What is heat shock?

Glass is susceptible to thermal shock.

Thermal shock describes the way in which some materials are prone to damage if they are exposed to a sudden change in temperature. Glass and certain other materials are vulnerable to this process, in part because they do not conduct thermal energy very well. This is readily observed when a hot glass is exposed to ice water — the result is a cracked, broken, or even shattered glass.

Damage is a reaction to a rapid and extreme temperature fluctuation, but the process is a little more complicated than that. The shock is a result of a thermal gradient, which refers to the fact that the temperature change occurs unevenly. A change in temperature causes an object’s molecular structure to expand, due to the weakening of the bonds that keep molecules in formation. The existence of the thermal gradient means that this expansion occurs unevenly, and glass in particular is very vulnerable to this process.

In the hot glass example, this means that the rapid change in temperature causes some parts of the glass to become much hotter than others. This, in turn, causes uneven expansion, which puts pressure on the molecular structure. If the tension becomes large enough, the material’s strength is overcome and the glass breaks.

Ceramics and glass are vulnerable to this process, in part because they are not good conductors of heat energy and also because they do not have high tensile strength. Even so, these materials are often used for applications where extreme temperatures are common as they have very high melting points. The problem then is how to avoid thermal shock while maintaining the temperature extremes required by the process.

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Improving the shock resistance of glass and ceramics can be achieved by improving the strength of the material or reducing its tendency to expand unevenly. An example of success in this area is Pyrex®, a brand of a type of glass that is better known to consumers as kitchen utensils, but which is also used to manufacture laboratory glassware. The type of glass traditionally used to make Pyrex® is called borosilicate glass, due to the addition of boron, which prevents shock by reducing the glass’s tendency to expand.

When materials must be tested for their ability to withstand extreme temperatures, they are tested inside a thermal shock chamber. Inside the chamber, they are exposed to rapid cycles of extreme heat and cold temperatures to determine the temperatures at which the material’s tensile strength is overcome. This type of testing is used in a wide range of industries, including land, air and spacecraft development, as well as industrial manufacturing.

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