Transmission electron microscopy on Zr- and Hf-borides with MoSi2 addition: Densification mechanisms

Abstract

The microstructures of two pressureless sintered ceramics, ZrB₂ and HfB₂ with 20 vol% MoSi₂ added, were analyzed by scanning and transmission electron microscopies. Carbides and oxides of the transition metals and MoB were observed to be well dispersed within the boride matrix. Mo₅Si₃ and Mo₅SiB₂, with Zr or Hf impurities, were observed at triple grain junctions and showed a partial wetting of the matrix. It was also noticed that the borides had a core-shell structure, which was especially pronounced in the ZrB₂-based composite. The experimental results suggest the formation of a Mo–Si–B liquid phase at high temperature, which strongly promoted the densification. The densification mechanisms are discussed in light of the microstructure evolution on sintering, thermodynamic considerations, and the phase diagrams of the species involved.