Development of carbon nanotubes reinforced hydroxyapatite composite coatings on titanium by electrodeposition method

Corrosion Science 73 (2013) 321–330

Contents lists available at SciVerse ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Development of carbon nanotubes reinforced hydroxyapatite composite coatings on titanium by electrodeposition method D. Gopi a,b,⇑, E. Shinyjoy a,b, M. Sekar a,c, M. Surendiran a, L. Kavitha b,c,d, T.S. Sampath Kumar e a

Department of Chemistry, Periyar University, Salem 636 011, Tamil Nadu, India Centre for Nanoscience and Nanotechnology, Periyar University, Salem 636 011, Tamil Nadu, India c Department of Physics, Periyar University, Salem 636 011, Tamil Nadu, India d The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy e Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Chennai 600 036, Tamil Nadu, India b

a r t i c l e

i n f o

Article history: Received 27 November 2012 Accepted 7 April 2013 Available online 18 April 2013 Keywords: A. Ceramic A. Titanium B. Potentiostatic B. Polarisation

a b s t r a c t Carbon nanotubes (CNTs) are outstanding reinforcement material for imparting strength and toughness to brittle hydroxyapatite (HAP). This work reports the electrodeposition of CNTs reinforced HAP on titanium substrate at 1.4 V vs. SCE during 30 min with the functionalised CNTs concentration ranging from 0 to 2 wt.%. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (EDX), high resolution transmission electron microscopy (HRTEM), mechanical and biological studies were used to characterise the coatings. Also, the corrosion resistance of the coatings was evaluated by electrochemical techniques in simulated body fluid (SBF) solution. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The foremost requirement for any material to be placed in the human body is that it should exhibit good biocompatibility and corrosion resistance in physiological body fluid [1]. The corrosion property of the implant is significant as it can adversely affect the mechanical and biological property [2]. Hence, the metal that is to be used as implant must possess high corrosion resistance and better biocompatibility. Till today titanium is the most commonly used biometal to orthopedic prostheses due to its excellent corrosion resistance and biocompatibility [3]. In order to enhance the cell implant material interaction and to increase the longevity of material, bioactive ceramic based coating have been applied to Ti [4]. HAP is an attractive biomaterial for human hard tissue implants since it contains the similar chemical composition of the natural bones and teeth [5,6]. HAP plays an excellent role in biomedical applications owing to their excellent biocompatible, osteoconductive and bioactive properties, and its close resemblance to mineral component of bone tissue [7–13]. Though HAP can bond directly to natural bones, the brittle nature and poor strength impedes its clinical applications under load-bearing conditions. One of the most common approaches to overcome this

⇑ Corresponding author at: Department of Chemistry, Periyar University, Salem 636 011, Tamil Nadu, India. Fax: +91 427 2345124. E-mail addresses: [email protected] (D. Gopi), [email protected] (L. Kavitha). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.04.021

weakness and to provide better corrosion resistance, incorporation of reinforcing materials like CNTs, TiO2, ZrO2 is followed [14–19]. CNTs are highly versatile materials with an enormous potential for biomedical applications. They are unique, one dimensional macromolecules, whose outstanding properties have sparked an abundance of research since their discovery in 1991 by Iijima [20]. They have excellent mechanical, good corrosion resistance and unique structural properties, with high aspect ratio, good biocompatibility and less toxicity that categorised them as outstanding reinforcement materials in the nanocomposites [21–24]. They also possess large surface area, low density, and high tensile strength. All these properties make CNTs a suitable material for variety of applications like high performance transistors, switches in nanoelectronic devices and incorporation in nanocomposites based on metals, ceramics, and polymers [25]. They may be used in orthopedics as mechanical reinforcement, to tailor surface properties and thus provide a nanostructured surface that promotes bone cell adhesion and function. The advantage of CNTs as a reinforcing material is that, they provide increased toughness and mechanical properties when used in composite [26]. CNTs have recently emerged as materials with exceptional properties exceeding those of any conventional material. These exceptional physical and mechanical properties make CNTs good candidates as reinforcements in composite materials to increase both stiffness and strength. Furthermore, the dispersion and structure of the nanotubes are the dominating factors for the mechanical improvement of CNTs reinforced HAP composites. CNTs as a reinforcing material could impart mechanical integrity to the composite without

322

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

diminishing the bioactivity of HAP [23,27]. Addition of CNTs can strengthen the composite coatings and the CNTs reinforced composite coatings might have excellent properties including high tensile strength and excellent bioactivity for the application in orthopedic implants. It is well known fact that, the nonfunctional CNTs have a systematic tendency to agglomerate and favors the formation of bundles. Further this aggregation tendency of CNTs is physically due to the Van der Waals forces between them and poor interaction with lack of interfacial bonding with HAP matrix due to their smooth surface [28]. Also, the non functionalised CNTs are insoluble in water and organic solvents which leads to an inhomogeneous distribution of pristine CNTs in the HAP matrix. Therefore functionalisation of CNTs is to be invoked to overcome these difficulties and to achieve a good dispersion of CNTs in HAP matrix [26]. Many functionalisation routes have been developed in recent years to solubilize CNTs and improve their biocompatibility [29]. Motivated by this we have performed the covalent functionalisation of CNTs by acid treatment [30] which resulted in an increased water solubility of CNTs in HAP matrix. Recently many methods have been developed for the coating of the CNTs–HAP composite on the metal substrate. Balani et al. [31] have reported the CNTs reinforced HAP coatings on Ti–6Al–4V alloy by plasma sprayed technique and compared the wear resistance of HAP and the CNTs–HAP composite coatings in SBF solution. But the long term stability of the CNTs–HAP composite coating cannot be achieved by this method because of its high dissolution rate and poor fracture toughness. Aerosol deposition of the CNTs–HAP composite coatings on titanium plate was evaluated by Hahn et al. [32]. From their study, it was observed that nanoindentation tests improved the mechanical properties of CNTs reinforced HAP composite coating. Pei et al. [33] reported the functionally graded CNTs–HAP composite coating by laser cladding method. These methods suffer from complex and time consuming procedures. Although many methods reported for the CNTs–HAP composites coating, invariably each method suffers from one to another. Gopi et al., has reported the coating of HAP on borate passivated 316L SS by dip coating and electrodeposition method and thereby reduced the release of metal ions [34,35]. The authors have also studied the combination of pulsed electrodeposition and addition of H2O2 into the electrolyte that promisingly improve the physicochemical properties of HAP [36]. In the recent years, electrodeposition has evolved as a successful technique for the HAP coatings over implantable materials [37,38]. In the present work we have achieved CNTs reinforced HAP composite coatings on titanium substrate using electrodeposition method with the aim of improving the mechanical, corrosion resistive and biological properties of HAP coatings. The mechanical properties of the CNTs–HAP composite coatings were measured by mean of nanoindentation technique as a function of CNTs concentrations (0–2 wt.%). Moreover, the effect of CNTs concentrations on the crystallinity, morphology, adhesion strength, hardness, elastic modulus and in vitro biological affinity of the CNTs–HAP composite coating on titanium for orthopedic applications was also analysed. The corrosion protection performance of the composite coatings on titanium in SBF solution was studied by different electrochemical techniques. 2. Materials and methods 2.1. Materials Commercially available calcium nitrate (Ca(NO3)24H2O), dipotassium hydrogen phosphate (K2HPO4) and single-walled CNTs with a diameter of 1.2–1.5 nm, purchased from Aldrich chemicals (Aldrich, India) were used for fabricating CNTs reinforced HAP composite coatings by electrodeposition method. All the chemicals

were of analytical grade and used as received and deionised water was used throughout the experiment. 2.2. Functionalisation of CNTs The surface treatment of CNTs with acid, oxidising agents and surfactants produces carboxylic, hydroxyl and ketonic groups which alter the properties of CNTs, such as improved dispersion stability and better interactions with HAP matrix [39,40]. The functionalisation of CNTs was carried out in a round-bottomed flask equipped with a reflux condenser and a thermometer. Before oxidation process, 4 g of raw CNTs was taken in the mixture of concentrated HNO3 and H2SO4 (200 ml) with the ratio of 1:3 at room temperature (28 ± 1 °C) and ultrasonicated during 60 min. Then, the mixture was poured into the round-bottomed flask and heated to 110 °C using an oil bath and kept refluxing during 1 h [41] and cooled to room temperature. The acid solution was filtered through a polytetrafluoroethylene membrane filter. The filtered substance was washed repeatedly with deionised water till the filtrate is neutral and finally rinsed with methanol. The final product was dried in a vacuum oven at 60 °C during 48 h. 2.3. Development of CNTs reinforced HAP composite coatings on titanium 2.3.1. Specimen preparation The pure titanium specimens (99.99%) of the size 10  10  3 mm were cut and embedded in epoxy resin, leaving area of 1 cm2 for exposure to the solution. Prior to electrodeposition, the samples were abraded with different grades of SiC emery papers from 400 to 1200 grit and washed with distilled water, degreased with acetone, then dried at room temperature. 2.3.2. Electrodeposition of HAP reinforced with CNTs on titanium The electrolyte for deposition was prepared by mixing a solution containing 0.042 mol/L of Ca(NO3)24H2O and 0.025 mol/L of K2HPO4 under constant stirring during 2 h. The different concentration (0–2 wt.%) of the functionalised CNTs were gradually added to the above solution and the pH was attuned to 4.7 using NaOH or HCl. Prior to electrodeposition, the electrolyte mixture was subjected to an intense ultrasonic treatment during 30 min to fully disperse the suspended CNTs into the electrolyte. The electrodeposition was performed in an individual cell using a three electrode configuration in which titanium served as cathode and platinum electrode acts as an anode. A saturated calomel electrode (SCE) was used as the reference electrode. The deposition was carried out in potentiostatic mode by applying a potential of 1.4 V vs. SCE during 1 h using an electrochemical system CHI 760C (CH instruments, USA). After the specimen coated with composite coating they were rinsed with deionised water and dried at 40 °C during 5 h. 2.4. Surface characterisation of the composite coatings The Fourier transform infrared spectroscopy of the samples were recorded using Nicolet 380 FT-IR Spectrometer (Perkin Elmer, USA) over the frequency range from 4000 cm1 to 400 cm1 with a number of 32 scans and spectral resolution of 4 cm1. The phase composition and the crystallinity of the CNTs–HAP composite coatings were identified by XRD (Bruker D8 advance diffractometer). For the XRD experiments, Cu Ka incident radiation, a tube voltage of 40 kV and a current of 30 mA was used and the scanning angle is ranged from 20° to 60°, with a scan rate (2h) of 0.02°. The surface morphology and elemental composition of the composite coatings were examined using SEM (JEOL JSM-6400, Japan) equipped with EDX. The microstructure of the composite coating was characterised using high resolution transmission electron microscopy

323

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

(HRTEM-JEOL JEM 2100 Co., Tokyo, Japan). The samples for HRTEM analysis was prepared by scraping the CNTs–HAP composite coating from the titanium substrate and dispersing in ethanol, followed by sonication step of 10 min. Then a drop of suspension was deposited on copper coated carbon grid with 200 meshes and the solvent was allowed to evaporate. 2.5. Electrochemical evaluation of the composite coatings The electrochemical measurements such as open circuit potential (OCP), potentiodynamic polarisation and electrochemical impedance spectroscopic (EIS) studies were carried out to access the anticorrosive characteristics of the coatings. For this purpose a conventional three electrode cell assembly was used. A saturated calomel electrode (SCE), platinum and titanium specimens were used as the reference, counter and working electrode, respectively. The SBF solution with ion concentration nearly equal to the human blood was used as electrolyte medium [42]. This solution was prepared according to the protocol suggested by Kokubo et al. (see Table 1). The OCP measurements were monitored with the exposure time of 1000 h and their monitoring will continue for the long term period of 5 years considering the fact that human body may prone to unexpected situations. Potentiodynamic polarisation studies were measured at a scan rate of 1 mV/s in the potential range between 1 and 1 V vs. SCE. The electrochemical impedance spectroscopic measurements were obtained at an OCP condition and the spectra were acquired in a frequency range of 101 Hz to 105 Hz with perturbation amplitude of 5 mV. The obtained data was recorded using internally available software and the each experiment was repeated three times to check the reproducibility. 2.6. Mechanical properties of the composite coatings

2.7. In vitro biocompatibility studies 2.7.1. Cell culture Mouse fibroblasts (L929) cells were obtained from National Centre for Cell Science (NCCS), Pune, India and were cultured in Table 1 Various reagents and composition for preparing 1000 ml of SBF solution.

1 2 3 4 5 6 7 8 9 10

2.7.2. Cell viability test Cell viability was determined using MTT (3-(4,5-dimethyl-2yl)-2,5-diphenyltetrozolium bromide) assay [44]. Cells with the density of 1  105/wells were seeded on the samples in 24 well culture plates. The culture medium was replaced with new medium every day. After the incubation period (48 h) the samples were removed from the respective wells and the wells were washed with phosphate buffered saline (pH = 7.4). Only those cells that are adherent to the well walls were found viable and incubated with 0.5% MTT solution. The viable cells reduce the MTT into insoluble formazan precipitate by mitochondrial succinic dehydrogenase. After 4 h incubation, 0.1% dimethyl sulfoxide (DMSO) was added to dissolve the formazan crystals. After these procedures, the absorbance of the content of each well was determined at 570 nm with a UV-spectrophotometer. Cell viability (%) related to the control wells containing cell culture medium without the samples was calculated based on the average of five replicates using the following equation:

%Cell viability ¼

½A570test ½A570control

3. Results and discussion

The adhesion strength between the composite coating and substrate was evaluated by the standard scratch tester and indentation using a Universal Instron Mechanical Testing system (Instron 5565, Instron Co.) according to ASTM F 1044-05 standard [43]. The specimens were subjected to tests at a constant cross-head speed. Nanoindentation tests were performed by an MTS Nanoindenter XP (MTS Corporation, nanoinstruments innovation centre, TN). A three-sided pyramid (Berkovich) diamond indenter was employed for the indentation experiments. In this test the indenter was pressed into the composite film with a constant strain rate of 0.05 s1. Twelve indentations were made during each test on each sample. The load-indentation depth profiles were recorded automatically during the indentation.

Order

minimal essential media (Hi Media Laboratories) supplemented with 10% Fetal Bovine Serum (FBS), Streptomycin (100 U/mL) and Penicillin (100 U/mL) (Cistron laboratories). The cell culture was then incubated under the humidified atmosphere (CO2) at 37 °C. The samples (uncoated and at different concentrations of CNTs (0, 0.1, 1, 2 wt.%) in the CNTs–HAP composite coated titanium) under examinations were sterilised in autoclave at 120 °C during 2 h and placed in 24 well cell culture plates.

Reagent

Composition

NaCl NaHCO3 KCl K2HPO43H2O MgCl26H2O 1.0 M HCl CaCl2 Na2SO4 Tris: ((OHCH2)3CNH2) Tris-hydroxymethyl aminomethane 1.0 M HCl

8.035 g 0.355 g 0.225 g 0.231 g 0.311 g 39.0 ml 0.292 g 0.072 g 6.118 g 0–5 ml

3.1. Surface characterisation 3.1.1. FT-IR spectra Fig. 1a–c illustrates the FT-IR spectra of functionalised CNTs, HAP and 1 wt.% CNTs–HAP composite coating on titanium. Fig. 1a represents the FT-IR spectrum for the oxidised CNTs and their corresponding characteristic groups such as AOH, AC@O and ACAO are observed at 3500, 1750 and 1094.76 cm1, respectively [45]. These negatively charged functional groups activate CNTs surface which attracts Ca2+ ion and helps to produce the arrival of HPO2 4 in the formation process of HAP coating on titanium surface. Fig. 1b shows the FT-IR spectrum of HAP coating on titanium where the peaks at 3446.13 and 1640.77 cm1 are due to the stretching and bending modes of absorbed water. The stretching and bending vibrational modes of hydroxyl (AOH) group were identified at 3565.12 and 634.23 cm1, respectively. The characteristic absorption peaks for phosphate group of HAP are observed at 560.13 and 604.15 cm1 (ˆ4), 1050.82 and 1099.33 cm1 (ˆ3) and 962.05 cm1 (ˆ1) which are assigned to P–O bending, asymmetric P–O stretching and symmetric P–O stretching vibrations, respectively. A peak at 1384.04 cm1 in Fig. 1c was identified as a strong absorption peak which is associated with the interaction of Ca2+ (in HAP) with COO group of oxidised CNTs. Thus, all these peaks confirm the formation of the CNTs–HAP composite coatings on titanium. 3.1.2. XRD analysis Fig. 2a–d corresponds to the XRD patterns at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in the CNTs–HAP composite coated titanium. Fig. 2a depicts the XRD pattern of HAP without CNTs content. The diffraction peak (2h) values of 25.88°, 31.77°, 32.2° and 32.9° were assigned to (0 0 2), (2 1 1), (1 1 2) and (3 0 0) planes of HAP, respectively. The remaining peaks at 46.71°, 49.4°

324

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

Fig. 1. FT-IR spectra of: (a) functionalised CNTs, (b) HAP, and (c) 1 wt.% CNTs–HAP composite coating on titanium.

and 53.14° are attributed to (2 2 2), (2 1 3) and (0 0 4) planes of HAP, respectively. All these peaks confirm the presence of HAP and are well consistent with International Centre for Diffraction Data (ICDD card No. 09-0432). A high intensity diffraction peak assigned to a graphite crystallographic (0 0 2) plane of CNTs is observed in Fig. 2b–d at 26.37° [46]. These results are in good agreement with ICDD card No. 41-1487. From this XRD pattern it is concluded that, as the CNTs concentration in the CNTs–HAP composite increases, the peaks become sharper and more intense thus suggesting the high crystallinity. 3.1.3. Morphological characterisation Fig. 3a–d represents the SEM micrographs at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in the CNTs–HAP composite coated titanium. The coating structure exhibited different crystal morphology at different concentrations of CNTs in the CNTs–HAP composite. Fig. 3a shows a non uniform dispersion of HAP matrix with the existence of aggregation on the surface of titanium. The coating morphology of 0.1 wt.% CNTs–HAP shows the formation of crystal nuclei which gradually grow into plate-like structure (Fig. 3b). In the addition of 1 wt.% CNTs, slanting and perpendicular flakes may develop covering the entire surface of titanium (Fig. 3c). On further concentration increases of CNTs to 2 wt.%, these flake-like structure grows outwardly and disorderly forming macro porous structure over the surface that paves the way for the detachment of the composite coating from the surface of titanium (Fig. 3d). Moreover, the increased crystallinity of the CNTs–HAP composite coating was observed and is also evident from the XRD results. It may be better to have the flake like structured coating for bone growth, since the inorganic apatite in the bone has plate shaped morphology. The cross-sectional SEM image of the

Fig. 2. XRD patterns at different concentrations of CNTs in the CNTs–HAP composite coating on titanium: (a) 0 wt.%, (b) 0.1 wt.%, (c) 1 wt.%, and (d) 2 wt.%.

1 wt.% CNTs–HAP composite coated titanium is shown in Fig. 3e. The coating is found to be compact and dense with the thickness of about 3 lm. The EDX spectra of showing the constituent elements of the HAP and the CNTs–HAP composite coated on titanium are presented in Fig. 4a and b. The spectrum for HAP shows the presence of Ca, P and O. The corresponding spectrum for the CNTs–HAP composite coating indicates the presence of C, O, P and Ca. This result confirms the existence of CNTs and Ca/P (HAP) on the surface of titanium. To further confirm the presence of CNTs in the CNTs–HAP composite coating, high resolution TEM analysis was performed and the micrograph is shown in Fig. 5. The tubular structure observed in the figure represents the presence of CNTs in the composite coating. Moreover, the spherical particles of HAP closely attached to the surface of CNTs were found. According to the above results, the CNTs obtained by electrodeposition method act as the reinforcing material in the HAP matrix.

3.2. Electrochemical characterisation 3.2.1. Open circuit potential measurements One of the ways to study the corrosion behavior of the CNTs– HAP composite coated titanium for different concentrations of CNTs is to check the OCP as a function of time. The potential-time measurements of various samples in SBF solution are shown in Fig. 6 [Inset: OCP plots for the immersion time of 1 h]. The OCP values of the samples extracted from the curves are given in Table 2.

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

325

Fig. 3. SEM micrographs at different concentrations of CNTs in the CNTs–HAP composite coating on titanium: (a) 0 wt.%, (b) 0.1 wt.%, (c) 1 wt.%, (d) 2 wt.%, and (e) cross sectional appearance of 1 wt.% CNTs–HAP composite coating on titanium.

Fig. 4. EDX spectra of: (a) HAP, and (b) 1 wt.% of CNTs–HAP composite coating on titanium.

From the table it was observed that the OCP of uncoated titanium (OCP = 0.66 ± 0.012 V vs. SCE) showed a slight shift of potential in the active direction during the initial period of time and then steady state potential is attained. Whereas, for all the coated samples the OCP values showed a shift of potential in the noble direction and then steady state potential is attained after a few minutes. The OCP value of 0 wt.% CNTs–HAP coated titanium was found to

be 0.44 ± 0.005 V vs. SCE, whereas for 0.1 and 1 wt.% CNTs–HAP composite coated titanium the values were found to be 0.41 ± 0.004 and 0.17 ± 0.007 V vs. SCE, respectively. Compared to all the other coated samples, OCP of 1 wt.% CNTs–HAP coated titanium showed an immediate shift of potential towards noble direction thus attained the steady state potential. This shift of potential towards the noble direction reveals the improved corrosion

326

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

resistance of the CNTs–HAP composite coatings on titanium. As the concentration of CNTs in the CNTs–HAP composite is increased further to 2 wt.%, the OCP value becomes 0.19 ± 0.003 V vs. SCE and this observed potential is more negative than the OCP value measured for 1 wt.% CNTs–HAP coated titanium. Thus, for 1 wt.% CNTs– HAP composite coated titanium, the OCP value increases towards more noble direction because of the corrosion protective nature of the composite coating.

the CNTs–HAP composite coated titanium were performed to assess the corrosion resistance in SBF solution (Fig. 7). The polarisation parameters like corrosion potential (Ecorr) and corrosion current (icorr) obtained from the curves are presented in Table 2. Compared with the uncoated titanium, all the coated titanium showed a drastic shift of corrosion potential in the anodic region thus suggesting the improved corrosion resistance of the coated titanium. The 0 wt.% CNTs–HAP coated titanium had the Ecorr value of 0.09 ± 0.003 V vs. SCE which indicates the slight shift of potential in the noble direction respect to the uncoated sample (Ecorr = 0.57 ± 0.005 V vs. SCE). The Ecorr values for 0.1 wt.% CNTs–HAP and 1 wt.% CNTs–HAP coated titanium samples were found to be 0.02 ± 0.007 and 0.02 ± 0.003 V vs. SCE, respectively. A more positive shift in OCP value was observed for the 1 wt.% CNTs–HAP coated titanium respect to 0 wt.% CNTs–HAP coated and uncoated titanium. However, a negative shift in the OCP value was observed when the concentration of CNTs in the CNTs–HAP composite reaches 2 wt.%. This may lead to the detachment of coating over the surface of titanium and thereby indicating the least corrosion performance of the composite coating. The icorr of 1 wt.% CNTs–HAP composite coated titanium showed the lowest value of 0.05 ± 0.006 lA/cm2, whereas icorr value of uncoated titanium was found to be 0.54 ± 0.003 lA/cm2. The lowest icorr value indicates the well dispersed structure of CNTs and coherence between CNTs and HAP in the CNTs–HAP composite. Based on the polarisation results, the bio-corrosion resistance of the samples is ranked below: 1 wt.% CNTs–HAP > 2 wt.% CNTs–HAP > 0.1 wt.% CNTs– HAP > 0 wt.% CNTs–HAP > Uncoated titanium. From the obtained results it could be determined that the CNTs–HAP composite coating exhibited much better corrosion resistance property than the HAP coated and uncoated titanium.

3.2.2. Potentiodynamic polarisation measurements Potentiodynamic polarisation measurements for the uncoated and at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in

3.2.3. Electrochemical impedance studies The impedance spectra for uncoated and at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in the CNTs–HAP composite

Fig. 5. HRTEM image of 1 wt.% CNTs–HAP composite coating.

Fig. 6. OCP time plots obtained in SBF solution for uncoated titanium (a) and at different concentrations of CNTs in the CNTs–HAP composite coated titanium: (b) 0 wt.%, (c) 0.1 wt.%, (d) 1 wt.%, and (e) 2 wt.%. [Inset: OCP plots during 1 h immersion time].

327

D. Gopi et al. / Corrosion Science 73 (2013) 321–330 Table 2 Electrochemical parameters obtained for uncoated and at different concentrations of CNTs in the CNTs–HAP composite coated titanium. Samples

OCP (V vs. SCE)

Ecorr (V vs. SCE)

icorr (lA/cm2)

Rp (kO cm2)

|Z| (kO cm2)

Uncoated titanium 0 wt.% CNTs–HAP coated titanium 0.1 wt.% CNTs–HAP coated titanium 1 wt.% CNTs–HAP coated titanium 2 wt.% CNTs–HAP coated titanium

0.66 ± 0.012 0.44 ± 0.005 0.41 ± 0.004 0.17 ± 0.007 0.19 ± 0.003

0.57 ± 0.005 0.09 ± 0.003 0.02 ± 0.007 0.02 ± 0.003 0.01 ± 0.002

0.54 ± 0.003 0.09 ± 0.005 0.08 ± 0.003 0.05 ± 0.006 0.06 ± 0.004

52.4 ± 0.03 65.2 ± 0.05 112.6 ± 0.07 160.5 ± 0.05 131.1 ± 0.04

65.0 ± 0.03 80.0 ± 0.05 140 ± 0.07 200 ± 0.05 160 ± 0.04

Fig. 7. Potentiodynamic polarisation curves in SBF solution registered at a scan rate of 1 mV/s for uncoated titanium (a) and at different concentrations of CNTs in CNTs–HAP composite coated titanium: (b) 0 wt.%, (c) 0.1 wt.%, (d) 1 wt.%, and (e) 2 wt.%.

coated titanium in SBF solution was obtained at OCP condition. The experimental data represented as Nyquist plots (Fig. 8a), Bode impedance plot and Bode phase plot (Fig. 8b and c) were fitted using the equivalent circuit as shown in Fig. 9a and b, respectively. The circuit for uncoated titanium in which Rs represents the solution resistance, Rp1 represents the polarisation resistance and Cdl1 represents double layer capacitance, is shown in Fig. 9a. Rp2 and Cdl2 in the second subsystem of circuit correspond to resistance and capacitances for the coated titanium (Fig. 9b). For the 0 wt.% CNTs–HAP coated titanium, the Rp value was found to be 65.2 ± 0.05 kO cm2 which showed higher corrosion resistance than the uncoated titanium (Rp = 52.4 ± 0.03 kO cm2). The Rp values of titanium coated with the CNTs–HAP composite containing different concentrations of CNTs (0.1, 1 and 2 wt.%) are 112.6 ± 0.07, 160.5 ± 0.05 and 131.1 ± 0.04 kO cm2, respectively. The maximum calculated Rp value (160.5 ± 0.05 kO cm2) was observed for 1 wt.% CNTs–HAP composite. Thus, the higher impedance values of the 1 wt.% CNTs–HAP composite coating suggest that the coating hinders the ion diffusion process and thereby enhance the corrosion resistance of titanium. Similarly, EIS spectra in the form of Bode impedance plot and Bode phase plot is also given for the uncoated and at different concentrations of the CNTs–HAP composite coated titanium (Fig. 8b and c). These plot clearly indicate a higher absolute impedance value of 200 ± 0.05 kO cm2 for 1 wt.% CNTs–HAP composite coated titanium. A shift in the phase angle value at the low frequency range was observed for uncoated titanium. The inhomogeneous coating show less absolute impedance value respect to all the other coated samples. The Bode phase plot (Fig. 8c) also confirms that 1 wt.% CNTs–HAP composite coated titanium posses high resistance in SBF solution. The higher phase angle is due to the

Fig. 8. Nyquist plot (a), Bode impedance plot (b), and Bode phase plot (c) for uncoated titanium and at different concentrations of CNTs in the CNTs–HAP composite coated titanium.

corrosion protective coating of 1 wt.% CNTs–HAP on titanium. Thus, the CNTs–HAP composite coated titanium serves as an effective barrier against corrosion attack in the aggressive environment.

328

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

Fig. 9. Equivalent circuit obtained for: (a) uncoated titanium, and (b) electrodeposited HAP and CNTs reinforced HAP on titanium.

3.3. Mechanical characterisation 3.3.1. Adhesion strength The adhesion strength at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in the CNTs–HAP composite coated titanium is based on the combination of adhesive and cohesive strength of the composite coating. The cohesive strength depends on the crystallinity and microstructure of the coating but the adhesive strength is based on the surface roughness and coating properties. The adhesion strength of the coating is measured by the Adhesion Test method (ASTM F1044-05) [43]. As the concentration of CNTs in the CNTs–HAP composite is increased, the coating becomes more crystalline and this is evident from the XRD and SEM results. From Table 3, it is observed that there is no significant difference in the adhesion strength of 0 wt.% CNTs–HAP and 0.1 wt.% CNTs–HAP composite coating. However, there is an appreciable improvement in the adhesion strength when the amount of CNTs in HAP composite is enhanced to 1 wt.% (24.2 ± 0.8 MPa) which is even higher than the adhesion strength of 2 wt.% CNTs in the CNTs–HAP (22.4 ± 0.6 MPa) composite coating on titanium substrate. These results were similar to the values reported for Hahn et al. [32], where the adhesion strength values of HAP, 1 and 3 wt.% CNTs– HAP were 28.5 ± 1.7, 29 ± 1.1 and 27.3 ± 1 MPa, respectively. In our present study we have observed that, for 2 wt.% CNTs–HAP, the cohesive strength is slightly decreased due to the crystalline nature of the composite coating and this may lead to the detachment of the coating over the titanium substrate.

3.3.2. Hardness and elastic modulus Mechanical properties like hardness and elastic modulus at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in the Table 3 Adhesion strength, hardness and elastic modulus values of the CNTs–HAP composite coating on titanium for different concentrations of CNTs in the composite. Samples

0 wt.% CNTs–HAP

Adhesion strength (MPa) Hardness (GPa) Elastic modulus (MPa)

18.1 ± 1.5

0.1 wt.% CNTs–HAP 19.0 ± 1.2

1 wt.% CNTs–HAP

2 wt.% CNTs– HAP

24.2 ± 0.8

22.4 ± 0.6

6.72 ± 0.45 125.7 ± 4.0

7.22 ± 0.52 130.50 ± 5.24

Fig. 10. Cell viability for control (a), uncoated titanium (b) and at different concentrations of CNTs in the composite: (c) 0 wt.%, (d) 0.1 wt.%, (e) 1 wt.%, and (f) 2 wt.%.

CNTs–HAP composite coated titanium were evaluated by nanoindentation test. The indentation depth profile was 0.5 lm. Hardness and elastic modulus were tabulated as a function of the CNTs contents in the CNTs–HAP composite coating in Table 3, respectively. It is observed from the tables that both hardness and elastic modulus of the CNTs–HAP composite coatings on titanium increase with increasing CNTs contents in the composite. Recent studies indicate that the mechanical property of HAP can be improved by the development of CNTs reinforced HAP composite [47]. The highest hardness (7.22 ± 0.52 GPa) and elastic modulus (130.50 ± 5.24 MPa) value is found for the 2 wt.% of the CNTs– HAP composite coating. These results are similar to the values obtained by Hahn et al. [32], who has reported a hardness of 9.02 GPa and an elastic modulus of 137.05 GPa for 3 wt.% CNTs–HAP composite coating, which were higher than the corresponding values of HAP coating.

3.3.3. In vitro cytotoxicity studies with L929 mouse fibroblast cells CNTs reinforced HAP coating on titanium showed enhanced cell viability (Fig. 10) than the 0 wt.% CNTs–HAP coated and uncoated titanium. The absorbance at 570 nm is directly proportional to the number of living cells in the culture. The cell viability is calculated for the uncoated and at different concentrations of CNTs (0, 0.1, 1 and 2 wt.%) in the CNTs–HAP composite coated titanium with respect to control. The uncoated titanium showed a reduced viability (40.4%.) indicating the toxic nature. The 0 wt.% CNTs– HAP coated specimen showed 46.6% viability, whereas, viability of cells was enhanced for the different concentration of the CNTs–HAP coated samples as can be seen in Fig. 10. A cell viability of 72.5% was obtained for 1 wt.% CNTs–HAP coated titanium, indicating the less/non-toxic nature of this composite respect to uncoated and the other coated titanium. As the amount of CNTs is increased further to 2 wt.% no significant change was observed. The viability of the CNTs–HAP composite coated samples is due to the presence of CNTs in the CNTs–HAP composite coating.

4. Conclusions 6.12 ± 0.56 115 ± 5.6

6.54 ± 0.43 121.42 ± 3.2

The functionalised CNTs reinforced HAP composite coating was successfully developed on the surface of titanium by electrodeposition method. The structural, morphological, electrochemical corrosion, mechanical and biological results exhibited improved

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

properties of CNTs reinforced HAP composite coating on titanium. The following observations were made. 1. It is obvious from the study that the CNTs–HAP composite coated titanium provide very good anti-corrosion resistance in SBF solution. 2. The SEM morphology of 1 wt.% CNTs–HAP composite coating is found to be uniform and adherent to the surface of titanium. 3. The tubular structure in the HRTEM micrograph confirms the presence of CNTs in the CNTs–HAP composite coating. 4. The electrochemical results confirm that the CNTs reinforced HAP composite coating provides an efficient corrosion protection of titanium substrate in SBF solution according to the low corrosion current density value. 5. The nanoindentation results confirm that the adhesive strength of CNTs reinforced HAP composite coating is increased with the incorporation of CNTs (until reach a concentration of 1 wt.%). It is also evident from the mechanical studies that the hardness and elastic moduli of CNTs reinforced HAP composite coatings. 6. The biological studies clarified that the CNTs plays an active role in the enhancement of cell viability of the CNTs–HAP composite coating on titanium. 7. The improved corrosion resistance and bio-mechanical properties of the CNTs–HAP composite coated titanium substrate will serve as a promising alternative for orthopedic and dental applications.

Acknowledgements The authors thank the unknown reviewers for their strenuous effort in bringing out the quality of the manuscript. One of the authors D. Gopi acknowledges the major financial support from the Indian Council of Medical Research (ICMR, IRIS ID No. 201008660, Ref. No.: 5/20/11(Bio)/10-NCD-I), Department of Science and Technology, New Delhi, India (DST-SERC, Ref. No.: SR/FTP/ ETA-04/2009 and DST-TSD, Ref. No.: DST/TSG/NTS/2011/73), CSIR, Ref. No: 01(2547)/11/EMR-II, Dated: 12.12.2011) and University Grants Commission (UGC, Ref. No.: 40-64/2011(SR)) in the form of major research projects. Also L. Kavitha acknowledges the financial support from ICTP, Italy in the form of Junior Associateship. E. Shinyjoy acknowledges the support received from Periyar University in the form of University research fellowship. References [1] C. Vasilescu, S.I. Drob, E.I. Neacsu, J.C. Mirza Rosca, Surface analysis and corrosion resistance of a new titanium base alloy in simulated body fluids, Corros. Sci. 65 (2012) 431–440. [2] M. Niinomi, Mechanical properties of biomedical titanium alloys, Mater. Sci. Eng. A 243 (1998) 231–236. [3] Anawati, H. Tanigawa, H. Asoh, T. Ohno, M. Kubota, S. Ono, Electrochemical corrosion and bioactivity of titanium–hydroxyapatite composites prepared by spark plasma sintering, http://dx.doi.org/doi.org/10.1016/j.corsci.2013.01.032. [4] P. Li, P. Ducheyne, Quasi-biological apatite film induced by titanium in simulated body fluid, J. Biomed. Mater. Res. 41 (1998) 341–348. [5] M. Wang, Developing bioactive composite materials for tissue replacement, Biomaterials 24 (2003) 2133–2151. [6] D. Tadic, F. Peter, M. Epple, Continuous synthesis of amorphous carbonated apatite, Biomaterials 23 (2002) 2553–2559. [7] X.M. Li, Q.L. Feng, W.J. Wang, F.Z. Cui, Chemical characteristics and cytocompatibility of collagen-based scaffold reinforced by chitin fibers for bone tissue engineering, J. Biomed. Mater. Res. 77B (2006) 219–226. [8] T.M. Sridhar, U. Kamachi Mudali, M. Subbaiyan, Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel, Corros. Sci. 45 (2003) 237–252. [9] X.H. Liu, X.M. Li, Y.B. Fan, G.P. Zhang, D.M. Li, W. Dong, Z.Y. Sha, X.G. Yu, Q.L. Feng, F.Z. Cui, F. Watari, Repairing goat tibia segmental bone defect using scaffold cultured with mesenchymal stem cells, J. Biomed. Mater. Res. 94B (2010) 44–52. [10] X.M. Li, Q.L. Feng, Y.F. Jiao, F.Z. Cui, Collagen-based scaffolds reinforced by chitosan fibers for bone tissue engineering, Polym. Int. 54 (2005) 1034–1040.

329

[11] X.M. Li, C.A.V. Blitterswijk, Q.L. Feng, F.Z. Cui, F. Watari, The effect of calcium phosphate microstructure on bone-related cells in vitro, Biomaterials 29 (2008) 3306–3316. [12] X.M. Li, H.F. Liu, X.F. Niu, Y.B. Yan, Q.L. Feng, F.Z. Cui, F. Watari, Osteogenic differentiation of human adipose-derived stem cells induced by osteoinductive calcium phosphate ceramics, J. Biomed. Mater. Res. 97B (2011) 10–19. [13] X.M. Li, Q.L. Feng, Dynamic rheological behaviors of the bone scaffold reinforced by chitin fibres, Mater. Sci. Forum 475–479 (2005) 2387–2390. [14] T.M. Sridhar, U. Kamachi Mudali, M. Subbaiyan, Sintering atmosphere and temperature effects on hydroxyapatite coated type 316L stainless steel, Corros. Sci. 45 (2003) 2337–2359. [15] X.H. Chen, C.S. Chen, H.N. Xiao, F.Q. Cheng, G. Zhang, G.J. Yi, Corrosion behavior of carbon nanotubes–Ni composite coating, Surf. Coat. Technol. 191 (2005) 351–356. [16] B.M. Praveen, T.V. Venkatesha, Y. Arthoba Naik, K. Prashantha, Corrosion studies of carbon nanotubes–Zn composite coating, Surf. Coat. Technol. 201 (2007) 5836–5842. [17] N. Ignjatovi, S. Tomi, M. Daki, M. Miljkovi, M. Plavsi, D. Uskokovi, Synthesis and properties of hydroxyapatite/poly-L-lactide composite biomaterials, Biomaterials 20 (1999) 809–816. [18] A. Bigi, E. Boanini, S. Panzavolta, N. Roveri, Biomimetic growth of hydroxyapatite on gelatin films doped with sodium polyacrylate, Biomacromolecules 1 (2000) 752–756. [19] J.D. Hartgerink, E. Beniash, S.I. Stupp, Self-assembly and mineralization of peptide-amphiphile nanofibers, Science 294 (2001) 1684–1688. [20] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [21] P.M. Ajayan, Nanotubes from carbon, Chem. Rev. 99 (1999) 1787–1799. [22] J. Cholpek, B. Czajkowska, B. Szaraniec, F. Beguin, In vitro studies of carbon nanotubes biocompatibility, Carbon 44 (2006) 1106–1111. [23] R.L. Price, M.C. Waid, K.M. Haberstroh, T.J. Webster, Selective bone cell adhesion on formulations containing carbon nanofibers, Biomaterials 24 (2003) 1877–1887. [24] A. Peigney, Composite materials: tougher ceramics with nanotubes, Nat. Mater. 2 (2003) 15–16. [25] A.R. Kamali, C. Schwandt, D.J. Fray, On the oxidation of electrolytic carbon nanomaterials, Corros. Sci. 54 (2012) 307–313. [26] Y. Chen, C.H. Gan, T.N. Zhang, G. Yu, P.C. Bai, A. Kaplan, Laser-surface-alloyed carbon nanotubes reinforced hydroxyapatite composite coatings, Appl. Phys. Lett. 86 (2005) 251905–251913. [27] L.P. Zanello, B. Zhao, H. Hu, R.C. Haddon, Bone cell proliferation on carbon nanotubes, Nano Lett. 6 (2006) 562–567. [28] H.J. Lee, S.J. Oh, J.Y. Choi, J.W. Kim, J. Han, L.S. Tan, J.B. Back, Microwaveassisted covalent sidewall functionalisation of multiwalled carbon nanotubes, Chem. Mater. 17 (2005) 5057–5064. [29] U.S. Shin, I.K. Yoon, G.S. Lee, W.C. Jang, J.C. Knowles, H.W. Kim, Carbon nanotubes in nanocomposites and hybrids with hydroxyapatite for bone replacements, J. Tissue Eng. (2011), http://dx.doi.org/10.4061/2011/674287. [30] O.K. Park, N.H. Kim, G.H. Yoo, K.Y. Rhee, J.H. Lee, Effect of the surface treatment on the properties of polyaniline coated carbon nanotubes/epoxy composites, Composites: Part B 41 (2010) 2–7. [31] K. Balani, R. Anderson, T. Laha, M. Andara, J. Tercero, E. Crumpler, E. Agarwal, Plasma-sprayed carbon nanotube reinforced hydroxyapatite coatings and their interaction with human osteoblasts in vitro, Biomaterials 28 (2007) 618–624. [32] B.D. Hahn, J.M. Lee, D.S. Park, J.J. Choi, J. Ryu, W.H. Yoon, B.K. Lee, D.S. Shin, H.E. Kim, Mechanical and in vitro biological performances of hydroxyapatitecarbon nanotube composite coatings deposited on Ti by aerosol deposition, Acta Biomater. 5 (2009) 3205–3214. [33] X. Pei, J. Wang, Q. Wan, L. Kang, M. Xiao, H. Bao, Functionally graded carbon nanotubes/hydroxyapatite composite coating by laser cladding, Surf. Coat. Technol. 205 (2011) 4380–4387. [34] D. Gopi, V.C.A. Prakash, L. Kavitha, Evaluation of hydroxyapatite on borate passivated 316L SS in Ringer’s solution, Mater. Sci. Eng. C 29 (2009) 955–958. [35] D. Gopi, V.C.A. Prakash, L. Kavitha, S. Kannan, P.R. Bhalaji, E. Shinyjoy, J.M.F. Ferreira, A facile electrodeposition of hydroxyapatite onto borate passivated surgical grade stainless steel, Corros. Sci. 53 (2011) 2328–2334. [36] D. Gopi, J. Indira, L. Kavitha, A comparative study on the direct and pulsed current electrodeposition of hydroxyapatite coatings on surgical grade stainless steel, Surf. Coat. Technol. 206 (2011) 2859–2869. [37] D.O. Flamini, S.B. Saidman, Electrodeposition of polypyrrole onto NiTi and the corrosion behaviour of the coated alloy, Corros. Sci. 52 (2010) 229–234. [38] C. Vasilescu, P. Drob, E. Vasilescu, I. Demetrescu, D. Ionita, M. Prodana, S.I. Drob, Characterisation and corrosion resistance of the electrodeposited hydroxyapatite and bovine serum albumin/hydroxyapatite films on Ti–6Al– 4V–1Zr alloy surface, Corros. Sci. 53 (2011) 992–999. [39] B. Philip, J. Xie, J.K. Abraham, V.K. Varadan, Polyaniline/carbon nanotubes composites: starting with phenylamino functionalised carbon nanotubes, Polym. Bull. 53 (2005) 127–138. [40] E.N. Konyushenko, J. Stejskal, M. Trochora, J. Hradil, J. Kovarova, K. Prokes, Multiwalled carbon nanotubes coated with polyaniline, Polymer 47 (2006) 5715–5723. [41] D.C. Wu, L. Shen, J.E. Low, S.Y. Wong, X. Li, W.C. Tjiu, Y. Liu, Multi-walled carbon nanotubes/polyimide composite film fabricated through electrophoretic deposition, Polymer 51 (2010) 2155–2160. [42] T. Kokubo, H. Takafama, How useful is SBF in predicting in vivo bone bioactivity?, Biomaterials 27 (2006) 2907–2915

330

D. Gopi et al. / Corrosion Science 73 (2013) 321–330

[43] ASTM standard F 1044-05, ASTM International, West Conshohocken, PA. [44] P. Hamerli, T. Weigel, T. Groth, D. Paul, Surface properties of and cell adhesion onto allylamine-plasma-coated polyethylenterephtalate membranes, Biomaterials 24 (2003) 3989–3999. [45] K. Yang, M. Gu, Y. Guo, X. Pan, G. Mu, Effects of carbon nanotube functionalisation on the mechanical and thermal properties of epoxy composites, Carbon 47 (2009) 1723–1737.

[46] G.M. Neelgund, A. Oki, Deposition of silver nanoparticles on dendrimer functionalised multiwalled carbon nanotubes: synthesis, characterisation and antimicrobial activity, J. Nanosci. Nanotechnol. 11 (2011) 3621– 3629. [47] A.A. White, S.M. Best, I.A. Kinloch, Hydroxyapatite–carbon nanotube composites for biomedical applications: a review, Int. J. Appl. Ceram. Technol. 4 (2007) 1–13.