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Failure modes of silicon nitride cylinders have been investigated under uniaxial compression at 1200 °C
in air. Samples with different aspect ratios (h/d) have been tested: 5/2, 4/2, 2/2, and 1/2 (mm/mm). In all
cases, the stress/strain curves evidence an initial linear portion followed by a peak and a slight softening.
Most of the tests were interrupted at about 3-4 % of load drop after the peak, and the samples observed
with optical and electronic microscope. Two samples catastrophically broke in correspondence of the test
stop and further observations were precluded. In all the other cases, failure patterns were observed and
involved modes which show intriguing symmetries. Particularly, surface exfoliation seems to play a
central role in limiting the load-bearing capacity of the sample. The reason of such an exfoliation may be
interpreted in different ways. A possibility is a surface buckling mechanism. However, in addition to this
surface mode, traces of localized patterns of deformations which initiated and propagated macro-cracks
can be found in some samples.