TEM characterization of carbide-based ultra-high temperature ceramics

摘要

Owing to their attractive properties such as high melting point, high thermal and electrical conductivity, high stiffness and hardness, the carbides of the IV group of transition metals are potential candidate materials for ultra-high temperature applications. Despite their good properties in aggressive environment, the use of monolithic materials is limited due to their poor sinterability. Recently, MoSi2 has been disclosed as an efficace sintering aid for sintering of hafnium and zirconium carbides, as it allows the densification of high density materials at ambient pressure and at temperatures below 2000?C. In order to understand the governing mechanisms during sintering, the microstructure of dense HfC and ZrC composites containing 20 vol% of MoSi2 was analyzed by scanning and transmission electron microscopy. In the HfC+MoSi2 system, besides the starting constituent compounds, a mixed phase was detected and identified as (Mo0,5Hf0,5)5Si3. The formation of this intermediate phase indicates the possibility of mutual solubility between the starting compounds. With regard to the ZrC-MoSi2 system, several ZrxSiy compounds and SiC were detected. For both the systems investigated, the high wettability of the silicide based phases (Mo-Si, Zr-Si, Hf-Si) on the matrix grains (HfC, ZrC) suggests that sintering is aided by a liquid-silicide phase, acting as a medium for matter transport via diffusion between neighbouring grains. The formation and evolution of intermediate phases and the densification mechanisms are discussed in the light of phase diagrams and thermodynamical calculations