| The relationship between fabrication processing, microstructure and mechanical properties of porous Ti has been investigated, and the effect of pore structure on the fracture behavior and electrical conductivity of porous Ti has been also analyzed, in order to utilize the adequate porous structure, good biocompatibility and appropriate mechanical properties of porous Ti for the application in clinic orthopaedics field such as the bone substitute.Adding polymethyl methacrylate powders as pore maker, porous Ti with controllable pore structure was successfully prepared by powder metallurgy at first. The porous Ti shows a three-dimensional open-cellular structure with two types of pore:inter-connection macro-pore with stepped pore walls and a small isolated micro-pore distributed on the macro-pore walls. Porosity, macro-pore size and the open pore ratio of sintered porous Ti increase with the increasing of volume fraction and particle size of the pore maker. The pore anisotropy decreases linearly as the size of the PMMA particles used increases and hence with macro-pore size. As the increase of sintering temperature and sintering time, the grain size increases, pore size and porosity decrease and micro-pore rounds gradually.The compressive stress-strain curve of porous Ti show linear elasticity at low stresses followed by a long collapse plateau, truncated by a regime of densification in which the stress rises steeply. While the tensile and bending stress-strain curves of porous Ti both show the linear-elastic feature. Fractography shows evidence of the brittle cleavage fracture in porous Ti. The stress is prior to concentrate on the weak macro-pore wall, thus resulting in the crack and then propagation. The failure with the formation of shear bands of 45°to the stress axis due to cracking (complete fracture) of the struts on porous Ti is controlled primarily by the macro-pores. The elastic modulus and strength of porous Ti decrease with the increase of porosity and macro-pore size. Compared to Mori-tanaka model, the present experimental results are in agreement with the theory of Gibson-Ashby satisfactory. Poisson's ratio measured is in the range of-0.92—-0.37, which is linear with macro-pore size. The elastic modulus and strength of porous Ti increase with the increasing sintering temperature, longer sintering time and higher vucuum.Electrochemical corrosion tests are performed on porous titanium in 0.1 M H2SO4,1 M NaOH and 37℃0.9% NaCl and 37℃Hank's solutions. It is shown that the anodic polarization curves of porous titanium exhibit active-passive transition behavior in 1 M NaOH and 0.1 M H2SO4 and 37℃Hank's solutions, while self-passivation in 37℃0.9% NaCl solution. As the porosity increases and macro-pore size decreases, the corrosion current density of porous titanium increases due to the higher effective specific area, while the corrosion potential does not change (<5%) remarkably.As the porosity increasing and the macro-pore size decreasing, the electrical conductivity of porous Ti decreases dramatically. The dependence of the electrical conductivity on the porosity could be well described by the Maxwell approximation. The differential effective medium approximation is only applicable to porous Ti with average pore size of 400μm in the porosity range of 40-70%. Taking the porosity, pore size and pore morphology into consideration, Maxwell approximation and differential effective medium approximation have been modified to beσ-σ0(1-ε)/(1+aε+bxε) andσ=σ0(1-ε)(c+dx), respectively. Where x represents pore size,εis porosity,σandσ0 are electrical conductivity of porous and solid Ti, respectively, a and b are the constant correlative to the experimental condition and pore morphology of porous materials, respectively, c and d represent the critical exponent that depend on the system parameters and pore morphology in a piecewise constant manner, respectively.
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