Oxidation Resistance of ZrB ₂ –SiC-WC Composites up to 2000 °C at High and Low Oxygen Partial Pressure

Composites of ZrB2–SiC-WC for hypersonic flight applications require the presence of a B2O3–SiO2-based surface layer covering a dense ZrO2 scale to suppress high-temperature oxidation. Previously, we showed that the addition of 3.6 vol % WC to ZrB2-20 vol % SiC enhances the stability of the composite at 1800 °C. Stabilization was mainly due to densification of the underlying ZrO2 scale. In this article, we ask whether increasing the SiC content can extend this stability? We increased the SiC content to 30% and determined whether the extra SiO2 formed during SiC oxidation can provide additional protection up to 2000 °C. Composites of ZrB2-30 vol % SiC-3 vol % WC were fabricated by SPS at 1950 °C and tested at various oxidation conditions. Non-WC-containing samples (Z30S) show significant surface degradation in air at 1800 °C, loss of protective B2O3–SiO2-based surface layers, and a thick porous ZrO2 scale that allows for oxygen penetration. WC-containing samples (Z30SW) preserve protective layers under the same conditions, with dense ZrO2 and B2O3–SiO2-based surface layers, thicker than those for Z30S and compositions previously studied. Following the exposure to air at 1800 °C for 15 min, Z30S and Z30SW lost 1.51 ± 0.18 mg cm–2 and gained 1.96 ± 0.08 mg cm–2, respectively. After 15 min at 2000 °C and low pO2 (∼10–6 Pa), samples undergo active oxidation of SiC and loss of the SiO2-based surface layer. A ZrO2 layer does not form under these low pO2 conditions; thus, a highly porous ZrB2 skeleton is formed at the surface, leading to a damaged Z30S surface layer that is 40 μm thick. In the Z30SW samples, the ZrB2 skeleton is 30% thinner, intact, and much denser, allowing significantly less oxygen to reach the base composite.