Transient liquid-phase to guide multiphase evolution in reactive-hot-pressed ZrB2–SiC–ZrC ceramics

Abstract

In reactive-hot-pressed ZrB2–SiC–ZrC ceramics, ZrO2 was found to replace ZrC phase, hence leading to confusion in designing ultra-high-temperature ceramics (UHTCs). We employ high-precision X-ray diffraction and electron microscopies to reassess the phase behavior during entire reaction and densification and to reveal the evolution of multiphase relationship at different stages before reaching the final ZrB2–SiC–ZrO2 composition. Frozen from transient liquid-phase, bulk glassy phase of 15 vol% was found to be constituted of Zr–Si–B–C–O with stable Zr:O ratio, which starts as early as in the intermediate stage to suppress ZrC in favor of SiC nucleation. Inhomogeneity in phase relations and microstructures results from variation in local transient liquid-phase to develop SiC phase in various modes and rates. As inferred from the earlier report of phase formation, competing reactions for ZrC and ZrB2 phases in the initial stage below 1000 °C were mediated via Zr–O–B–C liquid phase. Such liquid phase was moderated by stable B–O components, as initiated from surface oxides of starting powders. This picture under a continuous mother liquid phase can unify the reactions and sintering into a collective melting–nucleation–growth process, which enables and guides the evolution of multiphase relationship through several stages to reach final densification at relatively low temperature with the help of residual oxides.