Effects of fine particle peening on oxidation behavior of nickeltitanium shape memory alloy

The effects of fine particle peening (FPP) on oxidation behavior of nickeltitanium shape memory alloy (NiTi alloy) were evaluated. After FPP treatment, oxidation process was performed at 300, 400 and 500°C for 30 min in N220 vol%O2 atmosphere. Surface microstructures of oxidized specimens pre-treated with FPP were characterized using scanning electron microscope (SEM), glow discharge optical emission spectroscopy (GD-OES), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analysis (XRD). Oxide layers formed on the specimens pre-treated with FPP were thinner than those on the un-peened ones. Amorphous titanium oxides (TiO and TiO2) were formed on the un-peened specimens by oxidation process, whereas a nickel oxide (NiO) was created on the FPP-treated surface oxidized at temperature greater than 400°C as well as titanium oxides. This was because an amorphous structure created by FPP accelerated the outward growth of nickel oxide during the subsequent oxidation process. Moreover, the effects of surface oxide layers on the biocompatibility of NiTi alloy were investigated. Due to the formation of a thin oxide layer which contains nickel element, the oxidized specimen pre-treated with FPP showed higher amount of nickel ion elution than that of the oxidized one during the cell culture tests. These results suggest that microstructural feature of surface oxide layer strongly affects the biocompatibility of NiTi alloy.