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.

Effect of different sintering aids on thermo–mechanical properties and oxidation of SiC fibers – Reinforced ZrB2 composites

“…For the material densified with ZrSi 2 (Zf), ZreSi with various stoichiometries were concentrated at triple points and along the grain boundaries. In the MoSi 2 -doped system (Mf), clean grain boundaries were found along with newly formed MoB, Mo 5 Si 3 B phases, as reported in previous works [17]. The total amount of secondary phases deriving from the above mentioned reactions was lower than 1e2 vol%.…”

“…During cooling, reprecipitation resulted in formation of a ZrB 2 coreeshell microstructure, whilst the remainder of the liquid phase crystallized into MoB, Mo 5 Si 3 and Mo 5 SiB 2 phases, as observed earlier [17].…”

“…When MoSi 2 was added, the composite started to shrink at around 1600 C and achieved the theoretical density at 1750 C. The sintering process involved the reactions between MoSi 2 and BeO species with local formation of a transient liquid phase based on MoeSieBeO species according to [17]:…”

Spark plasma sintering of HfB2 with low additions of silicides of molybdenum and tantalum

“…2c. As previously reported, 23 during thermal treatment, MoSi 2 reacts with surface oxides, forming silica and other Mo-Si-B species which can be liquid at the sintering temperature. A possible reaction is: (1) Once borides grains are depleted of their surface oxide, they can sinter by solid state and by liquid phase sintering mechanisms.…”

In situ transmission electron microscopy studies of the kinetics of Pt-Mo alloy diffusion in ZrB2 thin films

“…6,12 Typically, these structural components are prepared out of ceramic powders, which are mixed with metals or metal silicides to promote sintering and to enhance their functionality. 8,11,[13][14][15][16][17][18][19] Growth and characterization of ZrB 2 thin films are also of interest for potential applications as decorative coatings and diffusion barriers in microelectronics. 20,21 In this letter, we report results from in situ transmission electron microscopy (TEM) studies of thermal stability of ZrB 2 thin films in contact with Pt and Mo.…”