First-principles study on the adsorption of oxygen at NiTi (110) surface

NiTi alloys with equiatomic compositions have been widely used as structural materials in aerospace, aviation and other fields due to their shape memory effects and good mechanical performances. At the same time, they are considered as excellent biomedical materials for their biocompatibilities and high fatigue resistances. As structural materials, the oxidation resistance of NiTi alloy should be improved. However, as biomedical materials, the formation of dense TiO₂ layers on the surface of NiTi alloy is required to suppress the release of Ni ions in body liquid. As a result, it is of great significance to study the oxidation mechanism of NiTi alloy. In this work, while the total number of Ti is kept the same as that of Ni atoms in the whole system, a series of defected c(2×2)-NiTi (110) surfaces with antisite of Ti are constructed to further understand the oxidation mechanism of NiTi alloy. The adsorption of oxygen atom at the NiTi (110) surface is investigated by the first-principles calculations. The calculated results show that the stability of the oxygen adsorption is strongly related to the enrichment of Ti atoms on the surface. The higher the enrichment of Ti atoms on the surface, the stronger the adsorption of oxygen atoms is. When the coverage of oxygen is high enough, the adsorption of oxygen atoms on the surface could cause the antisite of Ti atoms on the surface by the exchange of Ni atoms in the first layer with Ti atoms in other layers. Under the O-rich conditions (μ_O ≥-9.35 eV), it is the most stable that the oxygen atoms adsorbed on Ti antisite surface, with the whole Ni atoms in the first surface layer exchanged with the whole Ti atoms in the third surface layer. With the increase of the adsorbed oxygen atoms on the surface, the heights of Ti atoms in the surface layers are raised by the adsorption of oxygen. The TiO₂ layer is formed by the expansive growth, while Ni atoms are enriched beneath the TiO₂. As a result, the reason why the TiO₂ layer is formed on the NiTi alloy surface in the experimental conditions is well explained.