POWDER METALLURGY PREPARATION OF POROUS TITANIUM FOR MEDICAL IMPLANTS Pavel NOVÁK, Ladislav SITA, Anna KNAISLOVÁ, Dalibor VOJTĚCH Institute of Chemical Technology, Prague, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic, EU, panovak@vscht.cz Abstract Porous biomaterials are used for implants to improve the osseointegration and to achieve the mechanical properties closer to the human bone. In this work, conditions of the powder metallurgy process were optimized to obtain porous biomaterial. The effects of titanium particle size, pressure used for powder pressing and sintering temperature on the porosity and mechanical properties were determined. The most significant effect on the porosity was found in the case of the size of titanium particles. The addition of the pore-forming agent was also successfully tested. As the pore former, the water-soluble salt (NaCl) was applied. A mixture of titanium and salt was prepared, compressed and leached in distilled water. After that, sintering was carried out under the optimized conditions. Keywords: porous, titanium, biomaterial, implant 1. INTRODUCTION Titanium and its alloys are commonly used in medicine as the implant materials. Due to excellent corrosion resistance and low density, their use concerns both orthopedics and dentistry. To improve osseointegration, porous surface layers [1],[2] or bulk porous materials [3] are applied. The use of bulk porous materials also enables to adjust the mechanical properties to the values closer to the bone to prevent stress shielding [4]. Porous titanium alloys can be prepared by many way using both melting and powder metallurgy [5]. In melting metallurgy processes, porous material can be obtained by the addition of foaming agent, i.e. the compound that decomposes producing a gas. As the materials cools down during solidification, the solubility of the gas decreases. Evolved gas molecules produce pores in solidifying alloy. An example of kind of process is the addition of TiH 2 to the melt [5]. However, this technology produces mainly closed pores with no surface contact. Due to this fact, it can help to adjust the mechanical properties of the implant but not to improve the interaction with the bone [5]. For this reason, powder metallurgy processes play an important role in this field. These technologies can be divided to two main groups methods without pore-forming additives [6] and methods using space-holder agents [7]. When producing porous material without additives, partial sintering [6] or rapid prototyping [8] can be used. In the second group, space-holder agents are added to the titanium or alloy powder and compressed. After that, they are removed by thermal decomposition [7] or leaching in suitable agent [9]. In this work, the powder metallurgy processes were applied to commercial-purity titanium to obtain porous material. Partial sintering and the use of water-soluble spaceholder agent were tested. 2. EXPERIMENTAL In the first part of the work, green bodies of titanium without additives were prepared using two fractions of titanium powder (spherical <10 µm and coarse irregular powder 200-600 µm, 99.5 wt. % purity) were applied. Green bodies were prepared by pressing at the room temperature by a pressure of 350 and 600 MPa. Sintering temperature was 800 and 1100 C and duration was 5 h.
In the second part of the work, the application of pore-forming agent was tested. The water soluble salt (NaCl) was used as a space-holder. Titanium powder was blended with sodium chloride and compressed at room temperature. NaCl was removed by leaching in distilled water prior sintering. Density of prepared samples was determined from samples weight and dimensions and also by image analysis of the samples micrographs. Compressive strength was measured by LabTest 5.250SP1-VM universal loading machine. 3. RESULTS AND DISCUSSION In the first part of the work, the parameters affecting porosity of sintered titanium were determined. The dependence of the porosity on titanium particle size and pressure used during cold pressing is shown in Fig.1 and Tab. 1. It can be seen that the porosity of the material strongly increases with the size of the titanium particles and reduces with growing pressure used for cold pressing (Fig.1, Tab.1). The use of coarse particles also leads to the formation of larger pores with maximal percentage between 50 and 200 µm, see Fig.2. On the other hand, sintering of compressed fine particles (<10 µm) results mainly in small pores between 0 and 100 µm. To improve the osseointegration, the pores of 200 400 µm play the most important role due to the size of the bone cell. These pores can be found only in the material prepared from the coarse titanium powder (titanium particle size 200-600 µm), see Fig.1b and Fig.2. Fig.1 Microstructure of titanium after sintering at 800 C for 5 h a) Ti particle size >10 µm, pressure for cold pressing 600 MPa, b) Ti particle size 200-600 µm, pressure for cold pressing 600 MPa, c) Ti particle size >10 µm, pressure for cold pressing 350 MPa. The yield stress (R p0.2 ) was found to reduce with decreasing pressure used for cold pressing strongly. The use of coarse titanium particles results in higher yield stress than lowering the pressure during
pressing even though the porosity of the sample with coarse titanium is higher (Tab. 1). It is probably caused by the preparation route of coarse titanium particles. Coarse titanium particles were prepared by mechanical machining without any lubricant. By this procedure, fresh surface with low amount of oxides can be prepared. On the other hand, commercial fine particles may oxidize strongly during the storage and manipulation due to high surface area. Yield stress of the porous material can be also improved by the application of higher sintering temperature (Tab.1) Tab.1 Dependence of porosity and yield strength in compression on preparation conditions Ti size [µm] Temperature [ C] Pressure [MPa] w(nacl) [wt.%] Porosity [vol.%] R p0.2 [MPa] <10 800 600 0 5.9 280 200-600 800 600 0 46.2 230 <10 800 350 0 42.0 195 <10 1100 600 0 7.2 400 <10 800 600 10 18.7 185 <10 800 600 20 38.7 45 <10 800 600 30 53.8 38 Fig.2 Pore size distribution in sintered samples vs. titanium particle size and pressure for cold pressing
As the second step, the addition of pore-forming agent was tested. In this work, water-soluble salt (NaCl) was used as the space-holder. Powder mixture of titanium and NaCl was prepared and cold pressed. Before sintering, NaCl was removed by leaching in distilled water. The dependence of the porosity of sintered compacts on the weight fraction of NaCl is presented in Tab. 1 and Fig.3. As expected, the porosity increases with growing amount of NaCl. The amount of desirable large pores above 200 µm increases significantly by space-holder addition (Fig.4). By this technique, the pore size can be controlled by the size of the NaCl particles. Addition of sodium chloride particles strongly reduces the yield stress from 280 MPa for pure sintered titanium to less than 40 MPa for the sample with 30 wt. % of titanium. This decrease is desirable since it brings the properties of titanium-based biomaterial closer to the properties of the human bones [10]. Fig.3 Microstructure of titanium after sintering at 800 C for 5 h with the addition of a) 10 wt. % of NaCl, b) 20 wt. % of NaCl, c) 30 wt. % of NaCl.
Fig.4 Pore size distribution in sintered samples vs. NaCl weight fraction 4. CONCLUSION In this work, the parameters affecting the porosity of titanium were studied. The most crucial effect was found in the case of titanium particle size and pressure used for cold pressing prior sintering. To increase the porosity and to achieve the desired pore size distribution, the water-soluble pore-forming agent (NaCl) was applied. By the addition of NaCl, pore fraction and size as well as mechanical properties of resulting material can be controlled. ACKNOWLEDGEMENT This research was financially supported by Technology Agency of the Czech Republic, project No. TE01020390. REFERENCES [1] Braem A., Neirinck B., Schrooten J., Van der Biest O., Vleugels J., Biofunctionalization of porous titanium coatings through sol gel impregnation with a bioactive glass ceramic, Materials Science and Engineering: C, 32 (2012) 2292-2298. [2] Sun J., Han Y., Cui K., Innovative fabrication of porous titanium coating on titanium by cold spraying and vacuum sintering, Materials Letters, 62 (2008) 3623-3625. [3] Barbas A., Bonnet A.S., Lipinski P., Pesci R., Dubois G., Development and mechanical characterization of porous titanium bone substitutes, Journal of the mechanical behavior of biomedical materials, 9 (2012) 34-44.
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