**1. Introduction**

42 Ceramic Coatings – Applications in Engineering

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Electrophoretic Deposition (EPD) is one of the most outstanding coating techniques to be based on electrodeposition. Nowadays, increasing interest has been gained both from academic and industrial payers, due to its wide potential in ceramic coating processing technology.

The main advantages of this technique are:

its high versatility when used with different materials and their combinations; its cost effectiveness, because it requires simple and cheap equipment.

Moreover, it can be used both on a large scale, also to coat objects with a complex shape, and on small scale, to fabricate composite micro- and nanostructures, as well as near net-shape objects having accurate dimensions (micro- and nano-manufacturing).

The basic phenomena involved in EPD are well-known and have been the subject of extensive theoretical and experimental research. Nevertheless, further efforts have to be devoted in order to understand the fundamental mechanisms of EPD and to optimise the working parameters, especially when multicomponent suspensions are used.

EPD is a two-step process. In the first step, charged particles suspended in a liquid medium move towards the oppositely charged electrode under the effect of an externally applied electric field (electrophoresis). In the second step, the particles deposit on the electrode forming a more or less thick film, depending on the process conditions (concentration of particles in solution, applied electric field, time). The substrate acts as an electrode and the deposit of particles is the coating.

The aim of this paper is to provide an updated review of the wide potential of EPD as a technique to produce ceramic coatings, without omitting possible problems and drawbacks.

Applications of EPD, especially on nanoparticles, are in continuous expansion both in industry and academia, stimulating a great interest in developing of predictive analytical and numeric modelling of the EPD process. The proposed mechanisms have explained

Ceramic Coatings Obtained by Electrophoretic Deposition:

Fig. 1. Scheme of EPD process

**2.1 Kinetic models** 

planar geometry:

electrode (cm2).

process is going on.

conditions.

Fundamentals, Models, Post-Deposition Processes and Applications 45

The first model used to describe EPD process is attributed to Hamaker (Hamaker, 1940) who proposed a general expression for the deposited mass per area unit (*m,* g cm-2) in a cell with

where CS, solids concentration in the suspension (g cm-3); t, deposition time (s); µ, electrophoretic mobility (cm2 V-1 s-1); E, electric field strength (V cm-1); A, surface area of the

Several years later, Sarkar and Nicholson (Sarkar & Nicholson, 1996) considered again the first model of Hamaker and analysed the dependence of kinetics on some experimental

In eq. (1), µ and A can be evaluated numerically and one reasonably supposes that they are constant during the process. It is not so true for CS and E, that vary as the process while the

Sarkar and Nicholson considered the variation of the particles concentration in the suspension for long deposition time, starting with the condition that the only change in the concentration is due to the mass of powder deposited by EPD. It is equal to zero when the

where m0, initial mass of powder in suspension (g) and � � � ��� ⁄ defined as characteristic time (V is the volume of the suspension considered constant). Actually, is the reciprocal of k, the "kinetic parameter" that represents a key parameter in the modelling of EPD process.

process starts and varies with time according to the expression:

�������� (1)

�(�) � ��(� � ��� �� ) (2)

experimental results, but the current experts opinion is that a full understanding is still lacking, mainly due to the phenomena that are at the base of the interaction between the charged particles approaching both each other and to the electrode to form the solid deposit.

For some applications, a requirement is that the ceramic EPD deposit is dense, so a postdeposition treatment should be performed in order to densify it. Usually, this consists of a conventional heating treatment in a furnace, but some problems could occur such as delamination, cracks or residual stress due to the differential coefficient of thermal expansion. Moreover, the high densification temperature of ceramics can be detrimental for the substrate which could be damaged.

Sometimes, the sintering temperature can be decreased by adding some low melting additives. Alternative sintering methods could be considered, such as microwave, laser or electron beam.

Finally, the wide range of applications of EPD deposits will be mentioned. EPD process is very versatile, therefore porous, layered, and graded deposits can be obtained besides dense coatings. Recently, it has been clearly demonstrated the possibility of obtaining nanocomposite materials, especially those containing carbon nanotubes (CNTs), by using EPD. As a consequence, the applications are in a spread number of sectors: biomaterials, fuel cells, barrier coatings, electronics, catalysis, optical devices.
