Tape casting

Low viscosity

The rheology of the suspension plays a critical role in the tape casting process. The key rheological properties include viscosity (η), shear-thinning behavior (pseudoplasticity), yield stress, thixotropy, viscoelasticity, and time-dependent stability.

Low viscosity makes controlling the suspension during tape casting more difficult. Achieving higher thickness becomes more challenging, and the thickness may gradually decrease from the center toward the edges. It can also increase shrinkage during drying, and if the tape thickness is high, it may lead to warping during the drying process. In more severe cases, it can lead to non-uniform particle distribution in composite systems or a non-uniform particle size distribution in single-component systems.

High viscosity

If the solid loading of the suspension is high, controlling it becomes more difficult, especially for producing thin tapes. The likelihood of air entrapment increases, and a smooth, uniform surface may not be achieved.

Low wettability

This issue can be distinguished from high viscosity. It occurs when the cohesion of the suspension exceeds its adhesion, resulting in poor flowability and insufficient spreading of the suspension on the substrate. This effect becomes more pronounced, especially when nano-sized powders are used.

The selection of an appropriate substrate is crucial for achieving a balance between wettability and non-stick behavior. Silicone-coated MYLAR® films provide this capability.

Gas trapping

Entrapped air can lead to the formation of bubbles or pinholes. Even small bubbles can evolve into cracks during the drying or sintering process. Due to the mixing, stirring, or milling of the suspension, a certain amount of gas is always introduced into the suspension, which begins to escape once the system is at rest. Therefore, degassing (de-airing) is essential.

Skinning

A thin layer at the surface of the suspension rapidly loses its solvent and becomes stiff. This layer can hinder the release of entrapped air from the suspension. It may also pass under the blade and create discontinuities in the tape. Small bubbles are often observed around these discontinuities, and they can be identified by noticeable color differences. Appropriate additives or solvent mixtures can act as skin retarders (homogenizers).

Mixing inhomogeneity and segregation

Apart from the powder particles, the other components such as the binder, plasticizer, and dispersant are dissolved in the solvent. If this dissolution is incomplete, it can result in the formation of localized inhomogeneities upon the addition of the powder.

Even after complete dissolution, these components may come out of solution due to temperature changes or under static conditions and undergo segregation/migration. These inhomogeneities can result in the formation of porosity, cracks, and warping in the tape during drying and sintering.

Agglomeration or suspension instability

Powder agglomeration can lead to the formation of unwanted porosity, and in more severe cases, it can create drag lines (streaks or grooves) on the tape after passing under the blade.

Non-uniform dispersion of the powder in the solvent results in inhomogeneity in the final tape. This non-uniformity may appear in particle distribution, particle size distribution, and porosity, and can even lead to warping, cracking, or undesired deformation. In some cases, the use of filtration is necessary to remove contaminants or agglomerated particles.

Thickness non-uniformity

The build-up volume of the suspension behind the blade should be uniform and stable; otherwise, variations in the applied pressure can lead to non-uniform thickness. The flatness of the substrate is also of great importance.

To maintain a stable suspension pressure, double doctor blades with a reservoir are commonly used. The two blades help ensure that the pressure remains constant at the second blade. For thin tapes, it is sometimes necessary to use tape casting machines with a vacuum chuck/bed to keep the substrate surface flat.

Speckle

During tape casting, small spots may be observed on the tape that appear to grow slowly over time. Although they may not leave a visible trace after complete drying, they are not a good sign, as they can act as nucleation sites for cracks during handling, cutting, or sintering.

Contaminants such as pre-dried particles or water droplets can cause these features. They may also result from component segregation or contamination in the suspension.

Drying

Streaks

During drying or after complete drying, streak-like features can be observed in the tape. These features appear as grooves or regions with reduced thickness, and it is evident that they are rich in binder and depleted in powder. Although these are not actual cracks, they can evolve into cracks or even fractures during the sintering process. These streaks can be very closely spaced, forming an alligatoring (mud-crack precursor) pattern, or they may appear as a few isolated, narrow streaks.

To control this defect, it is necessary to optimize the amounts of binder, plasticizer, and solvent. In particular, the addition of a suitable plasticizer and sufficient mixing can significantly help to reduce or eliminate this issue.

Cracks

If the amounts of binder and plasticizer are not optimized, these streaks can evolve into cracks. In some cases, the tape may crumble during drying, leading to the formation of a mud crack pattern. Cracks may appear as longitudinal or transverse features, especially at the edges, and are clearly visible. In other cases, microcracks may form that are not visible to the naked eye but can grow during the sintering process.

Typically, the presence of even a small crack after drying can indicate that the entire tape is unsuitable for use, as it will be prone to cracking during handling or sintering.

Curling & Warping

Curling is usually caused by faster drying of the surface compared to the underlying layer. In this case, the edges tend to lift, and the tape may curl into a tube-like shape.

Warping is more often caused by non-uniform residual stresses and uneven distribution of suspension components. In this case, local deformations deviate the tape from its flat geometry. This wavy deformation, typically more pronounced at the edges, increases the likelihood of crack formation during handling or sintering.

Flattening the tape by applying pressure or ironing may not be able to eliminate the effect of these two defects. This phenomenon is often referred to as the “memory of clay.”

Bubble & Pin hole

Gas bubbles may not be visible during tape casting, but they can gradually coalesce during drying to form larger bubbles. Some of these bubbles reach the surface, while others remain beneath the surface, leading to blistering or swelling in the tape.

Binder Migration

As the solvent evaporates, polymeric species (binder) are transported toward the surface. This leads to the formation of a binder-rich, tacky surface layer, while the interior of the tape remains relatively weak and binder-deficient.

This phenomenon is commonly attributed to solvent-driven transport and capillary flow during drying, which can induce a non-uniform distribution of binder across the thickness of the tape. As a result, a density and composition gradient develops, often referred to as binder migration. Such gradients can adversely affect mechanical integrity, leading to differential shrinkage, internal stresses, and an increased likelihood of defects such as warping, delamination, or cracking during subsequent handling and sintering.

Delamination

Delamination can occur during drying due to binder migration, non-uniform solvent evaporation, and resulting internal stress gradients. These effects lead to differential shrinkage within the tape, weakening interfacial bonding and causing separation either within a single layer or between layers. The issue is particularly critical in co-cast (multilayer) systems, where differences in composition, drying behavior, and shrinkage between layers can significantly increase the risk of interfacial separation and failure during subsequent processing steps such as handling or sintering.

One common lamination method involves casting and drying the layers separately and then pressing them together at temperatures above the softening temperature of the binder.

Handling

Sticking to substrate

The tape casting substrate should provide a balance between wettability and non-stick behavior. Removing the tape from the substrate before it is fully dried can lead to tearing or the development of residual stresses.

Residual stress

Any applied stress to the tape, even if it does not cause visible deformation or dimensional change, can introduce residual stresses. These stresses may lead to cracking or distortion during sintering. This phenomenon is known as the “memory of clay” effect.

Cracks & Delamination

A brittle tape is prone to crack formation. Brittleness can create challenges during handling or cutting of the tape. The resulting cracks may be on a microscopic scale, but they can grow during sintering.

Sintering

Curling, Warping, and Cracking

The release of gases due to the decomposition of polymeric components, including binders and pore formers at low temperatures, before the powder particles begin to sinter, can lead to crack formation.

Non-uniform temperature distribution, especially through the thickness of the tape, can lead to curling. Regions that sinter earlier become rigid, and due to greater shrinkage, they generate internal stresses. For example, the edges and corners of the tape may bend upward or develop cracks.

Another contributing factor is non-uniform residual stresses, uneven powder distribution, and non-uniform constraint of the tape, all of which lead to non-uniform sintering behavior and result in warping, uneven dimensional changes, and microcracks growing.

blistering/swelling