Multicomponent (Hf-Zr-Ta)B₂ coatings for carbon/carbon composites and structural optimization enabling superior ablation resistance

Multicomponent (Hf-Zr-Ta)B₂ potentially provides improved ablation resistance compared with silicon-based ceramics. Here we deposited (Hf_0.5–x/2Zr_0.5–x/2Ta_x)B₂ (x = 0, 0.1, and 0.2) coatings onto C/C composites, and investigated their ablation behaviors under an oxyacetylene torch with a heat flux of 2.4 MW m^–2. It was observed that the x = 0.1 oxide scale bulged but was denser, and the x = 0.2 oxide scale was blown away due to the formation of excessive liquid. Based on these findings, we further developed a duplex (Hf-Zr-Ta)B₂ coating that showed a linear recession rate close to zero (0.11 μm s^–1) after two 120-s ablation cycles. It is identified that the resulting oxide scale is mainly composed of (Hf, Zr)₆Ta₂O₁₇ and (Hf, Zr, Ta)O₂ by performing aberration-corrected (scanning) transmission electron microscopy. The protective mechanism is related to the peritectic transformation of orthorhombic-(Hf, Zr)₆Ta₂O₁₇ to tetragonal-(Hf, Zr, Ta)O₂ plus Ta-dominated liquid. This study contributes to the development of Ta-containing multicomponent UHTC bulk and coatings for ultra-high temperature applications.