Adhesion properties of hydroxyapatite and octacalcium phosphate coating layers to AZ31 alloy formed at various pH values

Uniform hydroxyapatite (HAp) and octacalcium phosphate (OCP) with and without monetite layers were formed on Mg-3mass% Al-1mass% Zn (AZ31) alloy by a chemical solution deposition method at various pH values from pH 5.5 to 8.6. Adhesion strength of as-coated HAp and OCP layers was evaluated by a pull-off test, and that of HAp layers after immersing in a medium for 14 days was also evaluated. The as-coated pH 7.5-HAp layer showed higher adhesion strength of 6.7 MPa than 3.8 MPa of the as-coated pH 8.6-HAp layer, which was attributed to the former denser structure than the latter. The adhesion strength of the as-coated pH 5.5- and pH 6.2-OCP (+monetite) layers was both 4.2 MPa that was governed by the thick and fragile property of the layers. After immersing in a medium, the adhesion strength of pH 7.5- and pH 8.6-specimens decreased to 4.9 and 3.1 MPa, respectively. The pH 8.6-specimen showed a uniform Mg(OH)₂ corrosion layer and corrosion pits; while the pH 7.5-specimen showed mostly corrosion pits. The pitting caused the decrease of adhesion strength of the HAp layers and the formation of Mg(OH)₂ layer also weakens the adhesion. The adhesion strength of HAp and OCP layers was governed by the density and thickness of the layer, and was weaken by pitting and formation of Mg(OH)₂ corrosion product layer.