Microstructure and ablation behavior of hafnium carbide coating for carbon/carbon composites

Hafnium carbide (HfC) coating was prepared on carbon/carbon (C/C) composites by low pressure chemical vapor deposition (LPCVD) with HfCl4–CH4–H2–Ar system. The phase composition, microstructure and ablation properties of the coating were studied by X−ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and oxyacetylene flame torch. The evolution of morphology and microstructure of HfC coating during the ablation were investigated. The ablation behaviors of HfC coating in different ablation regions were discussed. The results showed that multi-layered HfC coating with compact structures can be prepared on C/C composites by LPCVD. The as-prepared coating exhibits dense surface and excellent ablation resistance. After ablation for 240 s, three distinct regions appeared on the coating surface. In the center and transitional ablation regions, the coating surface is compact and consists of HfO2 grains. While in the border ablation region, the coating surface is porous. During the ablation, a porous structure is formed within the oxide layer, resulting from the release of CO and CO2. Both molten HfO2 film and HfCxOy can play a role as an oxygen diffusion barrier, which is the major reason for the excellent oxidation and ablation resistance of the HfC coating. In addition, large amounts of heat can be consumed by the oxidation of HfC and the evaporation of the oxide, which can further protect the C/C composites from ablation.