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

The carbides and borides based ultra-high temperature ceramics (UHTCs) are materials of choice for hypersonic vehicles and scramjet engines. Nevertheless, these UHTCs are prone to oxidation in oxygen containing atmosphere. In addition, the high density of these UHTCs limit their widespread applications in aerospace industry. To address the urgent need for lightweight thermal protection material and mitigate oxidation induced volume change, a novel HfO₂-SiBOC ceramic was designed in this work, in which amorphous SiBOC is the matrix, while nano-sized HfO₂ acts as reinforcement phase. The advantage of this HfO₂-SiBOC ceramics is as follows. This HfO₂-SiBOC ceramic simultaneously achieves lightweight via tuning the SiBOC matrix content, and mitigates oxidation through nano-sized 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 Hf/Si ratio and pyrolysis temperature on SiHfBOC ceramic powders 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-non-ablation behavior, demonstrating exceptional ablation resistance. The ablation mechanism is elucidated. This work provides a new strategy for designing lightweight high-performance polymer-derived ceramics for ultra-high temperature applications.