Thermoablative resistance of ZrB₂-SiC-WC ceramics at 2400 °C

Although ZrB₂-SiC ceramics have been extensively researched for applications at ultra-high temperatures (>2000 °C), it is well known that at these temperatures the SiC oxidises actively yielding a gaseous sub-oxide, SiO, rather than the protective, passive oxide product, SiO₂. This limits the high-temperature range of SiC-bearing ceramics for ultra-high temperature applications. In the present work, the addition of 5 vol% WC has been shown to partially eliminate the active oxidation of SiC in ZrB₂-SiC ceramics, even when exposed to an oxyacetylene flame at 2400 °C. In contrast to the porous and fragmentary surface observed with ZrB₂-SiC ceramics tested under the same conditions, a dense oxide surface layer was observed that is believed to have resulted in decreasing pO₂ in the layers beneath. This had the effect of changing the chemistry of the system and hence the composition of the phases produced. Clear evidence of the presence of SiO₂ was observed, thus indicating that the oxidation of the SiC had been partially passive rather than active. A full volatility diagram for WB at 2400 °C was derived, and existing volatility diagrams for ZrB₂ and SiC were extended to the same temperature, in order to develop a theoretical understanding of the ablation mechanism. The significantly improved ablation resistance of ZrB₂-SiC-WC is consequently mainly attributed to a competitor transition from tungsten boride (WB) to metallic tungsten in the oxygen partial pressure range 10⁻⁸ Pa to 10⁻³ Pa, which retards the occurrence of the active oxidation of the SiC phase.