Microstructure evolution accompanying high temperature; uniaxial tensile creep of self-reinforced silicon nitride ceramics

Extensive transmission electron microscopy (TEM) has been performed to study the microstructure evolution of a self-reinforced silicon nitride associated with high temperature creep. A large population of strain whorls is observed in samples crept at relatively high temperatures and the strain whorls are not necessarily asymmetrical with respect to the grain boundary normal. Large angle convergent beam electron diffraction (LACBED) at the grain boundaries where strain whorl contrast is visible reveals severely curved Bragg lines, implying large residual strains. This indicates that grain boundary interlocking might be effective to enhance the creep resistance at high temperatures. Dislocation pile-ups, arrays and tangles are present in certain silicon nitride grains. However, a simple analysis rules out dislocations as the major creep mechanism. Most dislocations started from grain boundaries. The role of dislocations is to relieve the stress concentrations at the strain whorls. This adds to the diffusion mechanism of stress relaxation at the strain whorls and facilitates other creep mechanisms such as grain boundary sliding. A large density of multiple-junction cavities is observed in the samples crept at relatively high temperatures. It is proposed that grain boundary sliding and cavity formation, in addition to stress relaxation through nucleation of dislocations at the strain whorls act together to produce a much shorter life to failure at high temperatures. While at lower temperatures, the creep is more diffusion controlled which gives a stress exponent of unity.