Jinhe Dou

Research status of magnesium alloys by micro-arc oxidation: a review

ABSTRACT Magnesium and its alloys are fascinating candidates for biodegradable implants due to their suitable mechanical properties, biodegradability and biocompatibility. However, the rapid corrosion rate, generation of a large amount of hydrogen gas and increase in local pH of the body fluid limit their potential biomedical applications. Various techniques, including alloying, mechanical processing and surface treatment, have been used to reduce the corrosion rate of materials. Surface treatment is one of the widely used techniques to improve the corrosion resistance of the Mg alloys. Compared to other surface modification processes, micro-arc oxidation can provide good adherent, hard, scratch-resistant, wear-resistant, and corrosion-resistant coatings on magnesium alloys. This article mainly reviews the influence of key relatively process parameters, such as electrolytic composition and presence of additives. Normally, electrolytic system and additives can improve the corrosion behaviour of magnesium alloys using appropriate concentration. The future prospects are summarised as well.

Effect of phosphate additives on the microstructure, bioactivity, and degradability of microarc oxidation coatings on Mg-Zn-Ca-Mn alloy

Calcium phosphate coatings were prepared on the surface of self-designed Mg-Zn-Ca-Mn alloy using microarc oxidization technology. To characterize the microstructures, cross-section morphologies, and compositions of the coatings, the authors used scanning electron microscopy equipped with an energy-disperse spectrometer, x-ray diffraction, and Fourier transform infrared spectroscopy. Potentiodynamic polarization in the simulated body fluid (SBF) was used to evaluate the corrosion behaviors of the samples. An SBF immersion test was used to evaluate the coating bioactivity and degradability. After the immersion tests, some bonelike apatite formed on the coating surfaces indicate that bioactivity of the coatings is excellent. The coating prepared in electrolyte containing (NaPO3)6 had slower degradation rate after immersion test for 21 days.

Characterization and biodegradation behavior of micro-arc oxidation coatings formed on Mg–Zn–Ca alloys in two different electrolytes

MAO coatings were obtained on self-designed Mg alloy by micro-arc oxidation process in calcium phosphate and silicate based electrolytes. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) were used to analyze the microstructure, cross sectional morphology, elemental distribution and composition of the MAO coatings. The corrosion resistance of the MAO coatings was evaluated using potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in SBF solution. It was found that the electrolyte composition has a significant effect on the resulting coating characteristics, such as microstructure, composition, coating thickness, thus on the corrosion behavior. The corrosion resistance of the MAO coating formed in silicate electrolyte is superior at the initial immersion time while the CaP-containing coating demonstrates better corrosion behavior at the long-term time.

Effects of sintering temperature on the properties of alumina/hydroxyapatite composites

We have investigated the effects of sintering temperature on the properties of hot-pressed alumina/hydroxyapatite ceramic composites. The ceramic composites were characterized by field-emission scanning electron microscopy and X-ray diffraction. The mechanical properties of the composites were also assessed. The results showed that the sintering temperature is a critical factor influencing phase composition, sintered microstructure and hence the mechanical properties. The decomposition of Hydroxyapatite to calcium phosphate was accelerated and the compactness and grain size increased with the increasing sintering temperature. Samples sintered at 1250 °C exhibited better mechanical properties as compared to those sintered at other temperatures.Graphical abstractSecondary electron micrographs images of fracture surfaces of the composite ceramics sintered at different temperatures: (a) 1100 °C, (b) 1150 °C, (c) 1200 °C, (d) 1250 °C, (e) 1300 °C and (f) the SEM images of the residue extracted by dissolving the composite ceramic sintered at 1300 °C.

Effect of the second-step voltages on the structural and corrosion properties of silicon–calcium–phosphate (Si–CaP) coatings on Mg–Zn–Ca alloy

The applications of magnesium (Mg) alloys as biodegradable orthopedic implants are mainly restricted due to their rapid degradation rate in the physiological environment. In this study, Si–CaP micro-arc oxidation (MAO) coatings were prepared on a Mg–Zn–Ca alloy by a second-step MAO process at different voltages in order to decrease the degradation rate and increase the bioactivity of the alloy. The microstructure and morphology of the samples were characterized using XRD, FT-IR SEM and EDS. The degradation behaviours of samples were evaluated using electrochemical techniques, and immersion tests in simulated body fluid (SBF). The results indicate that the morphology of the Si–CaP coatings changed significantly with the increase in Ca/P ratio as the second-step voltage increased. The Si–CaP containing coating produced at 450 V could significantly decrease the degradation rate of Mg and caused a slow increase in pH of the SBF solution. The haemolysis test concluded that the coating C3 did not cause a haemolytic reaction. The corrosion resistance of Mg alloy was greatly improved with the Si–CaP coatings, and the Mg alloy with Si–CaP coating prepared at 450 V had the best corrosion resistance, which indicates that the Si–CaP coatings are promising for improving the biodegradation properties of Mg-based orthopedic implants. Haemolysis tests indicated that the Si–CaP coating prepared at 450 V conforms to the given standard (YY/T0127.1-93).

Corrosion behaviour of micro-arc oxidation coatings on Mg–2Sr prepared in poly(ethylene glycol)-incorporated electrolytes

Microarc oxidized calcium phosphate (CaP) ceramic coatings were fabricated on Mg–2Sr alloy from silicate electrolytes with different concentration gradient poly(ethylene glycol) (PEG1000). The microstructure, phase and degradability of the ceramic coatings were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion tests respectively. An electrochemical workstation was used to investigate the electrochemical corrosion properties of the coatings. It is found that microstructure, thickness, adhesive strength and degradation rate are influenced by PEG1000 incorporation through adjusting the electrolyte activity and then altering the coating growth mechanism. Similar thicknesses (39.0–42.2 μm) are observed in PEG1000-containing coatings while their PEG1000-free counterparts possess the maximum value (51.5 μm). The weight gain in the first two days of SBF immersion suggests that a new layer containing CaP apatites is generated. Results show that ceramic coatings prepared in the electrolyte containing 8 g L−1 PEG1000 exhibits the highest corrosion resistance and lowest degradation rate.