Thermocyclic protection performance of Si-Zr(Hf)B2-SiC coatings for oxidative and ablative environments up to 2300°C

Abstract

Ceramic coatings exhibiting superior thermocyclic oxidation and ablation resistance under broad thermal regimes are imperative for ensuring the durability of carbonaceous substrates in high-temperature oxidative and ablative environments. Herein, Si-Zr(Hf)B₂-SiC coatings were prepared on graphite surfaces by silicon vaporization reaction, the coatings have dense structure, with uniform distributions of Zr(Hf)B₂ and SiC grains in continuous silicon. After 100 h of oxidation (5 h × 20) at 1600°C, the coating with ZrB₂/HfB₂ molar ratio of 4:6 (Z4H6) exhibited better cyclic oxidation resistance (mass gain of 0.80%). A multicomponent Zr-Hf-Si-O protection layer that mainly consisted of ZrO₂/HfO₂, Zr(Hf)SiO₄, SiO₂ played an effective role in blocking oxygen, thus protecting graphite substrate from oxygen invasion. After 600 s of cyclic ablation (120 s × 5) at 2300°C, the mass and line ablation rates were 5.57 × 10² mg/s and 0.52 µm/s, respectively. During ablation, the Zr-Hf oxides formed on the surface effectively resisted flame erosion, while the SiO₂ with fluidity repaired the defects in the oxide layer, thereby reducing the coating consumption under plasma flame. Eventually, the volatilization and decomposition of ablative products, in combination with mechanical scouring caused by plasma flame, progressively degrade the Z4H6 coating, ultimately resulting in the protection failure.