Synergistic pore-filling and particle refinement: A ZrO2–SiC–Si/ZrB2–SiC–Si bilayer coating for ultrahigh-temperature oxidation protection at 1695 °C

Ceramic coatings capable of providing reliable protective barriers for carbon materials in oxidizing environments above 1650 °C are crucial for expanding their high-temperature applications. Herein, ZrO2–SiC–Si external coatings with varying zirconium–silicon source ratios were prepared via the slurry method on dense ZrB2–SiC–Si inner coatings fabricated by Si reactive infiltration. The composite coating comprised a dense inner layer and a porous outer layer, with ZrO2, ZrB2, SiC, and Si relatively uniformly dispersed throughout in the coating. The results showed that the external coating with a Si/Zr mass ratio of 7:3 (S7Z3) exhibited superior oxidation resistance at 1695 °C; after 30 h of oxidation, the coated sample had a mass gain of 6.24% with no obvious coating detachment. During oxidation, the fluid SiO2 phase effectively sealed pores and defects in the protective coating. Meanwhile, ZrO2 and ZrSiO4 particles in the oxide scale underwent progressive grain refinement and became increasingly uniformly dispersed over oxidation time. The resulting oxide microstructure was more homogeneous and thermodynamically stable, which markedly suppressed oxygen ingress toward inner coating. Consequently, the outer silicate glass layer containing thermally stable pinning phases (ZrO2 and ZrSiO4), combined with the dense oxygen-barrier inner layer (ZrB2–SiC–Si), thus provided reliable oxidation protection for the carbon substrate at 1695 °C in air.