Micromorphology and properties of novel phosphate conversion coatings based on zinc-magnesium and zinc-zirconium deposited on Ti6Al4V

Due to the high number of traumas and the increase in patients with osteoporosis/arthrosis, the biomaterials and implants market is growing rapidly. Consequently, bioactive phosphate coatings utilizing zinc and non-toxic metal ions have garnered increased interest in the surface modification of metallic implants. The primary focus remains on enhancing the body's biological response or reducing the adverse effects on the human body. Herein, two novel composite coatings based on Mg-Zn phosphate and Zr-Zn phosphate, respectively, have been developed on the surface of Ti6Al4V via the phosphate chemical conversion process. The effect of Mg and Zr ions on the morphology and the characteristics of the phosphate layers deposited on the Ti alloy substrate were analyzed. Studying the morphology of the layers, it was observed that the Zr ions promote the formation of a more compact structure while Mg ions will mainly create structures with acicular crystals and visible intercrystalline zones. The XRD results confirmed that both coatings contain hopeite (Zn 3 (PO 4 ) 2 ·4H 2 O) and phosphophyllite (Zn 2 Fe(PO 4 ) 2 ·4H 2 O) phases. Additionally, each coating includes phases specific to the metal ions added in each solution. In the coating with Mg addition, the specific phase identified are MgZn 2 (PO 4 ) 2 and reaphookhillite (MgZn 2 (PO 4 ) 2 ·4H 2 O). In contrast, the coating with Zr addition contains phases such as (ZnZr(PO 4 ) 2 ·2H 2 O, Zr(HPO 4 ) 2 , (ZrP 2 O 7 ). The results show that the coatings obtained with Zr-Zn compounds exhibited up to 9 times higher corrosion resistance in both Ringer and Dulbecco solutions. In addition, wettability tests demonstrated favorable cellular adhesion, while vitro cytotoxicity tests showed increased viability in the case of mouse fibroblasts L929 cells. Overall, this study elucidates the Zr and Mg ions-induced effects on the morphology and properties of the Zn-based phosphate coatings, promoting increased corrosion resistance and cell viability.