Reactive and non-reactive hot pressing of ZrB2-SiC-C UHTCs: Structure, mechanical and thermal properties

Four different precursor powder mixtures of ZrC-B₄C-Si, ZrC-B₄C-ZrSi₂, ZrC-B-ZrSi₂, and ZrB₂-SiC-C were hot-pressed at 1850 °C and 30 MPa for 3 min to produce ZrB₂-SiC-C heteromodulus Ultra-High Temperature Ceramics (UHTCs). Pure silicon, as one of the precursors, was shown to cause macroscopic reaction-induced stratification in the sintered sample, with the inner part enriched in oxygen impurities and carbon. Introducing silicon through ZrSi₂ did not lead to notable stratification. The materials' hardness decreased from 20 to 8 GPa, while the toughness rose from 4.2 to 5.4 MPa·m^1/2 with graphite content. The thermal conductivity of the reactively sintered samples increased from 38 to 73 W/mK with the increase in ZrB₂ content. The conventionally hot-pressed materials exhibited an opposite trend in the thermal conductivity coefficient. The different behaviours were explained based on the composite structure and the interface contact thermal resistance. The thermal shock tolerance improved significantly with the increase in graphite content.