The formation of PEO coatings on the superelastic Ti–18Zr–15Nb alloy in calcium-containing electrolytes

Abstract

The paper discusses the influence of the electrolyte composition on the characteristics of a biocompatible coating produced by plasma electrolytic oxidation (PEO) on titanium superelastic shape memory alloy Ti–18Zr–15Nb. The scientific novelty of the work is in the identification of the most effective electrolyte composition to form a PEO coating with improved functional properties for advanced metal implants. Having important scientific and social significance, the scientific results of the work will serve as the basis for the development of modern technologies for the production of new-generation implants for orthopedy and neurosurgery. To identify the most effective electrolyte composition, the authors studied the morphology and microstructure of coatings, phase and elemental composition, adhesive properties, and surface wear resistance; carried out electrochemical corrosion tests. The resulting coatings have a thickness in the range of ~15.5–17 µm, and porosity of ~12–18 %. The additive in the form of sodium silicate significantly smooths the surface and increases wear resistance, but, at the same time, reduces the adhesive properties of specimens. The coatings contain biocompatible calcium phosphate compounds, which is confirmed by the presence of an amorphous halo between ~25° and ~40° in the results of X-ray phase analysis and the identified elements Ca and P in elemental analysis. The electrochemical impedance spectroscopy results identified the difference in the structure of PEO coatings and the corrosion processes occurring in them. Coatings formed in phosphate electrolytes have two layers: the external porous and internal compact, and in the phosphate-silicate electrolytes – a single layer. The study identified that plasma-electrolytic oxidation reduces corrosion currents by 1–3 orders compared to a specimen without PEO treatment. The coating formed in a phosphate electrolyte with the addition of boric acid and calcium acetate has the best corrosion characteristics and the highest roughness, which positively affects biocompatibility. This electrolyte can be recommended for further research as the most effective one.