Electrophoretic deposition of nanocomposite (HAp + TiO₂) on titanium alloy for biomedical applications

Abstract

This paper reports on the corrosion and scratch behavior of TiO₂ + 50%HAp nanoceramic coated Ti–13Nb–13Zr orthopedic implant alloy. An adherent thin coating was obtained using the electrophoretic deposition (EPD) technique at 30 V and sintering at 850 °C. The microstructure of the coated surfaces was characterized by optical microscopy, AFM, and SEM, and the composition of the coating was examined using EDAX. The functional groups and formed phases analyzed using FT-IR, and XRD. Further, the adhesion strength of the coatings was evaluated using scratch tester and the corrosion behavior of all samples was tested in Simulated Body Fluid (SBF-Hank's solution) using a potentiodynamic polarization studies. The sintered coating exhibited higher adhesion, lower porosity and higher density compared to unsintered samples, and higher corrosion resistance compared to the substrate. However, the corrosion resistance of the unsintered coating was superior to that of the sintered one due to the presence of minimal interconnected porosity.

Introduction

Commercially pure Ti (CP-Ti) and Ti based alloys are extensively used for orthopedic and dental prosthetic applications [1], [2], [3] because of their superior mechanical and biocompatible properties. Currently, the alpha + beta Ti alloy viz., Ti–6Al–4V is the most widely used for hip, spinal and knee replacements due to its high hardness and the CP-Ti with alpha phase for dental applications owing to its low density and high corrosion resistance. The moduli of CP–Ti (100 GPa) and Ti–6Al–4V (114 GPa) are very much higher than human bone modulus (10–30 GPa). Beta Ti alloys and near beta Ti alloys are considered to be an alternative for Ti–6Al–4V as their modulus of Elasticity (55–77 GPa) is much closer to bone when compared with the CP-Ti and Ti–6Al–4V. Among different beta alloys, the near beta alloy Ti–13Nb–12Zr is well established biomedical material for joint allocations as it exhibits low modulus and in addition, this alloy possesses highly biocompatible and non-toxic alloying elements such as Zr and Nb. However, all Ti based alloys are classified as bioinert materials as they do not induce bone formation on their surface.

It is well known that hydroxyapatite (HAp) [Ca₃(PO₄)₃OH] coatings have been employed since few years as they are found to increase the bioactivity of the implant surface due to the fact that they possess similar chemical, structural and biological properties to that of the human bone, which in turn promotes osseointegration [4], [5]. The major advantage associated with nano HAp coatings is that the nanoparticles are similar to that of inorganic molecules of the human bone.

However, coating with HAp alone has some disadvantages such as (i) low melting point of phosphorous in the coating causing bioactive degradation of the coating [6] (ii) the HAp coated implants giving a better pinning to the bone compared with that of the adhesion between coating and the implant which will ultimately lead to loosening and failure of the implant. To overcome these problems, bioactive TiO₂ powders were added to HAp for improving bioactivity and adherence of the coating to the implant [6]. Koike et al. also have reported that TiO₂/HAp composite coatings on Ti–6Al–4V produced by plasma spraying technique led to increased corrosion resistance [7]. Further, Webster et al. showed that nanophase TiO₂ promotes osteoblastic adhesion compared to conventional powders [8]. The addition of nano TiO₂ to nano HAp is found to increase the adhesion of the coating with the substrate and also with the bone.

HAp/TiO₂ composite coatings on orthopedic biomaterial substrates have been obtained using several techniques such as sol–gel [9], [10], micro arc oxidation [11], and plasma spraying [12]. Among these techniques, the plasma spraying technique has some drawbacks, as coatings obtained by this method can be easily detached from the surfaces or resorbed into the body environment because of their unstable characteristics such as rapid solidification, inhomogeneous composition, melted and decomposed phases, etc., [13].

Electrophoretic Deposition (EPD) has been found to be an efficient technique to make ceramic coatings from powder suspensions and it is an easier process for obtaining nanostructural deposits from colloidal solutions. The other advantages of EPD technique are that it is less time consuming, less expensive and it leads to uniform coatings and that can be obtained on any complex shapes too [14], [15]. Nathanael et al. who worked on nanocomposite coatings with 50 as well as 80 vol% of TiO₂ added to HAp reported better mechanical properties compared to HAp. It should be noted that these coatings were produced on plain glass substrates [10], whereas in this work a 1:1 HAp/TiO₂ nanocomposite coating on the low-modulus FDA-approved Ti–13Nb–13Zr (ASTM F 1713–08) biomedical alloy was produced, hence these studies should be more relevant.