Preparation And Properties Of Porous Ti-Mg Matrix Bio-composites With Low Elastic Modulus

Titanium and its alloys have been used as implant materials in dental and orthopaedic applications owing to their excellent biocompatibility, corrosion resistance and mechanical properties. However, the fixation between implant and host bone remains a problem. The reason is that the mismatch of elastic modulus between implant and host bone. Having the mechanical mismatch, implant bone is not sufficiently loaded and becomes stress shielded, eventually leading to the loosening of the implant. One way to alleviate the problem is to introduce pores into the whole implant. Elastic modulus of porous implant could be tailored by controlling porosity to match the various modulus of the bone, thereby reducing the stress shielding. The newly born bone could be found in the pore space of porous implant thus it is beneficial to form the mechanical interlocking between the implant and natural bone.According to the current situation and existing flaws of titanium alloys for surgical implants, new Ti-Mg composites for surgical implants were prepared by powder metallurgiy in this work. The effects of the powder metallurgy process on the properties of samples were systematically studied in order to determine appropriate process parameters. Secondly, the influence of NH4HCO3 content and particle size on the pore characteristics and mechanical properties of porous composites were investigated respectively. In addition, Zr was added improved the strength of porous Ti-Mg. Scanning electron-microscope(SEM) equipped with an energy dispersive spectrometer (EDS),X-ray diffraction (XRD)were used to observe microstructure and composition; Corrosion stability and bioactivity of the material were evaluated by simulating body environment in vitro and potentiodynamic electrochemical technique. Based on the above experiments, the gradient porous composites were fabricated according to general structure of long bones. The main studies and results are summarized as follows:(1) After many experiments, the optimum parameters to prepare Ti^-10wt% Mg composite were obtained: milling speed of 300 r·min^-1, ball ratio of 10:1, milling time of 10 hours, cold compression at 550MPa, sintering temperature of 630℃, sintering time of 2 hours and heating rate of 5℃/min. Ti^-10wt%Mg had flexural modulus of 7.1GPa, bending strength of 250.7MPa, compressive elastic modulus of 8.2GPa, compressive strength of 562.1MPa, impact toughness of 6.8 kJ·m^-2 and hardness of 49HRA, which met the need of bone implant and showed good biomechanical compatibility. The finite element analysis of powder compression could reveal the powder flow and the density distribution. The simulation results were agreed with the experimental results. (2)The porous Ti-10wt%Mg composites were fabricated through powder metallurgy processing with ammonium acid carbonate (NH₄HCO₃) as a space-holder. The results indicated that the total porosity of the porous composites could be tailored effectively by changing the NH₄HCO₃ amount, and the open porosity could be controlled by changing NH₄HCO₃ amount and size. Mechanical properties decreased greatly with the amount of NH₄HCO₃ added, and slightly with the size of space-holder. When the NH₄HCO₃ content is 25wt%, the bending strength, the bending modulus, compressive strength, compressive Young's modulus and impact toughnessof the porous Ti-Mg composites with interconnected porous characteristics (pore size: 200um, porosity: 51.4%) were 54.5MPa, 1.2GPa, 43.5Mpa, 1.8GPa, 2.7 kJ·m^-2 respectively.(3) Owing to grain boundary strengthening of Zr, Ti^-10Mg composites added with 10 wt% Zr had higher bending strength of 121.0MPa, bending modulus of 2.0GPa, compressive strength of 75.4MPa, compressive modulus of 1.9GPa and impact toughnessof 3.1 kJ·m^-2.(4) Bending strength of Ti-Mg composites with graded porosity structure was higher than that of samples with uniform porosity. Five-layer gradient Ti-Mg composite with added Zr(10wt%) had bending strength of 168.3MPa and bending modulus of 2.3GPa.(5) In vitro tests showed that hydrogen evolution resulting from corrosion of Mg accompanied a pH increase. As the soaking time increased, the corrosion rate of magnesium decreased, and Mg-containing carbonate apatite deposited in the pores led to increased strength. In addition, composites had no hemolytic reaction .While apatite was only deposited on the hole edge of Ti nanopores prepared by anodic oxidation.From the mechanical and biological point of view, Ti-Mg composites, as degradable and absorbable implant materials under load-bearing conditions, exhibited lower corrosion rate than that of commercially compact pure Mg and mechanical properties met the requirements for implants.