Porous surface of NiTi alloy produced by plasma ion implantation

For several applications a nanoporous surface layer of titanium oxide on titanium or NiTi is desired, e.g. for surface-increase for catalytical or electrochemical applications, or as carrier in a drug release system. This application requires interconnected pores with the size in the same order of magnitude as the drug molecule, few nanometers. A possibility to produce such a nanoporous structure is seen in the high dose ion implantation of a non reactive gas, which aggregates to nano-bubbles. To our knowledge this type of treatment has not yet been done with the superelastic or memory shape NiTi alloy.
Helium ion implantation of the NiTi and Ti samples was performed by means of plasma immersion ion implantation at following parameters: ion energy of 20 keV, ion fluence of ~1e18 cm -2, substrate temperature (100 – 400 °C). The implanted layers were examined by elastic recoil detection analysis, Auger electron spectroscopy, grazing incidence X-ray diffraction analysis, atomic force microscopy and transmission electron microscopy.
The He+-implanted region of NiTi samples contains a Ni depleted surface layer of TiOx, a Ni enriched zone with fine Ni4Ti3 crystallites and a nanoporous amorphous layer. Structure and morphology of these layers after etching in 1% HF acid were studied in order to characterize a deeper porous layer. An influence of implantation temperature on Ni transport from surface to the deeper layers and nanoporous layer formation are discussed.