Introduction:

Magnesium alloys have been widely used in orthopedic implants due to their biocompatibility and mechanical properties. However, their high corrosion rate limits their application. Calcium phosphate coatings have been shown to improve the corrosion resistance of magnesium alloys. RF magnetron sputtering is a technique that has been used to deposit calcium phosphate coatings on magnesium alloys. This technique allows for control over coating thickness and composition by regulating sputtering parameters, such as deposition time and power level. The novelty of this research lies in its investigation of multilayered coatings either doped with various metallic ions such as Zn or Cu or having different crystallinity state from amorphous to crystalline which can modulate the corrosion resistance and functional properties of a composite. The relevance of this study is related to its potential application in bioresorbable orthopedic implants, where a programed corrosion rate and targeted biological performance is required.

Results:

In this study, we investigated the use of RF magnetron sputtering to deposit calcium phosphate-based coatings on Mg-Ca alloy. Our results showed that annealing of amorphous calcium phosphate (ACP) coatings at 400 °C for three hours in Ar atmosphere helps to crystallize a coating up to 1 µm in thickness to close to stoichiometric hydroxyapatite (HA) and significantly improve corrosion resistance. Furthermore, deposition of an additional layer of ACP on top of the crystalline HA further improves the corrosion resistance. Corrosion current was estimated using P40-X potentiostat galvanostat and was found to be 4143 nA for as deposited ACP coating, 872 nA for crystalline HA coating after annealing and 306 nA for crystalline HA coating with post deposition of additional ACP layer.

It was found that the corrosion resistance of a metastable ACP coating is time dependent if samples are stored under ambient conditions. Furthermore, corrosion resistance of Mg-Ca alloy could be significantly decreased if ACP coating is doped with Cu starting at 0.2 at.%. The further research is needed to explain the stability of an amorphous calcium phosphate coatings and future applications in clinical practice.

The work was performed according to the Government research assignment for ISPMS SB RAS, project FWRW-2021-0007.