蘆田 茉希

Ti6Al7Nb alloys have been widely used in the medical field, particularly in artificial hip joints, spinal fixators, and dental implants, owing to their light weight, low toxicity, and superior corrosion resistance. Grain refinement through a severe plastic deformation process under high pressure, such as high-pressure torsion (HPT) or high-pressure sliding, is widely employed for strengthening metallic materials. This overview presents the recent advances in the effect of HPT on the mechanical properties of the Ti6Al7Nb alloy. This alloy was grain-refined through HPT under applied pressures of 2 and 6 GPa, and the results revealed that the alloy subjected to HPT processing at 6 GPa exhibited a higher strength. To inhibit the decrease in the total elongation of the alloy, the number of revolutions in the HPT process was set to moderate. The tensile properties achieved after HPT processing were found to be dependent on the initial microstructure before the HPT treatment. Furthermore, an alloy with a bimodal equiaxed and acicular structure was subjected to grain refinement via the HPT process. The results revealed that fragmentation of the acicular structure during HPT further increased the strength. Moreover, the HPT-processed Ti6Al7Nb alloy exhibited superplasticity. It was thus confirmed that grain refinement by HPT is an effective method for strengthening the Ti6Al7Nb alloy, which is advantageous for medical applications.

A customized micro-arc oxidation (MAO) treatment technique was developed to obtain antibacterial properties with no toxicity on Ti surfaces. A two-step MAO treatment was used to fabricate a specimen containing both Ag and Zn in its surface oxide layer, and the optimal incorporation conditions were determined. Surface characterization by EDS was performed followed by the antibacterial properties against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) and osteogenic cell compatibility evaluations. In addition, metal ion release tests were performed to evaluate the contents of Ag and Zn and the ion release behavior in order to simulate practical usage. MAO-treated specimens prepared using proper concentrations of Ag and Zn (0.5Ag-5Zn: 0.5 mM AgNO3 and 5.0 mM ZnCl, respectively) exhibited excellent antibacterial properties against E. coli and S. aureus and no toxicity to MC3T3-E1 in antibacterial and cytotoxic evaluations, respectively. The antibacterial property of 0.5Ag-5Zn against S. aureus was sustained even after two months of immersion in physiological saline, simulating the in vivo environment.

In orthopedics, occasionally, different types of metals are used in applications in which they are in contact with each other. However, few studies have electrochemically investigated the galvanic corrosion of orthopedic implants formed of different metals. In this study, galvanic corrosion of Ti-6Al-4V ELI alloy, Co-Cr-Mo alloy, and 316L type stainless steel, which are used in orthopedics, and a newly developed Zr-1Mo alloy as a low-magnetic susceptibility material was evaluated in saline. Coupling of the Ti-6Al-4V ELI and Co-Cr-Mo alloys did not exhibit localized corrosion and maintained highly stable passive films. Coupling of the 316L type stainless steel and Co-Cr-Mo alloy, temporary localized corrosion occurred. Similarly, coupling of the Zr-1Mo and Co-Cr-Mo alloys, temporary localized corrosion occurred. However, both of 316L type stainless steel and Zr-1Mo alloy were finally repassivated spontaneously with the immersion time. The degree of the localized corrosion of the Zr-1Mo alloy was smaller than that of 316L type stainless steel. No galvanic current was observed when the Ti-6Al-4V ELI and Co-Cr-Mo alloys were coupled. A slight galvanic current flowed when 316L type stainless steel or the Zr-1Mo alloy was coupled with the other alloys; however, the galvanic current with the Zr-1Mo alloy coupling recovered to zero after a certain period owing to repassivation. No metal ions were detected from the couplings with Zr-1Mo

Titanium (Ti) is always covered by thin passive films. Thus, valence band (VB) spectra, obtained using X-ray photoelectron spectroscopy (XPS), are superpositions of the VB spectra of passive films and that of the metallic Ti substrate. In this study, to obtain the VB spectra only of passive films, angular resolution (for eliminating the substrate Ti contribution) and argon ion sputtering (for removing passive films) were used along with XPS. The passive film on Ti was determined to consist of a very thin TiO2 layer with small amounts of Ti2O3, TiO, hydroxyl groups, and water with a thickness of 5.9 nm. The VB spectra of Ti were deconvoluted into four peak components: a peak at similar to 1 eV, attributed to the Ti metal substrate; a broad peak in the 3-10 eV range, mainly attributed to O 2p (similar to 6 eV) and O 2p-Ti 3d hybridized states (similar to 8 eV), owing to the pi (non-bonding) and sigma (bonding) orbitals in the passive oxide film; and a peak at similar to 13 eV, attributed to the 3 sigma orbital of O 2p as OH- or H2O. The VB region spectrum between approximately 3 and 14 eV from Ti is originating from the passive film on Ti. In particular, characterization of VB spectrum obtained with a takeoff angle of less than 24 degrees is effective to obtain VB spectrum only from the passive film on Ti. The property as n-type semiconductor of the passive film on Ti is probably higher than that of rutile TiO2 ceramics.