200 (2006) 6350–6354www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Adhesion strength of sol–gel derived fluoridated hydroxyapatite coatings

Sam Zhang a,⁎, Zeng Xianting b, Wang Yongsheng a, Cheng Kui a, Weng Wenjian c

a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075

c Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, 310027, PR China

Available online 13 December 2005

Abstract

Dense and uniform fluoridated hydroxyapatite (FHA) coatings have been deposited on Ti6Al4V substrates by sol–gel dip coating method. X-ray photoelectron spectroscopy and X-ray diffraction analysis results show homogeneous FHA coatings with Ca /P molar ratio between 1.63 and1.70. A scanning scratch tester is used to evaluate the adhesion between the FHA coating and the substrate. The load at which complete removal ofthe coating occurs is taken as an indication of the adhesion strength. With increase of fluorine concentration and firing temperature, adhesionstrength increases; and at the same time, the coating-substrate interfacial failure mode changes from brittle to ductile. Based on the cross-sectionalanalysis, a mechanism is proposed for the increased adhesion.© 2005 Elsevier B.V. All rights reserved.

Keywords: Fluoridated hydroxyapatite coatings; Adhesion strength; Sol–gel; Interface

1. Introduction

Titanium and its alloys have been widely used as implantmaterials in orthopaedic and dental prosthesis for their excellentbiocompatibility, high corrosion resistance, lightweight andgood mechanical properties. However, the bone in-growthproperties and implant fixation behaviour need to be improvedin order to shorten the implant-tissue osseointegration time [1].Hydroxyapatite [Ca10(PO4)6(OH)2 or HA] is found to be thepreferred coating due to its chemical, structural and biologicalsimilarity to human bones [2] and to its direct bondingcapability to surrounding tissues [3]. HA coated titaniumalloy implants integrate the bioactivity of HA and themechanical properties of titanium alloy for a perfect combina-tion. In addition, HA coating provides protection to the titaniumalloy substrates against corrosion in the biological environment,and acts as a barrier against the release of toxic metal ions fromthe substrates into the living body [4].

However, pure HA suffers relatively high dissolution rate insimulated body fluid that affects its long-term stability: highdissolution may lead to disintegration of the coatings and hinder

⁎ Corresponding author. Tel.: +65 6790 4400; fax: +65 6791 1859.E-mail address: msyzhang@ntu.edu.sg (S. Zhang).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.11.033

the fixation of implant to the host tissue [5,6]. Fluorine ion,which exists in human bone and enamel, can be incorporatedinto HA crystal structure by substitution of fluorine ions for OH−

groups to form fluoridated hydroxyapatite (FHA, Ca10(PO4)6(OH)2−xFx, where 0bxb2 is the degree of fluoridation and x=0,pure HA; x=2, pure FA). Incorporation of fluorine into HA, or“fluoridation”, reduces the solubility while maintaining acomparable biocompatibility to that of HA [7]. Many methodshave been developed for processing of HA coatings, whichinclude pulsed laser deposition (PLD) [8], thermal spraying [9],electrophoretic deposition [10], biomimetic [11], sputtering [12]and sol–gel [13], etc. The sol–gel method is used in this studybecause of its advantages such as composition homogeneity, lowcost, ease in operation and doping of ions.

Adhesion strength between the coating and the substrate is acritical factor in successful implantation and long-term stabilityof any coated implant. This paper addresses this issue byevaluating adhesion strength of fluoridated HA and pure HA.The most commonly used methods employed to evaluateadhesion strength of hydroxyapatite on titanium alloys includetensile test (pull-out test) [14,15] and scratch test [16,17]. Giventhe porous microstructure and the high surface roughness ofthese coatings, scratch test deems a better approach because itavoids adverse influences such as glue infiltration which areinevitable in pull-out test [18]. In the present study, scratch

20 25 30 35 40 45 50

•

• •

••

• •••

Substrate

Apatite•

FHA4@700ºC

FHA4@500ºC

FHA6@600ºC

FHA4@600ºC

HA@600ºC

Inte

nsit

y (a

.u.)

Two Theta (degree)

Fig. 1. XRD phase identification of HA and FHA coatings at differenttemperatures.

6351S. Zhang et al. / Surface & Coatings Technology 200 (2006) 6350–6354

adhesion test is employed to study the influence of fluorine onthe adhesion strength of sol–gel derived FHA coatings ontitanium alloy substrates.

2. Experimental

2.1. Deposition of FHA coatings

Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, Sigma-Aldrich, AR), phosphorous pentoxide (P2O5, Merck, GR) andhexafluorophosphoric acid (HPF6, Sigma-Aldrich, GR) wereselected as Ca-precursor, P-precursor and F-precursor, respec-tively. In preparing the solutions, calcium nitrate tetrahydratewas dissolved in absolute ethanol giving rise to a 2 M Ca-containing solution; phosphorous pentoxide was dissolved inabsolute ethanol to form a 2 M P2O5 ethanol solution followedby a refluxing process for 24 h to obtain a clear P-containingsolution. A designed amount of F-precursor was mixed with P-containing ethanol solution to form the “P–F mixture”. Ca-containing ethanol solution was added drop-wise into the P–Fmixture to obtain a Ca /P ratio of 1.67. This mixed solution wasrefluxed for 24 h to obtain the sol. The designed degree ofsubstitution of OH− by F− was indicated by the x value in thegeneral formula of FHA (Ca10(PO4)6Fx(OH)2−x), where x wasselected as 0, 2/6, 4/6, 6/6, 8/6, 10/6 and 12/6. The subsequentcoatings obtained were named HA, FHA1, FHA2, FHA3,FHA4, FHA5, and FHA6, respectively.

Ti6Al4V substrates of 20×30×1.2 mm were polished withsilicon carbide sandpapers (grit range of 180–1000), and thenultrasonically washed in acetone for 15 min, followed bywashing in deionized water for 3 times before dip-coating: thesubstrates were dipped vertically into the sol and withdrawn at aspeed of 3 cm/min; the as-dipped coatings were dried at 150 °Cfor 15 min followed by firing at 500, 600 or 700 °C. Thedeposition run was repeated 4 times for the desired coatingsthickness of 1.5 μm.

2.2. Coating characterization and adhesion test

The phase characterization of the FHA coatings wasconducted by X-ray diffraction analysis (XRD, Philips X′pert1830) using monochromatic CuKα radiation with a step size of0.02°. The fluorine concentrations were determined by X-rayPhotoelectron Spectroscopy (XPS, Kratos-Axis Ultra System)using monochromatic Al Kα X-ray source (1486.7 eV). Thecross-section morphology and elemental analysis of theinterface were characterized using scanning electron microsco-py and Energy Dispersive Spectroscopy (SEM/EDS, JEOLJSM-5600LV).

The adhesion strength between the coating and the substratewas evaluated using a scanning scratch tester (SHIMADZU,SST-101). A spherical Rockwell C diamond stylus of 15 μmradium with a progressive load from 0 to 1000 mN was used.The stylus scanned the coating surface at a speed of 2 μm/swhile maintained scanning amplitude of 50 μm perpendicular tothe scratching direction. The load at which the first damage ismade in the coating is called the “lower critical load”; and the

load at which total peeling-off of the coating from the substrateoccurs is referred to as the “upper critical load”. In hardcoatings, usually the “lower critical load” is used a measure ofthe adhesion strength (in fact, the cohesion strength of thecoating). To evaluate the coating/substrate adhesion, we chooseto use the “upper critical load” as an indication of the adhesionstrength. Ten readings of such critical load values wereaveraged for each sample, and the standard deviation of thesedata was taken as the error. At each composition, two sampleswere tested. The scratch track was observed using SEM.

3. Results and discussion

The XRD patterns of the coatings fired at 500, 600 and700 °C are shown in Fig. 1. All the coatings have similardiffraction profile. The peaks (002), (211), (112), (300) arethose of the HA structure (JCPDS file card #9-432). Notricalcium phosphate (TCP), CaO, CaF2 or other phases aredetected in these coatings, thus high purity coatings areobtained.

The surface chemical compositions and the XPS spectra offluorine (F1s) are analyzed and the results are shown in Table 1and Fig. 2, respectively. With an increase of fluorine in the sols,an increasing intensity of F1s peak is observed, indicating thatmore fluorine ions are incorporated into the coating. All thesamples showed identical results, i.e., firing temperature haslittle influence on the incorporation of fluorine into HA coating,as shown in Fig. 2. Narrow scan analysis reveals only one peakat ∼684.3 eV belonging to F1s. That peak is the fingerprint forfluorine in FHA or FA structures [19]. The Ca /F molar ratioscalculated from the XPS peak area ratio approximately equal tothat of the designed Ca /F values of in the sols. The measuredCa /P ratio is in the range of 1.63∼1.70, close to thestoichiometric value of 1.67. The Ca /F and Ca /P values aretabulated in Table 1.

All the coatings have an identical coating thickness of∼1.5 μm. Fig. 3 shows the interface morphology of thesubstrate with a pure HA coating fired at 600 °C. The cross-

Substrate

Coating

2µµm

Epoxy

Fig. 3. Cross-section of a typical FHA coating.

Table 1Ca /F and Ca/P molar ratio of the coatings (firing at 600 °C)

FHA0 FHA1 FHA2 FHA3 FHA4 FHA5 FHA6

Ca/F \ 30.84 15.33 10.91 8.04 6.64 6.00Ca /P 1.65 1.64 1.66 1.63 1.63 1.70 1.67at.%F 0 0.64 1.36 1.96 2.44 3.07 3.29

6352 S. Zhang et al. / Surface & Coatings Technology 200 (2006) 6350–6354

section morphologies of the coatings show dense and uniformcoating structure, absence of delamination and/or cracks at theinterface.

A typical scratch track of the coating is shown in Fig. 4, andthe corresponding friction response is given in terms of relativevoltage as a function of load. Increasing voltage corresponds toincreasing coefficient of friction. Fig. 5 is that for HA and FHA6fired at 600 °C. At the beginning of the scratch, coefficient offriction increases as load increases due to cohesive failurebecause of the “soft” nature of the coating. Before point 1 (c.f.,Figs. 4 and 5), there are fluctuations as a result of the surfaceroughness. After point 1, the indenter starts to plough into thecoating, resulting in a steeper increase in coefficient of friction.As the load increases to point 2, or 370 mN for pure HA (curvea), the indenter completely peels off the coating and scratchesonto the substrate causing an abrupt increase in friction. Thisload is taken as an indication of the adhesion strength for thecoating represented by curve a and is entered in Fig. 6 as thefirst data point at 600 °C. Curve b records that for FHA6. Afterthe indenter digs into the coating (note the change of the slope inthe curve), there is no abrupt increase in friction till the indenterreaches the substrate at about 470 mN. Comparing curves a andb in Fig. 5, b appears smoother (less fluctuation in frictionbefore the indenter digs in) and the coating adheres to thesubstrate better (slower slope after the indenter digs in, and lackof abrupt increase). The abrupt increase of friction signals asudden and brittle peeling-off of the coating from the substrate.Lack of this abrupt change demonstrates that b has more ductileinterface thus better coating-substrate bonding [20].

800 600 400 200 0

FHA5@700ºC

FHA5@500ºC

Rel

ativ

e In

tens

ity

(a.u

.)

FHA6@600ºC

FHA5@600ºC

FHA2@600ºC

F KLL F1s

Binding energy (eV)

680 685 690 695

T=600ºCF1s

FHA6

FHA5

FHA2

Fig. 2. XPS profile of fired FHA coatings. Inset: narrow scan for F peak forcoatings of different fluorine content fired at 600 °C.

Fig. 6 summarizes the adhesion strength (the “upper criticalload” Lc) of all the FHA coatings as a function of fluorine andfiring temperature. Both fluorine content and firing tempera-tures have significant effect on the adhesion strength: withincreasing fluorine concentration or firing temperature, thereis an obvious increase of the critical load. For the samefluorine content coatings, higher firing temperatures give riseto jumps in adhesion strength. At the same firing temperature,the general trend is that adhesion increases with fluorinecontent. However, the increase is more profound at highertemperatures. At 500 °C, the critical load only increasemarginally with fluorine but at 600 and 700 °C the curves goup rapidly with fluorine content.

Coatings adhere to substrates through either mechanicalinterlocking or chemical bonding or both. In current study,since all substrates have the same finishing, mechanicalinterlocking is deemed identical, thus the increase in adhesionstrength is attributed to stronger chemical bonds developed atthe coating-substrate interface during firing. Fig. 7 shows anEDS cross-sectional elemental distribution of Ca, O, F and Pin FHA6 fired at 600 °C. Three regions are marked: thesubstrate (Rs), the coating (Rc) and in between is thetransition (Rt). Within the transitional region, the Ca and Pconcentration decrease significantly towards the substrate,while the F and O decreases slowly and gradually from thecoating to the substrate. In other words, there exists relativelylarge amount of F and O in the transition region. It has beenshown that along the cross section, certain Ti–P–Ca–O–Fchemical bonds form at the interface [15,16,21,22] that could

Fig. 4. Scratch track of an HA coating.

0 100 200 300 400 500 600

0

20

40

60

80

100

22

11

b

Rel

ativ

e V

olta

ge o

utpu

t (%

)

Load (mN)

Upper critical load

Peeling point

Tip radius: 15µmScratch Speed: 2µm/sScanning Amplitude: 50µm

a

Fig. 5. Coefficient of friction in terms of relative voltage as a function of normalload while scratching (a) pure HA coating; (b) fluoridated HA (FHA6) coatingon Ti6Al4V substrate.

F

O

Ca

P

FHA6

RcRtRs

Cross section

Inte

nsit

y (a

.u.)

Fig. 7. EDS element analysis of cross-section of FHA6 fired at 600 °C.

6353S. Zhang et al. / Surface & Coatings Technology 200 (2006) 6350–6354

contribute to the increase in adhesion. Formation of chemicalbonds at the interface may be also responsible for the changein failure mode (from brittle to more ductile).

The formation of the chemical bonding may be explained asfollows: First, chemi- and physisorption process during dippingand drying. Since native oxide (TiOy, y≤2) forms spontane-ously on titanium and titanium alloy surface upon exposure toair, hydroxide ions and water molecules are adsorbed by Tications that leads to formation of Ti–OH bonds on the outmostsurface [23]. XPS depth profiling confirmed a 5–8 nm titaniumoxide layer as soon as the Ti6Al4V substrates were prepared. Indipping sols, fluorine ions released by F-precursor react withCa2+ to form nano-crystalline CaF2 (nc-CaF2) [24]. The F in nc-CaF2 easily forms hydrogen bonding with H in OH group, i.e.O–H···F–Ca–F, etc. [21]. Therefore, when the Ti6Al4Vsubstrate is immersed into the dipping sol, nc-CaF2 could beeasily adsorbed onto the substrate surface through formation ofhydrogen bond. The higher the fluorine concentration in the

0.0 0.5 1.0 1.5 2.0300

350

400

450

500

550

600

500ºC

600ºC

700ºC

Adh

esio

n st

reng

th (

Lc,

mN

)

Degree of Fluoridation: x as in Ca10(PO4)6Fx(OH)2-x

Fig. 6. Adhesion strength of pure HA and fluoridated HA coatings on Ti6Al4Vsubstrates as indicated by upper critical load in scratch test. Firing temperaturesare indicated.

dipping sol, the more nc-CaF2 forms and adsorbs on thesubstrate surface. It has been reported that the adsorbed nc-CaF2on the titanium alloy substrate surface attract more O near to theinterface [21]. On the other hand, due to the amphotericproperty of Ti–OH, the substrate surface will be positivelycharged in current dipping sol (pHb1). Thus surface adsorptionof electronegative groups, such as NO3

−, P-containing groups,etc. will follow. Previous studies by Filiaggi et al. [25] haveindicated that the interactions at the interface of the substratesand the dipping sol have great influence on the ultimateadhesion property.

Following the adsorption, an important step is the diffusionduring firing when the surface-adsorbed nc-CaF2, P-containinggroups, etc. react with titanium to form certain chemical bondsif the temperature is high enough to activate the formationprocess. Diffusion of fluorine and oxygen from the coating intothe transition region encourages this reaction and results inaccumulation of O and F in the region (Fig. 7). Since higherfluorine content in the dipping sol attracts more O near to theinterface [21], firing in open atmosphere provides inexhaustibleoxygen for the process. As a result, complex Ti–P–O–F–Cabonds may be formed in the transient region that helps improvethe adhesion strength. Higher firing temperature obviouslypromotes diffusion of fluorine and oxygen as well as providingactivation for the chemical bonding process thus leading tohigher adhesion strength.

Residual stress at the interface resulted from the depositionprocess may exert adverse effect on adhesion strength. Thedifference in thermal expansion coefficient for the coating andsubstrate results in thermal mismatch that in turn contributesnegatively to adhesion strength via increased residual stress[26]. Incorporation of fluorine ion into apatite structuredecreases the coefficient of thermal expansion (CTE) from15×10− 6/°C to 9.1×10− 6/°C when HA is changed tofluorapatite (FA), which is much closer to that of the Ti6Al4Vsubstrate (8.9×10−6/°C): the difference in CTE between HAand Ti-alloy is 68.5% while between FA and Ti-alloy it is only2.2%! As a result, incorporation of fluorine in HA reduces thethermal mismatch and thus residual stress and consequentlyimproves the adhesion strength. The reduction in residual stressalso contributes to better ductility [19].

6354 S. Zhang et al. / Surface & Coatings Technology 200 (2006) 6350–6354

4. Conclusion

Dense crack-free FHA coatings (∼1.5 μm) have beendeposited through sol–gel dip coating on Ti6Al4V substrates.The Ca /P ratios of the coatings are nearly equal to that ofstoichiometric value (1.67). Scratch test reveals that the coatingadheres to Ti-alloy substrate up to 35% better as the fluorineconcentration increases in the coating. This adhesion enhance-ment is much more profound at higher firing temperature. Theincrease in adhesion likely comes from the formation ofchemical bonding at the interface and the relief of thermalmismatch due to incorporation of F into the HA structure. As aresult, the coating and substrate interface becomes more ductile.

Acknowledgements

This work is supported by the Agency for ScienceTechnology and Research, Singapore (A*Star) through project032101 0005 and SIMTech-NTU collaboration project U03-S-389B.

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