Synthesis and biodegradation evaluation of nano-Si and nano-Si/TiO2 coatings on biodegradable Mg-Ca alloy in simulated body fluid

In the present study, nano-Si and nano-Si/TiO2 composite coatings have been successfully synthesized on the surface of Mg-1 wt%Ca alloy by the physical vapor deposition (PVD) method. The surface morphology and compositions of the coated specimens were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results show the formation of thin and compact coating layers which homogeneously cover the surface of Mg alloy. Some micro-pores and micro-cracks were observed in the Si and Si/TiO2 films. It was also found that the Si and TiO2 nanoparticle had a spherical morphology with an average particle size of 30-40 nm and 70-80 nm, respectively. Electrochemical studies revealed that nano-Si/TiO2 coating offers a significant reduction in the corrosion rate (0.57 mm/year) compared to the Si coated (0.91 mm/year) and the uncoated alloys (6.21 mm/year) in simulated body fluid (SBF). Hydrogen evolution studies showed a lower degradation rate of nano-Si/TiO2 (1.57 ml/cm2/day) than that of nano-Si coated alloy (2.22 ml/cm2/day). Immersion test showed that the nano-Si/TiO2 coating presented a greater nucleation site of hydroxyapatite (HA) than the uncoated sample. Thus nano-Si/TiO2 composite coating prepared by PVD on the Mg-Ca alloy is more appropriate for biomedical applications.