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A new approach to the fabrication of porous magnesium with well-controlled 3D pore structure for orthopedic applications
Materials Science and Engineering C 43 (2014) 317–320
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Short communication
A new approach to the fabrication of porous magnesium with well-controlled 3D pore structure for orthopedic applications Guofeng Jiang, Guo He ⁎ State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 7 February 2014 Received in revised form 10 May 2014 Accepted 8 July 2014 Available online 15 July 2014 Keywords: Magnesium Porous material Biomaterials Biodegradability Implant
a b s t r a c t A new approach to the fabrication of porous magnesium by using the titanium wire space holder (TWSH) method has been developed. Since the entangled titanium wire structure is used as the space holder, the porous structure is pipe-like and interconnected, and the pore size is identical and equals to the diameter of the titanium wire. In particular, the pore size, porosity, and porous morphology of the porous magnesium can be exactly and individually controlled. When the porosity is in the range of 54.2–43.2%, the compressive yielding strength is in the range of 4.3–6.2 MPa, and the Young's modulus is in the range of 0.5–1.0 GPa. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Porous metallic materials have drawn widespread attention in various industrial fields due to their excellent physical and mechanical properties, such as high stiffness at extremely low density, high energy absorption, heat resistance, permeability and so on [1,2]. In particular, porous titanium and magnesium with very good biocompatibility have shown their potentials for the biomedical applications in surgical implants. The open-cellular structure in the porous materials can provide adequate spaces for the living tissue ingrowth and the nutrient transportation [3,4], and most importantly, the Young's modulus of the porous metals can be well controlled by adjusting the porosity, so as to match the stiffness of the natural bone [5]. Except for the above superiority, the porous magnesium also has very good biodegradability, which has been recently recognized as a very promising biomaterial for tissue engineering [6–9]. So far, many methods, such as investment casting [10], powder metallurgy with space holder [11], vacuum foaming [12], thixo casting [13], and directional solidification [14], have been developed to prepare porous magnesium. However, those traditional metallurgical methods are difficult to accurately control the pore morphologies (e.g., open and interconnected pores, 300–500 μm pore size, certain porosity ensuring appropriate stiffness and adequate strength) which are highly important for guaranteeing the biomechanical properties and the biocompatibility [4,15,16]. In recent years, a solid free-form fabrication
⁎ Corresponding author. Tel./fax: +86 21 3420 2643. E-mail address: [email protected] (G. He).
http://dx.doi.org/10.1016/j.msec.2014.07.033 0928-4931/© 2014 Elsevier B.V. All rights reserved.
(SFF) process for producing topologically ordered magnesium (TOPM) structures has been developed [17], which adopt a layer-by-layer fabrication process from computer aided design (CAD) models for manufacturing ordered pore architectures by accurate control of porous topology. This provides a possible route to fabricate the porous magnesium scaffolds with designed 3D pore structure and tailored mechanical properties. The drawback is that the SFF method needs six-steps which are complicated and cost-intensive. In this study, we report a new approach to the fabrication of porous magnesium with controlled 3D pore structure, which needs only threesteps: (1) the entangled (or woven) 3D structure was prepared by using titanium wire [18], (2) the 3D structure was used as a space holder to fabricate the titanium/magnesium composite by infiltrating cast, and (3) the 3D structure, i.e., titanium wire space holder (TWSH) material, was then removed from the composite by etching off. With this method, it is possible to fabricate the porous magnesium with well-controlled 3D pore structure. 2. Materials and methods The starting materials are commercially available titanium wire (purity: 99.9%, diameter: 0.27 mm) and magnesium ingot (purity: 99.9%). The 3D entangled titanium wire material with dimensions of Φ10 mm × 20 mm was prepared (details have been described elsewhere [18,19]), which was used as a space holder material. The solid magnesium was melted and kept at 700 °C under the atmosphere of the mixed gas of SF6 and CO2. Then the TWSH was soaked in the magnesium melts. In order to improve the melt infiltration into the porous structure in the TWSH material, the supersonic vibration (the source
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G. Jiang, G. He / Materials Science and Engineering C 43 (2014) 317–320
frequency: 40 kHz; output: 80 W; the vibration time is 60 s) was applied. After all the voids were completely filled with the magnesium melt, the sample was taken out slowly and cooled down in the protective atmosphere, so as to form titanium/magnesium composite. After that, the composite was immersed in the hydrofluoric acid (HF) solution (analytical reagent with the concentration of 40 wt.%) at room temperature to remove the TWSH material. During the etching, a supersonic vibration was applied on the sample from start to finish so as to quicken the Ti-etching and promote the discharge of the hydrogen gas. The etching time was about 72 h in this situation. When all the titanium wire in the composite was etched off, the sample (already becoming porous magnesium at this step) was taken out and cleaned by immersion in acetone with ultrasonic agitation. After the above steps (as illustrated in Fig. 1), the porous magnesium with well-controlled 3D pore structure could be fabricated. The porosity of the porous magnesium could be calculated with the following formula: P¼
1−
m1 νρMg
! 100%
ð1Þ
where ρMg is the density of the magnesium which is 1.74 g/cm3, v is the volume of the sample, and m1 is the weight of the magnesium in the composite (i.e., the weight of the porous magnesium), which can be directly measured or calculated by following relation: m1 ¼ m−m2
ð2Þ
where m denotes the weight of the as-prepared Ti/Mg composite, and m2 denotes the weight of the TWSH. Both could be measured directly. If the measured data (m, m2 and m1) could meet the above equation, the TWSH would be confirmed to be removed completely from the composite. The as-prepared porous structure was directly characterized by using the optical microscopy and the scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The pore size was evaluated by quantitative image analysis using the commercially available software Image-Pro Plus. The mechanical properties of the porous magnesium were evaluated by the uniaxial compressive tests that were performed by using Zwick AG-100KN testing machine at room temperature. The tests were conducted on the specimens with dimensions of Φ10 mm × 20 mm under displacement control with the crosshead speed of 1 mm/min. Since the porous magnesium deforms easily under a much lower loading, the elastic deformation occurs at relatively low stress levels. The elastic modulus evaluated from the stress–strain curve could be significantly influenced by the test machine compliance. Thus the
compliance corrections were conducted by using the known pure magnesium (its Young's modulus is 45 GPa) in this study (the details regarding the compliance correction were described in ref. [20]). Therefore, all the elastic moduli of the porous magnesium reported in this paper had been calibrated accordingly. 3. Results and discussion The appearance of the as-prepared porous magnesium was shown in Fig. 2a. Its porosity was evaluated to be about 51% according to the 49% porosity of the initial TWSH material. In order to verify the homogeneity of the porous structure, the sample was cut along the axial direction and shown in Fig. 2b. The porous structure was roughly homogenous, indicating that the infiltration of the magnesium melts into the porous TWSH material was well proceeded. Only one shrinkage cavity-like ‘large’ pore was found in the center of the cylindrical sample as indicated by the black arrow in Fig. 2b, suggesting that the local underfill occurred during the infiltrating cast. This perhaps could be attributed to the local high dense entangled titanium wire structure where the pores were too small to be filled with the magnesium melts. It also showed, as expected, that all the pores in the porous magnesium were pipe-like channels with the diameter of about 270 μm (approximately equivalent to the diameter of the titanium wire, used in this study). Fig. 2c–e showed the sectioned view of these channels, on which some flow lines were clearly shown on the ‘channel’ inner surface as indicated by the white arrows in Fig. 2c–e. These flow lines were reversely replicated from the surface plastic flowing deformation vestiges of the titanium wire. This just indicated that the titanium wire could be completely wetted by the magnesium melts at the infiltration temperature as reported in ref. [21]. The removal of the titanium wire from the titanium/magnesium composite could be achieved based on the different responses between titanium and magnesium in the HF solution. The titanium wire could be gradually eroded by the following chemical reactions [22]: 2Ti þ 6HF ¼ 2TiF3 þ 3H2 ↑
ð3Þ
Ti þ 4HF ¼ TiF4 þ 2H2 ↑
ð4Þ
−
TiF4 þ 2 F
2−
¼ ½TiF6
:
ð5Þ
The dissolution of titanium in the HF solution was suggested to occur via the electrochemical process [22] in which the solid state titanium was converted to ionized state [TiF6]2−. This process would be continually in progress until all the titanium wire was completely consumed. The release of hydrogen during the dissolution of titanium was just the sign indicating the titanium dissolving.
Fig. 1. Schematic illustration of the TWSH method for the fabrication of porous magnesium.
G. Jiang, G. He / Materials Science and Engineering C 43 (2014) 317–320
319
Fig. 2. Macroscopic appearance of as-prepared porous magnesium (a), the section view (b), the SEM micrographs of the section (c)–(e), and the EDS profile of the inner surface of the pipelike pore (f).
On the other hand, the magnesium could be rapidly and completely passivated in the HF solution by the formation of the MgF2 layer on the magnesium surface [23], which was confirmed by EDS analysis as shown in Fig. 2f. The basic reaction was: Mg þ 2HF ¼ MgF2 þ H2 ↑:
ð6Þ
Although the reaction also released hydrogen, it was rapidly slowing down as the formation of the MgF2 film [24]. The hydrogen bubbles were almost from the titanium dissolving at the latter stage, so that the bubbles directly indicated the dissolution of titanium. When the bubbles ceased to come out of the HF solution, the dissolution of the titanium wire finished. It was obvious that the size (i.e., diameter) of the pipe-like pores in the porous magnesium was identical which could be exactly controlled by selecting different diameters of the titanium wire. The bulk porosity of the porous magnesium could also be exactly controlled by adjusting the porosity of the initial TWSH structure. The porous morphology depended on the 3D structure of the TWSH material which could be designed and manufactured as various wire-entangled or multilayered truss-type porous titanium by the wire-woven method [25,26]. It is very favorable that the pore size, the bulk porosity, and the porous morphology are independent, which can be exactly and individually controlled. This makes it convenient to balance between the adequate mechanical properties and the obligatory porous structure (including pore size, porosity and interconnectivity) that is extremely important for the new tissue ingrowth and the body fluid transportation [4]. The mechanical properties of the as-prepared porous magnesium were evaluated by the compressive test. The nominal stress–strain curves were shown in Fig. 3. The continuous line and the dash line
showed the two tests of the specimens with the same porous structure, indicating the good repeatability of the mechanical behavior of the porous magnesium. Under the compressive loading the specimen exhibited a short elastic deformation at the initiation of strain followed by a very long strain-hardening region as shown in Fig. 3. Such stress–strain behavior was very similar with that of the porous magnesium fabricated by the mechanical perforation method [27], suggesting that the strain occurred mainly via the bulk deformation rather than via the porous structure local collapse that often happened in the porous magnesium
Fig. 3. Nominal stress–strain curves of the as-prepared porous magnesium. The two curves are from two tests of the specimens with the same porous structure.
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G. Jiang, G. He / Materials Science and Engineering C 43 (2014) 317–320
fabricated by the conventional powder sintering method [11] and in other metal foams [1]. The mechanical properties were directly evaluated from the stress–strain curves, which were about 4.6 MPa of yielding strength and about 0.6 GPa of Young's modulus for the porous magnesium with a 51% porosity. Another two groups of the porous magnesium samples with the porosities of 43.2% and 54.2% were also tested. Their yielding strengths were 6.2 MPa and 4.3 MPa, and Young's moduli were 1.0 GPa and 0.5 GPa, respectively. Obviously, such porous magnesium with the porosities in the range of 43.2% to 54.2% exhibits comparable strength and Young's modulus to the cancellous bone which has the strength in the range of 3–20 MPa [28], and the Young's modulus in the range of 0.01–3.0 GPa [29]. It should be mentioned that the MgF2 film on the Mg surface should affect the mechanical properties of the porous Mg, but this effect is relatively small. This is because the compressive mechanical properties strongly depend on the porosity and the pore morphology. The MgF2 film formed during the etching process is expected to be about 0–2 μm in thickness [24]. Such thin film accounts for a very small volume percentage of the total, So that its influence may be concealed in the systematic errors. To our knowledge, there is no data in the literature to show the effect of the conversion coating on the mechanical properties of Mg or its alloy. This needs to be clarified in the future work. So far, there are very few studies on the mechanical behaviors and properties of the porous magnesium. The available data of the materials fabricated by various methods [6,11,27,30–34] revealed an approximate linear relationship between the relative elastic modulus and the relative density in the logarithmic coordinates as shown in Fig. 4 although some scattered points were present in the figure. By comparing the data presented in Fig. 4, one can conclude that the mechanical properties of the porous magnesium fabricated by the TWSH method are similar to that of other porous magnesium. 4. Conclusions The TWSH method has been successfully used to fabricate porous magnesium, by which the porosity, pore size, and porous morphology can be exactly and individually controlled. In this study, the entangled titanium wire structure has been used as the space holder for the fabrication of the porous magnesium. The porous morphology is a pipe-like and interconnected pore structure, and the pore size is about 270 μm that equals to the diameter of the titanium wire. The as-prepared porous magnesium exhibits the compressive yielding strengths of 4.3, 4.6 and 6.2 MPa, and the Young's moduli of 0.5, 0.6 and 1.0 GPa at the
Fig. 4. Relationship between the relative elastic modulus and the relative density of the porous magnesium.
porosities of 54.2%, 51% and 43.2%, respectively. Such mechanical properties are comparable to that of the cancellous bone, suggesting potential applications in the orthopedic surgery. References [1] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, second ed. Cambridge University Press, Cambridge, 1997. [2] M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams: A Design Guide, Butterworth-Heinemann, Boston (MA), 2000. [3] L.D. Zardiackas, L.D. Dillon, D.W. Mitchell, L.A. Nunnery, R. Poggie, J. Biomed. Mater. Res. 58 (2001) 180–187. [4] J.T. Clemow, A.M. Weinstein, J.J. Klawitter, J. Koeneman, J. Anderson, J. Biomed. Mater. Res. 15 (1981) 73–82. [5] L.P. Lefebvre, J. Banhart, D.C. Dunand, Adv. Eng. Mater. 10 (2008) 775–787. [6] H. Zhuang, Y. Han, A. Feng, Mater. Sci. Eng. C 28 (2008) 1462–1466. [7] H. Zreiqat, C.R. Howlett, A. Zannettion, P. Evans, G. Schulze-Tanzil, C. Knabe, M. Shakibaei, Biomed. Mater. Res. 62 (2002) 175–184. [8] D. Persaud-Sharma, A. McGoron, J. Biomim. Biomater. Tissue Eng. 12 (2012) 25–39. [9] A.H. Yusop, A.A. Bakir, N.A. Shaharom, M.R. Abdul Kadir, H. Hermawan, Int. J. Biomater. 2012 (2012) 641430. [10] Y. Yamada, K. Shimojima, Y. Sakaguchi, M. Mabuchim, M. Nakamura, T. Asahina, J. Mater. Sci. Lett. 18 (1999) 1477–1480. [11] C.E. Wen, Y. Yamada, K. Shimojima, Y. Chino, H. Hosokawa, M. Mabuchi, Mater. Lett. 58 (2004) 357–360. [12] C. Korner, M. Hirschmann, V. Brautigam, R.F. Singer, Adv. Eng. Mater. 6 (2004) 385–390. [13] K. Renger, H. Kaufmann, Adv. Eng. Mater. 7 (2005) 117–123. [14] Z. Xie, M. Tane, S. Hyun, Y. Okuda, H. Nakajima, Mater. Sci. Eng. A 417 (2006) 129–133. [15] V. Karageorgiou, D. Kaplan, Biomaterials 26 (2005) 5474–5491. [16] C.M. Murphy, M.G. Haugh, F.J. O'Brien, Biomaterials 31 (2010) 461–466. [17] T.L. Nguyen, M.P. Staiger, G.J. Dias, T.B.F. Woodfield, Adv. Eng. Mater. 13 (2011) 872–881. [18] P. Liu, Q.B. Tan, L.H. Wu, G. He, Mater. Sci. Eng. A 527 (2010) 3301–3309. [19] G. He, P. Liu, Q.B. Tan, J. Mech. Behav. Biomed. 5 (2012) 16–31. [20] S.R. Kalidindi, A. Abusafieh, E. EI-Danaf., Exp. Mech. 37 (1997) 210–215. [21] K. Kondoh, M. Kawakami, H. Imai, J. Umeda, H. Fujii, Acta Mater. 58 (2010) 606–614. [22] M.E. Straumanis, P.C. Chen, J. Electrochem. Soc. 98 (1951) 234–240. [23] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88–113. [24] K.Y. Chiu, M.H. Wong, F.T. Cheng, H.C. Man, Surf. Coat. Technol. 202 (2007) 590–598. [25] S. Hyun, A.M. Karlsson, S. Torquato, A.G. Evans, Int. J. Solids Struct. 40 (2003) 6989–6998. [26] K. Kang, Acta Mater. 57 (2009) 1865–1874. [27] L. Tan, M. Gong, F. Zheng, B. Zhang, K. Yang, Biomed. Mater. 4 (2009) 015016. [28] F. Tencer, K.D. Johnson, Biomechanics in Orthopaedic Applications: Bone Fracture and Fixation, Martin Dunitz, London, 1994. [29] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Prog. Mater. Sci. 54 (2009) 397–425. [30] C.E. Wen, M. Mabuchi, Y. Yamada, K. Shimojima, Y. Chino, T. Asahina, Scripta Mater. 45 (2001) 1147–1153. [31] H. Okumura, K. Watanabe, S. Kamado, Y. Kojima, Mater. Trans. 44 (2003) 595–600. [32] J. Huadmai, A novel processing route for the fabrication of porous magnesium biomaterials, (Master thesis of University of Canterbury) Christchurch, New Zealand, 2005. [33] M. MohammadiZahrani, M. Meratian, Y. Kabiri, Mater. Lett. 85 (2012) 14–17. [34] M.H. Kang, H.D. Jung, S.W. Kim, S.M. Lee, H.E. Kim, Y. Estrin, Y.H. Koh, Mater. Lett. 108 (2013) 122–124.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Short communication
A new approach to the fabrication of porous magnesium with well-controlled 3D pore structure for orthopedic applications Guofeng Jiang, Guo He ⁎ State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 7 February 2014 Received in revised form 10 May 2014 Accepted 8 July 2014 Available online 15 July 2014 Keywords: Magnesium Porous material Biomaterials Biodegradability Implant
a b s t r a c t A new approach to the fabrication of porous magnesium by using the titanium wire space holder (TWSH) method has been developed. Since the entangled titanium wire structure is used as the space holder, the porous structure is pipe-like and interconnected, and the pore size is identical and equals to the diameter of the titanium wire. In particular, the pore size, porosity, and porous morphology of the porous magnesium can be exactly and individually controlled. When the porosity is in the range of 54.2–43.2%, the compressive yielding strength is in the range of 4.3–6.2 MPa, and the Young's modulus is in the range of 0.5–1.0 GPa. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Porous metallic materials have drawn widespread attention in various industrial fields due to their excellent physical and mechanical properties, such as high stiffness at extremely low density, high energy absorption, heat resistance, permeability and so on [1,2]. In particular, porous titanium and magnesium with very good biocompatibility have shown their potentials for the biomedical applications in surgical implants. The open-cellular structure in the porous materials can provide adequate spaces for the living tissue ingrowth and the nutrient transportation [3,4], and most importantly, the Young's modulus of the porous metals can be well controlled by adjusting the porosity, so as to match the stiffness of the natural bone [5]. Except for the above superiority, the porous magnesium also has very good biodegradability, which has been recently recognized as a very promising biomaterial for tissue engineering [6–9]. So far, many methods, such as investment casting [10], powder metallurgy with space holder [11], vacuum foaming [12], thixo casting [13], and directional solidification [14], have been developed to prepare porous magnesium. However, those traditional metallurgical methods are difficult to accurately control the pore morphologies (e.g., open and interconnected pores, 300–500 μm pore size, certain porosity ensuring appropriate stiffness and adequate strength) which are highly important for guaranteeing the biomechanical properties and the biocompatibility [4,15,16]. In recent years, a solid free-form fabrication
⁎ Corresponding author. Tel./fax: +86 21 3420 2643. E-mail address: [email protected] (G. He).
http://dx.doi.org/10.1016/j.msec.2014.07.033 0928-4931/© 2014 Elsevier B.V. All rights reserved.
(SFF) process for producing topologically ordered magnesium (TOPM) structures has been developed [17], which adopt a layer-by-layer fabrication process from computer aided design (CAD) models for manufacturing ordered pore architectures by accurate control of porous topology. This provides a possible route to fabricate the porous magnesium scaffolds with designed 3D pore structure and tailored mechanical properties. The drawback is that the SFF method needs six-steps which are complicated and cost-intensive. In this study, we report a new approach to the fabrication of porous magnesium with controlled 3D pore structure, which needs only threesteps: (1) the entangled (or woven) 3D structure was prepared by using titanium wire [18], (2) the 3D structure was used as a space holder to fabricate the titanium/magnesium composite by infiltrating cast, and (3) the 3D structure, i.e., titanium wire space holder (TWSH) material, was then removed from the composite by etching off. With this method, it is possible to fabricate the porous magnesium with well-controlled 3D pore structure. 2. Materials and methods The starting materials are commercially available titanium wire (purity: 99.9%, diameter: 0.27 mm) and magnesium ingot (purity: 99.9%). The 3D entangled titanium wire material with dimensions of Φ10 mm × 20 mm was prepared (details have been described elsewhere [18,19]), which was used as a space holder material. The solid magnesium was melted and kept at 700 °C under the atmosphere of the mixed gas of SF6 and CO2. Then the TWSH was soaked in the magnesium melts. In order to improve the melt infiltration into the porous structure in the TWSH material, the supersonic vibration (the source
318
G. Jiang, G. He / Materials Science and Engineering C 43 (2014) 317–320
frequency: 40 kHz; output: 80 W; the vibration time is 60 s) was applied. After all the voids were completely filled with the magnesium melt, the sample was taken out slowly and cooled down in the protective atmosphere, so as to form titanium/magnesium composite. After that, the composite was immersed in the hydrofluoric acid (HF) solution (analytical reagent with the concentration of 40 wt.%) at room temperature to remove the TWSH material. During the etching, a supersonic vibration was applied on the sample from start to finish so as to quicken the Ti-etching and promote the discharge of the hydrogen gas. The etching time was about 72 h in this situation. When all the titanium wire in the composite was etched off, the sample (already becoming porous magnesium at this step) was taken out and cleaned by immersion in acetone with ultrasonic agitation. After the above steps (as illustrated in Fig. 1), the porous magnesium with well-controlled 3D pore structure could be fabricated. The porosity of the porous magnesium could be calculated with the following formula: P¼
1−
m1 νρMg
! 100%
ð1Þ
where ρMg is the density of the magnesium which is 1.74 g/cm3, v is the volume of the sample, and m1 is the weight of the magnesium in the composite (i.e., the weight of the porous magnesium), which can be directly measured or calculated by following relation: m1 ¼ m−m2
ð2Þ
where m denotes the weight of the as-prepared Ti/Mg composite, and m2 denotes the weight of the TWSH. Both could be measured directly. If the measured data (m, m2 and m1) could meet the above equation, the TWSH would be confirmed to be removed completely from the composite. The as-prepared porous structure was directly characterized by using the optical microscopy and the scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The pore size was evaluated by quantitative image analysis using the commercially available software Image-Pro Plus. The mechanical properties of the porous magnesium were evaluated by the uniaxial compressive tests that were performed by using Zwick AG-100KN testing machine at room temperature. The tests were conducted on the specimens with dimensions of Φ10 mm × 20 mm under displacement control with the crosshead speed of 1 mm/min. Since the porous magnesium deforms easily under a much lower loading, the elastic deformation occurs at relatively low stress levels. The elastic modulus evaluated from the stress–strain curve could be significantly influenced by the test machine compliance. Thus the
compliance corrections were conducted by using the known pure magnesium (its Young's modulus is 45 GPa) in this study (the details regarding the compliance correction were described in ref. [20]). Therefore, all the elastic moduli of the porous magnesium reported in this paper had been calibrated accordingly. 3. Results and discussion The appearance of the as-prepared porous magnesium was shown in Fig. 2a. Its porosity was evaluated to be about 51% according to the 49% porosity of the initial TWSH material. In order to verify the homogeneity of the porous structure, the sample was cut along the axial direction and shown in Fig. 2b. The porous structure was roughly homogenous, indicating that the infiltration of the magnesium melts into the porous TWSH material was well proceeded. Only one shrinkage cavity-like ‘large’ pore was found in the center of the cylindrical sample as indicated by the black arrow in Fig. 2b, suggesting that the local underfill occurred during the infiltrating cast. This perhaps could be attributed to the local high dense entangled titanium wire structure where the pores were too small to be filled with the magnesium melts. It also showed, as expected, that all the pores in the porous magnesium were pipe-like channels with the diameter of about 270 μm (approximately equivalent to the diameter of the titanium wire, used in this study). Fig. 2c–e showed the sectioned view of these channels, on which some flow lines were clearly shown on the ‘channel’ inner surface as indicated by the white arrows in Fig. 2c–e. These flow lines were reversely replicated from the surface plastic flowing deformation vestiges of the titanium wire. This just indicated that the titanium wire could be completely wetted by the magnesium melts at the infiltration temperature as reported in ref. [21]. The removal of the titanium wire from the titanium/magnesium composite could be achieved based on the different responses between titanium and magnesium in the HF solution. The titanium wire could be gradually eroded by the following chemical reactions [22]: 2Ti þ 6HF ¼ 2TiF3 þ 3H2 ↑
ð3Þ
Ti þ 4HF ¼ TiF4 þ 2H2 ↑
ð4Þ
−
TiF4 þ 2 F
2−
¼ ½TiF6
:
ð5Þ
The dissolution of titanium in the HF solution was suggested to occur via the electrochemical process [22] in which the solid state titanium was converted to ionized state [TiF6]2−. This process would be continually in progress until all the titanium wire was completely consumed. The release of hydrogen during the dissolution of titanium was just the sign indicating the titanium dissolving.
Fig. 1. Schematic illustration of the TWSH method for the fabrication of porous magnesium.
G. Jiang, G. He / Materials Science and Engineering C 43 (2014) 317–320
319
Fig. 2. Macroscopic appearance of as-prepared porous magnesium (a), the section view (b), the SEM micrographs of the section (c)–(e), and the EDS profile of the inner surface of the pipelike pore (f).
On the other hand, the magnesium could be rapidly and completely passivated in the HF solution by the formation of the MgF2 layer on the magnesium surface [23], which was confirmed by EDS analysis as shown in Fig. 2f. The basic reaction was: Mg þ 2HF ¼ MgF2 þ H2 ↑:
ð6Þ
Although the reaction also released hydrogen, it was rapidly slowing down as the formation of the MgF2 film [24]. The hydrogen bubbles were almost from the titanium dissolving at the latter stage, so that the bubbles directly indicated the dissolution of titanium. When the bubbles ceased to come out of the HF solution, the dissolution of the titanium wire finished. It was obvious that the size (i.e., diameter) of the pipe-like pores in the porous magnesium was identical which could be exactly controlled by selecting different diameters of the titanium wire. The bulk porosity of the porous magnesium could also be exactly controlled by adjusting the porosity of the initial TWSH structure. The porous morphology depended on the 3D structure of the TWSH material which could be designed and manufactured as various wire-entangled or multilayered truss-type porous titanium by the wire-woven method [25,26]. It is very favorable that the pore size, the bulk porosity, and the porous morphology are independent, which can be exactly and individually controlled. This makes it convenient to balance between the adequate mechanical properties and the obligatory porous structure (including pore size, porosity and interconnectivity) that is extremely important for the new tissue ingrowth and the body fluid transportation [4]. The mechanical properties of the as-prepared porous magnesium were evaluated by the compressive test. The nominal stress–strain curves were shown in Fig. 3. The continuous line and the dash line
showed the two tests of the specimens with the same porous structure, indicating the good repeatability of the mechanical behavior of the porous magnesium. Under the compressive loading the specimen exhibited a short elastic deformation at the initiation of strain followed by a very long strain-hardening region as shown in Fig. 3. Such stress–strain behavior was very similar with that of the porous magnesium fabricated by the mechanical perforation method [27], suggesting that the strain occurred mainly via the bulk deformation rather than via the porous structure local collapse that often happened in the porous magnesium
Fig. 3. Nominal stress–strain curves of the as-prepared porous magnesium. The two curves are from two tests of the specimens with the same porous structure.
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fabricated by the conventional powder sintering method [11] and in other metal foams [1]. The mechanical properties were directly evaluated from the stress–strain curves, which were about 4.6 MPa of yielding strength and about 0.6 GPa of Young's modulus for the porous magnesium with a 51% porosity. Another two groups of the porous magnesium samples with the porosities of 43.2% and 54.2% were also tested. Their yielding strengths were 6.2 MPa and 4.3 MPa, and Young's moduli were 1.0 GPa and 0.5 GPa, respectively. Obviously, such porous magnesium with the porosities in the range of 43.2% to 54.2% exhibits comparable strength and Young's modulus to the cancellous bone which has the strength in the range of 3–20 MPa [28], and the Young's modulus in the range of 0.01–3.0 GPa [29]. It should be mentioned that the MgF2 film on the Mg surface should affect the mechanical properties of the porous Mg, but this effect is relatively small. This is because the compressive mechanical properties strongly depend on the porosity and the pore morphology. The MgF2 film formed during the etching process is expected to be about 0–2 μm in thickness [24]. Such thin film accounts for a very small volume percentage of the total, So that its influence may be concealed in the systematic errors. To our knowledge, there is no data in the literature to show the effect of the conversion coating on the mechanical properties of Mg or its alloy. This needs to be clarified in the future work. So far, there are very few studies on the mechanical behaviors and properties of the porous magnesium. The available data of the materials fabricated by various methods [6,11,27,30–34] revealed an approximate linear relationship between the relative elastic modulus and the relative density in the logarithmic coordinates as shown in Fig. 4 although some scattered points were present in the figure. By comparing the data presented in Fig. 4, one can conclude that the mechanical properties of the porous magnesium fabricated by the TWSH method are similar to that of other porous magnesium. 4. Conclusions The TWSH method has been successfully used to fabricate porous magnesium, by which the porosity, pore size, and porous morphology can be exactly and individually controlled. In this study, the entangled titanium wire structure has been used as the space holder for the fabrication of the porous magnesium. The porous morphology is a pipe-like and interconnected pore structure, and the pore size is about 270 μm that equals to the diameter of the titanium wire. The as-prepared porous magnesium exhibits the compressive yielding strengths of 4.3, 4.6 and 6.2 MPa, and the Young's moduli of 0.5, 0.6 and 1.0 GPa at the
Fig. 4. Relationship between the relative elastic modulus and the relative density of the porous magnesium.
porosities of 54.2%, 51% and 43.2%, respectively. Such mechanical properties are comparable to that of the cancellous bone, suggesting potential applications in the orthopedic surgery. References [1] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, second ed. Cambridge University Press, Cambridge, 1997. [2] M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams: A Design Guide, Butterworth-Heinemann, Boston (MA), 2000. [3] L.D. Zardiackas, L.D. Dillon, D.W. Mitchell, L.A. Nunnery, R. Poggie, J. Biomed. Mater. Res. 58 (2001) 180–187. [4] J.T. Clemow, A.M. Weinstein, J.J. Klawitter, J. Koeneman, J. Anderson, J. Biomed. Mater. Res. 15 (1981) 73–82. [5] L.P. Lefebvre, J. Banhart, D.C. Dunand, Adv. Eng. Mater. 10 (2008) 775–787. [6] H. Zhuang, Y. Han, A. Feng, Mater. Sci. Eng. C 28 (2008) 1462–1466. [7] H. Zreiqat, C.R. Howlett, A. Zannettion, P. Evans, G. Schulze-Tanzil, C. Knabe, M. Shakibaei, Biomed. Mater. Res. 62 (2002) 175–184. [8] D. Persaud-Sharma, A. McGoron, J. Biomim. Biomater. Tissue Eng. 12 (2012) 25–39. [9] A.H. Yusop, A.A. Bakir, N.A. Shaharom, M.R. Abdul Kadir, H. Hermawan, Int. J. Biomater. 2012 (2012) 641430. [10] Y. Yamada, K. Shimojima, Y. Sakaguchi, M. Mabuchim, M. Nakamura, T. Asahina, J. Mater. Sci. Lett. 18 (1999) 1477–1480. [11] C.E. Wen, Y. Yamada, K. Shimojima, Y. Chino, H. Hosokawa, M. Mabuchi, Mater. Lett. 58 (2004) 357–360. [12] C. Korner, M. Hirschmann, V. Brautigam, R.F. Singer, Adv. Eng. Mater. 6 (2004) 385–390. [13] K. Renger, H. Kaufmann, Adv. Eng. Mater. 7 (2005) 117–123. [14] Z. Xie, M. Tane, S. Hyun, Y. Okuda, H. Nakajima, Mater. Sci. Eng. A 417 (2006) 129–133. [15] V. Karageorgiou, D. Kaplan, Biomaterials 26 (2005) 5474–5491. [16] C.M. Murphy, M.G. Haugh, F.J. O'Brien, Biomaterials 31 (2010) 461–466. [17] T.L. Nguyen, M.P. Staiger, G.J. Dias, T.B.F. Woodfield, Adv. Eng. Mater. 13 (2011) 872–881. [18] P. Liu, Q.B. Tan, L.H. Wu, G. He, Mater. Sci. Eng. A 527 (2010) 3301–3309. [19] G. He, P. Liu, Q.B. Tan, J. Mech. Behav. Biomed. 5 (2012) 16–31. [20] S.R. Kalidindi, A. Abusafieh, E. EI-Danaf., Exp. Mech. 37 (1997) 210–215. [21] K. Kondoh, M. Kawakami, H. Imai, J. Umeda, H. Fujii, Acta Mater. 58 (2010) 606–614. [22] M.E. Straumanis, P.C. Chen, J. Electrochem. Soc. 98 (1951) 234–240. [23] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88–113. [24] K.Y. Chiu, M.H. Wong, F.T. Cheng, H.C. Man, Surf. Coat. Technol. 202 (2007) 590–598. [25] S. Hyun, A.M. Karlsson, S. Torquato, A.G. Evans, Int. J. Solids Struct. 40 (2003) 6989–6998. [26] K. Kang, Acta Mater. 57 (2009) 1865–1874. [27] L. Tan, M. Gong, F. Zheng, B. Zhang, K. Yang, Biomed. Mater. 4 (2009) 015016. [28] F. Tencer, K.D. Johnson, Biomechanics in Orthopaedic Applications: Bone Fracture and Fixation, Martin Dunitz, London, 1994. [29] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Prog. Mater. Sci. 54 (2009) 397–425. [30] C.E. Wen, M. Mabuchi, Y. Yamada, K. Shimojima, Y. Chino, T. Asahina, Scripta Mater. 45 (2001) 1147–1153. [31] H. Okumura, K. Watanabe, S. Kamado, Y. Kojima, Mater. Trans. 44 (2003) 595–600. [32] J. Huadmai, A novel processing route for the fabrication of porous magnesium biomaterials, (Master thesis of University of Canterbury) Christchurch, New Zealand, 2005. [33] M. MohammadiZahrani, M. Meratian, Y. Kabiri, Mater. Lett. 85 (2012) 14–17. [34] M.H. Kang, H.D. Jung, S.W. Kim, S.M. Lee, H.E. Kim, Y. Estrin, Y.H. Koh, Mater. Lett. 108 (2013) 122–124.