Characterization of oxide film formed on NiTi by laser oxidation

NiTi was surface treated by laser oxidation for improving corrosion resistance. While the corrosion behavior of laser oxidized NiTi is reported elsewhere, the present study aims at characterizing the oxide film formed in laser oxidation by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected-area diffraction (SAD). The naturally formed (NF) oxide film on a mechanically polished substrate is included in the study for comparison with the laser formed (LF) oxide film. The LF oxide film on NiTi carries a golden-yellowish interference color, and has a thickness of about 25 nm versus a thickness of 5 nm for the NF film, as determined by TEM or estimated from XPS depth profiling. The average surface roughness Raincreases from 1.02 nm for the NF oxide film to 13.1 nm for the LF oxide film. XPS analysis shows a surface Ni/Ti atomic ratio of 0.17 for laser oxidized samples, which is lower than that of 0.30 for mechanically polished samples. Depth profiling of composition by XPS reveals that the LF oxide film contains mainly titanium oxides, together with a small amount of Ni in oxidized and metallic states. The oxidation state of Ti changes from Ti4+(corresponding to TiO2) in the top layer of the LF oxide film to lower oxidation states Ti3+and Ti2+(corresponding to Ti2O3and TiO). The LF film is composed of nanocrystallites, as evidenced from SAD patterns. High-resolution TEM image shows that the crystallite size in the LF oxide film is about 10 nm. These findings show clearly that laser oxidation significantly changes the characteristics of the oxide film on NiTi and also provide an explanation for the improvement in surface properties due to laser oxidation.