A Study Upon Laser Surface Modifications On NiTi Shape Memory Alloy For Biomedical Applications

Nearly equiatomic NiTi alloys may get widely applications in biomedical field due to their unique properties such as shape memory effect, superelasticity and radiopacity. Shape memory alloys can be used to fabricate self-locking, self-expanding and self-compressing implants. Besides mechanics and working properties, it is necessary for a promising metal biomaterial to have good biocompatibility in order to ensuring the safety and the reliability of the implants during their long period implantation in body. The biocompatibility of metal biomaterials depends on two factors. One is the inherent toxicity of the components of the alloy, and another one is the corrosion behavior of the alloy in body environment. As we known, excessive nickel results in allergic, inflammation and even carcinogenicity. For these reasons, it is an effective way to improve the biocompatibility of NiTi alloys with enhanced corrosion resistance. In this thesis, a conventional used NiTi alloy in clinic with the composition of Ti-50.8 at% Ni was surface modified by Nd-YAG laser in both the mode of laser surface melting and laser gas nitriding. The chemical composition and microstructure modified surface layer were characterized with SEM, XRD and XPS. The corrosion and cavitation-erosion behavior of the surface modified NiTi alloy were investigated in analogous body fluid (37℃ Hank.s solution). The results show that the microstructure of the hot rolled Ti-50.8 at% Ni alloy after mechanical polishing was consisted mainly with single-phase of B2-NiTi, and with some secondary phases and inclusions. The surface chemical composition of the alloy was composed of C, O, Ti and Ni, in which C and part of O was resulted from atmospheric contamination and another part of O formed compounds with titanium. The surface layer of the alloy was constituted with NiTi+TiO 2+Ti 2O 3, and no nickel oxide was found. 4+ Ti /Ni ratio of the outmost surface was 1.19 for the alloy and the thickness of the oxide film on the top surface layer was 52.2 nm. The NiTi alloy was surface remelted by a Nd-YAG laser. The microstructure of the melted layer was clean, compact and with less inclusions and ultrafine grains. The interface between the melted layer and matrix metal exhibited high bonding strength. New phases such as NiTi2, Ni3Ti, NiTiO3 and martensitic phase B19′presented in the laser surface melted layer with lower microhardness. The chemical composition of the melted layer changed obviously after laser surface modification. The thickness of Ti-rich 4+ layer and of surface oxide layer increased significantly. Ti /Ti of the outmost surface raised to 5.11 after laser treatment from 1.19 prior to the treatment. Themathematics model of metals surface melt-solidification associated with laser heating process was III ...