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C composites by induction heating method
Surface & Coatings Technology 204 (2009) 115–119
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of hydrothermal temperature on hydroxyapatite coating transformed from monetite on HT-C/C composites by induction heating method Xiong Xin-bo a,⁎, Zeng Xie-rong a, Zou Chun-li a, Li Ping b, Fan Yun-bo b,⁎ a b
Shen Zhen Key Laboratory of Special Functional Materials, Department of Material, Science and Engineering, Shen Zhen University, Shen Zhen, 518060, PR China Bioengineering Department, Beihang University, Beijing, 100191, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2008 Accepted in revised form 29 June 2009 Available online 5 July 2009 PACS: 87.85.jf 81.05.Uw 81.15.-z 46.70.-p 81.65.-b
a b s t r a c t Hydrothermal post-treatment was used to convert monetite coating fabricated by induction heating method on H2O2 treated C/C (HT-C/C) composites to an adherent HA coating. The monetite coatings were hydrothermally treated for 4 h at 373 K, 403 K, 423 K, and 453 K in a 50 mL autoclave. After hydrothermal post-treatment, the structure, morphology and the chemical composition of these HA coatings were characterized with XRD, FTIR SEM and EDS. A scratch test was conducted to measure the strength of the adhesion of the coatings to the HT-C/C substrate. The results showed that the degree of crystallinity and the Ca/P ratio of the HA coatings increased with increasing hydrothermal temperature. The submicron-level morphology and adhesion of the HA coatings were highly affected by the hydrothermal temperature. From the results, it can be suggested that 423 K was the best hydrothermal treatment temperature for the HA coatings which were transformed from the monetite coatings produced by the induction heating method on HT-C/C composites. © 2009 Elsevier B.V. All rights reserved.
Keywords: Chemical conversion Carbon Cermets and composite Hydroxyapatite Scratch test
1. Introduction Carbon fiber reinforced carbon (C/C) composites have been considered as one alternative material to metallic implants for applications in loaded artificial bones due to their excellent biocompatibility and biomechanical properties [1,2]. Their mechanical properties, especially elastic modulus, are found to be closer to those of loaded human bones than other available bone repairing materials. Thus, stress shielding and sequential bone absorption caused by implant materials with high modulus could be avoided if C/C composites are applied [3]. However, C/C composites are bionert and easily encapsulated after implanted into the living body by fibrous tissue that isolates them from the surrounding bone. Also, due to friction damage during surgical procedures, they may release carbon particles which deposit in the neighborhood of the implant and the lymphatic node, causing “black skin effect” [4]. These drawbacks of C/C composites have limited their potential applications. Hydroxyapatite (HA) has a chemical composition similar to that of the inorganic part of human bones and has an excellent biocompability in comparison with other implant materials [5]. Never-
⁎ Corresponding authors. Xin-bo is to be contacted at Tel.: +86 755 26536239; fax: +86 755 26536239. Yun-bo, Tel./fax: +86 10 82339428. E-mail addresses: [email protected] (X. Xin-bo), [email protected] (F. Yun-bo). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.06.044
theless, its fracture toughness and strength are not high enough for application in human bones. Therefore, in recent years, many efforts have been made to coat bioactive HA or bonelike-apatite onto C/C substrate surfaces for improving the biocompability. Several calcium phosphate coating techniques have been used, for example, plasma spraying [6,7], electrodeposition (or sono-electrodeposition) processes [8], sol–gel [9] and biomimetic techniques [10]. The main problem of these methods is the weak adhesion strength between the HA coating and C/C substrate, which is difficult to improve. Recently, in authors' research group, a novel technique by the combination of induction heating deposition [11] with hydrothermal treatment was developed to coat adherent HA coating on C/C substrate [12]. In this method, specimens of C/C were firstly modified by immersed in an autoclave with H2O2 solution, and then coated with monetite coating by induction heating method. Subsequently, monetite coating on C/C was converted to HA coating by hydrothermal treatment in an autoclave with the ammonia solution. After these procedures, the adhesion of the HA coating was investigated by scratch test and the result showed that the critical load of resultant HA coating with the thickness of 50 μm could reach to 13.12 N, i.e. shear stress of 61.4 MPa. This shear strength is greater than the loading stress of HA coatings deposited on C/C composites by plasma spray (7.15 MPa). Besides this, the technique was found to have the following advantages: thicker films, better control over the deposited solid phase, the ability to deposit porous or complex shapes, and lower
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processing temperature. So it is worth further investigation for use in implants. The aim of this study is to investigate the influence of hydrothermal treatment on the phases, microstructures and adhesions of HA coatings transformed from monetite produced by the induction heating method on HT-C/C composites. 2. Experimental procedure 2.1. Experimental setup and mother solution Details of the experimental setup for induction heat deposition process are described elsewhere [12]. In the experiment, the mother solution was prepared by dissolving given amounts of reagent-grade chemicals of 0.04 M Ca(NO3)2 and 0.024 M NH4H2PO4 into distilled water. This solution was buffered to a pH of 4.5 with an adequate amount of ammonia. 2.2. C/C substrates and pretreatments C/C composites were prepared by chemical vapor infiltration (CVI) processing in Northwest Polytechnology University in China. The density and Shore scleroscope hardness of them are average 1.72 g/ cm3 and 36.1 respectively. C/C samples were cut from the block and had a diameter of 10 mm and a length of 10 mm. Prior to the coating runs, each sample was polished with No.600 and No.1000 abrasive paper, rinsed with distilled water, then cleaned ultrasonically in acetone, and dried in a desiccator. And H2O2 in analytical grade was dissolved in deinonized water to prepare 2.0 M solution at ambient temperature. Then, samples were pretreated in high pressure steam in a 50 mL autoclave with H2O2 solution at 433 K. After removal from H2O2 solution, these cylinders were rinsed ultrasonically with deionized water and dried in air. 2.3. Coating experiments and post-treatments All the deposition experiments were carried out at the applied current of 500 A at room temperature. At the end of each run, the coated cylinders was rinsed with distilled water and then hydrothermally treated in an autoclave with the ammonia solution at pH of 8 for 4 h. The temperatures of the hydrothermal treatment for the coatings are 373 K, 403 K, 423 K and 453 K respectively. After hydrothermal treatment, all the coated samples were annealed in vacuum to removal of water in the coatings at 473 K for 1 h. 2.4. Characterization The crystalline structure, morphologies and compositions of the coated samples were characterized by X-ray diffraction (XRD) using a D8 Advance X-ray diffractometer (Cu-Ka radiation), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis with a s-3400N (Japan) microscope. The functional groups of HA coatings were quantitatively identified by Fourier transform infrared (FTIR). The FTIR spectra were recorded in the 400–4000 cm− 1 range, resolution 4 cm− 1, using Perkin Elmer instruments Spectrum One Spectrometer and KBr pellet technology. The adhesion strength of the HA coatings deposited on C/C substrates were determined by the scratch test method using a s-3400N scratch tester fitted with a Rochwell C 0.2 mm-diamond stylus with a preload of 1 N, load speed 10 N/min, scratch speed 5 mm/min and the maximum load of 20 N. Finally, the scratch trace on HA coating was observed by stereomicroscope (STM).
Fig. 1. XRD spectra of (a) HT-C/C, (b) monetite coating on HT-C/C by induction heating deposition and (c), (d), (e), (f) transformed hydroxyapatite coating from monetite by hydrothermal post-treatment on HT-C/C at 373 K, 403 K, 423 K and 453 K respectively.
heating method consists of calcium hydrogen orthophosphate (CaHPO4, monetite). After hydrothermal post-treatment at different temperatures, monetite phases of all the coatings were transformed to hydroxyapatite with the three most intensive peaks ((211), (300), (112)) between 2θ = 30 – 35° [13] which are in a good agreement with the reference data for JCPDS 9-432. These results indicate that hydrothermal post-treatment method helps to convert monetite phase to well crystallised hydroxyapatite. From the XRD spectra of the coatings on HT-C/C, It can also be found that the intensity of HA peaks increases with increasing hydrothermal treatment temperatures (and this is especially true for 453 K) as a result of grain growth. Fig. 2(a) shows SEM images of monetite coating on HT-C/C composites of HA coating. It can be seen that the as-received monetite coating on HT C/C substrate is composed of sheet-like crystals with smooth surface, which agglomerated together to form dense morphology. After hydrothermal post-treatment, the morphologies of these crystals show no significant change and their dense microstructures still keep when all the coatings are observed at low times magnification, as presented in Fig. 2(b), (c), (d) and (e). But, at high times magnification differences in sub-micrometer morphologies can be found. When the hydrothermal temperature is lower than 403 K, nanometer sized grains occur in the HA crystals. However when the hydrothermal temperature was 423 K, besides nanometer sized grains, there are deposits on the surface of these grains. Surprisingly, when the hydrothermal temperature reaches to 453 K, plate-like grains besides some deposits are observed in the HA crystals. Also, it is clear that the size of these grains on the HA crystals increases with increasing hydrothermal post-treatment temperature. The transformation of monetite to HA was proposed to involve a continuous process of dissolution and a re-precipitation by Da silva et al. [14], as shown in Eqs. (1) and (2) 2+
CaHPO4 →Ca 2+
ð10−XÞCa
2−
ð1Þ
+ HPO4
2−
+ 6HPO4
−
+ 2OH →Ca10−X ðPO4 Þ6 ðOHÞ2 where 0bxb1 ð2Þ
or follow a topotactic mechanism, concerning incorportion of calcium ions into the monetite solid phase and HA crystallization by Lebugle et al. [15], as shown in Eq. (3).
3. Results and discussion 2+
Fig. 1 shows the XRD spectra of the as-deposited and hydrothermal post-treated coatings on HT-C/C. The coating deposited by induction
ð3−XÞCa
−
+ 6CaHPO4 + 2OH →Ca10−X ðPO4 Þ6 ðOHÞ2 where 0bxb1 ð3Þ
X. Xin-bo et al. / Surface & Coatings Technology 204 (2009) 115–119
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Fig. 2. Morphologies of the coatings on HT-C/C. (A) Induction heating deposition, and (B) at 373; (C) at 403 K; (D) at 423 K; (E) at 453 K hydrothermal post-treatment respectively.
According to these proposals, we suggest that the sheet-like crystals are composed of the nano-sized or plate-like grains which is as a result of the incorporation of calcium ions into the monetite solid phase and HA crystallization, while the deposits result from the reprecipitation of ions containing Ca and P elements which exist in the solution owing to dissolution of monetite crystals. The elemental compositions of the HA coatings were examined by EDS. Fig. 3(a) shows one of the typical EDS spectra of the coatings, which contained Ca, P and O except to Pd and Au from the Sputtering targets for SEM analysis. The exact Ca/P atomic ratio is also found to be dependent on the hydrothermal post-treatment temperature, given in Fig. 3(b). The Ca/P atomic ratios are less than 1.67 for all the as-
received HA coatings which means that the coating composition is calcium-deficient apatite. And the values of Ca/P atomic ratio increase as hydrothermal post-treatment temperature increases until a value of 1.59 is reached at a temperature of about 453 K. Generally, the Ca/P ratio of biological apatite varies from 1.55 to 1.85, depending on both the species and the functions [16]. So, it can be determined that the temperature at 425 K or 453 K is preferentially selected for preparing HA coating on C/C composites by this novel technology. The FTIR spectra of the HA coatings at different hydrothermal posttreatment temperature are given in Fig. 4. As it is seen, it displays a broad band between 2500 cm− 1 and 4000 cm− 1 corresponding to the presence of H2O in the layers [16]. The characteristic stretching
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Fig. 3. (A) One typical EDS spectrum of the coating on HT-C/C and (B) relation between Ca/P ratio and hydrothermal post-treatment temperature.
modes of the OH− 1 vibration in hydroxyapatite are observed to locate at around 3570 and 630 cm− 1 [16,17], the intensities of which increase with increasing hydrothermal post-treatment temperature. More, all the HA coatings have sharp and well-resolved peaks at 950– 1100 cm− 1 and 550–650 cm− 1, characteristic of a well-crystallized apatite phase [18]. The peaks at 1094 and 1031 cm− 1 are assigned to −1 υ3 stretching modes of PO3− are 4 , and the peaks at 961 and 473 cm assigned to υ1 stretching and υ2 bending modes of PO3− respectively. 4 There are two well-separated peaks at 603 and 564 cm− 1, both attributed to υ4 mode of PO3− [16]. The large separation of the two 4 peaks is another indicator of a highly crystallized apatite phase. These analysis results well agree with the analyses from the X-ray diffraction data. In addition, the IR absorption peak in the regions at 1410 cm− 1 can be assigned to the υ3a mode of the CO2− 3 ions in the apatite lattice which are typical of B-type CO2− 3 -containing apatite, while the peaks at 1574 and 879 cm− 1 are assigned to the υ3b modes of A-type CO2− 3 ions [16]. From the FTIR analyses, it can be confirmed that the CO2− 3 ions reside in both OH− and PO3− 4 sites. Thus, the apatite phases of all the as-prepared coatings are Ca-deficient hydoxyapatites containing CO2− ions, given the formula Ca10 − x(PO4)6 − a(CO3)a(OH)2 − b(CO3)b 3 with 0 b x, a and b b 1. Fig. 5(a) shows the critical load for scratch test of the HA coating on HT-C/C composites at different hydrothermal post-treatment temperature. As can be seen from the Fig. 5(a), samples A, B and C show the critical load of the coating increase with increasing hydrothermal
Fig. 4. FTIR spectra of the coatings on HT-C/C transformed at different hydrothermal post-treatment temperatures.
Fig. 5. (A) Plot of critical versus hydrothermal post-treatment temperature and (B) optical STM images of the scratch test performed on the HA coatings at different hydrothermal post-treatment temperatures.
X. Xin-bo et al. / Surface & Coatings Technology 204 (2009) 115–119
post-treatment temperature, and at 423 K, the highest critical load is achieved which reaches a value of 16.4 N. The shear stress at this stage was about 68.4 MPa calculated using the following expression [19]: 1=2
τc = ðHS Lc= πÞ
=R
ð4Þ
where τc is the shear stress, R is the radius of diamond stylus, Lc is the critical load, HS is the Shore scleroscope hardness of C/C substrate. However, at 453 K, the critical load of the coating on HT-C/C decreases to a minimum value of 6.9 N. According to the analysis results of XRD, SEM, FTIR and EDS, it is proposed that the tendencies of the adhesion strength may be attributed to cooperative effect of the structure and the submeter sized morphologies of the HA coating. Generally, better crystallization of apatite phase crystallized leads to its higher adhesion strength. Thus, when the morphologies of the HA coatings are alike at 373, 403 and 423 K, the critical load shows the increasing tendencies as the hydrothermal post-treatment temperature. However, at 453 K, the micrometer size of the crystal grains occurs which is far larger than that at the other treated temperature. These crystal grains show less dense morphologies than the other grains obtained at 373, 403 or 423 K. Therefore, the adhesion of the HA coating on C/C substrate arrives at a minimum load, as a result of these looser micrometer sized crystals in spite of the best crystallization of HA phase at 453 K. Fig. 5 (b) shows STM images of the scratch trace on HA coating, it can be seen that no fracture or chip is observed at the border or inside the scratch and the coating materials are squashed along the track. From the above results, it can be suggested that 423 K is the best hydrothermal post-treatment temperature for adherent HA coatings which were transformed from monetite coatings produced by the induction heating method on HT-C/C composites. At this hydrothermal post-treatment temperature, the stress strength of HA coating on HT-C/C is greater than the loading stress on the hip joint during gait (b35 MPa) [20], and that of HA coatings deposited on C/C composites by plasma spray (7.15 MPa) [6]. It is also higher than that of C/C composites-bone which is 2.44 MPa, 20 weeks after implantation in mouse [21] and thus is high enough for handling before implant and strong enough to survive in the living body. 4. Conclusion Well-crystallized and dense HA coatings were successfully prepared on H2O2 treated C/C substrate by depositing and subsequently transforming monetite coatings using induction heating deposition
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method and the hydrothermal technique at 373 K, 403 K, 423 K, and 453 K respectively. The degree of crystallinity and the Ca/P ratio of the HA coatings were found to increase with increasing hydrothermal temperature. When the hydrothermal temperature was lower than 423 K, the HA crystals contained nanometer sized grains, occasionally besides flocculation-like deposits. However, at 453 K, the HA crystals were composed of plate-like grains and sporadically flocculation-like deposits. From the scratch test, the highest critical load of 16.4 N was obtained for the as-prepared HA coating on C/C composites at 453 K. So, we reckoned that 423 K was the best hydrothermal treatment temperature for HA coatings which were transformed from monetite coatings produced by the induction heating method on HT-C/C composites.
Acknowledgements This work was supported by National Natural Science Foundation of China with grant no. 50702034.
References [1] P. Christel, A. Meunier, S. Leclercq, P. Bouquet, B. Buttazzoni, J. Biomed. Mater. Res. 21 (1987) 191. [2] N. Cao, Q.S. Ma, J.L. Sui, Q.X. Wan, Y.P. Lu, Y.M. Chen, M.S. Li, Sur. Rev. Lett. 13 (2006) 423. [3] L. Bacáková, V. Starý, O. Kofronová, V. Lisá, J. Biomed. Mater. Res. 54 (2000) 567. [4] N. Cao, Y.Q. Bai, Q.S. Ma, J.L. Sui, M.S. Li, Carbon 46 (2008) 3. [5] T. Kokubo, H.M. Kim, M. Kawashita, T. Nakamura, J. Mater. Sci: Mater in Med. 15 (2004) 99. [6] J.L. Sui, M.S. Li, Y.P. Lu, L.W. Yin, Y.J. Song, Surf. Coat. Technol. 176 (2004) 188. [7] J.L. Sui, M.S. Li, Y.P. Lu, Y.Q. Bai, Surf. Coat. Technol. 190 (2005) 287. [8] Y.Q. Zhai, K.Z. Li, H.J. Li, C. Wang, H. Liu, Mater. Chem. Phys. 106 (2007) 22. [9] T. Fu, L.P. He, Y. Han, K.W. Xu, Y.W. Mai, Mater. Lett. 4371 (2003) 1. [10] T. Fu, Y. Han, K.W. Xu, J.Y. Li, Z.X. Song, Mater. Lett. 57 (2002) 77. [11] J. Gómez Morales, R. Rodríguez Clemente, B. Armas, C. Combescure, R. Berjoan, J. Cubo, E. Martínez, J. García Carmona, S. Garelik, J. Murtra, O.D.N. Muraviev, X, Langmuir. 20 (2004) 5174. [12] X.B. Xiong, X.R. Zeng, C.L. Zhou, Mater. Chem. Phys. 114 (2009) 434. [13] I. Bogdanoviciene, A. Beganskiene, K. Tõnsuaadu, J. Glaser, H. Jürgen Meyer, A. Kareiv, Mater. Res. Bull. 41 (2006) 1754. [14] M.G. Ma, Y.J. Zhu, C. Jiang, J. Phys. Chem. B. 110 (2006) 14226. [15] A. Lebugle, B. Sallek, A. Tai Tai, J. Mater. Chem. 9 (1999) 2511. [16] G.F. Xu, I.A. Aksay, J.T. Groves, J. Am. Chem. Soc. 123 (2001) 2196. [17] E.A. De Maeyer, R.M. Verbeeck, I.Y. Pieters, Inorg. Chem. 35 (1996) 857. [18] E.A.P. De Maeter, R.M.H. Verbeeck, D.E. Naessens, Inorg. Chem. 32 (1993) 5709. [19] X. Cai, J.F. Gu, P.N. Zhou, X.Y. Yang, Trans. Met. Heat Treat. 18 (1997) 30 In Chinese. [20] M.C. Kuo, S.K. Yen, Mater. Sci. Eng. C 20 (2002) 153. [21] O. Chikara, K. Masanobu, M. Toshiki, J. Tissue Eng. Regen. Med. 1 (2007) 33.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of hydrothermal temperature on hydroxyapatite coating transformed from monetite on HT-C/C composites by induction heating method Xiong Xin-bo a,⁎, Zeng Xie-rong a, Zou Chun-li a, Li Ping b, Fan Yun-bo b,⁎ a b
Shen Zhen Key Laboratory of Special Functional Materials, Department of Material, Science and Engineering, Shen Zhen University, Shen Zhen, 518060, PR China Bioengineering Department, Beihang University, Beijing, 100191, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2008 Accepted in revised form 29 June 2009 Available online 5 July 2009 PACS: 87.85.jf 81.05.Uw 81.15.-z 46.70.-p 81.65.-b
a b s t r a c t Hydrothermal post-treatment was used to convert monetite coating fabricated by induction heating method on H2O2 treated C/C (HT-C/C) composites to an adherent HA coating. The monetite coatings were hydrothermally treated for 4 h at 373 K, 403 K, 423 K, and 453 K in a 50 mL autoclave. After hydrothermal post-treatment, the structure, morphology and the chemical composition of these HA coatings were characterized with XRD, FTIR SEM and EDS. A scratch test was conducted to measure the strength of the adhesion of the coatings to the HT-C/C substrate. The results showed that the degree of crystallinity and the Ca/P ratio of the HA coatings increased with increasing hydrothermal temperature. The submicron-level morphology and adhesion of the HA coatings were highly affected by the hydrothermal temperature. From the results, it can be suggested that 423 K was the best hydrothermal treatment temperature for the HA coatings which were transformed from the monetite coatings produced by the induction heating method on HT-C/C composites. © 2009 Elsevier B.V. All rights reserved.
Keywords: Chemical conversion Carbon Cermets and composite Hydroxyapatite Scratch test
1. Introduction Carbon fiber reinforced carbon (C/C) composites have been considered as one alternative material to metallic implants for applications in loaded artificial bones due to their excellent biocompatibility and biomechanical properties [1,2]. Their mechanical properties, especially elastic modulus, are found to be closer to those of loaded human bones than other available bone repairing materials. Thus, stress shielding and sequential bone absorption caused by implant materials with high modulus could be avoided if C/C composites are applied [3]. However, C/C composites are bionert and easily encapsulated after implanted into the living body by fibrous tissue that isolates them from the surrounding bone. Also, due to friction damage during surgical procedures, they may release carbon particles which deposit in the neighborhood of the implant and the lymphatic node, causing “black skin effect” [4]. These drawbacks of C/C composites have limited their potential applications. Hydroxyapatite (HA) has a chemical composition similar to that of the inorganic part of human bones and has an excellent biocompability in comparison with other implant materials [5]. Never-
⁎ Corresponding authors. Xin-bo is to be contacted at Tel.: +86 755 26536239; fax: +86 755 26536239. Yun-bo, Tel./fax: +86 10 82339428. E-mail addresses: [email protected] (X. Xin-bo), [email protected] (F. Yun-bo). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.06.044
theless, its fracture toughness and strength are not high enough for application in human bones. Therefore, in recent years, many efforts have been made to coat bioactive HA or bonelike-apatite onto C/C substrate surfaces for improving the biocompability. Several calcium phosphate coating techniques have been used, for example, plasma spraying [6,7], electrodeposition (or sono-electrodeposition) processes [8], sol–gel [9] and biomimetic techniques [10]. The main problem of these methods is the weak adhesion strength between the HA coating and C/C substrate, which is difficult to improve. Recently, in authors' research group, a novel technique by the combination of induction heating deposition [11] with hydrothermal treatment was developed to coat adherent HA coating on C/C substrate [12]. In this method, specimens of C/C were firstly modified by immersed in an autoclave with H2O2 solution, and then coated with monetite coating by induction heating method. Subsequently, monetite coating on C/C was converted to HA coating by hydrothermal treatment in an autoclave with the ammonia solution. After these procedures, the adhesion of the HA coating was investigated by scratch test and the result showed that the critical load of resultant HA coating with the thickness of 50 μm could reach to 13.12 N, i.e. shear stress of 61.4 MPa. This shear strength is greater than the loading stress of HA coatings deposited on C/C composites by plasma spray (7.15 MPa). Besides this, the technique was found to have the following advantages: thicker films, better control over the deposited solid phase, the ability to deposit porous or complex shapes, and lower
116
X. Xin-bo et al. / Surface & Coatings Technology 204 (2009) 115–119
processing temperature. So it is worth further investigation for use in implants. The aim of this study is to investigate the influence of hydrothermal treatment on the phases, microstructures and adhesions of HA coatings transformed from monetite produced by the induction heating method on HT-C/C composites. 2. Experimental procedure 2.1. Experimental setup and mother solution Details of the experimental setup for induction heat deposition process are described elsewhere [12]. In the experiment, the mother solution was prepared by dissolving given amounts of reagent-grade chemicals of 0.04 M Ca(NO3)2 and 0.024 M NH4H2PO4 into distilled water. This solution was buffered to a pH of 4.5 with an adequate amount of ammonia. 2.2. C/C substrates and pretreatments C/C composites were prepared by chemical vapor infiltration (CVI) processing in Northwest Polytechnology University in China. The density and Shore scleroscope hardness of them are average 1.72 g/ cm3 and 36.1 respectively. C/C samples were cut from the block and had a diameter of 10 mm and a length of 10 mm. Prior to the coating runs, each sample was polished with No.600 and No.1000 abrasive paper, rinsed with distilled water, then cleaned ultrasonically in acetone, and dried in a desiccator. And H2O2 in analytical grade was dissolved in deinonized water to prepare 2.0 M solution at ambient temperature. Then, samples were pretreated in high pressure steam in a 50 mL autoclave with H2O2 solution at 433 K. After removal from H2O2 solution, these cylinders were rinsed ultrasonically with deionized water and dried in air. 2.3. Coating experiments and post-treatments All the deposition experiments were carried out at the applied current of 500 A at room temperature. At the end of each run, the coated cylinders was rinsed with distilled water and then hydrothermally treated in an autoclave with the ammonia solution at pH of 8 for 4 h. The temperatures of the hydrothermal treatment for the coatings are 373 K, 403 K, 423 K and 453 K respectively. After hydrothermal treatment, all the coated samples were annealed in vacuum to removal of water in the coatings at 473 K for 1 h. 2.4. Characterization The crystalline structure, morphologies and compositions of the coated samples were characterized by X-ray diffraction (XRD) using a D8 Advance X-ray diffractometer (Cu-Ka radiation), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis with a s-3400N (Japan) microscope. The functional groups of HA coatings were quantitatively identified by Fourier transform infrared (FTIR). The FTIR spectra were recorded in the 400–4000 cm− 1 range, resolution 4 cm− 1, using Perkin Elmer instruments Spectrum One Spectrometer and KBr pellet technology. The adhesion strength of the HA coatings deposited on C/C substrates were determined by the scratch test method using a s-3400N scratch tester fitted with a Rochwell C 0.2 mm-diamond stylus with a preload of 1 N, load speed 10 N/min, scratch speed 5 mm/min and the maximum load of 20 N. Finally, the scratch trace on HA coating was observed by stereomicroscope (STM).
Fig. 1. XRD spectra of (a) HT-C/C, (b) monetite coating on HT-C/C by induction heating deposition and (c), (d), (e), (f) transformed hydroxyapatite coating from monetite by hydrothermal post-treatment on HT-C/C at 373 K, 403 K, 423 K and 453 K respectively.
heating method consists of calcium hydrogen orthophosphate (CaHPO4, monetite). After hydrothermal post-treatment at different temperatures, monetite phases of all the coatings were transformed to hydroxyapatite with the three most intensive peaks ((211), (300), (112)) between 2θ = 30 – 35° [13] which are in a good agreement with the reference data for JCPDS 9-432. These results indicate that hydrothermal post-treatment method helps to convert monetite phase to well crystallised hydroxyapatite. From the XRD spectra of the coatings on HT-C/C, It can also be found that the intensity of HA peaks increases with increasing hydrothermal treatment temperatures (and this is especially true for 453 K) as a result of grain growth. Fig. 2(a) shows SEM images of monetite coating on HT-C/C composites of HA coating. It can be seen that the as-received monetite coating on HT C/C substrate is composed of sheet-like crystals with smooth surface, which agglomerated together to form dense morphology. After hydrothermal post-treatment, the morphologies of these crystals show no significant change and their dense microstructures still keep when all the coatings are observed at low times magnification, as presented in Fig. 2(b), (c), (d) and (e). But, at high times magnification differences in sub-micrometer morphologies can be found. When the hydrothermal temperature is lower than 403 K, nanometer sized grains occur in the HA crystals. However when the hydrothermal temperature was 423 K, besides nanometer sized grains, there are deposits on the surface of these grains. Surprisingly, when the hydrothermal temperature reaches to 453 K, plate-like grains besides some deposits are observed in the HA crystals. Also, it is clear that the size of these grains on the HA crystals increases with increasing hydrothermal post-treatment temperature. The transformation of monetite to HA was proposed to involve a continuous process of dissolution and a re-precipitation by Da silva et al. [14], as shown in Eqs. (1) and (2) 2+
CaHPO4 →Ca 2+
ð10−XÞCa
2−
ð1Þ
+ HPO4
2−
+ 6HPO4
−
+ 2OH →Ca10−X ðPO4 Þ6 ðOHÞ2 where 0bxb1 ð2Þ
or follow a topotactic mechanism, concerning incorportion of calcium ions into the monetite solid phase and HA crystallization by Lebugle et al. [15], as shown in Eq. (3).
3. Results and discussion 2+
Fig. 1 shows the XRD spectra of the as-deposited and hydrothermal post-treated coatings on HT-C/C. The coating deposited by induction
ð3−XÞCa
−
+ 6CaHPO4 + 2OH →Ca10−X ðPO4 Þ6 ðOHÞ2 where 0bxb1 ð3Þ
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Fig. 2. Morphologies of the coatings on HT-C/C. (A) Induction heating deposition, and (B) at 373; (C) at 403 K; (D) at 423 K; (E) at 453 K hydrothermal post-treatment respectively.
According to these proposals, we suggest that the sheet-like crystals are composed of the nano-sized or plate-like grains which is as a result of the incorporation of calcium ions into the monetite solid phase and HA crystallization, while the deposits result from the reprecipitation of ions containing Ca and P elements which exist in the solution owing to dissolution of monetite crystals. The elemental compositions of the HA coatings were examined by EDS. Fig. 3(a) shows one of the typical EDS spectra of the coatings, which contained Ca, P and O except to Pd and Au from the Sputtering targets for SEM analysis. The exact Ca/P atomic ratio is also found to be dependent on the hydrothermal post-treatment temperature, given in Fig. 3(b). The Ca/P atomic ratios are less than 1.67 for all the as-
received HA coatings which means that the coating composition is calcium-deficient apatite. And the values of Ca/P atomic ratio increase as hydrothermal post-treatment temperature increases until a value of 1.59 is reached at a temperature of about 453 K. Generally, the Ca/P ratio of biological apatite varies from 1.55 to 1.85, depending on both the species and the functions [16]. So, it can be determined that the temperature at 425 K or 453 K is preferentially selected for preparing HA coating on C/C composites by this novel technology. The FTIR spectra of the HA coatings at different hydrothermal posttreatment temperature are given in Fig. 4. As it is seen, it displays a broad band between 2500 cm− 1 and 4000 cm− 1 corresponding to the presence of H2O in the layers [16]. The characteristic stretching
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Fig. 3. (A) One typical EDS spectrum of the coating on HT-C/C and (B) relation between Ca/P ratio and hydrothermal post-treatment temperature.
modes of the OH− 1 vibration in hydroxyapatite are observed to locate at around 3570 and 630 cm− 1 [16,17], the intensities of which increase with increasing hydrothermal post-treatment temperature. More, all the HA coatings have sharp and well-resolved peaks at 950– 1100 cm− 1 and 550–650 cm− 1, characteristic of a well-crystallized apatite phase [18]. The peaks at 1094 and 1031 cm− 1 are assigned to −1 υ3 stretching modes of PO3− are 4 , and the peaks at 961 and 473 cm assigned to υ1 stretching and υ2 bending modes of PO3− respectively. 4 There are two well-separated peaks at 603 and 564 cm− 1, both attributed to υ4 mode of PO3− [16]. The large separation of the two 4 peaks is another indicator of a highly crystallized apatite phase. These analysis results well agree with the analyses from the X-ray diffraction data. In addition, the IR absorption peak in the regions at 1410 cm− 1 can be assigned to the υ3a mode of the CO2− 3 ions in the apatite lattice which are typical of B-type CO2− 3 -containing apatite, while the peaks at 1574 and 879 cm− 1 are assigned to the υ3b modes of A-type CO2− 3 ions [16]. From the FTIR analyses, it can be confirmed that the CO2− 3 ions reside in both OH− and PO3− 4 sites. Thus, the apatite phases of all the as-prepared coatings are Ca-deficient hydoxyapatites containing CO2− ions, given the formula Ca10 − x(PO4)6 − a(CO3)a(OH)2 − b(CO3)b 3 with 0 b x, a and b b 1. Fig. 5(a) shows the critical load for scratch test of the HA coating on HT-C/C composites at different hydrothermal post-treatment temperature. As can be seen from the Fig. 5(a), samples A, B and C show the critical load of the coating increase with increasing hydrothermal
Fig. 4. FTIR spectra of the coatings on HT-C/C transformed at different hydrothermal post-treatment temperatures.
Fig. 5. (A) Plot of critical versus hydrothermal post-treatment temperature and (B) optical STM images of the scratch test performed on the HA coatings at different hydrothermal post-treatment temperatures.
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post-treatment temperature, and at 423 K, the highest critical load is achieved which reaches a value of 16.4 N. The shear stress at this stage was about 68.4 MPa calculated using the following expression [19]: 1=2
τc = ðHS Lc= πÞ
=R
ð4Þ
where τc is the shear stress, R is the radius of diamond stylus, Lc is the critical load, HS is the Shore scleroscope hardness of C/C substrate. However, at 453 K, the critical load of the coating on HT-C/C decreases to a minimum value of 6.9 N. According to the analysis results of XRD, SEM, FTIR and EDS, it is proposed that the tendencies of the adhesion strength may be attributed to cooperative effect of the structure and the submeter sized morphologies of the HA coating. Generally, better crystallization of apatite phase crystallized leads to its higher adhesion strength. Thus, when the morphologies of the HA coatings are alike at 373, 403 and 423 K, the critical load shows the increasing tendencies as the hydrothermal post-treatment temperature. However, at 453 K, the micrometer size of the crystal grains occurs which is far larger than that at the other treated temperature. These crystal grains show less dense morphologies than the other grains obtained at 373, 403 or 423 K. Therefore, the adhesion of the HA coating on C/C substrate arrives at a minimum load, as a result of these looser micrometer sized crystals in spite of the best crystallization of HA phase at 453 K. Fig. 5 (b) shows STM images of the scratch trace on HA coating, it can be seen that no fracture or chip is observed at the border or inside the scratch and the coating materials are squashed along the track. From the above results, it can be suggested that 423 K is the best hydrothermal post-treatment temperature for adherent HA coatings which were transformed from monetite coatings produced by the induction heating method on HT-C/C composites. At this hydrothermal post-treatment temperature, the stress strength of HA coating on HT-C/C is greater than the loading stress on the hip joint during gait (b35 MPa) [20], and that of HA coatings deposited on C/C composites by plasma spray (7.15 MPa) [6]. It is also higher than that of C/C composites-bone which is 2.44 MPa, 20 weeks after implantation in mouse [21] and thus is high enough for handling before implant and strong enough to survive in the living body. 4. Conclusion Well-crystallized and dense HA coatings were successfully prepared on H2O2 treated C/C substrate by depositing and subsequently transforming monetite coatings using induction heating deposition
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method and the hydrothermal technique at 373 K, 403 K, 423 K, and 453 K respectively. The degree of crystallinity and the Ca/P ratio of the HA coatings were found to increase with increasing hydrothermal temperature. When the hydrothermal temperature was lower than 423 K, the HA crystals contained nanometer sized grains, occasionally besides flocculation-like deposits. However, at 453 K, the HA crystals were composed of plate-like grains and sporadically flocculation-like deposits. From the scratch test, the highest critical load of 16.4 N was obtained for the as-prepared HA coating on C/C composites at 453 K. So, we reckoned that 423 K was the best hydrothermal treatment temperature for HA coatings which were transformed from monetite coatings produced by the induction heating method on HT-C/C composites.
Acknowledgements This work was supported by National Natural Science Foundation of China with grant no. 50702034.
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