Bioactive coating with hierarchical double porous structure on titanium surface formed by two-step microarc oxidation treatment Rui Zhou a , Daqing Wei a, ⁎, Wei Feng a , Su Cheng b , Haoyue Yang a , Baoqiang Li a , Yaming Wang a , Dechang Jia a , Yu Zhou a a Department of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China b Department of Mechanical Engineering, School of Architecture and Civil Engineering, Harbin University of Science and Technology, Harbin 150001, PR China abstract article info Article history: Received 13 January 2014 Accepted in revised form 23 April 2014 Available online xxxx Keywords: Double porous structure Microarc oxidation NaNO 3 NaOH concentration Apatite-inducing ability 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 ox- idation (MAO). The results reveal that the microarc oxidation and NO 3 - ion corrosion activated by plasma are the two key factors for the formation of macropores during the second MAO treatment. Meanwhile, the MAO process- ing 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 Ti 3 O 5 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Ti is proven to be a very suitable material for load bearing implants [1]. However, it shows certain drawbacks of poor biological activity. Fur- thermore, the interface bonding between Ti implant and tissue is very easy to become loose due to the “stress shielding” caused by the mis- match of Young's modulus [2,3]. Attempts to overcome the disadvantage of poor osseointegration have involved depositing bioactive coatings on Ti surface, including sol–gel [4,5], ion implantation [6,7], plasma spraying [8–10] and microarc oxidation (MAO) [11,12]. Recently, the bioactive coatings with double porous structure in micro and macro scales have attracted great attentions. This is attributed to the benefits of micropores with the size of 0.05–10 μm in the promoting of osteoblast attachment and pro- liferation on Ti implant [13–15]. Meanwhile, the macropores with the size of 50–400 μm on implant surface could improve the fixation strength between implant and tissue after implantation by providing good mechanical interlocking [16–19]. Among those technologies, MAO is a relatively convenient and effec- tive technique to cover coating with micro porous structure and desired elements [11,12,20]. Herein, a Ca, P, Si and Na incorporated MAO coating with micro porous structure (0.6–2 μm) has been fabricated on titanium by using a novel electrolyte containing Ca(CH 3 COO) 2 ·H 2 O, Na 2 SiO 3 , Ca(H 2 PO 4 ) 2 ·H 2 O, EDTA-2Na and NaOH. The previous studies have proved that the MAO coating containing Ca and P ions can induce apa- tite or other calcium phosphate minerals after stimulated body fluid im- mersion [12]. Meanwhile, the Si in bioactive glasses plays an important role in bone mineralization and formation, owing to the chemical stim- ulation of hydrated silica gel layer on the formation of apatite [21]. Fur- thermore, the existence of Na would lead to ion exchange which can benefit the formation of Si\OH [21]. Therefore, the formed MAO coat- ings are expected to obtain excellent bioactive ability. On the other hand, sandblast-acid etch is a normal technique to modified implant with desired roughness surface and fine macropores in size of 5–50 μm [22]. And the results reported by Xie et al. indicate that the MAO coating with irregular groove or macropores in the size of 5–20 μm can be formed on Ti after MAO treatment in acidic electrolyte containing CH 3 COOH or CH 3 COONa [23,24]. However, the macropores formed by these two methods don't contain any bioactive elements, such as Ca and P. And there are few reports about the formation of macropores with size of 50–400 μm on Ti surface via MAO technique. In order to form a bioactive coating with double porous structure in macro and micro scales on Ti surface, especially with macropore size be- tween 50 and 400 μm, a two-step MAO treatment technique is explored on Ti in this work. The first MAO treatment is performed in the above electrolyte to fabricate the Ca, P, Si and Na incorporated MAO coating Surface & Coatings Technology xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +86 451 8640 2040 8403; fax: +86 451 8641 4291. E-mail address: daqingwei@hit.edu.cn (D. Wei). SCT-19381; No of Pages 9 http://dx.doi.org/10.1016/j.surfcoat.2014.04.061 0257-8972/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Please cite this article as: R. Zhou, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.04.061