- Email: [email protected]
Apatite formed on the surface of plasma-sprayed wollastonite coating immersed in simulated body fluid
Biomaterials 22 (2001) 2007}2012
Apatite formed on the surface of plasma-sprayed wollastonite coating immersed in simulated body #uid Xuanyong Liu*, Chuanxian Ding, Zhenyao Wang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People+s Republic of China Received 8 June 2000; accepted 1 November 2000
Abstract Wollastonite coatings on titanium alloys substrates were prepared by plasma spraying and incubated in simulated body #uids for di!erent periods to investigate the nucleation and growth of apatite on their surface. Surface structural changes of the specimens were analyzed by XRD and IR technologies. SEM and EDS were used to observe surface morphologies and determine the composition of wollastonite coatings before and after immersion in simulated body #uid. The changes in the concentrations of calcium, silicon and phosphorus in the simulated body #uids due to the immersion of the specimens were measured by inductively coupled plasma atomic emission spectroscopy. The results obtained showed that hydroxycarbonate apatite can be formed on the surface of the coating soaked in SBF for 1 day. With longer immersion periods, the coating surface was covered by hydroxycarbonate apatite, which indicated that the wollastonite coating possesses good bioactivity. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Wollastonite coatings; Plasma spraying; Simulated body #uid; Hydroxycarbonate apatite
1. Introduction Wollastonite (CaSiO ), a common mineral of meta morphosed limestones and similar assemblages [1], is a raw material mainly used for traditional ceramics [2]. In addition, CaSiO ceramics is a candidate material for high-frequency insulator [3]. One of the other possible applications for CaSiO ceramics is as a medical material for arti"cial bone and dental root because some glasses, glass}ceramic and ceramics which include CaO}SiO components were reported to showed good biocompatibility [4}6]. Hench [7] and his colleagues discovered that bone could bond chemically to certain glass composition. Ono et al. [8] reported A}W glass}ceramic had higher bioactivity than sintered hydroxyapatite (HA). Some reports [9,10] pointed out that the rate of HAp formation on the surface of CaSiO ceramics is faster than those of the other biocompatible glass and glass}ceramics in SBF solution. Plasma spraying is most popular deposition technique due to its process feasibility as well as reasonably high coating bond strength and mechanical property [11].
* Corresponding author. Fax: #86-21-62513903. E-mail address: [email protected] (X. Liu).
Several bioactive materials, such as hydroxyapatite (HA) [12,13] and bioglass (BG) [14], have been coated onto metals and alloys substrates by plasma spraying. Therefore, an interest has been taken in wollastonite (CaSiO ) as a plasma-sprayed coating for applications in which bioactivity and biocompatibility are desired. The object of this work was to deposite wollastonite coatings on Ti}6Al}4 V substrate by atmospheric plasma spraying (APS) and investigate microstructure and phase composition of coatings. The bioactivity of the coatings also was evaluated by examining hydroxycarbonate apatite formation on their surface in simulated body #uid (SBF).
2. Experimental methods Commercially available wollastonite (CaSiO ) pow der, with a typical size range of 10}60 m was used. Plasma spraying of the powder was made onto Ti}6Al}4 V substrates with dimensions 20 mm; 10 mm;4 mm. An atmosphere plasma spray (APS) system (Sulzer Metco, Switzerland) was applied to fabricate wollastonite coatings under the modi"ed spray parameters. Argon (40 slpm) and hydrogen (12 slpm) were used as primary and auxiliary arc gas, respectively.
0142-9612/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 3 8 6 - 0
2008
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
The feeding rate of powders was about 20 g/min using argon (3.0 slpm) as carrying gas. The arc current and voltage were 600 A and 73 V, respectively. The spraying distance was 90 mm. Coatings thickness for all specimens was about 200 m. After being ultrasonically washed in acetone and rinsed in deionized water, specimens were soaked in the SBF solution whose ion concentrations nearly equal to those of the human body blood plasma, as shown in Table 1 [15]. The SBF solution was bu!ered at pH 7.4 with trimethanol aminomethane-HCl. Triplicate samples that would be analyzed by scanning electron microscopy (SEM), X-ray di!raction (XRD) and infrared spectroscopy (IR) were immersed in SBF which was renewed every day for 1, 3, 7, 10, 14 and 21 days at 36.53C without stirring. In order to measure the changes in the concentration of calcium, silicon and phosphorus in the SBF solution, some samples were immersed, respectively, in 40 ml SBF for 1, 2, 3, 7, 10, 14 and 21 days at 36.53C without stirring. SEM and EDS was used to observe the morphologies and determine the composition of coatings before and after immersion. The surface of samples was sputter coated with gold for morphological observation or with carbon for elemental analysis. Surface structural changes of the coating due to immersion in SBF were analyzed by a thin-"lm XRD. In the X-ray di!raction experiment the
Table 1 Ion concentration of SBF in comparison with human blood plasma Concentration (mM) Na> K> Ca> Mg> HCO\ Cl\ SBF 142.0 5.0 Blood 142.0 5.0 plasma
2.5 2.5
1.5 1.5
4.2 27.0
148.5 103.0
HPO\ SO\ 1.0 1.0
0.5 0.5
glancing angle of the incident beam against the surface of the specimen was "xed at 13. A few micrograms of the Ca}P layer formed on the coating in SBF were scraped o!. This was mixed with KBr and pressed into plates for structural analysis using infrared (IR) spectroscopy on a Bio Rad FTS-185. The changes in the concentrations of calcium, silicon and phosphorus in the SBF solution due to the immersion of specimens were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).
3. Results and discussion Fig. 1 showed SEM photographs of the surface and cross-sectional microstructure of the as-sprayed wollastonite coatings. From Fig. 1, it can be seen that the coating is characterized by a rough surface, with some partially melted particles (Fig. 1a). Under higher magni"cation, the structure of the coating appears to be highly melted and a few micro-cracks on the surface of the coating (Fig. 1b). The cross-sectional view of the coating revealed a lamellar structure with some pores and micro-cracks and without obvious cracks between coatings and substrate (Fig. 1c). Fig. 2 showed the XRD patterns of the wollastonite powder and coatings. From Fig. 2a, it can be seen that the wollastonite powder has high crystallinity. Some changes can be noted in Fig. 2b, including an increase in the background area under the sharp peaks. The sharp peaks represented crystalline wollastonite and the di!use background represented the amorphous phase. This indicated a lot of amorphous phase is present in the coating. In addition, the relative intensity of the peak at 2"28.93 increased manifestly comparing with Fig 2a. The observed strong peak could not be used to identify the proper crystalline phase. However, it may correspond to (2 1 0) crystal planes of wollastonite, though the intensity
Fig. 1. SEM photographs of as-sprayed wollastonite coatings: (a) and (b) surface morphology; (c) cross-section.
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
Fig. 2. XRD patterns of (a) wollastonite powder and (b) as-sprayed wollastonite coating.
of this re#ection in the JCPDS card is not the strongest. The phenomena may indicate that there is a probability of crystallite orientation in the plasma-sprayed wollastonite coatings. Roome et al. [16] reported that crystallite orientation could be found in thermally sprayed hydroxyapatite coatings. He thought that orientated coatings was a consequence of the thermal deposition process, where the signi"cantly molten powders undergoes rapid recrystallization (from remaining microscopic fragments or &seeds' of crystalline grains) and crystal growth upon striking the substrate. It was also noteworthy that the pseudowollastonite phase, the high-temperature form of wollastonite, did not appear in the coating as seen from Fig 2b. The accurate inversion temperature of wollastonite}pseudowollastonite is 1120$103C. However, the inversion rate is very slow because wollastonite is a chain silicate while pseudowollastonite is a ring silicate [1]. Therefore, pseudowollastonite cannot be formed during the plasma-spraying process. Fig. 3 showed the morphologies of the coatings immersed in SBF for various periods. From Fig. 3, it can be seen that granular crystals appeared on the surface of the coating soaked in SBF for 1 day (Fig. 3a). The EDS spectra of the granule indicated that the granules were mainly composed of calcium and phosphorus (Fig. 3g). After 3 days immersion in SBF, the surface of the wollastonite coating was completely covered by the ball-like particles, which changed the original morphology of the as-sprayed coating completely (Fig. 3c). With longer immersion periods, micro-cracks of tortoise shell character appeared on the newly formed layer and the granular apatite in the layer grew gradually (Fig. 3c}e). At a higher magni"cation, the apatite layer showed very small crystallites (Fig. 3f ). The EDS quantitative analysis of these
2009
small granulars on the surface of coatings gave a Ca/P ratio of around 1.65 which is nearly equal to the composition of apatite (Fig. 3h). Fig. 4 showed the changes in concentration of the calcium, silicon and phosphorus of the SBF due to the immersion of wollastonite coatings. From Fig. 4, it can be seen that the calcium and silicon concentration increased with an increase in the immersion time. The concentration of phosphorus of the SBF decreased very steeply by 3 days and became lower than one tenth of the starting concentration. The increases in the calcium and silicon concentration were attributed to the dissolution of the calcium and silicate ions from the wollastonite coatings. Although the formation of the Ca}P layer consumed some calcium ions, the calcium ions dissolution from the coatings were more than those consumed. The decrease in the phosphorus concentration was attributed to formation of both the amorphous calcium phosphate and crystalline apatite on the surfaces of wollastonite coatings by consuming the phosphate ion from the #uid. Fig. 5 showed the thin "lm-XRD patterns of the wollastonite coatings soaked in SBF solution for various times. The peak (2"323) of HA crystalline phase, could be observed in the XRD patterns of the coatings soaked in SBF solution for 1 day, and the peak was very broad resulting from super"ne grains of HA and amorphous calcium phosphate. With the increase of soaking time, the primary peak of the crystalline HA became higher and the secondary strong peak (2"263) of the crystalline HA can be seen from Fig. 5. These are attributed to the transition of amorphous calcium phosphate to a crystalline phase and the growth of crystalline HA grains by taking calcium and phosphate ions from the SBF solution. Fig. 6 showed the IR spectra of the surface layer of the coating immersed in SBF for 1, 7 and 21 days. The IR spectrum of the as-sprayed wollastonite coating is also given in Fig. 6a, showing spectral characteristics of wollastonite. The IR spectrum of the coatings soaked in SBF for various periods showed that the hydroxycarbonateapatite (HCA) appeared on the surface of the coatings. Bands at 602 and 563 cm\ are due to the bending vibration modes of the PO group [17]. A very broad OH\ absorption bands from 3700 to 2500 cm\ and a weak water absorption band around 1650 cm\ can be seen in these spectra. Bands between 1400 and 1550 cm\ are due to the carbonate IR absorption . The peak around 870 cm\ is due to the joint contribution of carbonate and HPO\ ions [18]. The broad bands around 1100 cm\ in Fig. 6b}d are di!erent from the bands around 1000 cm\ in Fig. 6a. The former is mainly attributed to the phosphate IR absorption [18], while the latter is attributed to the silicate IR absorption [19,20]. The formation of apatite on the surfaces of plasmasprayed wollastonite coatings is similar to that of CaO}SiO -based glasses. The calcium ion dissolved
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X. Liu et al. / Biomaterials 22 (2001) 2007}2012
Fig. 3. Surface morphologies of wollastonite coatings after immersion in SBF for various periods: (a) 1 d; (b) 3 d; (c) 7 d; (d) 14 d; (e) 21 d; (f ) higher magni"cation for apatite layer; (g) EDS for the granule in (a); (h) EDS for the apatite in (f ).
from the wollastonite coatings increase the ion activity product of the apatite in the SBF solution, and the hydrated silica on the surfaces of the wollastonite coatings provides favorable sites for apatite nucleation. Consequently, the apatite nuclei are rapidly formed on the surface of the wollastonite coatings. Once the apatite nuclei are formed, they spontaneously grow by consuming calcium and phosphate ions from the SBF solution. The calcium phosphate phase that accumulates on the surface of the wollastonite coatings is initially amorph-
ous. It later crystallizes to a hydroxycarbonate apatite (HCA) structure by incorporating carbonate anions from solution within the amorphous calcium phosphate phase [7].
4. Conclusions Wollastonite coatings on titanium alloys substrates were prepared by plasma spraying. The coatings are
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
2011
characterized by a lamellar structure without obvious cracks between coatings and substrate. The crystalline wollastonite which may be preferred orientation and the amorphous phase can be found in the coatings. The hydroxycarbonate apatite (HCA) was formed on the surface of the coatings soaked in SBF for 1 day. With longer immersion periods, the surface of coatings was covered by dense hydroxycarbonate apatite. Therefore, the plasma-sprayed wollastonite coatings possess the potential for excellent bioactivity and thus may be used as a candidate of biomaterials.
Fig. 4. The concentration of elements in SBF after immersion of wollastonite coatings.
Acknowledgements This work is supported by National Basic Research Fund under Grant G1999064706 and Shanghai Science and Technology R & D Fund under Grant 995211020.
References
Fig. 5. Thin-"lm XRD patterns of wollastonite coatings soaked in SBF for various periods.
Fig. 6. IR spectra of the wollastonite coatings: (a) as-sprayed and soaked in SBF for (b) 1 d, (c) 7 d and (d) 21 d.
[1] Deer Hoeie, Zussman, editors. Chain silicates. London: Longmans, Green and Co. Ltd, 1963. p. 167}75. [2] Milan Vukovich. E!ect of wollastonite substitute in a semivitreous dinnerware body. J Am Ceram Soc 1956;39:323}9. [3] Shigeo Hayashi N, Otsuka K, Akiyama Kokada, Yano T. Preparation of diopside "ne powders by spray pyrolysis and its sinterability. J Ceram Soc Jpn 1989;97:742}6. [4] Kokubo T, Ito S, Shigematsu M, Sakka S, Yamamuro T. Fatigue and life time of bioactive glass}ceramic A}W containing Apatite and Wollastonite. J Mater Sci 1987;22:4067}70. [5] Kokubo T. Surface chemistry of bioactive glass-ceramics. J NonCryst Solids 1990;120:138}51. [6] Abe Y, Kokubo T, Yamamuro T. Apatite coating on ceramics, metals and polymers utilizing a biological process. J Mater Sci Mater Med 1990;1:233}8. [7] Hench L, Anderson O. Bioactive glass. In: Hench L, Wilson J, editors. An introduction to bioceramics. USA: World Scienti"c, 1993. p. 41}62. [8] Ono K, Yamamuro T, Nakamura T, Kokubot T. Quantitative study on osteoconduction of Apatite-Wollastonite containing glass-ceramic granules, hydroxyapatite granules, and alumina granules. Biomaterials 1990;11:265}71. [9] Kokubo T, Ito S, Huang Z, Hayashi T, Sakka S, Kitsugi T, Yamamuro T. Ca}P-rich layer formed on high-strength bioactive glass}ceramic A}W. J Biomed Mater Res 1990;24:331}43. [10] Punama Siriphannon, Shigeo Hayashi, Atsuo Yasumori, Kiyoshi Okada. Preparation and sintering of CaSiO from coprecipitated powder using NaOH as precipitant and its apatite formation in simulated body #uid solution. J Mater Res 1999;14:529}36. [11] Ding SJ, Ju CP, Chern Lin JH. Morphology and immersion behavior of plasma-sprayed hydroxyapatite/bioactive glass coatings. J Mater Sci Mater Med 2000;11:183}90. [12] Lace"eld WR. Hydroxyapatite coatings. In: Hench LL, Wilson J, editors. An introduction to bioceramics. USA: World Scienti"c, 1993. p. 223}38. [13] De Groot K, Geesink RGT, Klein CPAT, Serekian P. Plasma sprayed coatings of hydroxyapatite. J Biomed Mater Res 1987; 21:1375}87. [14] Hench LL, Anderson O. Bioactive glass coatings. In: Hench LL, Wilson J, editors. An introduction to bioceramics. USA: World Scienti"c, 1993. p. 239}59.
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X. Liu et al. / Biomaterials 22 (2001) 2007}2012
[15] Sun-Back Cho, Kazuki Nakanishi, Tadashi Kokubo, Naohiro Soga. Dependence of apatite formation on silica gel on its structure: e!ect of heat treatment. J Am Ceram Soc 1995;78:1769}74. [16] Roome CM, Adam CD. Crystallite orientation and anisotropic strains in thermally sprayed hydroxyapatite coatings. Biomaterials 1995;16:691}6. [17] Radin SR, Ducheyne P. Plasma spraying induced changes of calcium phosphate ceramic characteristics and the e!ect on in vitro stability. J Mater Sci Mater Med 1992;3:33}42.
[18] Weng J, Liu Q, Wolke JGC, Zhang X, de Groot K. Formation and characteristics of the apatite layer on plasma-sprayed hydroxyapatite coatings in simulated body #uid. Biomaterials 1997;18:1027}35. [19] Almeida RM. Detection of LO mode in -SiO by infrared di!use re#ectance spectroscopy. J Non-Cryst Solids 1990;119:238}41. [20] Ohtsuki C, Kokubo T, Yamamura T. Mechanism of apatite formation on CaO}SiO }P O glasses in a simulated body #uid. J Non-Cryst Solids 1992;143:84}92.
Apatite formed on the surface of plasma-sprayed wollastonite coating immersed in simulated body #uid Xuanyong Liu*, Chuanxian Ding, Zhenyao Wang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People+s Republic of China Received 8 June 2000; accepted 1 November 2000
Abstract Wollastonite coatings on titanium alloys substrates were prepared by plasma spraying and incubated in simulated body #uids for di!erent periods to investigate the nucleation and growth of apatite on their surface. Surface structural changes of the specimens were analyzed by XRD and IR technologies. SEM and EDS were used to observe surface morphologies and determine the composition of wollastonite coatings before and after immersion in simulated body #uid. The changes in the concentrations of calcium, silicon and phosphorus in the simulated body #uids due to the immersion of the specimens were measured by inductively coupled plasma atomic emission spectroscopy. The results obtained showed that hydroxycarbonate apatite can be formed on the surface of the coating soaked in SBF for 1 day. With longer immersion periods, the coating surface was covered by hydroxycarbonate apatite, which indicated that the wollastonite coating possesses good bioactivity. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Wollastonite coatings; Plasma spraying; Simulated body #uid; Hydroxycarbonate apatite
1. Introduction Wollastonite (CaSiO ), a common mineral of meta morphosed limestones and similar assemblages [1], is a raw material mainly used for traditional ceramics [2]. In addition, CaSiO ceramics is a candidate material for high-frequency insulator [3]. One of the other possible applications for CaSiO ceramics is as a medical material for arti"cial bone and dental root because some glasses, glass}ceramic and ceramics which include CaO}SiO components were reported to showed good biocompatibility [4}6]. Hench [7] and his colleagues discovered that bone could bond chemically to certain glass composition. Ono et al. [8] reported A}W glass}ceramic had higher bioactivity than sintered hydroxyapatite (HA). Some reports [9,10] pointed out that the rate of HAp formation on the surface of CaSiO ceramics is faster than those of the other biocompatible glass and glass}ceramics in SBF solution. Plasma spraying is most popular deposition technique due to its process feasibility as well as reasonably high coating bond strength and mechanical property [11].
* Corresponding author. Fax: #86-21-62513903. E-mail address: [email protected] (X. Liu).
Several bioactive materials, such as hydroxyapatite (HA) [12,13] and bioglass (BG) [14], have been coated onto metals and alloys substrates by plasma spraying. Therefore, an interest has been taken in wollastonite (CaSiO ) as a plasma-sprayed coating for applications in which bioactivity and biocompatibility are desired. The object of this work was to deposite wollastonite coatings on Ti}6Al}4 V substrate by atmospheric plasma spraying (APS) and investigate microstructure and phase composition of coatings. The bioactivity of the coatings also was evaluated by examining hydroxycarbonate apatite formation on their surface in simulated body #uid (SBF).
2. Experimental methods Commercially available wollastonite (CaSiO ) pow der, with a typical size range of 10}60 m was used. Plasma spraying of the powder was made onto Ti}6Al}4 V substrates with dimensions 20 mm; 10 mm;4 mm. An atmosphere plasma spray (APS) system (Sulzer Metco, Switzerland) was applied to fabricate wollastonite coatings under the modi"ed spray parameters. Argon (40 slpm) and hydrogen (12 slpm) were used as primary and auxiliary arc gas, respectively.
0142-9612/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 3 8 6 - 0
2008
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
The feeding rate of powders was about 20 g/min using argon (3.0 slpm) as carrying gas. The arc current and voltage were 600 A and 73 V, respectively. The spraying distance was 90 mm. Coatings thickness for all specimens was about 200 m. After being ultrasonically washed in acetone and rinsed in deionized water, specimens were soaked in the SBF solution whose ion concentrations nearly equal to those of the human body blood plasma, as shown in Table 1 [15]. The SBF solution was bu!ered at pH 7.4 with trimethanol aminomethane-HCl. Triplicate samples that would be analyzed by scanning electron microscopy (SEM), X-ray di!raction (XRD) and infrared spectroscopy (IR) were immersed in SBF which was renewed every day for 1, 3, 7, 10, 14 and 21 days at 36.53C without stirring. In order to measure the changes in the concentration of calcium, silicon and phosphorus in the SBF solution, some samples were immersed, respectively, in 40 ml SBF for 1, 2, 3, 7, 10, 14 and 21 days at 36.53C without stirring. SEM and EDS was used to observe the morphologies and determine the composition of coatings before and after immersion. The surface of samples was sputter coated with gold for morphological observation or with carbon for elemental analysis. Surface structural changes of the coating due to immersion in SBF were analyzed by a thin-"lm XRD. In the X-ray di!raction experiment the
Table 1 Ion concentration of SBF in comparison with human blood plasma Concentration (mM) Na> K> Ca> Mg> HCO\ Cl\ SBF 142.0 5.0 Blood 142.0 5.0 plasma
2.5 2.5
1.5 1.5
4.2 27.0
148.5 103.0
HPO\ SO\ 1.0 1.0
0.5 0.5
glancing angle of the incident beam against the surface of the specimen was "xed at 13. A few micrograms of the Ca}P layer formed on the coating in SBF were scraped o!. This was mixed with KBr and pressed into plates for structural analysis using infrared (IR) spectroscopy on a Bio Rad FTS-185. The changes in the concentrations of calcium, silicon and phosphorus in the SBF solution due to the immersion of specimens were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).
3. Results and discussion Fig. 1 showed SEM photographs of the surface and cross-sectional microstructure of the as-sprayed wollastonite coatings. From Fig. 1, it can be seen that the coating is characterized by a rough surface, with some partially melted particles (Fig. 1a). Under higher magni"cation, the structure of the coating appears to be highly melted and a few micro-cracks on the surface of the coating (Fig. 1b). The cross-sectional view of the coating revealed a lamellar structure with some pores and micro-cracks and without obvious cracks between coatings and substrate (Fig. 1c). Fig. 2 showed the XRD patterns of the wollastonite powder and coatings. From Fig. 2a, it can be seen that the wollastonite powder has high crystallinity. Some changes can be noted in Fig. 2b, including an increase in the background area under the sharp peaks. The sharp peaks represented crystalline wollastonite and the di!use background represented the amorphous phase. This indicated a lot of amorphous phase is present in the coating. In addition, the relative intensity of the peak at 2"28.93 increased manifestly comparing with Fig 2a. The observed strong peak could not be used to identify the proper crystalline phase. However, it may correspond to (2 1 0) crystal planes of wollastonite, though the intensity
Fig. 1. SEM photographs of as-sprayed wollastonite coatings: (a) and (b) surface morphology; (c) cross-section.
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
Fig. 2. XRD patterns of (a) wollastonite powder and (b) as-sprayed wollastonite coating.
of this re#ection in the JCPDS card is not the strongest. The phenomena may indicate that there is a probability of crystallite orientation in the plasma-sprayed wollastonite coatings. Roome et al. [16] reported that crystallite orientation could be found in thermally sprayed hydroxyapatite coatings. He thought that orientated coatings was a consequence of the thermal deposition process, where the signi"cantly molten powders undergoes rapid recrystallization (from remaining microscopic fragments or &seeds' of crystalline grains) and crystal growth upon striking the substrate. It was also noteworthy that the pseudowollastonite phase, the high-temperature form of wollastonite, did not appear in the coating as seen from Fig 2b. The accurate inversion temperature of wollastonite}pseudowollastonite is 1120$103C. However, the inversion rate is very slow because wollastonite is a chain silicate while pseudowollastonite is a ring silicate [1]. Therefore, pseudowollastonite cannot be formed during the plasma-spraying process. Fig. 3 showed the morphologies of the coatings immersed in SBF for various periods. From Fig. 3, it can be seen that granular crystals appeared on the surface of the coating soaked in SBF for 1 day (Fig. 3a). The EDS spectra of the granule indicated that the granules were mainly composed of calcium and phosphorus (Fig. 3g). After 3 days immersion in SBF, the surface of the wollastonite coating was completely covered by the ball-like particles, which changed the original morphology of the as-sprayed coating completely (Fig. 3c). With longer immersion periods, micro-cracks of tortoise shell character appeared on the newly formed layer and the granular apatite in the layer grew gradually (Fig. 3c}e). At a higher magni"cation, the apatite layer showed very small crystallites (Fig. 3f ). The EDS quantitative analysis of these
2009
small granulars on the surface of coatings gave a Ca/P ratio of around 1.65 which is nearly equal to the composition of apatite (Fig. 3h). Fig. 4 showed the changes in concentration of the calcium, silicon and phosphorus of the SBF due to the immersion of wollastonite coatings. From Fig. 4, it can be seen that the calcium and silicon concentration increased with an increase in the immersion time. The concentration of phosphorus of the SBF decreased very steeply by 3 days and became lower than one tenth of the starting concentration. The increases in the calcium and silicon concentration were attributed to the dissolution of the calcium and silicate ions from the wollastonite coatings. Although the formation of the Ca}P layer consumed some calcium ions, the calcium ions dissolution from the coatings were more than those consumed. The decrease in the phosphorus concentration was attributed to formation of both the amorphous calcium phosphate and crystalline apatite on the surfaces of wollastonite coatings by consuming the phosphate ion from the #uid. Fig. 5 showed the thin "lm-XRD patterns of the wollastonite coatings soaked in SBF solution for various times. The peak (2"323) of HA crystalline phase, could be observed in the XRD patterns of the coatings soaked in SBF solution for 1 day, and the peak was very broad resulting from super"ne grains of HA and amorphous calcium phosphate. With the increase of soaking time, the primary peak of the crystalline HA became higher and the secondary strong peak (2"263) of the crystalline HA can be seen from Fig. 5. These are attributed to the transition of amorphous calcium phosphate to a crystalline phase and the growth of crystalline HA grains by taking calcium and phosphate ions from the SBF solution. Fig. 6 showed the IR spectra of the surface layer of the coating immersed in SBF for 1, 7 and 21 days. The IR spectrum of the as-sprayed wollastonite coating is also given in Fig. 6a, showing spectral characteristics of wollastonite. The IR spectrum of the coatings soaked in SBF for various periods showed that the hydroxycarbonateapatite (HCA) appeared on the surface of the coatings. Bands at 602 and 563 cm\ are due to the bending vibration modes of the PO group [17]. A very broad OH\ absorption bands from 3700 to 2500 cm\ and a weak water absorption band around 1650 cm\ can be seen in these spectra. Bands between 1400 and 1550 cm\ are due to the carbonate IR absorption . The peak around 870 cm\ is due to the joint contribution of carbonate and HPO\ ions [18]. The broad bands around 1100 cm\ in Fig. 6b}d are di!erent from the bands around 1000 cm\ in Fig. 6a. The former is mainly attributed to the phosphate IR absorption [18], while the latter is attributed to the silicate IR absorption [19,20]. The formation of apatite on the surfaces of plasmasprayed wollastonite coatings is similar to that of CaO}SiO -based glasses. The calcium ion dissolved
2010
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
Fig. 3. Surface morphologies of wollastonite coatings after immersion in SBF for various periods: (a) 1 d; (b) 3 d; (c) 7 d; (d) 14 d; (e) 21 d; (f ) higher magni"cation for apatite layer; (g) EDS for the granule in (a); (h) EDS for the apatite in (f ).
from the wollastonite coatings increase the ion activity product of the apatite in the SBF solution, and the hydrated silica on the surfaces of the wollastonite coatings provides favorable sites for apatite nucleation. Consequently, the apatite nuclei are rapidly formed on the surface of the wollastonite coatings. Once the apatite nuclei are formed, they spontaneously grow by consuming calcium and phosphate ions from the SBF solution. The calcium phosphate phase that accumulates on the surface of the wollastonite coatings is initially amorph-
ous. It later crystallizes to a hydroxycarbonate apatite (HCA) structure by incorporating carbonate anions from solution within the amorphous calcium phosphate phase [7].
4. Conclusions Wollastonite coatings on titanium alloys substrates were prepared by plasma spraying. The coatings are
X. Liu et al. / Biomaterials 22 (2001) 2007}2012
2011
characterized by a lamellar structure without obvious cracks between coatings and substrate. The crystalline wollastonite which may be preferred orientation and the amorphous phase can be found in the coatings. The hydroxycarbonate apatite (HCA) was formed on the surface of the coatings soaked in SBF for 1 day. With longer immersion periods, the surface of coatings was covered by dense hydroxycarbonate apatite. Therefore, the plasma-sprayed wollastonite coatings possess the potential for excellent bioactivity and thus may be used as a candidate of biomaterials.
Fig. 4. The concentration of elements in SBF after immersion of wollastonite coatings.
Acknowledgements This work is supported by National Basic Research Fund under Grant G1999064706 and Shanghai Science and Technology R & D Fund under Grant 995211020.
References
Fig. 5. Thin-"lm XRD patterns of wollastonite coatings soaked in SBF for various periods.
Fig. 6. IR spectra of the wollastonite coatings: (a) as-sprayed and soaked in SBF for (b) 1 d, (c) 7 d and (d) 21 d.
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