There are many reasons for thermal shock failure in industrial applications of ceramics. On analysis they usually come down to one or more of the following factors.
- Material selection
- Material processing
- Design of component
- Application/use of the product
It is often possible to improve the performance by changing one or more of these but as with all ceramic applications thermal shock is only part of the equation and changes must be looked at in context of all the performance requirements.
When designing any product in ceramic it is necessary to look at the overall requirement and often then to find the best compromise that will work.
In high temperature applications, thermal shock is often the main cause of failure. It is comprised of a combination of thermal expansion, thermal conductivity and strength. Rapid changes in temperature both up and down cause temperature differentials within the part, not unlike a crack occurring by putting an ice cube against a hot glass. Movement through differing expansion/contraction leads to cracking and failure.
There are no simple answers to the thermal shock issue however the following guidelines do tend to be beneficial.
- Select a material grade that has some inherent thermal shock characteristics but meets the needs of the application. Silicon carbides and silicates are excellent. Alumina based products are less good but can be improved with the right design.
- Porous products are generally better than impervious and will take larger changes in temperature.
- Thin walled products perform better than thick wall. Also avoid large transitions in thickness throughout the part. Sectional parts may be better as this provides less mass and offers a Pre cracked design alleviating stress raisers.
- Minimise the use of sharp corners as these provide ideal starting points for cracks.
- Avoid tension loading of the ceramic. Parts can be pre stressed through design to help alleviate this problem.
- ·Where possible look at the application process to see if it is possible to provide a more gentle change in temperature. Pre heating the ceramic or reducing the rate of temperature change.
The above points will help alleviate thermal shock problems but it is always best to discuss the situation with experts in the field.
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