Fabrication and Mechanical Properties of Si3N4/SiC Nanocompoisites under Pressureless Sintering

Abstract

To meet the practical application, a kind of commercially available Si3N4 powder by direct Si nitridation was used to fabricate dense Si3N4/SiC nanocomposite by pressureless sintering method. The price of this kind of powder is much lower than that of powder by imide decomposition. 5∼30 vol% of SiC powder (average particle size: 80 nm) were added in the composite to investigate the effect of SiC particle. Totally 15 wt% of MgAl2O4 and ZrO2 (50 wt% each) were used as sihtering additives. With increasing SiC content, the densities of the composites decreased. α-β Si3N4 phase transformation was enhanced at low temperatures, but prohibited at high temperatures for large SiC content. The sample with larger SiC content gave a relatively smaller Si3N4 grain size because SiC particles limit the Si3N4 grain growth by pinning and prohibiting the grain boundary movement. An improvement of Young's modulus, flexural strength and hardness by the SiC addition was found. Further increase in SiC content resulted in a decrease of these properties because of the decreasing density for the samples sintered at low temperatures. With an intermediate sintering temperature, the samples with 20 vol% SiC reached to high flexural strength as 1050 MPa as a result of good density and relatively fine grain size structure. The decreased fracture toughness with increasing the SiC content was observed by the refinement of Si3N4 grain. However, it was improved with the increasing sintering temperature. The result of this study provided an economic way to fabricate the Si3N4/SiC nanocomposite with a strength higher than 1 GPa and appropriate fracture toughness.