Fabrication and corrosion behavior of Si/HA nano-composite coatings on biodegradable Mg–Zn–Mn–Ca alloy

In this study, nano-silicon (Si) thin films were deposited on biodegradable Mg–6Zn–0.8Mn–3Ca substrates by physical vapor deposition (PVD) method. Subsequently, nano-hydroxyapatite (HA) was coated as a second layer on the nano-Si film using electrochemical deposition (ED). The surface morphology and the corrosion behavior of the composite coatings were evaluated using scanning electron microscope, X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, potentiodynamic polarization tests and immersion tests. The Si/HA nano-composite coating, with an average particle size of 72 nm, represented a uniform and dense film consisting of HA as the outer layer (8–10 μm) and Si as the inner layer (1.5–2 μm). However, some micropores and microflaws were observed in the Si thin film. The polarization test indicated that Si/HA coating could efficiently reduce the corrosion rate of Mg alloy in simulated body fluid from 2.57 to 0.12 mm/year. However, only moderate reduction in corrosion rate was observed in the case of single layer nano-Si coating. Hydrogen evolution studies showed greater reduction in degradation rate of Si/HA and Si coated samples, compared to uncoated alloy. Immersion test showed the formation of apatite layer on the surface of Si/HA film after immersion in SBF that resulted in noticeable improvement of bioactivity.