Mechanical properties and ablation resistance of HfC nanowire modified carbon/carbon composites

Hafnium carbide nanowires (HfCnws) were in-situ grown in carbon/carbon (C/C) composites, and subsquently the preforms were densified by isothermal chemical vapor infiltration to obtain HfCnws modified carbon/carbon (HfCnws-C/C) composites. Morphology and microstructure of HfCnws were examined, and the effect of HfCnws on the mechanical property and ablation resistance of C/C composites were also investigated. Results show that introducing HfCnws refined the grain size of pyrolytic carbon (PyC). The out-of-plane compression, interlaminar shear and flexual strength of HfCnws-C/C composites increased by 120.80%, 45.60% and 94.65%, respectively compared with pure C/C, and the HfCnws-C/C shows good ablation resistance under oxy-acetylene flame ablation.     

Integrative improvement on thermophysical properties and ablation resistance of laminated carbon/carbon composites modified by in situ grown HfC nanowires onto carbon fiber cloths

HfC nanowires modified carbon fiber cloth laminated carbon/carbon (HfC_nw-C/C) composites were fabricated by in situ growth of HfC nanowires on carbon cloths via catalytic CVD, followed with lamination of the cloths and densification by pyrolytic carbon (PyC). Morphologies, thermal conductivity, coefficient of thermal expansion (CTE), and ablation resistance of the composites were investigated. Due to the loading of HfC nanowires, the matrix PyC with low texture was obtained; the thermal conductivity of the composites in the Z direction was enhanced from 100℃ to 2500℃; CTE along the X–Y direction also decreased in the range of 2060 ℃ – 2500 ℃, which reaches the maximum of 24 % at 2500℃. Moreover, the 20s-ablation-resistance of HfC_nw-C/C composites exhibits mass and linear ablation rates of 5.3 mg/s and 21.0 μm/s, which are 40 % and 37 % lower than those of pure C/C composites, respectively. Our work shows laminated HfC_nw-C/C composites are a promising candidate for high-temperature applications.     

Microstructures and enhanced flexural properties of single-crystalline HfC nanowires in-situ modified carbon/carbon composites by electrophoresis-thermal evaporation using CNTs as the template

To improve the mechanical properties of carbon/carbon (C/C) composites, in-situ synthetized single-crystalline hafnium carbide nanowires (HfCnws) were introduced into the carbon fiber preforms by electrophoresis-thermal evaporation method. The Multi-walled carbon nanotubes (MWCNTs) were utilized as the carbon source and templates for forming HfCnws. The microstructure, chemical composition and mechanical properties of the HfCnws modified carbon/carbon (HfCnws-C/C) composites were characterized. Results reveal that HfC is produced preferentially in the inner nodular parts and end parts of MWCNTs. The raising heat-treatment temperature would influence the diffusion rate of Hf atoms and then the number of nucleation sites, which further changed the aspect ratio and morphology of HfCnws. The HfCnws have refined the grain size of pyrolytic carbon (PyC), and significantly improve the flexural strength of C/C composites by 79.3%.     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Effects of HfC nanowire amount on the microstructure and ablation resistance of CVD-HfC coating

To improve the ablation resistance of HfC coating for carbon/carbon (C/C) composites, various fractions of HfC nanowires were incorporated into the HfC coating by chemical vapor deposition (CVD). Effects of HfC nanowire amount on the microstructure and ablation resistance of the CVD-HfC coating were investigated. Results indicated that the HfC nanowire layer became thicker and denser with the deposition time extending. HfC nanowires could inhibit the formation of cracks and interlaminar gaps in the HfC coating. With the increase of HfC nanowire amount, the HfC coating became thicker, while its porosity and roughness firstly decreased and then increased. Ablation tests indicated that the incorporation of HfC nanowires could effectively improve the ablation resistance of the HfC coating, which could be ascribed to the decreasing surface temperature of the coated samples and the effective alleviation of cracking and delamination of the coating during ablation. The HfC coating with HfC nanowires deposited for 1?h exhibited better ablation resistance owing to its compact microstructure, and its mass and linear ablation rates were only 0.41?mg/s and ??1.53?µm/s after ablation for 120?s.     

Effects of deposition temperature and time on HfC nanowires synthesized by CVD on SiC-coated C/C composites

HfC nanowires were synthesized on SiC-coated carbon/carbon composites via a catalyst-assisted chemical vapor deposition (CVD) process from the HfCl₄–CH₄–H₂ system. The effects of deposition temperature (1273, 1323, 1373 and 1423 K) and time (20, 40, 60 and 90 min) on the formation and microstructure of HfC nanowires were investigated. The results showed that the diameter of HfC nanowires increased with the deposition temperature increasing; both the density and thickness of HfC nanowire films increased with the deposition time prolonging. The growth of HfC nanowires followed the bottom-type vapor–liquid–solid (VLS) mechanism.     

HfC nanowires to improve the toughness and oxidation resistance of Si-Mo-Cr/SiC coating for C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites, a dense HfC nanowire-toughened Si-Mo-Cr/SiC multilayer coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, thermal shock and isothermal oxidation resistance of the coating were investigated. HfC nanowires could improve the toughness of the coating and suppress the coating cracking. After incorporating HfC nanowires in the coating, both of the thermal shock and isothermal oxidation resistance of the coating were obviously improved. The multilayer coating with HfC nanowires could effectively protect C/C composites at 1773 K for 270 h, whose weight loss is only 0.19%. The good oxidation resistance is mainly attributed to the formation of a compound glass layer containing SiO₂ and Cr₂O₃.     

Awl-like HfC nanowires grown on carbon cloth via Fe-catalyzed in a polymer pyrolysis route

Awl-like hafnium carbide (HfC) nanowires were successfully synthesized on carbon cloth in a polymer pyrolysis route via Fe-catalyzed for the first time. A detailed study of the HfC nanowire morphology was performed. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to investigate the morphology and microstructure of the obtained awl-like HfC nanowires and the growth mechanism was also discussed in detail. Results show that HfC nanowires wrapped by amorphous HfO₂ layer with ~10 nm in thickness display awl-like structure and the nanowire's surface exhibit prismatic geometry. The diameter of the nanowire decreases from the bottom (~500 nm) to the top (~100 nm). The growth mechanism of the awl-like HfC nanowires was mainly affected by the combination of bottom-type vapor-liquid-solid (VLS) and solid-liquid-solid.     

Mechanical and thermophysical properties of carbon/carbon composites with hafnium carbide

Carbon/carbon (C/C) composites with addition of hafnium carbide (HfC) were prepared by immersing the carbon felt in a hafnium oxychloride aqueous solution, followed by densification and graphitization. Mechanical properties, coefficients of thermal expansion (CTE), and thermal conductivity of the composites were investigated. Results show that mechanical properties of the composites decrease dramatically when the HfC content is greater than 6.5 wt%. CTE of the composites increases with the increase of HfC contents. The composites with addition of 6.5 wt% HfC show the highest thermal conductivity. The high thermal conductivity results from the thermal motion of CO in the gaps and pores, which can improve phonon–defect interaction of the C/C composites. Thermal conductivities of the composites decrease when the HfC content is greater than 6.5 wt%, which is due to formation of a large number of cracks in the composites. Cracks increase the phonon scattering and hence restrain heat transport, which results in the decrease of thermal conductivity of the composites.     

Influence of graphitization temperature on microstructure and mechanical property of C/C-SiC composites with highly textured pyrolytic carbon

C/C-SiC composites with highly textured pyrolytic carbon (HT PyC) were prepared by a combining chemical vapor infiltration and liquid silicon infiltration. The effect of HT PyC graphitization before and after 2327 and 2723 K on C/C-SiC composites was investigated. The mechanical properties decreased with increasing graphitization temperature, but graphitization treatment changed the fracture behavior from brittle like to pseudo-ductile. The decrease in bending strength from 306.21 to 243.69 MPa resulted from the weak interfacial bonding between HT PyC and fiber, and the good orientation of graphite layers. The crack at border of fiber bundle and longitudinal crack in HT PyC shortened the path of crack propagation, resulting in fracture toughness decrease from 21.11 to 14.72 MPa·m^1/2. A more pseudo-ductile behavior was due to the longer pull-out of fibers, the better orientation of graphite layers, the sliding of sublayers, and the deflection and propagation caused by the transverse cracks.     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Ablation behavior of HfC coating with different thickness for carbon/carbon composites at ultra-high temperature

The thickness of the different HfC coatings from 20 μm to 50 μm were prepared on the surface of carbon/carbon (C/C) composites by low pressure chemical vapor deposition (LPCVD). The microstructure and thermal stress of the coatings after ablation were investigated, as well as the effect of thickness and thermal stress on the ablation resistance of the HfC coating was analyzed. After being ablated at a heat flux of 2.4 MW/m² for 60 s, the thermal stress gradually increased at first and then rapidly increased with the increasing thickness of coating. The results indicated that the moderate coating thickness can effectively release the thermal stress generated during the ablation process. The 40 μm-thick HfC coating showed the best ablation resistance with the mass ablation rate and line ablation rate were only 0.13 mg/s and 0.09 μm/s, respectively.     

Effect of co-deposited SiC nanowires and carbon nanotubes on oxidation resistance for SiC-coated C/C composites

To protect carbon/carbon composites (C/Cs) against oxidation, SiC coating toughened by SiC nanowires (SiCNWs) and carbon nanotubes (CNTs) hybrid nano-reinforcements was prepared on C/Cs by a two-step technique involving electrophoretic co-deposition and reactive melt infiltration. Co-deposited SiCNWs and CNTs with different shapes including straight-line, fusiform, curved and bamboo dispersed uniformly on the surface of C/Cs forming three-dimensional networks, which efficiently refined the SiC grains and meanwhile suppressed the cracking deflection of the coating during the fabrication process. The presence of SiCNWs and CNTs contributed to the formation of continuous glass layer during oxidation, while toughed the coating by introducing toughing methods such as bridging effect, crack deflection and nanowire pull out. Results showed that after oxidation for 45 h at 1773 K, the weight loss percentage of SiC coated specimen was 1.35%, while the weight gain percentage of the SiCNWs/CNTs reinforced SiC coating was 0.03052% due to the formation of continuous glass layer. After being exposed for 100 h, the weight loss percentage of the SiCNWs/CNTs reinforced SiC coating was 1.08%, which is relatively low.     

Microstructure,thermophysical properties,and ablation resistance of C/HfC-ZrC-SiC composites

Carbon/carbon composites modified by HfC-ZrC-SiC were fabricated by reactive melt infiltration with the aim of improving their ablation resistance for application in aerothermal environments. Their microstructure, thermophysical and ablation properties were investigated. Results show that the thermal diffusivity decreases with increasing temperature for all composites. The thermal conductivity of the C/HfC-ZrC-SiC composites decreasing with increasing HfC molar fraction is related to decreased grain size and increased porosity, which impede phonon interaction and increase the phonon scattering. High HfC content effectively improves the oxidation and ablation resistance of the composites. C/HfC-ZrC-SiC composites containing 8.8?mol.% HfC exhibited the best ablation resistance owing to a compact and continuous HfO₂-ZrO₂ mixed layer that formed on the ablated surface.     

MPCF增强的C/C复合材料及其在高超音速飞行器的应用

介绍了中间相沥青基碳纤维(MPCF)增强的碳/碳(C/C)复合材料在美国的研制及其在高超音速飞行器上应用概况。MPCF非均衡织物结构可实现2D-C/C复合材料单方向更高的热导率,K321 MPCF增强4∶1结构2D-C/C可达到X向室温368 W/(m˙K)的热导率。P30X MPCF 3∶1非均衡织物、转化法Si C及化学气相沉积Si C和HfC 3层涂层结构C/C复合材料用于X-43A马赫数(Ma)10飞行器C/C鼻锥部件,实现了2 200℃的临近空间环境中高超音速零氧化烧蚀。MER公司研制Ma 10飞行器鼻锥C/C时织物预处理温度是2 500℃,采用沥青碳和酚醛碳混合基体致密,C/C复合材料最终石墨化温度是2 800~3 000℃。     

碳/碳复合材料力学性能的研究进展

本文综述了碳/碳复合材料力学性能的研究进展,包括碳纤维、基体炭、界面性能、制备工艺及工艺参数等对碳/碳复合材料力学性能的影响。同时简单介绍了当今单向碳/碳复合材料力学性能的表征手段。希望对碳/碳复合材料力学性能的研究及应用提供帮助。     

Cyclic ablation behavior and microstructure evolution of multi-layer coating on C/C composites under oxyacetylene torch

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.     

Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf_1-xZr_xC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf_1-xZr_xC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf_1-xZr_xC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, K_c and bonding strength of Hf_1-xZr_xC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, K_c and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher K_c of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.     

针刺C/C复合材料拉伸性能和韧性的研究进展

综述了近年来国内外针刺C／C复合材料拉伸性能和韧性研究进展,归纳了影响针刺C／C复合材料拉伸性能和韧性的主要因素,即炭纤维、炭基体、界面和针刺工艺参数等,初步得到了提高材料拉伸性能和韧性的方法：提高针刺C／C材料拉伸性能的方法主要是提高纤维体积分数;提高针刺C／C材料韧性的方法主要是改善致密工艺。在此基础上,对未来的进一步研究进行展望。     

Influence of HfC nanowires on the growth behavior,microstructure and ablation resistance of CVD-HfC coating

HfC nanowire-toughened HfC ablation resistant coating was prepared on carbon/carbon composites by two steps of chemical vapor deposition. Effects of HfC nanowires on the growth behavior, microstructure and ablation resistance of the HfC coating were researched. Due to the incorporation of HfC nanowires, the deposition rate of the HfC coating was improved, the HfC coating was composed of particle-stacked crystals. After incorporating HfC nanowires, the bonding strength and fracture toughness of the HfC coating increased. HfC nanowires could restrain the crack propagation of HfC coating during ablation, contributing to improving the ablation resistance of HfC coating. After ablation for 60?s, the mass ablation rate of the HfC-coated C/C sample reduced from 0.44 to 0.26?mg/s because of the incorporation of HfC nanowires.