Biomedical Ultrafine-grained Titanium Alloys And Surface Modification

The improvement of chemical composition and microstructure, surface modification of titanium alloys with high strength and low elastic elastic modulus without the addition of harmful alloying elements have been widely investigated recently for biomedical implants applications. Ultrafine-grained titanium and titanium alloys, owing to their superior bio-conductibility and biocompatiblity, are usually frequently used for orthopedic and dental implants. The preparation process, microstructure, mechanism of grain refinement and response characteristics of ultrafine-grained/nano-crystaline titanium alloys during chemical and electrochemical reaction for surface modification were investigated:1. Three kinds of ultrafine-grained titanium materials were preduced by severe plastic deformation and alloying processes. An ultrafine-grained surface layer up to 40μm thick was formed on commercially pure (CP) titanium by means of the High energy shot peening (HESP) process. The microstructural features of the HESP Ti surface layer were systematically characterized by cross-sectional optical microscopy observations, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations. The grain refinement mechanism of the surface layer during the HESP will be analyzed:Firstly, a great deal of dislocations and twin crystals were formed in due order, while severe plastic deformation of the Ti surface layer; After that, the grain size of Ti decreased owing to the interwoven twin crystals cross each other; Finally grains further refined and equiaxed nano-crystaline or sub-micro grains were producted under high-frequency and multi-directional loading. The grain size stability can be maintained up to 450℃, crystal defects decrease after annaling at 450℃. Corrosion resistance of the HESP Ti surface layer with depth within 40μm in SBF was decreased because surface roughness increase and macroscopic defect exist. Accordingly, passive film forming ability and corrosion resistance of subsurface (depth within 40～70μm) enhanced evidently due to their high defect density and a large number of high-energy grain and sub-grain boundaries, which stored a large excess enery as extra driving force. Equal channel angular pressing (ECAP) results in ultrafine-grained (200～500 nm) Ti with superior mechanical properties without harmful alloying elements, which benefits medical implants.2. To further improve the bioactivity of Ti surfaces, Ca/P-containing porous titania coatings were prepared on ultrafine-grained and coarse-grained Ti by micro-arc oxidation (MAO) in electrolyte mixed with (CH3COO)2Ca·H2O and NaH2PO4·2H2O. The phase identification, composition, morphology and microstructure of the coatings and the thermal stability of ultrafine-grained Ti during MAO were investigated subsequently. The enhanced diffusivity of O, Ca, P and the improvement of the chemical reactivity of Ti may originate from a large volume fraction of non-equilibrium grain boundaries (GBs)/sub-GBs and a considerable amount of defects density (dislocation) in the present ultrafine-grained Ti sample processed by means of the ECAP technique.The amounts of Ca, P and the Ca/P ratio of the coatings formed on ultrafine-grained Ti were higher than those on coarse-grained Ti. Nanocrystalline hydroxyapatite (HA) and a-Ca3(PO4)2 phases appeared in the MAO coating formed on ultrafine-grained Ti for 20 min (E20). Incubated in a simulated body fluid, bone-like apatite was completely formed on the surface of E20 for 2 days, as evidence of preferable bioactivity. Compared with initial ultrafine-grained Ti, the microhardness of the E20 substrate was reduced by 8% to 2.9 GPa, which is considerably higher than that of coarse-grained Ti (～1.5 GPa). Therefore, the reuslts clearly demonstrate that the MAO coating formed on the ECAP-treated Ti exhibits an optimum combination of mechanical and bioactivity.3. The CP Ti and HESP Ti specimens were soaked in 5 mol/L solution of NaOH at temperatures of 100℃and 90℃. Lots of granular sodium titanate was formed on their surfaces at 100℃. The HESP process enhanced the chemical reactivity of the Ti, the sodium titanate particles become larger in size and amount. The acceleration in the rate of apatite formation is significant, as it should allow for earlier load bearing of prostheses following implantation. Surface roughness increasement narrowed the coating cracks, and enhanced the cohesion between the substrate and coating. A thin nano-porous network structural sodium titanate layer formed on the two kinds of substrate by the NaOH treatment at 90℃. Hemispherical HA deposition formed on the treated Ti surface when immersed in SBF for 14 days. The larger HA particles formed on the HESP Ti, which have already spread out and become layer on some areas.Nano-crystaline HA powders with grain size of 5-10 nm and HA coatings on HESP Ti and CP Ti were prepared by the sol-gel technique using Ca(NO3)2·4H2O and P2O5 as calcium and phosphorus precursors, respectively. The coating layer formed on HESP Ti was porous, and adhered to HESP Ti substrate strongly with adhesion strength of about 18.0 MPa. Adhesion strength between HA coating and CP Ti was 9.0 MPa. The coating on HESP Ti has better anticoagulant property, because of their small acicular structrue.To improve the bioactivity of Ti surfaces, porous titania coating were prepared on HESP and CP Ti by MAO. The phase identification, thickness, composition and morphology of the coating were analyzed. The amounts of Ca, P and Ca/P ratio of the MAO coating formed on the HESP-treated Ti were higher than those of CP Ti obviously. The a-Ca3(PO4)2 phase appeared in H10 MAO coating. Due to roughness of HESP Ti increased, the oxidation rate is different in different locations, which leads the micro-mechanical occlusion and cohesion improvement between substrate and coating.4. Nanostructured Ti2448 alloy (Ti-24Nb-4Zr-7.9Nb), which contains no harmful elements and has an elastic modulus close to human bone and high strength, the alloy has been processed by MAO in a solution containing Ca/P. The corrosion resistance, hemocompatibility of Ti2448 alloy with composite coatings was superior evidently to that with merely MAO film, but the influence of MAO process on the bioactivity improvment of the Ti2448 alloy substrate is not quite obvious. To further improve bioactivity of Ti2448 alloys, a surface layer containing porous titania and sodium titanate is successfully formed by subsequent Alkline treatment. The phase identification, composition and morphology of the MAO coatings before and after alkaline treatment (MAO-A) were analyzed by XRD, SEM equipped with EDX and XPS. The amounts of Ca, P in MAO coating increased with increasing oxidation time, but the P element dissolved into the NaOH solution after alkaline treatment. It was demonstrated that alkaline treatment significantly improves the bioactivity of coatings as compared with MAO alone. Soaking in the SBF, Na+ ions in the MAO-A samples were release via exchange with the H3O+ ions in the fluid to form Ti-OH (Nb-OH) groups, which accelerated the nucleation of hydroxyapatite in SBF. Compared with sample T05A, which contains 3.6% Ca, incubated in SBF within 7 days, bone-like apatite was formed on sample T20A (contains 12% Ca), together with improved bioactivity. Mineralization experiments show that a high concentration of Ca in the MAO coating leads to a preferred release into SBF to keep its high consistency of Ca2+ and leads to accelerated growth of hydroxyapatite.