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To improve the oxidation resistance of carbon/carbon (C/C) composites in 1473–1973 K, a HfSi 2 -HfB 2 -SiC coating was prepared on the surface of SiC-coated C/C composites by a combined method of slurry dipping and oxidation sintering at low temperature (below 1473 K). The Hf-Si-O multiphase layer generated by the oxidation of the coating acted as oxygen barriers at 1473–1973 K in air. The in-situ formed HfSiO 4 particles enhanced the thermal stability of the SiO 2 glass, terminated propagation of the crack, and relieved CTE mismatch between the inner and outer coating. Besides, B 2 O 3 filled pores in the early stage of oxidation and then SiO 2 took the place of the B 2 O 3 as a new filler with the increase time of oxidation. As a result, the coating can protect C/C composites for 516 h at 1473 K, 744 h at 1773 K, and 64 h at 1973 K with a mass gain of 0.63 wt%, 1.41 wt% and −1.91 wt%, respectively. [ABSTRACT FROM AUTHOR]

To improve the oxidation resistance of carbon/carbon (C/C) composites in a wide temperature range (1173–1773 K), a composite coating containing rich B 2 O 3 glass was prepared on SiC-coated C/C composites by slurry dipping-densifying at low temperature. Borosilicate and SiO 2 glasses acted as oxygen barriers at low and medium-high temperatures, respectively. Besides, Hf-oxides (HfO 2 , HfSiO 4) ceramic particles improved the thermal stability of the glass and enhanced the crack resistance of glass layer. Therefore, the composite coating can effectively protect C/C composites against oxidation for 403 h at 1173 K, 723 h at 1473 K and 403 h at 1773 K with the mass gain of 3.77 g·m−2, 21.41 g·m−2 and 0.42 g·m−2, respectively. After 50 times thermal cycles between room temperature and 1773 K, the mass gain of the coated sample was 3.95 g·m−2 and the mass retention rate was up to 98.19 % during the thermos-gravimetric test from room temperature to 1773 K. [ABSTRACT FROM AUTHOR]

HfB 2 -SiC coatings with 30–60 wt% HfB 2 additions were prepared by slurry-dipping on SiC-coated C/C composites for investigating their oxidation behavior at low (1173 K) and high (1773 K) temperatures. With the increase of HfB 2 content, more B 2 O 3 glass was produced in the coating and the coating became denser during oxidation at 1173 K. 50 wt% HfB 2 -SiC coating can protect C/C composites for 494 h against oxidation, which was attributed to the formation of the dense borosilicate glass with high thermal stability as an oxygen barrier layer. After oxidation at 1773 K, 30 wt% HfB 2 -SiC coating provided the longest protection time (460 h) for C/C composites among 30–60 wt% HfB 2 -SiC coatings. A few Hf-oxides inhibited the volatilization of SiO 2 glass as the main phase in the 30 wt% HfB 2 -SiC coating after oxidation. As for other coatings with higher content of HfB 2 , the violent oxidation of HfB 2 particles made more bubbles produced, resulting in the poor oxidation resistance at 1773 K. [ABSTRACT FROM AUTHOR]