Spontaneous Microcracking of Ceramics

In the previous section, the emphasis was on thermal shock, where failure was initiated by a rapid andjor severe temperature change. This is not always the case both single- and multiphase ceramics have been known to spontaneously microcrack upon cooling. Whereas thermal shock can be avoided by slow cooling, the latter phenomenon is unavoidable regardless of the rate at which the temperature is changed. 

Spontaneous microcracking results from the buildup of residual stresses which can be caused by one or more of the following three reasons  

In the remainder of this section each of these cases is explored in some detail. 

The thermal expansion coefficients of cubic materials are isotropic and hence do not exhibit this phenomenon. 

To estimate the critical grain size above which spontaneous microcracking would occur, the various energy terms have to be considered. For the sake of simplicity, the grains are assumed to be cubes with grain size d in which case the total energy of the system is  

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