Dip Coating of Nano Hydroxyapatite on Titanium Alloy with Plasma Assisted g-Alumina Buffer Layer: A Novel Coating Approach M. Khalid 1)* , M. Mujahid 1) , A. Nusair Khan 2) , R.S. Rawat 3) 1) School of Chemical and Materials Engineering, National University of Sciences and Technology, Sector H-12, Islamabad, Pakistan 2) Institute of Industrial Control Systems, Rawalpindi, Pakistan 3) NSSE, National Institute of Education, Nanyang Technological University, Singapore [Manuscript received May 3, 2012, in revised form August 3, 2012, Available online 9 February 2013] This paper reported a novel coating approach to deposit a thin, crack free and nano-structured hydroxyapatite (HA) film on Ti6Al4V alloy with Al 2 O 3 buffer layer for biomedical implants. The Al 2 O 3 buffer layer was deposited by plasma spraying while the HA top layer was applied by dip coating technique. The X-ray diffraction (XRD) and Raman reflections of alumina buffer layer showed a- to g-Al 2 O 3 phase transformation; and the fractographic analysis of the sample revealed the formation of columnar grains in well melted splats. The bonding strength between Al 2 O 3 coating and Ti6Al4V substrate was estimated to be about 40 MPa. The presence of dip coated HA layer was confirmed using XRD, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis. The SEM images exhibited that HA top layer enveloped homogenously the troughs and crests of the underneath rough (R a ¼ 2.91 mm) Al 2 O 3 surface. It is believed that the novel coating approach adopted might render the implant suitable for rapid cement-less fixation as well as biocompatible for longer periods. KEY WORDS: Ti6Al4V alloy; Biomaterials; Buffer layer; Ceramic coatings 1. Introduction To restore the functions of diseased and broken calcified tis- sues, like our bones and teeth, orthopedic and dental implants are commercially employed. The successful fixation of these im- plants in our body depends upon the formation of a stable and strong interface between the implant and the bone [1] . Hydroxy- apatite (HA) is chemically similar to the mineral part of human bone [2,3] , and is known for its bioactivity (ability to form a physiochemical bond with the surrounding bone) [4e6] . How- ever, it is mechanically weak and brittle [5] to be used in load bearing applications such as orthopedic endoprostheses. On the other hand, the titanium alloy (Ti6Al4V) is a light weight, cor- rosion resistant and high fracture toughness material but is not bioactive [4] ; susceptible to the formation of a fibrous layer be- tween the bone and the implant that may lead to implant loos- ening and ultimately its failure [1] . The application of HA coating on Ti alloy implants offers a possibility to combine the strength and ductility of a metal and the bioactivity of a ceramic like HA. The methods employed for the coating of HA include plasma spraying, dip coating, electrophoretic deposition, sputtering and electrochemical deposition which are reviewed elsewhere [7] . Amongst all, the plasma spraying is the most widely used pro- cess for such coatings and has advantages like fast deposition rate and sufficiently low cost [8,9] but it has certain drawbacks. In this method the particles are first melted and then solidified on the substrate surface. The similarity in coefficients of thermal expansion (CTE) of the depositing material and the substrate is important to obtain coatings with good adhesive strength. Since HA and Ti have quite different CTEs of about 11  10 6 e 15  10 6 K 1 and 8  10 6 e10  10 6 K 1 , respec- tively [10,11] , a strong bonding between them is not expected. Moreover, the HA coatings produced by plasma spraying have undesirable phases (such as CaO and tetracalcium phosphate (TTCP)) in varied proportions depending upon the temperature and velocity of the impinging particles [12] . In the present study, an interlayer of alumina was inserted between the HA top layer and Ti6Al4V substrate. The alumina and Ti6Al4V alloy have similar CTEs, 8.7  10 6 K 1 and 8.5  10 6 K 1 , respectively [5,13] . Hence, a strong interfacial bond is envisaged. The adhesion between the alumina coating and Ti has been reported to be improved when they were heat * Corresponding author. Tel.: þ92 333 5614466; E-mail address: semelkhalid@hotmail.com (M. Khalid). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.02.003 Available online at SciVerse ScienceDirect J. Mater. Sci. Technol., 2013, 29(6), 557e564