Formation and Biological Properties of Hydroxyapatite-Magnesium Oxide Composite Coatings on Mg-1Zn-1Gd Alloys Though Anodization, Electrodeposition, and Solution Treatment Processes
Authors: Lin Erli , Kure-Chu Songzhu Shochiku , Liu Jiacheng , WANG Peng , Kurita Noriaki , Li Xiaopei 
Published: December 2024

Magnesium and its alloys have received extensive attention in the biomedical field due to their excellent mechanical properties, good biocompatibility, and biodegradability. However, Mg alloys typically suffer from rapid degradation rates in physiological environments due to their active chemical properties and poor corrosion resistances of naturally formed oxide films, which greatly hinders their widespread applications in biomaterials. Besides, rapid degradation rates are usually accompanied by excessive release of hydrogen and alkaline substances in the vicinity of related tissues instantaneously. Consequently, the degradation rates of Mg alloys must be controlled to meet the requirements of biodegradable implants. Surface coating technologies including anodization and electrodeposition, and conversion coating are considered as effective approaches to improve the corrosion resistances of metallic materials. In special, it is known that making composite films, such as metal hydroxides and calcium phosphate salts, and adding other elements to the anodic porous films are effective methods to improve the wear resistance and biocompatibility of Mg alloys. Calcium phosphate compounds, particularly hydroxyapatite (HAp), are favorable and safe candidates for implants as they constitute the main components of human bones. Abundant calcium phosphate can fulfill the required porosity for bone ingrowth and provide certain mechanical strength and enhanced adhesive force, addressing the brittleness of hydroxyapatite.

In this study, various hydroxyapatite-magnesium oxide coatings were prepared on a Mg-1Zn-1Gd alloy base material using successive anodization, electrodeposition, and solution treatment, to enhance the long-term corrosion resistance and biocompatibility of Mg alloy. The effects of different treatment methods on the deposition of Ca-P substances, coating morphology, structure, and corrosion resistance were investigated. SEM results confirmed that the solution treatment method effectively repaired the pores and cracks on the anodic oxide film, and the morphology of the coating differed from that of the electrodeposited treatment. In particular, the samples treated with the solution method exhibited a denser coating with a greater thickness. Additionally, the corrosion performance of the coatings was evaluated through electrochemical characterization, immersion tests, XRD, and SECM. The samples treated with the solution method demonstrated the strongest long-term corrosion resistance during the immersion process, which can be attributed to the denser structure and greater thickness of the coating, as well as the reparative effect of hydroxyapatite on the coating. In summary, the anodic oxidation coatings improved the corrosion resistance of the magnesium alloy, while the hydroxyapatite coating further enhanced its long-term corrosion resistance.

Fig.1 shows the surface morphology of the anodized magnesium oxide film layer and the anodized hydroxyapatite composite coating. We used electrodeposition and solution treatment to prepare hydroxyapatite coating on anodized film, and the two methods produced different morphologies. This is due to the different driving forces of the two methods leading to different crystallization orientations of hydroxyapatite. The corrosion resistance of solution treated oxide film is better than that of electrodeposited samples, which is mainly due to the formation of a denser and thicker composite film on the surface of samples. In the cell proliferation experiment, hydroxyapatite coating can obviously promote the biocompatibility and cell activity of anodized samples, and cell morphology experiment shows that different microstructure characteristics and components can affect the cell survival rate, and the solution treated samples have more obvious promotion on cell proliferation and growth.

Figure 1