Mechanical Behavior And Biocompatibility Of Porous Titanium Fabricated By Diffusion Bonding

Titanium and its alloys are one of the promising metallic biomaterials because of their unique properties such as high specific strength,excellent biocompatibility and corrosion resistance.Unfortunately,the mechanical incompatibility and bioinert property has limited their applications in bone implant materials.For instance,Young's modulus of human bone is much lower than that of the solid titanium.This mismatch in Young's modulus may lead to bone resorption and implant loosening associated with the“stress shielding”effect.Porous structure in titanium is favored to decrease Young's modulus by increasing porosity,which can solve problems associated with the“stress shielding”effect.In addition,porous titanium can promote the transport of body fluid and stimulate bone ingrowth,which is helpful to improve the fixation of implants to the bone.In this dissertation,porous titanium was fabricated by vaccum diffusion bonding of titanium meshes.The pore structure,microstructure and diffusion interface of porous titnaium were characterized.The quasi-static compressive behavior and mechanical behavior in the phsilogical conditions were investigated.Moreover,the biocompatibility with trabecular bone of porous titanium was analysed.This dissertation aims to provide some experimental and theoretical guidance for the clinical application of porous titanium material and devices.The main research contents and conclusions are as follows:Porous titanium with average pore size of 100-700?m and porosity of 30-70%was fabricated by a novel method named vaccum diffusion bonding of titanium meshes.Pore structure and microstructure was characterized by Micro-CT scan,SEM and OM.The fabricated porous titanium possesses anisotropic structure with square pores in the in-plane direction and elongated pores in the out-of-plane direction.The microstructure of porous titanium is fine-grained equiaxed?phase with a few twins in some?grains.Moreover,the equiaxed?grains are very fine with size lower than 20?m.Diffusion interface of porous titanium was studied by nanoindentation.The diffusion interface is bonded well,with no infects and cracks in the mesh hinge.Quasi-static compressive properties of porous titanium were studied and compressive deformation mode was analyzed.Porous titanium shows anisotropic quasi-static compressive behavior in the in-plane and out-of-plane direction.The dominant compressive deformation mode in the out-of-plane direction is buckling of mesh wires,while it is slidding and friction of mesh wires and shearing of mesh hinges in the in-plane direction.By tailoring diffusion bonding temperature,relative density and pore size,quasi-static Young's modulus and yield stress can be adjusted in the range of 0.2~70GPa and 5~110MPa,respectively.Porous titanium with 50-70%porosity possesses quasi-static compressive properties in the range of corresponding properties for human trabecular bone from distal femur.Yield stress of porous titanium fabricated by diffusion bonding is close to that of fabricated by rapid prototyping,but higher than that of fabricated by powder sintering and space-holder method.Dynamic compressive behavior and strain rate sensitivity of porous titanium,together with the effect of porosity and pore size on the strain rate sensitivity,were investigated in this dissertation.The origin of strain rate sensitivity and mechanical compatibility in the range of physiological strain rate were elucidated.Porous titanium shows enhanced strain rate sensitivity in the out-of-plane direction and strain rate sensitivity exponent increases with increasing porosity.The strain rate sensitivity in the out-of-plane direction is originated from strain rate sensitivity of base materials and micro-inertia effect.The strain rate sensitivity of porous titanium in the in-plane direction is minor and porosity has no effect on the strain rate sensitivity exponent in the in-plane direction.The effect of pore size on strain rate sensitivity of porous titanium is negligible both in the out-of-plane and in-plane direction.The strain rate sensitivity exponent of porous titanium with 60-70%porosity is in the range of0.058-0.06 which is close to that of the trabecular bone.On the basis of quasi-static and dynamic compressive behavior,compressive fatigue behavior of porous titanium in the out-of-plane direction was investigated.The effect of pore structure on the compressive fatigue behavior and fatigue failure mechanism was also elucidated.Porosity and pore size have some effect on the S-N curve but this effect is negligible when the fatigue strength is normalized by the yield stress.The compression fatigue behavior is characteristic of strain accumulation.Both cyclic ratcheting and fatigue crack growth contribute to the fatigue failure mechanism,while the cyclic ratcheting is the dominant one.Porous titanium possesses higher normalized fatigue strength which is in the range of 0.5-0.55 at 10~6 cycles.The reasons for the higher normalized fatigue strength were analyzed based on the microstructure and fatigue failure mechanism.Titania nanotubes were prepared on the surface of porous titanium by anodic oxidization.The effect of titania nanotubes and pore size of porous titanium on the cell proliferation and bone ingrowth were studied by in vitro and in vivo.The titania nanotubes were homogeneous on the surface of porous titanium.The average diameter of titania nanotube is about 80nm and the microstructure is rutile.Titania nanotubes on the surface of porous titanium are beneficial to the cell proliferation but have minor effect on the bone ingrowth.Both cell proliferation and bone ingrowth are sensitive to the pore size of porous titanium.Pore size in the range of 500~650?m has better ability to stimulate cell proliferation and bone ingrowth.