Ablation properties of HfB₂ coatings prepared by supersonic atmospheric plasma spraying for SiC-coated carbon/carbon composites

In order to improve the ablation resistance of carbon/carbon (C/C) composites, hafnium boride (HfB₂) coatings were prepared on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying (SAPS). The anti-ablation property was investigated in an oxyacetylene torch environment under different heat flux. The phase composition, surface and cross-section microstructure and ablation behavior of the prepared coated C/C composites were characterized by using X-ray diffraction and scanning electron microscopy equipped with energy dispersion spectroscopy. The HfB₂ coating exhibited excellent ablation resistance under 2400 kW/m² after ablation for 30 s and no obvious penetrated crack and critical defect could be observed in the coating. The increase of ablation rates with the formation of rugged surface structure and pores indicated that HfB₂ coating suffered more serious mechanical denudation and thermochemical erosion. The formation of protective glassy Hf-O layer with low oxygen permeability, generated due to the oxidation of HfB₂ phase, inhibited the inward diffusion of oxygen and improved remarkably the ablation resistance of C/C composites.