Hydroxyapatite production on ultrafine-grained pure titanium by micro-arc oxidation and hydrothermal treatment A. Alsaran a, ⁎, G. Purcek b , I. Hacisalihoglu c , Y. Vangolu a , Ö. Bayrak a , I. Karaman d , A. Celik a a Ataturk University, Engineering Faculty, Mechanical Engineering Department, 25240-Erzurum, Turkey b Karadeniz Technical University, Engineering Faculty, Mechanical Engineering Department, 61080-Trabzon, Turkey c Gumushane University, Engineering Faculty, Mechanical Engineering Department, 29100-Gumushane-Turkey d Texas A&M University, Dept. of Mechanical Engineering, College Station, TX 77843, USA abstract article info Available online 16 March 2011 Keywords: ECAE MAO Hydrothermal treatment Hydroxyapatite Wear Strength In this study, pure-Ti (Grade-2) was severely plastically deformed by equal-channel angular extrusion (ECAE) to improve its strength, and then subjected to micro arc oxidation (MAO) and hydrothermal treatment (HT) to achieve improved surface behavior with hydroxyapatite (HA)-containing titania (TiO 2 ) coating. Microstructure, tensile properties and wear behavior of UFG Ti were investigated systematically before and after surface modification. ECAE considerably enhanced the strength of coarse-grained (CG) Ti by forming an ultra-fine grained (UFG) microstructure with high dislocation density. The application of MAO with or without HT did not cause remarkable decrease in strength and ductility of CG and UFG Ti, but resulted in a considerable increase in its wear resistance and osseointegration due to the formation of hard and adhered HA-containing TiO 2 coating on the surface. It can be concluded that the use of ECAE plus MAO/HA coating provides a simple and effective procedure to obtain pure Ti with high strength and high wear resistance. The improved bulk and surface properties can make this biocompatible material a good potential candidate for biomedical applications instead of alloys including toxic elements. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Titanium and its alloys have been widely utilized in many applications ranging from biomedical to aerospace due to their low density, high specific strength, excellent corrosion resistance and high biocompatibility [1,2]. Titanium alloys are generally preferred for use in biomedical applications like orthopedic and dental implants where high-load bearing capacity is demanded [3]. However, these alloys include some alloying elements that are generally harmful for the human body. For instance, the Ti–6Al–4 V alloy is the well-known type and widely used in some biomedical applications. The Al and V alloying elements in this alloy were reported to be toxic and may potentially cause a series ailments including cancer in long-term use in human body [4–7]. Therefore, it is desired to replace titanium alloys including toxic elements with higher strength and more biocompatible pure Ti in medical applications. However, coarse-grained (CG) commercially pure (CP) Ti lacks the strength needed for many implants. Strengthening pure Ti by forming ultrafine-grained (UFG) micro- structure via severe plastic deformation imposed by equal-channel angular extrusion (ECAE) method seems to be the most appropriate method to make Ti as a viable alternative to expensive and less biocompatible Ti alloys. Because, there are many published studies on the improvement of strength of pure Ti by forming UFG microstruc- ture [8,9]. However, Purcek et al. [10] showed that the strengthening of titanium by ECAE processing did not lead to any improvement in its wear resistance due to the unstable surface oxide film by the effect of tribo-chemical reaction leading to rapid oxidative wear loss. It is known that the worn particles from the implant may cause tissue inflammation and bone deterioration in medical applications. There- fore, the surface properties of UFG Ti and its osseointegration behavior should be improved by any means in order to obtain an implant material with high wear resistance along with high loading capacity. For this purpose, various surface-modification techniques such as plasma spraying, ion implanting, sol–gel, alkali heating, anode oxidation and micro-arc oxidation (MAO) have been developed so far in order to improve the wear resistance and the bioactivity of implant materials [11–16]. Among them, MAO can produce a porous and firmly adherent titania film on titanium surface [17–20]. In addition, a hydroxyapatite (HA)-containing porous titania coating can be formed on the surface of UFG Ti by the combination of MAO and hydrothermal (HT) processing. The HA film was found to be one of the coatings that have the best osseointegration and bioactivity [21–23]. In view of the above, the main purpose of the current study is to obtain a high-strength UFG Ti with improved wear behavior and good osseointegration behavior. Therefore, the ECAE, MAO and HT treat- ment processes were applied respectively to the grade-2 titanium. Surface & Coatings Technology 205 (2011) S537–S542 ⁎ Corresponding author. Tel.: +90 4422314859; fax: +90 4422360957. E-mail address: aalsaran@atauni.edu.tr (A. Alsaran). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.032 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat