Chemical and crystallographic characterizations of hydroxyapatite- and octacalcium phosphate-coatings on magnesium synthesized by chemical solution deposition using XPS and XRD

Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings deposited on a pure magnesium (Mg) substrate from an aqueous solution containing C10H12N2O8Na2Ca (Ca-EDTA) and KH2PO4 were precisely analyzed using X-ray photoelectron spectroscopy (XPS) and high-resolution X-ray diffractometry (XRD) to clarify their chemical and crystallographic characteristics. The calcium phosphate coatings comprised single-phase HAp or OCP containing small amounts of sodium (Na). The intermediate layer formed underneath the coatings consisted of Mg(OH)2 mixed with calcium phosphate. The atomic ratios of calcium to phosphorus ([Ca]/[P]) in the HAp and OCP coatings were notably smaller than those of stoichiometric compounds. The expansion along the a-axis compared to that of stoichiometric HAp was observed in XRD results. Also, we did not detect Na compounds in the coating. Therefore, it is presumed that Na existed as a substitute of Ca in the OCP and HAp structures.