Surface Modification Of Porous Niti Shape Memory Alloy By High Temperature Oxidation And Chemical Oxidation

In this thesis, surface modification of porous NiTi shape memory alloys were performed through high temperature oxidation, chemical oxidation and the combination of the above two precesses (i.e., the initial chemical oxidation followed by a high temperature treatment). The surface microstructures of the surface-modified porous NiTi alloys were characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The Ni ion release behavior and corrosion resistance of the surface-modified porous NiTi alloys immersed in physiological saline and electrochemical solution were evaluated.The results have shown that the oxide film mainly composed of anatase TiO2 and depleted in Ni formed on porous NiTi surface by high temperature oxidation. The content of anatase TiO2 phase increases with the oxidation temperature. However, an over high temperature (e.g., >500 oC) tends to induce cracking of the surface film caused by internal thermal stress and thus deteriorate the interface strength between the oxide film and NiTi substrate. The results of corrosion resistance tests indicate that the uniformity and integrity of the protective oxide film play a key in improving corrosion resistance of porous NiTi alloy.After a chemical oxidation in H2O2+HCl solution, the oxide film composed of the amorphous oxide and depleted in Ni formed on porous NiTi surface. The defect-free oxide film can significantly enhance corrosion resistance of the substrate and reduce Ni ion release, thus would greatly improve biocompatibility of porous NiTi alloys. The preliminary study of oxidation kinetics has shown that the mass variation of porous NiTi substrate, determined by the HCl concentration and oxidation time, depends on the balance of the kinetics between the oxidation and dissolution reactions during the chemical oxidation process.It has been shown that the amorphous titanium oxide formed through chemical oxidation followed by high temperature oxidation. At a relatively low temperature of 400oC, the amorphous titanium oxide changed into crystalline anatase TiO2. Although the follow-up high temperature treatment had an adverse influence on interface strength between the oxide film and NiTi substrate, the anatase TiO2 phase provides the possibility for the deposition of HA.