Evolution and performance of a MgO/HA/DCPD gradient coating on pure magnesium

•HA tends to aggregate together while DCPD changes from narrow bar to plate-like.•Plate-like DCPD weakens the bonding between PEO layer and CaPh compounds.•The corrosion resistance increases first and then drops with the deposition time.•The morphology of precipitated HA highly depends on the originated structures.•Phase conversion impedes the initial cell adhesion but then has limit influence. A composite calcium phosphate (CaPh) coating containing calcium hydrogen phosphate dihydrate (DCPD) and hydroxyapatite (HA) is fabricated on plasma electrolytic oxidized (PEO) magnesium via electro-assisted deposition. During immersion in simulated body fluid, the soluble DCPD stimulates the precipitation of HA and remains the degradability of the system, while the stable HA reinforces the corrosion resistance along with PEO layer. The morphology, thickness and corrosion property of the coating evolve remarkably with the deposition time. Various growth patterns of DCPD are presented in different period which noticeably influences the interfacial combination, as well as corrosion resistance. The composite coating after 60 min deposition shows most compact morphology and stable interfacial combination, leading to excellent corrosion resistance, apatite formation ability and cell attachment behavior.