Composite Microstructure and Formation Mechanism of Calcium Phosphate Conversion Coating on Magnesium Alloy

In order to improve the biodegradability and surface biocompatibility of magnesium (Mg) alloys, a calcium phosphate conversion coating (CPCC) was deposited onto an AZ60 Mg alloy. The coating's components and composite microstructure and its formation mechanism were studied using electrochemical measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy and by developing a thermodynamic stability diagram. The results revealed that the presence of Mg 2+ originating from the electrochemical reaction of the Mg alloy could affect the precipitation of different coating components. Because the concentration of Mg 2+ at the solution/coating interface continuously changed, three layers of the conversion coating were progressively produced. The inner layer was composed of magnesium hydrogen phosphate trihydrate (MHPT, MgHPO 4 ·3H 2 O) and had a thickness of ∼1.1 μm. The middle layer was ∼1.6 μm thick and consisted of dicalcium phosphate dihydrate (DCPD, CaHPO 4 ·2H 2 O), magnesium whitlockite (MWH, Ca 9 Mg(HPO 4 )(PO 4 ) 6 ), and MHPT. The top layer was ∼2.3 μm thick and was composed of a combination of DCPD and MWH.