2 forms various Ti–OHgroups at the Ti surface due to thermal energy originatedfrom the micro-arcing, and these Ti–OH groups induce

cro-arc oxidized (MAO) samples prepared using an electrolyte containing

M.-S. Kim et al. / Electrochemistry Communications 9 (2007) 1886–1891 1891

the formation of bone-like apatite. As the final step, Ti–OHgroups forming right on the CaTiO3/Ti attract the Ca2+

and PO3�4 ions from the electrolytes and form the HAp nu-

clei. The continuous supply of the Ca2+ and PO3�4 ions

from the electrolytes can cause the HAp crystal growth.The thermal energy from the micro-arcing can also assistthe diffusion of ions for the nucleation and crystal growthof the HAp. Fig. 5 presents the schematic diagram showingthe formation of crystalline HAp films on Ti substrates.

4. Conclusions

In this study, for the first time, only one single-step ofMAO coating was proposed as a method suitable to obtaincrystalline HAp films on Ti substrates. The crystallinity inthe HAp films showed high dependency on the concentra-tion of CaCl2 in the electrolytes. On the contrary to theK2HPO4-containing electrolyte used in the previousresearches, the KH2PO4-containing electrolyte allowed suc-cessful coating of the crystalline HAp on the Ti via MAO.A detail mechanism for the HAp coating was proposedbased upon the formation of amorphous CaTiO3 and itsdissociation to TiO(OH)2 by H+ ions. The high densityhydroxyl groups can incorporate Ca2+ and PO3�

4 ionsactively to form the crystalline HAp. The HAp films werefound to consist of HAp nanocrystals with 50–100 nm,dependent upon the CaCl2 concentration. Due to the highcrystallinity of the HAp films, it can be stated that thematerials possess strong biocompatibility and high poten-tial to be used for orthopedic and dental prosthesisapplications.

Acknowledgement

This study was supported by the Korea–Russia Interna-tional Collaboration Research & Development Program ofITEP (2006).

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