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Improvement of the mechanical properties of two-dimensional carbon/carbon composites
Abstract
The microstructure of carbon matrix was modified and the interfaces between filler/matrix and fiber/matrix were improved by the fillers including expanded graphite and vapor-grown spiral carbon fiber (VGSCF), used as a secondary reinforcement in the carbon matrix. The optimized two-dimensional carbon/carbon composites (C/Cs) were fabricated by adjusting the mass ratio of the two fillers, 5wt.% expanded graphite and 7wt.% VGSCF. The flexural strength, flexural modulus and fracture strain of the 2D C/Cs amount to 130.6MPa, 20.26GPa and 1.86%, respectively. The fracture mode of the 2D C/Cs exhibits a pseudo-plastic behavior. A secondary reinforcement model is proposed to explain the propagation of microcracks and the mechanism of toughness improvement in 2D C/Cs.
... Carbon/carbon (C/C) composites have recently attracted much interest due to their unique physical, mechanical and chemical properties. Some advantageous properties such as low weight, good high-temperature strength, high thermal conductivity, resistance to thermal shock and resistance to high-temperature erosion make C/C composites more useful [1][2][3][4][5][6]. The most important defect of these composites is oxidation at temperatures higher than 500ºC, which could be prevented by applying an appropriate coating on them. ...
Due to the mismatch of the thermal expansion coefficients between the matrix and yarns, thermal residual stress will appear in C/SiC composites. In this paper, a progressive damage model was used to predict the thermal-mechanical behavior of C/SiC composites and reveal the failure mechanism. Firstly, the properties of the composites under tensile load were tested at three different temperatures in vacuum. Then, the elastic-plastic progressive damage constitutive laws were used and implemented by a user-defined subroutine UMAT in ABAQUS. The thermal residual stress evolution in the cooling and heating processes was characterized. Finally, the stress-strain curves of the composites under tensile load at different temperatures were studied. The effects of thermal residual stress on the tensile properties and progressive damage process of C/SiC composites were revealed sequentially. This work can give design guidance for strengthening of C/SiC composites.
Using natural gas as carbon source, 2D needle felt as preform, 2D-C/C composites were prepared by thermal gradient chemical vapor infiltration. Their microstructures were observed under polarized light microscope (PLM) and scanning electron microscope (SEM), and the flexural behaviors before and after heat-treatment were studied with a universal mechanical testing machine. The fracture mechanism of the composites was discussed in detail. The results show that, carbon matrix exhibits pure smooth laminar (SL) characteristic including numerous wrinkled layered structures and some inter-laminar micro-cracks. With the decreasing density, the strength of the composites decreases and the toughness increases slightly; after 2500 degrees C heat-treatment, the inter-laminar micro-cracks in matrix increase, the strength decreases, and the toughness obviously increases. The fracture mode of the composites changes from brittle to pseudo-plastic characteristic due to more crack deflections in SL matrix.
A four directional carbon/carbon (4D C/C) composite was fabricated by first using liquid phase impregnation carbonization (LPIC), followed by hot isostatic pressure impregnation and carbonization (HIPIC) at 75 MPa, and finally high temperature treatment. The flexural properties and fracture behavior of the composite were investigated in the through-thickness direction under static and fatigue loading. The critical fatigue limit of the composite was 80% of the static flexural strength for one million loading cycles at 10 Hz. The failure mechanism of the composite under static flexural loading was dependent on the orientation of the carbon fibers in the tested specimen. Cyclic fatigue loading decreased the interfacial bonding strength and released the inherent stresses in the composite, which increased fiber pull-out, enhanced pseudo-ductility and increased the residual static flexural strength at the expense of a decrease in the flexural modulus. The fatigue loading increased the number of noncritical matrix cracks, increased interfacial debonding, and caused the fracture of filaments in the surviving fatigued C/C composite. These features of the fatigued composite internally accommodated expansion in long direction as the temperature was increased, which resulted in a decrease in its residual thermal expansion.
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.
Carbon/carbon (C/C) composites with multiple matrixes were fabricated by chemical vapor infiltration (CVI) and an impregnation and carbonization step. To study the effects of predeposited pyrocarbon on microstructure and the mechanical properties of C/C composites, different predeposition times (3 h, 5 h, 7 h and 9 h) were tested. This caused the predeposited pyrocarbon layer to have a large effect on the mechanical properties of C/C composites. C/C composites with 3 h predeposition exhibited the worst mechanical properties because the predeposited pyrocarbon layer is thin, which does not prevent deformation of the preform during impregnation with liquid pitch. For composites with a 5 h predeposition time, a pyrocarbon layer with proper thickness played an important role in protecting the preform; this sample possessed better mechanical properties. However, as the predeposition time was increased to 7 h and 9 h, due to the increased thickness of the deposited pyrocarbon, several pores were generated in the C/C composites that decreased their density and worsened their mechanical properties.
Carbon/carbon (C/C) composites with in situ grown carbon nanofibers (CNFs) were fabricated. Results showed that CNFs had significant benefit on improving mechanical properties of C/C composites. It was concluded that CNFs could be involved in serving as a second reinforcement for composites and absorbed more energy leading to higher mechanical properties.
Carbon/carbon (C/C) composites with multiple matrixes were fabricated by impregnation/carbonization and chemical vapor infiltration (CVI) using 2-dimensional (2D) carbon felts preform. In order to study the effects of microstructure on mechanical properties, C/C composites with single matrix of pitch based carbon were prepared for comparison. The mechanical properties were tested on CMT5304-30KN universal testing machine. Polarization microscope and scanning electron microscope were used to investigate the microstructures and fracture surface of C/C composites. It was resulted that the flexural strength of C/C composites with multiple matrixes was improved by 83% compared with that of C/C composites with single matrix. Meanwhile, better toughness and pseudo-plastic fracture behaviors were also obtained with multiple matrixes. Multi-layer interfaces between different constituents were formed in the composites which supplied variety of extension paths for cracks. This microstructure character improved the mechanical properties of C/C composites, especially the toughness, as more energy could be dissipated during cracks spread along multiple paths, such as the interface of different components or among the matrixes.
A comparative study has been carried out on mechanical behavior of two dimensionally (2D), three dimensionally (3D) and four dimensionally (4D) reinforced carbon/carbon composites (C/Cs) having 50, 48 and 37vol.% carbon fibers, respectively. The tensile and flexural strengths of 2D- and 3D-C/Cs, found as ∼25% higher than that of 4D-C/C, appear related to fiber volume fraction. The load–displacement plots obtained from three-point bend tests on single edge notch bend specimens appear non-linear, suggesting graceful failure. Scanning electron microscopy studies of crack paths have shown high tortuousity due to crack deflection, branching as well as fiber debonding and breakage. The results indicate that total fracture energy release rate (Jc) and displacement (δ0.6) at 60% of peak load are suitable parameters for evaluation of fracture resistance and damage tolerance, respectively of multi-directionally reinforced C/Cs. The values of Jc and δ0.6 have been found as maximum for 2D- and 4D-C/C, respectively.
Carbon/carbon (C/C) composites with two different matrixes of pitch carbon and pyrolytic carbon were fabricated using 2-dimensional
(2D) carbon felts preform. In order to study the effects of matrixes on mechanical properties, C/C composites with single
matrix of pitch carbon were prepared. The mechanical properties were tested on CMT5304-30KN universal testing machine. Polarization
microscope and scanning electron microscope were used to investigate the microstructures and fracture surface of C/C composites.
It was resulted that the flexural strength of C/C composites with two matrixes was improved by 96% compared with that of C/C
composites with single matrix. Meanwhile, better toughness was also obtained with two matrixes. For the composites, multilayer
microstructures were generated after filling up of voids caused during carbonization of mesophase pitch by pyrolytic carbon.
The multilayer microstructures were beneficial to the improvement of mechanical properties of C/C composites, especially the
toughness. More energy could be dissipated during mechanical tests while cracks might extend along multiple paths, such as
the interface between fiber and matrix or the interface between different matrixes.
The tensile behavior of four different brands of carbon fibers (a rayon-based, a PAN-based, and 2 pitch-based fibers) has been investigated at various temperatures up to 2400 °C. The tests were carried out using an original fiber testing apparatus. Various mechanical properties including strength and Young's modulus, as well as Weibull statistical parameters were extracted from test data. Typical tensile behaviors were evidenced such as an essentially linear elastic behavior at room temperature and intermediate temperatures up to 1400–1800 °C, then a nonlinear elastic delayed response at higher temperatures and ultimately an inelastic response with permanent deformations at very high temperatures. Such unusual nonlinear responses for homogeneous materials were related to structure and texture features at the nanometer scale, that were described through an X-ray diffraction technique.
Effect of interfacial carbon layers on the mechanical properties and fracture behavior of two-dimensional carbon fiber fabrics reinforced carbon matrix composites were investigated. Phenolic resin reinforced with two-dimensional plain woven carbon fiber fabrics was used as starting materials for carbon/carbon composites and was prepared using vacuum bag hot pressing technique. In order to study the effect of interfacial bonding, a carbon layer was applied to the carbon fabrics in advance. The carbon layers were prepared using petroleum pitch with different concentrations as precursors. The experimental results indicate that the carbon/carbon composites with interfacial carbon layers possess higher fracture energy than that without carbon layers after carbonization at 1000°C. For a pitch concentration of 0.15 g/ml, the carbon/carbon composites have both higher flexural strength and fracture energy than composites without carbon layers. Both flexural strength and fracture energy increased for composites with and without carbon layers after graphitization. The amount of increase in fracture energy was more significant for composites with interfacial carbon layers. Results indicate that a suitable pitch concentration should be used in order to tailor the mechanical behavior of carbon/carbon composites with interfacial carbon layers.
Tensile properties of carbon matrices for carbon/carbon composites made via chemical vapor infiltration (CVI) of a fiber pre-form, were determined from room temperature to 2200 °C. Microcomposite test specimens were used. For this purpose, a carbon coating was deposited on low modulus carbon fibers via chemical vapor deposition based techniques. The mechanical behavior of the carbon matrix was derived from the stress–strain curves obtained with the single filament reinforced microcomposites. It was related to features of nanostructure characterized using X-ray diffractometry. The trends are discussed with respect to those evi-denced on carbon fibers in a previous paper.
The relationship between the structure and mechanical properties of both polyacrylonitrile (PAN)and pitch-based carbon fibres has been studied in detail. Raman spectroscopic studies on the deformation of carbon fibres have shown that the rates of Raman band shift per unit strain, for both PAN- and mesophase pitch-based fibres, increase linearly with Young's modulus of the fibres. The shift rate for PAN-based fibres, however, was found to be almost twice that of mesophase pitch-based fibres of the same Young's modulus. To understand this difference, the microstructure of both types of fibre was examined using a combination of techniques, including scanning and transmission electron microscopy and Raman spectroscopy. The results demonstrate that there is a profound skin-core difference in PAN-based fibres, with more highly oriented and larger crystallites in the skin region, and less oriented and smaller crystallites in the core. In contrast, the mesophase pitch-based fibres show little change in microstructure across the fibre. The skin of the PAN-based fibres has a higher modulus than the core material. During deformation, this gives rise to a larger Raman band shift in the skin on the fibre than in the core, at the same level of overall strain.
A fracture mechanics study is reported for two different types of carbon fiber reinforced carbon matrix composites, a short-fiber felt composite and a satin weave laminated-composite. It is emphasized prior to discussing their fracture mechanisms that the first matrix cracking is the most important fracture process in these C/C composites. Fiber pull-out and bridging processes in the wake of the propagating crack tip are discussed in relation to experimental R curves. The bridging tractions are estimated by the Dugdale model, from which the fiber bridging-enhanced toughening in lamina composites is demonstrated.