Microstructural characterization, biocorrosion evaluation and mechanical properties of nanostructured ZnO and Si/ZnO coated Mg/HA/TiO2/MgO nanocomposites

Nano-ZnO monolayer and nano-Si/ZnO double-layer coatings were deposited on a Mg/HA/TiO2/MgO nanocomposite using radio frequency magnetron sputtering. The composition and surface morphology of the specimens were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, atomic force microscopy and field-emission scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Potentiodynamic polarization and immersion tests were used to investigate the corrosion behavior of samples. The Si/ZnO nanocomposite coating, which had an average thickness of 1.7 μm, exhibited a uniform and dense film, consisting of a ZnO outer layer (~ 1.1 μm thick) and a Si inner layer (~ 0.6 μm thick). However, some pores and cracks were observed in the ZnO monolayer coating (~ 1.5 μm thick). In addition, the Si and ZnO nanoparticles had a spherical morphology with an average particle size of 28-40 nm. Higher polarization resistance values were obtained for the nano-Si/ZnO coated sample (~ 2.55 kΩ cm2) compared with that of the ZnO coated (~ 1.34 kΩ cm2) and uncoated (~ 0.14 kΩ cm2) samples in a simulated body fluid (SBF). The results from the hydrogen evolution studies indicated that the nano-Si/ZnO-coated sample had a lower degradation rate (1.07 ml/cm2/day) than the ZnO-only coated sample (2.25 ml/cm2/day) and the uncoated sample (4.42 ml/cm2/day). After 168 h of immersion in a SBF solution, a larger amount of hydroxyapatite precipitated on the Si/ZnO coating than the ZnO coating, which resulted in an improvement in the bioactivity. The compressive strength and elongation of uncoated Mg/HA/TiO2/MgO decreased from 253 MPa and 9.8% to 104 MPa and 5.2% after 28 days of immersion in SBF solution, whereas the Si/ZnO coated sample indicated a compressive strength of 148 MPa and elongation of 7.2%. Therefore, the double-layer Si/ZnO composite coating prepared by magnetron sputtering on the Mg/HA/TiO2/MgO nanocomposite is more suitable for biomedical applications.