Bioactive coating with hierarchical double porous structure on titanium surface formed by two-step microarc oxidation treatment

The Ca, P, Si and Na incorporated bioactive coatings with double porous structure in macro (50–1000 μm) and micro (0.6–2.0 μm) scales have been successfully formed on Ti surface by a two-step treatment via microarc oxidation (MAO). The results reveal that the microarc oxidation and NO3− ion corrosion activated by plasma are the two key factors for the formation of macropores during the second MAO treatment. Meanwhile, the MAO processing current, mass loss, surface morphology and elemental concentration of the samples are significantly affected by the NaOH concentration in the electrolyte during the second treatment. With increasing of NaOH concentration from 5 to 20 g/L, the mass loss of the two-step MAO treated (TMAO) samples first increases from 4.3 to 6.7%, then decreases to 4.7%. The number of macropores increases dramatically from 26 to 806 mm− 2, but the size of macropores declines gradually in a range of 1000–143 μm with the varied NaOH concentration. Furthermore, an oxide film composed of Ti3O5 is formed on the macropore surface after treating in the electrolyte with 20 g/L NaOH concentration. Additionally, the sample shows apatite inducing-ability evaluated in simulated body fluid immersion.