Ceramics
introduction
properties
crack sensitivity
thermal shock
ceramics
glasses

Properties 3

Thermal shock

Most people have seen the results of thermal shock - have seen a glass or ceramic item from the oven fracture when it contacts cold water.
Most ceramics do not have good thermal shock resistance, for example glass will fracture under a sudden temperature change of > 80°C. Changes in temperature cause expansion or contraction, which lead to thermal stresses in the material. These thermal stresses in metals or polymers are alleviated by plastic deformation. But as ceramics are brittle, fracture occurs before plastic deformation.

bar chart of data for melting or softening temperatureMelting Temperature

Ceramics are able to operate at very high temperatures, because their melting temperature is very high. Most ceramics and glasses have melting temperatures above 2000°C, a temperature much higher than that for metals and polymers. As a result, ceramics are often used in high temperature applications.

Creep

Similarly to metals, ceramics creep when they are hot. The difference between ceramics and metals is the temperature at which creep occurs. Creep occurs when the temperature is above about 0.4-0.5 Tm. Metals will therefore creep at relatively low temperatures - lead begins to creep at room temperature and aluminium alloys will begin to creep from about 250°C. Ceramics have melting temperatures much higher than metals and will only creep when they are exposed to stresses at much higher temperatures. Therefore creep is not usually a limitation when designing with ceramics. Glasses will however creep at lower temperatures, but at very slow rates.

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