Theoretical Study of Ni Rich NiTi-Oxide Interface

J. JUAN; M. SANDOVAL; V. ORAZI; E. GONZALEZ; P. JASEN; HERNÁNDEZ-LAGUNA, A.; SAINZ-DÍAZ, C.I.

Santiago de Chile

Congreso; XLIV Congreso Internacional de QuimicosTe6ricosde Expresi6n Latina; 2018

Pontificia Universidad Católica de Chile

Nitinol, a nearly equiatomic nickel?titanium shape memory alloy, has wideapplications in cardiovascular stents, micro-actuators, and high damping devices [1].Nitinol exhibits two unique mechanical behaviors: thermal shape memory andsuperelasticity. This material is widely used in medical applications due to its goodbiocompatibility. Nitinol has extremely good biocompatibility, due to the formation ofa passive titanium-oxide layer (TiO2). This corroborates basic thermodynamics datathat specify that the free energy of formation of TiO2 is favored over other nickel ortitanium oxides [2]. This oxide layer serves for two purposes: increasing the stabilityof the surface layers by protecting the bulk material from corrosion and creating aphysical and chemical barrier against Ni oxidation and modifying the oxidationpathways of Ni [3]. However, the presence of Ni in metallic or oxidized form on itssurface or its release from the alloy core is a serious problem because of its allergenicproperties and toxicity to human tissue. For this reason, we performed a theoreticalstudy of Ni segregation for a Ni rich NiTi-oxide interface with and without vacancytype defects. Our results show that Ni diffusion is strongly dependent of the presenceof vacancies and the nature of an intermediary oxidation layer. We also analyzedchanges in the electronic structure and bonding at the interface.