Special requirements for slurry of zirconia ceramic molding process

The molding process of zirconia ceramics has always been a very important step, so the requirements for the slurry are also very strict, which will determine the performance of the ceramic after machining. The following are the main requirements of Pintejin Ceramics for the slurry.

Zirconia ceramic forming refers to adding an appropriate amount of water or organic liquid and a small amount of electrolyte to the powder to form a relatively stable suspension, and injecting the suspension into a plaster mold to absorb water to achieve the purpose of forming. The key to forming zirconia ceramic slurry is to obtain a good slurry, and its main requirements are:

• ①Good fluidity, small enough viscosity for pouring.
• ②When the solid-liquid ratio in the slurry changes to a certain extent, the viscosity change is small, so that the remaining slurry in the mold can be easily dumped when the hollow part is poured.
• ③Good suspension and sufficient stability, so that the slurry can be stored for a certain period of time, and at the same time, when pouring in large quantities, the performance of the slurry is consistent before and after.
• ④ The speed at which the water in the slurry is absorbed by the gypsum should be appropriate in order to control the wall thickness of the hollow blank and prevent the blank from cracking.
• ⑤ After drying, the blank is easy to be separated from the mold wall for demoulding.
• ⑥ The blank after demoulding must have sufficient strength and the greatest possible density.

Ceramic shaped parts

Among the above-mentioned zirconia ceramic slurry requirements, certain fluidity and stability are important. The fluidity of zirconia ceramic slurries is mainly determined by viscosity. The viscosity of zirconia ceramic slurry can be expressed by the following empirical formula: η=η0(1-C)+K1C “k2C” where: η, η0. respectively represent the viscosity of the slurry and the liquid medium, C represents the concentration of the solid phase in the slurry, η, m, K1, K2 are experimental constants (for example, for kaolin slurries, η=l, m=3, K1=0.03 , k=7.5).

When the concentration of zirconia ceramic glass-ceramic slurry is low, η is mainly affected by the first term η. Impact. But too low concentration is not suitable, because too much moisture will reduce the strength of the blank and make the sintering shrinkage larger. These are all things we don’t want. In addition to the solid phase concentration, the shape of the ceramic-mechanical solid phase particles also affects the viscosity of the slip. Because the slurry is in the process of flow, the resistance of particles of different shapes is also different. The following empirical formula applies to dilute suspensions in inert media. η=η0(1+Kv) where: v is the fraction of the solid phase in the suspension, and K is called the shape coefficient. The more irregular the shape, the greater the shape factor and the greater the flow resistance.

In addition to the solid content, particle shape, and medium viscosity of zirconia ceramic slurry, the factors that affect the fluidity of the slurry include: the temperature of the slurry, the machining method of the raw materials and the slurry, etc. Whether the solution can be suspended or not is determined by two conditions, one is the balance of Brownian motion, van der Waals force and electrostatic force, and the other is the formation of a hydration film. It is the electric double layer model commonly used in colloidal chemistry. A is the particle surface, called the interface of the adsorption layer, and C is the interface of the diffusion layer. So AB is the adsorption layer and Bc is the diffusion layer. c is also called sliding surface. E is the potential of A to C, and f is the potential of B to C. Because the potential of the solid phase and the potential of the medium are both fixed and determined by their type and state, it is the potential that can be changed.

Adding flocculants or deflocculating charged particles to the ceramic sheet solution can adjust the thickness of the electric double layer and thus the f-potential. When a deflocculant was added to the solution, the electric double layer thickened and the f potential increased. In the formula: e is the surface charge of the particle, d is the thickness of the electric double layer, which accounts for the dielectric constant of the medium, so when the particle and the medium are constant, f and d are proportional. The increase of the f potential increases the repulsion between the particles, and the industrial ceramics is beneficial to overcome the Van der Waals force (gravity) and Brownian motion, and obtain good suspension.