Influence of Annealing Temperature and Time on Surface Oxide Layers with a Thickness of ~60nm on NiTi

The performance of NiTi in biological environments results as a complex interplay of the conditions on the material's and the host's side. From the perspective of manufacturing and application, guidelines on the material's conditions are helpful that can both, warrant the materials performance and are straightforward to assess experimentally. Scientific discussions suggested the thickness of the surface oxide layer to be such a parameter, since it is linked to the release of Ni via the formation of the Ni-rich sublayer and the occurrence of cracks in the oxide during mechanical loading. An appropriate limit of the oxide layer thickness was suggested to be ~60nm. Obviously, various parameters, e.g. annealing temperature, annealing time, chemical pretreatment or the annealing environment affect the conditions of the formed surface oxide layer.

In the present work, surface oxide layers with a thickness of ~40nm were generated on NiTi via simply annealing in an air furnace. The range of annealing temperatures was chosen from 460°C to 540°C. It is shown that the difference in the required annealing time is unexpectedly large. The distribution of Ti, Ni, and O are assessed qualitatively and quantitatively using glow-discharge optical emission spectroscopy and transmission electron microscopy. Whereas the general conditions of the surface oxide layers and their subsurface regions appears similar at first glance, differences regarding formation and extension of Ni-rich phases below the surface oxide layer become apparent at a more detailed observation. Implications regarding the material's behavior will be discussed.