Fabrication of C f /HfC(N)–SiC Composites via O‐Free Precursor: Phase Evolution, Microstructure, and Ablation Resistance

ABSTRACT Ultra‐high temperature ceramic matrix composites (UHTCMCs) are promising candidates for thermal protection systems of hypersonic vehicles, yet conventional oxygen‐containing precursors for HfC matrices suffer from severe volume shrinkage, oxygen impurity, and poor densification efficiency via the polymer infiltration and pyrolysis (PIP) route. Herein, a dense and homogeneously structured C f /HfC–SiC composite was fabricated via PIP using a novel oxygen‐free precursor. Systematic characterization by Fourier transform infrared spectroscopy (FT‐IR), TG‐MS, x‐ray photoelectron spectroscopy (XPS), and XRD revealed that the precursor undergoes a stepwise ceramization pathway: Cross‐linking of the Hf–N backbone (200°C–900°C) → formation of an amorphous Hf–C–N network → crystallization of an HfC(N) solid solution at 1600°C. The as‐fabricated C f /HfC(N)–SiC composite has a bulk density of 2.47 g/cm 3 and an open porosity of 18.3 vol%, with a continuous HfC matrix uniformly distributed in the SiC secondary phase. During ablation under a heat flux of 5 MW/m 2 via air plasma test, the composite surface temperature exceeds 2800°C, while the linear recession rate is as low as 0.67 µm/s. The exceptional ablation resistance originates from a dense, continuous protective scale composed of cubic HfO 2 , SiO 2 glass, and HfSiO 4 , where trace N doping from the precursor stabilizes the cubic HfO 2 phase and suppresses cracking during cooling. Critically, the uniform HfC matrix refines the pore structure of the oxide scale, triggering a transition from molecular diffusion to Knudsen diffusion for oxygen transport, which drastically reduces oxygen penetration rates. This work provides a feasible oxygen‐free precursor strategy to overcome the intrinsic limitations of conventional PIP‐processed UHTCMCs for extreme service applications.