Formation of bioactive coatings on a Ti-6Al-7Nb alloy by plasma electrolytic oxidation

In an attempt to increase the bioactivity and corrosion resistance of a vanadium-free titanium alloy Ti-6Al-7Nb, the electrolytic plasma oxidation (PEO) process for surface modification was utilised. Select samples were subjected to further treatment, either thermal or alkali. The morphology, chemical composition and phase composition of the ground and treated Ti-6Al-7Nb alloy substrates were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was observed that during the anodic process under sparking discharge conditions, the simultaneous incorporation of calcium and phosphorus in the forming oxide layer occurs. The resulting layers were porous and exhibited the typical morphology for layers formed during the PEO process. The heat treatment of samples oxidised at 150 V resulted in the in surface oxide layer forming a crystalline phases anatase and rutile. After the alkali treatment of samples oxidised at 150 V, a gel-like titanate layer was formed. The bioactivity investigations in simulated body fluid (SBF) solution and with human bone marrow stromal cells (MSCs) indicated that after anodising at 150 V and following alkali treatment the Ti-6Al-7Nb alloy exhibits osteoinductive properties. The electrochemical investigations showed that application of the anodising process of the Ti-6Al-7Nb alloy significantly improved its corrosion resistance in Ringer solution. The samples anodised at 80 V presented the highest corrosion resistance because of the formation of the thin, compact oxide layer on the alloy surface. The approach presented here may be applied for fabricating Ti-6Al-7Nb-based implants.