Goto  Takashi

Sato  Mitsutaka

Tu  Rong

MATERIALS TRANSACTIONS

Sato, Mitsutaka

102659

Tu, Rong

102660

Goto, Takashi

102661

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Preparation of hydroxyapatite and calciumphosphate films by MOCVD著者 Sato  Mitsutaka, Tu  Rong, Goto  Takashijournal orpublication titleMaterials Transactionsvolume 48number 12page range 3149-3153year 2007URL http://hdl.handle.net/10097/52129Preparation of Hydroxyapatite and Calcium Phosphate Films by MOCVDMitsutaka Sato1, Rong Tu2 and Takashi Goto21Department of Materials Science, Graduate School, Tohoku University, Sendai 980-8577, Japan2Institute for Materials Research, Tohoku University, Sendai 980-8577, JapanCa-P-O films were prepared by MOCVD using Ca(dpm)2 and (C6H5O)3PO precursors. The crystal phase changed with changingdeposition conditions of substrate temperature (Tsub), total pressure (Ptot) and molar ratio of Ca and P precursors (RCa/P). -tricalcium phosphate(-Ca3(PO4)2) in a single phase was obtained at Tsub ¼ 973K, RCa/P < 0:3 and Tsub ¼ 1073K, RCa/P ¼ 0:1 to 0.5. Hydroxyapatite(Ca10(PO4)6(OH)2) in a single phase was first prepared by MOCVD at Tsub ¼ 973K, RCa/P ¼ 0:5 and Tsub ¼ 1073K, RCa/P ¼ 0:8 to 1. Themaximum deposition rate of -TCP and HAp films in a single phase were 6.0 and 4.0 nm s1 at Ptot ¼ 0:8 kPa and Tsub ¼ 1073K,respectively. [doi:10.2320/matertrans.MRA2007145](Received June 25, 2007; Accepted September 3, 2007; Published October 24, 2007)Keywords: metal-organic chemical vapor deposition, calcium phosphate, crystal structure, microstructure, deposition rate1. IntroductionTitanium and its alloy have been widely used as artificialimplants in a medical field because of their good biocompat-ibility and mechanical properties.1,2) It is known that thereproduction of bones on Ti implants can be promoted bybioceramic coatings such as hydroxyapatite (HAp,Ca10(PO4)6(OH)2), -tricalcium phosphate (-TCP, -Ca3(PO4)2), -tricalcium phosphate (-TCP, -Ca3(PO4)2)and calcium titanate (CaTiO3).3) Many studies on thesecoatings by sputtering,4,5) sol-gel,6,7) plasma spray deposi-tion,8,9) have been conducted to improve the biocompatibilityof Ti substrates.Although chemical vapor deposition (CVD) is an advanta-geous coating process due to relatively high deposition rateswith good morphology controllability and well-adherence tosubstrates as indicated in TiO210) and ZrO211) coatings. A fewreports on the preparation of Ca-P-O films by CVD has beenpublished.12,13) Allen et al. prepared -Ca2P2O7 pyrochlorefilm at 1123K using Ca(dpm)2 and P2O5 precursors by a hot-wall type CVD.12) The pyrochlore films were heat-treated at1373 to 1623K and then -TCP films were obtained. Darr etal. prepared fluorine-containing carbonated hydroxyapatitefilms by CVD.13) However, detailed deposition conditionshave not been reported in these reports, and no studies havesucceeded to prepare HAp, -TCP and -TCP films in asingle phase.In this study, Ca-P-O films were prepared by CVD and theeffects of deposition conditions on phases, morphology,preferred crystal orientation and deposition rate were inves-tigated.2. Experimental ProcedureCa-P-O films were prepared by a vertical cold-wall typeCVD apparatus.14) Ca(dpm)2 (bis-dipivaloylmethanato-cal-cium) and (C6H5O)3PO (triphenylphosphate) source powderswere heated at 523 to 573 and 493 to 533K, respectively.Their vapors were carried into the CVD reactor with Arcarrier gas. O2 gas was separately introduced by using adouble tube nozzle, and mixed with the precursor vapors in amixing chamber placed above the substrate holder. The totalgas flow rate (FRtot ¼ FRAr þ FRO2 þ FRsource vapor) wasfixed at 3:33 106 m3 s1. The total pressure (Ptot) in theCVD reactor was changed from 0.2 to 0.8 kPa. The substratetemperature (Tsub) was controlled between 873 and 1073K.Fused quartz glass plates of 10 15 0:5mm were used assubstrates due to the convenience to identify the crystal phaseand to observe the cross-section of films. Furthermore,alumina plate was used as substrate in the case of peel-off offilm on glass substrate due to the thermal expansionmismatch.The deposition conditions are summarized in Table 1. Thecrystal structure was analyzed by X-ray diffraction (XRD).Fourier transform infrared spectroscopy (FT-IR) was em-ployed to evaluate the O-H bond of HAp. The microstructureand thickness were examined by scanning electron microsco-py (SEM). The deposition rate (Rdep) was determined fromthe relationship between thickness and deposition time.3. Results and Discussion3.1 Crystal structureFigure 1 shows the relationship between precursor temper-ature (Tprec) and the evaporation rate of Ca(dpm)2 and(C6H5O)3PO. The evaporation rates exponentially increasedTable 1 Deposition conditions of Ca-P-O films.Precursor Temperature, TprecCa(dpm)2 : 533 to 573K(C6H5O)3PO : 493 to 533KTotal gas flow rate, FRtot : 3:33 106 m3 s1Carrier Gas : ArCa(dpm)2 : 0:83 106 m3 s1(C6H5O)3PO : 0:83 106 m3 s1O2 gas flow rate, FRO2 : 0.17 to 1:5 106 m3 s1Total pressure, Ptot : 0.2 to 1.0 kPaDeposition temperature, Tsub : 873 to 1123KDeposition time : 0.9 ksSubstrate : quartz glass, aluminaMaterials Transactions, Vol. 48, No. 12 (2007) pp. 3149 to 3153#2007 The Japan Institute of Metalswith Tprec. Although the evaporation rate of (C6H5O)3PO was10 times greater than that of Ca(dpm)2, the precursor molarratio of Ca to P (RCa/P) was precisely controlled by changingthe Tprec.Figure 2 shows X-ray diffraction patterns of Ca-P-O filmsprepared at Tsub ¼ 1073K and Ptot ¼ 0:8 kPa. -TCP andHAp films in a single phase were obtained at RCa/P ¼ 0:24(Tprec (Ca) = 553K and Tprec (P) = 533K) and 0.84 (Tprec(Ca) = 563K and Tprec (P) = 513K), respectively. A mix-ture film of -TCP and HAp was prepared at RCa/P ¼ 0:6.The HAp phase had a preferred orientation of (002) at2 ¼ 25:9.Figure 3 shows the FT-IR spectrum of the HAp film in asingle phase. The adsorption bands due to PO43 and O-Hstretching bonds of HAp were observed at 1100 and 3600cm1, respectively. It is well understood that HAp would losean OH group at high temperature, and may transform tooxyhydroxyapatite (OHAp, Ca10(PO4)6(OH)22xOxx, :defect, x < 1) or oxyapatite (OAp, Ca10(PO4)6O).15–18)OHAp and OAp were often obtained by sputtering andsintering because of high vacuum atmosphere and hightemperature.4) In this study, the formation of HAp wasconfirmed by the O-H stretching bond. The peak intensity ofthe O-H bond of HAp was almost the same as that in a HApfilm prepared by a sol-gel method.19,20)Figure 4 presents a CVD formation diagram of Ca-P-Ofilms as functions of Tsub and RCa/P at Ptot ¼ 0:8 kPa. AtTsub ¼ 873K, no calcium phosphate phase was obtained. Thefilms were a mixture of CaCO3 and CaO at RCa/P < 2 andCaO in a single phase at RCa/P > 2. At Tsub ¼ 973K, -TCPand HAp films in a single phase were obtained at RCa/P < 0:3and 0:5 < RCa/P < 0:6, respectively. The film were a mixtureof -TCP and HAp at 0:3 < RCa/P < 0:5. The films were aEvaporation rate, Revap / mol h-1Ptot = 0.8 kPaTprec-1 / 10-3 K-1Precursor temperature, Tprec / KEvaporation rate, Revap / mol s-1: Ca(dpm)210-410-31.6 1.7 1.8 1.9 2.0 2.1 2.210-610-5500550600: (C6H5O)3POFig. 1 Effect of precursor temperature on the evaporation rate of Ca(dpm)2and (C6H5O)3PO at PO2 ¼ 0:32 kPa and Ptot ¼ 0:8 kPa.10° 20° 30° 40° 50° 60° 70° 80°Intensity (arb. units)Ptot = 0.8 kPa αHAp2θ (CuKα)-TCP(a)(b)(c)Fig. 2 XRD patterns of Ca-P-O films prepared at PO2 ¼ 0:32 kPa,Ptot ¼ 0:8 kPa, Tsub ¼ 1073K and RCa/P ¼ 0:84 (a), 0.60 (b), 0.24 (c).Wavenumber (cm-1)Transmittance (arb. units)80012001600200024002800320036004000PO43-OH-Fig. 3 FT-IR spectrum of HAp film prepared at PO2 ¼ 0:32 kPa,Ptot ¼ 0:8 kPa Tsub ¼ 1073K and RCa/P ¼ 0:84.1 2 3800850900950100010501100Molar ratio of Ca and P precursor, RCa/P Deposition temperature, Tsub / K0α-TCPHApCaCO3CaOP2O5Fig. 4 CVD formation diagram of Ca-P-O films at PO2 ¼ 0:32 kPa andPtot ¼ 0:8 kPa.3150 M. Sato, R. Tu and T. Gotomixture of HAp and CaO at 0:6 < RCa/P < 2:1 and were CaOin a single phase at RCa/P > 2:1. At Tsub ¼ 1073K, -TCPfilm in a single phase was obtained at 0:1 < RCa/P < 0:4, andHAp film in a single phase was obtained at 0:8 < RCa/P <1:0. At RCa/P < 0:1, the film was a mixture of -TCP andP2O5. A mixture of -TCP and HAp were prepared at 0:4 <RCa/P < 0:8. A mixture of HAp and CaO were prepared atRCa/P > 1:0. In the present study, CaO-rich and P2O5-richphase tend to form at low and high Tsub, respectively.Although stoichiometrical RCa/P in -TCP and HAp are 1.5and 1.67, respectively, the films in a single phase wereobtained at lower RCa/P. This may be caused by homoge-neous nucleation in gas atmosphere, incomplete decomposi-tion of precursor and/or formation of by-product. In the CVDphase diagram of Ca-Ti-O system, CaTiO3 in a single phaseformed at 0:8 < RCa/Ti < 1:02.21) The RCa/Ti was lower thanthe stoichiometrical ratio of 1 in CaTiO3, consistent with theresults in this study. In addition, CaO-rich and TiO2-richphase was reported to be easily formed at low and high Tsub inCa-Ti-O system, respectively,21) showing almost same trendwith Ca-P-O system.3.2 MicrostructureFigure 5 shows the effect of PO2 on the surface morphol-ogy of Ca-P-O films prepared at Tsub ¼ 1073K, RCa/P ¼ 0:13(Tprec (Ca) = 543K and Tprec (P) = 533K) and Ptot ¼ 0:8kPa. Both films were -TCP almost in a single phasecontaining a small amount of HAp and showed a (510)orientation of -TCP. The films had a granular micro-structure. The grain size was less than 1 mm at PO2 ¼ 0:08 kPa(Fig. 5(a)) and increased to about 3 mm at PO2 ¼ 0:32 kPa(Fig. 5(b)). The thickness of the films was 0.4 atPO2 ¼ 0:08 kPa and 1.8 mm at PO2 ¼ 0:32 kPa. Therefore, itis suggested that the growth of film was promoted at high PO2because of enough quantum of oxygen for crystal growth.Figure 6 depicts the effect of Tsub on the surface and cross-sectional morphologies of -TCP films in a single phaseprepared at RCa/P ¼ 0:27 (Tprec (Ca) = 583K and Tprec (P) =583K), Ptot ¼ 0:8 kPa and PO2 ¼ 0:32 kPa. The -TCP filmprepared at Tsub ¼ 973K had a dense microstructure, and thegrain size was about 0.5 mm (Figs. 6(a) and (b)), showing anorientation of (510). The -TCP film prepared at Tsub ¼1073K also had a dense granular microstructure. The grainsize was about 2 mm in length (Figs. 6(c) and (d)), showing a(510) orientation as demonstrated in Fig. 7(a).Figure 8 depicts the effect of Tsub on the surface and cross-sectional morphologies of HAp films in a single phaseprepared at Ptot ¼ 0:8 kPa and PO2 ¼ 0:32 kPa. The HAp filmprepared at Tsub ¼ 973K (Tprec (Ca) = 543K and Tprec(P) = 503K) had a dense and fine microstructure with agrain size of about 0.2 mm (Figs. 8(a) and (b)). The HAp filmprepared at Tsub ¼ 1073K (Tprec (Ca) = 563K and Tprec(P) = 513K) had a dense and columnar microstructure witha coarser grain size of about 1 to 2 mm (Figs. 8(c) and (d)),while these grains consisted of smaller grains about 0.2 mm indiameter. With increasing Tsub, the mobility of chemicalspecies on the substrate surface increased, resulting in thegrowth of grain size. Both HAp films had a significantpreferred (002) orientation as demonstrated in Fig. 7(b).(b)5 µm(a)5 µmFig. 5 Effect of PO2 on the surface morphology of Ca-P-O films preparedat Tsub ¼ 1073K, Ptot ¼ 0:8 kPa and RCa/P ¼ 0:13. PO2 ¼ 0:08 (a), 0.36kPa (b).(b)5 µm(a)5 µm5 µm(c)5 µm(d)Fig. 6 Effect of Tsub on the surface and cross-sectional morphologies of -TCP films prepared at PO2 ¼ 0:32 kPa and Ptot ¼ 0:8 kPa. (a), (b)Tsub ¼ 973K and RCa/P ¼ 0:26, (c), (d) Tsub ¼ 1073K and RCa/P ¼ 0:27.10° 15° 20° 25° 30° 35° 40°Ptot = 0.8 kPaα-TCPHApIntensity (arb. units)2θ (CuKα)002102211112300202111130201 040202241132060312510 402, 170511530223043401113(a)(b)Fig. 7 XRD patterns of (510)-orientated -TCP (a) and (002)-orientatedHAp (b) films prepared at Tsub ¼ 1073K, Ptot ¼ 0:8 kPa and PO2 ¼0:32 kPa.Preparation of Hydroxyapatite and Calcium Phosphate Films by MOCVD 3151In the Ca-Ti-O system, the surface and cross-sectionalmorphology of CaTiO3 film changed from dense to columnarstructure with increasing Tsub. The CaTiO3 film having acauliflower-like complicated surface morphology and col-umnar structure showed a good adhesion with substrate.21)On the other hand, Ca-P-O film showed a dense and smoothmicrostructure at any deposition condition. However, Ca-P-Ofilm with a columnar structure and more complicated surfacemorphology may be obtained by further increment of Tsuband/or concentration of source precursor of Ca and P.3.3 Deposition rateFigure 9 depicts the effect of oxygen partial pressure (PO2)on the deposition rate (Rdep) at Tsub ¼ 1073K, Ptot ¼ 0:8 kPaand RCa/P ¼ 0:2 (Tprec (Ca) = 543K and Tprec (P) = 533K).At PO2 < 0:25 kPa, the Rdep increased with increasing PO2 ,and almost saturated at PO2 > 0:25 kPa. This increment ofRdep may be caused by the promotion of decomposition ofsource precursor by increasing of PO2 . This result suggeststhat the driving force of film deposition may be influenced bynot only the supersaturation degree of source precursor butalso the concentration of oxygen.Figure 10 shows the effect of Ptot on the Rdep of Ca-P-Ofilms at Tsub ¼ 1073K and RCa/P ¼ 0:2 (Tprec (Ca) = 543Kand Tprec (P) = 533K). The Rdep was almost constant atPtot < 0:6 kPa and slightly decreased at Ptot ¼ 0:8 kPa. It iscommonly observed in CVD that the deposition ratedecreases at a high Ptot mainly due to premature chemicalreaction in a gas phase.22)Figure 11 shows the relationship between Tsub and Rdep atRCa/P ¼ 0:25 (Tprec (Ca) = 583K and Tprec (P) = 583K) andPO2 ¼ 0:32 kPa in the Arrhenius format. The Rdep increased5 µm(a)5 µm(b)5 µm(c)5 µm(d)Fig. 8 Effect of Tsub on the surface and cross-sectional morphologies ofHAp films prepared at PO2 ¼ 0:32 kPa and Ptot ¼ 0:8 kPa. (a), (b)Tsub ¼ 973K and RCa/P ¼ 0:55, (c), (d) Tsub ¼ 1073K and RCa/P ¼ 0:84.O2 par tial pressure, PO2 / kPaDeposition rate, Rdep / nm s-1Depositionrate, Rdep /051015202502468100 0.1 0.3 0.4 0.50.2µ m h-1Fig. 9 Effect of PO2 on the deposition rate at Tsub ¼ 1073K andPtot ¼ 0:8 kPa (Ca(dpm)2: Tprec ¼ 543K, (C6H5O)3PO: Tprec ¼ 533K).Total pressure, P tot / kPaDepositionrate, Rdep /1100 0.2 0.4 0.6 0.8 1.0 1.2110Deposition rate, Rdep / nm s-1m h-1µFig. 10 Effect of Ptot on the deposition rate at PO2 ¼ 0:32 kPa andTsub ¼ 1073K. (Ca(dpm)2: Tprec ¼ 543K, (C6H5O)3PO: Tprec ¼ 533K).Substrate temperature, Tsub / K1101000.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5Deposition rate, Rdep / nm s-1Deposition rate, Rdep /TiO2 10)110Tsub-1 / 10-3 K-1CaTiO3 22)1000 80012001400 900 700Ptot = 0.8 kPaµm h-1Fig. 11 Effect of Tsub on the deposition rate at PO2 ¼ 0:32 kPa andPtot ¼ 0:8 kPa. (Ca(dpm)2: Tprec ¼ 543K, (C6H5O)3PO: Tprec ¼ 533K).3152 M. Sato, R. Tu and T. Gotowith increasing Tsub, and showed a maximum value of 6.0and 4.0 nm s1 for -TCP and HAp at Tsub ¼ 1073K, re-spectively. This Rdep is about 10 times greater than that ofusual sputtering.4) With further increasing of Tsub, the Rdepslightly increased. It is known that the rate-controlling step inCVD can be a diffusion-limited process in a high Tsub regionwith an activation energy (Ea) of a few kJmol1 and achemical reaction limited process in a low Tsub region withthe Ea of more than several 10 kJmol1. Since no literaturedata on the deposition rate of Ca-P-O films by CVD wasavailable, our results of CaTiO321) and TiO210) films by CVDwere compared in Fig. 11. The Rdep of Ca-P-O films showedalmost similar trend as those of CaTiO3 and TiO2 films. TheEa of CaTiO3 and TiO2 films were 70 and 100 kJmol1,implying the rate-controlling step of a chemical reaction. TheEa of 80 kJmol1 in the present study can also suggest achemical reaction limited process in the low Tsub region.4. ConclusionsCa-P-O films were prepared by MOCVD using Ca(dpm)2and (C6H5O)3PO precursors. At Tsub ¼ 973 to 1073K, -TCP and HAp films in a single phase were obtained bycontrolling mainly RCa/P and Tsub. HAp films had asignificant (002) orientation, and -TCP films showed(510) orientations at Tsub ¼ 973 and 1073K. The surface of-TCP and HAp films were dense and granular micro-structure. The grain size of HAp film (about 0.2 mm) wasmuch smaller than that of -TCP (2 to 3 mm). The depositionrate of Ca-P-O films increased with increasing PO2 and Tsub,and increased with decreasing Ptot. The highest depositionrate of -TCP film was 6.0 nm s1 and that of HAp film was4.0 nm s1 at Tsub ¼ 1073K, Ptot ¼ 0:8 kPa and PO2 ¼ 0:32kPa.AcknowledgementsThis research was partially supported by the Japan Societyfor the Promotion of Science (JSPS), Grant-in-Aids forScientific Research (B), 18360310, Grant-in-Aids for JSPSFellows and the JSPS Asian CORE program ‘‘Interdiscipli-nary Science of Nanometers’’.REFERENCES1) A. Yamamoto, R. Honma and M. Sumita: J. Biomed. Mater. Res. 39(1998) 331–340.2) M. Papakyriacou, H. Mayer, C. Pypen, H. Plenk Jr and S. Stanz-Tschegg: Int. J. Fatigue 22 (2000) 873–886.3) W. Suchanek and M. Yoshimura: J. Mater. Res. 13 (1998) 94–117.4) T. Narushima, K. Ueda, T. Goto, H. Masumoto, T. Katsube, H.Kawamura, C. Ouchi and Y. Iguchi: Mater. Trans. 46 (2005) 2246–2252.5) K. Yamashita, T. Arashi, K. Kitagaki, S. Yamada and T. Umegaki:J. Am. Ceram. 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Preparation of hydroxyapatite and calcium phosphate films by MOCVD

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Preparation of hydroxyapatite and calcium phosphate films by MOCVD

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