Titanium alloys such as Ti-6Al-4V and Ti-6Al-7Nb have been widely used as orthopedic implants such as artificial hip joint, because of their high mechanical strengths and good biocompatibilities. Recently, new kinds of titanium-based alloys free from elements such as V and Al, which are suspicious for cytotoxicities, are being developed. Ti-15Zr-4Ta-4Nb (Ti-15-4-4) is one of such alloys and shows high mechanical strength and corrosion resistance which are comparable to those of the Ti-6Al-4V alloy. In the present study, chemical treatments for providing bone-bonding ability to this alloy were investigated. Apatite-forming ability in a simulated body fluid (SBF) was used as an indication of the bone-bonding ability.

Ti-15-4-4 alloy plates 10×10×1 mm3 in size were soaked in 5M-NaOH solution at 60 °C for 24 h, soaked in 100mM-CaCl2 solution at 40 °C for 24 h, heated at 600 °C for 1 h and then soaked in hot water at 80 °C for 24 h. Surface structural changes of the alloy with these treatments were analyzed by a field emission scanning electron microscope (FE-SEM) attached with an energy-dispersive X-ray spectrometer (EDX), Thin-film X-ray diffraction (TF-XRD) and Fourier transform confocal laser Raman spectroscopy (FT-Raman). Scratch resistance of surface layer of the alloy was measured by a thin-film scratch tester. Apatite-forming ability of the specimens was examined by soaking them in SBF for 3 days. Long-term stability of the apatite-forming ability was examined after keeping the specimens in an incubator with relative humidity of 95 % at 80 °C for 1 week.

A sodium hydrogen titanate layer about 500 nm in thickness was formed on the surface of the alloy by the NaOH treatment. This specimen formed some amounts of apatite in SBF within 3 days, but its scratch resistance was as low as less than 10 mN. When the NaOH-treated specimen was subsequently heat treated, the sodium hydrogen titanate transformed into sodium titanate to give scratch resistance as high as 92 mN, but lost its apatite-forming ability.

When the NaOH-treated specimen was soaked in CaCl2 solution, the sodium hydrogen titanate was isomorphously transformed into calcium hydrogen titanate. Thus treated specimen increased its apatite-forming ability, but its scratch resistance was still low. When the NaOH- and CaCl2-treated specimen was subsequently heat treated, the calcium hydrogen titanate transformed into calcium titanate to give scratch resistance as high as 169 mN. However, its apatite-forming ability was lost. Thus treated specimen was then soaked in hot water. As a result, its apatite-forming ability remarkably increased without decreasing scratch resistance. It showed high apatite-forming ability even after a long-term-stability test.

The NaOH-, CaCl2-, heat- and hot-water-treated Ti-15-4-4 alloy is believed to be promising materials for artificial joints, because of its high apatite-forming ability with long-term stability as well as high scratch resistance.