Interface evaluation of bilayer nano-glass NiTi thin films during crystallization

Maryam Mohri, Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany
Julia Ivanisenko, Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany
Horst Hahn, Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany

In this research, the interface of the NiTiCu/Ni-rich bi-layer nano-glass thin films prepared by magnetron sputtering was evaluated. The as deposited bi-layer thin films has an amorphous structure and for crystallization thin films were annealed at 500 oC for 15, 30, and 60 min. For evaluation of the structure and interface of the thin films, X-ray diffraction (XRD), scanning and high resolution transmission electron microscopies were used. The profile composition gradient of the bi layer after crystallization was measured using secondary ion mass spectroscopy (SIMS).

The results displayed that the structure and interface between the layers in the crystallized thin films intensely are influenced by temperature and time of the heating. The as-prepared thin films are amorphous as they show broad diffraction peaks characteristic for glassy state in XRD pattern. The SEM and TEM images illustrated a globular nano structure with boundaries of few nanometers. HRTEM images exhibit that the deposited layers are completely amorphous. The interface between layers is smooth and no long range periodic lattice was observed. The NiTiCu was partially crystallized after annealing for 15 min, but the NiTi layer in this period keeps its amorphous structure. Three regions are exist in the NiTiCu layer. The first region is just up the interface has amorphous structure and second region is a precipitate free zone (PFZ). The presence of vacancies in the amorphous structures motivates the creation of the precipitates but near the interface, absorption of vacancies by the interface and drop of the vacancies concentration postpone formation of precipitation results to form the PFZ. The third region partly crystallized and contains Ti-rich nano precipitates. The NiTi layer is partially crystallized after 30 min annealing and the interface between layers includes amorphous phase and quasicrystal nano particles. After 60 min annealing, both of the layers were completely crystallized with the different structure and due to diffusion of the Ni and Cu through the interface, a full diffused interface was formed. The chemical composition profiles displayed that there is a compositional gradient in the bi layer which caused to gradual changes in microstructure through the thickness.

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