CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 41 (2015) 12668–12679 Fabrication and characterization of functionally graded hydroxyapatite/TiO 2 multilayer coating on Ti–6Al–4V titanium alloy for biomedical applications A. Araghi n , M.J. Hadianfard Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Blvd., Shiraz, Iran Received 26 April 2015; received in revised form 8 June 2015; accepted 20 June 2015 Available online 2 July 2015 Abstract Single layer, double layer and functionally graded coatings of hydroxyapatite and TiO 2 particles were deposited on Ti–6Al–4V titanium alloy substrate by electrophoretic deposition technique (EPD) in the acetylacetone medium. Optimum conditions to obtain stable suspensions for EPD of each type of coatings were estimated by investigating the effect of iodine concentration on the zeta-potential. In addition the effect of the applied voltage, during EPD process and polyethylenimine (PEI) concentration as binder on the coatings quality (formation of crack free coatings) was investigated. The results showed the formation of more intact coatings deposited at 20 V applied voltage from suspensions containing 0.6 g/L iodine and 4 g/L PEI additives. The coatings chemical composition, morphology and HA decomposition behavior were investigated using energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and X-ray diffraction (XRD) methods. The results showed that HA starts to decompose at lower temperature in functionally graded HA/TiO 2 coating (900 1C). The adhesion strengths of the coatings were measured by shear testing and the results showed that the HA/TiO 2 functionally graded coating has more adhesion strength (  31 MPa) compare to HA single layer and HA–TiO 2 double layer coatings. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Suspensions; C. Strength; D. Apatite; D. TiO 2 ; Functionally graded 1. Introduction Titanium and its alloys are extensively used in orthopedic and dental prosthetic applications [1,2] because of their good corrosion resistance, nontoxicalsoity, biocompatibility and good mechanical properties [1–4]. The Ti–6Al–4V alloy is the most widely used material for hip, spinal and knee implants due to its higher young modulus (114 GPa) than human bone (10–30 GPa) and good wear resistance [1]. Despite the fact that all Ti based alloys are regarded as bio inert materials but they do not allow bone formation on their surface leading to weak bonding between the implants and bone [1,4]. To overcome such problem, metallic implants are usually coated with bioactive materials, such as hydroxyapatite (HA) to facilitate the growth of natural bone on the implants surface [3]. It may be related to the fact that HA possesses similar chemical, structural and biological properties to that of the human bone [5,6]. The improvement in the Ti implant fixation properties, coated with HA, has been reported in previous studies [7–9]. On the other hand, it has been reported that HA coatings has poor adhesion on metallic substrates [1,2]. Several surface preparation techniques, such as sand blasting [10,11], grit blasting [12] and chemical methods like acid treatment [13,14], alkali treatment [15], hydrogen peroxide treatment [16], have been proposed to increase, marginally, coatings adhesion by increasing the substrate roughness. Another way of adhesion improvement in such coatings may be the use of high temperature driven processes such as thermal spray, plasma spray, electron beam deposition, sputter coating, pulse laser deposition based on melting/near melting of the coating materials during deposition process that will diffuse and solidify on the substrate. For instance, it has been reported that magnetron sputtered multilayered hydroxyapatite/ titanium coatings exhibit bond strength of as high as 60 MPa www.elsevier.com/locate/ceramint http://dx.doi.org/10.1016/j.ceramint.2015.06.098 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author. Tel.: þ98 917 416 1877; fax: þ 98 713 230 7293. E-mail address: araghi.alirezaa@gmail.com (A. Araghi).