High‐Temperature Strength and Microstructural Analysis in Si ₃ N ₄ /20‐vol%‐SiC Nanocomposites

Abstract

Si 3 N 4 /20‐vol%‐SiC nanocomposites with Al 2 O 3 and Y 2 O 3 as sintering additives have demonstrated very high strength at room temperature; however, the high‐temperature strength was drastically decreased, because of the low softening temperature of the grain‐boundary phase. To improve the high‐temperature strength, only Y 2 O 3 was used as a sintering additive. Results showed that the fracture strength of this nanocomposite at temperatures >1200°C was increased by adding Y 2 O 3 without Al 2 O 3 ; however, a distinct decrease in the high‐temperature strength still was observed for higher Y 2 O 3 contents. The fracture strength at room temperature (∼1 GPa) was maintained up to 1400°C in the sample that contained SiC particles (30 nm in size) and 4 wt% of Y 2 O 3 . Remarkably, the SiC particles at the grain boundaries were bonded directly to Si 3 N 4 grains without a glassy phase. This sample fractured in an elastic manner without exhibiting plastic deformation up to 1400°C and showed no evidence of subcritical crack growth on the fracture surface. The significant improvement of the high‐temperature strength in this nanocomposite can be attributed to inhibition of grain‐boundary 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 data revealed the presence of a secondary phase‐10Y 2 O 3 9SiO 2 Si 3 N 4 (h‐phase)‐in samples with Y 2 O 3 , whereas YSiO 2 N was present in the samples that contained both Y 2 O 3 and Al 2 O 3 .