**6.1 Refractory filler**

Refractory filler is the most important component of ceramic coatings. It determines metal penetration resistance by reducing the permeability of the surface to which it is applied, prevents sand blend erosion and reactions on the metal-mould contact surface. As mentioned above, the choice of the refractory filler depends primarily on the casting alloy type, casting wall thickness and weight, preferable inflow system, i.e. metalostatic pressure in the mould. Main physical-chemical and thermo-physical characteristics of refractory filler are the following:


The choice of the refractory filler highly depends on the metal casting temperature. In the foundry technology, fine grinded mineral raw materials are used for coatings; these mineral raw materials are based on olivine, chromite, zircon, siner magnesite, mica, corundum, cordierite and other refractory materials, Table 2.

Ceramic Coating for Cast House Application 273

Coating ability to retain its required properties and to be firmly adhered to the subject mould or core surfaces after a liquid component has been dried or expanded is ensured by addition of a binding agent. Binding agent is chosen in accordance with the refractory filler applied, as well as with ability to be dissolved in the liquid carrier of cast house coatings. The quantity of binding agent depends on the particle size of the refractory filler used. It must be carefully chosen, because gas development from the film of coating depends on it, as it is proportional to the excess of binding agent. Binding agents may be organic and nonorganic. Depending on the solidification temperature, binding agents are divided into the binding agents being solidified at the room temperature and binding agents being solidified through drying or curing. There are three mechanisms to form the coating hardness: binding agent drying, solidification after binding agent melting and solidification due to chemical processes. With the aqueous coatings containing bentonite, silica esters or water glass, coating solidification is carried out through the loss of a liquid component during heat treatment. With the coatings containing calophonium, bitumen, phurane, formaldehyde, phenol-formaldehyde binding agents and with the sugar-based or glucose-based binding agents, solidification of applied layer is carried out as a result of binding agent solidification after being heated up to its melting temperature. In case that a binding agent polymerisation is carried out or if various components chemically reacts when the applied layer is dried, the molecules of the binding agents are mutually connected into long chains or nets. That's how a binding agent solidifies. The coatings with a binding agent which is polymerised at the room temperature during a liquid component evapouration are very suitable because they do not require a thermal treatment. The coatings containing this type of binding agent (for example, polyvinilebutirol) easily solidifies once they have been applied on the mould or core surfaces, without heating. Basic requirements for the binding agent quality are the following: thermo-stability, i.e. maximum hardness maintenance at elevated temperatures, during either drying or inflow of liquid metal, in order to prevent coating stratification and crack, minimum gas separation, they must not soak humidity in and alike (Svarika 1977,

Suspension maintaining agent keeps the refractory powder particles in a dispersed state. They are divided into two groups: aqueous coating stabilizers and non-aqueous coating stabilizers. These agents prevent precipitation of refractory powder particles and have an important influence on the coating quality. Small amount of carriers causes a quick precipitation of the filler particles and other solid components contained in the coating composition. If there is an excess of carriers, these will cause an increment of coating density causing difficulties for the coating application on the mould or core surfaces; there will also be a crack risk for the thicker coating layers being dried. It is very important that solid matters used for suspension maintenance have the same mass as the liquid coating stage. In that case, suspension is maintained longer, the coating becomes more efficient and metal penetration is prevented. Aqueous coating stabilizers are the following: bentonite, carboxymethylcellulose, alghynate, and polyacrylamide. Alcohol coating stabilizers are the following: polyvinyl-butyral, polysobutylene, organic types of bentonite. Table 4 shows

some suspension maintaining agents (Clegg1978, Svarika 1977, Tomović 1990).

**6.2 Binding properties** 

Tomović 1990).

**6.3 Suspension maintaining agent** 

In order to use ceramic powder as a filler, it should have the grain size below 40 ·10-6 m to attain an appropriate coating rheology and adhesive properties. It has been shown in practice that rounded filler particles with different size (25-40·10-6 m) are suitable. These contribute to form a uniform, continuous film, i.e. a thinner coating layer, especially when applied for the EPC casting process, due to a better mutual stacking-packing of filler particles. Researches showed that refractory filler particles are more tiny after being finemilled and mechanically activated (individual particle size is below 15-20·10-6 m, even below 5·10-6 m); they are homogenously distributed in the solvent.

Having compared the coatings made either with or without mechanical activation of the filler, it has been noted that the mechanical activation process contributes to coating properties improvement; thus prepared coatings are easily applied and are more efficiently adhered to the sand mould and core surfaces, as well as to the polymer pattern surface. When being applied, they make a fine continuous film of coating on the subject surfaces, which is easily dried, without being either wiped off or broken after drying (Clegg 1978).


