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Internal friction behavior was utilized as an indirect metric to study structural change in carbon/carbon composites after fatigue tests. In this work, two kinds of unidirectional carbon/carbon composites with different densities were prepared by isothermal chemical vapor infiltration (ICVI), and loaded under stress level of their fatigue limit. The internal friction behavior of the composites after different fatigue cycles was studied. After the initial 104 fatigue cycles, since the matrix began to break and shed, frictional damping that happened between the fiber and matrix interfaces increased and the bulk internal friction increased rapidly. Between 104 and 5×105 fatigue cycles, holes formed at interfaces because of continued shedding of the matrix. Since the holes reduced contact area between different phases, frictional damping decreased, as along with the internal friction increment ratio. Between 5×105 and 106 fatigue cycles, interfacial bonding between carbon fiber and matrix was in a steady state, so structure and internal friction of the composites did not change. The results revealed that internal friction was an effective method to characterize structural change of carbon/carbon composites after fatigue tests. [ABSTRACT FROM AUTHOR]

Abstract: Carbon fiber reinforced paper-based friction (CFRPF) material is a new type wet friction material which has been used in automatic transmission. In this study, four kinds of CFRPF were prepared by paper-making process with cashew-modified phenolic resin. The resin contents ranged from 35wt.% to 50wt.%, and the effect of phenolic resin on properties of CFRPF was investigated. Porosity of the samples was measured by liquid permeation method. The degradation characteristics of the samples were analyzed using a thermal analyzer. The friction and wear properties were tested by a wet friction performance tester. Scanning electron microscopy (SEM) was also employed to study the wear mechanisms. Experimental results indicated that the reinforced fibers were dispersed uniformly in the phenolic matrix, containing some pores with different size. There were three stages in the thermal degradation of CFRPF, and the resin content had great effect on the degradation temperature at the second stage. Lots of tiny wear particles were found on the surface of the worn samples with lower resin content. Continuous friction film was formed on the worn surface with the increasing resin content, which efficiently enhanced wear resistance of the samples. Comprehensively, the samples with lower resin content (ranging from 35wt.% to 40wt.%) demonstrated better potential with high friction coefficient, excellent friction stability, good heat-resistance and reasonable strength. [ABSTRACT FROM AUTHOR]

Abstract: Based on the study of moving regulation of yarns in the carbon fiber preform, the yarn unit model with elliptical cross section was presented. This model involved the cross-section deformation and the space orientation of yarn through which the braided structure and the braiding parameters were connected. A software was developed by employing the model on which the integral geometric descriptions of 3D braided preform with different parameters were automatically generated. As a result, the pore’s shape and distribution among the yarns in the preform were expressed clearly. The effect rule of the braiding parameters on the shape and volume of the pores in the preform was that the pores became smaller and more separated with the increasing of the fiber volume fraction when the other parameters were invariant. The geometric descriptions are helpful for the calculation of whole permeability values for braided preform and the investigation of yarn stress distributions in 3D braided preform. [Copyright &y& Elsevier]

Abstract: Experimental studies were conducted to assess catalytic chemical vapor infiltration processing for preparing carbon/carbon composites as a potential improvement to conventional one. The catalyst was introduced into the carbon fiber preforms by wet impregnation. Using C3H6/Ar/H2 as the original gas, catalytic carbon was formed at 500–1000°C for 1–3h. It was found that carbon filaments were formed as the preparing temperatures were 500–700°C, and carbon particles could be obtained at 800–1000°C. The increasing rate of density was up to 0.916g/ml/h when the sample was formed at 600°C for 1h with the catalytic of 0.7wt.% Ni, and the carbon yield arrived to 90wt.% . According to the micrographs of catalytic carbon, the forming mechanism of carbon filaments agreed with that of carbon filaments due to a metal catalyst. The weighted average interlayer spacing of C/C composites with catalytic carbon decreased to 0.341. [Copyright &y& Elsevier]

Multi-wall carbon nanotubes (MWCNTs) were synthesized on carbon fiber cloth by chemical vapor deposition using natural gas as the carbon source and FeSO4 7H2O powder as the catalyst precursor. Scanning and transmission electron microscopy and Raman spectroscopy were used to characterize the product. Small catalyst particles were observed on the top of the tubes. The MWCNTs were dispersed uniformly and aligned perpendicular to the cloth. They had a high crystallinity, and few impurities and defects. The density and structure of the MWCNTs on the cloth can be controlled by deposition temperature and time. [New Carbon Materials 2013, 28(6): 421–427] [ABSTRACT FROM AUTHOR]

Precursor of copper (Cu) was introduced into 2D needle-punched carbon felts at room temperature to fabricate carbon/carbon (C/C) compotes modified with Cu (C/C Cu). Ablation property, heat erosion mechanism and flexural properties of C/C Cu composites were investigated. Bad wettability between copper and carbon faded their interface bonding and made flexural strength of C/C composites decreased by 11.5% in Z direction and 9.7% in X direction. By introducing the in-situ growth of CNTs, its flexural strength was back to that of C/C composites. The ablation test results showed that C/C Cu composites exhibited better ablation resistance than pure C/C composites. After ablation for 120 s, its mass and linear ablation rate decreased from 2.30 mg/s and 7.00 μm/s to 1.55 mg/s and 3.45 μm/s comparing with that of pure C/C composites. Analyses of ablation behavior depending on temperature and ablated surface micrographs indicated that copper liquid droplets escaped from sub surface and were blown away from ablation central zone to fringe zone, which all absorbed lots of heat and made the temperature of central zone decreased, keeping the temperature of ablation surface stable. Moreover, the central zone temperature of C/C Cu composites and C/C composites was 1943 °C and 2172 °C, respectively. Such high temperature (1943 °C) made copper evaporate with a small mount which also take away much heat. This temperature changes in ablation surface slowed the oxidation of carbon and made C/C Cu composites have less ablation rate. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanotubes (CNTs) were introduced into carbon fiber felts by electrophoretic deposition (EPD) and then pyrocarbon (PyC) was infiltrated in the felts by thermal gradient chemical vapor infiltration using methane as precursor to prepare carbon nanotube (CNT) reinforced carbon/carbon composites (CNT-C/C composites). The effects of CNTs by EPD on microstructure, mechanical and thermal properties of carbon/carbon (C/C) composites were investigated. Results showed that CNTs had complicated effects on the microstructure of PyC in CNT-C/C composites: PyC next to carbon fiber (CF) was isotropic texture and PyC far from CF showed medium texture, while PyC in C/C composites without CNTs (REF-C/C composites) presented low texture. Compared with REF-C/C composites, compressive strength and modulus of CNT-C/C composites were increased by 37% and 19%, respectively, and fracture mode of the composites changed from pseudo-plastic fracture to brittle fracture; coefficient of thermal expansion of CNT-C/C composites in the direction perpendicular to carbon fiber plies was decreased by 30%. [Copyright &y& Elsevier]

Abstract: The preparation of carbon fiber-reinforced cement-based composites involved two-step dispersions of carbon fibers. Both steps affected greatly the mechanical properties of the composites. With the aid of ultrasonic wave, a new dispersant hydroxyethyl cellulose was used to help fiber dispersion in the first step. The fracture surface of the composites was observed by scanning electron microscopy. The distribution of major elements was analyzed by the energy dispersive spectroscopy and the composition was analyzed through X-ray diffraction. The flexural strength, tensile strength, modulus, and compression strength were measured. Results showed that the distribution of major elements varied with the variation of the fiber dispersion status. The compressive strength increased by 20%, the tensile strength was 2.4 times that of the material without carbon fibers, the modulus increased by 26.8%, whereas the flexure stress decreased by 12.9%. [Copyright &y& Elsevier]

Abstract: A new technique, including optical imaging and data processing, was presented for measuring of the extinction angle of pyrocarbons deposited around carbon fibers. The experiment was performed using a rotating analyzer in the reflection polarized light microscope coupled with a digital imaging camera. The special specifications for adjusting each part in the optical arrangement were detailed. The principle of the proposed method was discussed from the crystal grain distribution of pyrocarbons and the extinction property of graphite. The researched C/C composites respectively contain smooth laminar, rough laminar and dark laminar pyrocarbons. The digital images were captured at intervals of 1°. The extinction curves were plotted by fitting the sampled pixel values, and the extinction angles were obtained corresponding to the lowest point of these curves. The correlation between the experimental results and the observation by eyes in PLM is consistent with Bourrat''s conclusion. [Copyright &y& Elsevier]

In the field of carbon fibers reinforced pyrocarbon matrix (C/C) composites fabrication, normally the carbon fiber preform is densified with the pyrocarbon matrix (PyC) after it is produced and weaved. Such two-step method is due to the different structures and different fabrication conditions that matrixes and reinforcements require. Here, we achieve the synchronous growth of matrix (PyC) and reinforcement (bamboo-shaped carbon nanotubes, CNTs) in one step using a controlled thermal gradient chemical vapor deposition (TGCVD). This proposed one-step method can greatly simplify the fabrication process of traditional C/C composites. And the fabricated bamboo-shaped carbon nanotubes reinforced pyrocarbon matrix (CNTs/PyC) nanocomposites achieve improved mechanical performance and show isotropy and super-elastic property. The CNTs/PyC nanocomposites can be the substitution of traditional C/C composites and isotropic PyC materials. And such method can provide a new technical strategy for synthesizing multi-carbon structures and other nanocomposites. Image 1 [ABSTRACT FROM AUTHOR]

Carbon/carbon (C/C) composites are considered the most promising materials for high-temperature structural applications owing to their excellent properties at high temperature. However, oxidation and ablation limits their applications in oxygen-containing environments at ultrahigh temperatures and high-speed gas flows. How to improve oxidation and ablation resistance of the C/C composites has become a critical issue. The methods for improving oxidation and ablation resistance of the C/C composites are reviewed, which mainly focus on optimizing the carbon fiber weave structure, controlling the pyrolytic carbon texture, modifying the matrix, and coating with anti-ablative ceramic layers. Matrix modification and coating technology have been proved to be the two effective ways of dealing with the problem. Development directions on producing oxidation and ablation resistive C/C composites are concentrated on decreasing preparation cost, controlling pyrolytic carbon texture, optimizing compositions of the doped ceramics used in the matrix and the coating layer, and combining matrix modification with an anti-ablation coating. [New Carbon Materials 2015, 30(2): 97–105] [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) were grown in situ on the surface of carbon fiber cloths by injection chemical vapor deposition using ethanol as a carbon source and ethylenediamine as a promoter. The nanotube-grown cloths were stacked to form multilayered preforms, and then densified by isothermal chemical vapor infiltration to prepare CNT reinforced carbon/carbon (CNT–C/C) composite laminates. The effect of CNTs on microstructure and mechanical properties of carbon/carbon (C/C) laminates were investigated. Results show that the existence of CNTs not only contributes to refine the pyrocarbon (PyC) grain size but also inhibits matrix cracking, resulting in high cohesion of PyC matrix. The grown CNTs possess high surface area thus modifying the fiber surface condition, which in turn improves the fiber/matrix (F/M) interface. Both the high matrix cohesion and strong F/M interface bonding give rise to the powerful interaction between adjacent layers in the CNT–C/C laminates. The measured out-of-plane, in-plane compressive strength and interlaminar shear strength of these laminates show a significant enhancement of 32%, 115% and 108%, respectively compared with the laminates without CNTs. Out-of-plane and in-plane compressive modulus of these laminates are also improved by 11% and 46%, respectively. [ABSTRACT FROM AUTHOR]

SiC–ZrC ablation resistance coating was prepared on the surface of carbon fiber reinforced carbon (C/C) composites by simple pack cementation combined with low-cost slurry infiltration method. The results showed that SiC–ZrC coating could effectively protect C/C composites from ablation for 45 s at 3723 K under oxyacetylene torch. The mass and linear ablation rates (0.038 ± 0.01 mg/(s cm 2 ) and 2.42 ± 0.15 μm/s) were largely reduced compared with that of uncoated C/C composites (0.530 ± 0.01 mg/(s cm 2 ) and 1.75 ± 0.15 μm/s) after ablation for 20 s. The good ablation protective ability of SiC–ZrC coating is mainly attributed to the volatilization of SiO 2 and the formation of ZrO 2 . [ABSTRACT FROM AUTHOR]

This paper reported simultaneous monitoring damage evolution of flexural fatigue in unidirectional carbon-fiber-reinforced carbon composites (C/C composites) by electrical resistance change (ERC) methods. The degree of irregularity in electrical resistance changes increased with stress levels increasing. The shapes of electrical resistance change rate-fatigue cycle curves can reflect stress levels and damage types of tested samples: sawtooth shapes reflected delamination at a higher stress level; and "peak" shapes reflected inner damages in one fiber bundle at the fatigue limit stress level. In addition, the similarity of initial electrical resistance-fatigue life curve and S-N curve was observed clearly. In summary, ERC methods can monitor the damage evolution and qualitatively estimate the fatigue life of unidirectional C/C composites. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanotubes (CNTs) with different orientation and lengths were grafted onto carbon fibers by catalytic chemical vapor deposition to produce hybrid preforms that were used to reinforce carbon/carbon (C/C) composites. Compressive property tests indicated that these C/C composites have obvious improvements in out-of-plane compressive strength, compared with pure C/C composites. Furthermore, the improvement has strongly influenced by the orientation and length of CNTs. Compared with entangled CNTs and radially-grown straight CNTs with smaller length, CNTs with a radial grafting morphology and longer length can reinforce the F/M interface and the whole matrix more efficiently and then increase the compressive strength better. [Copyright &y& Elsevier]

Abstract: The flexural properties of pyrocarbon-based carbon/carbon (C/C) composites reinforced by in situ grown carbon nanotubes are reported. The effect of nanotube growth position (on carbon fibers or in the spaces between them) was investigated. A method of reinforcing and toughening C/C composites is proposed and the corresponding mechanisms are discussed. [Copyright &y& Elsevier]

Abstract: Straight carbon nanotubes (CNTs) were grafted radially onto carbon fibers and they were used to enhance mechanical strength retention rate (i.e. the percentage of mechanical strength after graphitization as that before graphitization) of laminated carbon/carbon (C/C) composites after graphitization. Results show that out-of-plane compression strength and interlaminar shear strength retention rates of C/C composites increase from 34.7% and 42.1% to 76.2% and 84.9% after adding CNTs, showing 120% and 102% improvements. Analyses demonstrate that these great improvements are the result of simultaneous reinforcements to the fiber/matrix (F/M) interface and the matrix provided by the radially grafted CNTs, which can effectively inhibit the microstructure defects such as F/M interface debonding and matrix annular cracking occurred around carbon fiber during graphitization and then greatly increase the mechanical strength of graphitized C/C composites. [Copyright &y& Elsevier]

Abstract: Straight carbon nanotubes (CNTs) were grafted radially onto carbon fibers to produce hybrid materials that were used to reinforce carbon/carbon (C/C) composites. Mechanical property tests indicated that these C/C composites have improvements in out-of-plane and in-plane compressive strengths and interlaminar shear strength of 275%, 138% and 206%, respectively. They also have a large decrease in the anisotropy of mechanical properties, compared with pure C/C composites. This great improvement is the result of the simultaneous reinforcements to the fiber/matrix interface and the matrix provided by the grafted CNTs. [Copyright &y& Elsevier]