A novel lightweight near-non-ablation HfO₂–SiBOC ceramic: From SiHfBOC precursor synthesis to oxyacetylene flame testing at 2000 °C

Carbide- and boride-based ultrahigh temperature ceramics (UHTCs) are the materials of choice for hypersonic vehicles and scramjet engines. Nevertheless, these UHTCs are prone to oxidation in an oxygen-containing atmosphere. In addition, the high density of these UHTCs limits their widespread applications in the aerospace industry. To address the urgent need for lightweight thermal protection materials (TPMs) and mitigate oxidation-induced volume changes, a novel HfO₂–SiBOC ceramic was designed in this work, in which amorphous SiBOC is the matrix, while nanosized HfO₂ acts as the reinforcement phase. The advantages of these HfO₂–SiBOC ceramics are as follows. This HfO₂–SiBOC ceramic simultaneously achieves light weight by tuning the SiBOC matrix content and mitigates oxidation through nanosized HfO₂ uniformly dispersed in the matrix via preferential oxidation of Hf from the precursor. To achieve the above goals, a novel amber liquid SiHfBOC precursor was synthesized via a sol–gel and solvothermal method as the first step. The precursor, featuring Si–O–Si, Si–O–B main chains and Si–O–Hf side chains, achieves a high ceramic yield of 80.8 wt%. Its polymerization mechanism and properties were studied. The effects of the Hf/Si ratio and pyrolysis temperature on the SiHfBOC ceramic powder composition, microstructure evolution behavior, and oxidation resistance were systematically investigated. Based on the above results, HfO₂–SiBOC bulk ceramics were then prepared by hot pressing sintered powders. Oxyacetylene flame ablation tests at 2000 °C for 300 s confirmed their near-nonablation behavior, demonstrating exceptional ablation resistance. The ablation mechanism is elucidated. This work provides a new strategy for designing lightweight high-performance polymer-derived ceramics (PDCs) for ultrahigh temperature applications.