| Titanium and its alloys are preferential choices in application in the field of bonetissue repair and replacement due to their outstanding mechanical strength，goodcorrosion resistance and good biocompatibility. However, the metallic biomaterials havehigher stiffness and low bonding strength between implants and bone, which made aclaim for lower elastic modulus. Porous titanium materials overcome the problemsbecause of pores in the material, which ensures that the modulus of the materials isclose to that of human bone and the ingrowth of bone into the pores. A lot of researcheson mechanical and biological compatibility of porous titanium for biomedicalapplication were studied. But there are still many projects to research on the porestructure, mechanical behaviour and biological compatibility, which have greatsignificance to the clinical application.In this thesis, the powder metallurgy with space holder was used to prepare theporous titanium materials with different pore structure. The effect of space holder grainsize and volume fraction on the porosity, pore size and its distribution were studied aswell as the microstructure of porous titanium was examined; the mechanical propertiesof porous titanium with different pore structure and the relationships between therelative elastic modulus, relative strength and relative density were studied, and the poredeformation mechanism was investigated. The surfaces of porous titanium weremodified by alkali and heat treatment. The microstructure of bioactive layer on thesurfaces was examined and the degradation of the strength of porous titanium afteralkali and heat treatment was investigated. Finally, the bone bioactivity in vitro wasinvestigated and the formation mechanism of apatite on the surfaces after modificationwas discussed. The main work and conclusions in the thesis are as follows:(1) The effect of space holder grain size and volume fraction on the porosity, poresize and its distribution were studied. Porous titanium with porosity of <70%, with poresize distribution of100~500μm was prepared by powder metallurgy method with spaceholder. With increasing of the grain and volume fraction, the porosity, the pore size andthe interconnectivity of pores of porous titanium will increase. And the pore sizedistribution and the average pore size are more than that of the space holder.(2) The microstructure structural features of porous titanium were examined. there are two types of pores in the samples: the interconnected macrospores obtained by thedecomposition of the four kinds of space holder particles with the pore size of100~200μm、125~350μm、225~425μm and250~550μm; the microspores obtained by partiallysintering of Ti powders on the pore walls, the size of them is about several micrometers.And this kind of micropores is the optimum pore size for the attachment andproliferation of the new-bone tissues and the transport.(3) The mechanical properties of porous Ti were studied by the quasi-staticcompressive test. The strain–stress curves of porous titanium show the typical featuresof metallic foams, i.e. an elastic deformation stage, a long plateau stage and adensification stage. And the differences in the curves are that the porous titanium withlower porosity (<32%) exhibit a false plateau stage, and the slope of the false plateaustage decrease with increasing of the porosity. The elastic modulus, the yield strengthand the compressive strength of porous titanium are in the range of1-18GPa,50-800MPa and100~1600MPa, respectively. And with increasing of the porosity, the modulusand the strength will decrease.(4) Mechanical models of porous titanium were established under quasi-staticconditions. There are essential relationships between the relative elastic modulus/yieldstrength and the relative density in porous titanium prepared by powder metallurgy. C1and C2are constants for geometric effect from the different preparation methods, and n1and n2are the density exponent, which suggesting the deformation of porous titanium.The values of constants C1and C2achieved from the model are0.17and1.45, and thevalues of density exponent n1and n2are in the range of (1.2~1.8) and (2.6~3.0),respectively. The average value of n2is2.79, suggesting the deformation mode of theporous titanium, which is mainly of the cell struts buckling and some bending of cellstruts.(5) The microstructure of bioactive layer on the surfaces was examined and thestrength of porous titanium after alkali and heat treatment was investigated. Thesurfaces of porous titanium were modified by alkali and heat treatment. A bioactivecoating with no cracks was produced on the porous titanium by alkali-heat treatment. Amicro-network structure which was composed mainly of bioactive sodium titanate andrutile covered the interior and exterior of porous titanium cells and its size of200nm.And the mechanical properties porous titanium after modification decreased, however,the essential relationships between the relative elastic modulus/yield strength and therelative density of porous titanium remain unchanged. (6) The cross-sectional structure feature of bioactive layer on the surface of poroustitanium was studied. The cross-section of the bioactive layer consists of three layers:layer I is the titanium substrate; layer II is the intermediate layer; and layer III is thecoating containing Na2Ti5O11and TiO2. The thickness of the bioactive layer is1.64μm.The Ca and Na ions both reacted with porous titanium in the NaOH solution, and theredundant Ca advanced the bioactivity of the coating making the bone ingrowth into thepores and reducing the concrescence time.(7) The bone bioactivity in vitro was studied and the formation mechanism of apatiteon the surfaces after modification was discussed. After immersion into the SBF solutionfor3days, an unstable carbonate-hydroxylapatite containing Ti、O、Na、Ca、P、Cl andC on its surface,(Ca+Mg)/P=1.67, was detected on the cell surface of the poroustitanium, showing a good compatibility of the porous titanium with bone. And calciumtitanate with a white grain appearance was found on the surface of specimens and thecalcium titanate will induce the nucleation and growth of apatite. The Ti–OH groupforms immediately combine with Ca ions in the fluid to form an amorphous calciumtitanate on the surface and then this calcium titanate combines with phosphate ions toform an amorphous calcium phosphate which later transforms into bone-like apatitecrystal. And the bioactive layer with more Ca iron accelerated the ability of apatitedeposition. |
PDF Full Text Request