- Email: [email protected]
titania bioceramic composite coatings on titanium alloy and apatite deposition on their surfaces in a simulated body fluid
Surface & Coatings Technology 201 (2007) 8715 – 8722 www.elsevier.com/locate/surfcoat
Structure of calcium titanate/titania bioceramic composite coatings on titanium alloy and apatite deposition on their surfaces in a simulated body fluid Daqing Wei, Yu Zhou ⁎, Dechang Jia, Yaming Wang Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, PR China Received 15 December 2006; accepted 27 April 2007 Available online 10 May 2007
Abstract In this study, TiO2-based coatings containing Ca and P ions were prepared on titanium alloy surfaces by microarc oxidation (MAO). After soaking in aqueous NaOH solution and subsequent heat treatment at 700 and 800 °C, calcium titanate/titania bioceramic composite (CTBC) coatings were obtained. The results show that the outer layers (0–1.5 μm) of the CTBC coatings are mainly composed of Ca, Ti, O and Na constituents with a uniform distributions with increasing the depth near the surfaces. The surface phase compositions of the CTBC coating formed at 700 °C are anatase, rutile and CaTi21O38 phases, as well as a few CaTiO3, while those of the CTBC coating formed at 800 °C are anatase, rutile and CaTiO3. When incubated in a simulated body fluid (SBF), apatite was deposited on the CTBC coatings probably via formation of hydroxyl functionalized surface complexes on the CTBC coating surfaces by ionic exchanges between (Ca2+, Na+) ions of the CTBC coatings and H3O+ ions in the SBF. The CTBC coating formed at 800 °C seems to facilitate the deposition of Ca and P probably due to the good crystallographic match between perovskite CaTiO3 and HA on specific crystal planes. © 2007 Elsevier B.V. All rights reserved. Keywords: Coating; Microarc oxidation; Titania; Calcium titanate; Apatite; NaOH and heat treatment
1. Introduction Hydroxyapatite (HA) and bioactive glass-ceramic (SiO2– CaO–P2O5–MO (M = Na, Mg, etc.)) both exhibit good bioactivity [1,2]. Unfortunately, these bioactive ceramic materials are not suitable for load-bearing conditions on account of their poor mechanical properties [3,4]. Titanium and its alloys continue to be used extensively in skeletal repair and dental implants area [5]. Their excellent mechanical strength, toughness, biocompatibility and corrosion resistance have led to widespread clinical success [5]. However, the bioinert nature of titanium and its alloys must be modified towards bioconductivity if a strong interface bonding between implant and living bone is desired [3,5]. Hence one approach to overcome the ⁎ Corresponding author. Postal address: P. O. Box 433, Department of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 8641 5898; fax: +86 451 8641 4291. E-mail address: [email protected] (Y. Zhou). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.124
bioinertness of the metals is to produce bioconductive coatings on their surfaces [2,3–8]. Calcium titanate has been investigated extensively as a dielectric material [9–11]. In recent years, CaTiO3 has also gained much attention as a biomedical material. As reported the formation of CaTiO3 phase was observed during the preparation of HA coating on titanium and its alloys [12–14]. In addition, CaTiO3 has been proposed as an intermediate layer or an addition to improve the adhesion between HA and substrates [14–17]. Further investigations suggest that CaTiO3 material has potential biomedical applications [18,19]. Various surface modifying techniques have been developed to prepare CaTiO3 coating, such as sol–gel [17,19,20], ion beam assisted deposition and sputter-deposition techniques [21]. Microarc oxidation (MAO) is a relatively convenient and effective technique to deposit various functional coatings with porous structures on the surfaces of Ti, Al, Mg and their alloys [22–24]. Moreover, it is very suitable to fabricate uniform ceramic coatings on substrates with complex geometries.
8716
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
Recently, Song et al [25] used the MAO technique to prepare TiO2 coating containing CaTiO3 on titanium. However, no apatite was induced on this coating after incubation in a simulated body fluid (SBF). Subsequent hydrothermal treatment at 250 °C was proposed to improve the apatite-forming ability of this MAO coating. In this work, calcium titanate/titania bioceramic composite (CTBC) coatings were prepared by a novel process involving treatment of the MAO TiO2-based coatings containing Ca and P ions in aqueous NaOH solution and subsequent heating (NASH) at 700 and 800 °C. The results show that the CTBC coatings have high induction capability for the nucleation and growth of biomimetic apatite in the SBF. 2. Experimental procedure 2.1. Preparation of the CTBC coatings Ti6Al4V plates (10 × 10 × 1.5 mm 3 ) were ground with SiC abrasive papers, ultrasonically washed with acetone and deionized water, and dried at 40 °C. In the MAO process, the Ti6Al4V plates were used as anodes and stainless steel plates were used as cathodes in an electrolytic bath. An electrolyte was prepared by dissolving reagent-grade chemicals of Ca(CH3COO)2·H2O (6.3 g/l), Ca(H2PO4)2·H2O (13.2 g/l), EDTA-2Na (15 g/l) and NaOH (15 g/l) into deionized water. The applied voltage, frequency, duty cycle and oxidizing time were 300 V, 600 Hz, 8.0% and 5 min, respectively. The temperature of the electrolyte was kept at 40 °C by a cooling system. After the MAO pre-treatment, each sample was treated in 10 mL NaOH aqueous solution with concentration of 5 mol/l at 60 °C for 24 h, and then gently washed with deionized water and dried at 25 °C. The NaOH treated samples were kept at 700 and 800 °C for 1 h, with a heating rate of 10 °C/min and furnace cooling. The samples NASH treated at 700 and 800 °C were labeled as CTBC7 and CTBC8, respectively. 2.2. Immersion of the samples in the SBF The MAO, CTBC7 and CTBC8 samples were incubated in 15 mL SBF for 7, 14 and 28 days and the ion concentrations of the SBF and human blood plasma are shown in Table 1. The SBF was prepared by dissolving reagent-grade chemicals of NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2, Table 1 Ion concentrations of the SBF and human blood plasma Ion
+
Na K+ Mg2+ Ca2+ Cl− HCO2− 3 HPO2− 4 2− SO4
Concentration (mmol/l) SBF
Blood plasma
142.0 5.0 1.5 2.5 147.8 4.2 1.0 0.5
142.0 5.0 1.5 2.5 103.8 27 1.0 0.5
and Na2SO4 into deionized water and buffering at pH 7.40 with tris–hydroxymethyl–aminomethane ((CH2OH)3CNH2) and 1.0 mol/l HCl at 37 °C [26], and The SBF was refreshed every other day. 2.3. Analyses of the sample surfaces and the SBF Before and after incubation in SBF the phase composition of the surfaces of the MAO, CTBC7 and CTBC8 coatings were analyzed by glancing incidence X-ray diffraction (GIXRD) (Philips X'Pert, The Netherlands) using Cu Kα radiation (40 kV, 35 mA). In the GIXRD experiment, the angle of the incident beam was fixed at 1° against the surface of the sample and the measurements were performed with a continuous scanning mode at a rate of 2°/min. Chemical compositions of the surfaces of the MAO coatings before and after modifications were detected by X-ray photoelectron spectroscopy (XPS, PHI 5700, America Physical Electronics). In the XPS experiment, an Al Kα (1486.6 eV) X-ray source was used with an anode power of 250 W (12.5 kV, 20 mA) for the XPS survey work. Taking into account the presence of Na, an Mg Kα (1253.6 eV) X-ray source was used to analyze the atomic concentrations of Na, Ca, P and Ti, etc in a high-resolution mode. The XPS take-off angle was set at 45° and a region about 2 × 0.8 mm2 on the surface of each sample was analyzed with a hemispherical analyzer. Surface morphologies of the MAO, CTBC7 and CTBC8 coatings before and after SBF incubation were observed by a scanning electron microscopy (SEM, CamScan MX2600, CamScan Co., England). In addition, the surface constituents of the samples were also analyzed by an energy dispersive X-ray spectrometer (EDS, Oxford Model 7537, England) equipped on the SEM system. Scanning Auger electron spectroscopy (SAES) measurements were performed with a Scanning Auger Nanoprobe (PHI 700, ULVAC Inc., USA) to analyze the atomic concentrations of Ca, Na, P, Ti and O as a function of the depth near the surfaces of the MAO, CTBC7 and CTBC8 coatings. The Auger electron take-off angle in the SAES was 40° and the surfaces of the samples were sputtered by Ar+ ions with a sputtering rate of 144 nm/min referenced for SiO2 under a voltage and a current intensity of 3 kV and 10 nA, respectively. The sputtered and analyzed areas were about 2 × 2 mm2 and 40 × 40 μm2, respectively. The data acquisition was performed every 30 s. The etched depth was roughly calculated by this equation: Etched depth (nm) = Etch time (s)/60 × Etching rate (144 nm/min). Fourier transform infrared spectroscopy (FT-IR, Bruker Vector 22, Germany) was also used to analyze the structure of the CTBC7 and CTBC8 coatings after SBF incubation for 28 days. In the FT-IR experiment, the scanning resolution and range were 4 and 4000–400 cm− 1, respectively. An inductively coupled plasma/optical emission spectroscopy (ICP-OES, Optima 5300DV, Perkin-Elmer, America) was used to measure the Ca and P concentrations of the SBF after incubation of the CTBC7 and CTBC8 coatings for 0–96 h. In the ICP-OES measurement, the ion concentrations of 10 mL SBF after incubation of each sample (10 × 10 × 1.5 mm3) were
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
8717
measured, and two independent analyses were carried out for each solution. 3. Results 3.1. Character of the MAO and CTBC coatings Fig. 1 shows the GIXRD patterns of the surfaces of the MAO, CTBC7 and CTBC8 coatings. The surface of the MAO coating (Fig. 1a) shows a weak diffraction peak at 2θ = 25.3° and a board diffraction peak at 2θ = 22.5–40°, suggesting the presences of anatase with low crystallinity and amorphous phase. After NASH treatment at 700 °C, the major phase compositions of the modified surface are anatase, rutile and CaTi21O38 phases (JCPDS #42-1368), as well as a few CaTiO3 (JCPDS #42-0423) (Fig. 1b). However, the surface of the MAO coating NASH treated at 800 °C shows the presences of anatase, rutile and CaTiO3 phases (Fig. 1c). The results indicate that the phase composition of the CTBC coatings is highly dependent on the heat treatment temperature. The XPS survey shows that the major surface constituents of the MAO coating are O, Ca, Ti, P and C (Fig. 2a). However, the surfaces of the CTBC7 and CTBC8 coatings show only O, Ca, Ti, Na and C whereas the peaks of P have disappeared (Fig. 2c and d). Also, no peaks of P were observed on the surface of the MAO coating after NaOH treatment (Fig. 2b), suggesting the dissolution of P during NaOH treatment process. The Ca and Na concentrations of the surface of the MAO coating are about 11.8 and 0.2 at.%, and those on the surface of the MAO coating after NaOH treatment are about 6.1 and 2.2 at.%, suggesting the dissolution of Ca and introduction of Na during NaOH treatment, which is consistent with the EDS results (not shown). The sum of the atomic concentrations of Ca, Ti, O and C of the CTBC7 and CTBC8 coatings are about 98 at.%, and the atomic ratio levels of Ca/Ti, O/Ti, C/Ti, Na/Ti and Ca/Na are shown in
Fig. 2. XPS surveys of the surfaces of (a) MAO, (b) NaOH treated MAO, (c) CTBC7 and (d) CTBC8 coatings.
Table 2. The results indicate that the atomic ratios of the CTBC7 and CTBC8 are at approximate levels. Fig. 3 shows the surface morphologies of the MAO, CTBC7 and CTBC8 coatings. The MAO coating exhibits a porous surface where the micropores with size of ∼ 5 μm are distributed at regular intervals (Fig. 3a). At a higher magnification the MAO coating exhibits a smooth surface (Fig. 3b). However, both the surfaces of the CTBC7 and CTBC8 coatings exhibit a rough surface composed of numerous particles with ∼ 200 nm size, as well as nano-flakes with size of ∼ 100 nm in thickness (Fig. 3c and d). Though the XPS and EDS results show the presence of Na on the surfaces of the CTBC7 and CTBC8 coatings, no Na-containing phase was detected by GIXRD, probably due to the small content of Na-containing phase on the CTBC7 and CTBC8 coatings. Further analyses are needed to distinguish the Na-containing phase, calcium titanate and TiO2 phases of the surfaces of the CTBC7 and CTBC8 coatings. The whole thicknesses of the MAO coating before and after NASH treatments are about 4.5–6 μm. Fig. 4 shows the SAES sputter depth profile plots of the outer layers of the MAO, CTBC7 and CTBC8 coatings. In Fig. 4a, graded distributions in Ca (32–20 at.%), Ti (15–20 at.%) and O (40–50 at.%) were observed, whereas the Na and P concentrations do not change obviously within the outer layer (0–1.5 μm) of the MAO coating (3–5 at.% Na and 4.5–5 at.% P, Table 2 The atomic ratio levels of Ca/Ti, O/Ti, C/Ti, Na/Ti and Ca/Na of the surfaces of the CTBC7 and CTBC8 coatings
Fig. 1. GIXRD patterns of (a) MAO, (b) CTBC7 and (c) CTBC8 coatings.
Atomic ratio (at.%/at.%)
CTBC7
CTBC8
C/Ti O/Ti Ca/Ti Na/Ti Ca/Na Sum of Ca, Ti, O, C
1.1 3.0 0.3 0.1 2.6 97.7
1.2 2.8 0.2 0.1 2.0 98.4
8718
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
3.2. Apatite deposition on the MAO and CTBC coatings in the SBF Fig. 5 shows the surface morphologies of the MAO, CTBC7 and CTBC8 coatings after SBF incubation for 7 and 14 days. After incubation for 7 days, original surface micropores can still be distinguished without noticeable coverage of the surfaces of the MAO and CTBC7 coatings as shown in Fig. 5a and b. However, it is evident that the surface of the CTBC8 coating is covered by numerous sphere-like precipitates with size of ∼5 μm in diameter (Fig. 5c). After SBF incubation for 14 days, newly formed layers were observed on the surfaces of the MAO, CTBC7 and CTBC8 coatings (Fig. 5d–f). The EDS
Fig. 3. Surface morphologies of (a) MAO, (b) high magnification of (a), (c) CTBC7 and (d) CTBC8 coatings.
respectively). As can be seen in Fig. 4b and c, the atomic concentration levels of Ca, Ti, O and Na of both CTBC7 and CTBC8 coatings are respectively, 13–17 at.%, 24–26 at.%, 52– 55 at.% and 4–6 at.% without pronounced graded distributions, suggesting a uniform composition in the outer layers. In addition, it was found that the Ca concentration of the out layer is lower than that of the MAO coating, and P basically disappeared as results of the dissolutions of Ca and P during the NaOH modification. In the cases of the CTBC7 and CTBC8 coatings, P was observed after Ar+ ion etching for about 700 s, suggesting that the thickness of the CTBC coatings is about 1.5 μm.
Fig. 4. SAES sputter depth profile plots near the surfaces of (a) MAO, (b) CTBC7 and (c) CTBC8 coatings.
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
8719
Fig. 5. SEM micrographs of (a) MAO, (b) CTBC7 and (c) CTBC8 coatings after SBF incubation for 7 days and (d) MAO, (e) CTBC7 and (f) CTBC8 after SBF immersion for 14 days.
results further indicate that the new layers are Ca-and Pcontaining (CaP) precipitates (not shown here). The GIXRD patterns of the MAO, CTBC7 and CTBC8 coatings after SBF incubation for 14 and 28 days are shown in Fig. 6. After 14 days, the diffraction intensity of the CaP precipitates on the MAO coating was weak (Fig. 6a), suggesting a poor crystallinity of the CaP precipitates. However, the CTBC7 and CTBC8 coatings after SBF incubation for 14 days show the diffraction peaks of apatite with low crystallinity (Fig. 6a). After 28 days, the diffraction intensities of apatite increase, suggesting an enhanced crystallinity of apatite (Fig. 6b). In addition, the diffraction peaks of TiO2 were observed after SBF incubation for 14 days, while those of calcium titanate disappeared basically, suggesting a considerable dissolution of the calcium titanate phases. The absence of diffraction peaks of
calcium titanate could also be due to the limited penetration depth of the X-rays through a calcium phosphate layer deposited on top of the CTBC coatings. The SEM, EDS and GIXRD results indicate that the induction capability for apatite formation of the CTBC7 and CTBC8 coatings is higher than that of the MAO coating. Furthermore, the CTBC8 coating appears to facilitate the deposition of CaP precipitates at the early stage of the SBF incubation process compared to the CTBC7 coating. Fig. 7 shows the FT-IR spectra of the CTBC7 and CTBC8 coatings after SBF incubation for 28 days. Broad absorption bands at 3440–3443 cm− 1 and bending modes at 1641– 1653 cm− 1 were detected and attributed to H2O in the CTBC7 and CTBC8 coatings after SBF incubation [27]. The spectra clearly illustrate the triply degenerated asymmetric stretching
8720
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
Fig. 8. It can be seen that the Ca concentration of the SBF with immersion of the CTBC7 coating increases continuously, while that of the CTBC8 coating first increases at the early stage of incubation in SBF (0–48 h) and then begins to decrease after 48 h, suggesting absorption of Ca at the surface of the CTBC8 coating (Fig. 8a). On the other hand, the P concentration of the SBF immersed the CTBC7 coating does not change obviously, while that of the SBF immersed the CTBC8 coating exhibits a decreasing tendency, suggesting the absorption of P on the surface of the CTBC8 coating (Fig. 8b). The ICP-OES results suggest that the CTBC8 coating facilitate the depositions of CaP precipitates compared to CTBC7 coating, corresponding to the SEM results. 4. Discussion
Fig. 6. XRD patterns of the MAO, CTBC7 and CTBC8 coatings after SBF incubation for (a) 14 and (b) 28 days, respectively.
mode of v3PO4 bands at 1033–1035 cm− 1, triply degenerated bending mode of v4PO4 bands at 601–603 and 563–564 cm− 1 and double degenerated bending mode of v2PO4 bands at 470– 472 cm− 1 [27,28]. In the FT-IR spectrum, the CO32− absorption bands also were detected including bending mode of the v4CO32− group in A-type carbonated HA (CHA) at 1549–1552 cm− 1, characteristic stretching mode of v3CO32− group in CHA at 1508–1509 cm− 1, characteristic stretching mode of v1CO32− group in A-type CHA at 1460–1461 cm− 1, stretching mode of v1CO32−group in B-type CHA at 1423–1424 cm− 1 and bending mode of (v3 or v4) CO32− group in CHA at 871–872 cm− 1 [27,28]. In addition, the characteristic shoulders observed at 1101–1102, 957–960 and 871–872 cm− 1 also suggest the presence of HPO42− [27,28]. Also, the weak OH− stretch bands at 3560 cm− 1 indicate that the CO32− groups can also partially substitute for OH− groups (A-type HCA) during the apatite formation process [27]. The results indicate that the CTBC7 and CTBC8 coatings can induce the formation of carbonated apatite. The Ca and P concentrations of the SBF after incubation of the CTBC7 and CTBC8 coatings for 0–96 h are shown in
Upon implantation, calcium phosphate layers form on biomaterials that are osseoconductive. In this work, TiO2based coating containing Ca and P was introduced on the surface of titanium alloy by MAO technique. After NaOH treatment, phosphorous mostly dissolved from the surface of the MAO coating remaining Ca, Ti and O. Further NASH treatment at 700 and 800 °C of the MAO coating, the bioactive CTBC coatings were formed (Fig. 1). The current results indicate that the CTBC7 and CTBC8 coatings exhibit higher capability for apatite formation compared to the MAO coating (Figs. 5 and 6). As reported, the formation of biomimetic apatite is highly dependent on the structure and composition of substrates [23,32,33]. Generally, the formation of bonelike apatite on biomaterial surface in SBF requires a chemical stimulus; for instance, substrates with functionalized surfaces such as OH, PO4H2, COOH and CONH2-terminated surfaces could promote the apatite formation greatly [18,24,25,29–31].According to the SEM and ICP-OES results (Figs. 5 and 8), it is suggested that a variety of reactions such as ionic dissolution and precipitation occur on the surfaces of the CTBC7 and CTBC8 coatings during the SBF incubation process. Calcium is released from the surfaces of the CTBC7 and CTBC8 coatings according to the ICP-OES results. Based on this and previous works [18,24], an
Fig. 7. FT-IR spectra of (a) CTBC7 and (c) CTBC8 coatings after SBF incubation for 28 days.
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
Fig. 8. Ca and P concentrations of the SBF after incubation of the CTBC7 and CTBC8 coatings measured as a function of incubation time: (a) Ca and (b) P.
ionic exchange between Ca2+ ions of the CTBC coatings and H3O+ ions of the SBF may take place during SBF incubation process. As a result, abundant Ti–OH groups are formed on the surfaces of the CTBC coatings. The hydrolysis of the calcium titanate can produce numerous Ti–OH groups, which is one key factor for the apatite formation. The hydroxyl functionalized surfaces greatly enhance the nucleation and growth of apatite, which generate from an interfacial molecular recognition between functionalized surface and ions with respect to apatite in solutions [29,31]. The interfacial molecular recognition involves certain aspects, e.g. electrostatic potential interaction and crystallographic match, etc. [29,31]. According to this and previous researches [18,29,31], the Ti–OH groups can incorporate calcium ions, and then absorb phosphate and hydrogen carbonate ions in the SBF due to the electrostatic potential interaction. The absorbed calcium and phosphate ions further increase the degree of supersaturation of solution with respect to apatite near the vicinity of the Ti–OH group, which triggers the apatite nucleation. Furthermore, according to the ICP-OES results the CTBC coatings can release numerous Ca ions at the early stage of SBF incubation thus also increasing the degree of supersaturation near nucleation sites of apatite, promoting the nucleation and growth of apatite. Once the apatite nuclei are formed, they grow spontaneously by assembling the remaining calcium, phosphate and hydrogen carbonic acid ions around apatite nuclei in SBF.
8721
It was also noted that abundant perovskite CaTiO3 phase was formed in the CTBC8 coating, and this coating seems to facilitate the absorptions of Ca and P compared to the CTBC7 coating (Figs. 5 and 8). As mentioned above, the interfacial molecular recognition in the process of apatite formation involves crystal lattice matching. In other words, the lattice configuration of specific crystal plane of the perovskite is likely to provide good sites for the epitaxial adsorptions of Ca and P, which is derived from the inspiration of the researches on the crystal structure relation between TiO2 and HA [23,31–33]. Previous research also suggests that some type of surface structural arrangement of CaTiO3 may promote the nucleation of apatite [18,19]. In fact, the spatial arrangement of the surface groups plays a very important role in the formation of apatite [34,35]. Fig. 9 shows a schematic illustration of the twodimensional lattice match relations between 0¯ 22 plane of perovskite CaTiO3 (JCPDS #42-0423) and (0001) plane of HA (JCPDS #09-0432). As shown in Fig. 9, the perovskite CaTiO3 has lattice parameters of 2Xc = 9.345 Å and 3Yc = 16.326 Å on ¯ the 0 22 crystal plane. HA has lattice parameters of Xh = 9.418 Å and Yh = 16.312 Å on the (0001) crystal plane. The degrees of the crystallographic mismatch between 2Xc and Xh and between 3Yc and Yh are 0.8% and 0.09%, respectively. The results indicate that the perovskite CaTiO3 phase has good crystallographic matching relation with HA on the specific crystal planes, probably providing good sites for apatite nucleation by the epitaxial deposition process. Therefore, the presence of more perovskite CaTiO3 phase is likely to cause the favorable formation of apatite on the CTBC8 coating compared to the CTBC7 coating.
Fig. 9. Schematic diagrams of the arrangements of O atoms on the perovskite CaTiO3 ð0 ¯22Þ crystal plane and the arrangement of OH groups on the hydroxyapatite (0001) crystal plane.
8722
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
5. Conclusions Calcium titanate/titania bioceramic composite (CTBC) coatings were successfully prepared by chemical and heat treatment of the microarc oxidized (MAO) TiO2-based coating on titanium alloy. The CTBC coatings possess higher ability of apatite formation compared to the MAO coating, since the hydroxyl functionalized surface could be formed on the CTBC coatings during the SBF incubation, which enhances the nucleation and growth of apatite. In addition, the lattice configuration of specific crystal plane of perovskite CaTiO3 is likely to provide good sites for the epitaxial adsorption of Ca and P from the SBF, which promotes the nucleation and growth of apatite. References [1] M. Wang, Biomaterials 24 (2003) 2133. [2] L.L. Hench, J. Am. Ceram. Soc. 81 (7) (1998) 1705. [3] J.M. Gomez-V, E. Saiz, A.P. Tomsia, T. Oku, K. Suganuma, G.W. Marshall, S.J. Marshall, Adv. Mater. 12 (2000) 894. [4] C.Q. Ning, Y. Zhou, Biomaterials 23 (2002) 2909. [5] X.Y. Liu, K.C. Paul, C.X. Ding, Mater. Sci. Eng., R. 47 (2004) 49. [6] X.B. Zheng, M.H. Huang, C.X. Ding, Biomaterials 21 (2000) 841. [7] Q.Y. Zhang, Y. Leng, R.L. Xin, Biomaterials 26 (2005) 2857. [8] E. Milella, F. Cosentino, A. Licciulli, C. Massaro, Biomaterials 22 (2001) 1425. [9] A. Cohen, R.C. Bradt, G.S. Ansell, J. Am. Ceram. Soc. 53 (7) (1970) 396. [10] L.M. Troilo, D. Damjanovic, R.E. Newnham, J. Am. Ceram. Soc. 77 (3) (1994) 857. [11] F.M. Figueiredo, V.V. Kharton, J.C. Waerenborgh, A.P. Viskup, E.N. Naumovich, J.R. Frade, J. Am. Ceram. Soc. 87 (12) (2004) 2252. [12] X.F. Xiao, R.F. Liu, Y.Z. Zheng, Surf. Coat. Technol. 200 (2006) 4406.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
H. Li, K.A. Khor, P. Cheang, Biomaterials 24 (2003) 949. H. Li, K.A. Khor, P. Cheang, Biomaterials 23 (2002) 85. P. Zhou, M. Akao, Bio-Med. Mater. Eng. 1 (1997) 67. Y. Ohba, T. Watanabe, E. Sakai, M. Daimon, J. Ceram. Soc. Jpn. 107 (1999) 907. S. Kačiulis, G. Mattogno, L. Pandolfi, M. Cavalli, G. Gnappi, A. Montenero, Appl. Surf. Sci. 151 (1999) 1. J. Coreño, O. Coreño, J. Biomed. Mater. Res A 75 (2005) 478. M. Manso, M. Langlet, J.M. Martínez-Duart, Mater. Sci. Eng., C 23 (2003) 447. S. Holliday, A. Stanishevsky, Surf. Coat. Technol. 188–189 (2004) 741. N. Ohtsu, K. Sato, K. Saito, T. Hanawa, K. Asami, Mater. Trans. 45 (2004) 1778. A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. B.C. Yang, M. Uchida, H.-M. Kim, X.D. Zhang, T. Kokubo, Biomaterials 25 (2004) 1003. Y. Han, S.-H. Hong, K.W. Xu, Surf. Coat. Tech. 168 (2003) 249. W.-H. Song, H.S. Ryu, S.-H. Hong, J. Am. Ceram. Soc. 88 (2005) 2642. A. Oyane, H.-M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, T. Nakamura, J. Biomed. Mater. Res. A 65 (2003) 188. L. Müller, F.A. Müller, Acta. Biomater. 2 (2006) 181. S. Koutsopoulos, J. Biomed. Mater. Res. 62 (2002) 600. G.K. Toworfe, R.J. Composto, I.M. Shapiro, P. Ducheyne, Biomaterials 27 (2006) 631. M. Tanahashi, T. Matsuda, J. Biomed. Mater. Res. 34 (1997) 305. C.B. Mao, H.D. Li, F.Z. Cui, Q.L. Feng, C.L. Ma, J. Mater. Chem. 9 (1999) 2573. J.-M. Wu, S. Hayakawa, K. Tsuru, A. Osaka, J. Am. Ceram. Soc. 87 (9) (2004) 1635. M. Uchida, H.-M. Kim, T. Kokubo, S. Fujibayashi, T. Nakamura, J. Biomed. Mater. Res. A 64 (2003) 164. M. Uchida, H.-M. Kim, T. Kokubo, J. Am. Ceram. Soc. 84 (12) (2001) 2969. M. Wei, M. Uchida, H.-M. Kim, T. Kokubo, T. Nakamura, Biomaterials 23 (2002) 167.
Structure of calcium titanate/titania bioceramic composite coatings on titanium alloy and apatite deposition on their surfaces in a simulated body fluid Daqing Wei, Yu Zhou ⁎, Dechang Jia, Yaming Wang Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, PR China Received 15 December 2006; accepted 27 April 2007 Available online 10 May 2007
Abstract In this study, TiO2-based coatings containing Ca and P ions were prepared on titanium alloy surfaces by microarc oxidation (MAO). After soaking in aqueous NaOH solution and subsequent heat treatment at 700 and 800 °C, calcium titanate/titania bioceramic composite (CTBC) coatings were obtained. The results show that the outer layers (0–1.5 μm) of the CTBC coatings are mainly composed of Ca, Ti, O and Na constituents with a uniform distributions with increasing the depth near the surfaces. The surface phase compositions of the CTBC coating formed at 700 °C are anatase, rutile and CaTi21O38 phases, as well as a few CaTiO3, while those of the CTBC coating formed at 800 °C are anatase, rutile and CaTiO3. When incubated in a simulated body fluid (SBF), apatite was deposited on the CTBC coatings probably via formation of hydroxyl functionalized surface complexes on the CTBC coating surfaces by ionic exchanges between (Ca2+, Na+) ions of the CTBC coatings and H3O+ ions in the SBF. The CTBC coating formed at 800 °C seems to facilitate the deposition of Ca and P probably due to the good crystallographic match between perovskite CaTiO3 and HA on specific crystal planes. © 2007 Elsevier B.V. All rights reserved. Keywords: Coating; Microarc oxidation; Titania; Calcium titanate; Apatite; NaOH and heat treatment
1. Introduction Hydroxyapatite (HA) and bioactive glass-ceramic (SiO2– CaO–P2O5–MO (M = Na, Mg, etc.)) both exhibit good bioactivity [1,2]. Unfortunately, these bioactive ceramic materials are not suitable for load-bearing conditions on account of their poor mechanical properties [3,4]. Titanium and its alloys continue to be used extensively in skeletal repair and dental implants area [5]. Their excellent mechanical strength, toughness, biocompatibility and corrosion resistance have led to widespread clinical success [5]. However, the bioinert nature of titanium and its alloys must be modified towards bioconductivity if a strong interface bonding between implant and living bone is desired [3,5]. Hence one approach to overcome the ⁎ Corresponding author. Postal address: P. O. Box 433, Department of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 8641 5898; fax: +86 451 8641 4291. E-mail address: [email protected] (Y. Zhou). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.124
bioinertness of the metals is to produce bioconductive coatings on their surfaces [2,3–8]. Calcium titanate has been investigated extensively as a dielectric material [9–11]. In recent years, CaTiO3 has also gained much attention as a biomedical material. As reported the formation of CaTiO3 phase was observed during the preparation of HA coating on titanium and its alloys [12–14]. In addition, CaTiO3 has been proposed as an intermediate layer or an addition to improve the adhesion between HA and substrates [14–17]. Further investigations suggest that CaTiO3 material has potential biomedical applications [18,19]. Various surface modifying techniques have been developed to prepare CaTiO3 coating, such as sol–gel [17,19,20], ion beam assisted deposition and sputter-deposition techniques [21]. Microarc oxidation (MAO) is a relatively convenient and effective technique to deposit various functional coatings with porous structures on the surfaces of Ti, Al, Mg and their alloys [22–24]. Moreover, it is very suitable to fabricate uniform ceramic coatings on substrates with complex geometries.
8716
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
Recently, Song et al [25] used the MAO technique to prepare TiO2 coating containing CaTiO3 on titanium. However, no apatite was induced on this coating after incubation in a simulated body fluid (SBF). Subsequent hydrothermal treatment at 250 °C was proposed to improve the apatite-forming ability of this MAO coating. In this work, calcium titanate/titania bioceramic composite (CTBC) coatings were prepared by a novel process involving treatment of the MAO TiO2-based coatings containing Ca and P ions in aqueous NaOH solution and subsequent heating (NASH) at 700 and 800 °C. The results show that the CTBC coatings have high induction capability for the nucleation and growth of biomimetic apatite in the SBF. 2. Experimental procedure 2.1. Preparation of the CTBC coatings Ti6Al4V plates (10 × 10 × 1.5 mm 3 ) were ground with SiC abrasive papers, ultrasonically washed with acetone and deionized water, and dried at 40 °C. In the MAO process, the Ti6Al4V plates were used as anodes and stainless steel plates were used as cathodes in an electrolytic bath. An electrolyte was prepared by dissolving reagent-grade chemicals of Ca(CH3COO)2·H2O (6.3 g/l), Ca(H2PO4)2·H2O (13.2 g/l), EDTA-2Na (15 g/l) and NaOH (15 g/l) into deionized water. The applied voltage, frequency, duty cycle and oxidizing time were 300 V, 600 Hz, 8.0% and 5 min, respectively. The temperature of the electrolyte was kept at 40 °C by a cooling system. After the MAO pre-treatment, each sample was treated in 10 mL NaOH aqueous solution with concentration of 5 mol/l at 60 °C for 24 h, and then gently washed with deionized water and dried at 25 °C. The NaOH treated samples were kept at 700 and 800 °C for 1 h, with a heating rate of 10 °C/min and furnace cooling. The samples NASH treated at 700 and 800 °C were labeled as CTBC7 and CTBC8, respectively. 2.2. Immersion of the samples in the SBF The MAO, CTBC7 and CTBC8 samples were incubated in 15 mL SBF for 7, 14 and 28 days and the ion concentrations of the SBF and human blood plasma are shown in Table 1. The SBF was prepared by dissolving reagent-grade chemicals of NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2, Table 1 Ion concentrations of the SBF and human blood plasma Ion
+
Na K+ Mg2+ Ca2+ Cl− HCO2− 3 HPO2− 4 2− SO4
Concentration (mmol/l) SBF
Blood plasma
142.0 5.0 1.5 2.5 147.8 4.2 1.0 0.5
142.0 5.0 1.5 2.5 103.8 27 1.0 0.5
and Na2SO4 into deionized water and buffering at pH 7.40 with tris–hydroxymethyl–aminomethane ((CH2OH)3CNH2) and 1.0 mol/l HCl at 37 °C [26], and The SBF was refreshed every other day. 2.3. Analyses of the sample surfaces and the SBF Before and after incubation in SBF the phase composition of the surfaces of the MAO, CTBC7 and CTBC8 coatings were analyzed by glancing incidence X-ray diffraction (GIXRD) (Philips X'Pert, The Netherlands) using Cu Kα radiation (40 kV, 35 mA). In the GIXRD experiment, the angle of the incident beam was fixed at 1° against the surface of the sample and the measurements were performed with a continuous scanning mode at a rate of 2°/min. Chemical compositions of the surfaces of the MAO coatings before and after modifications were detected by X-ray photoelectron spectroscopy (XPS, PHI 5700, America Physical Electronics). In the XPS experiment, an Al Kα (1486.6 eV) X-ray source was used with an anode power of 250 W (12.5 kV, 20 mA) for the XPS survey work. Taking into account the presence of Na, an Mg Kα (1253.6 eV) X-ray source was used to analyze the atomic concentrations of Na, Ca, P and Ti, etc in a high-resolution mode. The XPS take-off angle was set at 45° and a region about 2 × 0.8 mm2 on the surface of each sample was analyzed with a hemispherical analyzer. Surface morphologies of the MAO, CTBC7 and CTBC8 coatings before and after SBF incubation were observed by a scanning electron microscopy (SEM, CamScan MX2600, CamScan Co., England). In addition, the surface constituents of the samples were also analyzed by an energy dispersive X-ray spectrometer (EDS, Oxford Model 7537, England) equipped on the SEM system. Scanning Auger electron spectroscopy (SAES) measurements were performed with a Scanning Auger Nanoprobe (PHI 700, ULVAC Inc., USA) to analyze the atomic concentrations of Ca, Na, P, Ti and O as a function of the depth near the surfaces of the MAO, CTBC7 and CTBC8 coatings. The Auger electron take-off angle in the SAES was 40° and the surfaces of the samples were sputtered by Ar+ ions with a sputtering rate of 144 nm/min referenced for SiO2 under a voltage and a current intensity of 3 kV and 10 nA, respectively. The sputtered and analyzed areas were about 2 × 2 mm2 and 40 × 40 μm2, respectively. The data acquisition was performed every 30 s. The etched depth was roughly calculated by this equation: Etched depth (nm) = Etch time (s)/60 × Etching rate (144 nm/min). Fourier transform infrared spectroscopy (FT-IR, Bruker Vector 22, Germany) was also used to analyze the structure of the CTBC7 and CTBC8 coatings after SBF incubation for 28 days. In the FT-IR experiment, the scanning resolution and range were 4 and 4000–400 cm− 1, respectively. An inductively coupled plasma/optical emission spectroscopy (ICP-OES, Optima 5300DV, Perkin-Elmer, America) was used to measure the Ca and P concentrations of the SBF after incubation of the CTBC7 and CTBC8 coatings for 0–96 h. In the ICP-OES measurement, the ion concentrations of 10 mL SBF after incubation of each sample (10 × 10 × 1.5 mm3) were
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
8717
measured, and two independent analyses were carried out for each solution. 3. Results 3.1. Character of the MAO and CTBC coatings Fig. 1 shows the GIXRD patterns of the surfaces of the MAO, CTBC7 and CTBC8 coatings. The surface of the MAO coating (Fig. 1a) shows a weak diffraction peak at 2θ = 25.3° and a board diffraction peak at 2θ = 22.5–40°, suggesting the presences of anatase with low crystallinity and amorphous phase. After NASH treatment at 700 °C, the major phase compositions of the modified surface are anatase, rutile and CaTi21O38 phases (JCPDS #42-1368), as well as a few CaTiO3 (JCPDS #42-0423) (Fig. 1b). However, the surface of the MAO coating NASH treated at 800 °C shows the presences of anatase, rutile and CaTiO3 phases (Fig. 1c). The results indicate that the phase composition of the CTBC coatings is highly dependent on the heat treatment temperature. The XPS survey shows that the major surface constituents of the MAO coating are O, Ca, Ti, P and C (Fig. 2a). However, the surfaces of the CTBC7 and CTBC8 coatings show only O, Ca, Ti, Na and C whereas the peaks of P have disappeared (Fig. 2c and d). Also, no peaks of P were observed on the surface of the MAO coating after NaOH treatment (Fig. 2b), suggesting the dissolution of P during NaOH treatment process. The Ca and Na concentrations of the surface of the MAO coating are about 11.8 and 0.2 at.%, and those on the surface of the MAO coating after NaOH treatment are about 6.1 and 2.2 at.%, suggesting the dissolution of Ca and introduction of Na during NaOH treatment, which is consistent with the EDS results (not shown). The sum of the atomic concentrations of Ca, Ti, O and C of the CTBC7 and CTBC8 coatings are about 98 at.%, and the atomic ratio levels of Ca/Ti, O/Ti, C/Ti, Na/Ti and Ca/Na are shown in
Fig. 2. XPS surveys of the surfaces of (a) MAO, (b) NaOH treated MAO, (c) CTBC7 and (d) CTBC8 coatings.
Table 2. The results indicate that the atomic ratios of the CTBC7 and CTBC8 are at approximate levels. Fig. 3 shows the surface morphologies of the MAO, CTBC7 and CTBC8 coatings. The MAO coating exhibits a porous surface where the micropores with size of ∼ 5 μm are distributed at regular intervals (Fig. 3a). At a higher magnification the MAO coating exhibits a smooth surface (Fig. 3b). However, both the surfaces of the CTBC7 and CTBC8 coatings exhibit a rough surface composed of numerous particles with ∼ 200 nm size, as well as nano-flakes with size of ∼ 100 nm in thickness (Fig. 3c and d). Though the XPS and EDS results show the presence of Na on the surfaces of the CTBC7 and CTBC8 coatings, no Na-containing phase was detected by GIXRD, probably due to the small content of Na-containing phase on the CTBC7 and CTBC8 coatings. Further analyses are needed to distinguish the Na-containing phase, calcium titanate and TiO2 phases of the surfaces of the CTBC7 and CTBC8 coatings. The whole thicknesses of the MAO coating before and after NASH treatments are about 4.5–6 μm. Fig. 4 shows the SAES sputter depth profile plots of the outer layers of the MAO, CTBC7 and CTBC8 coatings. In Fig. 4a, graded distributions in Ca (32–20 at.%), Ti (15–20 at.%) and O (40–50 at.%) were observed, whereas the Na and P concentrations do not change obviously within the outer layer (0–1.5 μm) of the MAO coating (3–5 at.% Na and 4.5–5 at.% P, Table 2 The atomic ratio levels of Ca/Ti, O/Ti, C/Ti, Na/Ti and Ca/Na of the surfaces of the CTBC7 and CTBC8 coatings
Fig. 1. GIXRD patterns of (a) MAO, (b) CTBC7 and (c) CTBC8 coatings.
Atomic ratio (at.%/at.%)
CTBC7
CTBC8
C/Ti O/Ti Ca/Ti Na/Ti Ca/Na Sum of Ca, Ti, O, C
1.1 3.0 0.3 0.1 2.6 97.7
1.2 2.8 0.2 0.1 2.0 98.4
8718
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
3.2. Apatite deposition on the MAO and CTBC coatings in the SBF Fig. 5 shows the surface morphologies of the MAO, CTBC7 and CTBC8 coatings after SBF incubation for 7 and 14 days. After incubation for 7 days, original surface micropores can still be distinguished without noticeable coverage of the surfaces of the MAO and CTBC7 coatings as shown in Fig. 5a and b. However, it is evident that the surface of the CTBC8 coating is covered by numerous sphere-like precipitates with size of ∼5 μm in diameter (Fig. 5c). After SBF incubation for 14 days, newly formed layers were observed on the surfaces of the MAO, CTBC7 and CTBC8 coatings (Fig. 5d–f). The EDS
Fig. 3. Surface morphologies of (a) MAO, (b) high magnification of (a), (c) CTBC7 and (d) CTBC8 coatings.
respectively). As can be seen in Fig. 4b and c, the atomic concentration levels of Ca, Ti, O and Na of both CTBC7 and CTBC8 coatings are respectively, 13–17 at.%, 24–26 at.%, 52– 55 at.% and 4–6 at.% without pronounced graded distributions, suggesting a uniform composition in the outer layers. In addition, it was found that the Ca concentration of the out layer is lower than that of the MAO coating, and P basically disappeared as results of the dissolutions of Ca and P during the NaOH modification. In the cases of the CTBC7 and CTBC8 coatings, P was observed after Ar+ ion etching for about 700 s, suggesting that the thickness of the CTBC coatings is about 1.5 μm.
Fig. 4. SAES sputter depth profile plots near the surfaces of (a) MAO, (b) CTBC7 and (c) CTBC8 coatings.
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
8719
Fig. 5. SEM micrographs of (a) MAO, (b) CTBC7 and (c) CTBC8 coatings after SBF incubation for 7 days and (d) MAO, (e) CTBC7 and (f) CTBC8 after SBF immersion for 14 days.
results further indicate that the new layers are Ca-and Pcontaining (CaP) precipitates (not shown here). The GIXRD patterns of the MAO, CTBC7 and CTBC8 coatings after SBF incubation for 14 and 28 days are shown in Fig. 6. After 14 days, the diffraction intensity of the CaP precipitates on the MAO coating was weak (Fig. 6a), suggesting a poor crystallinity of the CaP precipitates. However, the CTBC7 and CTBC8 coatings after SBF incubation for 14 days show the diffraction peaks of apatite with low crystallinity (Fig. 6a). After 28 days, the diffraction intensities of apatite increase, suggesting an enhanced crystallinity of apatite (Fig. 6b). In addition, the diffraction peaks of TiO2 were observed after SBF incubation for 14 days, while those of calcium titanate disappeared basically, suggesting a considerable dissolution of the calcium titanate phases. The absence of diffraction peaks of
calcium titanate could also be due to the limited penetration depth of the X-rays through a calcium phosphate layer deposited on top of the CTBC coatings. The SEM, EDS and GIXRD results indicate that the induction capability for apatite formation of the CTBC7 and CTBC8 coatings is higher than that of the MAO coating. Furthermore, the CTBC8 coating appears to facilitate the deposition of CaP precipitates at the early stage of the SBF incubation process compared to the CTBC7 coating. Fig. 7 shows the FT-IR spectra of the CTBC7 and CTBC8 coatings after SBF incubation for 28 days. Broad absorption bands at 3440–3443 cm− 1 and bending modes at 1641– 1653 cm− 1 were detected and attributed to H2O in the CTBC7 and CTBC8 coatings after SBF incubation [27]. The spectra clearly illustrate the triply degenerated asymmetric stretching
8720
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
Fig. 8. It can be seen that the Ca concentration of the SBF with immersion of the CTBC7 coating increases continuously, while that of the CTBC8 coating first increases at the early stage of incubation in SBF (0–48 h) and then begins to decrease after 48 h, suggesting absorption of Ca at the surface of the CTBC8 coating (Fig. 8a). On the other hand, the P concentration of the SBF immersed the CTBC7 coating does not change obviously, while that of the SBF immersed the CTBC8 coating exhibits a decreasing tendency, suggesting the absorption of P on the surface of the CTBC8 coating (Fig. 8b). The ICP-OES results suggest that the CTBC8 coating facilitate the depositions of CaP precipitates compared to CTBC7 coating, corresponding to the SEM results. 4. Discussion
Fig. 6. XRD patterns of the MAO, CTBC7 and CTBC8 coatings after SBF incubation for (a) 14 and (b) 28 days, respectively.
mode of v3PO4 bands at 1033–1035 cm− 1, triply degenerated bending mode of v4PO4 bands at 601–603 and 563–564 cm− 1 and double degenerated bending mode of v2PO4 bands at 470– 472 cm− 1 [27,28]. In the FT-IR spectrum, the CO32− absorption bands also were detected including bending mode of the v4CO32− group in A-type carbonated HA (CHA) at 1549–1552 cm− 1, characteristic stretching mode of v3CO32− group in CHA at 1508–1509 cm− 1, characteristic stretching mode of v1CO32− group in A-type CHA at 1460–1461 cm− 1, stretching mode of v1CO32−group in B-type CHA at 1423–1424 cm− 1 and bending mode of (v3 or v4) CO32− group in CHA at 871–872 cm− 1 [27,28]. In addition, the characteristic shoulders observed at 1101–1102, 957–960 and 871–872 cm− 1 also suggest the presence of HPO42− [27,28]. Also, the weak OH− stretch bands at 3560 cm− 1 indicate that the CO32− groups can also partially substitute for OH− groups (A-type HCA) during the apatite formation process [27]. The results indicate that the CTBC7 and CTBC8 coatings can induce the formation of carbonated apatite. The Ca and P concentrations of the SBF after incubation of the CTBC7 and CTBC8 coatings for 0–96 h are shown in
Upon implantation, calcium phosphate layers form on biomaterials that are osseoconductive. In this work, TiO2based coating containing Ca and P was introduced on the surface of titanium alloy by MAO technique. After NaOH treatment, phosphorous mostly dissolved from the surface of the MAO coating remaining Ca, Ti and O. Further NASH treatment at 700 and 800 °C of the MAO coating, the bioactive CTBC coatings were formed (Fig. 1). The current results indicate that the CTBC7 and CTBC8 coatings exhibit higher capability for apatite formation compared to the MAO coating (Figs. 5 and 6). As reported, the formation of biomimetic apatite is highly dependent on the structure and composition of substrates [23,32,33]. Generally, the formation of bonelike apatite on biomaterial surface in SBF requires a chemical stimulus; for instance, substrates with functionalized surfaces such as OH, PO4H2, COOH and CONH2-terminated surfaces could promote the apatite formation greatly [18,24,25,29–31].According to the SEM and ICP-OES results (Figs. 5 and 8), it is suggested that a variety of reactions such as ionic dissolution and precipitation occur on the surfaces of the CTBC7 and CTBC8 coatings during the SBF incubation process. Calcium is released from the surfaces of the CTBC7 and CTBC8 coatings according to the ICP-OES results. Based on this and previous works [18,24], an
Fig. 7. FT-IR spectra of (a) CTBC7 and (c) CTBC8 coatings after SBF incubation for 28 days.
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
Fig. 8. Ca and P concentrations of the SBF after incubation of the CTBC7 and CTBC8 coatings measured as a function of incubation time: (a) Ca and (b) P.
ionic exchange between Ca2+ ions of the CTBC coatings and H3O+ ions of the SBF may take place during SBF incubation process. As a result, abundant Ti–OH groups are formed on the surfaces of the CTBC coatings. The hydrolysis of the calcium titanate can produce numerous Ti–OH groups, which is one key factor for the apatite formation. The hydroxyl functionalized surfaces greatly enhance the nucleation and growth of apatite, which generate from an interfacial molecular recognition between functionalized surface and ions with respect to apatite in solutions [29,31]. The interfacial molecular recognition involves certain aspects, e.g. electrostatic potential interaction and crystallographic match, etc. [29,31]. According to this and previous researches [18,29,31], the Ti–OH groups can incorporate calcium ions, and then absorb phosphate and hydrogen carbonate ions in the SBF due to the electrostatic potential interaction. The absorbed calcium and phosphate ions further increase the degree of supersaturation of solution with respect to apatite near the vicinity of the Ti–OH group, which triggers the apatite nucleation. Furthermore, according to the ICP-OES results the CTBC coatings can release numerous Ca ions at the early stage of SBF incubation thus also increasing the degree of supersaturation near nucleation sites of apatite, promoting the nucleation and growth of apatite. Once the apatite nuclei are formed, they grow spontaneously by assembling the remaining calcium, phosphate and hydrogen carbonic acid ions around apatite nuclei in SBF.
8721
It was also noted that abundant perovskite CaTiO3 phase was formed in the CTBC8 coating, and this coating seems to facilitate the absorptions of Ca and P compared to the CTBC7 coating (Figs. 5 and 8). As mentioned above, the interfacial molecular recognition in the process of apatite formation involves crystal lattice matching. In other words, the lattice configuration of specific crystal plane of the perovskite is likely to provide good sites for the epitaxial adsorptions of Ca and P, which is derived from the inspiration of the researches on the crystal structure relation between TiO2 and HA [23,31–33]. Previous research also suggests that some type of surface structural arrangement of CaTiO3 may promote the nucleation of apatite [18,19]. In fact, the spatial arrangement of the surface groups plays a very important role in the formation of apatite [34,35]. Fig. 9 shows a schematic illustration of the twodimensional lattice match relations between 0¯ 22 plane of perovskite CaTiO3 (JCPDS #42-0423) and (0001) plane of HA (JCPDS #09-0432). As shown in Fig. 9, the perovskite CaTiO3 has lattice parameters of 2Xc = 9.345 Å and 3Yc = 16.326 Å on ¯ the 0 22 crystal plane. HA has lattice parameters of Xh = 9.418 Å and Yh = 16.312 Å on the (0001) crystal plane. The degrees of the crystallographic mismatch between 2Xc and Xh and between 3Yc and Yh are 0.8% and 0.09%, respectively. The results indicate that the perovskite CaTiO3 phase has good crystallographic matching relation with HA on the specific crystal planes, probably providing good sites for apatite nucleation by the epitaxial deposition process. Therefore, the presence of more perovskite CaTiO3 phase is likely to cause the favorable formation of apatite on the CTBC8 coating compared to the CTBC7 coating.
Fig. 9. Schematic diagrams of the arrangements of O atoms on the perovskite CaTiO3 ð0 ¯22Þ crystal plane and the arrangement of OH groups on the hydroxyapatite (0001) crystal plane.
8722
D. Wei et al. / Surface & Coatings Technology 201 (2007) 8715–8722
5. Conclusions Calcium titanate/titania bioceramic composite (CTBC) coatings were successfully prepared by chemical and heat treatment of the microarc oxidized (MAO) TiO2-based coating on titanium alloy. The CTBC coatings possess higher ability of apatite formation compared to the MAO coating, since the hydroxyl functionalized surface could be formed on the CTBC coatings during the SBF incubation, which enhances the nucleation and growth of apatite. In addition, the lattice configuration of specific crystal plane of perovskite CaTiO3 is likely to provide good sites for the epitaxial adsorption of Ca and P from the SBF, which promotes the nucleation and growth of apatite. References [1] M. Wang, Biomaterials 24 (2003) 2133. [2] L.L. Hench, J. Am. Ceram. Soc. 81 (7) (1998) 1705. [3] J.M. Gomez-V, E. Saiz, A.P. Tomsia, T. Oku, K. Suganuma, G.W. Marshall, S.J. Marshall, Adv. Mater. 12 (2000) 894. [4] C.Q. Ning, Y. Zhou, Biomaterials 23 (2002) 2909. [5] X.Y. Liu, K.C. Paul, C.X. Ding, Mater. Sci. Eng., R. 47 (2004) 49. [6] X.B. Zheng, M.H. Huang, C.X. Ding, Biomaterials 21 (2000) 841. [7] Q.Y. Zhang, Y. Leng, R.L. Xin, Biomaterials 26 (2005) 2857. [8] E. Milella, F. Cosentino, A. Licciulli, C. Massaro, Biomaterials 22 (2001) 1425. [9] A. Cohen, R.C. Bradt, G.S. Ansell, J. Am. Ceram. Soc. 53 (7) (1970) 396. [10] L.M. Troilo, D. Damjanovic, R.E. Newnham, J. Am. Ceram. Soc. 77 (3) (1994) 857. [11] F.M. Figueiredo, V.V. Kharton, J.C. Waerenborgh, A.P. Viskup, E.N. Naumovich, J.R. Frade, J. Am. Ceram. Soc. 87 (12) (2004) 2252. [12] X.F. Xiao, R.F. Liu, Y.Z. Zheng, Surf. Coat. Technol. 200 (2006) 4406.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
H. Li, K.A. Khor, P. Cheang, Biomaterials 24 (2003) 949. H. Li, K.A. Khor, P. Cheang, Biomaterials 23 (2002) 85. P. Zhou, M. Akao, Bio-Med. Mater. Eng. 1 (1997) 67. Y. Ohba, T. Watanabe, E. Sakai, M. Daimon, J. Ceram. Soc. Jpn. 107 (1999) 907. S. Kačiulis, G. Mattogno, L. Pandolfi, M. Cavalli, G. Gnappi, A. Montenero, Appl. Surf. Sci. 151 (1999) 1. J. Coreño, O. Coreño, J. Biomed. Mater. Res A 75 (2005) 478. M. Manso, M. Langlet, J.M. Martínez-Duart, Mater. Sci. Eng., C 23 (2003) 447. S. Holliday, A. Stanishevsky, Surf. Coat. Technol. 188–189 (2004) 741. N. Ohtsu, K. Sato, K. Saito, T. Hanawa, K. Asami, Mater. Trans. 45 (2004) 1778. A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. B.C. Yang, M. Uchida, H.-M. Kim, X.D. Zhang, T. Kokubo, Biomaterials 25 (2004) 1003. Y. Han, S.-H. Hong, K.W. Xu, Surf. Coat. Tech. 168 (2003) 249. W.-H. Song, H.S. Ryu, S.-H. Hong, J. Am. Ceram. Soc. 88 (2005) 2642. A. Oyane, H.-M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, T. Nakamura, J. Biomed. Mater. Res. A 65 (2003) 188. L. Müller, F.A. Müller, Acta. Biomater. 2 (2006) 181. S. Koutsopoulos, J. Biomed. Mater. Res. 62 (2002) 600. G.K. Toworfe, R.J. Composto, I.M. Shapiro, P. Ducheyne, Biomaterials 27 (2006) 631. M. Tanahashi, T. Matsuda, J. Biomed. Mater. Res. 34 (1997) 305. C.B. Mao, H.D. Li, F.Z. Cui, Q.L. Feng, C.L. Ma, J. Mater. Chem. 9 (1999) 2573. J.-M. Wu, S. Hayakawa, K. Tsuru, A. Osaka, J. Am. Ceram. Soc. 87 (9) (2004) 1635. M. Uchida, H.-M. Kim, T. Kokubo, S. Fujibayashi, T. Nakamura, J. Biomed. Mater. Res. A 64 (2003) 164. M. Uchida, H.-M. Kim, T. Kokubo, J. Am. Ceram. Soc. 84 (12) (2001) 2969. M. Wei, M. Uchida, H.-M. Kim, T. Kokubo, T. Nakamura, Biomaterials 23 (2002) 167.