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.     

Microstructure and ablation properties of zirconium carbide doped carbon/carbon composites

Carbon/carbon composites doped with zirconium carbide were prepared by a three-step process. Carbon fiber felts were first immersed in a zirconium oxychloride solution, followed by rapid densification using thermal gradient chemical vapor infiltration. The densified carbon/carbon composites were then graphitized at 2500 °C. The phase composition and morphology of the composites were investigated by X-ray diffraction and scanning electron microscopy. The ablation properties were tested in an oxyacetylene torch. The results show that the linear and mass ablation rates of the composites after doping with 4.14 wt.% zirconium carbide decreased by 83.0% and 77.0%, respectively. The ablated surface of the carbon matrix for pure carbon/carbon composites was very smooth and glossy, while that for doped carbon/carbon composites was honeycombed and dim. The bonding between carbon fibers and matrix decreased because of the formation of more zirconium dioxide, resulting in carbon fibers peeling off the matrix and the ablation resistance of carbon fibers could not be brought into play when the zirconium carbide contents achieved 4.14 wt.%. Although mechanical denudation does not seem to play a dominant role, the ablation was mainly controlled by heterogeneous mass transfer.     

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.     

Microstructure and ablation performances of dual-matrix carbon/carbon composites

The ablation performances of dual-matrix carbon/carbon composites were tested on a DJ-21 arc heater, and the morphologies of specimens were investigated by scanning electron microscopy (SEM). It is found that the composites have typical rough laminar pyrocarbon structure and ablation always tends to start at interfaces, defects and pores. In addition, the work indicates that ablation performances of parallel ablation are better than vertical ablation and parallel ablation is mainly controlled by block denudation while vertical ablation is more likely to be controlled by particle denudation.     

ZrC ablation protective coating for carbon/carbon composites

A zirconium carbide (ZrC) protective coating was deposited on carbon/carbon (C/C) composites by atmospheric pressure chemical vapor deposition. The phase compositions, surface and cross-section microstructures, and anti-ablative properties of the coatings were investigated. Results show that the method is an effective route to prepare a dense and thick ZrC coating on C/C composites. The coating can effectively protect C/C composites from ablation for 240 s in an oxy-acetylene torch system with a mass ablation rate of 1.1 × 10⁻⁴ g/cm² s and a linear ablation rate of 0.3 × 10⁻³ mm/s.     

Controlled nitrogen content synthesis of hafnium carbonitride powders by carbonizing hafnium nitride for enhanced ablation properties

In this study, a new method of carbonizing hafnium nitride was proposed to synthesize ultrahigh-temperature hafnium carbonitride (HfC_xN_y) powders. The new method helps to maintain both the purity of phases and control the content of nitrogen in the HfC_xN_y. The results show that the as-prepared HfC_xN_y powders have a single phase, with an average particle size of approximately 2 $\mu m$, and Hf, C and N are evenly distributed. Moreover, the microstructures, phase compositions, ablation properties and mechanism of the HfC_0.62N_0.38 composites under a plasma ablation environment were studied in detail. The results show that the HfC_0.62N_0.38 composites exhibited excellent ablation resistance at 3073 K for 60 s and the ablation mechanism of HfC_0.62N_0.38 can be identified as HfC_0.62N_0.38→HfC_xO_y→HfO₂. The mass ablation rate of the HfC_0.62N_0.38 composite is evaluated to be 1.36 mg/cm²∙s, which is lower than that of HfC ceramics. Our work is intended to provide new insight regarding the development of ultrahigh-temperature ceramics and widen their applications.     

Mechanical and ablation properties of 2D-carbon/carbon composites pre-infiltrated with a SiC filler

In order to improve the mechanical and ablation properties of 2D-carbon/carbon composites, a SiC filler was added to a 2D-preform before isothermal chemical vapor infiltration densification by using a powder infiltration technique. Backscattered electron images showed that the SiC filler was mainly concentrated between the fiber bundles and between the layers. The tensile and flexural strengths of the composites were improved by the addition of the SiC filler because of the increase of interfacial surface areas between the bundles and between the layers, the less residual open porosity, and also the strong bonding between the SiC particles and the pyrocarbon matrix. The composites with filler experienced a 15.2% lower thickness erosion rate and a 51.7% lower mass erosion rate, compared to those C/C without filler. This was attributed to the low oxygen permeability of the SiO₂ shielding the exterior inter-bundle pores as well as to a thermal barrier effect.     

Effects of zirconium carbide content on thermal stability and ablation properties of carbon/phenolic composites

Ceramic particles were utilized to improve thermal stability and ablation properties of carbon/phenolic (C/Ph) composites. In this study, zirconium carbide (ZrC) modified C/Ph composites were fabricated by vacuum impregnation method, and effects of ZrC content on thermal stability and ablation properties were investigated by thermogravimetry analysis and plasma wind tunnel test. Moreover, morphological characterization was carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. Experimental results showed that increasing ZrC content could lead to an evident increase in char yield, but an observable reduction in linear ablation rates and back-face temperatures because of the formation of ZrO₂ layer on the ablation surface. The work provided an effective way to improve thermal stability and ablation properties of C/Ph composites.     

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.     

Preparation and properties of multi-walled carbon nanotube/carbon/polystyrene composites

Multi-walled carbon nanotube (MWCNT)/C/polystyrene (PS) composite materials were prepared by in situ polymerization of monomer in preformed MWCNT/C foams. MWCNT/C foams were preformed using polyurethane foam as template. The preformed MWCNT/C foams had a more continuous conductive structure than the carbon nanotube networks formed by free assembly in composites. The structure of the MWCNT/C foam network was characterized with scanning electron microscopy. The MWCNT/C/PS composites have an electric conductivity higher than 0.01 S/cm for a filler loading of 1 wt.%. Enhancement of thermal conductivity and mechanical properties by the preformed MWCNT/C foam were also observed.     

炭黑/白炭黑并用增强NR/SSBR复合材料的结构与性能

研究白炭黑的用量5-15份,等量替代炭黑时,对NR/SSBR复合材料结构与性能的影响。白炭黑的用量为15份时,结合胶的含量最高;RPA,DMTA和TEM结果表明：白炭黑的用量为15份时,填料的分散最均匀;而且15份的白炭黑还一定程度提高了NR和SSBR的相容性。白炭黑的用量为5-10份时,NR/SSBR复合材料的拉伸强度、撕裂强度和拉断永久变形变化不大;白炭黑的用量为15份时,NR/SSBR复合材料的拉断伸长率和邵尔A硬度有所降低,300%定伸应力有所提高。在60℃下的tanδ和动态压缩温升最低,阿克隆磨耗量有所增加。     

Microstructure and ablation resistance of ZrCxNy-modified ZrC-SiC composite coating for carbon/carbon composites

Nitrogen-modified ZrC-SiC coatings were prepared by thermal evaporation, in situ reaction, and nitriding process, and the microstructure and ablation property of the coatings were studied. The results showed that nitrogen atoms could replace the carbon atoms and fill the vacancies of ZrC. In addition, the interface of the ZrC phase was optimized. The nitrogen atom solid solution was limited on the coating surface, and the interior of the coating was composed of high-melting point ZrC and SiC ceramics. The ablation test showed a reduction in the ablation rate of the coating after nitriding due to the formation of a dense ZrO₂ layer.     

碳／碳复合材料的性能和应用进展

碳／碳（c／c）复合材料是以碳为基体、碳纤维增强的复合材料,具有高比强度、高比模量、耐高温、耐腐蚀、耐疲劳、抗蠕变、导电、传热和线胀系数小等一系列优异性能,既可作为结构材料承栽负荷,又可作为功能材料发挥作用。同时,碳／碳（c／c）复合材料是一种能在超高温条件下工作的高温结构材料,所以在航空航天领域具有广阔的应用前景。本文综述了碳／碳（c／c）复合材料的力学、热学、化学性能及其在各领域的应用进展。     

Radiation effects on mechanical properties of carbon/carbon composites

Two types of carbon/carbon composites were irradiated to a fast-neutron fluence of 2 × 10¹⁸ n/cm² (E > 1 MeV) in a helium atmosphere at 175°C. These specimens were mechanically tested in flexure and in shear to determine resulting property changes. Shear strengths at room temperature were increased by 25 per cent or more by irradiation, and these increases in shear strengths allowed the composites to be flexed to higher stress levels (15–25 per cent) before undergoing permanent deformations. These property improvements were retained by the materials up to temperatures of at least 1000°C in an inert environment.     

Characterisation of the transverse mechanical properties of carbon/carbon composites by spherical indentation

The mechanical characterisation of the carbon/carbon composite in the transverse direction is essential for the design of braking discs. This paper presents a technique based on spherical indentation to identify the mechanical behaviour of such materials in the transverse direction.After the presentation of the material properties as determined from static and fatigue compression tests, the indentation technique is described in detail. The characterisation technique used takes into consideration the transverse isotropy of these materials. The method used allows to identify the material behaviour in elastic and inelastic fields. Characterisation of elastic parameters is carried out after identification of the Hertz law in the unloading indentation cycles. The hardening parameters and the elastic limit are identified by expressing the law of Hertz in strain/stress form.The identified parameters are used in a simulation of an indentation test by finite element method. A good agreement is found between numerical and experimental results.     

Anti-oxidation performance of carbon aerogel composites with SiCO ceramic inner coating

To improve the oxidative-resistant ability of carbon fibre-reinforced carbon aerogel composites (C/CAs), C/CAs were impregnated with a silicon oxycarbide (SiCO) precursor sol several times. After gelating, aging, solvent exchanging, ambient drying, and pyrolyzing, the SiCO precursor sol infiltrated the nanoporous C/CA, and anti-oxidation carbon aerogel composites with a SiCO ceramic inner coating (C/CA-SiCO) were prepared. It was found that the density of the material increases with the number of times that impregnation and pyrolysis were carried out. The mechanical properties and the anti-oxidant performance were significantly improved after the deposition with different numbers of the SiCO coating cycles. The bending strength increased from 2.2?MPa to 10.8–32.3?MPa and the compressive strength (10%ε) increased from 0.4?MPa to 3.6–44.4?MPa. The weight loss for the as-prepared composites after heating at 1600?°C for 60?min in air was only 7.6%, and the in-plane shrinkage was less than 2%. Additionally, after the oxidation resistance test, the surface of the C/CA-SiCO had abundant β-SiO₂ crystalline, whereas its inner region of the composite remained amorphous.     

Dielectric properties of carbon nanofibre/alumina composites

Carbon nanofibre (CNF)/Al₂O₃ composites with concentrations between 1 and 9 vol.% of CNF were prepared by the traditional ceramic processing route followed by spark plasma sintering. The dielectric properties of these composites have been studied in a broad frequency range from mHz to the infrared range. Unlike conventional composites, the percolation threshold in this system is more complex depending on the particles topology. Positive and negative variations by several orders of magnitude in the low frequency AC conductivity have been detected for concentrations near the threshold at ∼2 vol.% of CNF. To explain these results, a modified percolation model has been proposed which takes into consideration the effect of the concentration of the filler on the microstructure of the composite.     

The preparation and properties of SiCw/B4C composites infiltrated with molten silicon

Silicon carbide whisker (SiC_w) toughened B₄C composites have been prepared by pressureless infiltration of B₄C–SiC_w–C preforms with molten silicon under vacuum at 1500 °C. The effect of SiC_w addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiC_w/B₄C composites is discussed. It is revealed that the addition of SiC_w improves the fracture toughness of B₄C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiC_w/B₄C composite with 24 wt% SiC_w addition is 4.88 MPa m^1/2, which is about 9% higher than that of the one without SiC_w, but at the same time, the bending strength reduces to the minimum value 243 MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiC_w/B₄C composites is B₄C, SiC, Si, and B₁₂ (C, Si, B)₃, with no residual C. And the main toughening mechanism of SiC_w is whisker pulling up.