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Thin Solid Films 516 (2

Evaluation of adhesion strength and toughnessof fluoridated hydroxyapatite coatings

S. Zhang a,⁎, Y.S. Wang a, X.T. Zeng b, K.A. Khor a, Wenjian Weng c, D.E. Sun a

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

Available online 13 July 2007

Abstract

Adhesion strength and fracture toughness are two crucial mechanical properties for bioceramic coatings on metal implants directly affectingsuccessful implantation and long-term stability. In this study, the adhesion strength of sol–gel derived FHA coatings on Ti6Al4V substrates wasmeasured by pull-out tensile test, and the toughness was assessed by energy release method. With increase of the degree of fluoridation, theadhesion strength increases up to about 40% and the fracture toughness increases about 200 to 300%. Contrary to the wide-spread belief, it isinteresting to note that after soaking in the Tris-buffered physiological saline solution (for 21 days), the adhesion strength increases about 60% ascompared with the as-deposited coating, instead of decreasing. The mechanism of the increase is discussed.© 2007 Elsevier B.V. All rights reserved.

Keywords: Adhesion strength; Toughness; Fluoridated hydroxyapatite coating; Nanoindentation

1. Introduction

Hydroxyapatite (HA)-coated metallic implant has long beenrecognized as the preferred hard tissue replacement/repair, especiallyas load-bearing implants in orthopaedics and dentistry [1,2]. Thecombination possesses the biological properties of HA and theexcellent mechanical properties of metallic substrate thus delivers areliable implant for patients. The HA coating also serves asprotection against corrosion of the otherwise baremetal in biologicalenvironment [3]. It has been reported that the survival rate wasinitially high forHA-coated implants; however, the high degradationrate of HA coating in biological environment is a serious stabilityconcern, which could cause detrimental effect on adhesionproperties, resulting in undesirable debris and even delamination,which eventually leads to the failure of the implant [4–6].

Recently, fluoridated hydroxyapatite (FHA, Ca10(PO4)6(OH)2−xFx) has attracted much attention in replacement ofpure HA coating on metallic implants because it demonstratessignificant resistance to biodegradation while maintains com-parable biocompatibility [7,8]. Fluorine is an essential element

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

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.07.063

for the development of human bones and teeth as well as theprevention of dental caries [9,10]. Documented results indicatethat partial substitution of fluoride ions for OH− groups reducesthe solubility product of HA by ∼3.5 orders of magnitude [7].Presence of fluoride ions also enhances the proliferation anddifferentiation of osteoblastic cells and promotes bone regen-eration [11,12]. Coating of HA and FHA can be realized viathermal spraying [13], magnetron sputtering [14], pulsed laserdeposition [15] and sol–gel dip coating, [16] etc. In comparison,sol–gel method has the advantages of composition homogene-ity, low cost, ease in operation and doping of ions thus is usedwidely, which is also the choice of method in this study.

Long-term stability of the bioactive ceramic-coated implantscomes from at least two critical aspects: low solubility of thecoating and high adhesion strength between coating andsubstrate [17]. Solubility of HA can be reduced by incorpora-tion of fluoride ions into HA lattice structure. But very fewreports are available on adhesion improvements, not to mentionadhesion studies after in vitro dissolution test. The present studyconcentrates on the adhesion strength of sol–gel derivedfluoridated hydroxyapatite coatings on Ti6Al4V before andafter dissolution tests. Along with the adhesion measurement,coating toughness is also evaluated.

Fig. 2. Pull-out adhesion strength of FHA coating before and after soaking in TPSsolutions. ⁎ indicates a significant increase of adhesion strength with respect to F0(as prepared coatings); ⁎⁎ indicate a significant increase of adhesion strength withrespect to F0 (after soaking in TPS for 21 days).

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2. Experimental

2.1. Coating preparation and characterization

The preparation of dipping-sols and deposition of FHAcoatings were described in details in our previous work [16,18].In brief, the selected precursors, i.e. calcium nitrate tetrahydrate(Ca(NO3)2·4H2O, Sigma-Aldrich, AR), phosphorous pentoxide(P2O5, Merk, GR) and hexafluorophosphoric acid (HPF6,Sigma-Aldrich, GR) were dissolved in absolute ethanolrespectively for preparation of the dipping-sols. The designeddegree of substitution of OH− by F− was indicated by the xvalue in the general formula of FHA, (Ca10(PO4)6(OH)2−xFx),where x was selected as 0, 1 and 2, the subsequent coatingsobtained were labeled as F0, F1 and F2 respectively. Titaniumalloy (Ti6Al4V) slab of 20×30×1.2 mm polished to grade#1200 of SiC sandpaper were used as substrate. The dipping runwas repeated 4 times for a final coating thickness of ∼1.5 μm.

In order to investigate the influence of dissolution behavior onthe adhesion strength, in vitro dissolution tests were carried outby soaking FHA coatings in a Tris-buffered physiological salinesolution (TPS) (0.9%NaCl, pH7.4) at a constant temperature of37 °C for 3 weeks. After that, the samples were taken outfollowed by washing with DI water for 3 times, and then wereprepared for tensile tests as described in Section 2.2.

Surface roughness (Rq) of FHA coatings before and aftersoaking in TPS solutionwas determinedwith a non-contact opticalprofiler (WykoNT2000,Veecco Instruments Inc. USA).Results ofX-ray diffraction analysis, coating surface morphology as well ascomposition analysis were reported in our previous work [18,19].

2.2. Pull-out tensile test

The adhesion strength of the FHA coating on the metallicsubstrate was measured using a Universal Instron mechanicaltesting system (Instron 5569). A clamping fixture was designedto avoid misalignment during the uniaxial tensile test: An eight-

Fig. 1. Schematic illustration of the pull-out tensile test for the evaluation ofadhesion strength.

millimeter diameter Al rod was glued onto the coating surfacewith epoxy resin (Epoxy Adhesives DP460, 3M, Scotch-Weld™, USA) and cured at room temperature for 24 hours; therod-sample combination then slide into a steel holder such thatthe rod stuck out of the top opening for clamping onto theInstron (Fig. 1). In this way, the lower top surface of the hollowholder provided intimate contact with the sample surface whichtransmitted the downward pulling force via a fixture attached tothe lower housing of the holder. In testing, the rod was pulled ata cross-head speed of 1mm/min until the coating failure. TheSEM was used to evaluate the failure mode at the fracturesurface. A one-way ANOVA test was conducted to assess thestatistical significance of the adhesion and toughness results.

2.3. Toughness measurement

The toughness measurement of ceramic films and coatings isstill an unresolved issue since there is no international standardor test procedure so far [20]. Since the nanoindentation-basedenergy release method looks more convincing, we adopt theenergy method in this study. In the energy method, the energydifference is examined before and after the crack formation andpropagation. This energy difference is considered responsiblefor the through-thickness cracking in the coating. The energyrelease is obtained from a “step” that is observed in the load-displacement curve during the indentation, thus the toughnessof the coating is determined via [21,22]:

KIC ¼ DUt

dE

2pCRdð1� m2cÞ� �

ð1Þ

Where νc is the poisson's ration of the coating, 2πCR is thecrack length in the coating plane, t and E are the coatingthickness and elastic modulus respectively, ΔU is the strainenergy difference before and after cracking.

Nanoindentation was carried out using a NANO Indenter XPsystem (MTS Nano Instruments, USA) with a Berkovich

Fig. 3. Typical adhesion failure surface of the FHA coatings without TPS soaking.

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indenter. This instrument monitors and records the dynamicload and displacement of the indenter during indentationprocess. Indentations and associated cracks were observedusing SEM, and the length of cracks of the indentation wasanalyzed using Image-Pro Plus image measurement software.

3. Results and discussion

3.1. Adhesion strength of the as-prepared FHA coatings

The measured nominal adhesion strength between thecoating and the Ti6Al4V substrate is shown in Fig. 2. By“nominal” we do not distinguish “adhesion failure” and“cohesion failure”. Without fluoridation (sample F0), theadhesion strength is about 19 MPa. With incorporation offluoride ions (F1 and F2), the adhesion strength increasessignificantly (pb0.05) to about 26–27 MPa. There is nosignificant difference between F1 and F2 (pN0.05).

The adhesion strength between the coating and substratecomes from two aspects, the mechanical interlocking and thechemical bonding. In current work, since the substrates had thesame finish, the mechanical interlocking can be consideredidentical. Therefore, the increase in the adhesion strength isattributed to the stronger chemical bonds, which were

developed at the coating-substrate interface during coatingdeposition process, especially at the firing stage. Our previousstudies obtained by scratch testing show that increasing fluoridecontent accompanies increased scratch adhesion [18]. Mean-while, formation of chemical bonds at the interfacial areapromises higher shear strength [23]. We proposed a tow-stepprocess to explain the formation of chemical bonds at theinterface [18]. According to this model, the presence of fluorineions not only modify the chemisorption and physisorptionproperties of interface during the dipping and drying process,but also attract more oxygen near to the interface to formcomplex compound, e.g. Ti–O–Ca–P–F etc., during the firingprocess. Incorporation of fluorine ions reduces the residualstress (tensile) as much as 50%, thus will also contribute tohigher adhesion strength [23]. Weng et al. [24] also reportedhigher adhesion strength (20.6 MPa) of FHA coating ascompared to pure HA coating (15 MPa). Lee et al.'s [25]results confirmed that the adhesion strength improved from∼20 MPa to ∼40 MPa when HA was fluoridated.

As fluoridation is up to the extent of x=1 (sample F1),further increase in fluoridation has no significant effect onadhesion (Fig. 2). A typical adhesion failure topography of thecoatings is shown in Fig. 3. There are 3 different failure modes: 1)the adhesion failure between the coating and substrate; 2) the

Fig. 4. Typical adhesion failure topography of the FHA coatings after soaking in TPS for 21 days.

Fig. 5. Schematic illustration of the contact between the coating and the epoxyresin.

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cohesion failure which happens inside the coating and 3) the“adhesive failure” that happens between the epoxy and thecoating. Adhesion strength measurements the how strong thecoating bonds with the substrate; cohesion strengthmeasures howstrong the coating itself holds together; the “adhesive failure”signals poor bonding between the epoxy and the top (i.e., the outermost) coating surface. A mixed failure is commonly observed,consisting of all the three failures. From Fig. 3, the total areafraction of adhesion and cohesion failure is only about 20%–30%,the rest being “adhesive failure” (due to insufficient bonding oradhesiveness between the coating surface and the epoxy). Assuch, the adhesion strength should be a lot higher than 19MPa forHA and 27 MPa for FHA, thus the adhesion strength of the FHAcoatings are a lot more than the minimum 15MPa using the pull-out tensile test as stipulated by ISO standards (ISO 13779) [26] forbiomedical applications. As reference, the adhesion strength ofplasma sprayed HA coatings are usually in the range of ∼20–30 MPa [27], and that of sol–gel method is generally lower than30 MPa [8].

3.2. Adhesion strength of FHA coatings after soaking in TPS

Fig. 2 also plots the adhesion strength of the coating afterimmersion in TPS for 21 days. Surprisingly, all the coatings

show significant increase in adhesion strength.of 55%, 65% and66% for F0, F1 and F2 respectively. Though the smallimprovement (∼43 MPa to ∼46 MPa) from differentconcentrations of F is not believed to be real (pN0.05), theimprovement from HA to FHA is statistically significant(pb0.05). This is contrary to the published results. Usually itis believed that soaking in simulated body fluid results indegradation of adhesion. The reasons of our improvement willbe discussed later.

Fig. 4 depicts typical adhesion failure topography of FHAcoatings after soaking in TPS for 21 days before the pull-out test

Fig. 7. Calculated fracture toughness of FHA coatings in relation to the degree offluoridation: ⁎ indicates a significant increase of fracture toughness with respectto F0.

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is conducted. Similar to that without soaking, a mixed failure isobserved. However, the area fraction of the adhesive failuredrastically drops to 30–40% (from 70–80% before soaking). Inother words, the total area fraction of adhesion and cohesionfailure increased from around 25% to around 65%. Thisincrease contributes to the increase in the overall adhesionstrength (c.f., Fig. 2, marked as TPS) to a value of around45 MPa after soaking. The increase in area fraction of totaladhesion and cohesion failures after TPS soaking attributes tothe increase of surface roughness as a result of soaking: Thesurface roughness (Rq) was around 267±7, 293±9 and 315±15 nm for as-prepared F0, F1 and F2 respectively; afterimmersion in TPS for 21 days, these values increased to 358±10, 357±12 and 370±10 nm (or an increase of 34%, 22% and17% respectively). Rougher surface provides more contact areafor epoxy-coating interface, as illustrated in Fig. 5. Since the“adhesive failure” is a lot less after soaking, the “after soaking”adhesion results should be more representative of the realadhesion strength (still, the real adhesion strength should behigher than 43 MPa for our FHA coatings because the test stillhas some “adhesive failure”).

In the case of thermal sprayed HA coatings where “aftersoaking” adhesion always tend to decrease (up to 75%). Forinstance, the strength dropped from 27 MPa as tested beforesoaking to 19 MPa after soaking in SBF for 2 weeks [28]. Thereason behind the drop was the existence of coating cracks

Fig. 6. Typical SEMmicrograph of indentations (a) and load-displacement curveof indentation (b) on coating surface.

intrinsic to thermal spray deposited HA coatings. The impurityphases (e.g. CaO, TCP etc.) may serve as the crack initiationsource, and the transversal cracks across the coating thickness[13] will serve as the channels that lead the solution into thebulk of the coating and the coating/substrate interface. Chemicaldissolution of the coatings inside the coating and at the interfaceweakens the cohesion in the coating and the adhesion at theinterface, giving rise to decrease in overall adhesion strengthafter SBF soaking. In contrast, our coatings are completelydense, no surface cracks or through coating cracks exist (seeRef. [18] which has the surface and cross-section of the coatingsused here), sipping of the solution into the coating or into theinterface areas is not possible. However, chemical dissolutionthat takes place on the surface results in rougher surface, whicheffectively aids the epoxy/coating bonding which, in turn,helped reducing the area fraction of the “adhesive failure”.

3.3. Toughness of FHA coatings

A typical SEM micrograph and the corresponding indenta-tion curve of a coating are shown in Fig. 6. The coating aroundthe indenter bulges upwards (Fig. 6a), indicating delaminationand buckling of the coating. In the load-displacement curve, astep is (Fig. 6b) caused by energy release corresponding to thecoating delamination and buckling during nanoindentationprocess. Based on equation (1), the toughness of the coatingsare calculated and summarized in Fig. 7. Obviously, theincorporation of fluorine has significant effect on the coatingfracture toughness: ∼0.12 MPam1/2 of HA increases to0.26 MPam1/2 for F1 and to 0.31 MPam1/2 for F2. Thoughthe difference between F1 and F2 is insignificant (pN0.1), theincrease from “without” to “with” fluorine is more than doubledand statistically significant (pb0.01).

The reasons of increase in toughness are mainly attributed tothe following aspects caused by the incorporation of fluorideions. Firstly, the incorporation of fluoride ions causes a higherelastic modulus (E) (c.f. ""Eq. (1)). The elastic moduli weremeasured as about 47, 54 and 74 GPa for F0, F1 and F2

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respectively [23]. Similar trend is also reported regarding theenhancing effect of incorporated fluoride ions on Young'smodulus of FHA in Ref. [29]. Secondly, high adhesion strengthalso benefits the fracture toughness. As discussed above,fluoride ions incorporated into HA lattice structure give rise tohigher adhesion strength. From the scratch test results, thecoating-substrate interface becomes more ductile with fluori-dation [18]. As such, the crack propagation is more restricted,resulting in a smaller crack length (2πCR). Finally, residualstress also plays a role in toughness. The presence of tensilestress favors crack opening in indentation [30]. Therefore,reduction of tensile stress reduces crack-sensitivity, thusimproves toughness. The decrease in tensile residual stresswas indeed observed with increase of fluoridation degree [23].Meanwhile, the reduction of residual stress also benefits theadhesion strength, which also indirectly contributes to thecoating toughness.

4. Conclusions

The nominal adhesion strength of sol–gel derived fluoridat-ed hydroxyapatite (Ca10(PO4)6(OH)2−xFx) coatings on Ti6Al4Vsubstrates is measured by pull-out tensile test. The strengthranges from ∼19 MPa for pure hydroxyapatite (x=0) to∼26 MPa for x=1. After soaking in Tris-buffered physiologicalsaline solution for 21 days, the adhesion strength increases to∼30 MPa in the case of pure HA and to over 40 MPa in the caseof FHA. With incorporation of fluorine (x=1), the toughness ofthe coating doubles as compared with HA. Both adhesion andtoughness do not have statistically significant improvement asthe fluoridation increases from x=1 to x=2.

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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