Microstructure and Corrosion Behavior of Thermal-Sprayed Hydroxyapatite/Magnesium Composite Coating on the Surface of AZ91D Magnesium Alloy

To slow down the corrosion rate and increase the bioactivity of magnesium alloys, biodegradable hydroxyapatite/magnesium (HA/Mg) composite coating was deposited onto AZ91D alloy substrate by high-velocity suspension flame spray technique using nano-sized HA and micron-sized Mg powders. Microstructures and phase constitutions of HA/Mg composite coating were analyzed by scanning electron microscopy and x-ray diffraction. Corrosion resistance of composite coating was also evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy in Hanks’ balanced salt solution (HBSS). The result shows that the HA/Mg composite coating mainly consisted of Mg and HA in addition to a few of MgO phases. The coating showed a rough surface morphology with homogeneous elemental distribution and a lamellar structure in cross section as well as well-bonded coating/substrate interface. Mg particles presented superior deposition during spraying due to their higher melting degree compared to HA particles. AZ91D alloy substrate was significantly protected from corrosion in HBSS by the HA/Mg composite coating with Mg dissolution on surfaces of the composite coating. This strategy can offer a new route to fabricate HA/Mg composite coatings on other metal substrates for use in biodegradable implant fields.