The effect of NaOH concentration on the steam-hydrothermally treated bioactive microarc oxidation coatings containing Ca, P, Si and Na on pure Ti surface

• MAO coating covered Ti plates were steam-hydrothermally treated with NaOH solution.
• HA and anatase were generated from the previously amorphous MAO coating.
• The adding of NaOH can improve the densification of the formed coating.
• Ti–OH is formed on the steam-hydrothermally treated MAO coating.
• The coatings which are treated with NaOH solution show apatite-inducing ability.

The microarc oxidation (MAO) coating covered pure Ti plates are steam-hydrothermally treated in autoclaves containing NaOH solutions with different concentrations of 0, 0.001, 0.01, 0.1 and 1 mol·L− 1. Due to the composition of Ti, O, Ca, P, Si and Na elements in the MAO coating, anatase and hydroxyapatite (HA) crystals are generated from the previously amorphous MAO coating after the steam-hydrothermal treatment. Meanwhile, it is noticed that the amount of HA crystals increases but showing a decline trend in aspect ratio in morphologies with the increasing of NaOH concentration. Interestingly, the steam-hydrothermally treated MAO coatings exhibit better bonding strength with Ti substrate (up to 43.8 ± 1.1 MPa) than that of the untreated one (20.1 ± 3.1 MPa). In addition, benefiting from the corrosive attack of the dissolved NaOH in water vapor on the MAO coating, Ti–OH is also formed on the steam-hydrothermally treated MAO coating surface, which can trigger apatite nucleation. Thus, the steam-hydrothermally treated MAO coatings exhibit good apatite-inducing ability.