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.

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