**6.2 Binding properties**

272 Ceramic Coatings – Applications in Engineering

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

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

> Melting point, oC

oxide 3.30 9.0 2016 1.13 2.59 steel, Mg alloys Periclase 3.40 5.5-6.0 2800 1.08 3.32 Mn- steel Mullite 3.20 7.5 1830 1.00 2.02 cast iron, Al & Cu

Quartz 2.65 7.0 1716 1.05 1.73 cast iron, nodular

magnesite 2.85-3.5 5.5-6.0 2400 0.95 4.50 steel, Mg alloys

Specific heat, kJ/kgK Heat cond. W/mK on 827oC

Recommendation

alloys

cast iron

cast iron, nodular cast iron, steel, Al & Cu alloys

cast iron, nodular cast iron, steel, Al & Cu alloys

cast iron, nodular cast iron, steel, Al & Cu alloys

cast iron, nodular cast iron, Al & Cu alloys

cast iron, nodular cast iron, Al & Cu alloys

below 5·10-6 m); they are homogenously distributed in the solvent.

Mos hardness

Zircon 4.50 7.5 2500 0.54 2.31

oxide 5.5-6.1 2700 0.71 0.72

Chromite 4.5-4.8 5.5 2180 0.8-1.1 1.69

Talc 2.6-2.8 1-1.5 1547 0.19 2.40

Graphite 2.23 1-1.5 3000 0.75-1.4 1.50

Table 2. Type and properties of refractory fillers most frequently used

(Clegg 1978).

Refractory material

Aluminium

Zirconium-

Sinter

Specific weight, kg/m3

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, Tomović 1990).
