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Effect of electrical-discharging on formation of nanoporous biocompatible layer on titanium
Journal of Alloys and Compounds 492 (2010) 625–630
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of electrical-discharging on formation of nanoporous biocompatible layer on titanium Pei-Wen Peng a,b,c , Keng-Liang Ou b,c,d,∗∗ , Hsi-Chen Lin e , Yung-Ning Pan a , Chau-Hsiang Wang f,g,∗ a
Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan Research Center for Biomedical Implants and Microsurgery Devices, Taipei Medical University, Taipei 110, Taiwan Research Center for Biomedical Devices, Taipei Medical University, Taipei 110, Taiwan d Graduate Institute of Biomedical Materials and Engineering, Taipei Medical University, Taipei 110, Taiwan e Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taichung 406, Taiwan f School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan g Department of Dentistry, Kaohsiung Medical University Hospital, Kaohsiung 807, Taiwan b c
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 25 November 2009 Accepted 28 November 2009 Available online 5 December 2009 Keywords: Titanium alloy Recast layer Electrical-discharging Nanophase
a b s t r a c t In this study, we performed an electrical discharge machining (EDM), by which a recast layer was formed on a titanium surface. Subsequently, an ␣-phase and a ␥-TiH-(␥-hydride)-phase were formed on the recast layer by electrical-discharging. Nano-(␦ + ␥) hydrides play important roles in the formation of nanostructural oxide layers. Electrical-discharging not only generates a nanostructural recast layer but also converts the alloy surface into a nanostructured oxide surface, which increases the alloy biocompatibility. A ␥-hydride microstructure was also formed on the recast layer following electricaldischarging. The microstructure had a tetragonal structure with lattice constant a = 0.421 nm. In the recast layer, a transition, ␣ → (␣ + ␦) → (␦ + ␥) → ␥, occurred during electrical-discharging. This result has never been previously reported. The recast layer that contains nanophases was dissolved during electricaldischarging; by this process, electrical-discharging for a short duration yields nanoporous TiO2 . Hence, electric discharging for a short duration leads to the production of nanostructures as well as bioactive titanium. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Metals are becoming increasingly popular as surgical implants in cardiovascular, neurosurgical, maxillofacial, orthopedic, and dental fields [1–11]. Among metallic implants, titanium-based alloys such as CP–Ti and Ti–6Al–4V have been extensively applied because of their superior corrosion resistance and excellent biocompatibility [2,8–11]. However, body fluids contain amino acids and proteins that tend to accelerate metal corrosion [10,11]. Release of metallic ions leads to poor osseointegration and commonly causes clinical failure. Although the release of metallic ions may be harmful, the use of metals should continue because of their excellent strength. However, their corrosion resistance and bioactive characteristics have to be improved. The formation of titanium oxide films on implant surfaces is believed to be a prerequisite for bioactivity. Titanium oxide films
∗ Corresponding author at: School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan. Tel.: +886 73121101x7003; fax: +886 73210637. ∗∗ Co-corresponding author. E-mail address: [email protected] (C.-H. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.197
can be employed as apatite inducers and provide apatite nucleation. Furthermore, a nanoporous and thick titanium oxide surface can induce bony ingrowth into the porous structure, resulting in morphological fixation of the implants to the bone [8–10]. Previously, numerous procedures for forming titanium oxide films have been examined [8–14]. Among them, anodization is very popular. However, conventional anodization methods cannot generate implants with a thick oxide and sufficient nanoporosity [15–17]. Micro-arc oxidation is an anodization method that can lead to the formation of thick and porous anodic oxide films. But it cannot easily induce nanoporosity. Moreover, micro-arc oxidation often leads to high-voltage-induced surface damages, including microcracks [17]. The effects of hydrogen charging on the formation of porous TiO2 on Ti in alkaline solutions have been examined by many researchers [12,13]. But does not include information concerning the microstructural identification and phase transformation of titanium hydride. In this study, a new method is employed for forming nanoporous titanium oxide surfaces. Electrical-discharged machining (EDM) technology has been extensively adopted to remove and modify the properties of various materials [18–25]. Additionally, a machined surface modified by EDM using a silicon electrode exhibits excellent corrosion and wear resistance [25]. The microstructural varia-
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Fig. 2. GIXRD spectra of M-Ti, P-Ti and EDM-Ti.
2. Experimental procedures An ASTM F67 Grade IV Ti sheet with 1-mm thickness (Hung Chun Bio-S Co. Ltd., Taiwan) was used as a substrate. It was then cut into discs with 14.5-mm diameter for use in experimental tests. All the specimens were mechanically polished using 1500 grit paper and were further polished with diamond abrasives through 1 m. The specimens were finished by applying colloidal silica abrasives through 0.04 m. Before use, all the disks were degreased and pre-pickled in acid by washing them in acetone and processing them using: 2% ammonium fluoride, a solution of 2% hydrofluoric acid, and 10% nitric acid at room temperature for 60 s. All the specimens were washed with distilled water in an ultrasonic cleaner. The specimens underwent EDM using distilled water with a copper electrode and various pulse electrical currents (Ip , A/dm2 ). The surface morphologies of the specimens after the treatments were analyzed by scanning electron microscopy (SEM). The compositions of the films were determined by X-ray photoemission spectroscopy (XPS) using a monochromatic Mg K␣ source. The X-ray power was 250 W (15 kV at 16.7 mA). Secondary ion mass spectroscopy (SIMS) was applied to analyze the compositional depth profiles after EDM. An O2 + primary ion beam with an impact energy of 3 keV was applied. Grazing incidence X-ray diffractometry (GIXRD) was performed to identify the phases and thereby realize microstructural variations. The incident angle of the X-ray was fixed at three degrees. An X-ray diffractometer with Cu K␣ radiation was operated at 50 kV and 250 mA. The microstructure of the electrical-discharged Ti was determined by transmission electron microscopy (TEM). TEM samples with electron transparency were prepared by mechanical thinning followed by ion milling in a precision ion polishing system (PIPS). For ease of identification, in the present study, machined Ti, polished Ti, and Ti with EDM were denoted as M-Ti, P-Ti, and EDM-Ti, respectively.
Fig. 1. Cross-sectional optical morphology of (a) Ti, (b) EDM-Ti-1 and (c) EDM-Ti-5 alloy.
tions on alloy surfaces by EDM have been recently examined and discussed [26]. EDM is thus useful for machining implants and modifying their surfaces in situ. In this study, a new procedure for generating nanostructural surfaces was developed and elucidated. Electrical-discharging was employed to treat the titanium alloys. The specimens were evaluated by material analysis to determine the properties of treated and untreated titanium alloys. Furthermore, the biocompatibility of the alloy with and without treatments was also investigated.
Fig. 3. GIXRD spectra of Ti following various current densities.
P.-W. Peng et al. / Journal of Alloys and Compounds 492 (2010) 625–630
3. Results and discussion Fig. 1 shows cross-sectional optical morphologies of Ti, EDMTi-1, and EDM-5. An equiaxial microstructure consisting of the ␣-phase (Ti) is visible. Because ␣-phase grains have different orientations, they also have different gray colors. Fig. 1(a–c) show that grain sizes decrease as the electrical-discharging current density increases from 1 to 5 ASD (A/dm2 ). Based on optical microscopy and GIXRD analysis, the angle dependence of the width at halfmaximum was used to analyze the grain size effects after EDM. The Scherrer equation [27] was used for the calculation of the mean crystallite size D from the full width at half-maximum (FWHM) after corrections for instrumental contributions. The Scherrer equation was as follows: D=
0.94 FWHM cos
627
where: is the X-ray wavelength and is the diffraction angle. The grain size of Ti was approximately 110–150 m. The average grain size was 30–40 m with annealed twins and the recast layer (denoted by an arrow) was 40–110-m thick. This indicated a nodular structured surface due to the resulting metal droplet. The grain size of EDM-Ti-1 was slightly smaller after electricaldischarging at lower current densities. As the electrical-discharging current densities increased, the grain sizes in the case of EDM-Ti-1 and EDM-5 varied between 30 m and 40 m. This showed that the grain sizes of EDM-Ti were related to electrical-discharging current densities. Grain size is also associated with surface roughness. The surface roughness is one of the properties known to influence the quality of implantation. A smaller grain size and smoother surface generally induce cell differentiation which induces better biocompatibility.
Fig. 4. SEM photographs of the (a) Ti, (b) and (c) EDM-Ti-1, (d) EDM–Ti-3, and (e) EDM-Ti-5.
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Cross-sectional SEM morphologies of Ti, EDM-Ti-1, EDM-Ti-3, and EDM-Ti-5 can also be clearly observed in Fig. 1. The recast layer was investigated in the case of EDM-Ti—it was microscaled and similar to that described by Ou and co-workers [26]. The thickness of the recast layer can be attributed to the field-assisted migration of ions in dielectric fluid systems. As the field-assisted migration of ions proceeds continuously, a solidified layer and a molten alloy were rapidly formed. Fig. 2 shows the GIXRD spectra of M-Ti, P-Ti, and EDM-Ti. In the case of Ti without any treatment, only diffractions of (1 0 0), (0 0 2), (1 0 1), and (1 0 2) Ti peaks were observed and the structure was crystalline. In the case of P-Ti, the (0 0 2) diffraction peak of the ␥TiH phase, excluding Ti peaks, was obtained after the polishing. The crystalline structures of P-Ti and EDM-Ti comprised Ti and ␥-TiH phases. These results were similar to those reported by Cheng et al. [2]. Fig. 3 presents the GIXRD spectra of Ti with various current densities. The alloy that had not undergone electrical-discharging exhibited only Ti peak reflections and its structure was crystalline. Furthermore, numerous reflection peaks other than Ti peaks were obtained from EDM-Ti alloy. The crystalline structure of electricaldischarged Ti consisted of Ti and ␥-TiH phases. Results showed that hydrogen saturated the Ti-phase and that a larger thermal gradient was associated with the formation of a saturated hydride. The above investigation demonstrated that the recast layer was formed by the molten metal at high electrical-discharging energies. Higher energies may cause dissolution and impact more hydrogen and/or hydrogen species from the dielectric fluid. A super-saturated Tiphase may be formed, along with more hydrides, on the rapidly cooling alloy surface. The formation of the hydride and the phase transformation from ␣-phase to hydride was associated with thermal gradients during the electrical-discharging. Fig. 4 presents SEM photographs of the Ti alloy with and without EDM. The untreated alloy surface was not observably porous, and only machined textures can be observed on the surface. The porous structure was generally obtained by prolonged immersion in a chemical solution [2,8–12]. Nanoporous structures, and volcano-like and re-solidified molten metals, were observed on the EDM-treated surface. As widely believed, porosity are responsible for the improved implantation performance. Fig. 4 shows SEM photographs of Ti, EDM-Ti-1, EDM-Ti-3, and EDM-Ti-5. Fig. 4(a) presents the surface of Ti, revealing the machined tracks. In Fig. 4(b), EDM-Ti-1 surface resembles the volcano-like and re-solidified molten metals. Fig. 4(c) shows considerably scaffold nanoporosity on the EDM-Ti-1 surface (A in Fig. 4(b)). The porosity structure was obtained after prolonged immersion in an alkaline solution [12,13]. In previous studies, the effects of nano-(␥-TiH and ␦-TiH0.71 ) phases on the formation of nanoporous titanium oxide by chemical treatment had been explored. Titanium hydride acted as a sacrificial barrier on titanium after the HF pretreatment. The sacrificial barrier had nano-(␥-TiH and ␦-TiH0.71 ) phases [10,11]. Nanoporous structures were also observed on EDM-Ti-3 and EDM-Ti-5, as shown in Fig. 4(d and e). As widely believed, the improved implantation performance can be attributed to nanoporosity due to enhanced cell adhesion as well as cell differentiation. The thickness of TiO2 on Ti obtained by EDM was investigated in greater detail by examining SIMS depth profiles. Fig. 5 shows the depth profiles of EDM-Ti-1, EDM-Ti-3, and EDM-Ti-5. Observations show that the concentration of oxygen decreased as the Ar ion continued to sputter the oxide film. Oxygen then diffused toward the Ti surface after the EDM treatment. The diffused thicknesses of the titanium oxide film in the case of EDM-Ti-1 and EDM-Ti3 were more than 50 and 150 nm, respectively. EDM-Ti-5 had a thicker oxide film than EDM-Ti-1, and thicker than EDM-Ti-3. Furthermore, implant materials with thicker titanium dioxide films possess high biocompatibility [2,8–10]. A dense oxide layer is the
Fig. 5. Depth profiles of EDM-Ti-1, EDM-Ti-3 and EDM-Ti-5.
main factor that results in excellent biocompatibility. The tissue around an implant contacts with the native oxide layer and not with the metal or metal alloys itself. Titania films can be used as an apatite inducer and for providing apatite nucleation. Also, a thicker nanoporous titania surface could induce bony ingrowth into the porous structure and lead to morphological fixation of the implants onto the bone [2,8–10]. In this manner, the corrosion resistance of titanium-based alloys can be improved by thickening the oxide film. Ti films pretreated in H2 O2 have exhibited enhanced oxide growth and increased in the adsorption of plasma proteins [12,13]. The thickness of the oxide layer increased with time, and ions from the physiological environment become incorporated into the growing oxide [2,8–10,26]. Based on the above investigations, we can see that electrical-charged Ti has better biocompatibility and osseointegration than Ti without electrical-charging. Fig. 6 displays TEM micrographs and selected area diffraction patterns (SADPs) of EDM-Ti-1 and EDM-Ti-3. In Fig. 6(a), a darkfield electron micrograph of EDM-Ti-1 is shown; it was taken from the austenite matrix of EDM-Ti-1 under the two-beam condition in the [0 0 1] zone. The micrograph revealed the presence of a modulated structure within the matrix. These figures demonstrated the presence of various microstructures in the EDM-Ti-1 matrix. Fig. 6(a) also depicts an SADP of an area denoted as A. No diffraction spot was obtained from area A, which indicates that amorphous glass metal was formed on the recast layer. The diffraction ring pattern from the area B was an electron diffraction pattern, rather than a pattern of non-diffraction spots, which revealed that area B was a nanostructure. From the camera length and d-spacings of the reflection spots, the crystal structure of the plate-like precipitate was determined to be ␥-hydride having a tetragonal structure with the lattice parameter a = 0.421 nm. Therefore, the as-treated microstructure of the alloy was ␣-phase containing fine ␥-hydride. Hence, the microstructure of an electrical-discharged alloy was a mixture of nano-(␣-phase + ␥-hydride). The grain size of the hydrides was only 20–28 nm, indicating that not only was the nanophase formed during the electrical-discharging reactions, but electrical bombardment was also responsible for nanocrystallization. As previously stated, during electrical-discharging, nucleation and growth of solidified liquid metals and over-cooling can stabilize nucleation and control the nucleation rate. Liquid metals are easily nucleated preventing them from becoming metal glass because they are more viscous and crystallize quickly. An amorphous metal must typically have a high cooling rate [26]. Phase transformation during supercooling is responsible for the formation of complex microstructures in the recast layer. In addition, GIXRD and TEM investigations indicated that EDM-Ti-1 were titanium phases that
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contained extremely fine titanium hydrides. The titanium hydrides, ␥-TiH and ␦-TiH0.71, having tetragonal and orthorhombic structures, respectively, were formed within the titanium matrix during the electrical-discharging. After prolonged electrical-discharging, the coarse ␥-TiH grew into adjacent titanium grains through the hydride reaction ␣ → ␥ + ␦. When EDM-Ti-1 was electricaldischarged at low current densities, granular particles began to appear on the matrix and at the grain boundaries, as shown in Fig. 6(a). A dark-field electron micrograph of the fine granularshaped particles was obtained. Fig. 6 also presents two SADPs that were obtained for the matrix marked “B” and the particles marked “C” in Fig. 6(a). By using the camera length and d-spacings of the reflection spots, the crystal structure of the matrix was determined to be ␣-Ti with a hexagonal structure and lattice parameter a = 0.295 nm. The SADP of the particle marked “B” in Fig. 6(a) reveals that that the particle was ␦ hydride with an orthorhombic structure and lattice parameter a = 0.434 nm. These observations indicate that the microstructure of EDM-Ti-1 that was electrical-discharged under low current densities comprises (␥ + ␦) phases. Fine titanium hydrides underwent phase transformation during the electricaldischarging. Fig. 6(b) presents a bright-field image and SADP of EDM-Ti-3. The phase transformation and compositions of EDM-Ti-3 were analyzed by GIXRD. The results are shown in Figs. 2 and 3. In the Ti specimen, anatase TiO2 was detected. Therefore, it demonstrated that the structure was crystalline. Previously it had been reported that titanium metal was spontaneously covered by a surface oxide of ∼10-nm thickness if it was placed in open air at room temperature [4]. The titanium oxide film formed naturally in open air, and was dense and stable anatase TiO2 [12]. After treatment at 0.1 ASD, crystalline phases were detected including Ti, TiH2 , and anatase TiO2 . At 3 ASD, anatase was the prevailing phase, and the comparative intensity of the TiH2 peak decreased. As the current density increased, the preferred orientation of the peak of the anatase was visible. Fig. 6(b) shows the TEM micrograph (bright-field image) and SADP of EDM-Ti-3. An average multi-layered structure (the top layer was denoted as A, while the bottom layer was denoted as B) was obtained so that the ingrowth of bone cells was enhanced. Fig. 6(b) shows the diffraction ring pattern of the oxide layer (denoted as A). The first, second, third, and fourth rings correspond to the titanium oxide {1 1 1}, {2 0 0}, {1 1 2}, and {2 2 0} planes, respectively. No phases, other than the oxygen-deficient titania, were found in the oxide film. Fig. 6(c) displays the image, taken from the high-resolution transmission electron microscopy (HRTEM), of the region denoted as A. The d-spacing of the plane (1 1 1) for TiO was equal to 0.238–0.242 nm. Based on the results from Fig. 6(b and c), the surface oxide layer also existed with crystalline and amorphous oxide structures. In comparison with the GIXRD results, the phase transformation of specimens from amorphous to anatase phase occurred with increasing current density. The variations in the surface microstructures were determined during the electrical-discharging by the field-assisted migration of ion-electrolyte systems. Results show that the outer layer was a mixed oxide structure.
4. Conclusions
Fig. 6. TEM micrographs and selected area diffraction patterns (SADP) of (a) EDMTi-1, (b) and (c) EDM-Ti-3.
The effectiveness of hydrogen charging on titanium was investigated. Nano-(␥-TiH) and nano-(␥-TiH and ␦-TiH0.71 ) phases were formed by the natural penetration of hydrogen, and electricaldischarging, respectively. Nano-(␥-TiH and ␦-TiH0.71 ) phases were sacrificial particles when electrical-discharging and dipping were applied to treat Ti. A nanostructural surface and an oxidation layer were formed by the dissolution reactions of nano-(␥-TiH and ␦TiH0.71 ) phases. Nano-(␥-TiH and ␦-TiH0.71 ) phases are important
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in the formation of a nanoporous TiO2 layer. Electrical-discharging and hydrogen adsorption induced the evolution of hydrogen to form titanium hydrides. Furthermore, electrical-discharging was performed to dissolve nano-(␥-TiH and ␦-TiH0.71 ) phases and to simultaneously form a thicker nanoporous oxide layer at the same time. The presence of nano-(␥-TiH and ␦-TiH0.71 ) phases on titanium is critical in the preparation of a thick and nanoporous TiO2 layer by electrical-discharging. Acknowledgements The authors would like to thank Co-first author Keng-Liang Ou for financially supporting this research (contract 97-TMU-IAC-031) and supported partly by Taipei Medical University Hospital under contract 97TMU-TMUH-02. References [1] Y.C. Shyng, H. Devlin, K.L. Ou, Int. J. Prosthodont. 19 (5) (2006) 513. [2] H.C. Cheng, S.Y. Lee, C.M. Tsai, C.C. Chen, K.L. Ou, Electrochem. Solid-State Lett. 9 (2006) D25. [3] W.F. Ho, J. Alloys Compd. 464 (2008) 580. [4] B. Kasemo, J. Lausmaa, Int. J. C. Maxillofac. Surg. 3 (1988) 247. [5] A. López-Suárez, J. Rickards, R. Trejo-Luna, J. Alloys Compd. 457 (2008) 216. [6] P. Francois, P. Vaudaux, M. Taborelli, M. Tonetti, D.P. Lew, P. Descouts, Clin. Oral Implants Res. 8 (3) (1997) 217. [7] A. Klinger, D. Steinberg, D. Kohavi, M.N. Sela, J. Biomed. Mater. Res. 36 (3) (1997) 387.
[8] H.C. Cheng, S.Y. Lee, C.C. Chen, Y.C. Shyng, K.L. Ou, Appl. Phys. Lett. 89 (2006) 173902. [9] Y.H. Shih, C.T. Lin, C.M. Liu, C.C. Chen, C.S. Chen, K.L. Ou, Appl. Surf. Sci. 253 (2007) 3678. [10] H.C. Cheng, S.Y. Lee, C.C. Chen, Y.C. Shyng, K.L. Ou, J. Electrochem. Soc. 154 (1) (2007) E13. [11] H.H. Kim, F. Miyaji, T. Kokubo, S. Nishiguchi, T. Nakamura, J. Biomed. Mater. Res. 5 (1999) 100. [12] S.I. Tanaka, M. Aonuma, N. Hirose, T. Tanaki, J. Electrochem. Soc. 149 (11) (2002) D167. [13] S.I. Tanaka, T. Iwatani, N. Hirose, T. Tanaki, J. Electrochem. Soc. 149 (12) (2002) F186. [14] S. Sreekantan, Z. Lockman, R. Hazan, M. Tasbihi, L.K. Tong, A.R. Mohamed, J. Alloys Compd. 485 (2009) 478. [15] X.X. Wang, S. Hayakawa, K. Tsuru, A. Osaka, Biomater 23 (2002) 1353. [16] J. Yang, S. Mei, J.F.M. Ferreira, Scrip. Mater. 46 (2002) 101. [17] J.L. Xu, F. Liu, F.P. Wnag, D.Z. Yu, L.C. Zhao, J. Alloys Compd. 472 (2009) 276. [18] H.M. Kim, T. Kokubo, S. Fujibayashi, S. Nishiguchi, T. Nakamura, J. Biomed. Mater. Res. 52 (2000) 553. [19] F. Klocke, D. Lung, G. Antonoglou, D. Thomaidis, J. Mater. Process. Technol. 149 (2004) 191. [20] H.G. Lee, J. Simao, D.K. Aspinwall, R.C. Dewes, W. Voice, J. Mater. Process. Technol. 149 (2004) 334. [21] W.S. Zhao, Q.G. Meng, Z.L. Wang, J. Mater. Process. Technol. 129 (2002) 30. [22] Y.H. Guu, H. Hocheng, Mater. Manufact. Process. 16 (1) (2001) 91. [23] Y.C. Lin, B.H. Yan, Y.S. Chang, J. Mater. Process. Technol. 104 (2000) 171. [24] S.L. Chen, S.F. Hsieh, H.C. Lin, M.H. Lin, J.S. Huang, J. Alloys Compd. 464 (2008) 446. [25] L.C. Lim, L.C. Lee, Y.S. Wong, H.H. Lu, Maters. Sci. Tech. 7 (1991) 239. [26] S.L. Chen, M.H. Lin, C.C. Chen, K.L. Ou, J. Alloys Compd. 456 (2008) 413. [27] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1954.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of electrical-discharging on formation of nanoporous biocompatible layer on titanium Pei-Wen Peng a,b,c , Keng-Liang Ou b,c,d,∗∗ , Hsi-Chen Lin e , Yung-Ning Pan a , Chau-Hsiang Wang f,g,∗ a
Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan Research Center for Biomedical Implants and Microsurgery Devices, Taipei Medical University, Taipei 110, Taiwan Research Center for Biomedical Devices, Taipei Medical University, Taipei 110, Taiwan d Graduate Institute of Biomedical Materials and Engineering, Taipei Medical University, Taipei 110, Taiwan e Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taichung 406, Taiwan f School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan g Department of Dentistry, Kaohsiung Medical University Hospital, Kaohsiung 807, Taiwan b c
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 25 November 2009 Accepted 28 November 2009 Available online 5 December 2009 Keywords: Titanium alloy Recast layer Electrical-discharging Nanophase
a b s t r a c t In this study, we performed an electrical discharge machining (EDM), by which a recast layer was formed on a titanium surface. Subsequently, an ␣-phase and a ␥-TiH-(␥-hydride)-phase were formed on the recast layer by electrical-discharging. Nano-(␦ + ␥) hydrides play important roles in the formation of nanostructural oxide layers. Electrical-discharging not only generates a nanostructural recast layer but also converts the alloy surface into a nanostructured oxide surface, which increases the alloy biocompatibility. A ␥-hydride microstructure was also formed on the recast layer following electricaldischarging. The microstructure had a tetragonal structure with lattice constant a = 0.421 nm. In the recast layer, a transition, ␣ → (␣ + ␦) → (␦ + ␥) → ␥, occurred during electrical-discharging. This result has never been previously reported. The recast layer that contains nanophases was dissolved during electricaldischarging; by this process, electrical-discharging for a short duration yields nanoporous TiO2 . Hence, electric discharging for a short duration leads to the production of nanostructures as well as bioactive titanium. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Metals are becoming increasingly popular as surgical implants in cardiovascular, neurosurgical, maxillofacial, orthopedic, and dental fields [1–11]. Among metallic implants, titanium-based alloys such as CP–Ti and Ti–6Al–4V have been extensively applied because of their superior corrosion resistance and excellent biocompatibility [2,8–11]. However, body fluids contain amino acids and proteins that tend to accelerate metal corrosion [10,11]. Release of metallic ions leads to poor osseointegration and commonly causes clinical failure. Although the release of metallic ions may be harmful, the use of metals should continue because of their excellent strength. However, their corrosion resistance and bioactive characteristics have to be improved. The formation of titanium oxide films on implant surfaces is believed to be a prerequisite for bioactivity. Titanium oxide films
∗ Corresponding author at: School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan. Tel.: +886 73121101x7003; fax: +886 73210637. ∗∗ Co-corresponding author. E-mail address: [email protected] (C.-H. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.197
can be employed as apatite inducers and provide apatite nucleation. Furthermore, a nanoporous and thick titanium oxide surface can induce bony ingrowth into the porous structure, resulting in morphological fixation of the implants to the bone [8–10]. Previously, numerous procedures for forming titanium oxide films have been examined [8–14]. Among them, anodization is very popular. However, conventional anodization methods cannot generate implants with a thick oxide and sufficient nanoporosity [15–17]. Micro-arc oxidation is an anodization method that can lead to the formation of thick and porous anodic oxide films. But it cannot easily induce nanoporosity. Moreover, micro-arc oxidation often leads to high-voltage-induced surface damages, including microcracks [17]. The effects of hydrogen charging on the formation of porous TiO2 on Ti in alkaline solutions have been examined by many researchers [12,13]. But does not include information concerning the microstructural identification and phase transformation of titanium hydride. In this study, a new method is employed for forming nanoporous titanium oxide surfaces. Electrical-discharged machining (EDM) technology has been extensively adopted to remove and modify the properties of various materials [18–25]. Additionally, a machined surface modified by EDM using a silicon electrode exhibits excellent corrosion and wear resistance [25]. The microstructural varia-
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Fig. 2. GIXRD spectra of M-Ti, P-Ti and EDM-Ti.
2. Experimental procedures An ASTM F67 Grade IV Ti sheet with 1-mm thickness (Hung Chun Bio-S Co. Ltd., Taiwan) was used as a substrate. It was then cut into discs with 14.5-mm diameter for use in experimental tests. All the specimens were mechanically polished using 1500 grit paper and were further polished with diamond abrasives through 1 m. The specimens were finished by applying colloidal silica abrasives through 0.04 m. Before use, all the disks were degreased and pre-pickled in acid by washing them in acetone and processing them using: 2% ammonium fluoride, a solution of 2% hydrofluoric acid, and 10% nitric acid at room temperature for 60 s. All the specimens were washed with distilled water in an ultrasonic cleaner. The specimens underwent EDM using distilled water with a copper electrode and various pulse electrical currents (Ip , A/dm2 ). The surface morphologies of the specimens after the treatments were analyzed by scanning electron microscopy (SEM). The compositions of the films were determined by X-ray photoemission spectroscopy (XPS) using a monochromatic Mg K␣ source. The X-ray power was 250 W (15 kV at 16.7 mA). Secondary ion mass spectroscopy (SIMS) was applied to analyze the compositional depth profiles after EDM. An O2 + primary ion beam with an impact energy of 3 keV was applied. Grazing incidence X-ray diffractometry (GIXRD) was performed to identify the phases and thereby realize microstructural variations. The incident angle of the X-ray was fixed at three degrees. An X-ray diffractometer with Cu K␣ radiation was operated at 50 kV and 250 mA. The microstructure of the electrical-discharged Ti was determined by transmission electron microscopy (TEM). TEM samples with electron transparency were prepared by mechanical thinning followed by ion milling in a precision ion polishing system (PIPS). For ease of identification, in the present study, machined Ti, polished Ti, and Ti with EDM were denoted as M-Ti, P-Ti, and EDM-Ti, respectively.
Fig. 1. Cross-sectional optical morphology of (a) Ti, (b) EDM-Ti-1 and (c) EDM-Ti-5 alloy.
tions on alloy surfaces by EDM have been recently examined and discussed [26]. EDM is thus useful for machining implants and modifying their surfaces in situ. In this study, a new procedure for generating nanostructural surfaces was developed and elucidated. Electrical-discharging was employed to treat the titanium alloys. The specimens were evaluated by material analysis to determine the properties of treated and untreated titanium alloys. Furthermore, the biocompatibility of the alloy with and without treatments was also investigated.
Fig. 3. GIXRD spectra of Ti following various current densities.
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3. Results and discussion Fig. 1 shows cross-sectional optical morphologies of Ti, EDMTi-1, and EDM-5. An equiaxial microstructure consisting of the ␣-phase (Ti) is visible. Because ␣-phase grains have different orientations, they also have different gray colors. Fig. 1(a–c) show that grain sizes decrease as the electrical-discharging current density increases from 1 to 5 ASD (A/dm2 ). Based on optical microscopy and GIXRD analysis, the angle dependence of the width at halfmaximum was used to analyze the grain size effects after EDM. The Scherrer equation [27] was used for the calculation of the mean crystallite size D from the full width at half-maximum (FWHM) after corrections for instrumental contributions. The Scherrer equation was as follows: D=
0.94 FWHM cos
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where: is the X-ray wavelength and is the diffraction angle. The grain size of Ti was approximately 110–150 m. The average grain size was 30–40 m with annealed twins and the recast layer (denoted by an arrow) was 40–110-m thick. This indicated a nodular structured surface due to the resulting metal droplet. The grain size of EDM-Ti-1 was slightly smaller after electricaldischarging at lower current densities. As the electrical-discharging current densities increased, the grain sizes in the case of EDM-Ti-1 and EDM-5 varied between 30 m and 40 m. This showed that the grain sizes of EDM-Ti were related to electrical-discharging current densities. Grain size is also associated with surface roughness. The surface roughness is one of the properties known to influence the quality of implantation. A smaller grain size and smoother surface generally induce cell differentiation which induces better biocompatibility.
Fig. 4. SEM photographs of the (a) Ti, (b) and (c) EDM-Ti-1, (d) EDM–Ti-3, and (e) EDM-Ti-5.
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Cross-sectional SEM morphologies of Ti, EDM-Ti-1, EDM-Ti-3, and EDM-Ti-5 can also be clearly observed in Fig. 1. The recast layer was investigated in the case of EDM-Ti—it was microscaled and similar to that described by Ou and co-workers [26]. The thickness of the recast layer can be attributed to the field-assisted migration of ions in dielectric fluid systems. As the field-assisted migration of ions proceeds continuously, a solidified layer and a molten alloy were rapidly formed. Fig. 2 shows the GIXRD spectra of M-Ti, P-Ti, and EDM-Ti. In the case of Ti without any treatment, only diffractions of (1 0 0), (0 0 2), (1 0 1), and (1 0 2) Ti peaks were observed and the structure was crystalline. In the case of P-Ti, the (0 0 2) diffraction peak of the ␥TiH phase, excluding Ti peaks, was obtained after the polishing. The crystalline structures of P-Ti and EDM-Ti comprised Ti and ␥-TiH phases. These results were similar to those reported by Cheng et al. [2]. Fig. 3 presents the GIXRD spectra of Ti with various current densities. The alloy that had not undergone electrical-discharging exhibited only Ti peak reflections and its structure was crystalline. Furthermore, numerous reflection peaks other than Ti peaks were obtained from EDM-Ti alloy. The crystalline structure of electricaldischarged Ti consisted of Ti and ␥-TiH phases. Results showed that hydrogen saturated the Ti-phase and that a larger thermal gradient was associated with the formation of a saturated hydride. The above investigation demonstrated that the recast layer was formed by the molten metal at high electrical-discharging energies. Higher energies may cause dissolution and impact more hydrogen and/or hydrogen species from the dielectric fluid. A super-saturated Tiphase may be formed, along with more hydrides, on the rapidly cooling alloy surface. The formation of the hydride and the phase transformation from ␣-phase to hydride was associated with thermal gradients during the electrical-discharging. Fig. 4 presents SEM photographs of the Ti alloy with and without EDM. The untreated alloy surface was not observably porous, and only machined textures can be observed on the surface. The porous structure was generally obtained by prolonged immersion in a chemical solution [2,8–12]. Nanoporous structures, and volcano-like and re-solidified molten metals, were observed on the EDM-treated surface. As widely believed, porosity are responsible for the improved implantation performance. Fig. 4 shows SEM photographs of Ti, EDM-Ti-1, EDM-Ti-3, and EDM-Ti-5. Fig. 4(a) presents the surface of Ti, revealing the machined tracks. In Fig. 4(b), EDM-Ti-1 surface resembles the volcano-like and re-solidified molten metals. Fig. 4(c) shows considerably scaffold nanoporosity on the EDM-Ti-1 surface (A in Fig. 4(b)). The porosity structure was obtained after prolonged immersion in an alkaline solution [12,13]. In previous studies, the effects of nano-(␥-TiH and ␦-TiH0.71 ) phases on the formation of nanoporous titanium oxide by chemical treatment had been explored. Titanium hydride acted as a sacrificial barrier on titanium after the HF pretreatment. The sacrificial barrier had nano-(␥-TiH and ␦-TiH0.71 ) phases [10,11]. Nanoporous structures were also observed on EDM-Ti-3 and EDM-Ti-5, as shown in Fig. 4(d and e). As widely believed, the improved implantation performance can be attributed to nanoporosity due to enhanced cell adhesion as well as cell differentiation. The thickness of TiO2 on Ti obtained by EDM was investigated in greater detail by examining SIMS depth profiles. Fig. 5 shows the depth profiles of EDM-Ti-1, EDM-Ti-3, and EDM-Ti-5. Observations show that the concentration of oxygen decreased as the Ar ion continued to sputter the oxide film. Oxygen then diffused toward the Ti surface after the EDM treatment. The diffused thicknesses of the titanium oxide film in the case of EDM-Ti-1 and EDM-Ti3 were more than 50 and 150 nm, respectively. EDM-Ti-5 had a thicker oxide film than EDM-Ti-1, and thicker than EDM-Ti-3. Furthermore, implant materials with thicker titanium dioxide films possess high biocompatibility [2,8–10]. A dense oxide layer is the
Fig. 5. Depth profiles of EDM-Ti-1, EDM-Ti-3 and EDM-Ti-5.
main factor that results in excellent biocompatibility. The tissue around an implant contacts with the native oxide layer and not with the metal or metal alloys itself. Titania films can be used as an apatite inducer and for providing apatite nucleation. Also, a thicker nanoporous titania surface could induce bony ingrowth into the porous structure and lead to morphological fixation of the implants onto the bone [2,8–10]. In this manner, the corrosion resistance of titanium-based alloys can be improved by thickening the oxide film. Ti films pretreated in H2 O2 have exhibited enhanced oxide growth and increased in the adsorption of plasma proteins [12,13]. The thickness of the oxide layer increased with time, and ions from the physiological environment become incorporated into the growing oxide [2,8–10,26]. Based on the above investigations, we can see that electrical-charged Ti has better biocompatibility and osseointegration than Ti without electrical-charging. Fig. 6 displays TEM micrographs and selected area diffraction patterns (SADPs) of EDM-Ti-1 and EDM-Ti-3. In Fig. 6(a), a darkfield electron micrograph of EDM-Ti-1 is shown; it was taken from the austenite matrix of EDM-Ti-1 under the two-beam condition in the [0 0 1] zone. The micrograph revealed the presence of a modulated structure within the matrix. These figures demonstrated the presence of various microstructures in the EDM-Ti-1 matrix. Fig. 6(a) also depicts an SADP of an area denoted as A. No diffraction spot was obtained from area A, which indicates that amorphous glass metal was formed on the recast layer. The diffraction ring pattern from the area B was an electron diffraction pattern, rather than a pattern of non-diffraction spots, which revealed that area B was a nanostructure. From the camera length and d-spacings of the reflection spots, the crystal structure of the plate-like precipitate was determined to be ␥-hydride having a tetragonal structure with the lattice parameter a = 0.421 nm. Therefore, the as-treated microstructure of the alloy was ␣-phase containing fine ␥-hydride. Hence, the microstructure of an electrical-discharged alloy was a mixture of nano-(␣-phase + ␥-hydride). The grain size of the hydrides was only 20–28 nm, indicating that not only was the nanophase formed during the electrical-discharging reactions, but electrical bombardment was also responsible for nanocrystallization. As previously stated, during electrical-discharging, nucleation and growth of solidified liquid metals and over-cooling can stabilize nucleation and control the nucleation rate. Liquid metals are easily nucleated preventing them from becoming metal glass because they are more viscous and crystallize quickly. An amorphous metal must typically have a high cooling rate [26]. Phase transformation during supercooling is responsible for the formation of complex microstructures in the recast layer. In addition, GIXRD and TEM investigations indicated that EDM-Ti-1 were titanium phases that
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contained extremely fine titanium hydrides. The titanium hydrides, ␥-TiH and ␦-TiH0.71, having tetragonal and orthorhombic structures, respectively, were formed within the titanium matrix during the electrical-discharging. After prolonged electrical-discharging, the coarse ␥-TiH grew into adjacent titanium grains through the hydride reaction ␣ → ␥ + ␦. When EDM-Ti-1 was electricaldischarged at low current densities, granular particles began to appear on the matrix and at the grain boundaries, as shown in Fig. 6(a). A dark-field electron micrograph of the fine granularshaped particles was obtained. Fig. 6 also presents two SADPs that were obtained for the matrix marked “B” and the particles marked “C” in Fig. 6(a). By using the camera length and d-spacings of the reflection spots, the crystal structure of the matrix was determined to be ␣-Ti with a hexagonal structure and lattice parameter a = 0.295 nm. The SADP of the particle marked “B” in Fig. 6(a) reveals that that the particle was ␦ hydride with an orthorhombic structure and lattice parameter a = 0.434 nm. These observations indicate that the microstructure of EDM-Ti-1 that was electrical-discharged under low current densities comprises (␥ + ␦) phases. Fine titanium hydrides underwent phase transformation during the electricaldischarging. Fig. 6(b) presents a bright-field image and SADP of EDM-Ti-3. The phase transformation and compositions of EDM-Ti-3 were analyzed by GIXRD. The results are shown in Figs. 2 and 3. In the Ti specimen, anatase TiO2 was detected. Therefore, it demonstrated that the structure was crystalline. Previously it had been reported that titanium metal was spontaneously covered by a surface oxide of ∼10-nm thickness if it was placed in open air at room temperature [4]. The titanium oxide film formed naturally in open air, and was dense and stable anatase TiO2 [12]. After treatment at 0.1 ASD, crystalline phases were detected including Ti, TiH2 , and anatase TiO2 . At 3 ASD, anatase was the prevailing phase, and the comparative intensity of the TiH2 peak decreased. As the current density increased, the preferred orientation of the peak of the anatase was visible. Fig. 6(b) shows the TEM micrograph (bright-field image) and SADP of EDM-Ti-3. An average multi-layered structure (the top layer was denoted as A, while the bottom layer was denoted as B) was obtained so that the ingrowth of bone cells was enhanced. Fig. 6(b) shows the diffraction ring pattern of the oxide layer (denoted as A). The first, second, third, and fourth rings correspond to the titanium oxide {1 1 1}, {2 0 0}, {1 1 2}, and {2 2 0} planes, respectively. No phases, other than the oxygen-deficient titania, were found in the oxide film. Fig. 6(c) displays the image, taken from the high-resolution transmission electron microscopy (HRTEM), of the region denoted as A. The d-spacing of the plane (1 1 1) for TiO was equal to 0.238–0.242 nm. Based on the results from Fig. 6(b and c), the surface oxide layer also existed with crystalline and amorphous oxide structures. In comparison with the GIXRD results, the phase transformation of specimens from amorphous to anatase phase occurred with increasing current density. The variations in the surface microstructures were determined during the electrical-discharging by the field-assisted migration of ion-electrolyte systems. Results show that the outer layer was a mixed oxide structure.
4. Conclusions
Fig. 6. TEM micrographs and selected area diffraction patterns (SADP) of (a) EDMTi-1, (b) and (c) EDM-Ti-3.
The effectiveness of hydrogen charging on titanium was investigated. Nano-(␥-TiH) and nano-(␥-TiH and ␦-TiH0.71 ) phases were formed by the natural penetration of hydrogen, and electricaldischarging, respectively. Nano-(␥-TiH and ␦-TiH0.71 ) phases were sacrificial particles when electrical-discharging and dipping were applied to treat Ti. A nanostructural surface and an oxidation layer were formed by the dissolution reactions of nano-(␥-TiH and ␦TiH0.71 ) phases. Nano-(␥-TiH and ␦-TiH0.71 ) phases are important
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in the formation of a nanoporous TiO2 layer. Electrical-discharging and hydrogen adsorption induced the evolution of hydrogen to form titanium hydrides. Furthermore, electrical-discharging was performed to dissolve nano-(␥-TiH and ␦-TiH0.71 ) phases and to simultaneously form a thicker nanoporous oxide layer at the same time. The presence of nano-(␥-TiH and ␦-TiH0.71 ) phases on titanium is critical in the preparation of a thick and nanoporous TiO2 layer by electrical-discharging. Acknowledgements The authors would like to thank Co-first author Keng-Liang Ou for financially supporting this research (contract 97-TMU-IAC-031) and supported partly by Taipei Medical University Hospital under contract 97TMU-TMUH-02. References [1] Y.C. Shyng, H. Devlin, K.L. Ou, Int. J. Prosthodont. 19 (5) (2006) 513. [2] H.C. Cheng, S.Y. Lee, C.M. Tsai, C.C. Chen, K.L. Ou, Electrochem. Solid-State Lett. 9 (2006) D25. [3] W.F. Ho, J. Alloys Compd. 464 (2008) 580. [4] B. Kasemo, J. Lausmaa, Int. J. C. Maxillofac. Surg. 3 (1988) 247. [5] A. López-Suárez, J. Rickards, R. Trejo-Luna, J. Alloys Compd. 457 (2008) 216. [6] P. Francois, P. Vaudaux, M. Taborelli, M. Tonetti, D.P. Lew, P. Descouts, Clin. Oral Implants Res. 8 (3) (1997) 217. [7] A. Klinger, D. Steinberg, D. Kohavi, M.N. Sela, J. Biomed. Mater. Res. 36 (3) (1997) 387.
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