Ion-beam-sputtering/mixing deposition of calciumphosphate coatings. I. Effects of ion-mixing beams

C.-X. Wang,1,2 Z.Q. Chen,1 M. Wang,2 Z.Y. Liu,3 P.L. Wang 31Department of Dental Materials, College of Stomatology, West China University of Medical Sciences, Chengdu610041, Sichuan, China2School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore639798, Singapore3Laboratory of Radiation Physics and Technology, Institute of Nuclear Science and Technology, Sichuan University,Chengdu 610064, Sichuan, China

Received 26 November 1999; revised 11 December 2000; accepted 14 December 2000

Abstract: Ion-beam-sputtering/mixing deposition wasused to produce thin calcium phosphate coatings on tita-nium substrate from the hydroxyapatite target. The mixingbeam could be either Ar+ or N+ ions. It was found thatas-deposited coatings were amorphous. No distinct peak ofthe hydroxyl group was observed in FTIR spectra of thecoatings, but new spectral peaks, brought about during thedeposition process, were present for CO3

2−. Scanning elec-tron microscopy revealed that the deposited coatings had auniform and dense structure. The calcium-to-phosphorousratio of these coatings varied between 2.0 and 3.0. Compared

with the calcium phosphate coatings produced by Ar+

beam-mixing deposition, the calcium phosphate coatingsproduced by N+ beam-mixing deposition exhibited a higherdissolution rate in the physiologic saline solution andshowed a lower proliferation rate of osteoblast cells. © 2001John Wiley & Sons, Inc. J Biomed Mater Res 55: 587–595,2001

Key words: ion-beam-sputtering/mixing deposition; cal-cium phosphate coating; titanium; dissolution rate; cell cul-ture

INTRODUCTION

Due to their good biocompatibility and excellentmechanical properties, calcium phosphate ceramicssuch as hydroxyapatite- (HA) coated titanium and itsalloys are among the most promising implant materi-als for orthopedic and dental applications.1–3 Cur-rently, commercially available coated metal implantsare manufactured by using the plasma-spray tech-nique for depositing HA coatings. However, prob-lems, such as low bond strength between the coatingand the substrate and nonuniformity across the thick-ness of the coating, often are encountered withplasma-sprayed coatings.4 A variety of other coatingtechniques, such as electrochemical deposition,5 radio-frequency magnetron sputtering,6 excimer laser depo-sition,7 pulsed laser deposition,8 and dipping9 also

have been investigated for producing HA coatings onmetallic substrates. In addition to bioactivity, a cal-cium phosphate coating satisfactory for clinical usemust be dense, hard, adherent, and tough.10 Since themid-1970s, many surface modification techniquesbased on ion implantation, such as ion-beam deposi-tion (IBD), ion-beam-assisted deposition (IBAD), ion-beam mixing (IBM), and techniques that are based onplasma-assisted ion implantation, such as plasma-source ion implantation (PSII) and plasma-immersionion implantation (PIII), have been developed and noware used widely to modify the surfaces of materialssuch as metals, ceramics, and polymers.11 Ion-beam-sputtering deposition has been investigated as amethod for producing biocompatible ceramic coatingson metallic implants because it produces thin coatingswith high density and superior adhesion.12,13 In thisprocess, ionized argon gas was used to sputter atomsfrom a ceramic target. The sputtered atoms built up onthe metallic substrate that was placed in the path ofthe sputtered material.

Both argon ion beam and nitrogen ion beam can beused for ion-beam bombardment. It has been foundthat grafting −NH2

+ amidogen radicals on Al2O3 ce-

Correspondence to: C.-X. Wang, NTU, Singapore; e-mail:mcxwang@ntu.edu.sg

Contract grant sponsor: National 863 Program of NewTechnology

Contract grant sponsor: National Natural Science Foun-dation of China

© 2001 John Wiley & Sons, Inc.

ramic using an ion implantation technique can im-prove the biocompatibility of this ceramic.14 By usinga nitrogen ion beam to bombard Ca-P coatings, it ispossible to further improve the bone bonding betweenthe coating and the bone to hasten the wound healing.Moreover, during the bombardment process, nitrogenions are able to penetrate the whole coating layer andbond with the titanium substrate to form TixNy,

15

which improves the properties of the coatings. In thisinvestigation, the ion-beam-sputtering/mixing depo-sition technique was used to produce thin calciumphosphate coatings on titanium substrate. The influ-ences of argon and nitrogen ion-mixing beams on thestructure and properties of the resultant coatings arereported in this paper.

MATERIALS AND METHODS

Deposition method

A commercially available pure titanium substrate (platesdimensions 20 × 20 × 1 mm) was mechanically polished andultrasonically cleaned with acetone and alcohol. HA powderwas synthesized using the wet method. HA powder disks,which were to be used as the ion-beam-sputtering target,were cold pressed and sintered at 1150°C for 3 h. X-raydiffraction patterns of HA disks matched those of the stan-dard synthetic hydroxyapatite (PDFSM # 9-0432), indicatingthat the disks had well-crystallized HA with a microstruc-ture of purely random grain orientation.

The ion-beam-sputtering/mixing deposition system con-sisted mainly of one Kaufman ion source and one Freemanion source, one target holder, and one rotatable sampleholder in the path of both ion beams (Fig.1). The depositionchamber was evacuated to a base pressure of 2.8∼3.7 × 10−4

Pa. Prior to deposition, etching of the substrates with 800 eVand 40 mA/cm2 argon ions was performed for 30 min toclean the surface of the titanium substrate. The energetic ionbeam was produced by ionizing high-purity argon gas(99.999% pure). After cleaning, the stage was rotated so thatthe substrates were placed in the path of the sputtered at-

oms. The monolayer calcium phosphate coatings were sput-ter-deposited by an Ar+ beam with 1200 eV and 40 mA/cm2

for 60 min. The Ar+ beam sputtering- and Ar+ beam mixing-deposited calcium phosphate coatings were produced firstby sputtering the HA target with the 1400 eV and 40 mA/cm2 Ar+ beam for 1.5 h and then by using the second Ar+

beam with 60 keV to homogenize the coating. The dosage ofthe Ar+ mixing beam was 2.0 × 1016 ions/cm2. For the Ar+

beam sputtering- and N+ beam mixing-deposited calciumphosphate coatings, after sputtering the HA target with the1400 eV and 40 mA/cm2 Ar+ beam for 1.5 h, the N+ beamwith 60KeV was used to homogenize the coating, and itsdosage also was 2.0 × 1016 ions/cm2. The coated samplesused for Fourier transform infrared spectroscopy (FTIR) hadKCl crystal substrates and their coatings were sputtering/mixing-deposited from an HA target as well.

Characterization of coatings

X-ray diffraction (XRD) was employed to analyze thestructure of as-deposited coatings. A Rigaku D/max-tA X-ray diffractometer with Cu Ka radiation at 40 keV and 50∼80mA was used. FTIR analysis was performed on a NicoletFTIR 20SXB machine for characterizing various functionalgroups of the coatings, especially the hydroxyl and phos-phate groups. The FTIR spectra were obtained using thetransmittance mode from 4000 to 400 cm−1. The surface mor-phology of coatings was examined by using a scanning elec-tron microscope (SEM; Hitachi X-650, Hitachi, Japan) and ascanning tunneling microscope (STM; Explorer STM/AFM,TopoMetrix Co., USA). To prevent charging, the samples forSEM observations were coated with a thin layer of carbon.The elemental composition of coatings was determined byenergy dispersive X-ray analysis (EDX).

Transmission electron microscopy (TEM) also was used toexamine the microstructure of as-deposited coatings. Be-cause of the brittle nature of thin Ca-P coatings, it was verydifficult to prepare suitable specimens for TEM observationdirectly from coatings with titanium substrate by using anion beam thinning apparatus. An alternative method wasused, and TEM specimens successfully were produced. First,the Ca-P coatings were deposited on KCl crystal substratesfrom a HA target. The coated samples then were put into abeaker containing distilled water. As a result, the calciumphosphate coatings could be peeled off after the dissolutionof the KCl crystals. The peeled-off Ca-P coatings were ex-amined under a TEM (JEOL TEM-100CX, Japan).

Dissolution rate test

For the dissolution tests, the coated samples (with thetitanium as the substrate) were incubated in a physiologicsaline solution (0.9% NaCl) at pH 7.2 and at a temperature of37°C. At regular intervals, the solution was analyzed for theCa2+ concentration. The Ca2+ concentration in the solutionwas determined by flame atomic adsorption spectrometry

Figure 1. Schematic diagram showing the ion beam sput-tering/mixing deposition technique: 1 = lower energy ionsource for sputtering; 2 = higher energy ion source for mix-ing; 3 = sample holder; 4 = target holder.

588 WANG ET AL.

(Hitachi 180-80, Hotachi, Japan). Five replicates were com-pleted and expressed as means ± standard deviations.

Osteoblast cell culture

For the osteoblast cell culture tests, both the Ar+ beammixing- and the N+ beam mixing-deposited Ca-P coatingswere used. Pure titanium metal plates also were used as thepositive control. All samples were sterilized under dry heatat 180°C for 2 h. The osteoblast cell line MC3T3-E1 (suppliedby the Fourth Military University of Medical Sciences, Xi’an,China) was used for in vitro tests. It was cultured in DMEM(Gibco BRL, USA), containing 10% fetal bovine serum in ahumidified 5% CO2–air atmosphere at 37°C. Cells wereseeded on each specimen at 6000 cells/plate in 96-well tissueculture plates. The intracellular activity of MC3T3-E1 wasmeasured by the 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) formazan assay. Ab-sorbance at 490 nm of MTT-formazan in dimethyl sulphox-ide (DMSO) was measured using the Bio-Kinetics readermicroplate. The cell morphology was observed under SEM.

Statistical analysis

All measurements were collected in five replicates andexpressed as means ± standard deviations. The differencesbetween tested coatings were evaluated by Student’s t testand considered statistically significant with P < 0.05.

RESULTS

Characterization of coatings

A typical XRD pattern of as-deposited monolayercalcium phosphate coatings from the HA target is

shown in Figure 2. As can be seen from this pattern,the coatings only exhibited a broad bump and nopeaks other than those of titanium substrate were ob-served, indicating that as-deposited coatings wereamorphous.

The microstructure of as-deposited coatings was ex-amined under TEM. The TEM analysis provided in-formation on both the phases and the structure ofcoatings on a microscopic scale. Figure 3(a) shows thebright field TEM micrograph of the coating depositedwith the ion-beam energy of 1.2 keV and 40 mA. Thecorresponding selected area diffraction (SAD) patternis shown in Figure 3(b). A single halo in the SADpattern indicated that the as-deposited coatings wereamorphous, which confirmed XRD results.

FTIR was used mainly to characterize the hydroxyland phosphate groups in the as-deposited coatings.According to Dasarathy et al.3 and Walters et al.,16 inthe molecular structure of HA, the phosphate groupitself had a Td symmetry, resulting in four internalmodes (symmetric stretch n1: 956 cm

−1; asymmetricstretch n2: 430–460 cm

−1; bending n3: 1040–1090 cm−1;

bending n4: 575-–610 cm−1); and the hydroxyl group

with C`n had vibration modes at wavenumbers of3570 and 630 cm−1. Figure 4 and Figure 5 display typi-cal FTIR spectra of the Ar+ beam sputtering and Ar+

beam mixing Ca-P coating and the Ar+ beam sputter-ing and N+ beam mixing Ca-P coating from HA tar-gets that were deposited on the KCl crystal substrate,respectively. In Figure 4, spectral peaks at 1034 and565cm−1 were observed, indicating the existence ofPO4

3− in the as-deposited Ca-P coating. No distinctspectral peaks of the hydroxyl group were observedexcept for a peak at 3494 cm−1 for water. New peaks at1496, 1416, and 939 cm−1 were present for CO3

2−,which was brought about by the deposition process.From Figure 4 and Figure 5, it can be seen that therewas no shape change in FTIR spectra for Ca-P coatings

Figure 2. XRD pattern of as-deposited Ca-P coating from the HA target.

589EFFECTS OF ION-MIXING BEAMS

produced by Ar+ beam and N+ beam mixing deposi-tions. However, there were some alterations in the ab-sorption bands at 1034 and 565 cm−1, which were forthe Ca-P coating produced by Ar+ beam mixing de-position. These peaks moved to 1032 and 577 cm−1 forthe Ca-P coating produced by N+ beam mixing pro-cess.

Figure 6 shows the surface morphology of Ca-Pcoatings deposited on titanium substrate under differ-ent conditions. These coatings exhibited a clean andsmooth surface morphology. Energy-dispersive X-rayspot analysis (EDX) demonstrated that the obtainedcoatings consisted of calcium and phosphorous. The

semiquantitative EDX analysis of relative amounts ofcalcium and phosphorous revealed that the Ca/P ratioof the coatings varied between 2.0 and 3.0. The Ca-Pratio for the coating produced by Ar+ beam and Ar+

beam mixing depositions was 2.5, and it was 3.0 forthe coating produced by Ar+ beam and N+ beam mix-ing depositions.

Scanning tunneling microscopy (STM) is a relativelynew technique for mapping a surface with high reso-lution. A probe can be sharpened to a few angstromsin radius at the tip and then brought to within about 2nm above a flat surface. Piezoelectric mounts in theinstrument control both lateral and up-and-down

Figure 3. TEM micrograph of as-deposited Ca-P coating on the KCl crystal substrate: (a) bright field image; (b) correspond-ing selected area diffraction (SAD) pattern.

Figure 4. FTIR spectrum of Ca-P coating produced by Ar+ beam sputtering and Ar+ beam mixing deposition from the HAtarget.

590 WANG ET AL.

movements. This technique is based on the electron-tunneling phenomenon; that is, a current flows be-tween the probe and the surface due to an overlap ofrespective wave functions. The minute current, nano-amperes or less, varies exponentially with the tip-sample separation, about tenfold per angstrom. Thetunneling current is used to control the up-and-downmovement. As the surface is scanned, a relief map (i.e.,image) of the surface is obtained, with a resolutiondown to the atomic scale. A major advantage of STMover other imaging techniques is that the specimen

does not need to be in vacuum; it can be in air orimmersed in water or in some other fluids. STM can beused to study conducting and semiconducting sur-faces. In the present study, STM was used to investi-gate the surfaces of as-deposited Ca-P coatings. Figure7 shows typical STM images of as-deposited Ca-Pcoating on titanium substrate using the HA target.These images give detailed information of coating sur-faces at the atomic scale. The accumulation of sput-tered atoms and the formation of the coating layerclearly were shown. It also was observed that the

Figure 5. FTIR spectrum of Ca-P coating produced by Ar+ beam sputtering and N+ beam mixing deposition from the HAtarget.

Figure 6. SEM micrographs of Ca-P coatings deposited on titanium substrate: (a) coating produced by Ar+ beam sputteringand Ar+ beam mixing deposition, dosage 2 × 1016 ions/cm2 (original magnification ×1000); (b) coating produced by Ar+ beamsputtering and N+ beam mixing process, dosage 2 × 1016 ions/cm2 (original magnification ×1000).

591EFFECTS OF ION-MIXING BEAMS

grains in the coatings were very small (i.e., at nanom-eter scale).

Dissolution rate

Figure 8 shows the Ca2+ concentration of differentCa-P coatings in physiologic saline solution (0.9%NaCl) during a 5-h incubation period. It can be ob-served that, compared with coatings produced by Ar+

beam mixing deposition, coatings produced by N+

beam mixing deposition always exhibit higher Ca2+

concentration in the saline solution during the 5-h test-ing period.

Osteoblast cell culture

The intracellular activity of MC3T3-E1 cells on eachmaterial is shown in Figure 9. Statistically significantdifferences (P < 0.001) were observed in two testedcoatings. However, no statistical differences (P =0.137) were observed in the same materials incubated

for different times. After 2 days of incubation, the in-tracellular activity of MC3T3-E1 cells on the Ca-P coat-ing produced by Ar+ ion beam mixing depositionreached the peak rate. After 3 days of incubation, theintracellular activity of MC3T3-E1 cells on the samematerial slowed down sharply. The same phenom-enon was observed on the negative control sample. Asfor the other two materials, namely, titanium and theCa-P coating produced by N+ beam mixing deposi-tion, the intracellular activity of the osteoblastic cellswas increasing during the incubation period. Com-pared with the coating produced by Ar+ beam mixingdeposition, the intracellular activity of cells incubatedon titanium and the coating produced by N+ beammixing deposition were significantly lower. Figure 10exhibits the morphology of MC3T3-E1 cells on differentmaterials. No major difference was noted for thesecells with coatings tested.

DISCUSSION

XRD and TEM analyses showed that as-depositedCa-P coatings were amorphous. This “amorphous”

Figure 7. STM images of the Ca-P coating produced by Ar+ beam sputtering and Ar+ beam mixing deposition, dosage 2 ×1016 ions/cm2.

Figure 8. Ca2+

concentration resulting from different Ca-Pcoatings in physiologic saline solution (pH 7.2, 37°C).

Figure 9. Intracellular activity of MC3T3-E1 cells on differ-ent materials as a function of incubation time.

592 WANG ET AL.

appearance is a direct consequence of the ion-beamdeposition technique. Because of the high vacuum anda relatively low temperature as compared to theplasma-spray coating method, there was not enoughenergy for the growth of nano-crystallites in the coat-ings during the ion-beam-sputtering/mixing deposi-tion process, and hence the as-deposited coatings wereshown to be “amorphous.” This observation was veri-fied by the STM examination. The STM results re-vealed that the size of crystallites in the as-depositedcoatings was at the nanometer scale. The accumula-tion of the nano-crystallites also was shown in theSTM images. Using postdeposition heat treatment forthe amorphous Ca-P coatings, the crystallinity of theCa-P coatings could be increased.13 This means thatthe heat treatment provides energy for the growth ofnano-crystallites, and as a result, the crystallinity is

shown to be “increased.” In a separate study, part ofthe XRD peaks of HA was observed after heat treat-ment, and some crystal-like morphology was apparentin the surface of the coatings.17

Variations for the phosphate group and the loss ofthe hydroxyl group in FTIR spectra also were causedby the ion-beam deposition technique. During the de-position process, components of the target, such ashydroxide or oxygen and hydrogen, may not be trans-ferred completely to the substrate (or anchored ontothe substrate surface and thus remaining in the coat-ings) because of the requirement of maintaining a lowpressure in this particular coating method.

The wide variation in the Ca-P ratio among coatingswas attributed to the following: on the one hand, asphosphorus is very volatile, the low pressure duringthe sputtering process affected the anchoring of the

Figure 10. SEM micrographs of MC3T3-E1 cells incubated on different materials: (a) Ca-P coating produced by Ar+ ion beam

sputtering and Ar+ beam mixing deposition (original magnification ×2000); (b) Ca-P coating produced by Ar+ beam sput-tering and N+ beam mixing deposition (original magnification ×2000); (c) pure titanium (original magnification ×2000); (d)Ca-P coating produced by Ar+ beam sputtering and N+ beam mixing deposition (no cells seeded, original magnification×2000).

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sputtered phosphorus on the substrate; in addition,during the sputtering and the deposition process, oneAr+ ion sputtered different phosphorus and calciumatoms, indicating that the sputtering rate of calciumatoms and that of phosphorus atoms are different. Thehigher sputtering rate of calcium atoms led to a Ca-Pratio higher than the standard 1.67 of pure HA crystal.On the other hand, according to Cotell,18 one possiblecause was the substitution of the carbonate group forthe phosphate groups in the HA molecule. The pro-cess of ion-beam-sputtering HA target had been stud-ied using theoretical analysis.19 The results showedthat because of the collision of sputtering ions and HAmolecules, defects, such as lattice displacement andvacancy, were produced in the coating. These defectsmade it possible for the substitution of carbonategroups for the phosphate groups in the HA molecule.

The higher Ca2+ concentration in the saline solutionresulted from the coating produced by the N+ beammixing process and suggests that this coating is moresoluble and dissolves faster than the coating producedby the Ar+ beam mixing process. Previous studieshave shown that the dissolution of Ca-P coatings isinfluenced by the crystallinity of the deposited coat-ing.20,21 In the present study, it was found that themorphology of the coating after immersion in aqueoussolutions was the major factor affecting the dissolutionrate of the coating: the more cracks that were pro-duced on the coating surface, the higher the dissolu-tion rate of the coating. Yoshinari et al.22 studied thedissolution rate of calcium phosphate coatings afterrapid heating. They too found that surface morphol-ogy affects the solubility of the coatings.

It was confirmed that the MTT–formazan formationis proportional to the number of living cells in therange from 104 to 106 cells. After 2 days of incubation,the intracellular activity of MC3T3-E1 cells on theamorphous calcium phosphate coating produced bythe Ar+ beam mixing process reached peak intracellu-lar activity. After 3 days of incubation, the intracellu-lar activity of MC3T3-E1 cells on the same materialslowed down drastically. The same phenomenon wasobserved in the negative control sample. There wereno differences in the trend of the intracellular activitybetween the coating produced by the Ar+ beam mix-ing process and the negative control samples. As forthe other two materials, that is, titanium and the amor-phous calcium phosphate coating produced by N+

beam mixing process, the intracellular activity of theosteoblastic cells increased during the incubation pe-riod. However, compared with the coating producedby the Ar+ beam mixing process, the intracellular ac-tivity of cells incubated on titanium and the coatingproduced by the N+ beam mixing process were sig-nificantly lower. Although no major difference in themorphology of cells was noted among the materialstested during the in vitro test, the cells appeared to be

in the early stage of confluency on the coating pro-duced by the Ar+ beam mixing process.

It seems that no major differences exist between thecalcium phosphate coatings produced by the Ar+

beam mixing and the N+ beam mixing processes.However, the Ca-P coatings produced by the N+ beammixing process exhibited a higher dissolution rate andcaused a slower cellular activity of the osteoblasticcells. And preliminary animal tests have shown thatthere is a faster bone bonding between the implantsproduced by Ar+ beam mixing and N+ beam mixingprocesses than there is with coatings produced by theAr+ beam mixing and the Ar+ beam mixing processes .23

The possible reasons may be, first, that because themass of N+ ion is smaller than that of Ar+ ion, the sizeof nano-crystallites in the coatings produced by Ar+

beam mixing and N+ beam mixing processes wassmaller. Second, the higher Ca-P ratio in the coatingsproduced by Ar+ beam mixing and N+ beam mixingprocesses also contributed to the differences in disso-lution rate and intracellular activity. Further experi-ments that include the use of other analytical Ar+

beam mixing and N+ beam mixing processes tech-niques need to be performed to obtain more informa-tion about the composition and structure of the depos-ited Ca-P coatings. Such experiments may reveal thedifferences in the calcium phosphate coatings pro-duced by the two mixing processes.

CONCLUSIONS

Dense and homogeneous Ca-P coatings successfullywere produced on titanium substrate using the ion-beam deposition technique. The results showed thatthe as-deposited coatings were amorphous due to thesmall crystallite size. In comparison with the HA ce-ramic target, some variations in the chemical compo-sition of the coatings were brought about in the depo-sition process, such as the distortion of the phosphatelattice, loss of hydroxyl groups, and the incorporationof CO3

2−. Although no structure difference was foundin Ca-P coatings produced by Ar+ beam and N+ beammixing processes, the Ca-P coatings produced by theN+ beam mixing process were more soluble than theCa-P coatings produced by the Ar+ beam mixing pro-cess. In the in vitro test, the osteoblastic cells appearedto be in the early stage of confluency on the Ca-Pcoatings produced by the Ar+ beam mixing process.Further experiments are required to obtain more in-formation about the composition and structure of thedeposited Ca-P coatings.

Professor Deng Li of College of Pharmacy, West ChinaUniversity of Medical Sciences, is thanked for assistance inthe cell culture experiment.

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