Influence of grain size on the tensile creep behavior of ytterbium-containing silicon nitride

The effect of grain size on the tensile creep of silicon nitride was
investigated on two materials, one containing 5% by volume Yb2O3, the
other containing 5% by volume Yb2O3 and 0.5% by mass Al2O3. Annealing
of the Al2O3-free silicon nitride for a longer period during processing
increased the grain size by a factor of 2. This increase did not affect
the tensile creep rate; the grain size exponent of the creep rate
differed little from zero, p = -0.20 +/- 1.37 (95% confidence level).
This finding supports the more recent theories of tensile creep for
which p = 0 or -1 and rejects the more classical theory of
solution-precipitation. In compression, a more limited data set showed
p = -1.89 +/- 1.72 (95 % confidence level). In contrast to the
Al2O3-free material, a longer term anneal of the Al2O3-containing
material during processing did not increase its grain size. Despite
this, the longer-annealed Al2O3-containing material crept 10 to 100
times slower than the short-annealed material. The enhancement of creep
resistance may be a consequence of SiAlON formation during the
additional annealing, which reduces the Al content in the amorphous
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phase and increases its viscosity. Such changes in chemical composition
of the grain boundaries are more effective in controlling tensile creep
rate than is the grain size.