HIGH-TEMPERATURE MECHANICAL STRENGTH OF SI3N4/SIC NANOCOMPOSITE PREPARED FROM CARBON-COATED SI3N4 POWDER

Highly densified sintered Si3N4 bodies are conventionally obtained through liquid phase sintering by adding sintering aids to overcome difficulty of sintering due to the strong covalent bonding in Si3N4. Sintered Si3N4 ceramics contain a weak grain boundary phase, which is produced from a reaction between the sintering aids and the raw Si3N4 powder, especially oxide phase of surface contamination on the Si3N4 powder, Therefore, the grain boundary phase influences mechanical properties of Si3N4 ceramics. The present paper discusses a method to enhance mechanical properties by forming nano SIC particles in Si3N4 system with additives, especially in grain boundary phase. The Si3N4 ceramics with dispersed nano SiC particles were obtained by using a Si3N4 powder coated by carbon, which was obtained by thermal decomposition of methane (CH4) gas. The nano SiC particles were formed by in-situ reaction between the oxide phase on the surface of the Si3N4 powder and the coated carbon. The sintered bodies were obtained by hot-pressing at 1800 degrees C for Ih in Nz atmosphere under 30 MPa of applied pressure. Results of mechanical property measurements showed that the bending strength and fracture toughness increased about 20-25%, and the high temperature mechanical strength was also improved by the carbon coating method, TEM observation showed that the precipitated SiC particles were dispersed in three different types of locations, i.e., in the grain boundary layer, at the Si3N4-Si3N4 interface penetrating the grain boundary layer and inside of the Si3N4 grain. Sic particles dispersed at the Si3N4-Si3N4 interface block grain boundary sliding and slow crack growth at high temperatures. For carbon coated samples, the quantity of grain boundary phase was smaller than the non-coated samples as shown by cryogenic specific heat measurements. The quantity of grain boundary phase decreases, which leads the improvement of high temperature mechanical strength also. Therefore, the high temperature strength was improved by precipitated SiC particles dispersion and decreasing the quantity of grain boundary phase.