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|Title: ||Degradation mechanism and surface modification of biomedical magnesium alloy|
|Other Titles: ||Yi yong mei he jin jiang jie ji li ji biao mian gai xing yan jiu|
|Authors: ||Xin, Yunchang (信運昌)|
|Department: ||Department of Physics and Materials Science|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2010|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Magnesium alloys -- Biocompatibility.|
Metals in medicine.
|Notes: ||CityU Call Number: R857.M26 X56 2010|
xviii, 129 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2010.
Includes bibliographical references.
|Abstract: ||The degradability of magnesium and magnesium alloys in a physiological
environment makes them desirable biodegradable biomaterials in many applications.
However, their fast degradation rates in the human body impose a severe limitation. A
better understanding of the degradation process and corresponding mechanism can provide
important information for ensuing clinical trials and design of novel magnesium-based
biomedical materials. At the same time, development of the suitable strategies to enhance
the normally poor corrosion resistance of magnesium alloys is another important issue that
must be solved in order that magneiusm-based materials have wider biomedical
The aims of the studies in this thesis are: (1) to understand the degradation process and
mechanism of AZ91 magnesium alloy in a physiological environment in order to provide
the important information for future in vivo study and clinical applications and (2) to
provide a effective and economical strategy to enhance the surface corrosion resistance and
mechanical properties of the magnesium alloy.
In the work described in the thesis, the degradation mechanism of the biomedical
AZ91 magnesium alloy is systematically investigated via various studies conduced in
simulated physiological systems. Combining comprehensive electrochemical tests with
other characterization techniques such as X-ray photoelectron spectroscopy (XPS),
scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, the influence of aggressive ions, structure and composition
of the degradation layer, and degradation process associated with AZ91 magnesium alloy in
simulated body fluids (SBF) are systematically analyzed and determined. In addition, a
uniform and dense Al2O3/Al bi-layer coating with good bonding strength to the substrate
has been successfully fabricated on the AZ91 magnesium alloy to enhance its corrosion
resistance. The structure, electrochemical behavior, and mechanical properties of the
coated materials are determined.
The thesis is organized into four parts as shown in the following. Firstly, by
studying the degradation and electrochemical behavior in four designed test
solutions containing Cl−, HPO4
− and SO4
2−, the influence of these
aggressive ions in physiological environment on the degradation behavior of AZ91
magnesium alloy is systematically investigated. The corresponding mechanism is
proposed and discussed. Our results demonstrate that chloride ions can induce
porous pitting corrosion on the magnesium alloy but HPO4
2− retard magnesium
dissolution distinctly. HCO3
− stimulates magnesium dissolution progressively and
also can induce fast surface passivation which can strongly suppress development
of pitting corrosion. SO4
2− is also aggressive to the magnesium alloy to a certain
Secondly, the structure, composition, and phase distribution in the surface
corroded layer on the AZ91 magnesium alloy after exposure to SBF are
investigated systematically. The degradation products are amorphous and mainly composed of MgO/Mg(OH)2, magnesium/calcium phosphates, magnesium/calcium
carbonates, Al2O3, and Al(OH)3. Non-uniform corrosion results in the non-uniform
structure observed from the degradation product layer. In seriously corroded regions, a
thick corrosion product layer forms with a high content of Ca and P. The regions with less
corrosion have a thinner product layer with a smaller amount of P and nearly no Ca.
Thirdly, the degradation rate, magnesium ion leakage rate, and induced pH changes in
SBF are determined by immersion tests. The corrosion mechanism is studied by
electrochemical techniques. During early exposure, the alloy degrades fast with high rates
of magnesium leakage accompanied by increased pH values. After a certain period of
exposure to the SBF, the degradation and magnesium ion leaching rates drop dramatically.
In the SBF, weak pitting corrosion is observed, while pitting corrosion is self-limited.
Finally, a dense and uniform Al2O3/Al bi-layer coating is successfully deposited on
the AZ91 magnesium alloy using a filtered cathodic arc deposition system. The coating
can enhance the corrosion resistance of the treated Mg alloy in long term applications and
the coating bonds strongly to the substrate. The coating is not prone to fracture and
delamination when subjected to normal loads.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b3001172|
|Appears in Collections:||AP - Doctor of Philosophy |
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