Corrosion resistance and adhesion of poly(L-lactic acid)/MgF2 composite coating on AZ31 magnesium alloy for biomedical application

Abstract

Fluoride conversion layer was produced on AZ31 magnesium alloy by soaking in hydrofluoric acid solution and poly(L-lactic acid) (PLLA) film was prepared by spin-coating the PLLA solution. The as-prepared samples were comparatively characterized in phase structure, elements profile, morphology, adhesion force, and corrosion resistance. The results show that more MgF₂ was formed in the outer layer than at the interface which is likely to be composed of MgF₂ and Mg(OH)₂. The MgF₂ layer is of labyrinthine porosity with fine pores interconnected to larger ones, while the spin-coated PLLA film is dense and adhere to the substrate seamlessly. PLLA showed a higher adhesion force between the coating and AZ31 substrate than fluoride layer because of its ductility and higher contact area. PLLA was infused into the porous fluoride conversion layer forming an integrated inorganic/organic composite coating. Infiltration of PLLA into MgF₂ layer sealing pores and flaws contributes to reinforcement of the composite coating in favor of improvement of the interfacial adhesion force as well as corrosion resistance. The composite PLLA/MgF₂ coating outperforms either of the solely applied coatings with respect to anticorrosion and adhesion properties under the same condition.