45S5 bioactive glass–ceramic coated AZ31 magnesium alloy with improved corrosion resistance

To control the biodegradation rate, 45S5 glass–ceramic coatings were prepared on the commercial AZ31 magnesium alloy substrates by dip-coating method from a synthesized sol–gel. Sol concentration, calcination temperature and dip-coating cycle have been optimized to prepare the compact coatings. The results showed that homogeneous and crack-free coatings with a thickness of 0.48–1.00 μm, consisting of amorphous phase and Na2Ca2Si3O9, were successfully fabricated on AZ31 magnesium alloys. The effects of these coatings on the corrosion behavior of the magnesium alloy substrates have been investigated in vitro by soaking samples into modified simulated body fluid (m-SBF) for different periods. It was found that optimized 45S5 glass–ceramic coatings could slow down the degradation rate and decrease the mass loss of the magnesium alloy substrate from 78.04% to 2.31% in the 7th day test, showing a good anti-corrosion property in a certain period. Meanwhile, calcium-deficient hydroxyapatite deposition was observed on the surface of sample 3A500, indicating its biomineralization property in m-SBF. Nevertheless, cracking of the coating during the immersion test is the major factor for 45S5 glass–ceramic coatings to fail to protect the magnesium alloy substrates in the later immersion period.