How to avoid stresses in your component?

Have you ever had the problem that a metal door just wouldn’t close in the summer, and in the winter cold air came through gaps even more so?

Maybe not, but you’ve certainly had a glass shatter because you poured water inside that was too hot.

Whether it is the metal door or the glass: In both situations changes of the temperature caused the problem. Even ceramics that are best known for their good thermal stability can fail at rising temperatures. That’s because each material has its own characteristic thermal expansion that causes stresses in the microstructure. The thermal expansion of materials is measured by recording the changes in length during heating and cooling. One method is the mechanical dilatometry.

There are different mechanisms that cause changes in length of components with rising temperatures. In general, a distinction is made between two categories:

• Irreversible
• Reversible

In the case of irreversible changes in length the initial geometry and the geometry after thermal treatment of a sample or component do not match.

A classic example of ‘shrinkage’ is the sintering of ceramics. Due to the fusion of grains and the associated filling of pores compaction of the microstructure takes place. Other irreversible changes in length occur when glasses crystallize or (crystalline) phases change. Here, the dehydration of clay minerals or the volatilization of CO₂ from carbonates are typical examples.

However, there are cases where the sample ‘grows’ as well. Here, chemical reactions such as the oxidation of metals are mainly the cause.

Irreversible changes in length are usually intentional, as the sintering, or can be prevented by adjusting the maximum firing temperature or the atmosphere.

On the other hand, reversible thermal expansions are difficult to avoid – if they’re noticed at all! This is because, as hinted by the name, the initial and the final geometries are identical. During heating, atoms in every material – whether crystal or glass – start moving further apart which leads to the expansion of the material, a natural phenomenon when temperature increases.

A good example for a reversible thermal expansion is a single crystal of Al₂O₃ – also known as corundum or sapphire. The thermal expansion of this material follows an almost linear trend. When cooling, the material contracts again in the same way. For that reason, corundum is often used as a calibration standard for many high-temperature analyses. ZrO₂ is an equally important high-performance ceramic as Al₂O₃.The thermal behaviour of ZrO₂ is also characterized by a reversible thermal expansion. However, in contrast to Al₂O₃, this often becomes noticed in an unattractive way: Through damages.

This is due to the crystal lattice of ZrO₂. At room temperature, the material – also called zirconia or baddeleyite as mineral – has a monoclinic crystal structure. Monoclinic structures are characterized by a low symmetry with respect to the arrangement of the individual atoms. Consequently, monoclinic ZrO₂ crystals take up ‘a lot’ of space.

However, at higher temperatures – 1170 °C to be precise – the zirconium and oxygen atoms start moving, that leads to a change in the crystal lattice. The crystal structure is now ‘tetragonal’, meaning the symmetry increases. In addition, the crystal now requires ‘less’ space.

The change results in a sudden ‘shrinkage’ of about 3 % by volume, which causes stresses in the material. During cooling, the ‘jump’ back to the monoclinic phase takes place at lower temperatures of ~ 950 °C, depending on the grain size. In most cases, the double stress leads to damage of the component.

In the special case of zirconia, the problem is solved by ‘stabilizing’ the tetragonal phase with suitable dopants to room temperature. This way, the phase change at 1170 °C is prevented, because the tetragonal phase is already present.

However, other methods are used for most materials, as with cooktops made of glass-ceramics. Here, the components of the material are combined, so their thermal expansions compensate each other and the overall expansion equals zero.

Therefore, the thermal expansion of a material should be determined before it is used. That way, it can be taken into account in the design, e.g. by installing expansion joints, or, if in doubt, compensated by suitable material combinations.

Otherwise, damages occur or the metal door will not close properly in summer.

[1] Han, Y. & Zhu, J. (2013): Surface science studies on the zirconia-based model catalysts. Topics in Catalysis, 56. 1525-1541.