- Email: [email protected]
Influence of the hydrothermal temperature and pH on the crystallinity of a sputtered hydroxyapatite film
Applied Surface Science 256 (2010) 7027–7031
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of the hydrothermal temperature and pH on the crystallinity of a sputtered hydroxyapatite film K. Ozeki a,∗ , H. Aoki b , T. Masuzawa a a b
Department of Mechanical Engineering, Ibaraki University, 4-12-1, Nakanarusawa, Hitachi, Ibaraki, 316-8511 Japan International Apatite Co., Ltd., 20 Kanda-Ogawamachi 3-Chome, Chiyoda-ku, Tokyo 101-0052, Japan
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 4 May 2010 Accepted 4 May 2010 Available online 13 May 2010 Keywords: Hydroxyapatite Sputtering Hydrothermal Crystallinity
a b s t r a c t Hydroxyapatite (HA) was coated onto titanium substrates using radio frequency sputtering, and the sputtered films were crystallized under hydrothermal conditions at 110–170 ◦ C at pH values of 7.0 and 9.5. The crystallite size, the remnant film thickness, and the surface morphology of the films were observed using X-ray diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy, respectively. The crystallite size increased with the process temperature, and reached 123.6 nm (pH 9.5 and 170 ◦ C) after 24 h. All of the crystallite sizes of the film treated at pH 9.5 were higher than those treated at pH 7.0 at each process temperature. The film treated at pH 9.5 retained more than 90% of the initial film thickness at any process temperature. The ratio of the film treated at pH 7.0 did not reached 90% at less than 150 ◦ C, and tended to increase with the process temperature. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydroxyapatite (Ca10 (PO4 )6 (OH)2 ; HA) is known to be a ceramics that has uses in medical science because of its excellent osteocompatibility [1]. A composite of titanium and HA has been developed for dental implants and artificial joints. The plasma spraying technique is often used for this purpose, but the resulting HA coating is brittle because the coating is more than 50 m in thickness and has a low density [2–5]. As an alternative method, a radio frequency (RF) sputtering technique has been investigated to obtain thin HA films less than 1 m thick [6–9]. However, the sputtered HA film has low crystallinity, which accelerates the rate of dissolution of the HA film in the living body [6,10,11]. The sputtered HA film can be crystallized using a heat treatment to reduce the dissolution. However, the crystallization of HA requires a high temperature, over 600 ◦ C in air [12]. This high temperature heat treatment leads to cracks forming between the film and the titanium substrate because of differences in thermal expansion and the formation of TiO2 [13]. Therefore, the film must be crystallized at a low temperature. A hydrothermal technique has been used to synthesize HA crystals at less than 350 ◦ C [14,15]. In our previous study, it was reported that the sputtered HA film could be crystallized at 110 ◦ C in distilled water using a hydrothermal technique, without degradation of the film. The crystallized film showed higher bone bonding strength than the plasma spray
∗ Corresponding author. Tel.: +81 294 38 5040, fax: +81 294 38 5047. E-mail address: [email protected] (K. Ozeki). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.05.018
coating in animal test [16]. It was also clarified that an increase of pH value of the hydrothermal solution contributed to suppress dissolution of the film during the hydrothermal treatment [17]. However, the effect of the various hydrothermal conditions on the crystallinity of the HA film has not been elucidated yet. The accurate control of the crystallinity of the sputtered HA film is important to apply the film to orthopedic devices because nanosized HA crystals (corresponding to low crystallinity) have been reported to induce quickly a new bone formation at an early stage after implantation [18–20]. At the same time, the low crystallinity leads to the quick disappearance of the film [10,11]. This suggests that the crystallinity should be optimized for the medical application. In the present study, the sputtered film was crystallized at varying temperature and pH during the hydrothermal treatment to investigate the detail relation between the crystallinity and the hydrothermal conditions. 2. Materials and methods 2.1. Preparation of HA films on titanium The substrates used were titanium plates (10 mm × 10 mm × 1 mm). A ten-inch HA disc (>99.5%; Koujundo Chemical Laboratory Corp., Japan) was used for the sputtering target. RF magnetron sputtering was carried out using an SPF-410H (Canon Anelva Corp.) chamber. The distance between the target and the substrate was about 150 mm. The target was water-cooled during the sputtering process. The sputtering chamber was evac-
7028
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031
uated to a pressure below 1 × 10−5 Pa using a cryopump. Ar gas (99.9999%) was then introduced into the chamber using a mass flow controller. Before deposition took place, the substrate was covered with a shield, and the target was pre-sputtered with Ar ions for 10 min. The coating procedure was carried out at an Ar pressure of 0.2 Pa and a discharge power of 400 W. 2.2. Hydrothermal treatment Hydrothermal treatment was carried out in distilled water at 100–170 ◦ C and 0.1–0.79 MPa using an autoclave (TEM-D1500M; Taiatsu Techno Corp., Japan). The pH of the distilled water used as the hydrothermal solution was adjusted to 7.0 or 9.5 with NaOH. Four HA coated plates were placed in 500 ml of the distilled water at each hydrothermal condition. The treatment times selected were 6, 12, and 24 h. After the hydrothermal treatment, each sample was identified using X-ray diffraction (XRD; RINT2000; Rigaku Corp.) with a Cu K␣ radiation source operating at 40 kV and a 30 mA excitation current. The crystallite size (D0 0 2 ) of the film was calculated from the 0 0 2 reflections of the XRD patterns using Scherrer’s equation. The crystallite size was determined from an average of four samples. Surface observation of the films was carried out using scanning electron microscopy (SEM; JSM-5600LVB; JEOL) with an accelerating voltage of 20 kV. The Ca, P, and Ti atomic ratio of the samples were also analysed before and after the hydrothermal treatment using energy dispersive X-ray spectroscopy (EDX; JSX-3600M; JEOL) with an accelerating voltage of 30 kV. The film thickness was calculated from a calibration curve of the Ca/Ti ratio versus film thickness. The calibration curve had been prepared in advance by measuring some HA films with varying film thickness using EDX and a surface rough-
Fig. 1. Scanning electron micrograph of the as-sputtered film.
ness tester. The percentage of the remnant film thickness ratio was calculated by dividing the remnant film thickness by the initial film thickness. The initial film thickness on all films was 1.0–1.2 m. 3. Results 3.1. SEM observation of sputtered films before and after hydrothermal treatment Fig. 1 shows a scanning electron micrograph of the as-sputtered film. Any crystal was not observed on a smooth surface. After the treatment, the surface morphology was drastically changed (Fig. 2). The thin crystals with long needles were seen in all samples treated
Fig. 2. Scanning electron micrograph of the sputtered films after the hydrothermal treatment at pH 7.0.
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031
7029
Fig. 3. Scanning electron micrograph of the sputtered films after the hydrothermal treatment at pH 9.5.
at pH 7.0. The length of needles increased with the process temperature, and an increase of the width of the needle was accompanied by an increase of the process time. Generally, HA crystals preferentially grew along the c-axis early in the hydrothermal reaction, then grew along the a-axis, which corresponds to the HA crystals growing from needle-like shapes to flat-hexagonal shapes [21,22]. In all films treated at pH 9.5, no correlation was observed between the hydrothermal conditions (pH and temperature) and the surface morphology of the film (Fig. 3). The film treated at pH 9.5 also showed thicker needles than the film treated at pH 7.0.
and 170 ◦ C. All crystallite sizes of the film treated at pH 9.5 were larger than those treated at pH 7.0 at each process temperature. This can be explained by the pH dependence of the HA solubility. The solubility of HA decreases as the pH of the solution increases [23]. A driving force of crystal growth is the degree of supersaturation, which increases with pH. Consequently, the HA crystal growth rate increases with pH. The crystallite sizes were independent of the process time at each temperature at pH 7.0, whereas the sizes tended to increase with the process time at pH 9.5.
3.2. Crystallinity change of the HA after hydrothermal treatment
3.3. Thickness of the sputtered films after hydrothermal treatment
Fig. 4 shows the XRD patterns of the as-sputtered film and a sputtered film after the hydrothermal treatment. Before the treatment, a broad peak was observed around 2 = 30◦ , which corresponds to non-crystalline HA. Three strong titanium peaks were also observed at 2 = 35.1◦ , 38.4◦ , and 40.1◦ (Fig. 4(a)). After the hydrothermal treatment, HA peaks appeared (). In pH 7.0, a peak at 2 = 25.8◦ corresponding to the (0 0 2) plane, is relatively high, comparing with the standard XRD pattern of HA (JCPDS No. 090432) (Fig. 4(b)–(d)). This suggests that the crystal preferentially grew along the c-axis, which agrees with the SEM observation. In pH 9.5, each peak balance came to that of the standard XRD pattern of HA (Fig. 4(e)–(g)). The crystallite size (D0 0 2 ) can be calculated from the 0 0 2 reflections at 2 = 25.8◦ using Scherrer’s equation. Fig. 5 shows the crystallite size of the sputtered film after the hydrothermal treatment. In all samples, the crystallite sizes increased with the process temperature. The average crystallite size reached 123.6 nm after 24 h of hydrothermal treatment at pH 9.5
Fig. 6 shows the ratio of the remnant film thickness to the initial film thickness after the hydrothermal treatment. The ratio of the film treated at pH 9.5 was consistently higher than 90% at any process temperature. At pH 7.0, the ratio did not reached 90% at less than 150 ◦ C, and the remnant film thickness tended to increase with the process temperature. This result indicates that the pH of the solution greatly affected the solubility of the film as mentioned in Section 3.2. The thickness ratio of the remnant film treated at pH 7.0 decreased with the process time because the high solubility condition may precede dissolution of the film with increase of the process time. 4. Discussion Hydrothermal treatment is a useful method for the synthesis of inorganic materials. The method can increase the crystallite size of HA [24–26]. At the same time, the control of the crystallinity of the sputtered HA film is important because the crystallinity affects
7030
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031
Fig. 6. Remnant film thickness ratio as a function of the process temperature.
Fig. 4. XRD patterns of: (a) the as-sputtered film and (b)–(g) the sputtered film after the hydrothermal treatment. Key: (): HA and Ti: titanium substrate.
the solubility and the osteocompatibility of the film. In the present study, we focused on the crystallinity of the sputtered HA film under various conditions of the hydrothermal treatment. In Figs. 5 and 6, the crystallite sizes of the HA film and the remnant film thickness ratio increased with the process temperature.
The low solubility of HA leads to an increase in the HA crystal growth because a driving force of crystal growth is the degree of supersaturation. Therefore the solubility of HA decreases when the process temperature is increased. McDowell et al. reported on the influence of the solution temperature on the HA solubility during the hydrothermal treatment [27]. They suggested that the relationship between the HA solubility product (pKs) and the process temperature (T) as can be described by the following equation: Ks = −
8219.41 − 1.6657 − 0.098215T T
From this equation, Ks is 4.55 × 10−61 at 100 ◦ C and 1.83 × 10−64 at 170 ◦ C. An increase of 70 ◦ C causes Ks to decrease by three orders of magnitude. The pH value also influences the crystallinity on the HA film. The solubility product of HA is not dependent on the pH value [28,29]. The ion activity changes depending on the pH value. Larsen et al. reported that the ion activity product of HA (pI) changes lineally with pH value as follows [30]: pI = 63.1 − 1.42pH This equation indicates that the pI decreases as the pH increases. The saturation index (SI) can be calculated from the following equation: SI
Fig. 5. Crystallite size of the sputtered films as a function of the process temperature.
= log(I/Ks) = log I − log Ks = pKs − pI
The SI is the degree of saturation of the solution relative to the solute. An increase in the SI leads to an increase in precipitation of HA. Decrease of the pI leads to an increase in the SI, which corresponds to an increase of the supersaturation of the solution with respect to HA. From these equations, the SI dependence for the conditions tested in this experiment can be calculated as seen in Table 1. All of the SI values at pH 9.5 are higher than the SI values at pH 7.0. This suggests that the pH value (7.0 or 9.5) greatly influences the crystallite size of the HA and the remnant film thickness. The prediction agreed with the result in Fig. 6. In Fig. 6, the film treated
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031 Table 1 Calculated saturation indices (SI) as a function of the hydrothermal conditions. Process temperature (◦ C)
pH 7.0 pH 9.5
100
110
120
130
140
150
160
170
7.18 10.7
7.59 11.1
8.03 11.6
8.49 12.0
8.98 12.5
9.49 13.0
10.0 13.6
10.6 14.1
at pH 9.5 retained more than 90% of the initial film thickness at any process temperature, and the ratio of the film treated at pH 7.0 did not reached 90% at less than 150 ◦ C. This result indicates that the pH value has a greater influence on the remnant film thickness ratio than the process temperature does. In the SEM observation, the film treated at pH 7.0 showed the crystal growth increased with the process temperature (Fig. 2). This trend is consistent with the prediction from Table 1. However, the film treated at pH 9.5 showed similar sizes of the crystals at all process temperatures and times (Fig. 3). This result is inconsistent with the prediction from Table 1. This inconsistency can be explained by a difference in the amount of information that was obtained from the XRD and SEM analyses. The crystallinity analysed by XRD was from the entire film, whereas the SEM observation provided information for only the film surface. The surface morphology of the film did not show the crystallinity of the entire film layer. The XRD analysis might be more useful than the SEM analysis for the determination of the film crystallinity. The above results suggest that the crystallinity of the sputtered film can be changed by the altering the hydrothermal conditions such as the pH and temperature. These findings contribute to the control of the dissolution rate and osteocompatibility of the sputtered HA film in a living body. 5. Conclusions The influence of the process temperature and pH on the crystallinity and the remnant film thickness of the sputtered HA film under hydrothermal treatment was investigated. The following conclusions were reached: • The SEM observations showed that the HA crystal growth tended to increase with the process temperature at pH 7.0, whereas the film treated at pH 9.5 showed similar crystal growth at all process temperatures and times. • The XRD patterns showed that the HA crystallite size increased with the process temperature at both pH values (7.0 and 9.5), and reached 123.6 nm (pH 9.5 and 170 ◦ C) after 24 h. All crystallite sizes of the film treated at pH 9.5 were higher than those treated at pH 7.0. • The film treated at pH 9.5 retained more than 90% of the initial film thickness at any process temperature. The ratio of the film treated at pH 7.0 did not reached 90% at less than 150 ◦ C, and tended to increase with the process temperature. References [1] H. Aoki, Medical Applications of Hydroxyapatite, Ishiyaku EuroAmerica, Inc., St. Louis, 1994.
7031
[2] K. Hayashi, T. Inadome, H. Tsumura, Y. Nakashima, Y. Sugioka, Effect of surface roughness of hydroxyapatite-coated titanium on the bone–implant interface shear strength, Biomaterials 15 (1994) 1187–1191. [3] M. Ogiso, Y. Yamashita, T. Mastumoto, Comparative push-out test of dense HA implants and HA-coated implants: findings in a canine study, J. Biomed. Mater. Res. 39 (1998) 364–372. [4] G. Lewis, Hydroxyapatite-coated bioalloy surfaces: current status and future challenges, Bio-Med. Mater. Eng. 10 (2000) 157–188. [5] Y. Cao, J. Weng, J. Chen, J. Feng, Z. Yang, X. Zhang, Water vapour-treated hydroxyapatite coatings after plasma spraying and their characteristics, Biomaterials 17 (1996) 419–424. [6] K. Ozeki, T. Yuhta, H. Aoki, I. Nishimura, Y. Fukui, Crystal chemistry of hydroxyapatite deposited on titanium by sputtering technique, Bio-Med. Mater. Eng. 10 (2000) 221–227. [7] K. Ozeki, T. Yuhta, H. Aoki, I. Nishimura, Y. Fukui, Push-out strength of hydroxyapatite coated by sputtering technique in bone, Bio-Med. Mater. Eng. 11 (2001) 63–68. [8] J.G.C. Wolke, K. de Groot, J.A. Jansen, In vivo dissolution behavior of various RF magnetron sputtered Ca–P coatings, J. Biomed. Mater. Res. 39 (1998) 524–530. [9] M. Yoshinari, T. Hayakawa, J.G.C. Wolke, K. Nemoto, J.A. Jansen, Influence of rapid heating with infrared radiation on RF magnetron-sputtered calcium phosphate coatings, J. Biomed. Mater. Res. 37 (1997) 60–67. [10] K. Ozeki, T. Yuhta, H. Aoki, I. Nishimura, Y. Fukui, A functionally graded titanium/hydroxyapatite film obtained by sputtering, J. Mater. Sci. Mater. Med. 13 (2002) 253–258. [11] E.M. Burke, J.D. Haman, J.J. Weimer, A.B. Cheney, J.M. Rigsbee, L.C. Lucas, Influence of coating strain on calcium phosphate thin-film dissolution, J. Biomed. Mater. Res. 57 (2001) 41–47. [12] K. van Dijk, H.G. Schaeken, J.G.C. Wolke, J.A. Jansen, Influence of annealing temperature on RF magnetron sputtered calcium phosphate coatings, Biomaterials 17 (1996) 405–410. [13] L.C. Lucas, W.R. Lacefield, J.L. Ong, R.Y. Whitehead, Calcium phosphate coatings for medical and dental implants, Colloids Surf. A: Physiochem. Eng. Aspects 77 (1993) 141–147. [14] S. Ban, S. Maruo, Hydrothermal–electrochemical deposition of hydroxyapatite, J. Biomed. Mater. Res. 42 (1998) 387–395. [15] R.E. Riman, W.L. Suchanek, K. Byrappa, C.W. Chen, P. Shuk, C.S. Oakes, Solution synthesis of hydroxyapatite designer particulates, Solid State Ionics 151 (2002) 393–402. [16] K. Ozeki, A. Mishima, T. Yuhta, Y. Fukui, H. Aoki, Bone bonding strength of sputtered hydroxyapatite films subjected to a low temperature hydrothermal treatment, Bio-Med. Mater. Eng. 13 (2003) 451–463. [17] K. Ozeki, H. Aoki, Y. Fukui, Effect of pH on crystallization of sputtered hydroxyapatite film under hydrothermal conditions at low temperature, J. Mater. Sci. 40 (2005) 2837–2842. [18] T. Li, J. Lee, T. Kobayashi, H. Aoki, Hydroxyapatite coating by dipping method, and bone bonding strength, J. Mater. Sci.: Mater. Med. 7 (1996) 355–357. [19] T. Li, M. Akao, M. Takagi, Tissue reaction of hydroxyapatite sol to rat molar pulp, J. Mater. Sci.: Mater. Med. 9 (1998) 631–642. [20] T.J. Webster, R.W. Siegel, R. Bizios, Enhanced functions of osteoblasts on nanophase ceramics, Biomaterials 21 (2000) 1803–1810. [21] N. Kanzaki, K. Onuma, A. Ito, K. Teraoka, T. Tateishi, S. Tsutsumi, Direct growth rate measurement of hydroxyapatite single crystal by moiré phase shift interferometry, J. Phys. Chem. B 102 (1998) 6471–6476. [22] K. Sato, T. Kogure, Y. Kumagai, J. Tanaka, Crystal orientation of hydroxyapatite induced by ordered carboxyl groups, J. Colloid Interface Sci. 240 (2001) 133–138. [23] J.C. Elliot, Structure and Chemistry of Apatites and Other Calcium Orthophosphates, Elsevier Science Publishers, Amsterdam, 1994. [24] M. Mengeot, M.L. Harvill, O.R. Gilliam, Hydrothermal growth of calcium hydroxyapatite single crystals, J. Cryst. Growth 19 (1973) 199–203. [25] J.F. Kirn, H. Leidheiser, Progress in efforts to grow large single crystals of hydroxyapatite, J. Cryst. Growth 2 (1968) 111–112. [26] S. Ban, J. Hasegawa, Morphological regulation and crystal growth of hydrothermal–electrochemically deposited apatite, Biomaterials 23 (2002) 2965–2972. [27] H. McDowell, T.M. Gregory, W.E. Brown, Solubility of Ca5 (PO4 )3 OH in the system Ca(OH)2 –H3 PO4 –H2 O at 5, 15, 25, and 37 ◦ C, J. Res. Nat. Bur. Stand. 81 (1977) 273–281. [28] L.C. Bell, H. Mika, B.J. Kruger, Synthetic hydroxyapatite-solubility product and stoichiometry of dissolution, Arch. Oral Biol. 23 (1978) 329–336. [29] W.F. Jaynes, P.A. Moore Jr., D.M. Miller, Solubility and ion activity product of calcium phosphate minerals, J. Eviron. Qual. 28 (1999) 530–536. [30] M.J. Larsen, S.J. Jensen, The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6–7.6 at 20 ◦ C, Arch. Oral Biol. 34 (1989) 957–961.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of the hydrothermal temperature and pH on the crystallinity of a sputtered hydroxyapatite film K. Ozeki a,∗ , H. Aoki b , T. Masuzawa a a b
Department of Mechanical Engineering, Ibaraki University, 4-12-1, Nakanarusawa, Hitachi, Ibaraki, 316-8511 Japan International Apatite Co., Ltd., 20 Kanda-Ogawamachi 3-Chome, Chiyoda-ku, Tokyo 101-0052, Japan
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 4 May 2010 Accepted 4 May 2010 Available online 13 May 2010 Keywords: Hydroxyapatite Sputtering Hydrothermal Crystallinity
a b s t r a c t Hydroxyapatite (HA) was coated onto titanium substrates using radio frequency sputtering, and the sputtered films were crystallized under hydrothermal conditions at 110–170 ◦ C at pH values of 7.0 and 9.5. The crystallite size, the remnant film thickness, and the surface morphology of the films were observed using X-ray diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy, respectively. The crystallite size increased with the process temperature, and reached 123.6 nm (pH 9.5 and 170 ◦ C) after 24 h. All of the crystallite sizes of the film treated at pH 9.5 were higher than those treated at pH 7.0 at each process temperature. The film treated at pH 9.5 retained more than 90% of the initial film thickness at any process temperature. The ratio of the film treated at pH 7.0 did not reached 90% at less than 150 ◦ C, and tended to increase with the process temperature. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydroxyapatite (Ca10 (PO4 )6 (OH)2 ; HA) is known to be a ceramics that has uses in medical science because of its excellent osteocompatibility [1]. A composite of titanium and HA has been developed for dental implants and artificial joints. The plasma spraying technique is often used for this purpose, but the resulting HA coating is brittle because the coating is more than 50 m in thickness and has a low density [2–5]. As an alternative method, a radio frequency (RF) sputtering technique has been investigated to obtain thin HA films less than 1 m thick [6–9]. However, the sputtered HA film has low crystallinity, which accelerates the rate of dissolution of the HA film in the living body [6,10,11]. The sputtered HA film can be crystallized using a heat treatment to reduce the dissolution. However, the crystallization of HA requires a high temperature, over 600 ◦ C in air [12]. This high temperature heat treatment leads to cracks forming between the film and the titanium substrate because of differences in thermal expansion and the formation of TiO2 [13]. Therefore, the film must be crystallized at a low temperature. A hydrothermal technique has been used to synthesize HA crystals at less than 350 ◦ C [14,15]. In our previous study, it was reported that the sputtered HA film could be crystallized at 110 ◦ C in distilled water using a hydrothermal technique, without degradation of the film. The crystallized film showed higher bone bonding strength than the plasma spray
∗ Corresponding author. Tel.: +81 294 38 5040, fax: +81 294 38 5047. E-mail address: [email protected] (K. Ozeki). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.05.018
coating in animal test [16]. It was also clarified that an increase of pH value of the hydrothermal solution contributed to suppress dissolution of the film during the hydrothermal treatment [17]. However, the effect of the various hydrothermal conditions on the crystallinity of the HA film has not been elucidated yet. The accurate control of the crystallinity of the sputtered HA film is important to apply the film to orthopedic devices because nanosized HA crystals (corresponding to low crystallinity) have been reported to induce quickly a new bone formation at an early stage after implantation [18–20]. At the same time, the low crystallinity leads to the quick disappearance of the film [10,11]. This suggests that the crystallinity should be optimized for the medical application. In the present study, the sputtered film was crystallized at varying temperature and pH during the hydrothermal treatment to investigate the detail relation between the crystallinity and the hydrothermal conditions. 2. Materials and methods 2.1. Preparation of HA films on titanium The substrates used were titanium plates (10 mm × 10 mm × 1 mm). A ten-inch HA disc (>99.5%; Koujundo Chemical Laboratory Corp., Japan) was used for the sputtering target. RF magnetron sputtering was carried out using an SPF-410H (Canon Anelva Corp.) chamber. The distance between the target and the substrate was about 150 mm. The target was water-cooled during the sputtering process. The sputtering chamber was evac-
7028
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031
uated to a pressure below 1 × 10−5 Pa using a cryopump. Ar gas (99.9999%) was then introduced into the chamber using a mass flow controller. Before deposition took place, the substrate was covered with a shield, and the target was pre-sputtered with Ar ions for 10 min. The coating procedure was carried out at an Ar pressure of 0.2 Pa and a discharge power of 400 W. 2.2. Hydrothermal treatment Hydrothermal treatment was carried out in distilled water at 100–170 ◦ C and 0.1–0.79 MPa using an autoclave (TEM-D1500M; Taiatsu Techno Corp., Japan). The pH of the distilled water used as the hydrothermal solution was adjusted to 7.0 or 9.5 with NaOH. Four HA coated plates were placed in 500 ml of the distilled water at each hydrothermal condition. The treatment times selected were 6, 12, and 24 h. After the hydrothermal treatment, each sample was identified using X-ray diffraction (XRD; RINT2000; Rigaku Corp.) with a Cu K␣ radiation source operating at 40 kV and a 30 mA excitation current. The crystallite size (D0 0 2 ) of the film was calculated from the 0 0 2 reflections of the XRD patterns using Scherrer’s equation. The crystallite size was determined from an average of four samples. Surface observation of the films was carried out using scanning electron microscopy (SEM; JSM-5600LVB; JEOL) with an accelerating voltage of 20 kV. The Ca, P, and Ti atomic ratio of the samples were also analysed before and after the hydrothermal treatment using energy dispersive X-ray spectroscopy (EDX; JSX-3600M; JEOL) with an accelerating voltage of 30 kV. The film thickness was calculated from a calibration curve of the Ca/Ti ratio versus film thickness. The calibration curve had been prepared in advance by measuring some HA films with varying film thickness using EDX and a surface rough-
Fig. 1. Scanning electron micrograph of the as-sputtered film.
ness tester. The percentage of the remnant film thickness ratio was calculated by dividing the remnant film thickness by the initial film thickness. The initial film thickness on all films was 1.0–1.2 m. 3. Results 3.1. SEM observation of sputtered films before and after hydrothermal treatment Fig. 1 shows a scanning electron micrograph of the as-sputtered film. Any crystal was not observed on a smooth surface. After the treatment, the surface morphology was drastically changed (Fig. 2). The thin crystals with long needles were seen in all samples treated
Fig. 2. Scanning electron micrograph of the sputtered films after the hydrothermal treatment at pH 7.0.
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031
7029
Fig. 3. Scanning electron micrograph of the sputtered films after the hydrothermal treatment at pH 9.5.
at pH 7.0. The length of needles increased with the process temperature, and an increase of the width of the needle was accompanied by an increase of the process time. Generally, HA crystals preferentially grew along the c-axis early in the hydrothermal reaction, then grew along the a-axis, which corresponds to the HA crystals growing from needle-like shapes to flat-hexagonal shapes [21,22]. In all films treated at pH 9.5, no correlation was observed between the hydrothermal conditions (pH and temperature) and the surface morphology of the film (Fig. 3). The film treated at pH 9.5 also showed thicker needles than the film treated at pH 7.0.
and 170 ◦ C. All crystallite sizes of the film treated at pH 9.5 were larger than those treated at pH 7.0 at each process temperature. This can be explained by the pH dependence of the HA solubility. The solubility of HA decreases as the pH of the solution increases [23]. A driving force of crystal growth is the degree of supersaturation, which increases with pH. Consequently, the HA crystal growth rate increases with pH. The crystallite sizes were independent of the process time at each temperature at pH 7.0, whereas the sizes tended to increase with the process time at pH 9.5.
3.2. Crystallinity change of the HA after hydrothermal treatment
3.3. Thickness of the sputtered films after hydrothermal treatment
Fig. 4 shows the XRD patterns of the as-sputtered film and a sputtered film after the hydrothermal treatment. Before the treatment, a broad peak was observed around 2 = 30◦ , which corresponds to non-crystalline HA. Three strong titanium peaks were also observed at 2 = 35.1◦ , 38.4◦ , and 40.1◦ (Fig. 4(a)). After the hydrothermal treatment, HA peaks appeared (). In pH 7.0, a peak at 2 = 25.8◦ corresponding to the (0 0 2) plane, is relatively high, comparing with the standard XRD pattern of HA (JCPDS No. 090432) (Fig. 4(b)–(d)). This suggests that the crystal preferentially grew along the c-axis, which agrees with the SEM observation. In pH 9.5, each peak balance came to that of the standard XRD pattern of HA (Fig. 4(e)–(g)). The crystallite size (D0 0 2 ) can be calculated from the 0 0 2 reflections at 2 = 25.8◦ using Scherrer’s equation. Fig. 5 shows the crystallite size of the sputtered film after the hydrothermal treatment. In all samples, the crystallite sizes increased with the process temperature. The average crystallite size reached 123.6 nm after 24 h of hydrothermal treatment at pH 9.5
Fig. 6 shows the ratio of the remnant film thickness to the initial film thickness after the hydrothermal treatment. The ratio of the film treated at pH 9.5 was consistently higher than 90% at any process temperature. At pH 7.0, the ratio did not reached 90% at less than 150 ◦ C, and the remnant film thickness tended to increase with the process temperature. This result indicates that the pH of the solution greatly affected the solubility of the film as mentioned in Section 3.2. The thickness ratio of the remnant film treated at pH 7.0 decreased with the process time because the high solubility condition may precede dissolution of the film with increase of the process time. 4. Discussion Hydrothermal treatment is a useful method for the synthesis of inorganic materials. The method can increase the crystallite size of HA [24–26]. At the same time, the control of the crystallinity of the sputtered HA film is important because the crystallinity affects
7030
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031
Fig. 6. Remnant film thickness ratio as a function of the process temperature.
Fig. 4. XRD patterns of: (a) the as-sputtered film and (b)–(g) the sputtered film after the hydrothermal treatment. Key: (): HA and Ti: titanium substrate.
the solubility and the osteocompatibility of the film. In the present study, we focused on the crystallinity of the sputtered HA film under various conditions of the hydrothermal treatment. In Figs. 5 and 6, the crystallite sizes of the HA film and the remnant film thickness ratio increased with the process temperature.
The low solubility of HA leads to an increase in the HA crystal growth because a driving force of crystal growth is the degree of supersaturation. Therefore the solubility of HA decreases when the process temperature is increased. McDowell et al. reported on the influence of the solution temperature on the HA solubility during the hydrothermal treatment [27]. They suggested that the relationship between the HA solubility product (pKs) and the process temperature (T) as can be described by the following equation: Ks = −
8219.41 − 1.6657 − 0.098215T T
From this equation, Ks is 4.55 × 10−61 at 100 ◦ C and 1.83 × 10−64 at 170 ◦ C. An increase of 70 ◦ C causes Ks to decrease by three orders of magnitude. The pH value also influences the crystallinity on the HA film. The solubility product of HA is not dependent on the pH value [28,29]. The ion activity changes depending on the pH value. Larsen et al. reported that the ion activity product of HA (pI) changes lineally with pH value as follows [30]: pI = 63.1 − 1.42pH This equation indicates that the pI decreases as the pH increases. The saturation index (SI) can be calculated from the following equation: SI
Fig. 5. Crystallite size of the sputtered films as a function of the process temperature.
= log(I/Ks) = log I − log Ks = pKs − pI
The SI is the degree of saturation of the solution relative to the solute. An increase in the SI leads to an increase in precipitation of HA. Decrease of the pI leads to an increase in the SI, which corresponds to an increase of the supersaturation of the solution with respect to HA. From these equations, the SI dependence for the conditions tested in this experiment can be calculated as seen in Table 1. All of the SI values at pH 9.5 are higher than the SI values at pH 7.0. This suggests that the pH value (7.0 or 9.5) greatly influences the crystallite size of the HA and the remnant film thickness. The prediction agreed with the result in Fig. 6. In Fig. 6, the film treated
K. Ozeki et al. / Applied Surface Science 256 (2010) 7027–7031 Table 1 Calculated saturation indices (SI) as a function of the hydrothermal conditions. Process temperature (◦ C)
pH 7.0 pH 9.5
100
110
120
130
140
150
160
170
7.18 10.7
7.59 11.1
8.03 11.6
8.49 12.0
8.98 12.5
9.49 13.0
10.0 13.6
10.6 14.1
at pH 9.5 retained more than 90% of the initial film thickness at any process temperature, and the ratio of the film treated at pH 7.0 did not reached 90% at less than 150 ◦ C. This result indicates that the pH value has a greater influence on the remnant film thickness ratio than the process temperature does. In the SEM observation, the film treated at pH 7.0 showed the crystal growth increased with the process temperature (Fig. 2). This trend is consistent with the prediction from Table 1. However, the film treated at pH 9.5 showed similar sizes of the crystals at all process temperatures and times (Fig. 3). This result is inconsistent with the prediction from Table 1. This inconsistency can be explained by a difference in the amount of information that was obtained from the XRD and SEM analyses. The crystallinity analysed by XRD was from the entire film, whereas the SEM observation provided information for only the film surface. The surface morphology of the film did not show the crystallinity of the entire film layer. The XRD analysis might be more useful than the SEM analysis for the determination of the film crystallinity. The above results suggest that the crystallinity of the sputtered film can be changed by the altering the hydrothermal conditions such as the pH and temperature. These findings contribute to the control of the dissolution rate and osteocompatibility of the sputtered HA film in a living body. 5. Conclusions The influence of the process temperature and pH on the crystallinity and the remnant film thickness of the sputtered HA film under hydrothermal treatment was investigated. The following conclusions were reached: • The SEM observations showed that the HA crystal growth tended to increase with the process temperature at pH 7.0, whereas the film treated at pH 9.5 showed similar crystal growth at all process temperatures and times. • The XRD patterns showed that the HA crystallite size increased with the process temperature at both pH values (7.0 and 9.5), and reached 123.6 nm (pH 9.5 and 170 ◦ C) after 24 h. All crystallite sizes of the film treated at pH 9.5 were higher than those treated at pH 7.0. • The film treated at pH 9.5 retained more than 90% of the initial film thickness at any process temperature. The ratio of the film treated at pH 7.0 did not reached 90% at less than 150 ◦ C, and tended to increase with the process temperature. References [1] H. Aoki, Medical Applications of Hydroxyapatite, Ishiyaku EuroAmerica, Inc., St. Louis, 1994.
7031
[2] K. Hayashi, T. Inadome, H. Tsumura, Y. Nakashima, Y. Sugioka, Effect of surface roughness of hydroxyapatite-coated titanium on the bone–implant interface shear strength, Biomaterials 15 (1994) 1187–1191. [3] M. Ogiso, Y. Yamashita, T. Mastumoto, Comparative push-out test of dense HA implants and HA-coated implants: findings in a canine study, J. Biomed. Mater. Res. 39 (1998) 364–372. [4] G. Lewis, Hydroxyapatite-coated bioalloy surfaces: current status and future challenges, Bio-Med. Mater. Eng. 10 (2000) 157–188. [5] Y. Cao, J. Weng, J. Chen, J. Feng, Z. Yang, X. Zhang, Water vapour-treated hydroxyapatite coatings after plasma spraying and their characteristics, Biomaterials 17 (1996) 419–424. [6] K. Ozeki, T. Yuhta, H. Aoki, I. Nishimura, Y. Fukui, Crystal chemistry of hydroxyapatite deposited on titanium by sputtering technique, Bio-Med. Mater. Eng. 10 (2000) 221–227. [7] K. Ozeki, T. Yuhta, H. Aoki, I. Nishimura, Y. Fukui, Push-out strength of hydroxyapatite coated by sputtering technique in bone, Bio-Med. Mater. Eng. 11 (2001) 63–68. [8] J.G.C. Wolke, K. de Groot, J.A. Jansen, In vivo dissolution behavior of various RF magnetron sputtered Ca–P coatings, J. Biomed. Mater. Res. 39 (1998) 524–530. [9] M. Yoshinari, T. Hayakawa, J.G.C. Wolke, K. Nemoto, J.A. Jansen, Influence of rapid heating with infrared radiation on RF magnetron-sputtered calcium phosphate coatings, J. Biomed. Mater. Res. 37 (1997) 60–67. [10] K. Ozeki, T. Yuhta, H. Aoki, I. Nishimura, Y. Fukui, A functionally graded titanium/hydroxyapatite film obtained by sputtering, J. Mater. Sci. Mater. Med. 13 (2002) 253–258. [11] E.M. Burke, J.D. Haman, J.J. Weimer, A.B. Cheney, J.M. Rigsbee, L.C. Lucas, Influence of coating strain on calcium phosphate thin-film dissolution, J. Biomed. Mater. Res. 57 (2001) 41–47. [12] K. van Dijk, H.G. Schaeken, J.G.C. Wolke, J.A. Jansen, Influence of annealing temperature on RF magnetron sputtered calcium phosphate coatings, Biomaterials 17 (1996) 405–410. [13] L.C. Lucas, W.R. Lacefield, J.L. Ong, R.Y. Whitehead, Calcium phosphate coatings for medical and dental implants, Colloids Surf. A: Physiochem. Eng. Aspects 77 (1993) 141–147. [14] S. Ban, S. Maruo, Hydrothermal–electrochemical deposition of hydroxyapatite, J. Biomed. Mater. Res. 42 (1998) 387–395. [15] R.E. Riman, W.L. Suchanek, K. Byrappa, C.W. Chen, P. Shuk, C.S. Oakes, Solution synthesis of hydroxyapatite designer particulates, Solid State Ionics 151 (2002) 393–402. [16] K. Ozeki, A. Mishima, T. Yuhta, Y. Fukui, H. Aoki, Bone bonding strength of sputtered hydroxyapatite films subjected to a low temperature hydrothermal treatment, Bio-Med. Mater. Eng. 13 (2003) 451–463. [17] K. Ozeki, H. Aoki, Y. Fukui, Effect of pH on crystallization of sputtered hydroxyapatite film under hydrothermal conditions at low temperature, J. Mater. Sci. 40 (2005) 2837–2842. [18] T. Li, J. Lee, T. Kobayashi, H. Aoki, Hydroxyapatite coating by dipping method, and bone bonding strength, J. Mater. Sci.: Mater. Med. 7 (1996) 355–357. [19] T. Li, M. Akao, M. Takagi, Tissue reaction of hydroxyapatite sol to rat molar pulp, J. Mater. Sci.: Mater. Med. 9 (1998) 631–642. [20] T.J. Webster, R.W. Siegel, R. Bizios, Enhanced functions of osteoblasts on nanophase ceramics, Biomaterials 21 (2000) 1803–1810. [21] N. Kanzaki, K. Onuma, A. Ito, K. Teraoka, T. Tateishi, S. Tsutsumi, Direct growth rate measurement of hydroxyapatite single crystal by moiré phase shift interferometry, J. Phys. Chem. B 102 (1998) 6471–6476. [22] K. Sato, T. Kogure, Y. Kumagai, J. Tanaka, Crystal orientation of hydroxyapatite induced by ordered carboxyl groups, J. Colloid Interface Sci. 240 (2001) 133–138. [23] J.C. Elliot, Structure and Chemistry of Apatites and Other Calcium Orthophosphates, Elsevier Science Publishers, Amsterdam, 1994. [24] M. Mengeot, M.L. Harvill, O.R. Gilliam, Hydrothermal growth of calcium hydroxyapatite single crystals, J. Cryst. Growth 19 (1973) 199–203. [25] J.F. Kirn, H. Leidheiser, Progress in efforts to grow large single crystals of hydroxyapatite, J. Cryst. Growth 2 (1968) 111–112. [26] S. Ban, J. Hasegawa, Morphological regulation and crystal growth of hydrothermal–electrochemically deposited apatite, Biomaterials 23 (2002) 2965–2972. [27] H. McDowell, T.M. Gregory, W.E. Brown, Solubility of Ca5 (PO4 )3 OH in the system Ca(OH)2 –H3 PO4 –H2 O at 5, 15, 25, and 37 ◦ C, J. Res. Nat. Bur. Stand. 81 (1977) 273–281. [28] L.C. Bell, H. Mika, B.J. Kruger, Synthetic hydroxyapatite-solubility product and stoichiometry of dissolution, Arch. Oral Biol. 23 (1978) 329–336. [29] W.F. Jaynes, P.A. Moore Jr., D.M. Miller, Solubility and ion activity product of calcium phosphate minerals, J. Eviron. Qual. 28 (1999) 530–536. [30] M.J. Larsen, S.J. Jensen, The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6–7.6 at 20 ◦ C, Arch. Oral Biol. 34 (1989) 957–961.