Breviary Technical Ceramics


      From Powder to Part




4.1.4 From Forming to Firing

As a rule, green bodies made by a forming process like casting, plastic forming and pressing, contain, in addition to the ceramic powder mixture (including the permanent additives), moisture and often organic deflocculants, plasticisers, binders and other additives.
All components that are volatile at higher temperatures, i.e. which vaporise or decompose, must be removed from the green body with particular care before sintering in order to avoid damage.
Depending on the powder, the shape of the part and its size, and on the initial forming process, the green body incorporates greater or lesser density gradients. After sintering it is possible that these will make themselves noticeable in the tolerances of the part.


All particles in a formed moist body are surrounded by a film of water. The particles move closer together as the water is removed and a volume reduction takes place; this is referred to as drying shrinkage.
The drying shrinkage increases with increasing moisture content. Shrinkage is also dependent on the grain size, the type of raw materials and the forming process.
For example, the plate-like form of clay mineral particles in extruded porcelain results in less shrinkage in the length than in the cross-section. Such orientations, called textures, must be taken into careful consideration when drying, according to size and geometry of the ceramic product.
Different drying methods are applied depending on the particular ceramic products.

Burning out

Green bodies formed from unfired material that is naturally plastic are often sufficiently resistant to breakage in this state. The resistance can, however, be improved with the aid of organic additives. Such additives are, however, indispensable when the unfired material is not plastic.
Burning out the remaining plasticisers and binders along with other organic additives requires a carefully adapted temperature-pressure-atmosphere-time profile in order to achieve a non-destructive and reproducible removal of these additives from the finely porous green body.
One variation of burning-out is referred to as carbonising or coking, for example with SiC. Here, organic components are converted to carbon, which remains in the structure and is converted, with the help of added reagents, to a ceramic matrix during the sintering reaction.
After the drying and burning out (or carbonising/coking), the structure of the green body (pressed powder with the shape of component) is held together only by weak cohesive forces, and requires particularly careful handling during the subsequent process steps. For this reason, drying and burning out are integrated with the sintering wherever possible.


In order to reduce the level of risk during handling, and to allow the alternative white machining, the moulded green body can be strengthened by firing with relatively low shrinkage in preliminary firing. This requires strength and shrinkage to be made reproducible through control of the process parameters.


The goal of ceramic technology is the manufacture of a mechanically strong body able to withstand the widely differing requirements and conditions of the application. There is only a small degree of bonding between the particles of the green body. The ceramic bonding, and the very high strength associated with it, is obtained only by sintering at high temperatures. Firing allows sintering (with or without a liquid phase) to take place, and this is what actually creates the ceramic material.
The processes that occur during the sintering of the ceramic body are very complex. The sintering rate is dependent on purity, grain size, compaction and the sintering atmosphere. Products made of oxide ceramics with very high purity sinter via solid state reactions; they thus require much higher sintering temperatures than bodies containing feldspar, such as porcelain, that include a high proportion of glass phase material.

Figure 59: Grain growth during the sintering process


Through reactions that occur during sintering, a strengthening and densification of the ceramic takes place, resulting in a reduction in porosity. This process results in a volume reduction; this is called sintering shrinkage. The amount of shrinkage for the various ceramic materials is widely different.
Similar to drying, well-defined times and suitable atmospheres are required when firing ceramic products. Disregarding these can lead to increased internal stresses, to defects in the part, and to unsatisfactory characteristics. Thinner shapes and densely formed products behave better and can be sintered faster than large ones with greater wall thickness.

Figure 60:
Tunnel kiln

Typical sintering temperatures:

Table 5: Sintering temperatures of various ceramic materials

The energy required for the sintering process increases disproportionately as the firing temperature rises. In addition to the energy input, the kiln furniture used to stack the green bodies in the kiln is made of highly refractory material, and is a significant cost factor.

Special varieties of material can be created from some ceramic materials with the aid of particular types of sintering process:


Hot pressing (HP)

is used to manufacture components with a density close to the theoretical maximum. It is a sintering process supported by uniaxial pressing.

Hot isostatic pressing (HIP)

allows small parts particular to achieve the maximum density through the application of isostatic gas pressure of up to 3,000 bar at temperatures up to approximately 2,000°C (usually within a collapsible silicate glass envelope).

<< back   home   next >>