Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Electrophoretic deposition of ceramic materials for fuel cell applications

La_0.8Sr_0.2Ga_0.875Mg_0.125O_3-x (LSGM), La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ (LSCF), yttria stabilized zirconia (YSZ) and (Ce_0.8Gd_0.2)O_1.9 (CGO) were electrophoretically deposited on Ni foils and Ni-yttria stabilized zirconia substrates prepared by tape casting. It was demonstrated that the ethyl alcohol–phosphate ester–polyvinyl butyral system is an effective solvent–dispersant–binder system for electrophoretic deposition of these materials. The influence of dispersant, binder and current density on deposition efficiency and deposit morphology was studied. The microstructure of the deposits was examined by electron microscopy. The proposed solvent–dispersant–binder medium for electrophoretic deposition of LSGM, LSCF, YSZ and CGO has important advantages and implications in fuel cell design.     

Effects of solid loading on the fabrication of ceramic microparts by soft molding

The effects of solid loading on the fabrication of ceramic microparts by soft molding were studied. Alumina microchannel parts of different dimensions (60–160 µm) were fabricated from well-dispersed suspensions with different solid loadings (70, 75 and 80 wt%). The structural integrity of the green microchannel parts was examined to study the moldability of the suspensions. It was found that the minimum feature size and linear shrinkage of the microchannel parts decreased with increasing solid loading, while the green density and sintered density showed the opposite trend. The reasons for incomplete filling and demolding failures were also discussed.     

Electrophoretic deposition of ceramic slips under a pulse-periodic current

Experimental data on dectmphmetic precipitation of ceramic slips (for porcelain, faience, and household ceramics) onto an aluminum electrode under apulse-periodic current with different parameters (frequency. voltage, time) me presented, Advantages ofdoctrophnrmic technology with dbe use ofupulse-periodic current with u low-frequency spectrum are described. Tbc body grows 25 times faster than in gypsum molds, the density of the precipitate and the strength of the dried and fired specimens increase. and the total consumption of power is reduced. The intensification of the technological process is explained by electromagnetic activation of the working medium and the shaped mixture by the longwave spectrum.Translated from Steklo i Keramika, No. 5, pp. 13–15, 1996.     

Electrophoretic deposition studies of Ba(Zr-Ce-Y)O3 ceramic coating

Electrophoretic deposition (EPD) is now a well established colloidal processing technique which uses electrophoresis mechanism for the movement of suspended charged particles in a suspension in the presence of an electric field. In this work, electrophoretic deposition of BaZr_0.4Ce_0.4Y_0.2O_3-δ (BZCY) in ethanol medium was performed under different conditions on both conducting and non-conducting (porous anode) substrate without using any external additives in a suspension bath. Process parameters such as deposition time, voltage, and rate of deposition of suspended particles were studied under various conditions. Green coating deposited under different potential (30, 50, and 70V) was uniform and crack free, even at extended time of deposition. Surface roughnesses have also been evaluated to correlate it with deposition conditions. It is also found that the rate of deposition on conducting substrate was higher as compared to that on non-conducting substrate (anode). XRD studies of the calcined powder and coating exhibit an expected simple cubic perovskite structure. The deposition yield increases linearly with voltage for each deposition time for both conducting and non-conducting substrates. The coating on non-conducting porous anode heat treated at 1500°C for 2 hours was dense and well adherent to the anode substrate. A film thickness of about 13 μm was obtained at 70V. Such dense BZCY electrolyte coating on BZCY+NiO anode (Half cell) could be well utilized for fabrication of proton conducting SOFC single cell by applying suitable cathode layer on electrolyte film.     

Electrophoretic deposition of nanocrystalline TiO2 particles on porous TiO2-x ceramic scaffolds for biomedical applications

Nanocrystalline TiO₂ coated scaffolds offers the possibility to be used in bone tissue regeneration providing not only space for new tissue formation, but also to enhance bioactivity of the implant. In the present study, direct current electrophoretic deposition (EPD) was chosen as simple and low cost technique to coat 3D porous structure of TiO_2-x ceramic. Suspension for EPD was prepared suspending nanocrystalline TiO₂ particles in isopropanol and adding triethanolamine as dispersant. TiO₂ particles were electrophoretically deposited on the surface of TiO_2-x scaffolds through varying EPD time and applied voltage. The scaffold pore structure was maintained after applying the coating by EPD. The deposition of nanocrystalline TiO₂ coating can be a smart strategy to impart bioactive properties to the 3D scaffold, allowing formation of spherical hydroxyapatite particles on the coated scaffolds after immersion in simulated body fluid. In vitro cell studies does not show cytotoxic effect of nanocrystalline TiO₂ coated scaffolds.     

Electrophoretic fabrication of thermal barrier coatings

An electrochemical method of fabrication of (NiCoCrAlY)/MgO/yttria-stabilized zirconia (YSZ) multilayered coating was proposed. This multilayered coat is expected to work as a thermal barrier coating for nickel superalloy substrates. The (NiCoCrAlY) layer was deposited using the electrophoretic deposition technique, the MgO layer was deposited by the electrolytic deposition technique and the YSZ layer was electrophoretically deposited. The process of depositing (NiCoCrAlY) alloy particles revealed that the electrophoretic technique can be used for particles with submicron dimensions. The MgO intermediate layer was introduced to accommodate the difference in thermal expansion coefficient between the YSZ ceramic and the NiCoCrAlY metal layers.     

Electrophoretic deposition for fabrication of YSZ electrolyte film on non-conducting porous NiO–YSZ composite substrate for intermediate temperature SOFC

Electrophoretic deposition (EPD) of YSZ electrolyte films onto porous NiO–YSZ composite substrates that had been pre-coated with graphite thin layers was carried out in the following two means for solid oxide fuel cell application: one was EPD based on electrophoretic filtration by which YSZ films were formed on the reverse sides without the graphite layers; the other was EPD on a graphite thin layer pre-coated on the substrates. Dense YSZ electrolyte thin films were successfully obtained in both means, although it was difficult to form YSZ films that were strongly adherent to the substrates using the latter means. The densification of YSZ films was assisted by shrinkage of the substrates during co-firing. A single cell was constructed on ca. 5 μm thick dense YSZ films fabricated using the EPD based on electrophoretic filtration. Maximum power densities over 0.06, 0.35, 1.10 and 2.01 W/cm² were attained, respectively, at 500, 600, 700 and 800 °C on the cell.     

Electrophoretic deposition of zein coatings

In this study, crack free, well-adhered and transparent zein coatings were obtained on 316L stainless steel substrates by electrophoretic deposition (EPD) employing varying deposition voltages and times. Obtained films were studied by Fourier transform infrared spectroscopy and scanning electron microscopy, and it was shown that the obtained coatings exhibit homogeneous and smooth surfaces. The deposition yield was investigated at various EPD conditions; the highest yield was found at 10 V and 10 min deposition time. The deposition mechanism was discussed by considering chemical reactions occurring during EPD. The EPD method developed here is attractive for the surface modification of metal implants by zein layers aiming at functionalizing surfaces for biomedical applications.     

Electrophoretic deposition of carbon nanotubes

Electrophoretic deposition (EPD) has been gaining increasing interest as an economical and versatile processing technique for the production of novel coatings or films of carbon nanotubes (CNTs) on conductive substrates. The purpose of the paper is to present an up-to-date comprehensive overview of current research progress in the field of EPD of CNTs. The paper specifically reviews the preparation and characterisation of stable CNT suspensions, and the mechanism of the EPD process; it includes discussion of pure CNT coatings and CNT/nanoparticle composite films. A complete discussion of the EPD parameters is presented, including electrode materials, deposition time, electrode separation, deposition voltage and resultant electric field. The paper highlights potential applications of the resulting CNT and CNT/composite structures, in areas such as field emission devices, fuel cells, and supercapacitors.     

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 titanium nitride coatings

Electrophoretic deposition of the titanium nitride (TiN) coatings from suspensions prepared by dispersion of TiN particles in triethanolamine (TEA) containing butanol medium was studied. Effects of the TiN particles concentration (C_TiN) on the weight of the deposited coatings, triethanolamine concentration (C_TEA=0.25, 0.5, 0.75, and 1 mL/L) on the Zeta potential of the TiN particles, suspension electrical conductivity and pH, as well as effects of the deposition voltage (V_d=60, 90, and 120 V) and time (t_d =1, 2, and 3 minutes) on the microstructure and thickness of the deposited coatings were investigated. Variations in deposition current density, effective deposition voltage, electrical resistance, and deposited coating weight versus deposition time were recorded. The morphology of the as‐dried coatings was studied using Scanning Electron Microscope (SEM). The results indicated that by increasing the C_TiN the weight of deposits increases linearly up to 40 g/L. For suspensions containing C_TiN=40 g/L, the optimum C_TEA is obtained to be 0.5 mL/L leading to Zeta potential of 43.25 mV. Uniform and crack‐free as‐dried coatings obtained at V_d and t_d of 90 V and 2 minutes, respectively.     

Kneading and molding of ceramic microparts by precision powder injection molding (PIM)

This paper investigates the kneading and formability of microparts made using alumina in micro‐powder injection molding. In this study, quality feedstock with uniform powder dispersion was achieved when optimum kneading process was performed. In addition, the thin microplates were successfully manufactured using a custom‐made injection machine. Shrinkage was significantly reduced in microspecimens when the mold temperature was increased to 70°C. The results of flow visualization were conformed to that of experiments in this study. A very important result for flow visualization and experiment was molten polymer filled the cavity by shortest period producing a least shrinkage in microparts. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 892–899, 2006     

Electrophoretic deposition as rapid prototyping method

Electrophoretic deposition is a near-net shaping technique leading to excellent green body properties such as high green density and homogeneous pore size distribution that also affect the features of the sintered sample. It has been used in a wide range of applications from optical to biomedical ones. However, it was used as standardized method so that individually tailored geometries have not been fabricated yet. The present work modifies this promising method so that a rapid prototyping process is possible. First of all, the local application of an electric field was investigated and simulated in order to achieve a structure as fine as possible using commercially available coaxial cable. Afterwards, two different ways of fabricating prototype structures with optimized parameters were investigated. The first one was the combination of the developed electrode with a CAM unit. The second one was arranging 16 independent electrodes in a 4 × 4 array in order to parallelize deposition. In all cases, structural integrity as well as height distribution and shape were studied.     

Electrophoretic deposition of MgO from organic suspensions

MgO is extensively used as a basic refractory material, but in the last decade it has also been considered as a suitable material for technological applications in fields such as catalysis or electronics. Such applications have imposed new requirements regarding the processing techniques. In this way, the electrophoretic deposition (EPD) has been demonstrated to be a useful and simple deposition method among the available coating technologies. A difference with other coating techniques is that the material to be processed by EPD does not require defined properties and hence, there are a wide number of materials that could be coated and deposited by EPD. Major advantages of EPD are versatility, low-cost and reproducibility. The aim of this work was to obtain controlled deposits of MgO onto metallic electrodes by EPD. For this purpose the stability of MgO suspensions in ethanolic media was studied by electrokinetic sonic amplitude (ESA) at different pH conditions. The EPD kinetics was further studied at different electrical conditions. The characteristics of the deposits were studied in relation to the suspension properties and the electrical conditions.     

Electrophoretic deposition of chitosan in different alcohols

The electrophoretic deposition (EPD) of chitosan coatings on 316L stainless steel substrates using alcohol–water solutions was investigated in different electrical fields and times. The effect of water content in solution on the pH and conductivity of solution, current density during EPD, deposition rate and morphology, adhesion strength, and corrosion resistivity of coatings were studied. The difference between the conductivities obtained from conductivity meter and Ohm’s law was used as a criterion for the degree of water electrolysis during EPD. It was found that in the same water content, the degree of water electrolysis decreases as the molecular weight of alcohol increases. The optimum water content to achieve high quality coatings was 5, 15, and 20 vol% in methanol, ethanol, and isopropanol, respectively.     

Electrophoretic deposition of macroporous carbon nanotube assemblies for electrochemical applications

Electrophoretic deposition of macroporous assemblies of single-walled carbon nanotubes (SWCNTs) is described. The macroporous structure was created thanks to the presence of polystyrene (PS) beads which were co-deposited with the carbon nanotubes in a 60 V potential field. The ratio between the quantity of carbon nanotubes and polystyrene beads in the solution for deposition was found to be critical for the proper self-assembly of the composite film during electrophoretic deposition. The macroporous films have been characterized by scanning electron microscopy, atomic force microscopy and profilometry. The macroporosity was revealed after template removal (calcination of the PS beads). Access to the internal surface was assessed by electrochemical characterization using methylene green as a redox probe likely to adsorb on the SWCNT surface. Platinum nanoparticles and a sol–gel layer with encapsulated dehydrogenase and NAD⁺ cofactor have been deposited on the macroporous SWCNT electrodes in order to illustrate the use of the macropore texture for the detection of H₂O₂ and for biosensor applications, respectively.     

Novel application of ceramic precursors for the fabrication of composites

Preceramic polymers are enabling the development of a variety of advanced shaping methods which, in turn, make possible new and cost-effective approaches for the fabrication of composite materials. This opens new perspectives for the mass production of composites which might, for example, be used in cost-sensitive areas of application in the machine and automobile industries. In two examples it will be shown how preceramic polymers can be used to obtain both metal matrix composites (MMC) and ceramic matrix composites (CMC). Their properties will be discussed in particular with respect to the usage of a preceramic polymer.The first example shows an approach to manufacturing short-fibre-reinforced CMCs by means of a plastic forming technique which involves mixing of either carbon or SiC fibres, ceramic fillers and a viscous ceramic precursor. The precursor permits a fibre-reinforced ceramic with a low porosity to be obtained. The role of the precursor in the whole process and the resulting material properties will be discussed.The second example shows a method for fabricating porous SiC ceramic preforms which are subsequently infiltrated with aluminium to form a MMC. By using the precursor route, a machinable preform with tailored porosity can be produced. Correlations between precursor, preform and MMC properties will be drawn.