Study on the thermocyclic protection performance of ultrahigh temperature ceramic SiC–Si–Hf(Ta)B2 coating for static oxidation and ablation environments up to 2300 °C

Thermocyclic protection performance of ultrahigh temperature ceramic coatings is crucial for the reusability of carbon-based composites. In this work, the slurry method combined with vapor phase siliconizing was used to prepare SiC–Si–Hf(Ta)B2 ceramic coatings with changing Hf/Ta proportion on the surfaces of graphite substrates. The prepared coatings had a dense structure and were tightly bonded to the graphite substrates. (HfTa)B2 solid solution and SiC particles were collectively embedded in the consecutive Si. After 100 h of oxidation at 1600 °C (5 h × 20 cycles), the mass of the coated sample with a Hf/Ta ratio of 8:2 was increased by 0.84%, showing superior oxidation resistance and high temperature stability. This excellent cyclic oxidation protection performance was mainly ascribed to the formed Hf–Ta–Si–O multiphase oxide layer consisting of HfO2, Ta2O5, Hf6Ta2O17, HfSiO4, and SiO2, which possessed a low oxygen diffusion rate and appreciable high temperature stability, thus effectively preventing the invasion of oxygen. After thermocyclic ablation at 2300 °C for 1080 s (180 s × 6 cycles), the mass and line ablation rates of the coated sample were −6.02 × 10−3 mg/s (slight mass gain) and 0.189 μm/s, respectively. The high melting point oxides formed on the coating surface: HfO2 and Hf6Ta2O17 played roles in preventing mechanical scouring, and SiO2 and Ta2O5 with fluidity at high temperature filled some defects comprising pores and cracks, thus, enabling the coating to provide terrific cyclic protection.