(Digital Presentation) Formation Mechanism and Defects of Electrodeposited Ca-P Coating on Biodegradable Mg-Zn-Ca Alloy

Abstract

Biodegradable implants are a new concept to substitute current nondegradable medical materials. The biodegradable implant can decompose in the patients’ body when the treatment is finished and be discharged through metabolism. Mg alloys, with their biocompatibility and high reactivity, meet the requirements of a biodegradable material. Also, Mg has a similar density to the human bones and the lowest elastic modulus among the structural metallic biomaterials, which can mitigate the stress shielding effect. For Mg-Zn-Ca alloys, besides the merits of Mg, Zn is related to bone metabolism and osteopathic phosphate activity, while Ca is the main element of bones that is essential for the synthesis of bone compounds. However, the degradation rate of Mg alloy implants needs to be controlled, or the implant may corrode too fast, causing local hydrogen accumulation and alkalization that hurts the patients. One of the effective ways to improve the alloy corrosion resistance is by coating. Ca-P coatings, such as brushite (dicalcium phosphate dihydrate, DCPD) and hydroxyapatite (HA), can mitigate the degradation rate of Mg alloys and show good biocompatibility and osseointegration. Electrodeposition is a simple way to synthesize coatings on metallic materials with an easy experimental set-up, and the process parameters are highly controllable.Even though there are plenty of reports about the fabrication of Ca-P coatings on Mg alloys through electrodeposition, few of them focus on the film formation mechanism. Issues related to coating defect formation and inhomogeneity are rarely discussed. Therefore, this report focuses on the film formation mechanism and the relationship to the defects of the electrodeposited DCPD coating on a Mg-2.5wt%Zn-1wt%Ca alloy. By altering the deposition potential, the reduction reactions that proceed on the substrate are different. The involvement of H2O reduction affects the formation of DCPD and changes the microstructure of the coating. Combining electrochemical tests and microstructure analysis, the corrosion properties of the coatings are measured, and the results are linked to the difference in film microstructure and defects. With SEM/FIB cross-sectional observations, the formation mechanism of the coatings and the relationship to the process condition will be discussed. The degradation rate of Mg alloys can be tuned by understanding the DCPD film formation mechanism and the cause of the defects.