I. What is Tape Casting?

The tape casting method involves adding binders, solvents, and other components to ceramic powders, followed by ball milling, filtration, vacuum degassing, and viscosity control within a specified range. This viscous slurry is then cast under constant pressure through the gap between a doctor blade and a carrier substrate (coated with silicone) that moves at a controlled speed. The slurry flows onto the substrate, forming a wet film. After drying and edge trimming, the green tape is separated from the carrier substrate at the end of the casting machine (or left unseparated, depending on the process), and sequentially wound into rolls. The green tape then proceeds to subsequent processes such as cutting, lamination, printing, and sintering to produce the desired final products. This constitutes the tape casting preparation process.

Among the tape casting process steps, the most critical are slurry preparation and the tape casting forming process itself.

II. Applications of Tape Casting

Process flow diagram for preparing ceramic sheets via tape casting:

Through a series of steps including ball milling, organic and inorganic residues are removed from the slurry to obtain a stable and uniform casting slurry. The prepared slurry is then cast on a casting machine, followed by drying, cutting, debinding, and sintering to complete the production.

III. Introduction to Common Additives in Slurry

1. Solvents

Key factors to consider when selecting a solvent include:
(1) Must be capable of dissolving other additive components, such as dispersants, binders, and plasticizers;
(2) Must have chemical stability within the slurry and not react with the powder material;
(3) Should be easy to volatilize and burn off;
(4) Must be safe to use, non-toxic, and have minimal environmental impact.

The most common solvents fall into two categories: organic solvents and water.

Slurries prepared with organic solvents have higher viscosity, slower solvent evaporation, and longer drying times; therefore, organic solvents are more commonly used in tape casting. Common examples include ethanol, methyl ethyl ketone (MEK), trichloroethylene, toluene, and xylene. However, organic solvents have the disadvantages of being flammable and toxic.

Using water as a solvent offers advantages such as low cost, safety, non-toxicity, and ease of large-scale production. Its disadvantages include:
(1) Poor wetting ability for powder particles, rapid volatilization, and long drying times;
(2) Difficulty in degassing the slurry, as trapped bubbles can affect the quality of the green tape;
(3) Most binders used in water-based slurries are emulsions, and the limited variety available on the market restricts binder selection.

The wetting ability of a solvent toward powder particles is related to its surface tension: the higher the surface tension, the better the wetting ability. Organic solvents have higher surface tension than water, so their wetting performance is superior. Mixed solvents generally exhibit better overall properties (such as surface tension and dielectric constant) than single components, as well as higher boiling points and better dissolution of dispersants, binders, and plasticizers. To ensure co-volatilization during drying, binary azeotropic mixtures are commonly used in casting slurries, with ethanol/methyl ethyl ketone, ethanol/trichloroethylene, ethanol/water, and trichloroethylene/methyl ethyl ketone being the most common.

2. Dispersants

The uniformity of powder dispersion in the casting slurry directly affects the quality of the green tape, and subsequently influences the material’s density, porosity, mechanical properties, and other characteristics. Dispersants commonly used in tape casting fall into four categories: non-ionic, anionic, cationic, and amphoteric ionic surfactants. Generally, anionic surfactants are primarily used for neutral or weakly alkaline slurries where particle surfaces carry a positive charge, while cationic surfactants are used for neutral or weakly acidic slurries where particle surfaces carry a negative charge. Mikeska et al., through experimental studies on the dispersion effectiveness of 70 commercial dispersants, found that phosphate esters, ethoxylated compounds, and menhaden oil exhibited the best dispersion performance in ceramic powder slurries. Among these, the latter two are anionic surfactants, while menhaden oil is not classified as a surfactant.

3. Binders and Plasticizers

To facilitate separation from the carrier substrate and ease of handling, the cast tape must possess adequate strength, toughness, and flexibility. Therefore, binders and plasticizers must be added to the slurry. Factors to consider when selecting a binder include:
(1) The thickness of the green tape;
(2) Compatibility with the selected solvent type, to facilitate solvent volatilization and prevent bubble formation;
(3) Ease of burn-off, leaving no residue;
(4) Ability to stabilize the slurry and suppress particle sedimentation;
(5) A sufficiently high glass transition temperature (Tg) to prevent gelling at room temperature;
(6) Compatibility with the substrate material, ensuring no adhesion and easy separation.

Binders are classified into non-ionic, anionic, and cationic types based on their functional groups. In tape casting, the most commonly used binders are anionic and non-ionic, primarily vinyl and acrylic types. Common binders in non-aqueous slurries include polyvinyl butyral (PVB), polymethyl methacrylate, and ethyl cellulose. In aqueous media, common binders include polyvinyl alcohol (PVA), acrylic emulsions, and polyacrylamide salts.

The primary role of plasticizers in the slurry is to lower the glass transition temperature (Tg) of the binder, bringing it to or below room temperature, thereby ensuring good binder flowability and preventing gelling at ambient conditions. In addition, plasticizers also provide lubrication and bridging effects between powder particles, contributing to slurry dispersion stability. However, the addition of plasticizers tends to reduce the strength of the green tape. The most common plasticizers include polyethylene glycol, phthalates, and ethylene glycol, which have different effects on slurry rheology. Phthalates lubricate powder particles and reduce slurry viscosity, while polyethylene glycol forms organic bridges between particles, increasing slurry viscosity.

4. Other Additives

In addition, various functional organic additives are often incorporated during slurry preparation to impart specific slurry properties or optimize the quality of the dried tape.

Defoamers: Primarily used in aqueous media with low-molecular-weight solutions (e.g., PVA) or highly dispersed systems that tend to form stable foams, especially during agitation (using specific waxes or vacuum mixing). Preventing foam formation is more effective than eliminating it after formation. A common defoaming method combines mechanical and chemical approaches: adding defoamer to the slurry followed by vacuum mixing and degassing. A common defoamer is a 1:1 mixture of butanol and ethylene glycol.

Wetting Agents: Surface-active agents that dissolve in the liquid phase to reduce surface tension (especially in water) and improve wetting of powder particles and substrates. These surfactants are therefore also used as dispersants.

Homogenizers: Used to increase mutual solubility of components (e.g., cyclohexanone), thereby preventing skin formation during drying. They also increase the density and tensile strength of the substrate.

Flow Control Agents: Occasionally added in small amounts to prevent excessively slow surface drying of the substrate and to prevent cracking.

Flocculants: Agents used to prevent excessively low-density sedimentation in dispersed systems.

IV. Regarding Tape Casting Forming

Tape casting is a highly precise process with stringent quality requirements for the final product. The following points may serve as reference:

1. Surface Finish of the Doctor Blade

Casting doctor blades are typically made of tool steel, offering good wear resistance and long service life. However, proper maintenance is essential: the blade must be thoroughly cleaned after each use, and the surface must be protected from scratches by hard objects to maintain smoothness and flatness. A smooth, flat doctor blade is key to obtaining a tape with uniform thickness and a smooth surface.

2. Slurry Level in the Reservoir

An increase in the slurry level in the reservoir increases the pressure within the tank, causing the slurry to flow through the doctor blade gap at a higher rate, thereby increasing the cast film thickness. Therefore, maintaining a constant slurry level is very important for controlling cast film thickness uniformity. Large-scale casting equipment is typically equipped with level sensors to control the slurry supply valve and keep the level variation within a minimal range.

3. Uniformity of the Slurry

The casting slurry must be fully and uniformly dispersed. If undispersed hard lumps or agglomerates are not removed by filtration, they can cause pimple-like defects on the tape surface, or create depressions due to differential shrinkage during drying and sintering. Therefore, slurry preparation must be taken seriously, and the slurry must be screened to remove such lumps and agglomerates before use. If bubbles are present in the slurry, degassing treatment is required after casting.

4. Casting Thickness

The doctor blade gap thickness and the actual dried film thickness are not the same, due to solvent volatilization during drying. Under stable slurry conditions and constant casting parameters (such as flow rate and drying temperature), a consistent ratio between the two is generally established. Accurate parameters can be obtained through casting trials.

5. Establish and Implement an Optimal Drying Process

The cast slurry film can only be peeled from the substrate after drying. Therefore, establishing an appropriate drying process is a critical factor for obtaining high-quality tape. Improper drying parameters can cause defects such as bubbles, pinholes, wrinkles, cracking, and difficulty in peeling from the substrate. The principle for developing a drying process is to ensure gradual solvent evaporation, allowing the internal diffusion rate of the solvent to match the surface volatilization rate, thereby preventing defects such as cracking, blistering, and wrinkles caused by premature surface hardening.

V. Advantages of the Tape Casting Process

High degree of automation and high production efficiency.

Good product quality and high yield rate.

Suitable for large-scale production of substrates and a wide variety of substrate types.