Porous titanium products for biomedical applications
Two papers at the MPIF PowderMet 2011 conference, held in San Francisco on 18-21 May, presented novel PM-based techniques for the preparation of porous titanium structures for such applications. Dr David Whittaker reports for ipmd.net.
Processing and characterisation of porous titanium for biomedical applications
The paper by J Bris and S Lascano (Universidad del Norte, Barranquilla, Colombia); Y Torres, G Hernandez and J Rodriquez (Universidad de Sevilla, Spain) and J Pavon (Universidad de Antioquia, Medellin, Colombia) compared two techniques for the preparation of porous samples from commercial purity Titanium grade-4 – loose powder sintering, followed by vacuum sintering at either 1000°C or 1100°C, and uniaxial compaction of powder mixes incorporating space-holder additions of NaCl in the volume fraction range of 40-70%, followed by space-holder removal in warm distilled water (50-60°C) for 4 hours and vacuum sintering at 1250°C.
The aim of this project was to address the problem of bone resorption resulting from the stress shielding effect arising from the stiffness ratio between the bone and the implant by producing porous titanium with stiffness comparable to that of cortical bone.
With the loose powder sintering technique, optimum stiffness (Young’s modulus of 20-25GPa compared with around 20GPa for bone) was obtained with the lower sintering temperature (1000°C) and porosity level around 47%. (Fig. 1).
With the space-holder method, this appropriate stiffness level was achieved with a porosity level of around 45% (Fig. 2). This method gave porosity with a higher level of interconnectivity and better aspect ratio than loose powder sintering and hence improved mechanical properties because of the enhanced quality of sintering necks (compare Figs 1 and 2).
Pore geometry became more regular with increasing sintering temperature in loose powder sintering or with decreasing volume fraction of space holder addition in the space holder method.
Young’s modulus was determined by two techniques: from the results of uniaxial compression tests and by ultrasonic assessment of dynamic modulus. The authors concluded that the ultrasonic method was the more reliable on the basis of the comparative agreement of the results from the two methods with calculated values from the Nielsen theoretical model, based on experimentally determined porosity parameters (Fig. 3).
Development of porous/dense implant using titanium powder injection overmolding and Ti-foam inserts
The second paper, from E Baril, Y Thomas and L-P Lefebvre (NRC, Canada), presented a method in which titanium foam inserts were powder injection over-moulded with commercial purity Ti or Ti-6Al-4V.
The titanium foam inserts were produced by an NRC-patented method, involving filling of a powder feedstock in cylindrical moulds. In a subsequent thermal treatment, the insert is foamed, debound and pre-sintered. During foaming, the binder melts and forms a suspension with the other particles. The foaming agent then decomposes and forms a gas to expand the suspension. After debinding, pre-sintering takes place at a peak temperature of 800°C in argon. Two formulations at two levels of porosity and pore size were processed (Fig. 4).
The configuration for over-moulding is shown in Fig. 5. The pre-sintered Ti-foam insert was installed in a pocket of the moulding tool. The over-moulded part was ejected, debound and fully sintered (at 1300°C in vacuum).
Examination of an etched metallographic section of a sample over-moulded with Ti-6Al-4V demonstrated the penetration of the PIM feedstock into the porous Ti-foam matrix (Fig. 6). The EDS elemental profiles in Fig. 7 demonstrate the Al and V depletion in the dense PIM portion of the samples and the diffusion of these elements into the Ti matrix of the foam. The transition zone is around 600µm wide.
On applying Hot Isostatic Pressing after sintering, residual porosity can be removed from the MIM portion of the structure without influencing the interconnected porosity in the foam structure (Fig. 8 and 9).
Tensile testing of the bond between foam and dense structure was carried out demonstrating that bond strengths in excess of 85 MPa are achieved, well above the FDA requirement of 22 MPa for porous coatings on orthopaedic implants.
The authors have proposed that this new production approach may enable the design of complex implants with foams integrated in various locations.
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