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Electrophoretic Deposition (EPD)

Problem statement

Starting from ceramic powders, a first problem to be solved consists in shaping these powders into a defect free powder compact. Colloidal processing offers the potential to reliably produce ceramic films and components through control of the initial suspension and its evolution during shaping. Electrophoretic deposition is a colloidal processing technique that allows not only to shape free standing objects but also allows to deposit thin films and coatings on substrates. One of the most attractive features however is the possibility to engineer step-graded and continuously graded functionally graded materials and components. The presence of the electric field also offers the possibility to orient anisotropic powders allowing texturing of materials. The equipment is very application specific, but can be home made at low cost. Modelling of the EPD process, especially for functionally graded materials has been successful.

At present, the EPD technique is being developed for the shaping of :

  • Layered materials (laminates)
  • Thin films and Coatings
  • Functionally graded materials (FGMs)
  • Textured materials

The material classes (micometer-sized down to nanopowders) currently being investigated are:

  • Oxides (ZrO2, Al2O3, PZT, etc.)
  • Nitrides (TiN, TiCN, etc.)
  • Carbides (WC, SiC, TiC, graphite, etc.)
  • Borides (TiB2, cBN, etc. )
  • Cermets (WC-Co)
  • Bioactive glass
  • Organic materials
  • Living cells

Electrophoretic deposition

Electrophoretic deposition (EPD) is a fairly rapid low cost two-step process. In a first step, particles having acquired an electric charge in the liquid in which they are suspended, are forced to move towards one of the electrodes by applying an electric field to the suspension (electrophoresis). In a second step (deposition), the particles collect at one of the electrodes and form a coherent deposit on it. The deposit takes the shape imposed by this electrode. Hence, after drying and removal from the electrode, a shaped green ceramic body is obtained. Firing this green body then results in a ceramic component. The only shape limitation is the feasibility to remove the deposit from the electrode after deposition.

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Free-standing objects

plate.jpg   hemisphere.jpg

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More complex geometries require appropriate design of the counter electrode in order to generate a constant electric field. The depositing electrode has the actual shape of the component to be made. The design of the most suitable counter electrodes is supported by electrical field calculations.

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Functionally graded materials

To combine irreconcilable mechanical properties in a component, functionally graded materials can be a solution, for example by developing a graded material with a tough core and a hard surface or a material with a gradient in optical, magnetic, electrical, thermal expansion, etc. A very wide variety of processes have been reported for FGM production. Amongst these processes, electrophoretic deposition (EPD) is a fairly rapid low cost process, capable of producing continuously and step graded materials.

Since the local composition of the deposit during the EPD process is directly related to the concentration and composition of the suspension at the moment of deposition, the EPD technique allows processing of functionally graded materials with a continuous gradient in composition by judiciously adjusting the suspension composition in time.

 

As an example, the measured composition profile on a cross-sectioned Al2O3/ZrO2 FGM disk (Ø = 55 mm) after sintering, together with the predicted composition profile and a general view on the cross-sectioned FGM is shown below. This specific FGM consists of homogeneous outer and central parts with intermediate graded regions. It is possible to modify the compressive stress in the outer pure alumina layer of the Al2O3/ZrO2 FGM discs by engineering the widths of the constituent regions and the slope of the profiles in the graded layers in the FGM disc through adjustment of the experimental EPD parameters.

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 Another example are graded hardmetal cutting insert blanks with a hard homogeneous WC-Co-Ti(C,N) surface layer, a homogeneous WC/Co core and intermediate graded layer.

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More complex shaped FGM components that are currently under investigation are functionally graded femoral ball-heads and acetabular cups for biomedical applications. A number of sintered complex shaped deposits, without finish machining, are shown below.

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Laminates

The main goal of this research is to formulate an answer to the problem of shaping ceramic/ceramic laminates. If all ceramic forming methods are considered, taking into account their cost and the requirement that complex shaped laminates need to be produced, electrophoretic deposition comes forward as an elegant solution. The main advantage of EPD for ceramic/ceramic laminates, is that the process allows to form a layered shape with great ease: by alternating deposition from 2 suspensions, the layered microstructure and shape is immediately obtained. In contrast, e.g. tape casting requires that the layers are first made, then cut, stacked and compressed. Moreover, electrophoretic deposition is a fast process – 100 µm sintered thickness per minute of deposition is easily obtained – and does not involve high costs.

To illustrate the feasibility of EPD, plate and tube shaped SiC/graphite laminates were produced. Additionally, attention was given to produce laminates with weakened interlayers of the same material via electrophoretic deposition. The weakening is obtained by introducing a controlled porosity in the interlayers. Controlled in this context means that the amount, shape and size of pores would be designed.

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Tubes containing 19 layers of SiC (100 µm) and 18 graphite interlayers (18 µm)

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 Compressive force-displacement trace of the failure of a multiple laminated SiC tube, illustrating its “gracefull failure” under load

Thin films and coatings

Depending on the powder composition and size, thin films of a few µm up to more than 300 µm can be directly deposited on an electrically conductive substrate of any shape from a stable colloidal suspension by means of EPD. Subsequent sintering allows to densify the deposits on the substrate. Interlayers between substrate and final coating can be easily incorporated. Examples are hardmetal (left) and ZrO2 (right) coatings on steel substrates :

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ZrO2 coating.jpg

 

Texturing

To investigate the texturing possibilities of EPD, a mixture of alumina platelets and powders was deposited on a flat electrode. After removal of the deposit, drying and sintering, the material was investigated by means of SEM and orientation imaging microscopy (OIM), clearly revealing a preferred orientation of the alumina grains with respect to the depositing electrode, as illustrated below.

texture.jpg   OIM.jpg

Equipment

 

Because the electric field and therefore the deposition rate is strongly influenced by the relative distance between the electrodes, a rigid positioning system is essential in order to shape complex geometries in a reproducible way by means of EPD. Moreover, it is desirable to control the operation of the suspension pumps and voltage and to monitor the current by computer.

During the years, MTM gathered experience in the construction of rigid positioning devices with temperature control, allowing to reproducibly position the electrodes very accurately with respect to each other. Full automation of the process is established incorporating the displacement of the counter-electrode, application and monitoring of the voltage and control of the suspension supply pumps. The full EPD process can actually be programmed as a function of time. Additionally, the current, conductivity and temperature can be logged during the EPD process.

An overview of an automated EPD set-up is shown below, together with some of the electrode configurations used.

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plates setup.jpg   tubes setup.jpg  

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Modeling the electrophoretic deposition process

A procedure has recently been developed to measure the electric field drop during EPD by means of current and conductivity measurements during EPD.
The kinetics of the EPD process has been successfully modelled based on EPD and powder specific parameters. A very good relationship between experiments and the model has been found.

 

MTM publications on EPD

see list of publications