preparation and properties of magnesium...

18. - 20. 5. 2011, Brno, Czech Republic, EU

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PREPARATION AND PROPERTIES OF MAGNESIUM POROUS MATERIALS FOR MEDICAL APPLICATIONS

Jaroslav CAPEK1,a, Dalibor VOJTECH1

1Department of metals and corrosion engineering, Institute of Chemical Technology, Prague,

Technická 5, 166 28 Prague 6, Czech Republic [email protected]

Abstract

Magnesium is a biocompatible and biodegradable metal which possesses suitable mechanical properties for load-bearing applications in orthopaedics. Moreover, magnesium ions, the products of magnesium corrosion, improve growth of the new bone tissue.

Implementation of the porosity into these materials compromises values of mechanical properties closer to the values of nature bone and allows the incorporation of the new tissue into the implant and the transport of body fluids to the healing tissue.

In this work porous magnesium samples with the porosity of approximately 11, 14, 26, 29 and 44% were prepared and their structures and flexural strengths were studied.

Keywords: Porous magnesium, biocompatibility, biodegradability, powder metallurgy.

1. INTRODUCTION

Recently, demand for surgery implants increases. There are four types of materials possible to produce surgery implants: metals, ceramics, polymers and composites. Due to their mechanical properties, metals are more suitable for load-bearing applications than the other above mentioned materials [1]. In comparison with commonly used alloys as stainless steel or titanium alloys, magnesium-based alloys are biocompatible and biodegradable materials which possess mechanical properties more close to them of a nature bone. Moreover, magnesium ions are necessary for human body and they improve growth of the new bone tissue. Therefore magnesium-based materials appear to be suitable for using in orthopaedic applications [2]. Unfortunately corrosion rate of pure magnesium in human body is too high, so it has to be decreased by alloying or coating [2].

Implementation of the porosity to these materials compromises values of mechanical properties more closely to the values of a nature bone. In addition, the weight of the implant decreases and the porosity allows better incorporation of a new bone tissue into the implant. The porosity allows body fluids to flow to the healing tissue too [3]. The influence of corrosion on mechanical properties is less negative in the case of porous samples than that of compact magnesium [3]. On the other hand, the corrosion rate increases by increased porosity [4].

Generally, there are many techniques for manufacturing of metal foams [5], but only some of them are suitable for producing of metal foams for medical applications, because the products cannot be polluted by harmful impurities. Three techniques are commonly used for manufacture of porous metal equipment for medical applications: injection of an inert gas to the melt, the plaster casting method and powder metallurgy techniques [2].

In this work, the porous magnesium samples were prepared by a powder metallurgy technique using space-holder particles. Solid compounds which decompose to form gaseous products at elevated temperatures can be used as space-holder particles. For example, urea or ammonium bicarbonate can be used for preparation


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metal foams [4, 6]. Naturally, space-holder particles and products of their decomposition cannot be harmful and react with metal powder [4, 6]. For the preparation of magnesium foams ammonium bicarbonate is more suitable then urea, because decomposition of ammonium bicarbonate occurs between 36 and 60°C [7], whereas total decomposition of the urea takes place at temperatures over 700°C [8], which is higher than melting point of magnesium [9].

In this work, porous magnesium samples were prepared by the powder metallurgy technique. Ammonium bicarbonate was used as space-holder particles. Structure, porosity, flexure strength and their relations were studied.

2. EXPERIMENTAL

Samples were prepared by the powder metallurgy technique using ammonium bicarbonate as space-holder particles. As starting materials commercial pure magnesium powder of irregular shape and analytical pure ammonium bicarbonate powder were used. The starting materials were studied by grain size analysis and fractions having the desired particle size (500 – 1000 µm for Mg, 250 – 500 µm for NH4HCO3) were used. Magnesium and ammonium bicarbonate powder were mixed in volume ratio Mg:NH4HCO3 = 100:0, 98:2, 95:5, 90:10 and 85:15. These mixtures were pressed by Heckert FPZ 100/1 machine by a pressure of approximately 265 MPa into the cylindrical green compacts with 16 mm in diameter and about 30 mm in length. The green compacts were treated by three-step heat treatment: 1. decomposition of ammonium bicarbonate at 130°C for 4 h in a muffle furnace on the air, 2. sintering at 550°C for 6 h in a tube furnace in argon atmosphere, 3. decomposition of residual ammonium bicarbonate at 130°C for 4 h in a muffle furnace on the air. Cylindrical sample of casted magnesium with 4 mm in diameter and about 30 mm in length was used for the comparison of the flexural strengths of samples prepared by powder metallurgy and by casting process. The flexural strengths of these samples were measured by Heckert FPZ 100/1 machine. Metallographic cross-sections were prepared by grinding on SiC papers P 400 - P 4000 and polishing on D 2 and D 0.7 diamond pastes. These metallographic cross-sections were documented by a Neophot 2 light metallographic microscope and Tescan Vega-3 LMU scanning electron microscope. Porosity of the samples was determined from optical micrographs by the Lucia G 4.8 image analyzer.

3. RESULTS AND DISCUSSION

3.1. Structure and porosity

In fig. 1 cross-section optical and SEM micrographs of the prepared samples are shown. It can be seen, that the sample prepared by powder metallurgy without using space-holder particles contains pores, too. These pores form through incomplete pressing and thermal expansion and contraction which take place during sintering and cooling. The samples prepared by the use of space-holder particles contain these “micro-pores” together with pores of a size corresponding with particles size of ammonium bicarbonate. Some pores were bigger than 500 µm (maximal size of used ammonium bicarbonate). It can be due to incomplete homogenization or due to the effects of evolved gasses.


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a)

b)

c)


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Fig. 1. Optical and SEM (BSE detector) micrographs of cross-sections of the samples prepared by pressing of the mixture containing magnesium and a) 0, b) 2, c) 5, d) 10 and e) 15 vol. % NH4HCO3

d)

e)


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Influence of the amount of added ammonium bicarbonate on the real porosity is shown in Fig. 2.

Fig. 2. Dependence of the real porosity on the volume ratio Mg:NH4HCO3

From Fig. 2 it is obvious, that the porosity increases almost linearly with growing content of ammonium bicarbonate. The total porosity is higher than sum of volume of added ammonium bicarbonate and porosity of the sample prepared from magnesium powder only, and this difference increases with the increasing amount of ammonium bicarbonate. It may be due to the increasing amount of generated gasses.

3.2. Mechanical properties The flexural strength of prepared samples was measured and its dependence on porosity was correlated (Fig. 3).

Fig. 3. Influence of the porosity on the flexural strength The flexural strength of the sample prepared without addition of ammonium bicarbonate (porosity of approximately 11%) was immeasurable. By the addition of ammonium bicarbonate the flexural strength


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rapidly decreased. It may be due to bigger pores, which have more negative influence on mechanical properties than “micro-pores” formed in materials prepared by powder metallurgy technique without using space-holder particles. It can be concluded from differences of flexural strengths of the samples with 11 and 14% porosity (0 or 2 vol.% of ammonium bicarbonate added to pressed magnesium). Generally, the flexural strength decreases with increasing porosity. In comparison with the flexural strength of the cast magnesium (approximately 190 MPa), observed flexural strengths of prepared samples are markedly lower (approximately 5; 4,6; 4,1 and 1,8 MPa for porosity 14; 26; 29 and 44% respectively).

4. CONCLUSIONS Porous magnesium samples were prepared by a powder metallurgy technique using ammonium bicarbonate as space-holder particles. The obtained porosities were not too homogenous and the flexural strengths were too low in comparison with cast magnesium. These insufficiencies would be improved by the use of the spherical magnesium powder or by changing and by optimization of pressing and sintering conditions. It will be investigate in future. ACKNOWLEDGEMENT This work was financially supported by Ministry of Education, Youth and Sports of the Czech Republic (project no. MSM6046137302 and 21/2011).

5. BIBLIOGRAPHIES [1] ZHENG, H., et al. Progress and Challenge for Magnesium Alloys as Biomaterials. Advanced

Engineering Materials, 2008, vol. 10, p. B1–B32. [2] STEIGER, M. P., et al. Magnesium and its alloys as orthopedic biomaterials: A review. Biomaterials,

2006, vol. 27, p. 1728–1734. [3] GU, X., et al. Degradation and cytotoxicity of lotus-type porous pure magnesium as potential tissue

engineering scaffold material. Mater. Lett., 2010, vol. 64, p. 1871–1874. [4] ZHUANG, H., YONG, H., AILING, F. Preparation, mechanical properties and in vitro biodegradation of

porous magnesium scaffolds. Mater. Sci. Eng., C, 2008, vol. 28, p. 1462–1466. [5] DAVIES, G., ZHEN, S. Metallic foams: their production, properties and applications. J. Mater. Sci.,

1983, vol. 18, p. 1899–1911. [6] WEN, C. E., et al. Processing of biocompatible porous Ti and Mg. Scripta Materialia, 2001, vol. 45, p.

1147–1153. [7] http://www.jtbaker.com/msds/englishhtml/a5616.htm, 8.3.2011 [8] SCHABER, P.M. Thermal decomposition (pyrolysis) of urea in an open reaction vessel. Thermochim.

Acta, 2004, vol. 424, p. 131–142. [9] FRIEDRICH, H. E., MORDIKE, B. L. Magnesium Technology - Metallurgy, Design Data, Applications.

1st ed. 2006. ISBN 978-1-60119-031-4.

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