Microwave plasma oxidation of near-equiatomic NiTi alloy for obtaining low-Ni TiO2 coating

In this paper, microwave plasma oxidation (MPO) treatment was carried out on near-equiatomic NiTi alloy for the first time, in order to reduce Ni content and improve cytocompatibility on its surface. The specimens were oxidized at 600 degrees C for 10, 30, and 60 min, named MPO-1, MPO-2, and MPO-3, respectively. The differences in surface properties, corrosion resistance in DMEM, protein adsorption, and cytocompatibility between the MPO-treated specimens and original substrate were investigated. The results showed that uniform and dense rutile TiO2 coatings with nano-sized granular structures could be formed on the NiTi surface under all MPO time. The Ni/Ti ratio on the MPO-treated surfaces was reduced to similar to 0.04, close to zero. For MPO-1 and MPO-2, a Ni-rich layer with Ni content of similar to 70 at.% was formed underneath the outermost TiO2 layer, while an extra high Ni-rich layer with Ni content of similar to 90 at% was formed between them for MPO-3. Due to the dense TiO2 coating and suitable nanostructure on the surface structure, MPO-2 possessed better corrosion resistance in DMEM and biocompatibility than the original substrate, MPO-1, and MPO-3. Besides, the oxidation rate of MPO treatment is higher than that of other oxidation methods, such as thermal oxidation. That is, the proposed MPO treatment is an oxidation method with high efficiency and effectiveness.