Failure of silicon nitride under uniaxial compression at high temperature

Failure modes of silicon nitride cylinders have been investigated under uniaxial compression at 1200°C in air.
Samples with different aspect ratios (h/d=5/2, 4/2, 2/2, and 1/2 mm/mm) have been tested. In all cases, the stress/strain
curves evidence an initial linear portion followed by a peak and a slight softening, denoting a plastic behaviour. Surface
exfoliation is the dominant failure mode, although traces of localized patterns of deformations--which initiated and
propagated macrocracks--can be found in some samples. A bifurcation analysis has been carried out in order to describe
the onset of the specific failure mode. The first failure mode predicted by this approach is an antisymmetric mode,
while symmetric modes almost immediately follow. However, antisymmetric modes may be partially hampered by
friction at the specimen/cushion contact, while symmetric modes could be triggered by residual stress. Therefore, an
interpretation of the observed failure mode is that the exfoliation mechanism may result as an evolution of a first
antisymmetric mode into a symmetric one and that localized deformations follow to produce final macrocracks
growth.