電化學沉積MgO/ZrO2及CaP/ZrO2鍍層於AZ91D鎂合金之研究

Abstract

In order to promote the corrosion resistance of magnesium alloy, an MgO/ZrO2 duplex-layer coating has been prepared on AZ91D magnesium alloy as a protective film against corrosion by a two-step fabrication process of electrodeposition and annealing treatment in the study. Owing to the chemical bonding formed after the condensation of precursory hydroxides, the adhesion strength, thickness and compactness of MgO coating on the substrate are significantly enhanced by the intermediate ZrO2 layer which prevents the formation of corrosion product Mg2(OH)3Cl·4H2O. As a result, the MgO/ZrO2 duplex-layer coated specimen reveals relatively high corrosion resistance and superior stability in 3.5 wt % NaCl solution with respect to the MgO single-layer coated specimen. Recently, magnesium alloys have been further proposed as a new class of biodegradable metallic biomaterials. However, their corrosion resistance restricts further applications in medical devices. In order to control the degradation rate and enhance the biocompatibility of magnesium alloys, calcium phosphate (CaP) top layer with ZrO2 interlayer composing CaP/ZrO2 coating was carried out on AZ91D magnesium alloy by electrodeposition and annealing. ZrO2 interlayer makes unstable dicalcium phosphate anhydrous (DCPA) for CaP single layer transform into stable hydroxyapatite (HA) for CaP/ZrO2 composite coating and enhance the adhesion strength of CaP from 12.1 to 24.4 MPa, owing to OH bonds provided by the precursory of intermediate layer Zr(OH)4 condensed with OH bonds in HA and on Mg alloy surface after annealing, also leading to a dense and compact under layer which effectively reduces the corrosion current density from 84.30 to 0.49 μA/cm2 in potentiodynamic polarization tests and weight increase in immersion tests. Besides, the in vitro cell assays demonstrate that CaP/ZrO2 and ZrO2 coatings can enhance more cell adhesion and proliferation whereas cell numbers on uncoated specimen decreases with culture time due to the corrosion accompanied with evolution of hydrogen, the rise in Mg2+ and pH of solution near the specimen surface, and the formation of corrosion products, revealing that CaP/ZrO2 or ZrO2 coated AZ91D magnesium alloy can be a promising candidate as a biodegradable implant.