journal of the mechanical behavior of biomedical materials 37 (2014) 125–132
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Characterization and nanomechanical properties of novel dental implant coatings containing copper decorated-carbon nanotubes N. Sasanin, J. Vahdati Khaki, S. Mojtaba Zebarjad Department of Materials Science and Metallurgical Engineering, Engineering Faculty, Ferdowsi University of Mashhad, Mashhad, Iran
art i cle i nfo
ab st rac t
Article history:
Fluorapatite–titania coated Ti-based implants are promising for using in dental surgery for
Received 12 February 2014
restoring teeth. One of the challenges in implantology is to achieve a bioactive coating with
Received in revised form
appropriate mechanical properties. In this research, simple sol–gel method was developed
29 April 2014
for synthesis of fluorapatite–titania–carbon nanotube decorated with antibacterial agent.
Accepted 3 May 2014
Triethyl phosphate [PO4(C2H5)3], calcium nitrate [Ca(NO3)2] and ammonium fluoride (NH4F)
Available online 17 May 2014
were used as precursors under an ethanol–water based solution for fluorapatite (FA)
Keywords:
production. Titanium isopropoxide and isopropanol were used as starting materials for
Nanomechanical properties
making TiO2 sol–gels. Also, Copper acetate [Cu(C2H3O2)2 H2O] was used as precursor for
Fluorapatite
decoration of multi walled carbon nanotubes (MWCNTs) with wet chemical method. The
Titania
decorated MWCNTs (CNT(Cu)) were evaluated by transmission electron microscopy (TEM).
Copper decorated
The phase identification of the FA–TiO2–CNT(Cu) coating was carried out by XRD analysis.
Carbon nanotube
Morphology of coated samples was investigated by SEM observations. The surface elastic
Sol–gel
modulus and hardness of coatings were studied using nanoindentation technique. The results indicate that novel dental implant coating containing FA, TiO2 and copper decorated MWCNTs have proper morphological features. The results of nanoindentation test show that incorporation of CNT(Cu) in FA–TiO2 matrix can improve the nanomechanical properties of composite coating. & 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Synthetic Hydroxyapatite [HA, Ca10(PO4)6(OH)2] shows to possess exceptional biocompatibility and bioactivity properties. It has been widely used clinically in the form of powders, granules, dense and porous blocks, and coating (Cunniffe
n
Corresponding author. E-mail address: [email protected] (N. Sasani).
http://dx.doi.org/10.1016/j.jmbbm.2014.05.003 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
et al., 2010). Fluorine ion, which exists in human bone and enamel, can be incorporated into HA crystal structure by substitution of fluorine ions with OH groups to form Fluorapatite, [FA, Ca10 (PO4)6F2]. Substitution of F in HA structure plays an important role because of its influence on the physical and biological properties of HA (Nikč evic et al.,
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journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
2004). Incorporation of fluorine into HA, or “fluoridation”, reduces its solubility (Darroudi et al., 2010), increases chemical stability and promotes bone cell proliferation (Ebrahimikahrizsangi et al., 2010) while maintaining a comparable biocompatibility to that of HA (Darroudi et al., 2010). The results of in vitro showed that FA could provide better protein adsorption, comparable or better cell attachment, and significantly improved alkaline phosphates activity (Fathi and Mohammadi Zahrani, 2009). HA coated implant sometimes failed in long term stability, because the coated film deteriorated partly owing to fracture at the HA–titanium interface, due to the poor adherence between the ceramic film and the substrate. Thus, improvement of chemical bonding between the HA and Ti implant is mostly desirable particularly for clinical applications. Over the past decades, hydroxyapatite–titania (HA–TiO2) nanocomposites have been widely used as Ti-based implant coatings. The addition of TiO2 is a possible method for improvement the bonding of coated HA to Ti implants (Lim et al., 2001). Most recently, for tissue engineering applications the trend is to develop nanosized apatites with properties closer to those of living bone (Cunniffe et al., 2010). In natural tissues or organs, cells directly interact with nanostructures extracellular matrices (ECM), which are mainly composed of nanofibrous collagen fibrils. Nanomaterials, resembling the natural ECM in some features and possessing unique physiochemical properties, play a key role in stimulating cell growth as well as guide tissue regeneration (Zhang and Webster, 2009). In a biomimetic viewpoint, the three-dimensional CNTs resembles the nanofibrous network of natural ECM (Lin et al., 2004). Also, the extraordinary mechanical properties of CNTs are expected to have a contribution for improvement the poor mechanical properties of apatite coatings when used as reinforcement in novel apatite/CNT composites (Kaya et al., 2008). However, pristine CNTs tend to bundle up and are insoluble in most types of solvents, making them difficult to be used in biological systems (Worsley et al., 2009). Recently, Smart et al. (2006) reported that unrefined CNT possess some degree of toxicity (in vivo and in vitro), predominately due to the presence of transition metal catalysts, while chemically functionalized CNT applicable for drug delivery have not demonstrated any toxicity (Smart et al., 2006). Therefore CNTs need to be functionalized and purified by the removal of residual metal catalysts to improve their solubility and biocompatibility properties. The controlled functionalization and decoration of metal nanoparticles onto CNTs were done for reduction its toxicity as well as prevention of aggregation (Wang et al., 2006). The success and long-term survival of the biomaterials are also dependent on the prevention of bacterial putrefaction after implant placement. In the case of dental implants, complications derived from an implant-associated infection lead to a harm in the patient quality of life and satisfaction because it is one of the reasons for early failures by lack of osseointegration and the major cause of late failures (Chen and Darby, 2003). The antiseptic activity of antibacterial nanoparticles is influenced by their size. The smaller particles, lead to the greater antibacterial effect (Panacek et al., 2006; Chen et al., 2006) although the problems of aggregation can not be neglected. A common solution to avoid this disadvantage is
to support the nanoparticles on the surface of different substrates. Decoration of carbon nanotubes with antibacterial metals such as silver and copper are of considerable interest for their application in the biotechnology. In fact, the decoration of MWCNTs with antibacterial agent can prevent from carbon nanotubes and antibacterial nanoparticles agglomeration. The present study were investigated the preparation and characterization of FA–TiO2–CNT nanocomposite coatings containing antibacterial agent by sol–gel process. Ti–6Al–4V as substrate was subjected to a sol–gel spin coating process. The aims of this study are to investigate the role of decorated CNTs in the microstructural and nanomechanical evolution of FA-based composite coatings. This has been carried out by XRD analysis and SEM observations. The effect of CNTs modified microstructure on elastic modulus and Hardness of the composite coatings is also elucidated.
2.
Materials and methods
2.1.
Decoration of MWCNTs
Carbon nanotubes with outside and inside diameters 40– 60 nm and 10–20 nm respectively were used as starting material. Mean length of MWCNTs reported by manufacture was about 2–10 μm. Nitric acid (HNO3, Merck, PurityE64.3– 66.4%) was used for modification of surface of carbon nanotubes. 0.1 g of as-received MWCNTs were added to 50 ml of HNO3. The mixture was mixed ultrasonically for 2 h and then refluxed for 24 h at room temperature. After 24 h refluxing, the reaction mixture was diluted with distilled water and then filtered through a filter paper (Watchman, 42). Dilution was repeated until the pH of mixture reached to pH of distilled water about 6. The functionalizing process followed by drying in a vacuum oven at 90 1C for 24 h (Hiang et al., 2001). These conditions have been employed to non-covalently functionalize CNTs for binding the nanoparticles. In fact the final products may be nanotube fragments whose ends and sidewalls are decorated with mainly carboxyl groups. About 0.03 g of functionalized MWCNTs was added to 50 ml of ethanol (PurityE96%) and then the solution was stirred for 2 h. Also, the solution of copper acetate (CuAc) in ethanol with 1 g/10 ml concentration was prepared and mixed for 2 h. The mixtures was added together and mixed ultrasonically until the ethanol was evaporated completely. This process leads to impregnation of MWCNTs with copper acetate. The weltered MWCNTs then heat treated at 200 1C temperatures at air atmosphere for 30 min (Lin et al., 2012). The suitable temperature for heat treatment was determined from thermogravimetric analysis.
2.2.
Preparation of FA–TiO2–CNT (Cu) composite sol–gel
The sol–gel process started with the preparation of a TiO2 sol. Tetraethylorthotitanate (C8H20TiO4, Merck, PurityZ98%) was first diluted with isopropanol (Dr. Mojallali chemical laboratories, PurityZ99%), and then a small amount of distilled water mixed for hydrolysis, followed by vigorous stirring for 24 h. Subsequently, the mixed sol was aged for 24 h. For preparation of fluorapatite sol–gel, controlled amounts of triethyl phosphate
journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
(TEP) ([PO4(C2H5)3], Merck, PurityZ99%) and ammonium fluoride (NH4F, Merck, PurityZ95%) were first dissolved in ethanol (PurityE96%) and distilled water and then the solution was stirred for 48 h. After hydrolysis process, phosphate sol slowly was added to a solution of calcium nitrate ([Ca (NO3)2], Merck, PurityZ98%) and was allowed to mix for a 1 h at 60 1C. Ammonium hydroxide (NH4OH, Merck, PurityZ99%) was used as alkali catalyst to set pHE11. For preparation of FA–10 wt% TiO2 nanocomposite, FA and TiO2 sols were mixed together and stirred vigorously for 2 h (Ramires et al., 2001). After the preparation of FA–10 wt% TiO2 sol, 1 wt% of Cu decorated MWCNTs were added and mixed for 2 h with magnetic mixer (Najafi et al., 2009). Subsequently, composite sol was allowed to age for 24 h. After aging process, composite sols were spin coated on Ti–6Al–4V.
2.3.
Fig. 1 – XRD diffraction of MWCNT weltered by copper acetate after heat treatment at 200 1C.
Spin coating processing
A spin coating protocol has been established for coating the sol–gels onto substrates. Commercially available Ti–6Al–4V plate were cut into 10 mm 10 mm 1 mm in size using a cutting machine. All samples were polished successively using SiC grinding papers from 120 to 1200 grit size to remove original oxides and surface defects. 0.5 ml of the sol–gels was centered over the plates spinning at 3000 rpm and dispensed at a distance of 20 mm. The plates was allowed to spin for a further 30 s and then removed from the platform. The plates were dried at 80 1C then calcinated at 450 1C in an air oven with a ramp of 5 1C per minute, dwell time of 60 min and cool down rate of 10 1C per minute (Tredwin et al., 2013).
2.4.
127
Characterization techniques
The phase identification of the decorated MWCNTs, fluorapatite and composite samples was carried out by a Philips X'pert diffractometer with Cu Kα (λ¼ 1.54184 Å) radiation by the step of 0.021. X'Pert HighScore software (PANalytical B.V. Almelo, the Netherlands) used for analysis of XRD patterns. ICDD reference standard data supplied with X'Pert HighScore. The decorated MWCNTs were evaluated by transmission electron microscopy (TEM). Studies were performed in a Leo (Zeiss) 912 instrument at an acceleration voltage of 120 kV. Morphology of nanocomposites coatings was evaluated by scanning electron microscopy (SEM) (model VP 1450, LEO, Germany). Nanomechanical properties of coatings was performed by nanoindentation instrument with a constant loading/unloading rate and the peak load of 950 μN. Nanohardness (H) and elastic modulus (E) properties of composite coatings have been characterized by means of Instrumented Indentation Techniques, such as Atomic Force Microscopes (AFM) (Nanoscope II Digital Instruments CA, USA) and nanoindentation machines (Triboscopes Nanomechanical Test-Instrument, Hysitron Inc, Germany).
3.
Results and discussion
3.1.
XRD analysis
The XRD patterns for the decorated MWCNTs shown in Fig. 1. The diffraction peak at 2θ¼ 26.71 can be assigned to a
Fig. 2 – XRD pattern of FA powder synthesized at pH E11 and heat treated at 450 1C. reflection from the (002) plane of graphite in MWCNTs. The diffraction angles at 2θ E491 and 741 can be indexed to (111) and (220) planes of the cubic Cu structure, respectively, confirming the presence of Cu in the materials. Besides the peaks arising from Cu, the peaks corresponding to Cu2O appears at 2θ E29, 37 and 621, indicating that Cu2O phase was formed on the surface of MWCNTs. Also, diffraction peaks of CuO can be observed at 2θE32, 35 and 391. X-ray diffraction (XRD) analyses reveal a crystallite size of about 4.6, 10 and 13 nm for Cu, Cu2O and CuO respectively, estimated by Scherer method. The results indicated that after thermal treatment at 300 1C, Copper acetate decomposes to a mixture of phases containing Cu, Cu2O and CuO. The XRD spectra show typical peaks at 2θr141. By reason of lack of sufficient accuracy of X-ray diffractometer at this range of 2θ, we can dispense these peaks. The FA sols were transformed to gels through solvent evaporation and then were heat treated at 450 1C. Fig. 2 shows the XRD pattern of FA nanopowders. It can be seen that pure FA was synthesized at process conditions mentioned above. Mean particle size of FA phase after heat treatment at 450 1C was calculated by particle size analyzer to be about 552.85 nm. Fig. 3 shows the XRD spectra of the FA–TiO2–CNT(Cu) after heated treatment at 450 1C. The characteristic peaks of the FA (matrix of composite) have seen at XRD spectra. Also, analysis of XRD data by X'Pert highScore software showed that TiO2 reinforcement phase has acceptable purity.
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journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
Fig. 3 – XRD diffraction of FA–10 wt% TiO2–1%wtCNT(Cu) after heat treatment at 450 1C.
Fig. 4 – TEM micrographs of MWCNTs decorated with copper and copper oxides at two magnifications.
The presence of main peaks of MWCNTs at 2θ ¼26.0, 42.6 and 53.51 corresponding to (002), (100) and (004) planes proof retention of the CNTs (1 wt%) after heat treatment in air at 450 1C. As shown in Fig. 3, the diffraction angle at 2θ¼ 50.41 can be indexed to (200) plane of the cubic Cu structure, confirming the presence of Cu in composite. Beside this peak, the peaks corresponding to Cu2O and CuO appears at 2θ¼ 67.5, 59.0 and 40.01. Antibacterial activity experiments performed on various microorganisms clearly demonstrated the effectiveness of copper oxide nanoparticles against bacterial growth (Azam et al., 2012). XRD spectra confirmed the formation and acceptable purity of FA–TiO2–CNT composite containing copper and copper oxides as antibacterial agents. Mean particle size of this composite after heat treatment at 450 1C was calculated by particle size analyzer to be about 400.22 nm. In fact, presence of carbon nanotube as heterogeneous nucleation sites for FA, TiO2 or both decrease the particle size of composite.
3.2.
TEM images
For TEM observations, the decorated MWCNTs with copper and copper oxides was sonicated in ethanol and deposited on a holey copper grid. Fig. 4 shows TEM micrographs obtained from sample. Clearly, copper and copper oxides nanoparticles with an average size of about 10 nm show an almost uniform distribution on the surface of MWCNTs. From TEM studies, it is observed that the copper acetate have ability to wet the functionalized MWCNTs surfaces. Also TEM studies confirmed the successful attachment of copper and copper oxides to MWCNTs by wet chemical method. Fig. 5 illustrates the distribution of as-received, acid treated and copper decorated MWCNTs in FA–TiO2 aged sols. As see in pictures, as-received CNTs have dispersed as coarse clusters in sol. Also, acid treatment of MWCNTs (for removing catalyst particles) causes the agglomeration of tubes in sol. In spite of acid treated MWCNTs, the decorated MWCNTs have
journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
129
Fig. 5 – Distribution of as-received (left), acid treated (center) and copper decorated (right) MWCNTs in FA–TiO2 aged sols after 2 h stirring.
Fig. 6 – SEM images of (a) FA, (b) FA–TiO2, (c) FA–TiO2–CNT and (d) FA–TiO2–CNT(Cu) coatings on Ti–6Al–4V substrate after heat treatment at 450 1C.
appropriate and homogenous distribution in matrix of composite. With due attention to TEM micrograph and Fig. 5 one may conclude that decoration of MWCNTs with copper and copper oxides can inhibit from agglomeration of nanotubes and improve their distribution in sol–gel composites.
3.3.
SEM observations
The SEM morphologies of FA, FA–TiO2, FA–TiO2–CNT and FA–TiO2–CNT(Cu) coating layers on Ti–6Al–4V substrate are shown in Fig. 6a–d. As shown in Fig. 6, the addition of TiO2, MWCNTs and MWCNTs decorated with copper and copper oxides caused to make the coatings with different morphological features. The FA coating layer has a non-uniform morphology with scarce number of large porosities (Fig. 6a).
The addition of 10 wt% TiO2 phase to FA result in a fine structure composite coating (Fig. 6b). It can be attributed to nucleation of FA crystallites on surface of TiO2 particles. Based on Surface morphology of composite coating, chemical bonding is suggested as the main bonding mechanism between FA and TiO2 phases. The higher rate of hydrolysis and gelation of TiO2 than FA, confirm possibility of heterogeneous nucleation of FA phase on TiO2 particles. Fig. 6c shows the morphology of composite coating containing FA, 10 wt% TiO2 and 1 wt% MWCNTs. The morphological feature of this coating has not significant difference with FA–TiO2 coating, but SEM observation of this samples show the agglomerated zone (arrow head). In fact, the agglomeration of non-functionalized MWCNTs and trapping in composite structure may be the reason of creating these zones. Fig. 6d
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journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
illustrates the morphology of FA–10% TiO2 containing 1 wt% of decorated MWCNTs. As motioned above, decoration of MWCNTs cause the proper distribution of tubes and prevention from agglomerated zone formation. As shown in Fig. 6d, this sample has flower-like morphology in most part of layer. Also, the roughness of coating has increased in comparison to previous coating. The porosities in this sample included with both micro and macro-scales and appropriate distribution in surface. The morphology, roughness and porosity size of apatite coating are known to govern its bio-physiological and mechanical properties. Literature review indicated that osteoblastic cells attach faster to rougher surfaces (Jokinen et al., 1998; Jokinen et al., 1998). Therefore, FA–TiO2–CNT(Cu) composite coating on Ti–6Al–4V with high degree of roughness and porosity can improve the osteoblast cells attachment and accelerate oseointegration process.
3.4.
of 10 wt% TiO2 and 1 wt% CNT can increase the elastic modulus of FA up to 60%. Ratio of the elastic to total work during indentation for the composite coatings is given by the following expressions (Cheng and Cheng, 2004): Wp þ We ¼ Wt " # 1 3ðhf =hm Þ2 þ 2ðhf =hm Þ3 we ¼ wt 1 ðhf =hm Þ2 Where Wp, We and Wt are the plastic, elastic and total work respectively, hm is the depth of the indent at the peak load
Nanomechanical properties
The nanoindentation method was developed to measure the hardness and elastic modulus of FA and composite coatings from indentation load–displacement data obtained during one cycle of loading and unloading. Indentation was performed with a constant loading/unloading rate for 30 s and 10 s hold at the peak load of 950 μN (Fig. 7, left). Elastic modulus (E) had been calculated using Oliver–Pharr method (Oliver and Pharr, 1992; Lahiri et al., 2010). Fig. 7 shows the 3D topographic image acquired on the surface of coatings at 1000 nm depth. Getting an impression of the mechanical properties of the composites coatings, 5 indents were made at randomly chosen regions throughout the coatings surface at an appropriate distance from each other. The load–displacement curves acquired for indentation tests are shown in Fig. 8. As seen in Fig. 8, indentation depth of samples is as follow: FA4FA–TiO2–CNT(Cu)4FA–TiO2. This pattern in indentation depth indicates the hardness of samples. Elastic modulus, calculated from the unloading part of the load–displacement curves shows about 23% improvement with addition of copper decorated MWCNTs to FA–TiO2. Also, presence of TiO2 as reinforcement can increase the Elastic modulus of FA matrix. In fact, simultaneous presence
Fig. 8 – The load–displacement curves for indentation tests of coating samples.
Table 1 – Nanomechanical properties measured by nanoindentation and Vickers indentation methods for FA and composite coatings. Sample
E (Gpa)
H (Gpa) (Vickers)
We/Wtot
FA FA–TiO2 FA–TiO2–CNT(Cu)
11.8 14.5 19.3
0.37 0.72 0.58
0.74 0.75 0.57
Fig. 7 – load–time graph (left) and the 3D topographic images of indented coatings surface (right).
journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
131
Fig. 9 – SEM micrographs of polished FA–TiO2–CNT(Cu) coaing layer.
and hf is the final depth of indentation (Lahiri et al., 2010). The We/Wtot and other nanoindentation results for FA, FA–TiO2 and FA–TiO2–CNT(Cu) coatings are compared in Table 1. Inclusion of MWCNTs in fluorapatite matrix result in higher elastic modulus compared with the single phase FA and FA–TiO2 coating. As seen in Table 1, presence of CNT(Cu) in fluorapatite increase the fraction of plastic work during stress applying on coating surface. In fact, the addition of copper decorated MWCNTs can increase the nanomechanical properties of FA–TiO2–CNT(Cu) composite by three effective factors: (1) reinforcement effect of CNTs with higher elastic modulus than matrix (2) the homogeneous distribution of decorated CNTs in FA matrix and (3) strong interface between FA and CNTs reinforcements. Fig. 9 illustrate the SEM micrographs of polished FA–TiO2–CNT(Cu) coating layer. Polishing process apply the stress against the composite particles. As shown in these micrographs, the bridging of MWCNTs in ceramic matrix can increase the toughness of composite through resisting the crack propagation and reducing radial crack length.
4.
Conclusion
In the current research, the sol–gel technique has been applied for fabrication of fluorapatite, FA–TiO2 and FA–TiO2–CNT(Cu) composite. The decoration of MWCNTs was accomplished by simple wet chemical method. Chemical, morphological and nanomechanical characterization of coatings was performed with XRD, SEM and TEM and nanoindentation analysis. In summary, the following conclusions are established: – XRD study clearly reveals that the synthesis of FA, FA–TiO2 and FA–TiO2–CNT(Cu) nanocomposites is successfully performed through mentioned reaction after heat treatment at 450 1C for 1h. The results demonstrate that calcium titanium oxide impurity produced in prepared FA–TiO2 nanocomposite due to reaction between precursors during gelation. Mean crystallite size of FA and FA–10 wt% TiO2 after heat treatment in 450 1C was calculated by Scherer method about 70 and 34 nm respectively. – TEM images demonstrate that the surface of MWCNTs is uniformly decorated with a certain amount of copper and copper oxides Nanoparticles.
– SEM micrographs illustrate that addition of decorated MWCNTs to FA–TiO2 composite sol–gel cause the morphological changes. FA–TiO2–CNT(Cu) composite coating have flower-like morphology, macro and micro-scale porosity with uniform distribution and high degree of roughness. These morphological features make this coating suitable for application in bioimplants coatings. – Nanomechanical evaluation of coatings showed that copper decorated MWCNTs have a key role in improvement of mechanical properties. Based on the results of the work found in this paper, there are some proposals for future research and clinical uses of the materials developed. The results of this work have suggested in vitro properties of FA–TiO2–CNT(Cu). In fact, in vitro experiments have their limitations and therefore we suggested the in vivo investigation of this composite. Also, the effects of coating thickness on the biocompatibility and nanomechanical properties of the coatings can be study. One of the most important issues when considering apatites for biological coatings is the dissolution rate in an environment where human body fluids exist (Tredwin et al., 2013). The dissolution rate of composite coating in SBF can also compare with fluorapatite.
r e f e r e nc e s
Azam, A., et al., 2012. Size-dependent antimicrobial properties of CuO nanoparticles against gram-positive and -negative bacterial strains. Int. J. Nanomed. 7, 3527–3535. Chen, S., Darby, I., 2003. Dental implants: maintenance, care and treatment of peri-implant infection. Aust. Dent. J. 48, 212–220. Chen, W., et al., 2006. In vitro anti-bacterial and biological properties of magnetron co-sputtered silver-containing hydroxyapatite coating. Biomater 27, 5512–5517. Cheng, Y.T., Cheng, C.M., 2004. Scaling, dimensional analysis, and indentation measurements. Mater. Sci. Eng. R 44, 91–149. Cunniffe, G.M., et al., 2010. The synthesis and characterization of nano phase hydroxyapatite using an ovel dispersant-aided precipitation method. J. Biomed. Mater. Res. 95A, 1142–1149. Darroudi, M., et al., 2010. Preparation and characterization of fluorohydroxyapatite nanopowders by non-alkoxide sol–gel method. Dig. J. Nanomater. Biostruct. 5, 29–33.
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Ebrahimi-kahrizsangi, R., et al., 2010. Synthesis and characterization of fluorapatite–titania (FAp–TiO2) nanocomposite via mechanochemical process. Solid State Sci. 12, 1645–1651. Fathi, M.H., Mohammadi Zahrani, E., 2009. Fabrication and characterization of fluoridated hydroxyapatite nanopowders via Mechanical alloying. J. Alloy. Compd. 475, 408–414. Hiang, C., et al., 2001. Purification and characterization of singlewall carbon, nanotubes. J. Phys. Chem. B 105, 1157–1161. Jokinen, M., et al., 1998. Influence of sol and surface properties on in vitro bioactivity of sol–gel derived TiO2 and TiO2–SiO2 films deposited by dip coating method. J. Biomed. Mater. Res. 42, 295–302. Kaya, C., et al., 2008. Multi-walled carbon nanotube-reinforced hydroxyapatite layers on Ti6Al4V medical implants by electrophoretic deposition (EPD). Adv. Eng. Mater. 10, 131–138. Lahiri, D., et al., 2010. Carbon nanotube toughened hydroxyapatite by spark plasma sintering: microstructural evolution and multiscale tribological properties. Carbon 49, 3103–3120. Lim, Y.M., et al., 2001. Sol–gel derived functionally graded TiO2/HAP films on Ti–6Al–4V implants. J. Sol–Gel Sci. Technol. 21, 123–128. Lin, Y., et al., 2004. Advances toward bioapplications of carbon nanotubes. J. Mater. Chem. 14, 527–541. Lin, Z., et al., 2012. Study on thermal decomposition of copper(II) acetate monohydrate in air. J. Therm. Anal. Calorim. 107, 471–475. Najafi, H., et al., 2009. Inclusion of carbon nanotubes in a hydroxyapatite sol–gel matrix. Ceram. Int. 35, 2987–2991.
Nikcˇevic, I., et al., 2004. Mechanochemical synthesis of nanostructured fluorapatite/fluorhydroxyapatite and carbonated fluorapatite/fluorhydroxyapatite. J. Solid State Chem. 177, 2565–2574. Oliver, W.C., Pharr, G.M., 1992. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 7, 1564–1583. Panacek, A., et al., 2006. Silver colloid nanoparticles: synthesis, characterization, and their antibacterial activity. J. Phys. Chem. 110, 16248–16253. Ramires, P.A., et al., 2001. The influence of titania/hydroxyapatite composite coatings on in vitro osteoblasts behavior. Biomaterials 22, 1467–1474. Smart, S.K., et al., 2006. The biocompatibility of carbon nanotubes. Carbon 44, 1034–1047. Tredwin, C.J., et al., 2013. Hydroxyapatite, fluor-hydroxyapatite and fluorapatite produced via the sol–gel method: bonding to titanium and scanning electron microscopy. Dent. Mater. 29, 521–529. Worsley, K.A., et al., 2009. Functionalization and dissolution of nitric acid treated single-walled carbon nanotubes. J. Am. Chem. Soc. 131, 18153–18158. Wang, Y., et al., 2006. Selective heterogeneous nucleation and growth of size-controlled metal nanoparticles on carbon nanotubes in solution. Chem. Eur. J. 12, 2542–2549. Zhang, L.J., Webster, T.J., 2009. Nanotechnology and nanomaterials: promises for improved tissue regeneration. Nano Today 4, 66–80.
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Characterization and nanomechanical properties of novel dental implant coatings containing copper decorated-carbon nanotubes N. Sasanin, J. Vahdati Khaki, S. Mojtaba Zebarjad Department of Materials Science and Metallurgical Engineering, Engineering Faculty, Ferdowsi University of Mashhad, Mashhad, Iran
art i cle i nfo
ab st rac t
Article history:
Fluorapatite–titania coated Ti-based implants are promising for using in dental surgery for
Received 12 February 2014
restoring teeth. One of the challenges in implantology is to achieve a bioactive coating with
Received in revised form
appropriate mechanical properties. In this research, simple sol–gel method was developed
29 April 2014
for synthesis of fluorapatite–titania–carbon nanotube decorated with antibacterial agent.
Accepted 3 May 2014
Triethyl phosphate [PO4(C2H5)3], calcium nitrate [Ca(NO3)2] and ammonium fluoride (NH4F)
Available online 17 May 2014
were used as precursors under an ethanol–water based solution for fluorapatite (FA)
Keywords:
production. Titanium isopropoxide and isopropanol were used as starting materials for
Nanomechanical properties
making TiO2 sol–gels. Also, Copper acetate [Cu(C2H3O2)2 H2O] was used as precursor for
Fluorapatite
decoration of multi walled carbon nanotubes (MWCNTs) with wet chemical method. The
Titania
decorated MWCNTs (CNT(Cu)) were evaluated by transmission electron microscopy (TEM).
Copper decorated
The phase identification of the FA–TiO2–CNT(Cu) coating was carried out by XRD analysis.
Carbon nanotube
Morphology of coated samples was investigated by SEM observations. The surface elastic
Sol–gel
modulus and hardness of coatings were studied using nanoindentation technique. The results indicate that novel dental implant coating containing FA, TiO2 and copper decorated MWCNTs have proper morphological features. The results of nanoindentation test show that incorporation of CNT(Cu) in FA–TiO2 matrix can improve the nanomechanical properties of composite coating. & 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Synthetic Hydroxyapatite [HA, Ca10(PO4)6(OH)2] shows to possess exceptional biocompatibility and bioactivity properties. It has been widely used clinically in the form of powders, granules, dense and porous blocks, and coating (Cunniffe
n
Corresponding author. E-mail address: [email protected] (N. Sasani).
http://dx.doi.org/10.1016/j.jmbbm.2014.05.003 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
et al., 2010). Fluorine ion, which exists in human bone and enamel, can be incorporated into HA crystal structure by substitution of fluorine ions with OH groups to form Fluorapatite, [FA, Ca10 (PO4)6F2]. Substitution of F in HA structure plays an important role because of its influence on the physical and biological properties of HA (Nikč evic et al.,
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journal of the mechanical behavior of biomedical materials 37 (2014) 125 –132
2004). Incorporation of fluorine into HA, or “fluoridation”, reduces its solubility (Darroudi et al., 2010), increases chemical stability and promotes bone cell proliferation (Ebrahimikahrizsangi et al., 2010) while maintaining a comparable biocompatibility to that of HA (Darroudi et al., 2010). The results of in vitro showed that FA could provide better protein adsorption, comparable or better cell attachment, and significantly improved alkaline phosphates activity (Fathi and Mohammadi Zahrani, 2009). HA coated implant sometimes failed in long term stability, because the coated film deteriorated partly owing to fracture at the HA–titanium interface, due to the poor adherence between the ceramic film and the substrate. Thus, improvement of chemical bonding between the HA and Ti implant is mostly desirable particularly for clinical applications. Over the past decades, hydroxyapatite–titania (HA–TiO2) nanocomposites have been widely used as Ti-based implant coatings. The addition of TiO2 is a possible method for improvement the bonding of coated HA to Ti implants (Lim et al., 2001). Most recently, for tissue engineering applications the trend is to develop nanosized apatites with properties closer to those of living bone (Cunniffe et al., 2010). In natural tissues or organs, cells directly interact with nanostructures extracellular matrices (ECM), which are mainly composed of nanofibrous collagen fibrils. Nanomaterials, resembling the natural ECM in some features and possessing unique physiochemical properties, play a key role in stimulating cell growth as well as guide tissue regeneration (Zhang and Webster, 2009). In a biomimetic viewpoint, the three-dimensional CNTs resembles the nanofibrous network of natural ECM (Lin et al., 2004). Also, the extraordinary mechanical properties of CNTs are expected to have a contribution for improvement the poor mechanical properties of apatite coatings when used as reinforcement in novel apatite/CNT composites (Kaya et al., 2008). However, pristine CNTs tend to bundle up and are insoluble in most types of solvents, making them difficult to be used in biological systems (Worsley et al., 2009). Recently, Smart et al. (2006) reported that unrefined CNT possess some degree of toxicity (in vivo and in vitro), predominately due to the presence of transition metal catalysts, while chemically functionalized CNT applicable for drug delivery have not demonstrated any toxicity (Smart et al., 2006). Therefore CNTs need to be functionalized and purified by the removal of residual metal catalysts to improve their solubility and biocompatibility properties. The controlled functionalization and decoration of metal nanoparticles onto CNTs were done for reduction its toxicity as well as prevention of aggregation (Wang et al., 2006). The success and long-term survival of the biomaterials are also dependent on the prevention of bacterial putrefaction after implant placement. In the case of dental implants, complications derived from an implant-associated infection lead to a harm in the patient quality of life and satisfaction because it is one of the reasons for early failures by lack of osseointegration and the major cause of late failures (Chen and Darby, 2003). The antiseptic activity of antibacterial nanoparticles is influenced by their size. The smaller particles, lead to the greater antibacterial effect (Panacek et al., 2006; Chen et al., 2006) although the problems of aggregation can not be neglected. A common solution to avoid this disadvantage is
to support the nanoparticles on the surface of different substrates. Decoration of carbon nanotubes with antibacterial metals such as silver and copper are of considerable interest for their application in the biotechnology. In fact, the decoration of MWCNTs with antibacterial agent can prevent from carbon nanotubes and antibacterial nanoparticles agglomeration. The present study were investigated the preparation and characterization of FA–TiO2–CNT nanocomposite coatings containing antibacterial agent by sol–gel process. Ti–6Al–4V as substrate was subjected to a sol–gel spin coating process. The aims of this study are to investigate the role of decorated CNTs in the microstructural and nanomechanical evolution of FA-based composite coatings. This has been carried out by XRD analysis and SEM observations. The effect of CNTs modified microstructure on elastic modulus and Hardness of the composite coatings is also elucidated.
2.
Materials and methods
2.1.
Decoration of MWCNTs
Carbon nanotubes with outside and inside diameters 40– 60 nm and 10–20 nm respectively were used as starting material. Mean length of MWCNTs reported by manufacture was about 2–10 μm. Nitric acid (HNO3, Merck, PurityE64.3– 66.4%) was used for modification of surface of carbon nanotubes. 0.1 g of as-received MWCNTs were added to 50 ml of HNO3. The mixture was mixed ultrasonically for 2 h and then refluxed for 24 h at room temperature. After 24 h refluxing, the reaction mixture was diluted with distilled water and then filtered through a filter paper (Watchman, 42). Dilution was repeated until the pH of mixture reached to pH of distilled water about 6. The functionalizing process followed by drying in a vacuum oven at 90 1C for 24 h (Hiang et al., 2001). These conditions have been employed to non-covalently functionalize CNTs for binding the nanoparticles. In fact the final products may be nanotube fragments whose ends and sidewalls are decorated with mainly carboxyl groups. About 0.03 g of functionalized MWCNTs was added to 50 ml of ethanol (PurityE96%) and then the solution was stirred for 2 h. Also, the solution of copper acetate (CuAc) in ethanol with 1 g/10 ml concentration was prepared and mixed for 2 h. The mixtures was added together and mixed ultrasonically until the ethanol was evaporated completely. This process leads to impregnation of MWCNTs with copper acetate. The weltered MWCNTs then heat treated at 200 1C temperatures at air atmosphere for 30 min (Lin et al., 2012). The suitable temperature for heat treatment was determined from thermogravimetric analysis.
2.2.
Preparation of FA–TiO2–CNT (Cu) composite sol–gel
The sol–gel process started with the preparation of a TiO2 sol. Tetraethylorthotitanate (C8H20TiO4, Merck, PurityZ98%) was first diluted with isopropanol (Dr. Mojallali chemical laboratories, PurityZ99%), and then a small amount of distilled water mixed for hydrolysis, followed by vigorous stirring for 24 h. Subsequently, the mixed sol was aged for 24 h. For preparation of fluorapatite sol–gel, controlled amounts of triethyl phosphate
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(TEP) ([PO4(C2H5)3], Merck, PurityZ99%) and ammonium fluoride (NH4F, Merck, PurityZ95%) were first dissolved in ethanol (PurityE96%) and distilled water and then the solution was stirred for 48 h. After hydrolysis process, phosphate sol slowly was added to a solution of calcium nitrate ([Ca (NO3)2], Merck, PurityZ98%) and was allowed to mix for a 1 h at 60 1C. Ammonium hydroxide (NH4OH, Merck, PurityZ99%) was used as alkali catalyst to set pHE11. For preparation of FA–10 wt% TiO2 nanocomposite, FA and TiO2 sols were mixed together and stirred vigorously for 2 h (Ramires et al., 2001). After the preparation of FA–10 wt% TiO2 sol, 1 wt% of Cu decorated MWCNTs were added and mixed for 2 h with magnetic mixer (Najafi et al., 2009). Subsequently, composite sol was allowed to age for 24 h. After aging process, composite sols were spin coated on Ti–6Al–4V.
2.3.
Fig. 1 – XRD diffraction of MWCNT weltered by copper acetate after heat treatment at 200 1C.
Spin coating processing
A spin coating protocol has been established for coating the sol–gels onto substrates. Commercially available Ti–6Al–4V plate were cut into 10 mm 10 mm 1 mm in size using a cutting machine. All samples were polished successively using SiC grinding papers from 120 to 1200 grit size to remove original oxides and surface defects. 0.5 ml of the sol–gels was centered over the plates spinning at 3000 rpm and dispensed at a distance of 20 mm. The plates was allowed to spin for a further 30 s and then removed from the platform. The plates were dried at 80 1C then calcinated at 450 1C in an air oven with a ramp of 5 1C per minute, dwell time of 60 min and cool down rate of 10 1C per minute (Tredwin et al., 2013).
2.4.
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Characterization techniques
The phase identification of the decorated MWCNTs, fluorapatite and composite samples was carried out by a Philips X'pert diffractometer with Cu Kα (λ¼ 1.54184 Å) radiation by the step of 0.021. X'Pert HighScore software (PANalytical B.V. Almelo, the Netherlands) used for analysis of XRD patterns. ICDD reference standard data supplied with X'Pert HighScore. The decorated MWCNTs were evaluated by transmission electron microscopy (TEM). Studies were performed in a Leo (Zeiss) 912 instrument at an acceleration voltage of 120 kV. Morphology of nanocomposites coatings was evaluated by scanning electron microscopy (SEM) (model VP 1450, LEO, Germany). Nanomechanical properties of coatings was performed by nanoindentation instrument with a constant loading/unloading rate and the peak load of 950 μN. Nanohardness (H) and elastic modulus (E) properties of composite coatings have been characterized by means of Instrumented Indentation Techniques, such as Atomic Force Microscopes (AFM) (Nanoscope II Digital Instruments CA, USA) and nanoindentation machines (Triboscopes Nanomechanical Test-Instrument, Hysitron Inc, Germany).
3.
Results and discussion
3.1.
XRD analysis
The XRD patterns for the decorated MWCNTs shown in Fig. 1. The diffraction peak at 2θ¼ 26.71 can be assigned to a
Fig. 2 – XRD pattern of FA powder synthesized at pH E11 and heat treated at 450 1C. reflection from the (002) plane of graphite in MWCNTs. The diffraction angles at 2θ E491 and 741 can be indexed to (111) and (220) planes of the cubic Cu structure, respectively, confirming the presence of Cu in the materials. Besides the peaks arising from Cu, the peaks corresponding to Cu2O appears at 2θ E29, 37 and 621, indicating that Cu2O phase was formed on the surface of MWCNTs. Also, diffraction peaks of CuO can be observed at 2θE32, 35 and 391. X-ray diffraction (XRD) analyses reveal a crystallite size of about 4.6, 10 and 13 nm for Cu, Cu2O and CuO respectively, estimated by Scherer method. The results indicated that after thermal treatment at 300 1C, Copper acetate decomposes to a mixture of phases containing Cu, Cu2O and CuO. The XRD spectra show typical peaks at 2θr141. By reason of lack of sufficient accuracy of X-ray diffractometer at this range of 2θ, we can dispense these peaks. The FA sols were transformed to gels through solvent evaporation and then were heat treated at 450 1C. Fig. 2 shows the XRD pattern of FA nanopowders. It can be seen that pure FA was synthesized at process conditions mentioned above. Mean particle size of FA phase after heat treatment at 450 1C was calculated by particle size analyzer to be about 552.85 nm. Fig. 3 shows the XRD spectra of the FA–TiO2–CNT(Cu) after heated treatment at 450 1C. The characteristic peaks of the FA (matrix of composite) have seen at XRD spectra. Also, analysis of XRD data by X'Pert highScore software showed that TiO2 reinforcement phase has acceptable purity.
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Fig. 3 – XRD diffraction of FA–10 wt% TiO2–1%wtCNT(Cu) after heat treatment at 450 1C.
Fig. 4 – TEM micrographs of MWCNTs decorated with copper and copper oxides at two magnifications.
The presence of main peaks of MWCNTs at 2θ ¼26.0, 42.6 and 53.51 corresponding to (002), (100) and (004) planes proof retention of the CNTs (1 wt%) after heat treatment in air at 450 1C. As shown in Fig. 3, the diffraction angle at 2θ¼ 50.41 can be indexed to (200) plane of the cubic Cu structure, confirming the presence of Cu in composite. Beside this peak, the peaks corresponding to Cu2O and CuO appears at 2θ¼ 67.5, 59.0 and 40.01. Antibacterial activity experiments performed on various microorganisms clearly demonstrated the effectiveness of copper oxide nanoparticles against bacterial growth (Azam et al., 2012). XRD spectra confirmed the formation and acceptable purity of FA–TiO2–CNT composite containing copper and copper oxides as antibacterial agents. Mean particle size of this composite after heat treatment at 450 1C was calculated by particle size analyzer to be about 400.22 nm. In fact, presence of carbon nanotube as heterogeneous nucleation sites for FA, TiO2 or both decrease the particle size of composite.
3.2.
TEM images
For TEM observations, the decorated MWCNTs with copper and copper oxides was sonicated in ethanol and deposited on a holey copper grid. Fig. 4 shows TEM micrographs obtained from sample. Clearly, copper and copper oxides nanoparticles with an average size of about 10 nm show an almost uniform distribution on the surface of MWCNTs. From TEM studies, it is observed that the copper acetate have ability to wet the functionalized MWCNTs surfaces. Also TEM studies confirmed the successful attachment of copper and copper oxides to MWCNTs by wet chemical method. Fig. 5 illustrates the distribution of as-received, acid treated and copper decorated MWCNTs in FA–TiO2 aged sols. As see in pictures, as-received CNTs have dispersed as coarse clusters in sol. Also, acid treatment of MWCNTs (for removing catalyst particles) causes the agglomeration of tubes in sol. In spite of acid treated MWCNTs, the decorated MWCNTs have
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Fig. 5 – Distribution of as-received (left), acid treated (center) and copper decorated (right) MWCNTs in FA–TiO2 aged sols after 2 h stirring.
Fig. 6 – SEM images of (a) FA, (b) FA–TiO2, (c) FA–TiO2–CNT and (d) FA–TiO2–CNT(Cu) coatings on Ti–6Al–4V substrate after heat treatment at 450 1C.
appropriate and homogenous distribution in matrix of composite. With due attention to TEM micrograph and Fig. 5 one may conclude that decoration of MWCNTs with copper and copper oxides can inhibit from agglomeration of nanotubes and improve their distribution in sol–gel composites.
3.3.
SEM observations
The SEM morphologies of FA, FA–TiO2, FA–TiO2–CNT and FA–TiO2–CNT(Cu) coating layers on Ti–6Al–4V substrate are shown in Fig. 6a–d. As shown in Fig. 6, the addition of TiO2, MWCNTs and MWCNTs decorated with copper and copper oxides caused to make the coatings with different morphological features. The FA coating layer has a non-uniform morphology with scarce number of large porosities (Fig. 6a).
The addition of 10 wt% TiO2 phase to FA result in a fine structure composite coating (Fig. 6b). It can be attributed to nucleation of FA crystallites on surface of TiO2 particles. Based on Surface morphology of composite coating, chemical bonding is suggested as the main bonding mechanism between FA and TiO2 phases. The higher rate of hydrolysis and gelation of TiO2 than FA, confirm possibility of heterogeneous nucleation of FA phase on TiO2 particles. Fig. 6c shows the morphology of composite coating containing FA, 10 wt% TiO2 and 1 wt% MWCNTs. The morphological feature of this coating has not significant difference with FA–TiO2 coating, but SEM observation of this samples show the agglomerated zone (arrow head). In fact, the agglomeration of non-functionalized MWCNTs and trapping in composite structure may be the reason of creating these zones. Fig. 6d
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illustrates the morphology of FA–10% TiO2 containing 1 wt% of decorated MWCNTs. As motioned above, decoration of MWCNTs cause the proper distribution of tubes and prevention from agglomerated zone formation. As shown in Fig. 6d, this sample has flower-like morphology in most part of layer. Also, the roughness of coating has increased in comparison to previous coating. The porosities in this sample included with both micro and macro-scales and appropriate distribution in surface. The morphology, roughness and porosity size of apatite coating are known to govern its bio-physiological and mechanical properties. Literature review indicated that osteoblastic cells attach faster to rougher surfaces (Jokinen et al., 1998; Jokinen et al., 1998). Therefore, FA–TiO2–CNT(Cu) composite coating on Ti–6Al–4V with high degree of roughness and porosity can improve the osteoblast cells attachment and accelerate oseointegration process.
3.4.
of 10 wt% TiO2 and 1 wt% CNT can increase the elastic modulus of FA up to 60%. Ratio of the elastic to total work during indentation for the composite coatings is given by the following expressions (Cheng and Cheng, 2004): Wp þ We ¼ Wt " # 1 3ðhf =hm Þ2 þ 2ðhf =hm Þ3 we ¼ wt 1 ðhf =hm Þ2 Where Wp, We and Wt are the plastic, elastic and total work respectively, hm is the depth of the indent at the peak load
Nanomechanical properties
The nanoindentation method was developed to measure the hardness and elastic modulus of FA and composite coatings from indentation load–displacement data obtained during one cycle of loading and unloading. Indentation was performed with a constant loading/unloading rate for 30 s and 10 s hold at the peak load of 950 μN (Fig. 7, left). Elastic modulus (E) had been calculated using Oliver–Pharr method (Oliver and Pharr, 1992; Lahiri et al., 2010). Fig. 7 shows the 3D topographic image acquired on the surface of coatings at 1000 nm depth. Getting an impression of the mechanical properties of the composites coatings, 5 indents were made at randomly chosen regions throughout the coatings surface at an appropriate distance from each other. The load–displacement curves acquired for indentation tests are shown in Fig. 8. As seen in Fig. 8, indentation depth of samples is as follow: FA4FA–TiO2–CNT(Cu)4FA–TiO2. This pattern in indentation depth indicates the hardness of samples. Elastic modulus, calculated from the unloading part of the load–displacement curves shows about 23% improvement with addition of copper decorated MWCNTs to FA–TiO2. Also, presence of TiO2 as reinforcement can increase the Elastic modulus of FA matrix. In fact, simultaneous presence
Fig. 8 – The load–displacement curves for indentation tests of coating samples.
Table 1 – Nanomechanical properties measured by nanoindentation and Vickers indentation methods for FA and composite coatings. Sample
E (Gpa)
H (Gpa) (Vickers)
We/Wtot
FA FA–TiO2 FA–TiO2–CNT(Cu)
11.8 14.5 19.3
0.37 0.72 0.58
0.74 0.75 0.57
Fig. 7 – load–time graph (left) and the 3D topographic images of indented coatings surface (right).
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Fig. 9 – SEM micrographs of polished FA–TiO2–CNT(Cu) coaing layer.
and hf is the final depth of indentation (Lahiri et al., 2010). The We/Wtot and other nanoindentation results for FA, FA–TiO2 and FA–TiO2–CNT(Cu) coatings are compared in Table 1. Inclusion of MWCNTs in fluorapatite matrix result in higher elastic modulus compared with the single phase FA and FA–TiO2 coating. As seen in Table 1, presence of CNT(Cu) in fluorapatite increase the fraction of plastic work during stress applying on coating surface. In fact, the addition of copper decorated MWCNTs can increase the nanomechanical properties of FA–TiO2–CNT(Cu) composite by three effective factors: (1) reinforcement effect of CNTs with higher elastic modulus than matrix (2) the homogeneous distribution of decorated CNTs in FA matrix and (3) strong interface between FA and CNTs reinforcements. Fig. 9 illustrate the SEM micrographs of polished FA–TiO2–CNT(Cu) coating layer. Polishing process apply the stress against the composite particles. As shown in these micrographs, the bridging of MWCNTs in ceramic matrix can increase the toughness of composite through resisting the crack propagation and reducing radial crack length.
4.
Conclusion
In the current research, the sol–gel technique has been applied for fabrication of fluorapatite, FA–TiO2 and FA–TiO2–CNT(Cu) composite. The decoration of MWCNTs was accomplished by simple wet chemical method. Chemical, morphological and nanomechanical characterization of coatings was performed with XRD, SEM and TEM and nanoindentation analysis. In summary, the following conclusions are established: – XRD study clearly reveals that the synthesis of FA, FA–TiO2 and FA–TiO2–CNT(Cu) nanocomposites is successfully performed through mentioned reaction after heat treatment at 450 1C for 1h. The results demonstrate that calcium titanium oxide impurity produced in prepared FA–TiO2 nanocomposite due to reaction between precursors during gelation. Mean crystallite size of FA and FA–10 wt% TiO2 after heat treatment in 450 1C was calculated by Scherer method about 70 and 34 nm respectively. – TEM images demonstrate that the surface of MWCNTs is uniformly decorated with a certain amount of copper and copper oxides Nanoparticles.
– SEM micrographs illustrate that addition of decorated MWCNTs to FA–TiO2 composite sol–gel cause the morphological changes. FA–TiO2–CNT(Cu) composite coating have flower-like morphology, macro and micro-scale porosity with uniform distribution and high degree of roughness. These morphological features make this coating suitable for application in bioimplants coatings. – Nanomechanical evaluation of coatings showed that copper decorated MWCNTs have a key role in improvement of mechanical properties. Based on the results of the work found in this paper, there are some proposals for future research and clinical uses of the materials developed. The results of this work have suggested in vitro properties of FA–TiO2–CNT(Cu). In fact, in vitro experiments have their limitations and therefore we suggested the in vivo investigation of this composite. Also, the effects of coating thickness on the biocompatibility and nanomechanical properties of the coatings can be study. One of the most important issues when considering apatites for biological coatings is the dissolution rate in an environment where human body fluids exist (Tredwin et al., 2013). The dissolution rate of composite coating in SBF can also compare with fluorapatite.
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