Ablation behavior of atmospheric plasma sprayed (Hf_0.2Zr_0.2Nb_0.2Ta_0.2Sc_0.2)B₂-SiC hybrid coating for carbon/carbon composites

Hybrid coatings composed of (Hf_0.2Zr_0.2Nb_0.2Ta_0.2Sc_0.2)B₂ (high-entropy diboride, HEB) and SiC on carbon/carbon substrate prepared by atmospheric plasma spray (APS) was tested by oxyacetylene flame at 2000 °C. The hybrid coating, referred as HEB-20, shows very low mass and line ablation rates after 180 s ablation. By analyzing the compositional changes on both the surface and cross-section of the coating upon ablation, it is found that the oxidized products of HEB play the critical role in the later stage of the ablation at 2000 °C due to the sluggish-diffusion effect of high-entropy oxides when the volatile filing phases, like SiO₂, are consumed. The formation of different oxides in the hybrid coating prevents oxygen from entering the coating during ablation continuously and is, therefore, suggested to be the main mechanism for long-time ablation resistance. Our study shows that the HEB-SiC coating is effective in protecting C/C composites during ablation at 2000 °C for at least 150 sec.