Effect of Pseudo Elastic Deformation of NiTi on Protective Surface Oxide Layers

The success of NiTi as material for medical implants in minimal invasive surgery is due to the unique combination of biocompatibility and shape memory/ pseudo elasticity. Despite the nickel content of ~50at.%, good biocompatibility has been confirmed in numerous studies, which is related to the formation of a surface oxide layer that consists essentially of titanium oxide. It is generally assumed that the (dense and defect free) oxide layer acts as a barrier to nickel diffusion and thus reduces or prevents Ni-release from the bulk. However, the oxide layer has been shown to be prone to cracking and flaking upon pseudo elastic deformation of the bulk. In the present work it is described how defects in the oxide layer resulting from pseudo elastic deformation can be identified. Depending on the pretreatment, different kinds of defects are observed. The underlying mechanisms of defect formation as e.g. stress states and localization of deformation and the possibilities to reduce or avoid potentially critical defects are discussed.