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Applied Surface Science 256 (2010) 4677–4681

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

xcellent stability of plasma-sprayed bioactive Ca3ZrSi2O9

eramic coating on Ti–6Al–4V

ing Lianga,b,∗, Youtao Xiea,b, Heng Ji a,b, Liping Huanga,b, Xuebin Zhenga,b,1

Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR ChinaShanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China

r t i c l e i n f o

rticle history:eceived 22 December 2009eceived in revised form 19 February 2010ccepted 22 February 2010vailable online 1 March 2010

a b s t r a c t

In this work, novel zirconium incorporated Ca–Si based ceramic powder Ca3ZrSi2O9 was synthesized. Theaim of this study was to fabricate Ca3ZrSi2O9 coating onto Ti–6Al–4V substrate using atmospheric plasma-spraying technology and to evaluate its potential applications in the fields of orthopedics and dentistry.The phase composition, surface morphologies of the coating were examined by XRD and SEM, which

eywords:lasma sprayinga3ZrSi2O9

hemical stabilitypatite formation

revealed that the Ca3ZrSi2O9 coating was composed of grains around 100 nm and amorphous phases.The bonding strength between the coating and the substrate was 28 ± 4 MPa, which is higher than thatof traditional HA coating. The dissolution rate of the coating was assessed by monitoring the ions releaseand mass loss after immersion in the Tris–HCl buffer solution. The in vitro bioactivity of the coating wasdetermined by observing the formation of apatite on its surface in simulated body fluids. It was found thatthe Ca3ZrSi2O9 coating possessed both excellent chemical stability and good apatite-formation ability,

se as

suggesting its potential u

. Introduction

With the aging of population and the growth of accidents,emands for artificial materials that can replace diseased or lostones grow rapidly in the past 20 years and are anticipated to

ncrease over the next two decades [1]. Ti–6Al–4V is consideredo be one of the best metallic materials for orthopedic implants.urface coatings produced by plasma spraying on Ti–6Al–4V haveeen brought into effect on account of their potential to enhancesseoconduction and osseointegration [2,3].

Plasma-sprayed titanium and hydroxyapatite (HA) coatings oni–6Al–4V substrates have been in fact used in clinical applica-ions. However, both of them have drawbacks. Titanium coatingannot chemically bond to living bone as bioactive ceramics can,ut through a morphological fixation at the bone–implant interface

hich requires a long immobilization time and may cause mechan-

cal loosening [4]. HA tends to decompose during plasma-sprayingrocess, which reduces the physiological stability of HA coating.urthermore, the poor bonding strength between HA coating and

∗ Corresponding author at: Key Laboratory of Inorganic Coating Materials, Chinesecademy of Sciences, 1295 Dingxi Road, Shanghai 200050,R China. Tel.: +86 21 52411050; fax: +86 21 52414104.

E-mail addresses: yliang@student.sic.ac.cn (Y. Liang),bzheng@mail.sic.ac.cn (X. Zheng).1 Tel.: +86 21 52414104; fax: +86 21 52414104.

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.02.071

bone implants.© 2010 Elsevier B.V. All rights reserved.

substrates often results in failure in long-term conditions [5–7].Given the above situations, synthetic implants still have limita-tions in clinical practice. As many as half of hip joint replacementsloose after 10 years implantation, and over 10% of products needrevision operations. Consequently, there is a need to develop newcoated implant materials with both good bioactivity and long-termfunctional stability.

In the past decade, calcium silicate (Ca–Si) based ceramics havereceived great attentions as materials for bone tissue regenerationdue to their excellent bioactivity [8,9]. Plasma-sprayed calcium sil-icate coatings such as CaSiO3 and Ca2SiO4 show not only goodbioactivity but also enhanced bonding strength with Ti–6Al–4Vas compared with HA coating [10]. However, high dissolutionrate of the coatings makes their long-term stability still ques-tionable [11,12]. Recently, several metal ions such as Ti and Znwere incorporated into Ca–Si system and formed sphene (CaTiSiO5)and hardystonite (Ca2ZnSi2O7) with improved chemical stability inphysiological environment [13,14].

In this study, we have incorporated Zr into Ca–Si systemand synthesized Ca3ZrSi2O9 ceramic powder. Ca3ZrSi2O9 is a cal-cium zirconium silicate mineral originally discovered in Iraq andwas called baghdadite. The excellent cytocompatibility of bagh-

dadite ceramic disks have been demonstrated lately [15]. Yetthere are no available reports regarding the potential use ofbaghdadite coating on metal substrates as bone implants. Thebaghdadite powder we prepared was used as a plasma-sprayingfeedstock. The aim of the present work was to investigate the

4678 Y. Liang et al. / Applied Surface Science 256 (2010) 4677–4681

Table 1Plasma-spraying parameters.

Argon plasma gas flow rate (slpm) 40Hydrogen plasma gas flow rate (slpm) 10Spray distance (mm) 100Argon powder carrier gas (slpm) 3.5Current (A) 650Voltage (V) 68

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during plasma-spraying process often result in the formation of

Powder feed rate (g/min) 25

lpm: standard liter per minute.

ioactivity and long-term stability of the Ca3ZrSi2O9 coating oni–6Al–4V.

. Experimental details

.1. Powder preparation and plasma spraying

Ca3ZrSi2O9 powder was prepared by high-temperature solid-tate reaction using CaCO3 (A.R., SCRC, China), SiO2 (A.R., SCRC,hina) and ZrO2 (99.99% purity, Hangzhou Veking, China) asaw materials. The mixture of raw materials (molar ratio:aCO3/SiO2/ZrO2 = 3:2:1) was calcined at 1400 ◦C for 6 h. Thebtained Ca3ZrSi2O9 powder was sieved to 200 mesh and then theowder was sprayed onto Ti–6Al–4V substrates with dimensions of0 mm × 10 mm × 2 mm. An atmospheric plasma-spraying systemF4-MB, Sulzer Metco, Switzerland) was applied and the detailedarameters are listed in Table 1. The thickness of the coating wasround 120 �m.

.2. Characterization and bonding strength

The crystalline phases of the obtained Ca3ZrSi2O9 powder andoating were analyzed by X-ray diffraction (XRD, D/max 2500 V,igaku, Japan), using Cu K� radiation at 40 kV and 100 mA. The sur-

ace morphology of the coating was investigated by field emissioncanning electron microscope (FE-SEM, JSM-6700F, JEOL, Japan),nd average surface roughness (Ra) was measured using surfaceest apparatus (T8000, Hommel werke, Germany). The cross-ection of the coating was polished and ground before analyzingy electron probe micro-analyzer (EPMA, JXA-8100, JEOL, Japan).

The bonding strength between the Ca3ZrSi2O9 coating andi–6Al–4V substrate was assessed according to ASTM C633-01. Onei–6Al–4V rod (˚25.4 mm × 25.4 mm) was coated with Ca3ZrSi2O9approximately 380 �m) on the surface and the other identical oneithout. A layer of adhesive glue (E-7, Shanghai institute of syn-

hetic resin, China) was applied to join the two rods. After drying 3 ht 100 ◦C, the bonding strength was measured using a mechanicalester (Instron-5592, SATEC, USA) and the average of five measure-

ents was taken as the bonding strength.

.3. Chemical stability of coating

For evaluating the chemical stability of the Ca3ZrSi2O9 coat-ng, the specimens were immersed in 50 mL Tris–HCl bufferolution. The buffer solution was prepared by dissolving 50 mMris-hydroxymethyl-aminomethane ((CH2OH)3CNH2) in deionizedater and then buffered at pH 7.40 with hydrochloric acid (HCl)

t 36.5 ◦C. After soaking for 14 days, the mass loss of the coatingas measured. The Ca, Si and Zr ion concentrations in Tris–HCl

uffer solution were determined using inductively coupled plasmatomic emission spectroscopy (ICP-AES, Vista AX, Varian, USA). Foromparison with pure calcium silicate coating, Ca2SiO4 coating fab-icated in the same condition and spraying parameters was tested.

Fig. 1. XRD patterns of the synthesized Ca3ZrSi2O9 powder and coating.

2.4. In vitro bioactivity of coating in simulated body fluids (SBF)

Simulated body fluids (SBF) is composed of 142.0 mM Na+,5.0 mM K+, 1.5 mM Mg2+, 2.5 mM Ca2+, 147.8 mM Cl−, 4.2 mMHCO3

−, 1.0 mM HPO42−, and 0.5 mM SO4

2−, which has similar ionconcentrations to human blood plasma. The bone-bonding abilityof a material is often evaluated by examining the ability of apatite-formation on its surface in SBF [16]. The coating was soaked in SBF at36.5 ◦C for two weeks. The soaked coating was dried and observedby SEM. After immersion for 28 days, the surface and cross-sectionof the coating were characterized using XRD and EPMA. The com-position of the deposits on the coating was tested using energydispersive spectrometer (EDS) attached to EPMA.

3. Results and discussion

3.1. Characterization of the powder and coating

The phase composition of the Ca3ZrSi2O9 powder and the coat-ing are shown in Fig. 1. XRD analysis shows that the Ca3ZrSi2O9pattern was in agreement with the data in the JCPDS file (standardcards no.: 39-195). The plasma-sprayed coating exhibited a muchlower crystallinity than that of the powder. During the plasma-spraying process, ceramic powders were heated up to fusion rapidlyand re-solidified onto the surface of the substrates at a high coolingrate of 106–107 K/s [17,18], most of the molten particles could notrecrystallize, and therefore the obvious diffraction peaks broaden-ing with low intensities can be often observed in the patterns ofplasma-sprayed coatings. Moreover, Ca3ZrSi2O9 has a complicatedcrystal structure of silicate, which takes longer time for each atomto come back to its original lattice position. For this reason, theamorphous phases increased during the plasma-spraying process.

The morphology of the plasma-sprayed Ca3ZrSi2O9 coatingis shown in Fig. 2. The coating was mainly composed of fullymelted and partially melted particles with a rough and unevensurface (Fig. 2(a)). The surface roughness (Ra) of the coating was9.844 ± 1.215 �m. Grains with around 100 nm in size and amor-phous phases made up of the microstructure of the coating surface,as presented in Fig. 2(b). As mentioned above, rapid cooling rates

amorphous phases. Meanwhile, high cooling rates give rise to highnucleation rates and the plasma-sprayed coating prepared fromconventional micrometer-sized feedstock can also present nano-structure [19]. The small size effect of the microstructure of the

Y. Liang et al. / Applied Surface Science 256 (2010) 4677–4681 4679

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Table 2Bonding strength of plasma-sprayed Ca3ZrSi2O9 coating and HA coating.

Coating Bonding strength (MPa) Researchers

Ca3ZrSi2O9 28 ± 4HA 5.97 ± 0.78 Tsui et al. [6]

ig. 2. Surface micrograph of the Ca3ZrSi2O9 coating (a), and its higher magnificationb).

oating also induced the peaks broadening of the XRD pattern. Theross-section of the as-sprayed coating is displayed in Fig. 3. It man-fests the typical lamella structure of plasma-sprayed coatings, withome micropores and microcracks existed in the coating.

.2. Bonding strength

As an implant coating material, sufficient bonding strength playsn important role in maintaining the long-term stability of the

aterial. The currently used HA coating possesses very good bio-

ogical properties, but the main drawback of HA coating is its lowonding strength, which causes delamination and deteriorationf the coating in long-term use. The bonding strength betweenhe plasma-sprayed Ca3ZrSi2O9 coating and Ti–6Al–4V substrate

ig. 3. Cross-section microstructure of the plasma-sprayed Ca3ZrSi2O9 coating.

HA/Ti–6Al–4V 8.0 Khor et al. [20]HA 13 Zheng et al. [21]HA (spheroidized powder) 16.6 Khor et al. [22]

was 28 ± 4 MPa, which was much higher than that of the plasma-sprayed HA coatings reported in previous studies, as listed inTable 2.

3.3. Chemical stability of the coating in Tris–HCl buffer solution

ICP-AES analysis demonstrates a much lower Ca and Si ionsrelease of the Ca3ZrSi2O9 coating than that of the Ca2SiO4 coat-ing in Tris–HCl buffer solution, as shown in Fig. 4(a) and (b). No Zrion was detected in the Tris–HCl solution for the Ca3ZrSi2O9 coat-ing. Also, it can be seen from Fig. 4(c) that the mass loss of theCa3ZrSi2O9 coating was much lower as compared with the Ca2SiO4coating. The Ca and Si ions release of the Ca2SiO4 coating were ashigh as 7.71 and 2.33 mM after soaking for 14 days, and the curveincreased fast at the beginning of immersion. For the Ca3ZrSi2O9coating, the concentration of Ca and Si ions in the buffer solutionincreased gradually in a small range, and got to 0.821 and 0.068 mMafter two weeks immersion.

The chemical stability of the plasma-sprayed coatings is anothercrucial factor affecting the long-term stability of the implants. Thehigh dissolution rate of pure calcium silicates limits their applica-tions as coated implants. In this study, plasma-sprayed Ca3ZrSi2O9coating exhibited an obviously improved chemical stability ascompared with Ca2SiO4 coating and CaSiO3 coating studied inour previous work [23], which might be related to the impact ofzirconium addition on the crystal structure. Ca3ZrSi2O9 ceramicbelongs to sorosilicates, which have two silicate tetrahedrons thatare linked by one oxygen ion and thus the basic chemical unit is[Si2O7]6−. Zr4+ is in [6] coordination and it is speculated that ZrO6octahedral and Si2O7 group can form a more stable network thationically binds Ca ions [24], which may improve the chemical sta-bility of the calcium silicate-based ceramics. It can be summarizedfrom literatures that hardystonite, sphene and baghdadite in thisstudy show better chemical stability altogether than that of purecalcium silicates. However, the incorporation of varied metal ionsinto calcium silicate always results in complicated structure of dif-ferent silicates and therefore different bond strength between ions,for which among these materials Ca, Si and metal ions release arenot the same [13,14].

3.4. Apatite-formation ability of the coating in SBF solution

The surface morphology of the plasma-sprayed Ca3ZrSi2O9 coat-ing after soaking in SBF solution for 14 days is presented in Fig. 5.Apatite formation was obvious on the surface of the coating afterimmersion. Under higher magnification, it could be observed thatthe particles were composed of plenty of worm-like micrograins,which was one of the typical morphologies of the apatite formed invitro [25]. Fig. 6 shows the XRD pattern of the coating immersedin SBF for 28 days, the appearance of the characteristic diffrac-tion peaks of apatite near 26◦ and 32 ◦ confirmed the formation

of apatite on the surface of the Ca3ZrSi2O9 coating. It also can beseen clearly from Fig. 7 that the coating was covered by a newlyformed layer. EDS analysis revealed that the new layer was mainlycomposed of Ca and P, indicating the formation of apatite.

4680 Y. Liang et al. / Applied Surface Science 256 (2010) 4677–4681

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When zirconium was incorporated into calcium silicate, the dis-solution rate of the Ca3ZrSi2O9 coating decreased and the chemicalstability of the material was greatly improved. However, due tofaster dissolution of Ca ion and more Si–OH group formation, purecalcium silicate coatings always present a higher efficiency for the

ig. 4. Ca ions (a), Si ions (b) release in Tris–HCl buffer solution and mass losses (c)f the coatings after 14 days immersion.

The plasma-sprayed Ca3ZrSi2O9 coating showed apatite-ormation ability in SBF solution, which indicated its bioactivity.he mechanism of apatite formation on the Ca3ZrSi2O9 coating isuggested to be through the incongruent dissolution of Ca and Sions, which is similar to that of CaO–SiO2-based glasses. It can bebserved from Fig. 4(a) and (b) that Ca ion was released at a higherate than Si ion, which was determined by the structure of sili-

ates. In silicate minerals, the bond strength of Si–O in the basiciO4 group is much stronger than that of Ca–O. Therefore, theseeakly bonded Ca ions dissolve into SBF solution quickly and ini-

ially exchange with H+ leading to the formation of Si–OH on theurface of the coating. Subsequently, Ca2+ and HPO4

2− in the SBF

Fig. 5. Surface SEM image of the Ca3ZrSi2O9 coating after 14 days immersion in SBFsolution (a) and its higher magnification (b).

solution are attracted to the surface and consequently the ionicactivity product of the apatite is high enough to precipitate apatiteon the coating surface [26].

Fig. 6. XRD patterns of the Ca3ZrSi2O9 coatings: as-sprayed coating (a), after soakingin SBF solution for 28 days (b).

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ig. 7. Cross-section micrograph and EDS analysis of the Ca3ZrSi2O9 coating soakedn SBF solution for 28 days.

patite formation in SBF compared with the Ca3ZrSi2O9 coating23,10]. It can be seen that the chemical stability and the apatite-ormation ability of the Ca–Si based materials are mutual restraintsnd balances. The zirconium incorporation decreased Ca ion releasend the amount of Si–OH group which could act as the nucle-tion sites of apatite; hence the apatite could not precipitate onhe Ca3ZrSi2O9 coating in the initial immersion. After a period ofmmersion, Ca2+ and HPO4

2− became supersaturated on the sur-ace and the apatite began to crystallize and grew gradually. Theense apatite layer formed on the surface of the coating was sug-ested to be very important for direct bonding with bone tissues16].

. Conclusion

Ca3ZrSi2O9 ceramic powder synthesized by high tempera-ure solid-state reaction was deposited onto Ti–6Al–4V substrate.uring the plasma-spraying process, grains around 100 nm andmorphous phases formed on the coating surface. The coating pre-ented tight bonding with the substrate and excellent chemicaltability. Apatite was formed on the surface of the coating afteroaking in SBF solution, indicating its good bioactivity. Our resultshow that the plasma-sprayed Ca3ZrSi2O9 coating possessed bothood bioactivity and long-term stability, which might have poten-ial use as orthopedic and dental implants.

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