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|Title: ||Microstructure and biological properties of NiTi shape memory alloy with surface nanostructures|
|Other Titles: ||Nie tai xing zhuang ji yi he jin biao mian na mi jie gou de wei guan zu zhi ji sheng wu xue xing neng yan jiu|
|Authors: ||Hu, Tao ( 胡濤)|
|Department: ||Department of Physics and Materials Science|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2011|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Nickel-titanium alloys -- Microstructure.|
Shape memory alloys -- Microstructure
|Notes: ||CityU Call Number: TA480.N63 H8 2011|
155 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references.
|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
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
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.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b4086287|
|Appears in Collections:||AP - Doctor of Philosophy |
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