Si3N4/20-vol%-SiC nanocomposites with Al2O3 and Y2O3 as sintering additives
have demonstrated very high strength at room temperature; however, the hig
h-temperature strength was drastically decreased, because of the low soften
ing temperature of the grain-boundary phase. To improve the high-temperatur
e strength, only Y2O3 was used as a sintering additive. Results showed that
the fracture strength of this nanocomposite at temperatures >1200 degrees
C was increased by adding Y2O3 without Al2O3; however, a distinct decrease
in the high-temperature strength still was observed for higher Y2O3 content
s, The fracture strength at room temperature (similar to 1 GPa) was maintai
ned up to 1400 degrees C in the sample that contained SiC particles (30 nm
in size) and 4 wt% of Y2O3. Remarkably, the SiC particles at the grain boun
daries were bonded directly to Si3N4 grains without a glassy phase. This sa
mple fractured in an elastic manner without exhibiting plastic deformation
up to 1400 degrees C and showed no evidence of subcritical crack growth on
the fracture surface. The significant improvement of the high-temperature s
trength in this nanocomposite can be attributed to inhibition of grain-boun
dary sliding and cavity formation, primarily by intergranular SiC particles
that are bonded directly to the matrix grains, as well as crystallization
of the grain-boundary phase. Rietveld analysis of the X-ray diffraction dat
a revealed the presence of a secondary phase-10Y(2)O(3). 9SiO(2). Si3N4 (h-
phase)-in samples with Y2O3, whereas YSiO2N was present in the samples that
contained both Y2O3 and Al2O3.