Degradation mechanism and surface modification of biomedical magnesium alloy

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 applications. 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 2−, HCO3 − 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 extent. 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.