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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]

• SiO 2 gradient distribution in the gradient composite coating reduces CTE mismatch between the coating and SiC coating to inhibit cracking. • The outer dense layer and the inner microporous layer in the gradient composite coating play a synergistic antioxidant role. • The coating can protect C/C composites against oxidation at mid and high temperature for long-life service. To improve the oxidation resistance of carbon/carbon (C/C) composites at mid and high temperature, a gradient composite coating was designed and prepared on SiC-coated C/C composites by in situ formed-SiO 2 densifying the porous SiC-ZrSi 2 pre-coating. SiO 2 gradient distribution was conducive to inhibiting the cracking of the coating. A dual-layer structure with the outer dense layer and the inner microporous layer was formed in the coating during densifying. The dense layer had excellent oxygen diffusion resistance and the microporous layer alleviated CTE mismatch between SiC inner coating and dense layer. Moreover, ZrSiO 4 particles inhibited crack propagation and stabilized SiO 2 glass. Therefore, the coating can protect the C/C composites from oxidation at 1473 K, 1573 K and 1773 K for 810 h, 815 h and 901 h, respectively. The coated samples underwent 30 thermal cycles between room temperature and 1773 K without mass loss, exhibiting good thermal shock resistance. [ABSTRACT FROM AUTHOR]

• More SiO 2 can be produced in the porous coating than the dense coating. • The SiO 2 -SiC inlay structure are formed during the oxidation process. • The SiO 2 -SiC inlay structure can inhibit the cracking of SiO 2 glass. • The crack propagation is prevented by the micro-pores of the coating. To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperatures, a SiC-Si coating with micro-pores was prepared by slurry and heat-treatment on the surface of C/C composites with SiC-Si inner coating acquired by pack cementation (PC). The microstructure, phase composition, element distribution, and anti-oxidation properties of the dual-layer SiC-Si coating were investigated. The results show that a SiO 2 -SiC inlay structure was formed during the oxidation process, due to a large amount of SiO 2 rapidly generated by the oxidation of SiC particles in the porous coating. The coating with this structure could inhibit the cracking of SiO 2 glass and had a good resistance to oxygen diffusion. Moreover, the crack propagation was blocked by the remaining micro-pores of the coating. The coating could protect C/C composites against oxidation for 846 h only with the mass loss of 0.16 % at 1773 K in air. [ABSTRACT FROM AUTHOR]

A HfB 2 -SiC skeleton was designed on SiC-coated C/C composites using hollow SiO 2 microspheres as sacrificial templates, which was densified by SiC via chemical vapor infiltration to fabricate a dense SiC-HfB 2 oxidation protective coating. In this way, the uniform distribution of HfB 2 in the coating was realized. Cracks in the SiC inner coating were covered by the SiC-HfB 2 coating; Hf-oxides improved the stability and inhibited the volatilization of SiO 2 in the compound glass layer. Therefore, the coating can protect C/C composites against oxidation for 59 h at 1473 K, 503 h at 1773 K, and 166 h at 1973 K. • A HfB 2 -SiC skeleton is prepared on SiC-coated C/C composites using hollow SiO 2 microspheres as sacrificial templates. • A dense SiC-HfB 2 coating with uniform distribution of HfB 2 is fabricated by SiC densifying HfB 2 -SiC skeleton at low temperature. • Hf-oxides improve the high-temperature stability of SiO 2 glass for reducing its volatilization. • The SiC-HfB 2 coating can protect C/C composites against oxidation at 1473, 1773, and 1973 K. [ABSTRACT FROM AUTHOR]

A dense SiC/HfB 2 -based ceramic/SiC multilayer coating was designed on C/C composites by a combined technique of pack cementation, slurry brushing-pressureless reaction sintering and chemical vapor deposition. The coating can protect C/C composites against oxidation at 1473 K for 544 h, 1573 K for 597 h and 1773 K for 382 h. Such good performance was ascribed to the synergistic oxidation resistance of three layers. The cracks were healed by the borosilicate glass generated from the oxidation of HfB 2 -based ceramic intermediate layer at medium temperatures, and covered by SiO 2 glass produced from the oxidation of SiC outer layer at high temperatures. • The multilayer coating composed of HfB 2 -based ceramic internmate layer and two SiC layers is designed by a combined technique. • The dense HfB 2 -based ceramic internmate layer is prepared by introducing MoSi 2 and resin carbon as reaction sintering additives. • The cracks are healed by the borosilicate glass at 1473 K and SiO 2 glass at 1773 K during oxidaiton, respectively. • The coating can protect C/C composites against oxidation at medium and high temperatures for long-life service. [ABSTRACT FROM AUTHOR]

A compound glass coating with micro-pores, derived from the pre-oxidation of the porous HfSi 2 -SiC-Si coating, was fabricated on SiC-coated C/C composites. The coating resisted cracking by introducing micro-pores and compressive stress, which blocked oxygen diffusion and covered penetrable cracks in the SiC inner coating. A dual-layer structure for synergistic anti-oxidation was formed during oxidation at 1773 K and uniformly distributed Hf-oxides reduced the volatilization of SiO 2 at 1973 K in the compound glass coating. Therefore, the coating could protect C/C composites against oxidation for 447 h at 1773 K and 45 h at 1973 K. • The micro-pores and the compressive stress are conducive to inhibiting cracking of the compound glass coating. • Hf-oxides particles are evenly distributed in the compound glass coating. • A dual-layer structure for synergistic anti-oxidation is formed in the compound glass coating during oxidation at 1773 K. • Hf-oxides reduce the volatilization of SiO 2 in the compound glass coating during oxidation at 1973 K. [ABSTRACT FROM AUTHOR]

To improve the oxidation resistance of C/C composites for long-life service at high temperatures, a SiC/SiC-ZrSi 2 coating with micro-pore was prepared by pack cementation and slurry, respectively. The coating could protect C/C composites for 1194 h at 1773 K in air with a mass gain of 1.42 wt.%. Such good antioxidant property was ascribed to the SiO 2 -ceramic inlay structure with the ability of healing and covering the penetrating crack formed during oxidation, which had excellent oxygen diffusion resistance. Besides, the thermal stress was reduced by the micropores and the crack propagation was inhibited by ZrSiO 4 particles. • The SiO 2 -ceramic inlay structure is formed due to abundant SiO 2 generated in the porous ceramic coating during oxidation. • The SiO 2 -ceramic inlay structure can prevent oxygen diffusion and heal the penetrating crack in the inner coating. • The micropores can alleviate the thermal mismatch between the inner and outer coating. • ZrSiO 4 particles of the glass layer can inhibit the crack propagation. [ABSTRACT FROM AUTHOR]

• SiC-HfB 2 -Si ternary coating was made via slurry panting plus gaseous Si infiltration. • The as-prepared coating exhibited obvious dense and multi-phase mosaic structure. • The coating prevented C/C substrate against oxidation at 1773 K for more than 1507 h. • The linear, mass erosion rates up to 2392 K for 60 s were -0.72 μm/s, 0.07 mg/s. An in-situ SiC-HfB 2 -Si ternary coating was deposited on C/C composites (C/Cs) via slurry panting plus gaseous Si infiltration composite method, to improve the oxidation and ablation resistance of C/Cs above 1773 K. The coating formation mechanism was investigated by microstructural analyses and thermo-dynamic calculations. The oxidation behavior of the coated specimens subjected either to high-temperature testing at 1773 K and 1973 K in static air furnace or to ablation testing with oxyacetylene torch upon ultra-high temperature service were studied, base on thermo-dynamic computations, numerical simulations and microstructure evolution. The SiC-HfB 2 -Si coating protected C/Cs against oxidation at 1773 K for more than 1507 h which is longer than that of the reported SiC-HfB 2 -based coatings, due to the as-prepared compact mosaic coating filled with HfB 2 -rich Si-based multiphase and the consequently formed dense Hf-Si-O oxide layer. Moreover, a good ablation resistance with relatively low linear and mass ablation rates of −0.72 μm/s and 0.07 mg/s, respectively, was achieved due to the stable oxide scale with high viscosity. [ABSTRACT FROM AUTHOR]

A rigorous understanding of the structural evolution of oxidation-resistant coatings in service can pave the way for the development of coatings for carbon/carbon composites, and therefore a typical HfB 2 -SiC/SiC coating was investigated here. The initial oxidation of the coating resulted in a SiO 2 /HfO 2 scale, and the subsequent structural evolution was dominated by the growth of HfSiO 4 , forming a SiO 2 /HfSiO 4 /HfO 2 protective layer that was stable at 1700 °C with long-term exposure to air. Ultimately, the SiC transition layer gradually degraded due to the active oxidation of SiC. Based on these findings, feasible strategies to improve coating longevity are proposed. • Effective HfB 2 -SiC/SiC coating was fabricated by a readily implementable method. • The coating can protect C/C composites in air at 1700 °C for 100 h. • The structure evolution of the coating in service was investigated in-depth. • Feasible strategies for further improving the coating longevity were proposed. [ABSTRACT FROM AUTHOR]

Traditional HfC, ZrC, and HfC-ZrC coatings cannot resist long-term ablation above 2100 °C because their oxides (HfO 2 and ZrO 2) express a loose structure and poor anti-oxygen infiltration ability after ablation. While Ti-rich oxides with a lower melting point can seal this loose structure during ablation. In this work, a HfC-ZrC-TiC multi-phases coating was proposed to solve the ablation failure problem of these coatings. It was prepared by supersonic atmospheric plasma spraying and tested by ablation with a heat flux of 2.38 MW/m2. With time ranging from 30 s to 120 s, the mass and linear ablation rates decreased from 0.85 mg/s to 0.18 mg/s and from 2.58 µm/s to 0.71 µm/s, respectively. All positive values indicated an increase in all coating thicknesses, so this coating can effectively protect C/C composites for more than 120 s. The surface of the coating was mainly composed of a loose oxide m-(Hf, Zr, Ti)O 2 skeleton within 120 s. As oxide skeletons had been gradually destroyed by mechanical denudation, the liquid phase of (Hf, Zr)TiO 4 was formed under oxide skeletons to fill pores. Then the surface of the oxide layer was dense after 120 s. The self-healing ability of the HfC-ZrC-TiC coating improved the ablation resistance of C/C composites above 2100 °C. • The HfC-ZrC-TiC coating can effectively protect C/C composites for more than 120 s during ablation above 2100 °C. • A loose oxide m-(Hf, Zr, Ti)O 2 skeleton was appeared and destroyed by mechanical denudation within 120 s. • The sealing role of Ti-rich phase ((Hf, Zr)TiO 4) can fill pores and cracks, so the coating was dense after 120 s. [ABSTRACT FROM AUTHOR]

To improve the ablation resistance of HfC coating, HfC-MoSi 2 coatings with different MoSi 2 content were prepared on SiC-coated carbon/carbon (C/C) composites by supersonic atmospheric plasma spraying (SAPS). The effects of MoSi 2 content on the microstructure, phase composition and ablation behavior of HfC-MoSi 2 coating were investigated. The ablation resistance of as-prepared coatings was tested under the oxyacetylene flame with a heat flux of 2.4 MW/m2. The results showed that the HfC-(25 vol%) MoSi 2 coating exhibited better ablation resistance, and its mass and linear ablation rate are 0.23 mg/s and 0.48 µm/s, respectively. Such good performance was ascribed to the formation of Hf-Si-O compound phases, which effectively filled the diffusion channel of oxygen and improved the compactness of the coating. • In present study, the ablation resistance and behavior of the HfC-MoSi 2 multiphase coatings were researched in detail. • Compared other HfC multiphase coatings, MoSi 2 was helpful to form Hf-Si-O phases, inhibiting the permeation of oxidizing gas. • The ablation behavior of HfC-MoSi 2 coatings was analyzed and HfC-25vol%MoSi 2 coating showed the best ablation resistance. [ABSTRACT FROM AUTHOR]