This article is one of eight papers to be presented exclusively on the web as part of the January 2000 JOM-e the electronic supplement to JOM.
	The following article appears as part of JOM-e, 52 (1) (2000), http://www.tms.org/pubs/journals/JOM/0001/Zhitomirsky/Zhitomirsky-0001.html JOM is a publication of The Minerals, Metals & Materials Society

Functional Coatings: Overview

Ceramic Films Using Cathodic Electrodeposition

I. Zhitomirsky


TABLE OF CONTENTS
INTRODUCTION CATHODIC ELECTROPHORETIC DEPOSITION CATHODIC ELECTROLYTIC DEPOSITION APPLICATIONS References

Electrodeposition is evolving as an important method in ceramic processing. Two processes for forming ceramic films by cathodic electrodeposition are electrophoretic deposition, in which suspensions of ceramic particles are used, and electrolytic deposition, which is based on the use of metal salts solutions. Electrolytic deposition enables the formation of thin ceramic films and nanostructured powders; electrophoretic deposition is an important tool in preparing thick ceramic films and body shaping.

INTRODUCTION

Electrophoresis was discovered in 1809 by Reuss of Moscow University. Many processes based on electrophoretic deposition have been described,^1,2 including deposition of thick films, laminates, and body shaping. Some of these processes are in commercial use. Significant interest has recently focused on cathodic electrodeposition, which offers important advantages for various applications;³ cathodic electrolytic deposition is a new technique in ceramic processing⁴ that has been used to produce a variety of ceramic thin films.^3-22

Electrodeposition offers rigid control of film thickness, uniformity, and deposition rate and is especially attractive owing to its low equipment cost and starting materials. Due to the use of an electric field, electrodeposition is particularly suited for the formation of uniform films on substrates of complicated shape, impregnation of porous substrates, and deposition on selected areas of the substrates. Two electrodeposition processes have been developed for forming ceramic films: electrophoretic deposition (EPD)^1-3 and electrolytic deposition (ELD) (Figure 1).^3,4 Features of the two processes are shown in Table I.

Table I. Electrophoretic and Electrolytic Deposition of Ceramic Materials
	Electrophoretic Deposition	Electrolytic Deposition
Medium	Suspension	Solution
Moving Species	Particles	Ions or complexes
Electrode Reactions	None	Electrogeneration of OH- and neutralization of cationic species
Preferred Liquid	Organic solvent	Mixed solvent (water-organic)
Required Conductivity of Liquid	Low	High
Deposition Rate	1-10³ mm/min	10^-3-1 mm/min
Deposit Thickness*	1-10³ mm	10^-3-10 mm
Deposit Uniformity^†	Limited by size of particles	On nm scale
Deposit Stoichiometry	Controlled by stoichiometry of powders used for deposition	Can be controlled by use of precursors

*Controlled by variation of deposition time, voltage, or current density. † Controlled by electric field.

Figure 1. A schematic of electrolytic deposition and electrophoretic deposition.

CATHODIC ELECTROPHORETIC DEPOSITION

Electrophoretic deposition, a process in which ceramic particles, suspended in a liquid medium, migrate in an electric field and deposit on an electrode, has been the subject of considerable interest; review papers are now available.^1,2 Electrophoretic deposition offers important advantages in the deposition of complex compounds and ceramic laminates. The degree of stoichiometry in the electrophoretic deposit is controlled by the degree of stoichiometry in the powder used. According to Reference 1, particle/electrode reactions are not involved in EPD, and ceramic particles do not lose their charge on being deposited. The reversal of the electric field results in stripping-off the deposited layer. Therefore, it is important to use similarly charged particles and similar solvent-binder-dispersant systems for forming laminates of various ceramic materials and gaining better control of layer thickness.

A suspension for EPD is a complex system in which each component has a substantial effect on deposition efficiency. There are two principal types of solvents used: water and organic liquids. Organic liquids are superior to water as a suspension medium since the use of water-based suspensions causes gas formation from the hydrolysis of water. In general, suspensions can be dispersed by electrostatic, steric, or electrosteric stabilization mechanisms. Ceramic particles must be electrically charged to permit forming by electrophoretic deposition. The charge on a colloidal particle could originate from various sources, such as from adsorbed simple inorganic ions or from dispersants. A binder is also added to the liquid to increase the adherence and strength of the deposited material and prevent cracking.

When testing a new ceramic material in the laboratory, polyvinyl butyral as a binder, phosphate ester as a dispersant, and ethyl alcohol as a solvent were generally used. Experimental results presented in Reference 23 indicate that phosphate ester is one of the most effective commercial dispersants, acting as a steric dispersant by anchoring the long chain molecules to the particle surfaces. Moreover, phosphate ester is an effective electrostatic stabilizer, which charges the particles positively in organic liquids by donating protons to the surface.^23,24

Table II. The Compositions of Suspensions (SP) and Solutions (SL) and Experimental Conditions for Constant-Current EPD and ELD
Suspension or Solution	Material	Additives	Solvent	Temperature (° C)	Current density (mA/cm²)

SP1	100 g/l TiO₂^A	2.2 g/l PVB^G + 2.5 g/l PE^H	Ethyl alcohol	20	0.1
SP2	100 g/l YSZ^B	3 g/l PVB^G + 3.5 g/l PE^H	Ethyl alcohol	20	0.3
SP3	100 g/l Al₂O₃^C	2.3 g/l PVB^G + 2.7 g/l PE^H	Ethyl alcohol	20	0.2
SL1	5 mM TiCl₄^D	0.01 M H₂O₂^I	Methyl alcohol-water (3:1 volume ratio)	1	20
SL2	5 mM ZrOCl₂^E	-	water	20	20
SL3	5 mM Al(NO₃)₃^F	-	Ethyl alcohol-water (19:1 volume ratio)	20	5
SL4	2.5 mM TiCl₄^D + 2.5 mM ZrOCl₂^E	0.02 M H₂O₂^I	Methyl alcohol-water (3:1 volume ratio)	1	20
SL5	0.02M SnCl₄^F	0.15 M H₂O₂^I	Ethyl alcohol-water (19:1 volume ratio)	20	10
A Cerac (-325 mesh) B yttrium-stabilized zirconia (YSZ) ,TZ-8Y, Tosoh C Venton, Alfa Division (-400 mesh) D Merck E Fluka Chemie AG F Aldrich Chemical Company G polyvinyl butyral, average M_w = 50,000-80,000, Aldrich Chemical Company H phosphate ester, Emphos PS-21A, Witco I 30 wt.% in water, Carlo Erba Reagenti

Figure 2. Deposit weight versus time for (a-top) electrophoretic deposits obtained from suspensions SP1-SP3 and (b-bottom) electrolytic deposits obtained from solutions SL1-SL3 at constant current regimes.

Suspensions for EPD are produced by breaking down agglomerates and uniformly distributing a dispersing agent on the surfaces of the ceramic particles. The particle deagglomeration is carried out by milling and ultrasonic treatment. The preparation of suspensions is carried out in two stages. The dispersant must be added before the binder to prevent competitive adsorption. Figure 2a shows deposit weight versus time dependencies for titania, zirconia, and alumina deposits obtained from suspensions SP1, SP2, and SP3, respectively (Table II). It is seen that deposit weight increases with time at a constant current density. The experimental data presented in Figure 2a demonstrate a manner in which the amount of deposited material can be controlled.

Experiments indicate that the ethyl alcohol-phosphate ester-polyvinyl butyral system is an effective system for cathodic deposition of various ceramic materials. This is especially important for deposition of consecutive ceramic layers of controlled thickness in multilayer processing. Problems related to the application of toxic solvents, the chemical compatibility of powders and additives, and deposit contamination and corrosion of electrodes could be eliminated or diminished. Prepared suspensions exhibited high stability, and a relatively high deposition rate could be achieved. Due to the use of an effective binder, obtained deposits adhered well to the substrates and exhibited enhanced stability against cracking.

The deposition rate depends on applied electric field, suspension concentration, and electrophoretic mobility of articles.^1,2,25-30 When considering other possible factors that can influence the deposition yield, it is important to note that a certain potential distribution needs to be achieved in the electrophoretic cell in order to supply sufficient voltage at the electrode interface and obtain high deposition rates.²⁶ Such potential distribution can be realized by adding an appropriate amount of phosphate ester or electrolyte. It was shown^31-33 that uniformity and adhesion of the deposits can be improved by the use of electrolytes. However, an increase in the electrolyte concentration caused significant aggregation of ceramic particles and their sedimentation.³¹ Particle sedimentation resulted in decreased suspension concentration and was accompanied by a decrease in the deposition rate.^25,31 The deposition process resulted in porous deposits that included a significant amount of agglomerates.³¹ It is in this regard that the DLVO theory^34,35 explains the existence of a critical electrolyte concentration (flocculation value) for coagulation, below which the suspension is stable and above which it is kinetically unstable. The flocculation value decreases with the valence of the electrolyte ions of a charge opposite to that of the colloidal particles (rule of Schulze and Hardey).

Figure 3. SEM micrographs of (a-top) hollow alumina fiber obtained via EPD and sintered at 1,400°C and (b-bottom) green zirconia deposit obtained via ELD on carbon fiber felt ( photo courtesy of Technimat, Lydall Technical Papers).

Constant-current or constant-voltage regimes could be used for EPD. The electric field drives ceramic particles toward the electrode and exerts a pressure on the deposited layer. It is desirable to maintain a high potential difference between the anode and the cathode. The use of high voltages has the advantage of smaller deposition times and higher deposit thickness. It should be noted that in the case of relatively large particles (~1 mm) stirring the suspension is usually performed to prevent settling. In this respect, higher voltages and smaller deposition times are preferable, because shorter deposition times allow deposition without stirring. It was demonstrated that electrophoretic phenomena have distinctive features for relatively large particles (several micrometers) and for particles on a submicrometer scale.²⁵ A high electric field and stirring can induce aggregation and sedimentation of submicrometer particles, detracting from the deposition process efficiency. It should be noted that high electric fields bring about porosity in the deposits.²⁵

The use of the electrophoretic process for the deposition of ceramic materials enables the deposition of uniform coatings on substrates of complex shapes. Figure 3a shows hollow alumina fiber obtained via the EPD of submicrometer alumina particles (Baikalox SM-8, Baikowski Ceramic Aluminas) on a carbon fiber and sintering in air at 1,400°C. The obtained deposit was uniform in diameter along the entire fiber length (5 cm). The uniform deposition results from the insulating properties of the deposit and electric field dependence of the deposition rate.^3,27,28 However, deposit uniformity is limited by the particle size of the powders used for the deposition process.^3,27-29 The possibility to form multilayer structures with controlled layer thickness and sharp interfaces between the layers has been demonstrated.³⁰ Such composites are attracting considerable interest due to their advanced mechanical properties.¹ In multilayer fibers obtained via EPD, crack propagation can be deflected at the laminate interfaces.²⁷

CATHODIC ELECTROLYTIC DEPOSITION

Electrolytic deposition produces ceramic materials and provides their deposition. In the cathodic electrodeposition method,⁴ the following reactions are used to generate base at an electrode surface:

2H₂O + 2e^– <==> H₂ + 2OH^–	(1)
NO₃^– + H₂O + 2e^– <==> NO₂^– +2OH^–	(2)
O₂ + 2H₂O + 4e^– <==> 4OH^–	(3)

Some other cathodic reactions available for the generation of base have been discussed in the literature.⁴ Reactions 1-3 consume H₂O, generate OH^–, and increase the pH at the electrode.

Figure 4. The (a-top) electrolytic deposition of ceramic particles and (b-bottom) intercalation of cationic polyelectrolytes into electrolytic deposits.

In cathodic ELD, metal ions or complexes are hydrolyzed by electrogenerated base (Figure 4a) to form oxide,^4-6 hydroxide,^7-10 or peroxide^11-15 deposits on cathodic substrates. Hydroxide and peroxide deposits can be converted to corresponding oxides by thermal treatment. Hydrolysis reactions result in the accumulation of colloidal particles near the electrode. Turning again to the DLVO theory of colloidal stability,^34,35 it may be concluded that the formation of a deposit is caused by flocculation introduced by the electrolyte. The coagulation of colloidal particles near the cathode can be enhanced by the electric field,²⁵ electrohydrodynamic flows,^36,37 and pressure resulting from the formation of new particles.

Cathodic ELD is governed by Faraday's law. The amount of the deposited material can be controlled by varying deposition time or current density. Figure 2b shows deposit weight versus time dependencies for titania, zirconia, and alumina deposits obtained from solutions SL1, SL2, and SL3, respectively (Table II). Turning to the data on the EPD of the same materials (Figure 2a), it is seen that the deposition rate in EPD is much faster (by about 1-2 orders of magnitude) than that in ELD (Figure 2b), resulting in higher deposit thickness (Table I).

The amount of material deposited from solution SL2 increased with time in a decelerating manner. This result is inconsistent with Faraday's law. Possible reasons for the deviation of experimental deposit weights from Faraday's law have been discussed in previous papers.^4,5,7 Owing to the use of ionic species instead of ceramic particles, electrolytic deposition allows better control of the deposition rate and deposit uniformity.³ The deposits obtained via the electrolytic process have lower particle sizes and exhibit higher sintering activity. Figure 3b shows an electrolytic zirconia deposit on a carbon-fiber felt. Electrolytic deposition results in the formation of uniform deposits on substrates of complex shape. Deposit uniformity is controlled by electric field.⁴

Aqueous or mixed solvents can be used for electrolytic deposition. It should be noted that the adsorbed water in as-prepared deposits leads to cementation of small particles to form aggregates. However, the deposition process needs a certain amount of water for base generation and prevention of the formation of nonstoichiometric oxides.¹¹

Figure 5. X-ray diffraction patterns of deposits obtained from solutions (a) SL1, (b) SL2, (c) SL4, and (d) SL5 and thermally treated at 400°C (SL1, SL2, and SL5) and 700°C (SL4) for 1 h. (O--TiO₂, --ZrO₂, --ZrTiO₄, D--SnO₂).

Figure 6. Crystallite sizes of electrolytic titania (anatase) deposits (solution SL1) determined from x-ray data at different temperatures.

The formation of oxide materials via corresponding hydroxides and peroxides constitute two different chemical routes in electrodeposition. The peroxoprecursor method has been designed in order to solve problems associated with cathodic electrolytic deposition of TiO₂^11,12,17 and Nb₂O₅^13,15 from aqueous solutions. The major problem with the electrodeposition of these oxides is related to the use of water for base generation (Reactions 1-3). Titanium and niobium salts immediately react with water to form precipitates.

The problem of titania electrodeposition was solved^11,12,17 by use of a titanium peroxocomplex. The peroxocomplex of titanium is stable under certain conditions in water and has a cationic character. Electrodeposition of TiO₂ films is based on hydrolysis of a peroxocomplex at the cathode and formation of hydrated peroxide. Oxide films were obtained by thermal dehydration of the peroxoprecursors. As-prepared titania films and powders were found to be amorphous. After thermal treatment at 400°C, peaks of an anatase structure were observed (Figure 5). The feasibility of cathodic electrolytic deposition of niobium-oxide films via the peroxoprecursor method has recently been demonstrated.^13,15 This approach has been further expanded to electrodeposition of SnO₂; ZrTiO₄ (Table II, Figure 5); and other individual oxides, complex compounds, and composites.^4,8,14-20

The hydrogen-peroxide additive has a number of effects on the deposits, as discussed in References 8 and 15. The important finding was that complex compounds^14-18 can be deposited via the peroxoprecursor method. The results of titania and zirconia electrodeposition indicate that the deposits remains amorphous up to ~300-350°C.^8,11,12,17 At higher temperatures, crystallization of nanostructured titania and zirconia was observed (Figures 5 and 6).

ZrTiO₄ has been deposited via the peroxoprecursor method.^14,17 It was established that the use of a peroxoprecursor provides an equal deposition rate of the individual components and allows a deposit of desired stoichiometry to be obtained. The deposits obtained from mixed titanium and zirconium salts solutions in the presence of hydrogen peroxide remained amorphous up to 600°C. This is in contrast to the experimental data on the electrodeposition of individual components. ZrTiO₄ crystallizes directly from the amorphous phase, as shown in Figure 5. No peaks of individual components were observed. It was concluded that obtained green deposits are not a simple mixture of individual components, but have a complex nature. This approach has been further expanded to the formation of other complex compounds, such as PZT and BaTiO₃.^4,15,16,18

As pointed out in References 19 and 20, the peroxoprecursor method cannot be applied for depositing such materials as RuO₂. Ruthenium species bring about the decomposition of H₂O₂ in solution, and the electrodeposition of RuO₂ films was performed via a hydroxide precursor. SnO₂, ZrO₂, La₂O₃, PbO, and some other materials can be deposited via hydroxide or peroxide precursors. The important finding was that composites^{9,10,15,19,20} can be deposited via cathodic ELD. Electrolytic deposition of ceramic composites, such as ZrO₂-Al₂O₃, Al₂O₃-Cr₂O₃, Al₂O₃-TiO₂, and TiO₂-RuO₂, was performed via hydroxide or mixed hydroxide/peroxide precursors.

Figure 7. The deposit weight of alumina versus cetyltrimethylammonium bromide concentration, 0.1 M Al(NO₃)₃ solution in ethyl alcohol, deposition time 20 min., current density 5 mA/cm².

The influence of additives on the deposition rate and morphology of electrolytic deposits has been studied.^9,10,15,18 Deposit cracking associated with drying shrinkage is a common problem among wet chemical methods once thick coatings are formed. Oxide films deposited via hydroxide and peroxide precursors exhibited cracking when deposit thickness exceeded ~0.2-0.3 mm. The cracking problem was approached by multiple deposition.^16,19,20 It should be noted that the most common binders used in EPD are nonionic-type polymers (polyvinyl alcohol, polyvinyl butyral, ethyl cellulose, and polyacrylamide). The polymeric molecules adsorb onto the surfaces of ceramic particles. Positively charged ceramic particles provide electrophoretic transport of the polymeric molecules to form deposits on cathodic substrates. However, the application of these polymers for electrolytic deposition presents difficulties, as the formation of ceramic particles is achieved near the electrode surface (Figure 4a). However, it is possible to perform electrochemical intercalation of charged polyelectrolytes into electrolytic deposits (Figure 4b). By using cationic polyelectrolytes, such as poly(dimethyldiallylammonium chloride) (PDDA) or polyethylenimine (PEI) with inherent binding properties, problems related to cracking in electrolytic deposits could be diminished. Moreover, various organoceramic nanocomposites, such as Y(OH)₃-PDDA, Zr(OH)₄-PDDA, and Y(OH)₃-PEI can be obtained via electrodeposition. The intercalation of polymer particles is achieved by their adsorption on the surface of colloidal particles, which are produced near the cathode and form a cathodic deposit. In the cathodic electrolytic deposition process, the pH in the bulk of solutions is low; whereas Reactions 1-3 result in a significant increase of pH value near the cathode. Therefore, a negative charge of colloidal particles formed near the electrode surface can be expected:

M - OH + OH^– <==> M - O^– + H₂O

(4)

APPLICATIONS
There is a growing interest in electrodeposition of various ceramic materials.^1-22,38-59 Electrodeposition has been used for the preparation of thin (ELD^4,6,16,40,42) and thick (EPD^{1,2,38,39,41,43,44}) films of ferroelectric,^16,38 piezoelectric,^6,39 magnetic materials,^40,41 superconductors,^42,43 and semiconductors.^4,44 The interest in EPD^3,25,28 and ELD^45,46 for biomedical applications stems from a variety of reasons, such as the possibility of deposition of stoichiometric, high-purity material to a degree not easily achievable by other processing techniques and the possibility of forming coatings and bodies of complex shape.^3,28 EPD^1-3,47-49 and ELD^3,4,21,22,50 are especially attractive for the design of solid-oxide fuel cells,^21,22,⁴⁷ solar cells,⁴⁸ electrochromic devices,^49,50 microelectronic devices,^1,2,4 fiber-reinforced composites,^1,3,4 and batteries.^1,4 Protective coatings and electrode materials were deposited via EPD^1,2,51,52 and ELD.^{4,7,9,10,19,20,22} Electrolytic TiO₂, RuO₂, SnO₂, Nb₂O₅, and composite films^{4,7,12,13,15,19,20} are of considerable interest for fabrication of dimensionally stable anodes, supercapacitors, and for other electrochemical and catalytic applications.⁴ Substantial interest in EPD^38,43,53 and ELD^54,55 has evolved for the deposition of oriented and patterned films. One of the important capabilities provided by EPD⁵⁶ and ELD⁵⁷ is the ability to achieve particle impregnation into a porous substrate and composite consolidation. EPD has been demonstrated as a suitable technique for the fabrication of laminar ceramic composites,^27,30 functionally gradiented composites,⁵⁸ hollow fibers and coated fibers,³ phosphor screens,⁵⁹ and shaping of ceramic bodies.^1,2 Electrolytic deposition can be considered as an important tool in the formation of nanostructured materials.^4,8,12,17 Other applications of electrophoretic and electrolytic films are discussed in References 1, 2, and 4.

On the other hand, the electric field provides electrophoretic motion of cationic polyelectrolytes toward the cathode. In this case, the adsorption can be achieved via electrostatic attraction of oppositely charged ceramic particles and polyelectrolytes. Cationic surfactants are of considerable interest for application in ELD. Figure 7 shows that the deposit weight of alumina increases with the increase of surfactant concentration and remains relatively constant for concentrations higher than 20 mg/dm³. It is suggested that the surfactant acts like an electrolyte in compressing the double layer of ceramic particles, resulting in particle flocculation and increasing the deposition process efficiency. The increase in yield of the deposit with increasing concentration of surfactant could also be related to the retarded diffusion of OH^– ions away from the cathode region.

Coating resistivity is a limiting factor of the ELD method for development of thick films. As the coating process progresses, an insulating layer is formed, which prevents OH^– generation. Some individual oxides (RuO₂, IrO₂, SnO₂, and Cr₂O₃) and composites (RuO₂-TiO₂ and Al₂O₃-Cr₂O₃) exhibit high conductivity, and thick deposits (up to ~10 mm) were obtained.^4,7,15,19,20 Insulating ceramics formed very thin deposits (up to 1-2 mm).

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Igor Zhitomirsky is with the Department of Materials Science and Engineering, McMaster University.

For more information, contact I. Zhitomirsky, Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4L7; fax (905) 528-9295; e-mail zhitom@mcmaster.ca.

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