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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]

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]

• 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]

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 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 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]

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]

• Highly efficient porous HEA coatings is fabricated through the plasma spraying, which could be favorable to develop highly efficient and stable non-noble metal electrocatalysts. • CoCrFeNiMo catalyst shows a low overpotential of 273 mV for OER at 100 mA cm−2 in 1.0 M KOH electrolyte. • DFT result shows that CoCrFeNiMo can enhance the transformation of *O to *OOH and improve the OER performance. Developing highly efficient and stable non-noble metal electrocatalysts has always been a grand challenge for water splitting. Herein, the promising and highly efficient porous high entropy alloy (HEA) coatings is fabricated through the plasma spraying, which could be favorable to solve the above issues. Abased on the synergistic effect of different metals and the porous structure within the HEA coatings, it can provide the superior catalytic activities. Hence, the CoCrFeNiMo catalyst displays a low overpotential of 273 mV (for oxygen evolution reaction, OER) at 100 mA cm−2 in 1.0 M KOH electrolyte. As hydrogen evolution reaction (HER) catalysts, the CoCrFeNiMo presents a small overpotential. When the catalysts can be utilized to the overall water splitting, it delivers an excellent durable for 24 h. DFT result shows that CoCrFeNiMo is conducive to the hydrogen adsorption/desorption process, and it can enhance the transformation of *O to *OOH and improve the OER performance. This study not only offers a universal method to prepare high performance electrocatalysts, but also demonstrates the great potential of HEA coatings in energy conversion fields. [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]

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]

HfC coatings with different microstructure were successfully fabricated by chemical vapor deposition. The mechanical properties and ablation resistance of HfC coatings were evaluated, and the coefficient of thermal expansion (CTE) and ablation behavior of the as-prepared coatings were investigated in detail. Results showed that HfC coating with the dense coarse columnar structure exhibits good mechanical properties, and the equiaxed structure has a low CTE. The fine columnar structure coating has better ablation resistance compared with other microstructures, and the mass ablation rate and linear rate are only 2.02 mg/s and 1.25 µm/s after ablation for 90 s, respectively. • HfC coatings with different microstructure were successfully fabricated on C/C composites by CVD. • The effects of different microstructures on the bending strength, hardness, thermal expansion coefficient, and ablation behavior of HfC coatings have been investigated. • HfC coating with fine columnar structure coating had better ablation resistance compared with other structures. [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]

A calcium phosphate (Ca–P) coating is prepared on carbon/carbon (C/C) substrate by supersonic atmospheric plasma sprayed (SAPS) method. After that, microwave-hydrothermal (MH) method is used as the postprocessing method to lower surface defects for sprayed coating as well as obtaining uniform decomposition phase. The aim of this work is an extensive study on the preferred growth of hydroxyapatite (HA) crystals existing in the coating after MH treatment which can rapidly form in-situ growth structures to repair surface defects. The preferred orientation of these generated HA crystals is discussed. Chemical components, length-diameter ratio, particle diameter distribution, microstructure, formula derivation, thermal properties and protein adsorption of the HA crystals prepared by the MH method on sprayed coating are also studied. As a result, it is found that the MH heating time on the preferred orientation of the HA crystal appeared on sprayed coating plays an important role and the optimal crystal growth time during the MH process is 35 min. The coating after MH treatment for 250 min has the most excellent protein adsorption ability of 553 μg/disc. [ABSTRACT FROM AUTHOR]