Microstructure and surface performance of hydroxyapatite-modified multilayer amorphous coating on Ti-rich TiNbZrSn medium entropy alloy: A comparative study

The present study focuses on the formation of a hydroxyapatite-modified amorphous micro-arc oxidized (MAO) coating on the TiNbZrSn medium entropy alloy (TNZS MEA) through micro-arc oxidation and microwave hydrothermal treatment (MHT). The TNZS MEA primarily consists of a body-centered cubic (bcc) and hexagonal closed-packed (hcp) structure, exhibiting a low elastic modulus and high yield strength. A porous coating with an amorphous structure was obtained after MAO treatment, displaying superior corrosion resistance to MAOed Ti. After MHT, the MAOed TNZS MEA exhibited a multilayered structure consisting of three layers: an external growth layer (hydroxyapatite nanorods), an inner layer (amorphous and crystalline structures), and a substrate layer. This composite structure demonstrated enhanced superior apatite-inducing ability and corrosion resistance compared to MHTed Ti. The improvement in bioactivity and a slight decrease in corrosion resistance can be attributed to the incomplete structure of hydroxyapatite nanorods when compared to MHTed Ti. Consequently, this work successfully achieved a hydroxyapatite-modified multilayered oxidized coating with gradient performance in term of bioactivity and corrosion resistance, establishing it as a promising candidate material for hard tissue repair.