M. Rettenmayr, F. Schrempel, W. Wesch, Friedrich-Schiller-University Jena, Jena, Germany; A. Undisz, Massachusetts Institute of Technology, Cambridge, MA
Manufacturing of NiTi implants and devices requires annealing procedures to set the characteristics and the final shape of the material. During annealing a surface oxide layer forms that consists mainly of Titanium dioxide. The properties of the oxide layer are crucial for the corrosive stability and the biocompatibility of NiTi. The interplay of annealing and surface oxide layer has been investigated extensively: thickness, composition and crystal structure of the oxide layer depend on the annealing conditions, e.g. temperature and partial pressure of Oxygen. However, the oxidation mechanism that results in the characteristic concentration profiles of Nickel, Titanium and Oxygen is not entirely understood. Open questions concern diffusion rate and chemical bonding of the constituent elements.
To trace the oxidation process in detail, polished NiTi sheets were annealed in a two-step procedure. During each step the annealing atmosphere consisted of a different oxygen isotope (18O or 16O, respectively). The annealed NiTi sheets were characterized by Rutherford-Backscattering (RBS) and Resonant-Rutherford-Backscattering (RRBS) techniques. The concentration distribution of the different Oxygen isotopes in the oxide layer indicates that Oxygen is integrated into the oxide layer from the surface while the previously bonded oxygen atoms are shifted to Titanium atoms in deeper regions of the oxide layer. Transport of Oxygen atoms occurs through the oxide layer towards the oxide/metal interface where the Oxygen atoms bond to Titanium atoms. Nickel was detected exclusively on top of and below the oxide layer, respectively. Transport of Nickel atoms in or through the oxide layer was not observed. The concentration of Nickel on top of the oxide layer depends on the early stages of annealing and remains unaffected later.
Summary: Manufacturing of NiTi implants and devices requires annealing procedures to set the characteristics and the final shape of the material. During annealing a surface oxide layer forms that consists mainly of Titanium dioxide. The properties of the oxide layer are crucial for the corrosive stability and the biocompatibility of NiTi. The interplay of annealing and surface oxide layer has been investigated extensively: thickness, composition and crystal structure of the oxide layer depend on the annealing conditions, e.g. temperature and partial pressure of Oxygen. However, the oxidation mechanism that results in the characteristic concentration profiles of Nickel, Titanium and Oxygen is not entirely understood. Open questions concern diffusion rate and chemical bonding of the constituent elements.
To trace the oxidation process in detail, polished NiTi sheets were annealed in a two-step procedure. During each step the annealing atmosphere consisted of a different oxygen isotope (18O or 16O, respectively). The annealed NiTi sheets were characterized by Rutherford-Backscattering (RBS) and Resonant-Rutherford-Backscattering (RRBS) techniques. The concentration distribution of the different Oxygen isotopes in the oxide layer indicates that Oxygen is integrated into the oxide layer from the surface while the previously bonded oxygen atoms are shifted to Titanium atoms in deeper regions of the oxide layer. Transport of Oxygen atoms occurs through the oxide layer towards the oxide/metal interface where the Oxygen atoms bond to Titanium atoms. Nickel was detected exclusively on top of and below the oxide layer, respectively. Transport of Nickel atoms in or through the oxide layer was not observed. The concentration of Nickel on top of the oxide layer depends on the early stages of annealing and remains unaffected later.
