| In order to improve the ablation resistance performance of carbon/carbon (C/C) composites and meet the higher requirements for nozzle materials of the new type solid rocket motor, HfC coating, multilayerd-structure HfC coating and Hf(Ta)C coating with outstanding anti-ablation properties were successfully prepared on C/C composites by chemical vapor deposition (CVD).A novel powder carrier was designed and developed to solve the technical problem of HfCl4transport. The accurate control of HfCl4flow at low pressure condition provides technical support for the structural control of HfC coating.The influences of deposition process on the microstructure of HfC coating were investigated systematically, the basic laws of the structural formation of HfC coating are explored and the controlled formation of coating structure is achieved. HfC coatings with typical structures were prepared, the type1with equiaxed-crystal structure, the type T with needle-crystal structure and the type2with columnar-ctystal structure.Based on the deposition rates and homogeneities of HfC coating, the relatedness between homogeneity of HfC coating and the controlled mechanisms of deposition process were established. The deposition rate increases with the increased temperature, C/Hf ratio and concentration of H2. With the decline of deposition interval, the deposition rate decreases due to the effects of the loss of reactive gases and the diffusion barrier of boundary layer. HfC coating has favorable homogeneity with the deposition process controlling by surface reaction, while has lower homogeneity with the deposition process controlling by diffusion process.The mechanical properties of HfC coating were investigated by nanoindentor. The relatedness between mechanical properties and coating structure were established and the influence mechanisms of the structure on the mechanical properties were explored. The reduced capacity of load resulting from the pores is the main reason for the lower mechanical properties of HfC1coating. The increased mechanical properties of HfC T and HfC2coatings are mainly attributed to the dense crystal-arranged structures.The ablation properties were investigated by oxyacetylene torch. The structural evolution of HfC coating during ablation process was investigated. The ablation process and the controlled mechanism of HfC coating were discussed, the ablation mechanism of HfC coating at ultral high temperature was proposed. Among the HfC coatings with single structure, the HfC1coating has the favorable ablation properties with mass ablation rates of-0.05mg·cm-2·s-1, while part of the HfC T and2coatings cracks and peels during ablation. Moreover, the multilayerd HfC coating exhibits outstanding ablation resistance. The HfC coating can protect the C/C composites by sacrificial oxidation, the ablation rate of HfC coating depends on the controlled mechanism of oxidation. The oxidation mechanism changes with formation of porous HfO2, HfCxOy and molten HfO2, the ablation rate of HfC coating decreases gradually.Hf(Ta)C and Hf(Zr)C coatings were co-deposited on C/C composites by CVD. The phase composition, microstructure and ablation properties of these coatings were investigated. The Hf(Ta)C coating which formed with equiaxed-crystal structure, is composed of HfC and HfTaC2-Compared with HfC coating, the formation of HfTaC2is reasonable to the formation of dense oxide layer during ablation and the thermal shock resistance of Hf(Ta)C coating increases due to the formation of stabled Hf6Ta2O17. The Hf(Zr)C coating is composed of cubic HfC and ZrC. The density of Hf(Zr)C coating increases with the reduced concentration of CH4. The Hf(Zr)C coating with equiaxed-crystal structure has the favorable thermal shock resistance during ablation.In a word, HfC coating, multilayerd HfC coating and Hf(Ta)C coating were prepared on C/C composites with excellent anti-ablation performance, which is benefical to the further research of ablation protection for high performance C/C nozzle materials. |
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