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Effect of morphology on the electrophoretic deposition of hydroxyapatite nanoparticles
Journal of Alloys and Compounds 741 (2018) 211e222
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Effect of morphology on the electrophoretic deposition of hydroxyapatite nanoparticles Morteza Farrokhi-Rad Department of Materials Engineering, Faculty of Engineering, Azarbaijan Shahid Madani University, P.O. Box: 53751-71379, Tabriz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2017 Received in revised form 29 December 2017 Accepted 8 January 2018 Available online 11 January 2018
Electrophoretic deposition (EPD) was performed from alcoholic suspensions of spherical (SHA) and fiber (FHA) hydroxyapatite nanoparticles. FHA particles were horizontally aligned in deposits prepared at lower voltages since they can find enough space and time to rotate upon reaching the deposit and sit horizontally on it. While, they are more randomly oriented at higher voltages since more FHA particles aligned parallel to electric field reach to the substrate and also particles cannot find enough time and space for rotation. FHA particles are highly interlocked together in FHA deposits reinforcing them and also the drying shrinkages are lower for them resulting in their less cracking. The cracks are parallel to each other in FHA coating deposited from methanolic suspension at 5 V due to the more horizontal and parallel alignment of FHA particles in it. The crack-free SHA coating decreased the corrosion rate of substrate more efficiently due to its less porosity. © 2018 Elsevier B.V. All rights reserved.
Keywords: Spherical hydroxyapatite nanoparticles (SHA) Fiber hydroxyapatite particles (FHA) Electrophoretic deposition (EPD) Alcoholic suspensions Corrosion
1. Introduction Hydroxyapatite (HA) is the calcium phosphate ceramic very similar to the inorganic part of the human bone and teeth both in morphology and composition [1,2]. HA has been widely used in biomedical applications because of its good bioactivity, biocompatibility, osteoconductivity and biodegradability [3e6]. However, compared to natural bone, HA has poor mechanical properties like low fracture toughness and hardness [7e12]. To overcome this problem, usually HA is applied as the coating on the metallic implants like titanium and 316L stainless steel. By this method the bioactivity of the implant is also promoted since HA surface has higher bioactivity than metallic surfaces. Also the corrosion rate of implant decreases considerably in body fluid when it is coated by HA or other corrosion resistant coatings [13,14]. The corrosion of implant releases the metallic ions into the surrounding tissues and can results in the implant failure [15]. Various techniques have been used to deposit HA coatings on the metallic substrate such as solgel, electrodeposition, thermal and plasma spray and so on [16e21]. Electrophoretic deposition (EPD) is another method which has been widely used to deposit HA coatings on the metallic substrates [22e33]. EPD is based on the movement of charged particles
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dispersed in a suitable solvent towards the oppositely charged electrode under the influence of an applied electric field and then their deposition on it [34]. EPD is a simple, low-cost and fast technique capable to control the microstructure and thickness of coatings by simple adjustment of process parameters like voltage and time [34]. The porous coatings can accelerate implant fixation by bone ingrowths into the pores [35e37]. One of the inherent characteristics of EPD is the porous nature of the coatings obtained by it [38]. So EPD is attractive method to deposit bioactive coatings on the metallic implants. In this work the effects of HA particles morphology (spherical and fiber) on the EPD process and the characteristics of obtained coatings have been investigated. 2. Experimental Hydroxyapatite nanoparticles with spherical (SHA) and fiber (FHA) morphologies were synthesized by wet chemical methods described in Refs. [39] and [40], respectively. The synthesized powders were characterized by XRD, FTIR and SEM analysis. Different alcoholic (methanol (Merck, 99.99%), ethanol (Merck, 99.8%), isopropanol (Merck, 99.8%) and butanol (Merck, 99%)) suspensions of SHA and FHA particles (10 g/L) were prepared using triethanolamine (TEA, Merck, reagent grade) as the dispersing agent. Firstly, different concentrations of TEA (0, 0.67, 1.33 and 2.67 mL/L) were dissolved in alcohols by stirring magnetically for
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15min. Then, 10 g/L of SHA and FHA particles were added into them followed by magnetic stirring for 24 h and finally ultrasonic dispersing for 10min. The zeta potential of particles was determined in different suspensions using Malvern instrument. The samples were diluted by the procedure described in Ref. [41]. Electrophoretic deposition (EPD) was performed at 60 V using a D.C power supply. In-situ kinetics of deposition and the current density during EPD were recorded according to the method reported in Ref. [41]. Voltage was applied for 6min and then switched off for 2min during in-situ recording of the EPD kinetics. Sticking parameter (f factor) was also calculated at initial times of EPD using the following equation [41]:
f ¼
Ji Ji þ Jd
(1)
where Ji and Jd are the mass flow of particles incorporated in and detached from the deposit, respectively. Ji and Jd are equal to the slope of kinetics curve at initial times of EPD and after switching off the voltage, respectively. The wet density and wet thickness of deposits prepared at 60 V from different alcoholic suspensions of SHA and FHA particles with optimum concentration of TEA (TEA*) was determined against EPD times according to the method described in Ref. [42]. The dry thickness of coatings was also measured using a coating thickness gauges (Qnix 8500, Germany). The microstructure of the SHA and FHA coatings deposited at 60 V for 1min from the suspensions with optimum concentration of TEA (TEA*) was observed by optical (OM) and electron (SEM) microscopes. The corrosion rate of bare and coated substrates in simulated body fluid (SBF) at 37.5 C was measured by electrochemical polarization method. SBF was synthesized according to the method described in Ref. [43]. The coatings for corrosion studies were deposited at 60 V for 1min from different alcoholic suspensions of SHA and FHA particles with TEA*. The coatings were dried overnight at room temperature and then sintered at 800 C for 1 h (heating rate: 4 C/min) under the flowing argon gas atmosphere. 3. Results and discussion Fig. 1 shows the XRD pattern of the synthesized SHA and FHA powders. As can be seen the spectra of both powders only show the
Fig. 1. XRD pattern for synthesized (a) SHA and (b) FHA powders.
characteristic peaks of hydroxyapatite proving their high purity. Also the spectra of FHA particles have more intensive and narrower peaks implying their more crystallinity and/or coarser primary particles size. The SEM image from the synthesized SHA and FHA particles is shown in Fig. 2. The SEM images verify the spherical and fiber morphologies of synthesized SHA and FHA particles, respectively. Also FHA particles are relatively coarser than SHA particles in agreement with the XRD pattern. The FTIR spectra of the SHA and FHA particles are shown in Fig. 3. The related bonds are also specified for each peak. The peaks at 565, 600, 1038 and 1088 cm1 are attributed to phosphate groups ((PO4)3-) [44]. The symmetric stretching vibration of P-O bond results in the peak at 961 cm1 [45]. The peaks at 1628 and 3420 cm1 are due to the water adsorbed on the particles [46,47]. The peaks around 632 and 3573 cm1 are related to OH groups of HA [48]. The peaks around 1420, 1450 and 873 cm1 are due to the carbonate groups (CO2 3 ) in HA [45,47]. In contrast to SHA particles, there are no peaks at 1420 and 1450 cm1 in the spectra of FHA particles. Also the intensity of peaks attributed to OH groups is weaker in case of SHA particles. These results imply that OH groups of SHA particles are partially substituted by carbonate ions; so SHA particles are partially carbonated. The zeta potential of SHA and FHA particles in different alcoholic suspensions against TEA concentration is shown in Fig. 4. As can be seen, firstly the zeta potential of both SHA and FHA particles increases with TEA addition until reaching the maximum value (at TEA*) and then decreases with its further addition. It was found in author previous work [28] that HþTEA ions are generated through the protonation of TEA in alcoholic suspensions; these HþTEA ions are then chemically adsorbed on the surface of HA particles via hydrogen bonding with their surface P-OH groups leading to the increase in their surface charge and so zeta potential. The surface of particles is saturated by HþTEA ions at TEA* so that it’s further addition only results in the reduction in double layer thickness and so zeta potential. The optimum concentrations of TEA are 0.67, 0.67, 1.33 and 2 mL/L in methanolic, ethanolic, isopropanolic and butanolic suspensions of SHA particles, respectively. In case of FHA particles, the optimum concentrations are 0.67, 2, 0.67 and 1.33 mL/ L in methanolic, ethanolic, isopropanolic and butanolic suspensions, respectively. The variations of current density during EPD at 60 V from different alcoholic suspensions of SHA and FHA particles are shown in Fig. 5. As can be seen the current density decreases during EPD for all suspensions except the TEA containing methanolic suspensions of SHA particles. Also current density decreases more rapidly in case of FHA suspensions. The reduction in current density during EPD is due to the formation of layer with higher resistivity than the corresponding suspension. The wet EPD deposits are composed of ceramic particles with extremely large resistivity dispersed in the continuous liquid phase with less resistivity; so the resistivity of deposits is nearly equal to that of their continuous interparticles liquid phase. The higher the volume fraction of liquid phase in the deposit the lower is its resistance. Compared to SHA deposits, the volume fraction of interparticles liquid phase is less in FHA deposits due to the coarser size of FHA particles leading to their higher resistance and so faster current density reduction for them. Fig. 6 shows the in-situ kinetics of EPD at 60 V from different alcoholic suspensions of SHA and FHA particles. The weight of deposits continuously increases in the period of time when the voltage is connected (0e6min). The deposition weight remains constant in case of all FHA and SHA suspensions without TEA after switching off the voltage. However, it decreases with time for TEA containing SHA suspensions after switching off the voltage. Reduction in deposition weight after switching off the voltage is because of the particles detachment from deposit into the
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Fig. 2. SEM images for synthesized (a) SHA and (b) FHA nanoparticles.
Fig. 3. FTIR spectra for synthesized (a) SHA and (b) FHA nanoparticles.
suspension at the interface between them. Particles are more closely packed in the deposit than the suspension; also as mentioned TEA molecules as well as HþTEA ions are adsorbed on the surface of particles in all alcoholic suspensions; so a sharp gradient is developed in the concentration of TEA molecules and HþTEA ions at the interface between deposit and suspension. These concentration gradients develop an electrochemical potential
difference at the interface leading to the force exerted on the deposited particles and detach them into the suspension. The higher the concentration of TEA in the suspension the higher is the adsorbed TEA molecules and HþTEA ions on the surface of particles. Thus the gradient in their concentration at the interface increases leading to the higher electrochemical potential difference and so the stronger force exerted on the particles. So deposition weight decreases more rapidly with TEA concentration in SHA suspensions after switching off the voltage. FHA particles are coarser than SHA particles so that less surface is available for the adsorption of TEA and HþTEA species leading to the smaller concentration gradient and thereby smaller electrochemical potential difference and finally weaker force exerted on the particles at the interface. Moreover, long FHA particles are mechanically interlocked together in FHA deposits so that their detachment is not as easy as that of SHA particles. So the weight of FHA deposits remains constant after switching off the voltage even in case of TEA containing suspensions. The EPD rate increases as TEA is added into the SHA suspensions (except the butanolic suspension with 0.67 mL/L TEA) because of the increase in the zeta potential and so the electrophoretic mobility of SHA particles. Also it can be seen that the deposition rate declines more rapidly with time and deviate from linearity in case of FHA suspensions. The deviation from linearity is because of the voltage drop over the
Fig. 4. Zeta potential of (a) SHA and (b) FHA particles in various alcoholic suspensions versus TEA concentration.
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Fig. 5. Variations of current density during EPD at 60 V from methanolic (a, b), ethanolic (c, d), isopropanolic (e, f) and butanolic (g, h) suspensions of SHA (a, c, e and g) and FHA (b, d, f and h) particles.
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Fig. 6. In-situ kinetics of EPD at 60 V from methanolic (a, b), ethanolic (c, d), isopropanolic (e, f) and butanolic (g, h) suspensions of SHA (a, c, e and g) and FHA (b, d, f and h) particles (voltage was connected for 6min and then switched off for 2min).
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deposit. Voltage drop is larger over the FHA deposits due to their larger resistance so that the EPD rate deviates from linearity more prominently for them. Fig. 7 shows the values of sticking parameter (f factor) at initial times of EPD at 60 V from different alcoholic suspensions of SHA particles against TEA concentration (the f factor is 1 in for all FHA suspensions). As can be seen the f factor decreases with TEA concentration for all alcoholic suspensions of SHA particles. On one hand, TEA increases the EPD rate via enhancing the zeta potential and thereby the electrophoretic mobility of particles; on the other hand, it decreases the EPD rate by decreasing the f factor. The net influence of TEA on the EPD rate is determined by the competition of these two opposite effects. For instance, while the zeta potential of SHA particles increases with the addition of 0.67 mL/L TEA into their butanolic suspension but the EPD rate decreases due to the more reduction in f factor. Fig. 8 shows the wet density of deposits prepared at 60 V from different alcoholic suspensions of SHA and FHA particles with TEA* as a function of time. As can be seen, at constant EPD time and for all alcoholic suspensions, the wet density of FHA deposits is higher than that of SHA ones. So it can be concluded that the volume fraction of interparticles liquid phase is lower in FHA deposits due to the coarser size of FHA particles leading to the higher wet density for them (sine the density of liquid phases (the density of methanol, ethanol, isopropanol and butanol is 0.792, 0.789, 0.786 and 0.81g/ cm3, respectively) is less than that of hydroxyapatite (z3.2 g/cm3)). The wet density of deposits also increases slightly with time due to the rearrangement of particles in them caused by electro-osmotic flows [42,49]. The wet and dry thickness of deposits prepared at 60 V from different alcoholic suspensions of SHA and FHA particles with TEA* are shown in Fig. 9 as a function of time. It can be seen that the difference between the wet and dry thicknesses is higher for SHA deposits implying the higher drying shrinkages for them. SHA particles with spherical morphology can easily slide over each other and rearrange due to the capillary forces exerted on them during drying of their deposits as the interparticles liquid phase is removed from them. However, long FHA particles are highly interlocked to each other restricting their freely sliding and rearrangement during drying and so results in less drying shrinkages for them. So although FHA deposits have higher wet densities than SHA ones, but their microstructure is more porous after drying. The SEM images from the coatings deposited at 60 V for 1min from different alcoholic suspensions of SHA and FHA particles with TEA* are shown in Fig. 10. The SEM images of FHA coatings deposited at 5 V for 1min are also shown in Fig. 10 to investigate the effect of applied voltage on the arrangement of FHA particles in the deposit. As can be seen all SHA coatings have the fine and even microstructures with low agglomeration. This is due to the high zeta potential of SHA particles in alcoholic suspensions with TEA*. However, among the SHA coatings, the one deposited from the butanolic suspension has the finest microstructure. SHA particles have high zeta potential in butanolic suspension with 2 mL/L TEA (86 mV). Also butanol has the highest viscosity among the all alcohols used in this work leading to the lower diffusion coefficient for particles in it. So the agglomeration of SHA particles in butanolic suspension with TEA* is very low. Moreover, SHA particles move slowly towards the substrate electrode during EPD because of their lower mobility in butanolic suspension. So SHA particles have longer time to site in their best position in deposit yielding the best microstructure for it. FHA particles are mostly aligned horizontally in the coatings deposited at 5 V. FHA particles with different orientations move towards the substrate during EPD. Those aligned non-parallel to the
Fig. 7. Sticking parameter at initial times of EPD at 60 V from different alcoholic suspensions of SHA particles versus TEA content.
applied electric field can rotate as soon as one of their head reach to and collide with deposit as a result of torque exerted on their opposite head. However, for the rotation to occur, particles must have enough time and space upon their incorporation in the deposit. At low voltages, FHA particles move so slowly towards the substrate that they find enough time for rotation; also the mass flow of particles towards the substrate electrode is less in lower voltages. So the number of particles reaching to and incorporating in the deposit per unit of time and area is also less in lower voltages leading to a more free available space adjacent to the deposit for particles so that they can freely rotate and incorporate horizontally on it. The rotation results in the horizontal incorporation of most FHA particles in deposit prepared at 5 V. However, the number of vertically aligned FHA particles is higher in the coatings deposited at 60 V resulting in their more random microstructure. The mobility of insulator cylindrical particles is as follow [50]:
m¼
2ε0 εr z fðkɑÞ 3h
(2)
where ε0 is the permittivity of vacuum, εr is the relative dielectric constant of medium, h is the viscosity of suspension and x is the zeta potential of particles. f(kɑ) is Henry's function, k is the inverse of Debye length and ɑ is the radius of cylinder. For cylinders aligned parallel to the applied electric field the values of f(kɑ)k ¼ 1.5 for all values of kɑ so mk ¼ ε0hεr z. However, f(kɑ) for cylinders aligned perpendicular to the electric field depend on the value of kɑ. The values of kɑ can be calculated by the following equation [51]:
sffiffiffiffiffiffiffiffiffiffiffiffi sa2 kɑ ¼ Dε0 εr
(3)
where D is the diffusion coefficient of ions and s is the electrical conductivity of suspension. The typical value for D is in the order of z109 m2/s. The conductivity of suspensions is also 8.3, 3.6, 2.5 and 2.2mS/cm, respectively, for methanolic, ethanolic, isopropanolic and butanolic suspensions with TEA*. The average radius of FHA particles was also considered about 100 nm. The approximate values of kɑ obtained using these data and equation (3) are 5.3, 4.1, 3.8 and 3.8 for methanolic, ethanolic, isopropanolic and butanolic suspensions of FHA particles, respectively. These values of kɑ yield f(kɑ) ⊥z0.9 for FHA particles aligned perpendicular to the applied electric field in the alcoholic suspensions so: m⊥ ¼ 0:6 ε0hεr z. Thus the
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Fig. 8. Wet density of deposits prepared at 60 V from different alcoholic suspensions of SHA (a) and FHA (b) particles with TEA* as a function of time.
mobility of FHA particles aligned parallel to the applied electric field is about 70% more than theirs aligned perpendicular to it. The mobility of FHA particles with other random (mr) orientation with respect to the applied electric field is between mk and m⊥: m⊥
orientation of FHA particles in the deposit, the cracks generated in the FHA coatings deposited at 5, 100 and 200 V from methanolic suspension with TEA* were also investigated by optical and electron microscopy. To exclude the effect of thickness on the cracking, the duration of EPD was chosen in a manner that all coatings have the nearly same thickness (z150 mm: (5 V; 6min), (100 V; 30 s) and (200 V; 15 s)). The optical and electron microscopy images of these coatings are shown in Fig. 12. As can be seen the number of cracks decreases and their parallel arrangement to each other diminishes at higher voltages. As can be seen in SEM images, FHA particles are mostly horizontally deposited on the substrate and are parallel to each other in the microstructure of coating deposited at 5 V for 5min. The interlocking of parallel aligned FHA particles is the least in the direction perpendicular to them. So there is no selfreinforcement action in the direction perpendicular to FHA particles aligned parallel to each other. Moreover, the drying shrinkage is large in the direction perpendicular to parallel aligned FHA particles; as mentioned the interlocking of FHA particles is the least in this direction so that the particles can easily rearrange and get close to each other due to the increasing capillary forces exerted on them as the liquid phase is gradually removed from their wet deposits resulting in the large shrinkages in this direction. So due to the less self-reinforcement as well as large shrinkages in the direction perpendicular to parallel aligned FHA particles, the cracks are easily generated and propagated in the direction parallel to FHA particles during their drying resulting in the development of parallel cracks in the coating. As mentioned the number of vertically aligned FHA particles increases with increasing the EPD voltage. So as the SEM images also show, the FHA particles are in average more randomly oriented in the coatings deposited at higher voltages; thus the selfreinforcement is not so directional leading to the nearly random propagation of cracks in the FHA coatings deposited at higher voltages. The results obtained for the electrochemical polarization analysis of bare substrate and those coated with sintered SHA and FHA deposits in SBF solution at 37.5 C are shown in Fig. 13. The values of corrosion parameters (current density (icorr) and potential (Ecorr)) extracted from polarization curves using Tafel analysis are summarized in Table 1. It is interesting to note that the corrosion rate of the substrate increases as it is coated with SHA deposits from methanolic and ethanolic suspensions. As it was seen in Fig. 11, the SHA coatings deposited from methanolic and ethanolic suspensions are highly cracked; the width of cracks is ranged from few micrometers up to around 100 and 50 mm in SHA coatings deposited from methanolic and ethanolic suspensions, respectively. The corrosive fluid can enter into the gaps between the cracks and get in
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Fig. 9. Wet and dry thickness of deposits prepared at 60 V from methanolic (a, b), ethanolic (c, d), isopropanolic (e, f) and butanolic (g, h) suspensions of SHA (a, c, e and g) and FHA (b, d, f and h) particles with TEA* as a function of time.
direct contact with the metallic surface of the substrate and corrode it. The 316L stainless steel is also prone to localized corrosion. The corrosive fluid can remain stagnant in the gaps between the narrow
cracks (width in the order of few micrometers). However, the fluid present inside the wider cracks can flow. In this case, the substrate area below the narrow and wide cracks acts as the anode and
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Fig. 10. SEM images from SHA (a, b, c and d) and FHA (i, j, k and l) coatings deposited at 60 V for 60s and FHA (e, f, g and h) coatings deposited at 5 V for 60s from methanolic (a, e and i), ethanolic (b, f and j), isopropanolic (c, g and k) and butanolic (d, h and l) suspensions with TEA*.
Fig. 11. The macrograph and optical microscopy images of SHA (a, b, c and d) and FHA (e, f, g and h) coatings shaped at 60 V for 1min from methanolic (a, e), ethanolic (b, f), isopropanolic (c, g) and butanolic (d, h) suspensions with TEA*.
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Fig. 12. Optical and electron microscopy images of FHA coatings deposited at (a) 5 V for 6min, (b) 100 V for 30s and (c) 200 V for 15s from methanolic suspension of FHA particles with TEA*.
Fig. 13. Electrochemical polarization curves for bare substrate (1) and those coated with sintered SHA (a) and FHA (b) coatings deposited at 60 V for 1min from methanolic (2), ethanolic (3), isopropanolic (4) and butanolic (5) suspensions with TEA* in SBF solution at 37.5 C.
Table 1 Values of corrosion current density (icorr) and potential (Ecorr) for the bare substrate and those coated with SHA and FHA deposits at 60 V for 60 s from different alcoholic suspensions with TEA* in SBF solution and 37.5 C. Sample Parameter
Ecorr (mV Vs SCE) Icorr (mA/cm2)
Bare substrate
470 43.68
Methanolic
Ethanolic
Isopropanolic
Butanolic
SHA
FHA
SHA
FHA
SHA
FHA
SHA
FHA
391.6 123.2
410 7.47
361.2 87.3
304 0.72
345.6 8.42
278 0.94
288 0.38
322 1.12
cathode, respectively. So the intensive localized crevice corrosion occurs inside the narrow cracks. It can be said that the increase in corrosion rate due to the intensive localized corrosion inside the cracks is dominant to its reduction by the blocking of the corrosive fluid from reaching to the substrate surface in the uncracked area of coating; so the corrosion rate of the substrate increases when it is coated with SHA coatings from methanolic and ethanolic suspensions. Despite the cracked microstructure of SHA coating deposited
from isopropanolic suspension, it decreases the corrosion rate of the substrate. All cracks in this coating are very narrow and there is no wide crack in it. So the localized corrosion does not occur inside these cracks due to the absence of cathodic sites (wide cracks) in this coating. The SHA coating deposited from butanolic suspension has a crack-free microstructure so that its deposition on the substrate considerably decreases its corrosion rate. The corrosion rate of the substrate decreases as it is coated with
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FHA coatings from all alcoholic suspensions. However, the reduction in corrosion rate is less in case of FHA coating deposited from methanolic suspension due to the presence of few cracks in its microstructure. All FHA coatings deposited from ethanolic, isopropanolic and butanolic suspensions have crack-free microstructures so that their applying on the substrate decreases its corrosion rate substantially. Also, among these FHA coatings, the one deposited from ethanolic suspension yields the least corrosion rate followed by those deposited from isopropanolic and butanolic suspensions, respectively. The FHA coatings deposited at same voltage and time from smaller molecular size alcohols are thicker due to the faster EPD rate from them. The crack-free and thick FHA coating deposited from ethanolic suspension can more efficiently block the corrosive fluid from reaching to the substrate surface and prevent from its corrosion followed by those deposited from isopropanolic and butanolic suspensions, respectively. Due to their crack-free microstructures and nearly same thicknesses, the SHA and FHA coatings deposited from butanolic suspension were selected to investigate the effect of particles morphology on the corrosion resistance of the coated substrates. The icorr values are 0.38 and 1.12mA/cm2 for the substrates coated with SHA and FHA deposits from their corresponding butanolic suspensions, respectively. It was mentioned that the drying shrinkages is lower for FHA coatings than SHA ones. So after drying, the FHA coatings will have more open microstructure than SHA coatings (as the SEM images (Fig. 10) verify this). The corrosive fluid can penetrate through the pores of coatings and reach the substrate surface to corrode it. So SHA coatings more efficiently prevent from substrate corrosion due to its more closely packed microstructure. Thick FHA coatings with crack-free microstructure can be easily prepared by EPD method. These coatings are strengthened by selfreinforcement action of interlocked FHA particles and have more porous microstructure accelerating the implant fixation. Also FHA particles can potentially act as the nano-needles and promote cell adhesion to implant surface. So FHA coatings are very good candidate to be applied on the metallic implants. 4. Conclusion The suspension of spherical (SHA) and fiber (FHA) hydroxyapatite nanoparticles were prepared in various alcohols using triethanolamine (TEA) as the dispersing agent. The rate of electrophoretic deposition (EPD) from FHA suspensions decreased more rapidly with time due to the larger resistance of FHA deposits leading to the higher voltage drop over them. Sticking parameter (f factor) was unity for all FHA suspensions while it decreased with TEA concentrations for all SHA suspensions. FHA particles were horizontally aligned in the deposits prepared at lower voltages (5 V); FHA particles not aligned parallel to the applied electric field move so slowly towards the substrate that they can find enough space and time to rotate upon reaching the deposit and incorporate horizontally on it. FHA particles are more randomly oriented at higher voltages. Since more number of FHA particles aligned parallel to applied electric field reach to the substrate at higher voltages. Also particles not aligned parallel to electric field move so fast that they cannot find enough time and space for rotation upon reaching the deposit leading to their non-horizontal incorporation in the deposit. All SHA coatings deposited at 60 V for 60s were cracked during drying except the one deposited from butanolic suspension. While only the FHA coating deposited at 60 V for 60s from methanolic suspension was slightly cracked. FHA particles are highly interlocked together in FHA deposits resulting in their strengthening via self-reinforcement mechanism and the drying shrinkages are also lower for FHA deposits leading to their less cracking. The cracks are parallel to each other in FHA coating
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deposited from methanolic suspension at 5 V due to the more horizontal and parallel alignment of FHA particles in it. Comparing the SHA and FHA coating deposited at 60 V for 60s from butanolic suspension, the SHA coating decreased the corrosion rate of substrate in SBF environment more efficiently due to its less porous microstructure. References [1] M. Ferraz, F. Monteiro, C. Manuel, Hydroxyapatite nanoparticles: a review of preparation methodologies, J. Appl. Biomater. Biomech. 2 (2004) 74e80. [2] A. Singh, Hydroxyapatite, a biomaterial: its chemical synthesis, characterization and study of biocompatibility prepared from shell of garden snail, Helix aspersa, Bull. Mater. Sci. 35 (2012) 1031e1038. [3] L.L. Hench, Bioceramics, J. Am. Ceram. Soc. 81 (1998) 1705e1727. [4] J.W. Vehof, P.H. Spauwen, J.A. Jansen, Bone formation in calcium-phosphate coated titanium mesh, Biomaterials 21 (2000) 2003e2009. [5] J.M. Gomez-Vega, E. Saiz, A.P. Tomsia, G.W. Marshall, S.J. 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[15] N.J. Hallab, K. Mikecz, C. Vermes, A. Skipor, J.J. Jacobs, Differential lymphocytereactivity to serum-derived metal-protein complexes produced from cobalt-based and titanium- based implant alloy degradation, J. Biomed. Mater. Res. 56 (2001) 427e436. [16] S. Hirai, K. Nishinaka, K. Shimakage, M. Uo, F. Watari, Hydroxyapatite coating on titanium substrate by the sol-gel process, J. Am. Ceram. Soc. 87 (2004) 29e34. [17] S. Dyshlovenko, L. Pawlowski, P. Roussel, D. Murano, A. Le Maguer, Relationship between plasma spray operational parameters and microstructure of hydroxyapatite coatings and powder particles sprayed into water, Surf. Coating. Technol. 200 (2006) 3845e3855. [18] W. Weng, J.L. Baptista, Preparation and characterization of hydroxyapatite coatings on Ti6Al4V alloy by a sol-gel method, J. Am. Ceram. Soc. 82 (1999) 27e32. [19] R. Gadow, A. Killinger, N. Stiegler, Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques, Surf. 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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of morphology on the electrophoretic deposition of hydroxyapatite nanoparticles Morteza Farrokhi-Rad Department of Materials Engineering, Faculty of Engineering, Azarbaijan Shahid Madani University, P.O. Box: 53751-71379, Tabriz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2017 Received in revised form 29 December 2017 Accepted 8 January 2018 Available online 11 January 2018
Electrophoretic deposition (EPD) was performed from alcoholic suspensions of spherical (SHA) and fiber (FHA) hydroxyapatite nanoparticles. FHA particles were horizontally aligned in deposits prepared at lower voltages since they can find enough space and time to rotate upon reaching the deposit and sit horizontally on it. While, they are more randomly oriented at higher voltages since more FHA particles aligned parallel to electric field reach to the substrate and also particles cannot find enough time and space for rotation. FHA particles are highly interlocked together in FHA deposits reinforcing them and also the drying shrinkages are lower for them resulting in their less cracking. The cracks are parallel to each other in FHA coating deposited from methanolic suspension at 5 V due to the more horizontal and parallel alignment of FHA particles in it. The crack-free SHA coating decreased the corrosion rate of substrate more efficiently due to its less porosity. © 2018 Elsevier B.V. All rights reserved.
Keywords: Spherical hydroxyapatite nanoparticles (SHA) Fiber hydroxyapatite particles (FHA) Electrophoretic deposition (EPD) Alcoholic suspensions Corrosion
1. Introduction Hydroxyapatite (HA) is the calcium phosphate ceramic very similar to the inorganic part of the human bone and teeth both in morphology and composition [1,2]. HA has been widely used in biomedical applications because of its good bioactivity, biocompatibility, osteoconductivity and biodegradability [3e6]. However, compared to natural bone, HA has poor mechanical properties like low fracture toughness and hardness [7e12]. To overcome this problem, usually HA is applied as the coating on the metallic implants like titanium and 316L stainless steel. By this method the bioactivity of the implant is also promoted since HA surface has higher bioactivity than metallic surfaces. Also the corrosion rate of implant decreases considerably in body fluid when it is coated by HA or other corrosion resistant coatings [13,14]. The corrosion of implant releases the metallic ions into the surrounding tissues and can results in the implant failure [15]. Various techniques have been used to deposit HA coatings on the metallic substrate such as solgel, electrodeposition, thermal and plasma spray and so on [16e21]. Electrophoretic deposition (EPD) is another method which has been widely used to deposit HA coatings on the metallic substrates [22e33]. EPD is based on the movement of charged particles
E-mail address: [email protected]. https://doi.org/10.1016/j.jallcom.2018.01.101 0925-8388/© 2018 Elsevier B.V. All rights reserved.
dispersed in a suitable solvent towards the oppositely charged electrode under the influence of an applied electric field and then their deposition on it [34]. EPD is a simple, low-cost and fast technique capable to control the microstructure and thickness of coatings by simple adjustment of process parameters like voltage and time [34]. The porous coatings can accelerate implant fixation by bone ingrowths into the pores [35e37]. One of the inherent characteristics of EPD is the porous nature of the coatings obtained by it [38]. So EPD is attractive method to deposit bioactive coatings on the metallic implants. In this work the effects of HA particles morphology (spherical and fiber) on the EPD process and the characteristics of obtained coatings have been investigated. 2. Experimental Hydroxyapatite nanoparticles with spherical (SHA) and fiber (FHA) morphologies were synthesized by wet chemical methods described in Refs. [39] and [40], respectively. The synthesized powders were characterized by XRD, FTIR and SEM analysis. Different alcoholic (methanol (Merck, 99.99%), ethanol (Merck, 99.8%), isopropanol (Merck, 99.8%) and butanol (Merck, 99%)) suspensions of SHA and FHA particles (10 g/L) were prepared using triethanolamine (TEA, Merck, reagent grade) as the dispersing agent. Firstly, different concentrations of TEA (0, 0.67, 1.33 and 2.67 mL/L) were dissolved in alcohols by stirring magnetically for
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15min. Then, 10 g/L of SHA and FHA particles were added into them followed by magnetic stirring for 24 h and finally ultrasonic dispersing for 10min. The zeta potential of particles was determined in different suspensions using Malvern instrument. The samples were diluted by the procedure described in Ref. [41]. Electrophoretic deposition (EPD) was performed at 60 V using a D.C power supply. In-situ kinetics of deposition and the current density during EPD were recorded according to the method reported in Ref. [41]. Voltage was applied for 6min and then switched off for 2min during in-situ recording of the EPD kinetics. Sticking parameter (f factor) was also calculated at initial times of EPD using the following equation [41]:
f ¼
Ji Ji þ Jd
(1)
where Ji and Jd are the mass flow of particles incorporated in and detached from the deposit, respectively. Ji and Jd are equal to the slope of kinetics curve at initial times of EPD and after switching off the voltage, respectively. The wet density and wet thickness of deposits prepared at 60 V from different alcoholic suspensions of SHA and FHA particles with optimum concentration of TEA (TEA*) was determined against EPD times according to the method described in Ref. [42]. The dry thickness of coatings was also measured using a coating thickness gauges (Qnix 8500, Germany). The microstructure of the SHA and FHA coatings deposited at 60 V for 1min from the suspensions with optimum concentration of TEA (TEA*) was observed by optical (OM) and electron (SEM) microscopes. The corrosion rate of bare and coated substrates in simulated body fluid (SBF) at 37.5 C was measured by electrochemical polarization method. SBF was synthesized according to the method described in Ref. [43]. The coatings for corrosion studies were deposited at 60 V for 1min from different alcoholic suspensions of SHA and FHA particles with TEA*. The coatings were dried overnight at room temperature and then sintered at 800 C for 1 h (heating rate: 4 C/min) under the flowing argon gas atmosphere. 3. Results and discussion Fig. 1 shows the XRD pattern of the synthesized SHA and FHA powders. As can be seen the spectra of both powders only show the
Fig. 1. XRD pattern for synthesized (a) SHA and (b) FHA powders.
characteristic peaks of hydroxyapatite proving their high purity. Also the spectra of FHA particles have more intensive and narrower peaks implying their more crystallinity and/or coarser primary particles size. The SEM image from the synthesized SHA and FHA particles is shown in Fig. 2. The SEM images verify the spherical and fiber morphologies of synthesized SHA and FHA particles, respectively. Also FHA particles are relatively coarser than SHA particles in agreement with the XRD pattern. The FTIR spectra of the SHA and FHA particles are shown in Fig. 3. The related bonds are also specified for each peak. The peaks at 565, 600, 1038 and 1088 cm1 are attributed to phosphate groups ((PO4)3-) [44]. The symmetric stretching vibration of P-O bond results in the peak at 961 cm1 [45]. The peaks at 1628 and 3420 cm1 are due to the water adsorbed on the particles [46,47]. The peaks around 632 and 3573 cm1 are related to OH groups of HA [48]. The peaks around 1420, 1450 and 873 cm1 are due to the carbonate groups (CO2 3 ) in HA [45,47]. In contrast to SHA particles, there are no peaks at 1420 and 1450 cm1 in the spectra of FHA particles. Also the intensity of peaks attributed to OH groups is weaker in case of SHA particles. These results imply that OH groups of SHA particles are partially substituted by carbonate ions; so SHA particles are partially carbonated. The zeta potential of SHA and FHA particles in different alcoholic suspensions against TEA concentration is shown in Fig. 4. As can be seen, firstly the zeta potential of both SHA and FHA particles increases with TEA addition until reaching the maximum value (at TEA*) and then decreases with its further addition. It was found in author previous work [28] that HþTEA ions are generated through the protonation of TEA in alcoholic suspensions; these HþTEA ions are then chemically adsorbed on the surface of HA particles via hydrogen bonding with their surface P-OH groups leading to the increase in their surface charge and so zeta potential. The surface of particles is saturated by HþTEA ions at TEA* so that it’s further addition only results in the reduction in double layer thickness and so zeta potential. The optimum concentrations of TEA are 0.67, 0.67, 1.33 and 2 mL/L in methanolic, ethanolic, isopropanolic and butanolic suspensions of SHA particles, respectively. In case of FHA particles, the optimum concentrations are 0.67, 2, 0.67 and 1.33 mL/ L in methanolic, ethanolic, isopropanolic and butanolic suspensions, respectively. The variations of current density during EPD at 60 V from different alcoholic suspensions of SHA and FHA particles are shown in Fig. 5. As can be seen the current density decreases during EPD for all suspensions except the TEA containing methanolic suspensions of SHA particles. Also current density decreases more rapidly in case of FHA suspensions. The reduction in current density during EPD is due to the formation of layer with higher resistivity than the corresponding suspension. The wet EPD deposits are composed of ceramic particles with extremely large resistivity dispersed in the continuous liquid phase with less resistivity; so the resistivity of deposits is nearly equal to that of their continuous interparticles liquid phase. The higher the volume fraction of liquid phase in the deposit the lower is its resistance. Compared to SHA deposits, the volume fraction of interparticles liquid phase is less in FHA deposits due to the coarser size of FHA particles leading to their higher resistance and so faster current density reduction for them. Fig. 6 shows the in-situ kinetics of EPD at 60 V from different alcoholic suspensions of SHA and FHA particles. The weight of deposits continuously increases in the period of time when the voltage is connected (0e6min). The deposition weight remains constant in case of all FHA and SHA suspensions without TEA after switching off the voltage. However, it decreases with time for TEA containing SHA suspensions after switching off the voltage. Reduction in deposition weight after switching off the voltage is because of the particles detachment from deposit into the
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Fig. 2. SEM images for synthesized (a) SHA and (b) FHA nanoparticles.
Fig. 3. FTIR spectra for synthesized (a) SHA and (b) FHA nanoparticles.
suspension at the interface between them. Particles are more closely packed in the deposit than the suspension; also as mentioned TEA molecules as well as HþTEA ions are adsorbed on the surface of particles in all alcoholic suspensions; so a sharp gradient is developed in the concentration of TEA molecules and HþTEA ions at the interface between deposit and suspension. These concentration gradients develop an electrochemical potential
difference at the interface leading to the force exerted on the deposited particles and detach them into the suspension. The higher the concentration of TEA in the suspension the higher is the adsorbed TEA molecules and HþTEA ions on the surface of particles. Thus the gradient in their concentration at the interface increases leading to the higher electrochemical potential difference and so the stronger force exerted on the particles. So deposition weight decreases more rapidly with TEA concentration in SHA suspensions after switching off the voltage. FHA particles are coarser than SHA particles so that less surface is available for the adsorption of TEA and HþTEA species leading to the smaller concentration gradient and thereby smaller electrochemical potential difference and finally weaker force exerted on the particles at the interface. Moreover, long FHA particles are mechanically interlocked together in FHA deposits so that their detachment is not as easy as that of SHA particles. So the weight of FHA deposits remains constant after switching off the voltage even in case of TEA containing suspensions. The EPD rate increases as TEA is added into the SHA suspensions (except the butanolic suspension with 0.67 mL/L TEA) because of the increase in the zeta potential and so the electrophoretic mobility of SHA particles. Also it can be seen that the deposition rate declines more rapidly with time and deviate from linearity in case of FHA suspensions. The deviation from linearity is because of the voltage drop over the
Fig. 4. Zeta potential of (a) SHA and (b) FHA particles in various alcoholic suspensions versus TEA concentration.
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Fig. 5. Variations of current density during EPD at 60 V from methanolic (a, b), ethanolic (c, d), isopropanolic (e, f) and butanolic (g, h) suspensions of SHA (a, c, e and g) and FHA (b, d, f and h) particles.
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Fig. 6. In-situ kinetics of EPD at 60 V from methanolic (a, b), ethanolic (c, d), isopropanolic (e, f) and butanolic (g, h) suspensions of SHA (a, c, e and g) and FHA (b, d, f and h) particles (voltage was connected for 6min and then switched off for 2min).
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deposit. Voltage drop is larger over the FHA deposits due to their larger resistance so that the EPD rate deviates from linearity more prominently for them. Fig. 7 shows the values of sticking parameter (f factor) at initial times of EPD at 60 V from different alcoholic suspensions of SHA particles against TEA concentration (the f factor is 1 in for all FHA suspensions). As can be seen the f factor decreases with TEA concentration for all alcoholic suspensions of SHA particles. On one hand, TEA increases the EPD rate via enhancing the zeta potential and thereby the electrophoretic mobility of particles; on the other hand, it decreases the EPD rate by decreasing the f factor. The net influence of TEA on the EPD rate is determined by the competition of these two opposite effects. For instance, while the zeta potential of SHA particles increases with the addition of 0.67 mL/L TEA into their butanolic suspension but the EPD rate decreases due to the more reduction in f factor. Fig. 8 shows the wet density of deposits prepared at 60 V from different alcoholic suspensions of SHA and FHA particles with TEA* as a function of time. As can be seen, at constant EPD time and for all alcoholic suspensions, the wet density of FHA deposits is higher than that of SHA ones. So it can be concluded that the volume fraction of interparticles liquid phase is lower in FHA deposits due to the coarser size of FHA particles leading to the higher wet density for them (sine the density of liquid phases (the density of methanol, ethanol, isopropanol and butanol is 0.792, 0.789, 0.786 and 0.81g/ cm3, respectively) is less than that of hydroxyapatite (z3.2 g/cm3)). The wet density of deposits also increases slightly with time due to the rearrangement of particles in them caused by electro-osmotic flows [42,49]. The wet and dry thickness of deposits prepared at 60 V from different alcoholic suspensions of SHA and FHA particles with TEA* are shown in Fig. 9 as a function of time. It can be seen that the difference between the wet and dry thicknesses is higher for SHA deposits implying the higher drying shrinkages for them. SHA particles with spherical morphology can easily slide over each other and rearrange due to the capillary forces exerted on them during drying of their deposits as the interparticles liquid phase is removed from them. However, long FHA particles are highly interlocked to each other restricting their freely sliding and rearrangement during drying and so results in less drying shrinkages for them. So although FHA deposits have higher wet densities than SHA ones, but their microstructure is more porous after drying. The SEM images from the coatings deposited at 60 V for 1min from different alcoholic suspensions of SHA and FHA particles with TEA* are shown in Fig. 10. The SEM images of FHA coatings deposited at 5 V for 1min are also shown in Fig. 10 to investigate the effect of applied voltage on the arrangement of FHA particles in the deposit. As can be seen all SHA coatings have the fine and even microstructures with low agglomeration. This is due to the high zeta potential of SHA particles in alcoholic suspensions with TEA*. However, among the SHA coatings, the one deposited from the butanolic suspension has the finest microstructure. SHA particles have high zeta potential in butanolic suspension with 2 mL/L TEA (86 mV). Also butanol has the highest viscosity among the all alcohols used in this work leading to the lower diffusion coefficient for particles in it. So the agglomeration of SHA particles in butanolic suspension with TEA* is very low. Moreover, SHA particles move slowly towards the substrate electrode during EPD because of their lower mobility in butanolic suspension. So SHA particles have longer time to site in their best position in deposit yielding the best microstructure for it. FHA particles are mostly aligned horizontally in the coatings deposited at 5 V. FHA particles with different orientations move towards the substrate during EPD. Those aligned non-parallel to the
Fig. 7. Sticking parameter at initial times of EPD at 60 V from different alcoholic suspensions of SHA particles versus TEA content.
applied electric field can rotate as soon as one of their head reach to and collide with deposit as a result of torque exerted on their opposite head. However, for the rotation to occur, particles must have enough time and space upon their incorporation in the deposit. At low voltages, FHA particles move so slowly towards the substrate that they find enough time for rotation; also the mass flow of particles towards the substrate electrode is less in lower voltages. So the number of particles reaching to and incorporating in the deposit per unit of time and area is also less in lower voltages leading to a more free available space adjacent to the deposit for particles so that they can freely rotate and incorporate horizontally on it. The rotation results in the horizontal incorporation of most FHA particles in deposit prepared at 5 V. However, the number of vertically aligned FHA particles is higher in the coatings deposited at 60 V resulting in their more random microstructure. The mobility of insulator cylindrical particles is as follow [50]:
m¼
2ε0 εr z fðkɑÞ 3h
(2)
where ε0 is the permittivity of vacuum, εr is the relative dielectric constant of medium, h is the viscosity of suspension and x is the zeta potential of particles. f(kɑ) is Henry's function, k is the inverse of Debye length and ɑ is the radius of cylinder. For cylinders aligned parallel to the applied electric field the values of f(kɑ)k ¼ 1.5 for all values of kɑ so mk ¼ ε0hεr z. However, f(kɑ) for cylinders aligned perpendicular to the electric field depend on the value of kɑ. The values of kɑ can be calculated by the following equation [51]:
sffiffiffiffiffiffiffiffiffiffiffiffi sa2 kɑ ¼ Dε0 εr
(3)
where D is the diffusion coefficient of ions and s is the electrical conductivity of suspension. The typical value for D is in the order of z109 m2/s. The conductivity of suspensions is also 8.3, 3.6, 2.5 and 2.2mS/cm, respectively, for methanolic, ethanolic, isopropanolic and butanolic suspensions with TEA*. The average radius of FHA particles was also considered about 100 nm. The approximate values of kɑ obtained using these data and equation (3) are 5.3, 4.1, 3.8 and 3.8 for methanolic, ethanolic, isopropanolic and butanolic suspensions of FHA particles, respectively. These values of kɑ yield f(kɑ) ⊥z0.9 for FHA particles aligned perpendicular to the applied electric field in the alcoholic suspensions so: m⊥ ¼ 0:6 ε0hεr z. Thus the
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Fig. 8. Wet density of deposits prepared at 60 V from different alcoholic suspensions of SHA (a) and FHA (b) particles with TEA* as a function of time.
mobility of FHA particles aligned parallel to the applied electric field is about 70% more than theirs aligned perpendicular to it. The mobility of FHA particles with other random (mr) orientation with respect to the applied electric field is between mk and m⊥: m⊥
orientation of FHA particles in the deposit, the cracks generated in the FHA coatings deposited at 5, 100 and 200 V from methanolic suspension with TEA* were also investigated by optical and electron microscopy. To exclude the effect of thickness on the cracking, the duration of EPD was chosen in a manner that all coatings have the nearly same thickness (z150 mm: (5 V; 6min), (100 V; 30 s) and (200 V; 15 s)). The optical and electron microscopy images of these coatings are shown in Fig. 12. As can be seen the number of cracks decreases and their parallel arrangement to each other diminishes at higher voltages. As can be seen in SEM images, FHA particles are mostly horizontally deposited on the substrate and are parallel to each other in the microstructure of coating deposited at 5 V for 5min. The interlocking of parallel aligned FHA particles is the least in the direction perpendicular to them. So there is no selfreinforcement action in the direction perpendicular to FHA particles aligned parallel to each other. Moreover, the drying shrinkage is large in the direction perpendicular to parallel aligned FHA particles; as mentioned the interlocking of FHA particles is the least in this direction so that the particles can easily rearrange and get close to each other due to the increasing capillary forces exerted on them as the liquid phase is gradually removed from their wet deposits resulting in the large shrinkages in this direction. So due to the less self-reinforcement as well as large shrinkages in the direction perpendicular to parallel aligned FHA particles, the cracks are easily generated and propagated in the direction parallel to FHA particles during their drying resulting in the development of parallel cracks in the coating. As mentioned the number of vertically aligned FHA particles increases with increasing the EPD voltage. So as the SEM images also show, the FHA particles are in average more randomly oriented in the coatings deposited at higher voltages; thus the selfreinforcement is not so directional leading to the nearly random propagation of cracks in the FHA coatings deposited at higher voltages. The results obtained for the electrochemical polarization analysis of bare substrate and those coated with sintered SHA and FHA deposits in SBF solution at 37.5 C are shown in Fig. 13. The values of corrosion parameters (current density (icorr) and potential (Ecorr)) extracted from polarization curves using Tafel analysis are summarized in Table 1. It is interesting to note that the corrosion rate of the substrate increases as it is coated with SHA deposits from methanolic and ethanolic suspensions. As it was seen in Fig. 11, the SHA coatings deposited from methanolic and ethanolic suspensions are highly cracked; the width of cracks is ranged from few micrometers up to around 100 and 50 mm in SHA coatings deposited from methanolic and ethanolic suspensions, respectively. The corrosive fluid can enter into the gaps between the cracks and get in
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Fig. 9. Wet and dry thickness of deposits prepared at 60 V from methanolic (a, b), ethanolic (c, d), isopropanolic (e, f) and butanolic (g, h) suspensions of SHA (a, c, e and g) and FHA (b, d, f and h) particles with TEA* as a function of time.
direct contact with the metallic surface of the substrate and corrode it. The 316L stainless steel is also prone to localized corrosion. The corrosive fluid can remain stagnant in the gaps between the narrow
cracks (width in the order of few micrometers). However, the fluid present inside the wider cracks can flow. In this case, the substrate area below the narrow and wide cracks acts as the anode and
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Fig. 10. SEM images from SHA (a, b, c and d) and FHA (i, j, k and l) coatings deposited at 60 V for 60s and FHA (e, f, g and h) coatings deposited at 5 V for 60s from methanolic (a, e and i), ethanolic (b, f and j), isopropanolic (c, g and k) and butanolic (d, h and l) suspensions with TEA*.
Fig. 11. The macrograph and optical microscopy images of SHA (a, b, c and d) and FHA (e, f, g and h) coatings shaped at 60 V for 1min from methanolic (a, e), ethanolic (b, f), isopropanolic (c, g) and butanolic (d, h) suspensions with TEA*.
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Fig. 12. Optical and electron microscopy images of FHA coatings deposited at (a) 5 V for 6min, (b) 100 V for 30s and (c) 200 V for 15s from methanolic suspension of FHA particles with TEA*.
Fig. 13. Electrochemical polarization curves for bare substrate (1) and those coated with sintered SHA (a) and FHA (b) coatings deposited at 60 V for 1min from methanolic (2), ethanolic (3), isopropanolic (4) and butanolic (5) suspensions with TEA* in SBF solution at 37.5 C.
Table 1 Values of corrosion current density (icorr) and potential (Ecorr) for the bare substrate and those coated with SHA and FHA deposits at 60 V for 60 s from different alcoholic suspensions with TEA* in SBF solution and 37.5 C. Sample Parameter
Ecorr (mV Vs SCE) Icorr (mA/cm2)
Bare substrate
470 43.68
Methanolic
Ethanolic
Isopropanolic
Butanolic
SHA
FHA
SHA
FHA
SHA
FHA
SHA
FHA
391.6 123.2
410 7.47
361.2 87.3
304 0.72
345.6 8.42
278 0.94
288 0.38
322 1.12
cathode, respectively. So the intensive localized crevice corrosion occurs inside the narrow cracks. It can be said that the increase in corrosion rate due to the intensive localized corrosion inside the cracks is dominant to its reduction by the blocking of the corrosive fluid from reaching to the substrate surface in the uncracked area of coating; so the corrosion rate of the substrate increases when it is coated with SHA coatings from methanolic and ethanolic suspensions. Despite the cracked microstructure of SHA coating deposited
from isopropanolic suspension, it decreases the corrosion rate of the substrate. All cracks in this coating are very narrow and there is no wide crack in it. So the localized corrosion does not occur inside these cracks due to the absence of cathodic sites (wide cracks) in this coating. The SHA coating deposited from butanolic suspension has a crack-free microstructure so that its deposition on the substrate considerably decreases its corrosion rate. The corrosion rate of the substrate decreases as it is coated with
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FHA coatings from all alcoholic suspensions. However, the reduction in corrosion rate is less in case of FHA coating deposited from methanolic suspension due to the presence of few cracks in its microstructure. All FHA coatings deposited from ethanolic, isopropanolic and butanolic suspensions have crack-free microstructures so that their applying on the substrate decreases its corrosion rate substantially. Also, among these FHA coatings, the one deposited from ethanolic suspension yields the least corrosion rate followed by those deposited from isopropanolic and butanolic suspensions, respectively. The FHA coatings deposited at same voltage and time from smaller molecular size alcohols are thicker due to the faster EPD rate from them. The crack-free and thick FHA coating deposited from ethanolic suspension can more efficiently block the corrosive fluid from reaching to the substrate surface and prevent from its corrosion followed by those deposited from isopropanolic and butanolic suspensions, respectively. Due to their crack-free microstructures and nearly same thicknesses, the SHA and FHA coatings deposited from butanolic suspension were selected to investigate the effect of particles morphology on the corrosion resistance of the coated substrates. The icorr values are 0.38 and 1.12mA/cm2 for the substrates coated with SHA and FHA deposits from their corresponding butanolic suspensions, respectively. It was mentioned that the drying shrinkages is lower for FHA coatings than SHA ones. So after drying, the FHA coatings will have more open microstructure than SHA coatings (as the SEM images (Fig. 10) verify this). The corrosive fluid can penetrate through the pores of coatings and reach the substrate surface to corrode it. So SHA coatings more efficiently prevent from substrate corrosion due to its more closely packed microstructure. Thick FHA coatings with crack-free microstructure can be easily prepared by EPD method. These coatings are strengthened by selfreinforcement action of interlocked FHA particles and have more porous microstructure accelerating the implant fixation. Also FHA particles can potentially act as the nano-needles and promote cell adhesion to implant surface. So FHA coatings are very good candidate to be applied on the metallic implants. 4. Conclusion The suspension of spherical (SHA) and fiber (FHA) hydroxyapatite nanoparticles were prepared in various alcohols using triethanolamine (TEA) as the dispersing agent. The rate of electrophoretic deposition (EPD) from FHA suspensions decreased more rapidly with time due to the larger resistance of FHA deposits leading to the higher voltage drop over them. Sticking parameter (f factor) was unity for all FHA suspensions while it decreased with TEA concentrations for all SHA suspensions. FHA particles were horizontally aligned in the deposits prepared at lower voltages (5 V); FHA particles not aligned parallel to the applied electric field move so slowly towards the substrate that they can find enough space and time to rotate upon reaching the deposit and incorporate horizontally on it. FHA particles are more randomly oriented at higher voltages. Since more number of FHA particles aligned parallel to applied electric field reach to the substrate at higher voltages. Also particles not aligned parallel to electric field move so fast that they cannot find enough time and space for rotation upon reaching the deposit leading to their non-horizontal incorporation in the deposit. All SHA coatings deposited at 60 V for 60s were cracked during drying except the one deposited from butanolic suspension. While only the FHA coating deposited at 60 V for 60s from methanolic suspension was slightly cracked. FHA particles are highly interlocked together in FHA deposits resulting in their strengthening via self-reinforcement mechanism and the drying shrinkages are also lower for FHA deposits leading to their less cracking. The cracks are parallel to each other in FHA coating
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