Breviary Technical Ceramics






5.4.3 Thermal Shock Resistance

A large number of ceramic materials are sensitive to thermal shock, which means that sudden changes of temperature can lead to failure. Aluminium titanate, sintered fused silica and materials based on cordierite are notable exceptions.

The causes of this sensitivity to thermal shock are the internal mechanical stresses induced by temperature gradients, and the highly brittle nature of the ceramic material. Whereas high local thermal stresses in metals merely lead to a slight local plastic deformation, they can lead to the propagation of cracks in ceramic materials. For this reason sudden, large changes of temperature should be avoided whenever possible.
The thermal stresses responsible for the response to temperature stress depend on:

  • geometrical boundary conditions
  • thermal boundary conditions
  • physical parameters, such as
      coefficient of thermal expansion
      modulus of elasticity E
      thermal conductivity
      and others.  

Determination of the sensitivity to thermal shock can be carried out using a method suggested by Hasselmann. Test pieces – in the simplest case these are bending bars – are quenched to drop them from a temperature T0 to a temperature Tu. The strength of the samples is measured after the quenching. The curve of strength against the temperature difference, T = T0 - Tu, has the shape shown on Figure 93. Up to a temperature difference of Tc the strength does not alter. The strength then drops sharply within a narrow range, T. Up to Tc’ this reduced length then remains constant, falling away again at higher temperature differences.

Figure 93: The strength of thermally shocked bending samples according to Hasselman

Resistance to thermal shock or stress is characterised in the literature by what are known as R parameters (RS = resistance to thermal shock).

The values listed there for thermal shock behaviour must only be looked on as general guidelines for the various materials, because the individual physical and mechanical data can vary widely. Other factors such as the fracture toughness, KIC, and the specific thermal capacity, Cp, also play a part.
Values obtained from homogenous ceramic material have only limited relevance to the associated porous ceramics.
In practice, the geometry also plays an important part. For this reason we have included a qualitative assessment in our tables of materials.

Test procedures for determining the resistance to temperature change are specified in DIN V ENV 820-3.


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