Self-healing behavior of sodium polyacrylate-hydroxyapatite coatings on biodegradable magnesium alloy

The effect of the anionic polymer sodium polyacrylate (SPA) on the healing behavior of a hydroxyapatite (HAp) coating formed on Mg-3 mass% Al-1 mass% Zn (AZ31) alloy was examined. Static immersion tests of pre-scratched specimens and slow strain rate tensile (SSRT) tests were performed in simulated body fluid (Hanks’ solution), for HAp-coated AZ31 with and without SPA. The static immersion tests revealed that the deposition of calcium phosphate (Ca-P) inside the scratches was enhanced by the presence of SPA, and the Ca-P layer deposited with SPA prevented local corrosion from the scratches. The SSRT tests revealed that the time to fracture of HAp-coated AZ31 increased by 45% in the presence of SPA. The fracture surface in the presence of SPA showed ductile regions to depths of 10 μm to 20 μm from crack initiation sites, and deeper regions showed a cleavage morphology. The cleavage morphology occurred directly from crack initiation sites in HAp-coated AZ31. The cleavage morphology suggested that hydrogen embrittlement occurred for AZ31 in Hanks’ solution. The combination of SPA and HAp coating promoted the in situ healing of coating cracks upon formation. This helped to suppress corrosion under the cracks, which increased the time to fracture of HAp-coated AZ31.