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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]

Obtaining coatings with excellent ablation resistance is a longstanding challenge for the ablation protection of carbon/carbon (C/C) composites. In this work, a single phase Hf 0.75 Zr 0.25 N solid solution ultra-high temperature ceramic (UHTC) coating was proposed, and prepared on the surface of C/C composites by chemical vapor deposition (CVD) for the first time. The results revealed that the hardness and Young's modulus of Hf 0.75 Zr 0.25 N coating are higher than those of HfN and ZrN coatings, reaching 33.35 GPa and 316.42 GPa, respectively. After ablation for 180 s under oxygen acetylene flame with a temperature exceeding 2200 °C, the Hf 0.75 Zr 0.25 N coating retained its structural integrity and exhibited lower ablation (R m : 0.26 mg/s; R h : 0.32 μm/s). This is mainly attributed to the formation of the dense Hf-Zr-O oxide scale promoted by the oxidation sintering reaction during ablation. The above results indicate the nitride solid solution UHTC has great potential as ablation resistance coating at ultra-high temperatures. [ABSTRACT FROM AUTHOR]

A thermodynamic calculation on the HfB 2 coating prepared by chemical vapor deposition (CVD) through HfCl 4 -BCl 3 -H 2 -Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB 2 coating could be obtained above 900 °C with the ratios of BCl 3 /HfCl 4 and H 2 /HfCl 4 higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H 2 and BCl 3 flow rates had significant effects on the grain size, growth rate and phase composition of HfB 2 coatings. A dense and uniform HfB 2 coating was prepared at 1150 °C with a BCl 3 /HfCl 4 ratio of 3 and a H 2 /HfCl 4 ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 μm/s under oxyacetylene flame. [ABSTRACT FROM AUTHOR]

The design of a new multilayer anti-ablation coating is inseparable from the performance assessment in multiple environments. This work investigates the effects of different ablation flux on the microstructure evolution and phase composition of multilayer TaC/HfC coating. The results show that the different temperature regions were generated on ablated coating surface, resulting in the formation of different oxidation products. At heat flux of 2.4 MW/m2, the formation of dense Hf-Ta-O solid solution phase tightly adhered on substrate. As heat flux increased to 4.2 MW/m2, a large amount of molten oxidation phase was produced in the central area, which caused secondary damage to the coating under the action of high-speed flame. • Different temperature regions were formed on the coating surface under different heat flux. • The generation of a large amount of molten oxide phase caused secondary damage to the coating. • The ablation process of the TaC/HfC multi-layer coating can be divided into two stages. [ABSTRACT FROM AUTHOR]

The cyclic ablation resistance of coated carbon/carbon (C/C) composites play crucial roles in their further engineering applications and development due to the cyclic ablation environment accompanied by rapid heating and cooling and high-speed heat flow scouring, which can reflect the performance stability of the coating. In this research, a (SiC/HfC) 4 /SiC (SHS) multi-layer coating was prepared on C/C composites. Compared with single layer (SiC and HfC coating) coated sample, the mass and linear ablation rate of SHS coated sample after three ablation cycles (60 s × 3) were only 0.64 mg/s and 0.53 μm/s, respectively. This is mainly because the introduction of many interfaces inhibits the propagation of cracks, the irregular cracks region only exists in the outer layer. Besides, the oxide layer with dense structure was formed near the C/C substrate, which could prevent oxygen from penetrating into the coating and continue to play a protective role. [ABSTRACT FROM AUTHOR]

HfC-SiC coatings have promising applications in the field of ablation protection for carbon-carbon (C/C) composites. In this work, HfC-SiC coatings with different ratios were designed and successfully prepared by low pressure chemical vapor co-deposition using HfCl 4 -MTS-CH 4 -H 2 -Ar system under the guidance of thermodynamic calculations. The phase composition, microstructure, and ablation behavior of HfC-SiC coating with different phase proportions were investigated. The coating with HfC/SiC ratio of 1:1 showed the best ablation resistance after ablation for 90 s, and its mass and linear ablation rates of HfC-SiC coating was only 1.016 ± 0.12 mg/s and 0.998 ± 0.08 µm/s, respectively. [Display omitted] • HfC-SiC coatings were co-deposited on C/C composites by low pressure chemical vapor co-deposition using HfCl 4 -MTS-CH 4 -H 2 -Ar system. • The influences of process parameters on coating phase composition and content were investigated under the guidance of thermodynamic calculations. • The coating with the HfC/SiC ratio of 1:1 exhibited excellent ablation resistance after ablation for 90 s. [ABSTRACT FROM AUTHOR]

HfC coatings were deposited on the surface of carbon/carbon (C/C) composites and CVD-SiC coating by low-pressure chemical vapor deposition (CVD), respectively. The effects of two substrates on the microstructure, phase composition and preferred orientation of HfC coatings were investigated in detail. Results showed that HfC coatings deposited on two substrates inherit their own substrate surface characteristics. After ablation for 90 s, the HfC coating prepared on CVD-SiC coating has better ablation resistance under oxyacetylene flame with heat flux of 2.4 MW/m2, because the transition layer can effectively relieve the damage caused by thermal stress. • HfC coating was successfully deposited on the surface of CVD-SiC and C/C composite. • The influence of two substrates on the growth mechanism of CVD-HfC coatings was systematically discussed. • The ablation resistance of CVD-HfC coating on the surface of C/C composites and CVD-SiC coating was tested. [ABSTRACT FROM AUTHOR]

• In present study, a detailed investigation on the ablation behavior of the layered structure by computational and experimental methods. • The multilayer alternating structure HS coating performed better ablation resistance compared with single layer SiC and HfC coating. • The multi-layer structure can effectively inhibit the propagation of cracks in the coating during ablation. To improve the service life of carbon/carbon (C/C) composites under ultra-high temperature ablation environments (> 2200 ℃), SiC/HfC coating with multi-layer alternating structure was designed and prepared on C/C composites by low-pressure chemical vapor deposition (LPCVD). The microstructure and phase compositions of the as-prepared and ablated coatings were investigated, and the influence of the multi-layer structure on the ablation behavior and thermal stress distribution of the coatings were analyzed as well. After ablation for 120 s, the (SiC/HfC) 3 coating has better ablation resistance due to its multi-layer structure and the formation of Hf-Si-O sticky glass phase. [ABSTRACT FROM AUTHOR]

HfB 2 -MoSi 2 /SiC-Si coating was fabricated via pack cementation (PC) combined with slurry painting (SP) followed by heat treatment, where MoSi 2 was in-situ synthesized from Mo and Si. The microstructure evolution and high-temperature oxidation resistance at 1773 K and 1973 K of the HfB 2 -MoSi 2 /SiC-Si coated specimens were comparatively studied. The protection mechanism of the coating was investigated, based on the structure and thermodynamic transformation of the oxide scale. The results revealed that the HfB 2 -MoSi 2 /SiC-Si coating possessed better oxidation resistance at 1973 K than that at 1773 K, mainly due to the changed properties of the multiphase Hf-Si-O oxide scale. At 1973 K, the formed Hf-Si-O oxide scale has low oxygen permeability, superior high-temperature stability and self-healing ability. While the Hf-Si-O oxide scale formed at 1773 K is loose, which caused oxygen penetrating into the inner layer and C/C substrate, finally resulting in a high mass loss. This study can provide valuable information and induce facile method to develop high-temperature environmental barrier coatings for C/C composites. • HfB 2 -MoSi 2 /SiC-Si coating was made by PC, SP and in-situ synthesis of MoSi 2. • Varying Hf-Si-O film led to the changed oxidation behaviors at 1773 K and 1973 K. • Hf-Si-O with self-healing ability formed at 1973 K caused good oxidation resistance. [ABSTRACT FROM AUTHOR]