Heat resistance
When heat resistance , the resistance of a material is the component , precursor or finished part to high temperatures designated. If the temperature of a material or object reaches the so-called upper usage temperature, the temperature-dependent properties change so much that the material or object no longer meets the requirements or is destroyed (overheating). The duration of the temperature exposure also plays a role.
Influence on material with short-term exposure to temperature
Obvious examples for exceeding the service temperature of a material are the melting or ignition of a solid or the boiling of liquids (e.g. boiling cooling water). But even at lower temperatures, there can be a negative influence. For example, plastics and glass have a glass transition temperature which, if exceeded, softens the material and can therefore be plastically or elastically deformed and lose strength . Other material properties such as dielectric strength also change significantly.
In particular with objects made of different materials with different thermal expansion coefficients exist, can thermal stresses occur. With moving parts, dimensional changes can lead to increased friction.
The electrical resistance of materials is also temperature-dependent, which is why electrical components and circuits can fail at high temperatures.
Aging with long-term exposure to temperature
Many aging processes of materials such as embrittlement , discoloration or decomposition are accelerated by increased temperatures. A material that can withstand a certain temperature for a short time is therefore not necessarily permanently heat-resistant. Rather, the aging processes can take place so quickly due to the high temperature that the service life is significantly shortened.
In the case of metals, extremely high temperatures (more than 40% of the melting temperature) can lead to metal-physical processes such as diffusion, structural changes, creeping or oxidation.
In the temperature weathering of rocks , the brittleness of the minerals or the petrological association often plays a decisive role.
Technical importance
Heat resistance is an essential property of a material or technical product and is specified in the relevant documentation and standards. The expected operating temperatures, the materials used and the desired service life must always be coordinated. An example of this are the insulation classes for electrical insulation materials.
Overheating can be avoided by cooling , temperature controls and temperature safeguards. Safety and alarm systems with connected temperature sensors can indicate the risk of overheating and, if necessary, measures such as B. trigger an emergency shutdown.
See also
Individual evidence
- ^ R. Greiner: Technical properties . In: Ludwig Bottenbruch, Rudolf Binsack (Hrsg.): Polyamides (= technical thermoplastics ). tape 4 . Hanser Verlag, Munich / Vienna 1998, ISBN 978-3-446-16486-4 , chapter 2.6.2.2.2, p. 225 ( limited preview in Google Book Search).
- ^ Ralf Bürgel, Hans Jürgen Maier, Thomas Niendorf: Handbook high temperature materials technology . Basics, material stresses, high temperature alloys and coatings. 4th edition. Vieweg & Teubner, no location 2011, ISBN 978-3-8348-1388-6 ( limited preview in Google book search).