Electrophoretic deposition of hydroxyapatite nanostructured coatings with controlled porosity

Hydroxyapatite (HA) coatings with controlled porosity were prepared by electrophoretic deposition (EPD) method. Carbon black (CB) particles were used as the sacrificial template (porogen agent). Two component suspensions containing different concentrations of HA and CB particles were prepared in isopropanol. It was found that the finer and positively charged HA nanoparticles are heterocoagulated on the coarser and negatively charged CB particles to form CB–HA composite particles with net positive charge. The deposition rate from the suspensions with WR (C_CB/C_HA ratio) of 0.25 was faster than that of those with WR: 0.5 at initial times of EPD. However the situation was reversed at longer EPD times. It was also found that the amount of porosity in the coatings increases as the CB concentration in the suspension increases (15%, 24%, 31%, 43% for the coatings deposited from the suspensions with 20 g/L HA nanoparticles and 0, 5, 10 and 20 g/L CB particles, respectively).     

Electrophoretic deposition of hydroxyapatite-chitosan-CNTs nanocomposite coatings

Three component suspensions of hydroxyapatite (HA), chitosan and CNTs were prepared in ethanol base solution (15 vol% water and 0.05 vol% acetic acid). The adsorption of HA nanoparticles on CNTs was investigated by FTIR and SEM analysis. It was found that HA nanoparticles are adsorbed on CNTs via chemical bonding between -NH₂ groups of chitosan (adsorbed on their surface) and -COOH groups of CNTs. Current density as well as kinetics of EPD was studied at 60 V. It was found that current density increases or remains nearly constant during EPD due to the rise in water electrolysis as deposit grows on the substrate. Deposition weight against EPD time showed a linear trend due to the absence of any voltage drop over the deposit during EPD. The incorporation of chitosan and CNTs in the microstructure of coatings was confirmed by TG/DTA and SEM analysis. CNTs exhibited high efficiency in reinforcing the microstructure of coatings and preventing from their cracking. CNTs incorporation in the coatings improved their mechanical properties (adhesion strength, hardness and elastic modulus) and corrosion resistance.     

Electrophoretic deposition of fiber hydroxyapatite/titania nanocomposite coatings

Hydroxyapatite/titania nanocomposite coatings were electrophoretically deposited from ethanolic suspensions of titania and fiber shaped hydroxyapatite (FHA) nanoparticles. Triethanolamine (TEA) was used to enhance the colloidal stability of particles in suspensions. Electrophoretic deposition (EPD) was performed using the suspensions with different concentrations (wt%) of titania/FHA particles. EPD rate decreased more rapidly with time for suspensions with higher wt% of FHA due to the higher voltage drop over the deposits shaped from them. Stacking of long FHA particles on the substrate during EPD resulted in the formation of coarse pores in the deposits. It was found that titania nanoparticles can more efficiently infiltrate through and fill the pores in TEA containing suspensions due to the stronger electrostatic repulsion force between pore walls (FHA) and titania nanoparticles in them. The coatings deposited from the suspensions with 50 wt% of FHA or more did not crack during drying due to the significant reinforcement action provided by high wt% of FHA in them. Nanocomposite coatings deposited from TEA containing (2 mL/L) suspensions with 50 and 75 wt% of FHA had the best corrosion resistance in simulated body fluid (SBF) solution due to their crack-free microstructure and efficiently filled pores.     

Electrophoretic deposition of titania nanostructured coatings for photodegradation of methylene blue

The suspensions of titania nanoparticles were prepared in different alcohols (methanol, ethanol, isopropanol and butanol) using polyethyleneimine (PEI) as dispersant. The results of sedimentation, conductivity, zeta potential, FTIR and thermal analysis showed that PEI macromolecules are protonated in the suspensions and then adsorbed on the particles enhancing their positive surface charge and so colloidal stability. Optimum concentration of PEI (PEI^*) was lower in large molecular size alcohols due to its higher adsorption efficiency. PEI^* was 0.75, 0.5, 0.5 and 0.25?g/l in methanolic, ethanolic, isopropanolic and butanolic suspensions, respectively. Electrophoretic deposition (EPD) was performed at 60?V in different alcoholic suspensions. EPD rate was the fastest in the suspensions with PEI^* due to the highest mobility of particles. In contrast to the coatings deposited for 1?min from isopropanolic and butanolic suspensions, those deposited at same conditions from methanolic and ethanolic ones severely cracked during drying owing to their higher thickness and lower PEI contents (acting as the binder). The coatings deposited from methanolic and ethanolic suspensions with PEI^* had finer and rougher microstructures. The sintered coating deposited at 60?V for 10?s from ethanolic suspension with PEI^* had crack-free microstructure with the thickness of ≈?130?μm. This coating degraded ≈?24% of methylene blue from its aqueous solution (30?ml with the concentration of 5?mg/l) within 2?h under UV illumination. Photodegradation of MB on the surface of titania coating obeyed the first order kinetics law.     

AC vs. DC electrophoretic deposition of hydroxyapatite on titanium

In this study, electrophoretic deposition (EPD) of hydroxyapatite (HA) powder on titanium plate was performed using butanol as solvent under direct current (DC) and alternating current (AC) fields. The zeta potential of the suspensions was measured to define their stability and the charge on the particles. Coating thickness was varied by adjusting the voltage and time of deposition. Surface morphology and cross section thickness were studied using scanning electron microscopy and image analysis software. Surface crack density was calculated from the micrographs. The results showed that the samples of similar thickness have higher grain density when coated using AC as compared to DC EPD. This facile but novel test proves the capability of AC-EPD to attain denser and uniform HA coatings from non-aqueous medium.     

Fabrication of aluminum nitride coatings by electrophoretic deposition: Effect of particle size on deposition and drying behavior

Electrophoretic technique was used to deposit micro- and nano-sized aluminum nitride coatings on stainless steel surfaces by using a well-dispersed stable suspension produced by addition of AlN powder plus a small amount of iodine to ethanol. Parabolic regime governed the deposition. Electrophoretic deposition for 240 s at 100 V resulted in formation of a uniformly dense film on the top, but a porous inhomogeneous layer at the bottom. This was attributed to fast deposition of coarse particles and/or agglomerates at large electric fields. After drying, micro-sized particles led to a uniform crack-free interface while nano-particles resulted in fragmented non-cohesive layers. Weight loss measurements revealed higher drying rates for micro-layer as compared to nano-cover. This seemed owing to the larger pore sizes and lower specific surfaces of the former. Stress inducement by lateral drying of small capillaries led to crack initiation from the edges and its propagation across the surfaces. This resulted in fragmentation of the samples due to their delamination. Effect of deposition rate on particles packability was also investigated.     

Suspension characterization and electrophoretic deposition of Yttria-stabilized Zirconia nanoparticles on an iron-nickel based superalloy

Yttria-Stabilized Zirconia (YSZ) is the most common material for thermal barrier coatings. Suspensions of 3 mol% YSZ nanoparticles in acetone medium have been prepared in presence of different amounts of iodine as dispersant. Size distribution of particles in the suspensions and zeta potential were measured as a function of dispersant concentration. Adding 1.2 g/l iodine was found to be effective for the dispersion of YSZ nanoparticles in acetone. The stability of YSZ suspension in acetone increased with iodine content increasing until reached 1.2 g/l. Mean diameter of particles and zeta potential of the YSZ suspension in acetone were 912 nm and 2.4 mV respectively, and with addition of 1.2 g/l iodine shifted to 111.6 nm and 50.2 mV respectively. Electrophoretic deposition (EPD) process has been carried out from this suspension at different applied voltages and deposition times. A uniform green coating was obtained at voltage of 6 V and deposition time of 2 min the thickness of the green coating is measured about 25 µm.     

Effects of suspension properties on the fabrication of Yb2Si2O7 coatings using electrophoretic deposition

This study examines the electrophoretic deposition of Yb₂Si₂O₇ particles on SiC substrates to produce Environmental Barrier Coatings. To prepare crack-free and homogeneous green coatings, the effect of the solvent, dispersant concentration, and pH were investigated. Ethanol provided a well-dispersed suspension and crack-free coating which was shown by sedimentation tests and microstructure analysis. The effect of the dispersant concentration was investigated with zeta potential measurement and microstructure analysis with a concentration above 0.5 g/L resulting in higher ionic strength and producing cracked and uneven coatings. The ionic strength was also associated with the powder packing density with larger indentation impressions measured for loosely packed coatings. The deposition rate depended on the suspension properties influenced coating integrity with delamination evidenced by analysing the current density drop during deposition. Sintering of the green coatings having different densities and microstructure showed their importance in the preparation of uniform and dense sintered coatings.     

On the role of the indifferent electrolyte LiCl in electrophoretic deposition of hydroxyapatite from 2-propanol dispersions

In the present work, the effect of an indifferent electrolyte (LiCl) on the electrical conductivity of dispersions for the electrophoretic deposition (EPD) of thick layers (>1 mm) of hydroxyapatite (HA) with low surface roughness was studied. The indifferent electrolyte was added at concentrations of 0.10 and 0.25 g/L into dispersions, which contained HA particles, 0–21.25 wt% monochloroacetic acid (MCAA) and 2-propanol. The stability of the dispersions, which was determined by measuring the zeta potential, increased with the MCAA content in the dispersion (up to −31.4 mV). An addition of LiCl slightly reduced the stability of the dispersions but significantly increased their electrical conductivity. The increase in electrical conductivity caused the decrease in deposition rate during the EPD, which affected the deposition yields and deposit thickness. The relative density of the HA deposits increased with the MCAA content in the dispersion and further increased when 0.10 g/L of LiCl was added. Moreover, using a highly conductive dispersion, thick HA layers were deposited with low surface roughness and no surface cracks. An optimum content of indifferent electrolyte in the dispersion enables the preparation of relatively dense and thick (>2 mm) HA deposits with a low surface roughness (~3 µm) via EPD, which can be used in medical applications.     

Electrophoretic deposition of titania nanostructured coatings with different porous patterns

Titania nanostructured coatings with different porous patterns were fabricated by electrophoretic deposition (EPD) in isopropanolic suspension including different concentrations of carbon active (CA) or carbon black (CB) particles as the porogen additives. Finer and negatively charged CA particles were electrostatically adsorbed on the coarser and positively charged titania particles and formed CA-titania particles. While, finer and positively charged titania particles were electrostatically adsorbed on the coarser and negatively charged CB particles to form titania-CB particles. Both CA-titania and titania-CB particles had the net positive surface charge and so cathodic EPD was applicable. EPD was carried out at optimized conditions of 60?V and 10?s. Thermogravimetry (TG) analysis showed that CA and CB burn out between 450?°C and 600?°C. The higher the carbon content in the suspension the higher was their content in the coating. The coatings were characterized by SEM, AFM, adhesion strength and bioactivity tests. Even coatings with interconnected fine pores and low roughnesses were obtained after the heat treatment of CA-titania coatings. While, rough coatings with coarse and isolated pores were obtained after the heat treatment of titania-CB coatings. The porosity of coating increased as the carbon content increased in the suspension. The hydroxyapatite layer grew on the coatings after their soaking in simulated body fluid for 1week at 37.5?±?1.5?°C.     

Thermal behaviour of vitreous ceramic coatings obtained by electrophoretic deposition for furnace components

In this study, three different vitreous ceramic coatings have been designed to improve radiation heat transfer and thereby increase the thermal efficiency of fired heaters or furnaces working at high temperatures. The vitreous ceramic coatings were produced through Electrophoretic Deposition technique (EPD) of ceramic suspensions. These ceramic formulations were designed based on components which increase emissivity, such as SiO₂ and a Black dye (based on chromium, copper and iron oxides), added in 25 wt%. These coatings showed emissivity values around 0.89 at room temperature and around 0.82 at 550 °C in the middle infrared (MIR) spectral range, with slight differences between them. The SiO₂ and Black dye additions provide an important protective effect on the coatings’ thermal stability as it was proved by the absorbance level at long times, higher than 85% in the near infrared (NIR) spectral range. These results were also supported by microstructural characterisation, substrate-coatings adhesion strength and thermal stability tests.     

Vancomycin loaded-mesoporous bioglass/hydroxyapatite/chitosan coatings by electrophoretic deposition

In the current research, vancomycin loaded-mesoporous bioglass/hydroxyapatite whiskers/chitosan composite coatings with drug delivery capability were fabricated on titanium substrate via electrophoretic deposition. Hydroxyapatite with whisker morphology demonstrated improvement in microhardness of the coatings. Among different initial vancomycin concentrations of 1, 5, 15, 25 and 50 mg/ml to load mesoporous bioglass particles, maximum loading efficiency and drug content (95%) were achieved at 25 mg/ml. Based on the quality of the coatings, bioglass loaded with initial concentrations of 1, 5 and 15 mg/ml drug were chosen as optimal samples for further investigations. Synergistic effect of bioglass and hydroxyapatite caused the composition with 20% and 50%Wt MBG to have faster bioactivity response. Regarding the drug delivery capability of the composite coatings, sample containing 50%Wt bioglass encapsulated with 15 mg/ml drug exhibited rapid and maximal drug release due to the facile diffusion of the solved bioglass particles. The achieved results showed the acceptable antibacterial potency of vancomycin-loaded coating to prevent infection. Moreover, existence of antibiotic did not have any toxic effect over the osteoblast cells cultured on the coatings.     

Liquid metal corrosion resistant LaPO4 coating with metallophobic characteristics fabricated on 316 stainless steel using electrophoretic deposition technique

A liquid metal corrosion (LMC) resistant and metallophobic lanthanum phosphate (LaPO₄) coating was prepared on SUS316 stainless steel using electrophoretic deposition (EPD) technique. A specific hierarchical surface structure was created on coating surface by adjusting EPD process parameters. LMC test was performed using three different metal melts, Al–Zn–Mg alloy, Mg–Al–Mn alloy, and pure Zinc. Results indicated that steel bare surface was severely attacked by all three melts. The mechanism of corrosion process was explained in each case. After coating, the LaPO₄ covered steel showed an excellent resistance against all three liquid metals. Besides, wetting of steel surface by liquid metals was strongly decreased by application of LaPO₄ surface coating. This can be attributed to the intrinsic metallophobic characteristic of LaPO₄ as well as the hierarchical surface structure developed on coating surface.     

Structural changes of hydroxyapatite coating electrophoretically deposited on NiTi shape memory alloy

The surface of the NiTi shape memory alloy was functionalized through the deposition of hydroxyapatite (HAp) coatings using the electrophoretic method (EPD). The electrophoresis carried out at the voltage of 40?V during the time of 120?s did not affect the crystalline structure of the initial HAp powder and, at the same time, ensured obtaining a homogeneous layer without visible cracks or discontinuities. Next, the coatings were subjected to heat treatment at 800?°C for 2?h in vacuum, wherein the applied conditions did not affect the decomposition of the deposited hydroxyapatite. The heat treatment resulted in the formation of carbonate apatite (C-HAp) in the HAp layer and in ceramic particles’ coalescence. Changes in the morphology and roughness of the layer as well as partial decomposition of the NiTi substrate parent phase into Ti₂Ni and Ni₄Ti₃ phases were also observed.     

Electrophoretic deposition of titania–carbon nanotubes nanocomposite coatings in different alcohols

The suspensions of titania nanoparticles in different alcohols (methanol, ethanol and butanol) were prepared using triethanolamine (TEA) as a dispersant. The optimum concentration of TEA was 16.67, 8 and 0.33 mL/L in methanol, ethanol and butanol, respectively. Two component suspensions of titania (20 g/L) and carbon nanotubes (CNTs) (0.1, 0.2, 0.5 and 1 g/L) were prepared in different alcohols without and with optimum concentration of TEA. The finer and positively charged titania nanoparticles were heterocoagulated on the surface of coarser and negatively charged CNTs and generated the titania–CNT composite particles with the net positive charge. In the presence of TEA, titania nanoparticles completely covered CNTs surface due to their higher positive surface charge. At same CNT concentration, the deposition rate was faster for suspensions with TEA additive due to the faster mobility of the composite particles. The photocatalysis efficiency of coatings for methylene blue degradation increased as CNTs were incorporated in their microstructure.     

Electrophoretic deposition of biomimetic zinc substituted hydroxyapatite coatings with chitosan and carbon nanotubes on titanium

In this study, a biomimetic method was used to prepare hydroxyapatite (HAP) and zinc substituted HAP (ZnHAP) nanoparticles, in which silk fibroin was used as template. The morphology of HAP is rod-like, while ZnHAP changes to wrinkled sheets. HAP and ZnHAP nanoparticles were used to coat titanium by EPD with additional chitosan and multiwalled carbon nanotubes. Phase composition, morphology and structure were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared and Raman spectroscopy. The results showed that the composite coatings containing HAP and ZnHAP had homogeneous morphology and good apatite formation ability. The ZnHAP composite coating possessed class 5B adhesive strength using tape test. Furthermore, the ZnHAP composite coating had better corrosion resistance compared to the HAP composite coatings.     

Influence of anionic stabilization of alumina particles in 2-propanol medium on the electrophoretic deposition and mechanical properties of deposits

The ceramic dispersions were prepared using 0.85, 1.70, 4.25, 12.75 or 21.25 wt.% of monochloroacetic, dichloroacetic or trichoroacetic acid, 15 wt.% alumina and 2-propanol. The mechanism of anionic stabilization in 2-propanolic media was described. Alumina green bodies were prepared from the stable dispersion via electrophoretic deposition (EPD). It was found that increasing dispersion conductivity significantly influenced the EPD yields. The most effective electrophoretic depositions were performed from dispersions with conductivity in range 4.0–5.3 × 10⁻⁴ S m⁻¹. Deposits with the highest green density were prepared from the dispersion stabilized by trichloroacetic acid. This behavior was explained by low voltage drop during deposition. The surface roughness was high at low dispersion conductivity and with increasing acid concentration in dispersion the surface of deposits was smoother. The mechanism of particle arrangement in deposit was discussed. Influence of stabilizer amount in the dispersion on the hardness and fracture toughness was described.     

Filling behavior of ZnO nanoparticles into opal template via electrophoretic deposition and the fabrication of inverse opal

Combining colloidal crystal template (artificial opal) and electrophoretic deposition (EPD) process, well-ordered ZnO inverse opal can be formed by finding the optimum driving potential of EPD. Through providing the various driving potentials from −25 V, −10 V, −5 V to −2.5 V, the different mechanism of electrophoretically depositing ZnO nanoparticles into the colloidal crystal template was determined by the SEM observation of the filled templates. Because the nano-channels of colloidal crystal template are the network type, the results of surface jam, incomplete filling and perfect filling are found under specific applied voltages. The high-quality ZnO inverse opal can be only fabricated under the perfect nano-channel-filling condition. The filling behavior can be monitored dynamically by tracing the current transients, and the optimum conditions for filling the interstitial spaces of templates constructed from colloidal particles with 180 nm and 300 nm diameter can be obtained by applying a voltage of −5 V and −15 V, respectively. After the complete filling of ZnO nanoparticles into the colloidal crystal template consisting of 300 nm colloids, high-quality ZnO photonic crystal possessing an absorptive peak at the wavelength of 560 nm can be fabricated by removing the template. It is expected that the EPD can find extensive applications for preparing photonic crystals of various oxides only if their nanoparticles are available.     

Alumina ceramics based on seeded boehmite and electrophoretic deposition

It is shown that seeding a commercial boehmite sol with crystallographically suitable seeds reduces both the crystallization temperature for the final -A1₂O₃ phase and the sintering densification temperature. The seeding component was a tailored combination of δ- and -alumina particles in nanometre and micrometer ranges, respectively, dispersed in water. Electrophoretic deposition (EPD) provided a quick, simple and cost-effective processing route to prepare dense monolithic alumina ceramics from the seeded boehmite suspensions. EPD-formed green bodies could be sintered and densified at temperatures as low as 1250 °C for 2 h.     

Influence of deposition temperature on the properties of hydroxyapatite obtained by electrochemical assisted deposition

Surface functionalization of pure titanium (cp-Ti) with hydroxyapatite (HAp) was successfully achieved by means of electrochemical deposition (ED) in a solution containing calcium nitrate and ammonium dihydrogen phosphate. The aim of this study is to evaluate the influence of the deposition temperature on the elemental and phase composition, chemical bonds, morphology, and in vitro electrochemical behaviour in biological simulated media (simulated body fluid - SBF). The roughness and wettability of the developed coatings are also investigated. By increasing the deposition temperature from 50 °C to 75 °C, the HAp coatings present a well-crystalized structure, denser and a nobler behaviour in terms of electrochemical behaviour in SBF at 37 °C. Also, by increasing the deposition temperature from 50 °C to 75 °C, the contact angle has decreased from 76.1° to 27.4°, exhibiting a highly hydrophilic surface. Taking into consideration all the obtained data, electrodeposition of HAp at 75 °C was found preferable when compared to 50 °C. The characteristics of the HAp coatings can be easily adjusted by optimizing the electrochemical deposition parameters and/or controlling specific features like pH, temperature, or ionic concentration of electrolyte, etc.