The tensile properties of carbon matrices at temperatures up to 2200 °C

Tensile properties of carbon matrices for carbon/carbon composites made via chemical vapor infiltration (CVI) of a fiber preform, 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 evidenced on carbon fibers in a previous paper.     

Internal friction behavior of carbon–carbon composites

The fundamental internal friction behavior of carbon–carbon composites is studied. Two internal friction mechanisms are proposed according to the special internal friction characteristics in carbon–carbon composites. A thermoelastic mechanism, which is independent of amplitude, mainly leads to the internal friction increase with increasing frequency. The other is a static hysteresis mechanism that internal friction depends on the amplitude but is independent of frequency. Moreover, it is very interesting that some abnormal internal friction phenomena can be observed. The variation characteristics of internal friction and dynamic modulus versus temperature in carbon–carbon composites are quite different from other materials. This special behavior may be a result of interfacial CTE effects, as well as the coordination effects of the individual response of the fibers, matrix and interface of carbon–carbon composites. Finally, the validity of internal friction analysis methods for densification process monitoring and non-destructive inspection of carbon–carbon composites is discussed for the first time. The results indicate that internal friction testing methods have great potential for monitoring process and inspecting components of carbon–carbon composites non-destructively.     

Evolution of the structure and mechanical behaviour of a carbon foam at very high temperatures

Mechanical behaviour of a low density carbon/carbon composite at very high temperature is studied in relation with its microstructure. This composite is a syntactic foam made of carbon microbeads with a binder and voids. The resulting geometrical density is 0.3 g cm⁻³. Compressive tests from room temperature up to 3100 °C with a very high heating rate (180 °C s⁻¹) have been conducted. Intermediate temperature tests have also been performed and show an obvious modification of mechanical behaviour from around 2000 °C. This result is related to a sudden modification of structure and texture of the carbonaceous matter during the high temperature mechanical test. A strong plastic deformation occurs when the mechanical experiment is performed at 3100 °C whereas the material elastically deforms at room temperature.     

Mechanical behavior of two-dimensional carbon/carbon composites with interfacial carbon layers

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.     

Surface characteristics of fluorine-modified PAN-based carbon fibers

Different fluorination methods were applied to modify the surface properties of carbon fibers. The relationship between the degree of fluorination and the physicochemical properties of carbon fibers was studied using a combination of mechanical tests, elemental analysis (EA), X-ray photoelectron spectrometry (XPS), and X-ray diffraction (XRD). EA and XPS analyses of fluorinated carbon fibers showed that treatment with mixtures of F₂/O₂ introduced a much higher fluorine concentration than that with F₂ only. However, XRD analysis showed that there was no increase in the interlayer distance, due to the mild fluorination condition applied. Consequently, the oxyfluorination was one of the more effective methods to increase surface polarity of carbon fibers, which probably played an important role in improving the tensile properties of the fibers in the epoxy resin system.     

The pull-out and failure of a fiber bundle in a carbon fiber reinforced carbon matrix composite

The interfacial failure is examined for a unidirectionally reinforced carbon fiber/carbon matrix composite. A novel tensile test is conducted which realizes the processes of interfacial debonding and subsequent pull-out of a fiber bundle from the surrounding composite medium. The critical stress at the onset of delamination cracking is related to the fracture energy (the critical energy release rate for mode II cracking). A force-balance equation of a fiber bundle, which is quasi-statically pulled-out of the composite socket, is formulated in terms of the inter- and intra-laminar shear strengths of the composite. This equation is successfully used to estimate the delamination crack length along the debonded fiber bundle, as a function of the stress applied to the bundle.     

Prediction of elastic properties of carbon fibers and CVI matrices

The mechanical properties of carbon materials are highly dependent upon nanostructure and orientation distribution of graphitic layer planes. A model of deformations based upon theory of elasticity for anisotropic solids is proposed. Then it is used for prediction of elastic modulus and Poisson coefficient from intrinsic elastic constants for particles and orientation distribution of graphitic planes. It was applied to carbon fibers and to carbon matrices produced via Chemical Vapor Deposition based techniques. The orientation distribution of graphitic planes was determined using the distribution of intensity of X-ray scattering I(?). The predictions were compared to Young’s moduli measured on single fibers and matrices deposited on single fibers (microcomposites). The results underline the key role played by the modulus for shear between the graphitic layer planes. Influence of graphitic layer Poisson coefficient and Young’s modulus and nanostructure parameters is discussed.     

Oxidation behavior of fine-grained SiC-B4C/C composites up to 1400 °C

Fine-grained B₄C-SiC/C composites were fabricated using a ball-milling dispersion process. The oxidation behaviors of both fine-grained B₄C-SiC/C composites and coarse-grained B₄C-SiC/C composites at temperatures of up to 1400 °C were analyzed by the differential thermal analysis technique, and the surface morphology of the composites after isothermal oxidation at 800, 1200 and 1400 °C was examined by scanning electron microscopy (SEM). The results indicated that fine-grained B₄C-SiC/C composites had excellent oxidation resistance with self-healing properties at 1400 °C. A general model and mechanism for self-protection against oxidation of carbon materials were proposed.     

Sharp indentation behavior of carbon/carbon composites and varieties of carbon

Sharp indentation tests on carbon fiber and carbon matrix composites (C/C composite) were carried out over a wide load range from 0 to 2 N on three different cross sections: normal, parallel and inclined to the fiber axis. For comparison purposes, a variety of carbons including HOPG, glassy C, and pyrocarbon films was also examined. Both the fibers and the matrices displayed first a purely elastic response and second crack-induced damage. A purely elastic behavior was also observed with most of the varieties of carbon considered. Young’s modulus was extracted from the indentation curves either at maximum or at various forces, using the Sneddon equation of elastic response on loading (elastic indentation) or a classical equation based on elastic recovery on unloading (elastoplastic indentation). Results are discussed with respect to features of structure and heterogeneity of material in the stressed volume.     

Effect of oxidation pre-treatment at 220 to 270 °C on the carbonization and activation behavior of phenolic resin fiber

The effect of oxidation pre-treatment of a phenolic resin fiber was examined from two aspects: one is to examine if the pre-treatment can be a means to increase the yield of carbon fiber and activated carbon fiber (ACF), and the other is to study the effect of the pre-treatment on the carbonization and activation behavior. A phenolic resin fiber was oxidized in air at 220 to 270 °C and it was subsequently carbonized at 900 °C and activated by steam at 900 °C. The oxidation was found to affect significantly the subsequent carbonization process in the way that the yield of the carbonized fiber increased with the severity of the oxidation. On the other hand, the oxidation was found not to affect the chemical and physical properties of the carbonized fiber. The ACF produced from the oxidized fiber had almost same pore structure as the ACF produced from the non-treated fiber when compared at a same activation level. The maximum yield of ACF produced from the oxidized fiber was 1.13 times larger than the yield of ACF produced from the non-treated fiber. Thus we could increase the production yield of ACF significantly without losing its high adsorption performance.     

Comparison of 2D and 3D carbon/carbon composites with respect to damage and fracture resistance

The static mechanical responses of two- and three-dimensionally reinforced carbon/carbon composites (2D- and 3D-C/Cs) were compared. The mechanical properties examined included tensile and shear stress-strain (S-S) relations, and fracture behavior using compact tension and double edge notch configurations. Compared with 2D-C/Cs, 3D-C/Cs were shown to possess a similar tensile S-S relation, lower shear strength, higher ultimate deformation in shear, and much higher fracture resistance. The differences in shear and fracture resistance were shown to be derived from a weaker fiber/matrix interface and weaker bonding between fiber bundles in the 3D-C/Cs. These weak interface characteristics of 3D-C/Cs are due to the high value of residual stresses caused by the three-dimensional fiber constraint of 3D-C/Cs.     

Studies on the mechanical and mechanical interfacial properties of carbon–carbon composites impregnated with an oxidation inhibitor

In this work, the relationships between work of adhesion and fracture toughness parameters, such as work of fracture (W_f), the critical stress intensity factor (K_IC), and the specific fracture energy (G_IC), of carbon–carbon composites (C/C composites) were investigated. The impact properties of the composites were also studied in the context of differentiating between the initiation and propagation energies for failure behavior. Composites consisting of different contents of the oxidation inhibitor MoSi₂ displayed an increase of the work of adhesion between the fibers and the matrix, which improved both the fracture toughness and impact properties of the composites. The 12 wt% MoSi₂ composites exhibited the highest mechanical and mechanical interfacial properties. This was probably due to the improvement of the London dispersive component, W_A^L, of the work of adhesion, resulting in an increase in the interfacial adhesion force among the fibers, filler, and matrix in this system.     

Core structure of vapor grown carbon fibers and morphology dependence of tensile strength

Vapor grown carbon fibers (VGCFs) grown via a liquid pulse injection technique mostly have a turbostratic structure with a smooth surface (ca. 90%). The rest are connected or fused fibers, and fibers with spheroidal grains on their surfaces. It was clarified by TEM observation that the central core of the VGCFs whose diameter is 0.2-0.5 μm has an amorphous structure and is surrounded by graphitic sheets. Graphite layers with polygonal plates were found to be contained among circular graphite layers. The tensile strengths of individual fibers depend on their morphologies and are in the range from 0.6 to 3.7 GPa.     

Biopitch-based general purpose carbon fibers: Processing and properties

Eucalyptus tar pitches are generated on a large scale in Brazil as by-products of the charcoal manufacturing industry. They present a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units common to lignin. The low aromaticity (60-70%), high O/C atomic ratios (0.20-0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches. In the present work, eucalyptus tar pitches are evaluated as precursors of general purpose carbon fibers (GPCF) through a four-step process: pitch pre-treatment and melt spinning, and fiber stabilization and carbonization. Homogeneous isotropic fibers with a diameter of 27 μm were obtained. The fibers had an apparent density of 1.84 g/cm³, an electrical resistivity of 2 × 10⁻⁴ Ω m, a tensile strength of 130 MPa, and a tensile modulus of 14 GPa. Although the tensile properties advise against using the produced fibers as structural reinforcement, other properties give rise to different potential applications, as for example in the manufacture of activated carbon fibers or felts for electrical insulation.     

Numerical modeling of the carbonization process in the manufacture of carbon/carbon composites

A method for the numerical simulation of the carbonization process is introduced. A general model for the transient analyses of heat and mass transfer together with stress and displacement predictions is constructed using two-dimensional FEM (finite element method) for arbitrary geometry. The established model is applied to the carbonization of a single-phase, homogeneous, isotropic phenolic foam, and an anisotropic, two-phase composite material. A damage model is introduced to account for the development of shrinkage cracks, and a CDM (continuum damage mechanics) model is implemented for the calculation of mechanical property degradation due to crack evolution. The established model is verified by comparison with experimental results, and is applied to various numerical examples.     

Effect of air oxidation of Rayon-based activated carbon fibers on the adsorption behavior for formaldehyde

The tailoring of pore surface chemistry of activated carbon fibers is shown to be an effective method for improving the adsorption efficiency of various volatile chemical compounds (VOCs). An oxidation treatment with air resulted in a significant increase in the adsorption capacities and breakthrough time for Rayon-based activated carbon fibers (ACFs) in removal of formaldehyde. The porous structure parameters of Rayon-based ACFs were determined with standard nitrogen adsorption analysis. The pore surface chemistry of samples under study was analyzed by Fourier Transform Infrared spectra (FTIR). Thus to some extent, the relationship between the adsorption properties, porous structure and pore surface chemistry was revealed.