REPOSITORY > RESULTS

Abstract

Existing production techniques of commercially available Nitinol (50Ni50Ti) are numerous, complicated and time-consuming. Due to this fact, the aim of the doctoral work was to produce a NiTi alloy in the shape of a rod with the smallest diameter possible in one-step using a combination of Vacuum Induction Melting and Continuous Casting. In these frames, requirements connected to the chemical composition and final properties of the as-cast NiTi rod should fulfilled and they have to be as close as possible to the Nitinol. With the deeper studies, and after many experiments carried out, a 3 m long NiTi rod with diameter 11.5 mm was produced. We found out that the optimal parameters during melting and casting were: 1450 °C melting temperature, 8 mm step, 0,5 s waiting time and 26 l/min mould cooling.
Chemical composition of as-cast NiTi rod was determined with XRF methods and semi-quantitative EDS analysis. Results showed some deviations to the required composition (± 15%), which could be attributed to the non-optimised production techniques and measurements` error of the chosen methods. The microstructure of the NiTi rod was observed using Optical and Scanning Electron Microscopy. It was identified that, in the microstructure, there are no visible macro-cracks and or other type of porosity. Detailed microstructure investigations revealed dendritic character, where inside of the dendrites was phase NiTi, and in the inter-dendritic space, NiTi3 and NiTi phases. Due to the contact between the clay graphite crucible, containing carbon, and the melt of nickel and titanium at high temperature (˃ 1450OC), individual inclusions of titanium carbide were formed. In continuation, the mechanical properties of the as-cast NiTi rod were examined (hardness, tensile, compressive and bending strength). Measurements of the mechanical properties showed that the NiTi rod is hard (˃ 550 HV). With the assistance of a tensile, compressive and bending test, it has also been found that the NiTi rod is also brittle, due to the fracture specificity, and that it lies only in the elastic region of the stress-strain curve. As it does not pass into the plastic region, the NiTi rod cannot be strain hardened. Homogenization heat treatment should follow after the CVC procedure.
As the original purpose of the NiTi rod is an application for dental purposes, it was necessary to perform additional tests, which are required for biomaterials. The surface of NiTi rod and commercially available Nitinol, was modified by a layer of titanium oxide (50 nm), which was deposited by the Atomic Layer Deposition technique. Such modified and nonmodified samples were then tested for their biocompatibility with the contact of HUVEC cells, and for their corrosion properties with physiological saline. All tests were performed at body temperature. Results showed that biocompatibility of base NiTi rod, as well as its corrosion properties, are poor in comparison with the commercially available Nitinol. In the case where a layer of titanium oxide was deposited on the NiTi rod, its biocompatibility and corrosion properties were improved considerably.