1. Ceramic shrinkage during drying

A small drying shrinkage is actually desirable in ceramic bodies, as it allows the formed article to shrink away from its plaster mould, helping its removal. However, excessive shrinkage, and particularly differentia or anisotropic shrinkage, is responsible for a considerable proportion of manufacturing losses. High shrinkage also results in loss of dimensional accuracy and size variations, which may result in problems during the later stages of manufacture or final quality control.

How does ceramic shrinkage occur during drying?

When a plastic body dries, water is removed from the surface and replaced by water from the interior. As the water content reduces, particles are drawn closer together, resulting in shrinkage. Eventually, the particles will touch each other, and though water is still present in the voids, no further shrinkage can occur on removal of the water.

Careful drying conditions are required at the point this water is still present, to prevent distortion or cracking. And as well as the materials and drying conditions, the shape of the article will impact on this drying rate.

The greater the surface area of an article relative to its volume, the more readily it will dry. Any alignment of particles in the shaped article will result in anisotropic shrinkage, increasing the stresses within the article during drying and enhancing the likelihood of losses.

2. Ceramic shrinkage during firing

The firing process encompasses chemical and physical changes in the ceramic body accompanied by a loss of porosity, with an associated increase in density, and subsequent shrinkage. The formed ceramic powder becomes bonded together in a rigid matrix by vitrification or sintering.

How does vitrification affect shrinkage rates?

Vitrification is the progressive reduction and elimination of porosity of a ceramic composition with the formulation of a glass phase as a result of heating. Glass formation typically starts around 1100°C and accelerates with a further increase in temperature. As the amount of glassy phase increases, the open pores are reduced, closed pores start to develop and the amount of shrinkage increases.

Once the desired porosity is attained, cooling is started – and during the cooling the glassy phase freezes and becomes rigid, forming a strong bond with the crystalline phase of the body.

Sintering and shrinkage

Sintering is a process of permanent chemical and physical change accompanied by reduced porosity by the mechanism of grain growth and bonding. During the sintering process, large grains grow at the expense of smaller ones, leading to the average size increasing, while the number of grains decreases.

Sintering can start at relatively low temperatures, but doesn’t become significant until temperatures of 1200°C are reached for most ceramic materials. Sintering can occur in three ways, but shrinkage only occurs in two:

1. During material transfer by solid state diffusion, which relies on the movement of atoms along or through the grains.
2. During liquid phase sintering, where the solid phase shows a certain limited solubility in the liquid at sintering temperatures.

Aside from vitrification and sintering, a number of other reactions take place during firing, including loss of physical water, oxidation, de-composition and phase transformations.

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