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Applied Surface Science 261 (2012) 37– 42

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

AO-derived hydroxyapatite/TiO2 nanostructured multi-layer coatings onitanium substrate

. Abbasia,∗, F. Golestani-Farda,b, H.R. Rezaiea,b, S.M.M. Mirhosseini c

School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box 16845-161, Tehran, IranCenter of Excellence for Advanced Materials, Iran University of Science and Technology, P.O. Box 16845-195, Tehran, IranDepartment of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 6 June 2012eceived in revised form 8 July 2012ccepted 9 July 2012vailable online 16 July 2012

eywords:icro arc oxidation

a b s t r a c t

In this study, titanium substrates which previously oxidized through Micro arc oxidation method, wascoated by Hydroxyapatite (HAp) coating once more by means of the same method. Morphology, topogra-phy and chemical properties as well as phase composition and thickness of layers were studied to revealthe effect of the electrolyte concentration on coating features. According to results, the obtained coat-ings are consisted of HAp and titania as the major phases along with minor amounts of calcium titanateand �-tri calcium phosphate. Ca and P are present on surface of obtained layers as well as predictableTi and O based on the XPS results. Thickness profile of coatings figured out that by increasing the elec-

ydroxyapatiteiO2

ayered structuresanocompositesorosity

trolyte concentration, especially by addition of more Calcium Acetate (CA) to electrolyte, the thicknessof HAp layer would rise, consequently. However, the influence of coating time on thickness of obtainedcoatings would be more considerable than electrolyte concentration. High specific area coatings withnest morphology were obtained in Electrolyte containing 5 g/L �-Glycero Phosphate (�-GP) and 5 g/L CA.Increasing coating duration time in this kind of coatings would cause deduction of the nesting in theirstructure.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Micro arc oxidation (MAO) is a modern technique for pro-ucing thick and hard ceramic layers on valve metals and theirlloys. This technique is an electrochemical process along withlasma in order to form conversion oxide coatings and has beenaken survey by many scientists [1,2,3]. In this process, electri-al discharges are formed and disappeared quickly (10−4–10−5 s)djacent to electrodes. Local temperature and pressure insidehe discharge channels, formed by means of electrical sparks,each to 103–104 K and 102–103 MPa, respectively [3]. The men-ioned high temperature and pressure is sufficient for maximizinglasma thermo-chemical interaction between the substrate andhe electrolyte. These internal actions could cause fast melting andolidifying of the substrate and consequently formation of oxidesnd high-temperature complex compositions on the surface. These

xides contain the constitutional elements of both the electrolytend the substrate [2,4].

∗ Corresponding author. Tel.: +98 21 77240291; fax: +98 21 77240291.E-mail address: abbasi.iust.material@gmail.com (S. Abbasi).URL: http://rrg.iust.ac.ir (S. Abbasi).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.07.044

MAO is an inexpensive, simple, controllable and efficientmethod for formation of oxide layers with a coarse and porousstructure and high cohesivity on the surface of titanium and itsalloys [5]. Biological properties improvement by modifying outerlayer composition and morphology is the main aim of MAO processusage as a coating route on Ti implants surface. HAp is producible ontitanium substrates via MAO process. Apart from high resistance tocorrosion, high strength-to-weight ratio, good fracture toughnessand high bio compatibility, titanium is considered as a bio-neutralmaterial. In addition, HAp as a prominent bioactive material is com-monly synthesized on titanium substrate to improve its bioactivity[6].

HAp(Ca10(PO4)6(OH)2) is the principle component of hard bio-logical textures and the most applicable bio ceramic. After HApcoating being created on titanium, a composition of mechanicalproperties of titanium and biological properties of HAp is provided.HAp layers coated on titanium implants would be employed as den-tal implants and orthopedics (artificial bone bonds) [6]. To producethese layers through MAO, it is necessary to provide calcium andphosphor ions in the electrolyte [7]. The noted problem is that tita-

nium has metal bonds, while HAp possesses covalent bonds. As aresult, there are different chemical bonds in the interface betweentitanium substrate and HAp layer. Furthermore, Young’s modulusand thermal conductivity of titanium and HAp are totally different.

38 S. Abbasi et al. / Applied Surface

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ig. 1. XRD patterns of (a) Depth of coating grown in electrolyte containing 5 g/L CAnd 1 g/L �-GP, samples coated for 3 min in electrolytes containing: (b) 5 g/L CA and

g/L �-GP, (c) 10 g/L CA and 1 g/L �-GP, and (d) 5 g/L CA and 5 g/L �-GP.

he mentioned parameters weakens the bonding between the coat-ng and the substrate but the presence of MAO titania layer betweenitanium substrate and HAp coating leads to strengthening of theironds [8,9].

In previous studies [10,11], usually the composite coatings ofAp-titania (titania coating containing calcium and phosphor ions)re prepared and according to the information available, limitedumber of surveys have been done through this technique on multi-

ayer coatings. In this study, hybrid layers containing a layer ofully crystalline HAp on a layer of MAO titania have been grownn a titanium substrate, so that other than providing good cohesionetween the coating and the substrate, bioactivity of coating would

ncrease by the presence of higher percentage of HAp on the surfacef the coating in comparison with composite coatings. Microstruc-ure (such as pores size and porosity fraction), crystalline structurend the MAO coatings roughness and thickness are controlled byetting the electrolyte nature and properties, the applied potential,he electrolyte temperature, duration of the process and eventuallyy the flow density [1,2,3]. In the present study, a systematic survey

s performed on the chemical and phase composition, morphology,opography and thickness of MAO HAp layers on MAO titania oness a function of time and the electrolyte concentration.

. Methods and materials

Plates of commercial pure titanium (grade 2) of 30 mm × 30 mmnd with of 0.5 mm thickness were employed as the substrateanode) and a cylinder of stainless steel ASTM 316 as the cathode.node was placed in the middle of cathode, so that a uniform elec-

rical field could be produced between them. At first, an aqueousolution containing 10 g/L trisodium phosphate (Na3PO4·12H2O,erck) was used as the electrolyte and during the process its tem-

erature was maintained at about 70 ± 3 ◦C by water circulationystem. MAO process was accomplished by a DC source and under

voltage of 350 V. Synthesis time was considered 3 min for eachample. Polishing, washing by 1 M NaOH solution, acid washing by0 wt.% HF solution for 30 s and eventually 15 s ultrasonic processere used for samples surface preparation before coating via MAO

oute).Electrolytes containing various concentrations of �-GP

C3H7Na2O6P, Merck) and CA (Ca(CH3COO)2·xH2O, Merck) weremployed in the second stage. MAO process was still performednder voltage of 350 V and the growing period of the layers wereonsidered to be constant. As a result, two layers were laid on

Science 261 (2012) 37– 42

titanium. The produced samples in 5 g/L �-GP and 5 g/L CA con-taining electrolyte was chosen to be sputtered for 75 min, in orderto study the phase structure and chemical depth of the fabricatedcoating.

Phase structure and chemical composition of the layers wereinvestigated by X-ray diffractometer (XRD, Rigaku, Multiflex) andX-ray photoelectron spectroscopy (XPS, VG Microtech, Twin anode,XR3E2 X-ray source, using Al K� = 1486.6 eV). The XRD scanningrate was 0.02◦ � step size and in the range of 0◦ to 60◦ 2�. Surfacemorphology and topography of layers were scrutinized throughscanning electron microscopy (SEM,TESCAN, Vega II, Check) andatomic force microscopy (AFM, Veeco auto probe) with silicon pinof 10 nm tip in contact mode with air. Pore percentage and meansize of the pores were defined by means of visual analysis andCelemax software. Roughness of layers was obtained by AFM andProscan software (Ver. 1.7). Eventually, in order to study the crys-tal structure, field emission scanning electron microscope (FESEM,Hitachi S-4160) was used with high magnification.

3. Results and discussion

Fig. 1 shows the X-ray diffraction patterns of hybrid layer coat-ings that their second stage HAp layers was produced in 3 minthrough application of 350 V in the electrolyte with concentra-tion of 1 and 5 g/L �-GP and 5 and 10 g/L CA. These layers areconsisted of HAp, anatas, calcium titanate (CaTiO3), and �-TCP(Ca3(PO4)2).

Presence of peaks with 2� values of 26, 29.3, 35.6, and 42.7◦ cor-respond to the (0 0 2), (2 1 0), (2 1 1), and (3 1 0) diffraction planesof crystalline HAp [12]. As shown in Fig. 1, the HAp peaks are inten-sified by increasing the concentration of the electrolyte, either foran increase in the concentration of �-GP or CA; however, the raisein concentration of �-GP is more effective than that of CA. In otherwords, the effect of increasing �-GP on forming more HAp is muchmore than CA. This is followed by observing the most intense peaksof HAp in the spectrum of the sample prepared in the electrolytecontaining 5 g/L CA and 5 g/L �-GP.

CaTiO3 and Ca3(PO4)2 are the probable by-products of thechemical reactions which occurred in high temperature resultedby electrical discharges on the surface during formation of coat-ing. These two products could be generated through the reactionbetween HAp and TiO2. Weng and co-workers [13] suggested thatHAp and anatas react with each other in a temperature of 900 ◦C.�-TCP is considered to be a bio-resorbent material and possesses agreat absorption speed and erosion in alive environment. Strongerand faster connections between bones and implants are achievedas a result of minute amounts of �-TCP along with HAp [14]. Forthe titania layers fabricated through MAO it is reported that in SBF,apatite layer has the potential to be produced on the layers con-taining calcium titanat and �-TCP and these layers only displaybioactivity characteristic [15]. Changing the intensity of the peaksrelated to these two phases is similar to HAp. The reason behindintensification of peaks of all phases in terms of more concen-trated electrolytes is alteration of electrolyte electrical resistance.By increasing the concentration of the electrolyte, its resistanceand consequently the resistance of the entire circuit reduces whichresults in higher current passing through the circuit and formationof bigger and more electrical sparks. This causes increase in the tem-perature of plasma region and chemical reactions could intensifythe formation process of so called phases.

Fig. 1 also represents the trend of X-ray diffraction related to the

coated sample in 5 g/L �-GP and 5 g/L CA after sputtering process.As shown in this figure, the layer is mostly contained of titaniumoxide; the issue that proves the sole presence of titanium oxide indeep layers.

S. Abbasi et al. / Applied Surface Science 261 (2012) 37– 42 39

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2p3/2 and 2p1/2, prove the existence of phosphor in HAp structure[19].

ig. 2. XPS survey spectrum of the coating grown for 3 min in electrolyte containin(2p) core levels.

In order to study the chemical composition of the formed layers,he selected sample was exposed to XPS. XPS survey spectrum ofhis sample is displayed in Fig. 2a, which confirms the existence ofitanium, oxygen, phosphor and calcium in the composition of theayer. It is known that this analysis is especially used for the surfacend displays the chemical composition of the material in a depth ofnly a few angstroms. It should be considered that titanium existsn the surface of the coating. On the second stage, there would be a

elted flow resulted by titania layer getting melted through electri-al sparks which leads to titanium maintaining in the second layer.he existing carbon is result of preparation process and keeping theample and washing it by acetone before conducting the test.

In Fig. 2, the peaks related to the bonding energy of each elementere also represented. The entire interpretations are extracted

ased on the C(1s) with bonding energy of 284.8 eV. Fig. 2b sug-ests that O(1s) peak could be changed into a secondary peak. Peak

demonstrates the existence of oxygen in form of Ti O bonding by bonding energy of 529.9 eV [16]. Peak B which is lied on a bond-ng energy of 532.35 eV, could be related to O− species absorbedn layer surfaces. This feature can be explained by oxygen plasmahich surrounds samples during the MAO process [17]. In addition,

he C peak (on 533.11 eV) may be related to the water moleculesrapped in porosities of most external oxide layer [17]. On the curvetted on Ti 2p peak (Fig. 2c), peaks of A and C with bonding ener-ies of 458.33 and 463.95 eV are due to division of Ti 2p peak intowo component, 2Ti1/2 and 2Ti3/2, caused by coupling of the cir-uit spin. These two peaks specify the presence of titanium in theorm of Ti4+ in Ti O bonds [16]. In fact, presence of titanium oxiden the form of TiO2 is confirmed. The peak of Ti(2p3/2) is occurredn a bonding energy of 458.33 eV and by employing XPS softwareas not been converted into secondary peaks. But, conversion ofi(2p1/2) into two peaks B (with a bonding energy of 461.61 eV) and

could represent the existence of titanium with capacities otherhan Ti4+. In general, despite of TiO2 being the dominant oxide inhe layer, having small amount of TiO and Ti2O3, could be accepted18]. This is because it may be Ti3+ and Ti2+other than Ti4+ in the

L CA and 5 g/L �-GP, binding energies of (b) O(1s), (c) Ti(2p3/2), (d) Ca(2p), and (e)

coating due to non-equilibrium conditions of the coating growthin melting and freezing steps and under high process temperaturesleading to releasing oxygen from the surface of TiO2. It should benoticed that titanium is just in form of Ti4+ in TiO2/HAp compositelayer [10,11]. This feature is a result of thickening of oxide layerwhich results in fewer and then more strong discharges happen-ing around the samples. Fig. 2d, shows the energy of bonds relatedto calcium. Peaks A and B, with bonding energies of 347.72 and351.14 eV, are correlated to the spin–orbit coupling Ca(2p), assum-ing Ca(2p3/2) and Ca(2p1/2), which are the so called calcium bondsenergy in HAp structure. Fig. 2e shows the energy of P(2p) bondswhich have been separated into two peaks A and B in bondingenergies of 285.13 and 287.93 eV. These peaks, showing orbitals

Fig. 3. XPS O(1s) core level binding energy at depth of the layer grown in electrolytecontaining 5 g/L CA and 5 g/L �-GP.

40 S. Abbasi et al. / Applied Surface Science 261 (2012) 37– 42

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ig. 4. SEM micrographs of the coating formed for 3 min in electrolytes containing:

According to XPS software and also the area under these curves,he proportion of calcium to phosphor (Ca/P) was calculated andbtained equal to 1.65. HAp possesses calcium-to-phosphor ratiof 1.67 theoretically. �-TCP phase alongside of HAp leads to a reduc-ion in the ratio of Ca/P in the final coating, because of lower Ca/Patio.

Furthermore, the selected sample was characterized by XPSechnique after being sputtered for 75 min; the result of which isisplayed in Fig. 3. It was observed that the peak O(1s) could note converted into smaller peaks, so that only one oxygen bond-

ng would exist in lower depths of the coating. Bonding energy ofhis peak is defined to be 530.4 eV that could be correlated to Ti–Oompositions.

Microstructures of the produced layers are displayed in Fig. 4.orous structure of the surface plays an important role in HApeeding [20]. During the growing period and through applying aonstant voltage, the pores diameter and also their density wouldncrease by increasing the concentration of CA from 5 g/L to 10 g/L;his is the way that the mean diameter of the pores of the fabricatedamples would be 280 nm in an electrolyte containing 5 g/L CA and60 nm in the one with 10 g/L CA. The reason for such behavior isorming stronger electrical sparks due to a rise in the concentrationf the electrolyte which will eventually lead to destruction of theoating surface and increasing the mean diameter of the cavities.owever, by increasing the concentration of �-GP from 1 to 5 g/L,

t is surprisingly observed that the nesting phenomenon hasccurred [21]. In other words, multi-sediment layer, each of whichaving porosity, has been produced. Nesting phenomenon occursecause electrical discharges always occur in places having less

hickness, due to lower break down voltage possesses in such posi-ions. On the other hand, it could be interpreted from the nestinghenomenon that super large micro porosities would be produceds a result of joining several adjacent plasma channels. In addition,

Fig. 5. SEM micrographs of the grown layers for 6 min in electrolytes containing: (a) 5

g/L CA and 1 g/L �-GP. (b) 10 g/L CA and 1 g/L �-GP. (c) 5 g/L CA and 5 g/L �-GP.

nesting morphology with high specific area and increasing theprobability of the body liquid to have contact with the implantwould provide a better bio activity characteristic for it.

To obtain more certainty of the result, especially in the elec-trolyte with a concentration of 5 g/L CA and 5 g/L �-GP, the sampleswere also prepared in a time of 6 min. Microscopic pictures of thesesamples are displayed in Fig. 5. These figures are similar to theones for the samples prepared in 3 min. However, the pores densityreduced in the samples coated for 6 min in electrolytes containing1 g/L �-GP and 5 and 10 g/L CA in comparison with samples coatedfor 3 min. The pores density was 28 and 16% for samples producedin 3 min and 10 and 9% for those made in 6 min. Concentratingand repeating of electrical sparks in some specific points couldcause the pores density to reduce by increasing the coating processduration. Pores location in the previously obtained titania layersare preferred for occurring of following sparks. Breaking potentialdecrease is assumed as the responsible for this phenomenon due tothe reduction in the thickness of the surface layers in such points.Breaking potential reduction is a significant factor in anode voltageincrease which is followed by discharges occurred in thinner pointsof the layer easily. Accordingly, the sparks are not scattered all overthe surface of the specimen in contrast with the initial moments ofthe process which this circumstance can be considered as one ofthe most important factors of the pores density reduction.

Furthermore, it should be considered that in the fabricated sam-ples in the electrolyte containing 5 g/L CA and 5 g/L �-GP, nestingamount has dropped by time. This could be result of the localmelting of the layer due to the heat produced by electrical sparks.Intensive electron avalanches in the growing coating are responsi-

ble for temperature rise of the oxide layer until it melts down.

Based on cross sectional microscopic investigation of samplescoated for 3 and 6 min in different electrolytes, the layers thicknesswas defined. Fig. 6 represents the trend of changing the second layer

g/L CA and 1 g/L �-GP. (b) 10 g/L CA and 1 g/L �-GP. (c) 5 g/L CA and 5 g/L �-GP.

S. Abbasi et al. / Applied Surface Science 261 (2012) 37– 42 41

Ff

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Fig. 8. FESEM image of the grown layers for 3 min in electrolytes containing 5 g/LCA and 5 g/L �-GP.

the titanium substrate could lead to better growth of the bone in

ig. 6. Statistical information of top layer (HAp layer) thickness of the coatingormed for 3 and 6 min in different electrolyte concentrations.

hickness, HAp layer. Thickness of the formed HAp layers at theame time but in different electrolytes has been close to each othern the way that it alters between 4.8 and 6.6 �m for the samplesabricated in 3 min and between 8.2 and 10.1 �m for those made in

min. It could be concluded that the thickness of the coatings is aunction of their growing time and is not so much dependant on theoncentration of the employed electrolyte. However, the thicknessf the created layers has been increased by a rise in the electrolyteoncentration. It could be also mentioned that increasing the thick-ess of the produced layers is more obvious with raising the CAoncentration in the electrolyte rather than �-GP.

Microstructure in Fig. 7 confirmed the coatings consisted of twoistinct layers. In this figure, the interface of two titania and HAp

ayers of the selected sample is represented. According to XRDnd XPS results that agreed upon the formation of TiO2-HAp bio-eramic coating, it was assumed that the bottom layer of 2 ± 0.2 �mhick had been a titania layer and the top one had been mainly con-tituted of HAp phase. The cohesion of bottom layer (titania) andhe top layer (HAp) in the coating is noticeable and porosity of HApayer is obviously displayed.

In Fig. 8, the FESEM image of the selected sample shows theAp crystals with high magnification. The coating is made up ofigh amount of spherical seeds of 30–60 nm. According to the idea

hat the formation of HAp is performed in an electrolyte with lowemperature and there is high rate of cooling in this environment,t is assumed that the crystals of the so called material do not have

Fig. 7. SEM-BS cross-sectional and tow layer interface micrograp

Fig. 9. AFM micrographs (three dimensional topography) of layer fabricated for3 min in electrolyte containing 5 g/L CA and 5 g/L �-GP.

the chance for appropriate growth after seeding and nanostructurecrystals (1–100 nm) are formed. Improvement of the cellularcohesion on the surface of HAp nano-crystals has been proved bymeans of in vitro test on fibroblast cell line L929 of a mouse [22].

In addition to the fact that a good chemical cohesion of HAp to

the implant, increasing roughness of the implant surface is consid-ered as a positive feature for bones growing in the implant [23].As a result, roughness of the sample surface was evaluated by AFM

hs of fabricated layer for 3 min in 5 g/L CA and 5 g/L �-GP.

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2 S. Abbasi et al. / Applied Su

echnique. Fig. 9 shows the surface topography of the sample in thecale of 3 �m × 3 �m. The conclusion expresses the formation of aoarse surface with mean roughness (Ra) of 11.40 nm. The grownayer would melt locally by electrons avalanche motions on theurface of anode. It is predictable that after the sparks getting dis-ppeared, the melted regions are frozen around the electrolyte and

rough layer is produced.

. Conclusion

In this survey, in order to provide the excellent bioactivityharacteristic for titanium implants, formation of a two-layer coat-ng containing titania and HAp has been employed through MAO

ethod. The electrolyte composition and coating time were consid-red as the variable parameters of coating process and their effectsn chemical and phase composition, morphology, topography andhickness of the obtained HAp layers were investigated. The resultshowed that the produced coatings contained HAp, titania, calciumitanate and �-TCP phases. They also showed that by an increase inA or �-GP concentration in the consuming electrolyte, HAp peaks

ntensity in XRD pattern would face a rise. This increase is specif-cally affected by increasing �-GP concentration in a way that theroduced coating in the electrolyte containing 5 g/L �-GP and 5 g/LA has mainly consisted of HAp.

According to XPS results, along with Ti and O which are observedn all the coating layers, Ca and P are also visible on the pro-uced coating. On the other hand, investigating morphology of theesulted coatings showed that by increasing the coating time inlectrolytes containing 1 g/L �-GP as well as 5 and 10 g/L CA, poreizes will increase and yet there would be a fall in their number pernit area. It was also proved that employing 5 g/L �-GP and 5 g/LA contained electrolyte would lead to produce a coating with nestorphology and high specific area. Yet, by increasing the coating

ime in these samples the nesting amount would also decrease.Through investigating the variation in the thickness of surface

ayers of HAp, it was apparent that an increase in the coatinguration time from 3 to 6 min likewise a rise in the electrolyteoncentration (specially an increase in CA concentration) led to

hickening the eventual coating, although the effect of growth timen thickness has been more considerable. In all the cases, pro-uced HAp coatings possessed high surface roughness and haveeen formed from spherical nanoparticles.

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Science 261 (2012) 37– 42

Acknowledgements

The authors would like to thank A. Ziaee and H.R. Zargar forhis kind helps. The authors would like to express their sincereappreciations. Meanwhile, financial support of Iran National Sci-ence Foundation (INSF) is highly appreciated.

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