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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]

To solve the problem of thermal protection posed by the long working time of Mg/CO 2 powder rocket engines, an experimental system is designed to study the heat-transfer characteristics of liquid CO 2 in the regenerative cooling channel. How the heat flux, coolant temperature, mass flux, and back pressure influence the heat-transfer coefficient, the thermodynamic quality, and the gas-phase formation point of nearly saturated CO 2 is studied. The heat-transfer coefficient is found to increase with increasing mass flux or decreasing inlet temperature. Higher heat flux increases the rate of bubble formation and promotes nucleate-boiling heat transfer, but it also intensifies the film boiling, thereby degrading the heat-transfer performance. Lower back pressure does not affect the heat-transfer performance in the nucleate-boiling region, but it worsens it in the film-boiling region. It is also found that the classical boiling-heat-transfer models do not predict the experimental heat-transfer coefficient well. Combined with experimental data, how reduced pressure affects heat transfer is considered, and a new empirical correlation formula is proposed for the heat-transfer coefficient of CO 2 near saturation. The optimized model can successfully predict 90.23% of the experimental data, and the prediction accuracy is improved greatly. The present research provides a theoretical basis for designing a regenerative cooling scheme for an Mg/CO 2 powder rocket engine. • Liquid CO 2 is used as coolant for the first time to conduct heat transfer experiments in a regenerative cooling channel. • A method for calculating the heat transfer coefficient along the microchannel is established. • The effects of different heat fluxes, coolant temperatures, mass fluxes and back pressures on the heat transfer performance of CO 2 are obtained. • An empirical correlation for heat transfer of CO 2 boiling flow in a regenerative cooling channel at near saturation state is obtained. [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]