Oxide coating containing apatite formed on Ti-25Nb-25Ta alloy treated by Two-Step Plasma Electrolytic Oxidation

Abstract

Hydroxyapatite (HA) is a bioactive calcium phosphate capable of enhancing the implant/bone connection improving osteoconductivity and osseointegration process. However, the HA presents mechanical properties limiting its application. A good way to resolve this limitation is to combine the excellent biological properties of HA with materials with suitable mechanical behavior, like Ti-25Nb-25Ta alloy. The Ti-25Nb-25Ta alloy is cornposted by non-toxic and corrosion-resistant elements, presenting good biological compatibility. In this work, Plasma Electrolytic Oxidation (PEO) (using direct current-DC) was applied in conventional mode and Two-Step PEO aiming to produce a porous coating containing HA. It was not possible to produce a satisfactory coating applying conventional PEO due to successive spalling during the oxidation process. Adding a pretreatment to the conventional PEO changed the process to Two -Step PEO allowing to form a porous coating containing HA. The pretreatment was made by PEO using phosphoric electrolyte to produce a pre-coating. After that, the pre-coating was re-oxidized with calcium/phosphorus electrolyte. The Two -Step oxidized surface presented well-known good characteristics to applications in osseous implant devices such as porous formation, roughness in the micrometrical range, surface containing calcium and phosphorus, bioactive crystalline titanium oxide, and well adhered HA formation. However, the coating morphology and chemical composition of pre-coating and Two Step oxidized surfaces were not uniform. The non-uniformity of the Two -Step oxidized surface follows a similar non-uniformity pattern of the pre-coating surface. Two distinct morphologies were identified on the Two-Step oxidized surface: a velvety morphology with HA formation and a highly porous morphology. Regarding the Ti-25Nb-25Ta alloy, the Two-Step oxidation produced an adhered coating with porous apatite distribution interspersed with calcium-rich porous oxide.