Meaning
Internal mechanical forces arising from the unequal expansion of neighboring crystals belong to the class of microstructural defects. Grain boundary thermal stress develops when a polycrystalline material is heated and the individual grains expand at different rates. It leads to the formation of microcracks at the interfaces.
This phenomenon limits the thermal shock resistance of high temperature ceramics.
Interfacial Tension
High pressure builds up where two grains with different orientations meet. The grain boundary thermal stress is proportional to the difference in the expansion coefficients of the adjacent crystals. If the grains are large, the localized stress can exceed the fracture toughness of the material.
Microstructural Integrity
Cohesion between grains is essential for the mechanical stability of the part. Grain boundary thermal stress can cause a gradual degradation of the material over many thermal cycles. A pilot result may show initial strength, but the demonstrated rate of failure increases over time.
Fracture Risk
Cracking begins when the internal tension pulls the grains apart. Within a production environment, the grain boundary thermal stress must be managed by controlling the grain size during the sintering process. Small grains help to distribute the stress more evenly and prevent catastrophic failure.
The final quality is measured against the theoretical density of the material.
