Microstructure and deformation behavior of biocompatible TiO 2 nanotubes on titanium substrate q G.A. Crawford a , N. Chawla a, * , K. Das b , S. Bose b , A. Bandyopadhyay b a School of Materials, Arizona State University, Tempe, AZ 85287-8706, USA b School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA Received 20 April 2006; received in revised form 7 August 2006; accepted 16 August 2006 Abstract Titanium oxide coatings have been shown to exhibit desirable properties as biocompatible coatings. We report on the quantitative microstructure characterization and deformation behavior of TiO 2 nanotubes on Ti substrate. Nanotubes were processed using anodic oxidation of Ti in a NaF electrolyte solution. Characterization of the as-processed coatings was conducted using scanning electron microscopy and focused ion beam milling. Increases in anodization time had no significant effect on tube diameter or tube wall thickness. Coating thickness, however, increased with time up to 2 h of anodization, at which point an equilibrium thickness was established. Nanoindentation was used to probe the mechanical response in terms of Young’s modulus and hardness. Progressively higher values of elastic modulus were obtained for thinner films consistent with increasing effects of the Ti substrate. A possible deformation mechanism of densification of the porous oxide and wear of the dense surface is suggested and discussed. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanoindentation; Biocompatible; TiO 2 ; Porosity; Coating 1. Introduction Titanium (Ti) and Ti alloys have been used extensively as bone-implant materials due to their high strength-to- weight ratio, good biocompatibility and excellent corrosion resistance [1,2]. Titanium, however, being relatively inert, cannot directly bond to bone, and osseointegration via the natural oxide (TiO 2 ) is a long process [3,4]. Hence, there is an increased interest in reducing the time needed for osseointegration. In an effort to enhance the cell- implant material interaction and increase lifetime, bioac- tive ceramic based coatings have been applied to Ti implants, most notably hydroxyapatite (HA) [5–8]. Recently, TiO 2 has been suggested as a potential alter- native to HA coatings. The advantage of using TiO 2 is that it can be grown directly on the Ti surface, by cost-effective techniques such as anodic oxidation [9–11]. Also, it is well known that one problem with HA coatings is poor adhe- sion strength at the HA/Ti interface [5–8]. Using anodic oxidation, TiO 2 is formed with a chemical bond between the oxide and Ti substrate that likely results in enhanced adhesion strength. Indeed, a bone-like apatite layer is formed on TiO 2 in simulated body fluid (SBF) [9,10]. Fur- thermore, researchers have suggested that TiO 2 with a 3-D micro/nanoporous structure may enhance apatite form- ability when compared to dense TiO 2 [11]. Recently, Gong et al. [12] have shown that TiO 2 coatings, consisting of reg- ular self-assembled nanotube arrays, may be formed on the surface of Ti by anodic oxidation. Treating the TiO 2 nano- tubes with NaOH solution induces the growth of nanosized HA when subjected to SBF [13]. Furthermore, nanophase ceramics such as HA, Al 2 O 3 and TiO 2 enhance long-term osteoblast functions relative to typical microstructure forms [14]. Thus, the apatite-forming and bone-bonding ability of titanium bone-implant materials may be 1742-7061/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2006.08.004 q Research presented at the TMS 2006 Biological Materials Science Symposium. * Corresponding author. E-mail address: nchawla@asu.edu (N. Chawla). Acta Biomaterialia 3 (2007) 359–367 www.elsevier.com/locate/actabiomat