- Email: [email protected]
Ti composite coatings
Biomaterials 21 (2000) 841}849
Bond strength of plasma-sprayed hydroxyapatite/Ti composite coatings Xuebin Zheng*, Minhui Huang, Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People+s Republic of China
Abstract One of the most important clinical applications of hydroxyapatite (HA) is as a coating on metal implants, especially plasma-sprayed HA coating applied on Ti alloy substrate. However, the poor bonding strength between HA and Ti alloy has been of concern to orthopedists. In this paper, an attempt has been made to enhance the bonding strength of HA coating by forming a composite coating with Ti. The bioactivity of the coating has also been studied. HA/Ti composite coatings were prepared via atmospheric plasma spraying on Ti}6Al}4V alloy substrates. The bond strength evaluation of HA/Ti composite coatings was performed according to ASTM C-633 test method. X-ray di!ractometer and scanning electron microscopy were applied to identify the phases and the morphologies of the coatings. The bioactivity of HA/Ti composite coating was quali"ed by immersion of coating in simulated body #uid (SBF). The obtained results revealed that the addition of Ti to HA improved the bonding strength of coating signi"cantly. In the SBF test, the coating surface was covered by carbonate-apatite, which was testi"ed by X-ray photoelectron spectroscope, indicating good bioactivity for HA/Ti composite coating. The bioactivity of the coating has not been reduced by the addition of Ti. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyapatite; Composite coating; Plasma spraying; Bonding strength; SBF
1. Introduction Ceramics such as bioglasses in the Na O}CaO} 2 SiO }P O (Bioglass) [1], sintered hydroxyapatite 2 2 5 (Ca (PO ) (OH) ) [2] and glass-ceramics containing 10 46 2 apatite and Wollastonite (CaO}SiO ) [3] systems have 2 been found to bond to living bone, and meet wide clinical applications. However, they cannot be used at highly loaded places such as fermoral and tibial cortical bones. For these reasons, various hydroxyapatite (HA)-based composites have been fabricated and the HA-coated Ti or Ti alloy by plasma spraying has found wide application for these places [4}6]. These coatings combine the mechanical advantages of metal alloy with the excellent biocompatibility and bioactivity of HA. Nevertheless, the long-term stability of this material is still questionable. Despite the strong bonding between the HA coating and bone structure, it has been recognized that the mechanical stability of the interface between the coating and the metallic substrate could be a problem either during surgi-
* Corresponding author. Fax: 86-21-62513903. E-mail address: [email protected] (X. Zheng)
cal operation or after implantation for a given time [7,8]. To overcome this problem, several attempts have been made [9}11]. One of these approaches is to form a composite coating by mechanically strong bioinert, biocompatible metals such as Ti and the bioactive but mechanically fragile HA [12,13]. In this paper, the preparation of HA/Ti composite coatings by atmospheric plasma spraying (APS) is described. The bonding strengths of fabricated specimens were tested by ASTM C-633 method. Microstructure and phase composition of HA/Ti coatings were examined by scanning electron microscopy (SEM) and X-ray di!ractometer (XRD), respectively. The bioactivity of coatings was evaluated by examining carbonated apatite formation on their surface in simulated body #uid (SBF).
2. Experimental procedure Commerially available HA and Ti powders, with typical size ranges of 45}160 and 60}100 lm respectively, were used. The HA and Ti powders were mixed in a ball mill pulverizer for 5 h. Two shapes of Ti}6Al}4V substrate were used: one plate-like (20]10]4 mm) for micro-
0142-9612/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 9 9 ) 0 0 2 5 5 - 0
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Table 1 Spray parameters Plasma gas Ar Plasma gas H 2 Spray distance Coating thickness
40 slpm 10 slpm 90 mm 200 lm
Powder carrier gas Powder feed rate Current Voltage
Ar, 2.0 slpm 25 g/min 650 A 58 V
Table 2 Ion concentration of SBF in comparison with human blood plasma Concentration (mM) Na` K` SBF 142.0 5.0 Blood plasma 142.0 5.0
Ca2` Mg2` HCO~ Cl~ 3 2.5 2.5
1.5 1.5
4.2 27.0
HPO2~ SO2~ 4 4
148.5 1.0 103.0 1.0
0.5 0.5
structure observation, phase analyses and SBF test and the other cylindrical (H25.4 mm) for bonding strength measurements according to ASTM C-633. APS system (Sulzer Metco, Swizerland) was applied to fabricate HA/Ti composite coatings under modi"ed spray parameters listed in Table 1. Pure HA coating was also fabricated for comparison. Coating thickness for all specimens was about 200 lm. After being ultrasonically washed in acetone, rinsed in deionized water, specimens were soaked in the SBF solution, whose ion concentrations are given in Table 2. The SBF solution was bu!ered at PH 7.4 with trimethanol aminomethane}HCl. Duplicate samples were immersed in SBF for 1, 3, 7, 14 d at 36.53C without stirring. SEM and XRD were used to observe surface morphologies and determine phase composition of HA/Ti before and after immersion. X-ray photoelectron spectroscopy (XPS) was used for qualitative determination of the elements which are present on the sample surface after immersion. Spectra were referenced to C 1s peak of adventitious carbon "xed at 284.6 eV.
3. Results and discussion 3.1. Morphology and phase composition Fig. 1 shows the as-sprayed surfaces of HA/Ti composite coating with 20 wt% Ti, which is characterized by many pores and some microcracks. The electron microprobe Ca K and Ti K analyses of the composite coating a a shown in Fig. 1b and c respectively indicate that Ca and Ti are relatively well distributed in the coating. Under higher magni"cation the coating is characterized by some cracks, as shown in Fig. 1d where a splat, testi"ed to be a Ti particle according to Ti K picture shown in a
Fig. 1e, appears on one crack. The Ti splat is not split by the crack, suggesting that Ti has the ability to bridge cracks and thus hinder the spread of cracks in the coating to some degree. Morphologies of HA/Ti composite coating with 60 wt% Ti are shown in Fig. 2, where a rougher surface with less cracks is seen as compared with the HA/Ti 20 wt% coating shown seen in Fig. 1a. The Ti K and Ca K pictures demonstrate that Ca and Ti are a a dispersed evenly in the composite coating. The higher magni"cation shown in Fig. 2b and the corresponding Ti K picture shown in Fig. 2c reveal that some cracks in the a coating ended by a Ti splat. The ability of Ti particle to impede the propagation of crack is further testi"ed. XRD patterns for HA coating and HA/Ti composite coatings with 20 and 60 wt% Ti are shown in Fig. 3. From Fig. 3a, it can be seen that the HA coating contains some amorphous phases and new phases such as CaO and Ca (PO ) (TCP). Amorphous phases resulting 3 42 from the quenching of molten particles on substrate during spraying reduce with increasing Ti content in the coatings owing to the di!erences in the crystal structures of HA and Ti. HA has a complicated crystal structure with a space group of P6 /m, while Ti has a simpler one, 3 hexagonal-close-packing. Therefore, it takes shorter time for Ti to arrange atoms to "t the crystal structure. This helps Ti restore its crystal structure in the process of coat formation. For this reason, with the increase in Ti content the amorphous phases decrease. It is noteworthy that the TiO phase appeared in the HA/Ti composite coatings as seen from Fig. 3b and 3c; this is explained in terms of the oxidation of Ti during plasma spraying in air. However, the XPS Ti 2p pattern shown in Fig. 4 illustrates that the binding energy determined for Ti 2p is 453.7 eV (another peak 459.6 eV) and 456.8 eV (another peak 462.8 eV). The "rst value coincides with Ti very well and the second one is between the values for TiO, 455.1 eV, and TiO , 458.7 eV [14], 2 suggesting the oxide of Ti is TiO (x"1}2). The reason x for the di!erence between XRD and XPS results is that the testing depth in the surface of specimen for XPS is limited to several nanometers but that for XRD is much deeper [15]. In the outermost layer of the coating surface, some TiO is further oxidized to TiO (x"1}2) in air, x which is detected by XPS. However, the inner layer of the coating remained as Ti and TiO; therefore, the XRD pattern can illustrate the peaks only for Ti and TiO. 3.2. Bonding strength evaluation The bonding strength evaluation shown in Fig. 5 indicates that the mean bond strength increased from 12.9 to 14.5 and 17.3 MPa while reinforcing the coating with 20 and 60 wt% Ti, respectively. The improvement in the bonding strength of the coatings by the addition of Ti to HA has been testi"ed from Fig. 5, and with the increase in Ti content the adhesion of the coating to the substrate is
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Fig. 1. SEM pictures of HA/Ti 20 wt% composite coating: (a) morphology of coating surface; (b) Ca K picture for (a); (c) Ti K picture for (a); a a (d) higher magni"cation; (e) Ti K picture for (d). a
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Fig. 2. SEM pictures of HA/Ti 60 wt% composite coating: (a) morphology of coating surface; (b) Ca K picture for (a); (c) Ti K picture for (a); a a (d) higher magni"cation; (e) Ti K picture for (d). a
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Fig. 3. XRD patterns for HA coating and HA/Ti composite coatings: (a) HA coating; (b) HA/Ti 20 wt% coating; (c) HA/Ti 60 wt% coating.
Fig. 5. Bonding strength for HA coating and HA/Ti composite coatings. Fig. 4. XPS Ti 2p spectrum for HA/Ti 20 wt% composite coating.
further enhanced. The relatively poor adhesion of pure HA coating mainly arises from the mismatch of the coe$cients of thermal expansion between the Ti alloy substrate and the HA coating [16]. The mismatch of the coe$cients of thermal expansion is reduced and most of
the residual stresses that occurred during spraying process are prevented by the addition of Ti to HA coating. Therefore, the fabrication of HA/Ti composite coating can reinforce the adhesion of the coating. The images of fracture surfaces after bond strength test are shown in Fig. 6. Although adhesive failure is the main fracture mechanism for the HA coating, some HA
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Fig. 6. Failure surface images for HA coating and HA/Ti composite coatings: (a) HA coating; (b) HA/Ti 20 wt% coating; (c) HA/Ti 60 wt% coating.
Fig. 7. XRD patterns for HA coating and HA/Ti composite coatings after immersion in SBF for 14 d: (a) HA coating; (b) HA/Ti 20 wt% coating; (c) HA/Ti 60 wt% coating.
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Fig. 8. Morphology of HA coating after immersion in SBF for 14 d.
remains on the surface, as shown in Fig. 6a, which indicates that cohesive failure also exists during the bond strength evaluation owing to weak mechanical properties such as fracture toughness for HA coating. However, it can be seen from Fig. 6b and c that the fracture mechanism for HA/Ti composite coatings is simply the adhesive failure, which means that the addition of Ti not only increases the adhesive strength between the coating and the substrate but also enhances the cohesive strength of particles in the coating. 3.3. SBF test The results of XRD analysis of the HA coating and HA/Ti 20 wt%, HA/Ti 60 wt% composite coatings, immersed in SBF for 14 d, are shown in Fig. 7. The observation of XRD patterns reveals that impurity phases such as TCP and CaO disappear after 14 d of immersion, which is explained in terms of the reaction of TCP and CaO with water and their dissolution in water. Besides Ti and TiO, TiO is also observed in XRD patterns, which 2 indicates that not only the TiO in the outermost layer of the coating but also that in the inner layer is further oxidized during the SBF test. The existence of TiO 2 seems to induce apatite nucleation on the specimen's surface in SBF [17,18]. SEM examination of the HA coating after immersion in SBF for 14 d, as shown in Fig. 8, reveals that the coating surface is completely covered by a dune-like layer. Microcracks of tortoiseshell character appeared on the newly formed layer, similar to the cracks formed naturally on a dry mud deposit. Morphologies of the HA/Ti 20 wt% composite coating after immersion in SBF for 1, 3, 7, and 14 d are shown in Fig. 9. In Fig. 9a, a loose and thin apatite layer constituted by small granules is observed on the coating surface after soaking in SBF for 1 d. With longer immersion periods, the layer becomes dense and the granular apatite in the layer
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grows gradually as seen from Fig. 9b}d. At a higher magni"cation (Fig. 9e), the apatite layer shows very small crystallites. The energy dispersive spectroscopy analysis (EDS) indicates that the layer is composed of Ca and P and XPS analysis is used to further determine the chemical composition of the apatite layer. XPS analysis results of the elements seen on the surface of the composite after immersion in SBF for 1 d are recorded in Fig. 10. From Fig. 10, O 1s, Ca 2p, P 2p, and C 1s level spectra are identi"ed, indicating that the apatite layer formed on the coating comprises O, Ca, P, and C. Quantitative elemental analysis revealed that the Ca/P mole ratio is 1.29, indicating that the apatite formed is Ca-de"cient, which is in accordance with the work of Panjian Li [19]. Since the identi"ed C 1s peak is very strong and broad, it is deconvoluted using the Gaussian curve "tting process. The deconvoluted peaks shown in Fig. 11 reveal that the C 1s peak is composed of two peaks: one at 284.6 eV and another at 287.8 eV, corresponding to the carbon contamination and the carbonate group CO2~ [20]. The 3 SEM pictures and XPS results reveal that the layer formed on the coating surface is carbonate-apatite, which is considered to be the symbol of the activity for biomaterials [20,21]. The surface picture for HA/Ti 60 wt% composite coating after immersion in SBF is shown in Fig. 12. Carbonate-apatite layer also appears on the surface of the coating. The good bioactivity of the composite coatings is con"rmed by the SBF test. Although the addition of Ti reduces the HA content in the coating, it does not distinctly a!ect the formation of carbonate-apatite layer in SBF. This pheonenomen may be related to the ability of TiO to induce apatite nucleation in SBF. More works 2 are required to verify this.
4. Conclusion In the plasma-sprayed HA/Ti composite coatings, Ti and HA are well distributed. The addition of Ti could help to improve the mechanical properties of the coatings, especially elevate the bonding strength. The bonding strength evaluation indicates that HA/Ti composite coatings possess much higher bonding strength than the HA coating, suggesting that the fabrication of HA/Ti composite coating is a useful way of improving the adhesive strength of the coating. The behavior of the composite coating in SBF shows that the composite coatings are reactive in SBF. After immersion in SBF for a period as short as 1 d, apatite layer appears on the surface of the coating, which is testi"ed by XPS to be carbonate-apatite. This result proves the good bioactivity of the HA/Ti composite coatings. This work shows that the fabrication of HA/Ti composite coatings can enhance the bonding strength, and that it does not a!ect the bioactivity of the coatings signi"cantly.
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Fig. 9. Morphologies of HA/Ti 20 wt% composite coating after immersion in SBF for 1, 3, 7, and 14 d: (a) 1 d; (b) 3 d; (c) 7 d; (d) 14 d; (e) higher magni"cation for apatite layer.
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Fig. 10. XPS spectrum for HA/Ti 20 wt% composite coating after immersion in SBF for 1 d.
Fig. 11. XPS C 1s spectrum for HA/Ti 20 wt% composite coating after immersion in SBF for 1 d.
References [1] Hench LL, Splinter RJ, Allen WC, Greenlee TK. Bonding mechanism at interface of ceramics prosthetic materials. J Biomed Mater Res Symp 1972;2:117}41. [2] Jarcho T, Kay JF, Gumaer KI, Doremus RH, Drobeck HP. Tissue, cellular and subcellular events at a bone}ceramic hydroxyapatite interface. J Bioeng 1977;1:79}92. [3] Kokubo T, Shigematsu M, Nagashima Y, Tashire M, Nakamura T, Yamamuro T, Higashi S. Apatite and wollastonite-containing glass-ceramics for prosthetic application. Bull Inst Chem Res (Kyoto Univ) 1982;60:260}8. [4] Suchanek W, Yoshimura M. Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants. J Mater Res 1998;13:94}117. [5] Klein CPAT, Patka P, Van der Lubbe HBM, Wolke JGC, De Groot K. Plasma sprayed coating of tetracalcium phosphate, hydroxyapatite, and a-TCP on titanium alloy: an interfacial study. J Biomed Mater Res 1991;25:53}65. [6] De Groot K, Geesink RGT, Klein CPAT, Serekian P. Plasma sprayed coatings of hydroxyapatite. J Biomed Mater Res 1987;21:1375}87. [7] Cook SD, Thomas KA, Kay JF. Experimental coating defect in hydroxyapatite coated implants. Clin Orthop 1991;265: 280}90.
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Fig. 12. Morphology of HA/Ti 60 wt% composite coating after immersion in SBF for 14 d. [8] Collier JP, Suprenant VA, Mayor MB, Wrona M, Jensen RE, Suprenant HP. Loss of hydroxyapatite coating on retrieved total hip components. J Arthroplasty 1993;8:389}92. [9] Wang CK, Lin JHC, Ju CP, Ong HC, Chang RPH. Structural characterization of pulsed laser-deposited hydroxyapatite "lm on titanium substrate. Biomater 1997;18:1331}8. [10] Chang E, Chang WJ, Wang BC, Yang CY. Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium. J Mater Sci Mater Med 1997;8:193}200. [11] Khor KA, Yip CS, Cheang P. Ti}6Al}4V/hydroxyapatite composite coatings prepared by thermal spray techniques. J Therm Spray Technol 1997;6:109}15. [12] Bishop A, Lin CY, Navaratnam M, Rawlings RD, McShane HB. A functional gradient material produced by a powder metallurgy process. J Mater Sci Lett 1993;12:1516}8. [13] Oki S, Gohda S, Sohmura T, Kimura T, Ono K, Yoshioka T. Preparation of porous spray coating for biomaterials with a titanium-calcium phosphate hybridized powder. In: Berndt CC, editor. Thermal spray: international advances in coatings technology, ASM International, 1992. p. 447}51. [14] Wagner CD, Riggs WM, Davies LE, Moulder JF, Muilenberg GE. Handbook of X-ray photoelectron spectroscopy. Eden Prairie, Minnesota: Perkin-Elmer, 1979. [15] Hench LL. Characterization of bioceramics. In: Hench LL, Wilson J, editors. An Introduction to Bioceramics. USA: World Scienti"c, 1993. p. 319}34. [16] Heimann RB, Kurzweg H, Vu TA. Hydroxyapatite-bond coat systems for improved mechanical and biological performance of hip implants. Proceedings of the 15th International Thermal Spray Conference, France, 1998. p. 999}1005. [17] Kokubo T, Miyaki F, Kim H, Nakamura T. Spontaneous formation of bonelike apatite layer on chemically treated titanium metals. J Am Ceram Soc 1996;79:1127}9. [18] Wen HB, De Wijn JR, Cui FZ, De Groot K. Preparation of bioactive Ti6Al4V surfaces by a simple method. Biomaterials 1998;19:215}21. [19] Li P, Kangasniemi I, De Groot K. Bone like hydroxyapatite induction by a gel-derived titania on a titanium substrate. J Am Ceram Soc 1994;77:1307}12. [20] Santos JD, Jha LJ, Monteriro FJ. In vivo calcium phosphate formation on SiO }NaO}CaO}P O glass reinforced hy2 2 5 droxyapatite composites: a study by XPS analysis. J Mater Sci Mater Med 1996;7:181}5. [21] LeGeros RZ, LeGeros JP. Dense hydroxyapatite. In: Hench LL, Wilson J, editors. An introduction to bioceramics. USA: World Scienti"c, 1993. p. 162}5.
Bond strength of plasma-sprayed hydroxyapatite/Ti composite coatings Xuebin Zheng*, Minhui Huang, Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People+s Republic of China
Abstract One of the most important clinical applications of hydroxyapatite (HA) is as a coating on metal implants, especially plasma-sprayed HA coating applied on Ti alloy substrate. However, the poor bonding strength between HA and Ti alloy has been of concern to orthopedists. In this paper, an attempt has been made to enhance the bonding strength of HA coating by forming a composite coating with Ti. The bioactivity of the coating has also been studied. HA/Ti composite coatings were prepared via atmospheric plasma spraying on Ti}6Al}4V alloy substrates. The bond strength evaluation of HA/Ti composite coatings was performed according to ASTM C-633 test method. X-ray di!ractometer and scanning electron microscopy were applied to identify the phases and the morphologies of the coatings. The bioactivity of HA/Ti composite coating was quali"ed by immersion of coating in simulated body #uid (SBF). The obtained results revealed that the addition of Ti to HA improved the bonding strength of coating signi"cantly. In the SBF test, the coating surface was covered by carbonate-apatite, which was testi"ed by X-ray photoelectron spectroscope, indicating good bioactivity for HA/Ti composite coating. The bioactivity of the coating has not been reduced by the addition of Ti. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyapatite; Composite coating; Plasma spraying; Bonding strength; SBF
1. Introduction Ceramics such as bioglasses in the Na O}CaO} 2 SiO }P O (Bioglass) [1], sintered hydroxyapatite 2 2 5 (Ca (PO ) (OH) ) [2] and glass-ceramics containing 10 46 2 apatite and Wollastonite (CaO}SiO ) [3] systems have 2 been found to bond to living bone, and meet wide clinical applications. However, they cannot be used at highly loaded places such as fermoral and tibial cortical bones. For these reasons, various hydroxyapatite (HA)-based composites have been fabricated and the HA-coated Ti or Ti alloy by plasma spraying has found wide application for these places [4}6]. These coatings combine the mechanical advantages of metal alloy with the excellent biocompatibility and bioactivity of HA. Nevertheless, the long-term stability of this material is still questionable. Despite the strong bonding between the HA coating and bone structure, it has been recognized that the mechanical stability of the interface between the coating and the metallic substrate could be a problem either during surgi-
* Corresponding author. Fax: 86-21-62513903. E-mail address: [email protected] (X. Zheng)
cal operation or after implantation for a given time [7,8]. To overcome this problem, several attempts have been made [9}11]. One of these approaches is to form a composite coating by mechanically strong bioinert, biocompatible metals such as Ti and the bioactive but mechanically fragile HA [12,13]. In this paper, the preparation of HA/Ti composite coatings by atmospheric plasma spraying (APS) is described. The bonding strengths of fabricated specimens were tested by ASTM C-633 method. Microstructure and phase composition of HA/Ti coatings were examined by scanning electron microscopy (SEM) and X-ray di!ractometer (XRD), respectively. The bioactivity of coatings was evaluated by examining carbonated apatite formation on their surface in simulated body #uid (SBF).
2. Experimental procedure Commerially available HA and Ti powders, with typical size ranges of 45}160 and 60}100 lm respectively, were used. The HA and Ti powders were mixed in a ball mill pulverizer for 5 h. Two shapes of Ti}6Al}4V substrate were used: one plate-like (20]10]4 mm) for micro-
0142-9612/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 9 9 ) 0 0 2 5 5 - 0
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Table 1 Spray parameters Plasma gas Ar Plasma gas H 2 Spray distance Coating thickness
40 slpm 10 slpm 90 mm 200 lm
Powder carrier gas Powder feed rate Current Voltage
Ar, 2.0 slpm 25 g/min 650 A 58 V
Table 2 Ion concentration of SBF in comparison with human blood plasma Concentration (mM) Na` K` SBF 142.0 5.0 Blood plasma 142.0 5.0
Ca2` Mg2` HCO~ Cl~ 3 2.5 2.5
1.5 1.5
4.2 27.0
HPO2~ SO2~ 4 4
148.5 1.0 103.0 1.0
0.5 0.5
structure observation, phase analyses and SBF test and the other cylindrical (H25.4 mm) for bonding strength measurements according to ASTM C-633. APS system (Sulzer Metco, Swizerland) was applied to fabricate HA/Ti composite coatings under modi"ed spray parameters listed in Table 1. Pure HA coating was also fabricated for comparison. Coating thickness for all specimens was about 200 lm. After being ultrasonically washed in acetone, rinsed in deionized water, specimens were soaked in the SBF solution, whose ion concentrations are given in Table 2. The SBF solution was bu!ered at PH 7.4 with trimethanol aminomethane}HCl. Duplicate samples were immersed in SBF for 1, 3, 7, 14 d at 36.53C without stirring. SEM and XRD were used to observe surface morphologies and determine phase composition of HA/Ti before and after immersion. X-ray photoelectron spectroscopy (XPS) was used for qualitative determination of the elements which are present on the sample surface after immersion. Spectra were referenced to C 1s peak of adventitious carbon "xed at 284.6 eV.
3. Results and discussion 3.1. Morphology and phase composition Fig. 1 shows the as-sprayed surfaces of HA/Ti composite coating with 20 wt% Ti, which is characterized by many pores and some microcracks. The electron microprobe Ca K and Ti K analyses of the composite coating a a shown in Fig. 1b and c respectively indicate that Ca and Ti are relatively well distributed in the coating. Under higher magni"cation the coating is characterized by some cracks, as shown in Fig. 1d where a splat, testi"ed to be a Ti particle according to Ti K picture shown in a
Fig. 1e, appears on one crack. The Ti splat is not split by the crack, suggesting that Ti has the ability to bridge cracks and thus hinder the spread of cracks in the coating to some degree. Morphologies of HA/Ti composite coating with 60 wt% Ti are shown in Fig. 2, where a rougher surface with less cracks is seen as compared with the HA/Ti 20 wt% coating shown seen in Fig. 1a. The Ti K and Ca K pictures demonstrate that Ca and Ti are a a dispersed evenly in the composite coating. The higher magni"cation shown in Fig. 2b and the corresponding Ti K picture shown in Fig. 2c reveal that some cracks in the a coating ended by a Ti splat. The ability of Ti particle to impede the propagation of crack is further testi"ed. XRD patterns for HA coating and HA/Ti composite coatings with 20 and 60 wt% Ti are shown in Fig. 3. From Fig. 3a, it can be seen that the HA coating contains some amorphous phases and new phases such as CaO and Ca (PO ) (TCP). Amorphous phases resulting 3 42 from the quenching of molten particles on substrate during spraying reduce with increasing Ti content in the coatings owing to the di!erences in the crystal structures of HA and Ti. HA has a complicated crystal structure with a space group of P6 /m, while Ti has a simpler one, 3 hexagonal-close-packing. Therefore, it takes shorter time for Ti to arrange atoms to "t the crystal structure. This helps Ti restore its crystal structure in the process of coat formation. For this reason, with the increase in Ti content the amorphous phases decrease. It is noteworthy that the TiO phase appeared in the HA/Ti composite coatings as seen from Fig. 3b and 3c; this is explained in terms of the oxidation of Ti during plasma spraying in air. However, the XPS Ti 2p pattern shown in Fig. 4 illustrates that the binding energy determined for Ti 2p is 453.7 eV (another peak 459.6 eV) and 456.8 eV (another peak 462.8 eV). The "rst value coincides with Ti very well and the second one is between the values for TiO, 455.1 eV, and TiO , 458.7 eV [14], 2 suggesting the oxide of Ti is TiO (x"1}2). The reason x for the di!erence between XRD and XPS results is that the testing depth in the surface of specimen for XPS is limited to several nanometers but that for XRD is much deeper [15]. In the outermost layer of the coating surface, some TiO is further oxidized to TiO (x"1}2) in air, x which is detected by XPS. However, the inner layer of the coating remained as Ti and TiO; therefore, the XRD pattern can illustrate the peaks only for Ti and TiO. 3.2. Bonding strength evaluation The bonding strength evaluation shown in Fig. 5 indicates that the mean bond strength increased from 12.9 to 14.5 and 17.3 MPa while reinforcing the coating with 20 and 60 wt% Ti, respectively. The improvement in the bonding strength of the coatings by the addition of Ti to HA has been testi"ed from Fig. 5, and with the increase in Ti content the adhesion of the coating to the substrate is
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Fig. 1. SEM pictures of HA/Ti 20 wt% composite coating: (a) morphology of coating surface; (b) Ca K picture for (a); (c) Ti K picture for (a); a a (d) higher magni"cation; (e) Ti K picture for (d). a
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Fig. 2. SEM pictures of HA/Ti 60 wt% composite coating: (a) morphology of coating surface; (b) Ca K picture for (a); (c) Ti K picture for (a); a a (d) higher magni"cation; (e) Ti K picture for (d). a
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Fig. 3. XRD patterns for HA coating and HA/Ti composite coatings: (a) HA coating; (b) HA/Ti 20 wt% coating; (c) HA/Ti 60 wt% coating.
Fig. 5. Bonding strength for HA coating and HA/Ti composite coatings. Fig. 4. XPS Ti 2p spectrum for HA/Ti 20 wt% composite coating.
further enhanced. The relatively poor adhesion of pure HA coating mainly arises from the mismatch of the coe$cients of thermal expansion between the Ti alloy substrate and the HA coating [16]. The mismatch of the coe$cients of thermal expansion is reduced and most of
the residual stresses that occurred during spraying process are prevented by the addition of Ti to HA coating. Therefore, the fabrication of HA/Ti composite coating can reinforce the adhesion of the coating. The images of fracture surfaces after bond strength test are shown in Fig. 6. Although adhesive failure is the main fracture mechanism for the HA coating, some HA
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Fig. 6. Failure surface images for HA coating and HA/Ti composite coatings: (a) HA coating; (b) HA/Ti 20 wt% coating; (c) HA/Ti 60 wt% coating.
Fig. 7. XRD patterns for HA coating and HA/Ti composite coatings after immersion in SBF for 14 d: (a) HA coating; (b) HA/Ti 20 wt% coating; (c) HA/Ti 60 wt% coating.
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Fig. 8. Morphology of HA coating after immersion in SBF for 14 d.
remains on the surface, as shown in Fig. 6a, which indicates that cohesive failure also exists during the bond strength evaluation owing to weak mechanical properties such as fracture toughness for HA coating. However, it can be seen from Fig. 6b and c that the fracture mechanism for HA/Ti composite coatings is simply the adhesive failure, which means that the addition of Ti not only increases the adhesive strength between the coating and the substrate but also enhances the cohesive strength of particles in the coating. 3.3. SBF test The results of XRD analysis of the HA coating and HA/Ti 20 wt%, HA/Ti 60 wt% composite coatings, immersed in SBF for 14 d, are shown in Fig. 7. The observation of XRD patterns reveals that impurity phases such as TCP and CaO disappear after 14 d of immersion, which is explained in terms of the reaction of TCP and CaO with water and their dissolution in water. Besides Ti and TiO, TiO is also observed in XRD patterns, which 2 indicates that not only the TiO in the outermost layer of the coating but also that in the inner layer is further oxidized during the SBF test. The existence of TiO 2 seems to induce apatite nucleation on the specimen's surface in SBF [17,18]. SEM examination of the HA coating after immersion in SBF for 14 d, as shown in Fig. 8, reveals that the coating surface is completely covered by a dune-like layer. Microcracks of tortoiseshell character appeared on the newly formed layer, similar to the cracks formed naturally on a dry mud deposit. Morphologies of the HA/Ti 20 wt% composite coating after immersion in SBF for 1, 3, 7, and 14 d are shown in Fig. 9. In Fig. 9a, a loose and thin apatite layer constituted by small granules is observed on the coating surface after soaking in SBF for 1 d. With longer immersion periods, the layer becomes dense and the granular apatite in the layer
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grows gradually as seen from Fig. 9b}d. At a higher magni"cation (Fig. 9e), the apatite layer shows very small crystallites. The energy dispersive spectroscopy analysis (EDS) indicates that the layer is composed of Ca and P and XPS analysis is used to further determine the chemical composition of the apatite layer. XPS analysis results of the elements seen on the surface of the composite after immersion in SBF for 1 d are recorded in Fig. 10. From Fig. 10, O 1s, Ca 2p, P 2p, and C 1s level spectra are identi"ed, indicating that the apatite layer formed on the coating comprises O, Ca, P, and C. Quantitative elemental analysis revealed that the Ca/P mole ratio is 1.29, indicating that the apatite formed is Ca-de"cient, which is in accordance with the work of Panjian Li [19]. Since the identi"ed C 1s peak is very strong and broad, it is deconvoluted using the Gaussian curve "tting process. The deconvoluted peaks shown in Fig. 11 reveal that the C 1s peak is composed of two peaks: one at 284.6 eV and another at 287.8 eV, corresponding to the carbon contamination and the carbonate group CO2~ [20]. The 3 SEM pictures and XPS results reveal that the layer formed on the coating surface is carbonate-apatite, which is considered to be the symbol of the activity for biomaterials [20,21]. The surface picture for HA/Ti 60 wt% composite coating after immersion in SBF is shown in Fig. 12. Carbonate-apatite layer also appears on the surface of the coating. The good bioactivity of the composite coatings is con"rmed by the SBF test. Although the addition of Ti reduces the HA content in the coating, it does not distinctly a!ect the formation of carbonate-apatite layer in SBF. This pheonenomen may be related to the ability of TiO to induce apatite nucleation in SBF. More works 2 are required to verify this.
4. Conclusion In the plasma-sprayed HA/Ti composite coatings, Ti and HA are well distributed. The addition of Ti could help to improve the mechanical properties of the coatings, especially elevate the bonding strength. The bonding strength evaluation indicates that HA/Ti composite coatings possess much higher bonding strength than the HA coating, suggesting that the fabrication of HA/Ti composite coating is a useful way of improving the adhesive strength of the coating. The behavior of the composite coating in SBF shows that the composite coatings are reactive in SBF. After immersion in SBF for a period as short as 1 d, apatite layer appears on the surface of the coating, which is testi"ed by XPS to be carbonate-apatite. This result proves the good bioactivity of the HA/Ti composite coatings. This work shows that the fabrication of HA/Ti composite coatings can enhance the bonding strength, and that it does not a!ect the bioactivity of the coatings signi"cantly.
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Fig. 9. Morphologies of HA/Ti 20 wt% composite coating after immersion in SBF for 1, 3, 7, and 14 d: (a) 1 d; (b) 3 d; (c) 7 d; (d) 14 d; (e) higher magni"cation for apatite layer.
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Fig. 10. XPS spectrum for HA/Ti 20 wt% composite coating after immersion in SBF for 1 d.
Fig. 11. XPS C 1s spectrum for HA/Ti 20 wt% composite coating after immersion in SBF for 1 d.
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