Microstructure and high-temperature strength of Si 3N 4-SiC nanocomposite

Abstract

The microstructural evolution and the high-temperature strength of Si 3N 4-SiC nanocomposite hot pressed with Yb 2O 3 as a sintering aid were investigated. The microstructure of the composite was influenced by the nucleation step prior to the densification. When the number of nucleation sites was small, large elongated grains were formed and large fraction of the SiC particles was trapped inside the grains. On the other hand, when there were many nucleation sites, the grains became small because of the inhibition of growth of β-Si 3N 4 grains by the SiC particles, and most of the SiC particles were located on the grain boundaries. The number of nucleation site was controlled by the heat treatment temperature before the densification. The location of the SiC particles was important in determining the high-temperature strength of this composite material. The strength was improved only when the SiC particles were located on the grain boundaries. The flexural strength of Si 3N 4 with 20 vol% SiC was 1150 ± 50 MPa at room temperature. The strength was maintained at elevated temperatures up to 1200 °C and decreased to 1000 M Pa at 1400 °C in air.