Effect of pH of coating solution on the adhesion strength and corrosion resistance of hydroxyapatite and octacalcium phosphate coated WE43 alloy

Abstract:
Hydroxyapatite (HAp) coatings were formed at pH 7.5 and 8.6 and octacalcium phosphate (OCP) coatings were formed at pH 5.5 and 6.2 on Mg-4mass% Y-3mass% rare earth (WE43) alloy by a chemical solution deposition method. The adhesion strength of the HAp and OCP coating layers was measured by a pull-off test, before and after immersing in a medium for 14 days. The as-coated pH7.5-HAp specimen showed adhesion strength of 8.6 MPa that was higher than 6.5 MPa of the as-coated pH8.6-HAp specimen and 3.9 MPa and 7.1 MPa of the as-coated pH5.5- and pH6.2-OCP specimens, respectively. The relatively high adhesion strength of the as-coated HAp coatings was attributed to the dense structure. The relatively low adhesion strength of the as-coated OCP coatings was attributed to the thick and fragile property. After immersing in the medium, the adhesion strength of pH7.5- and pH8.6-specimens decreased to 5.8 MPa and 5.6 MPa, respectively. The pitting corrosion and formation of Mg(OH)₂ under the HAp layers were responsible for the decrease of adhesion strength. It was revealed that the HAp coatings showed higher adhesion strength than the OCP coatings.