Microstructure and biological properties of NiTi shape memory alloy with surface nanostructures

Abstract

Surface modification has been widely applied to NiTi shape memory alloy (SMA) to improve its biological properties for biomedical applications. Coatings such as titanium oxide and titanium nitride are suitable candidates due to their excellent biocompatibility. However, the coatings are normally thin and have low bond strength with the substrate. Hence, they may not be able to withstand the fretting and wear during long-term clinic applications. In order to produce a thick and adherent coating on the surface of NiTi alloy, a novel approach is to introduce a nanocrystalline surface layer which has a large volume of grain boundaries which act as diffusion channels to enable nitrogen or oxygen to penetrate deeply into the substrate. This effect resembles that of a thick coating. In the work described in this thesis, NiTi specimens are subjected to surface mechanical attrition treatment (SMAT) to form a surface nanocrystalline layer, followed by the fabrication of a thick titanium oxide and/or TiN layer by glow discharge. The microstructure, phase transformation behavior, and mechanical properties of the nanocrystalline NiTi as well as the composition and biological properties of the SMAT NiTi after undergoing glow discharge treatment are systematically investigated. SMAT is first employed to produce a graded structure on the surface of the NiTi sample. Dislocation activities via formation of highly dense dislocations dominate plastic deformation at the depth of below 200 µm in the SMAT NiTi specimen and in this region, the strain and strain rate are low. At a depth of about 150 µm, stress induced martensite transformation occurs forming martensite bands with 40 to 200 nm wide and several micrometers long. Plastic deformation also generates dislocation lines and dense dislocation walls inside the martensite bands and thus the reverse martensite transformation is suppressed. In the subsurface layer (from 20 to 70 µm) of SMAT NiTi specimen, the martensite bands are refined into submicrometer grains. Meanwhile, severe plastic deformation induces amorphization of the NiTi alloy. In the top surface layer, nanocrystallines with an average grain size about 20 nm are formed and coexist with the amorphous phase. SMAT is found to induce the formation of the B2 phase from martensite B19' in the surface layer of the NiTi alloy with B19' as the initial phase. A graded phase structure layer is observed and the amount of the B2 phase decreases with depth. In the sub-surface layer, SMAT stabilizes the martensite B19' phase and in the NiTi alloy with the initial B2 phase, a graded phase structure is also formed in the surface layer after SMAT. At a depth of about 150 µm, the plastic strain is small and stress induced martensite (SIM) transformation is triggered to produce the martensite B19' as the main phase. With increased strain and strain rate, a transient phase layer comprising the B19' and B2 phases is formed due to the reverse martensite transformation. In the top surface layer, only the B2 phase exists. The nanocrystallines and fine grains lower the friction coefficients of the NiTi and improve the wear resistance due to the lubrication of abrasive particles. The surface hardness in SMAT NiTi is significantly improved by grain refinement in lieu of the work hardening effect. Electrochemical results disclose that the corrosion resistance of the nanocrystalline NiTi specimen is significantly improved compared to that of the coarse-grained NiTi. The improved corrosion resistance arises from the quick formation of a passive oxide film on the nanocrystalline surface. Finally, the SMAT NiTi specimen is further processed by glow discharge to produce titanium oxide or titanium nitride coatings. XPS results indicate that the thickness of the oxidized layer is about 165 nm, which is ten times larger than that in NiTi treated by plasma immersion ion implantation alone. The nitrided layer thickness is about 54 nm. The corrosion resistance of the nanocrystalline NiTi specimens as well as those with thick oxide and nitride layers is studied in SBF solutions and observed to be enhanced. Actin cytoskeleton results reveal that the surface nanocrystallines and subsequent glow discharge treatment do not adversely influence the cytocompatibility of the NiTi alloy. In recapitulation, the surface nanocrystalline structure on the NiTi alloy formed by SMAT possesses good mechanical strength and corrosion resistance. By additionally introducing a coating by glow discharge treatment, the materials boast excellent mechanical and biological properties and are suitable for surgical implants.