Mechanical behaviour of cp-magnesium with duplex hydroxyapatite and PEO coatings

Hydroxyapatite-magnesia coatings were formed on cp-magnesium by plasma electrolytic oxidation (PEO) followed by cathodic electrodeposition (CED). The static tensile and cyclic fatigue performance of the coated samples were investigated. The cracking behaviour of the coatings during the tensile tests was studied by fracture analysis. The effects of the surface treatment on the fatigue performance of the magnesium substrate were addressed. Tensile strength of cp-Mg was not significantly affected, whereas the fatigue performance was improved by the PEO+CED coatings in the low-cycle region, possibly due to compressive residual stress induced to the metal substrate by the surface treatment. However, reduced fatigue strength was observed in the high-cycle region, which might be attributed to the defects at the coating/substrate interface produced during the surface modification. The in vitro corrosion reduced the fatigue strength in both the low- and high-cycle regions. Finally, the applicability of surface engineered magnesium for biomedical applications was demonstrated from the mechanical standpoint.