Fabrication of carbon/carbon composites by an electrified preform heating CVI method

Carbon/carbon composites are manufactured using the electrified preform producing directly heat CVI process. The preforms are prepared by laminating the carbon fiber felts with crossply reinforcement, and infiltrated with carbon using natural gas or propylene as a reactant, with nitrogen as diluent at atmospheric pressure. The relations between the resistivity of samples and infiltration time are determined under the operating conditions. The results indicate that the preforms have gained a high infiltration rate by this technology, and the samples have higher densities using natural gas rather than propylene. Their highest average bulk densities are up to 1.71 g/cm³ after the preforms of 1100×500×35 mm size have been densified for 80 h using natural gas. The carbon fibres in the preforms have not been damaged by this technology as yet, and the composites prepared have sufficiently high flexural properties. As the brake angular velocity is increased with the constant brake moment inertia and specific pressure, the average coefficient of friction for the composites prepared using natural gas is linearly and greatly decreased, but the variations of the brake moment inertia have a slight influence on the average coefficient of the friction when the brake angular velocity and specific pressure are kept constant. Their average thickness wear is 13×10⁻⁴ mm/surface per stop.     

Effect of additives on mechanical properties of oxidation-resistant carbon/carbon composite fabricated by rapid CVD method

The optimum composition of additives was obtained by the orthogonal design test method for oxidation-resistant carbon/carbon composites (C/C) fabricated by the rapid CVD method. The effect of additives on mechanical properties was examined. The additives used in this test included silicon carbide, silicon nitride, and metal borides. Additives doped into the matrix of C/C increase not only the initial oxidation temperature from 400 to 657°C (64%), but also its flexural strength from 121 to 254 MPa (110%), and flexural modulus from 25 to 45 GPa (96%). The increase of mechanical properties is considered to be due to the formation of a metallic carbon–boron compound in the microstructure.     

Kinetics of pyrolytic carbon infiltration for the preparation of ceramic/carbon and carbon/carbon composites

Carbon/carbon and zeolite/carbon composites have been prepared by pyrolytic carbon infiltration of organic and inorganic substrates with different porous structures. The chemical vapour infiltration kinetics of these substrates has been studied in a thermogravimetric system at atmospheric pressure, using benzene as pyrolytic carbon precursor. The rate of pyrolytic carbon infiltration seems to depend on the porosity of the substrate available to the pyrolytic carbon precursor, irrespective of the nature of the substrate studied. Activation energy values of about 180 kJ/mol were found for the different substrates used in the temperature range of 700-800 °C, where the cracking reaction of benzene takes place, predominantly, in a heterogeneous form. At higher temperatures homogeneous reactions compete with heterogeneous ones and higher values of activation energies (280-380 kJ/mol) were obtained. The oxidation of the pyrolytic carbon deposited on the different substrates studied takes place in the same range of temperature, which suggests the presence of a similar pyrolytic carbon structure on substrates of different nature or a similar accessibility to the deposited layer.     

Hierarchical material modeling of carbon/carbon composites

Unidirectional, long fiber carbon/carbon composites fabricated by chemical vapor infiltration (CVI) consisting of carbon fibers in a pyrolytic carbon matrix are anisotropic materials. It is practically impossible to identify experimentally the elastic properties (modules) of this anisotropic material. The aim of this investigation is to predict the elastic properties of this composite theoretically. The study of this material with the help of microscopy gives information about the very complicated anisotropic structure of this composite at each length scale. That is the reason that a hierarchical model for this material is developed, which consists of four length levels. A methodology for identification of the elastic properties for such composites is proposed. The problem is solved with the help of a homogenization procedure for each level.     

Effects of infiltration conditions on the densification behavior of carbon/carbon composites prepared by a directional-flow thermal gradient CVI process

Multiple quasi three-dimensioned carbon fibre preforms with disk-like shape were simultaneously densified by a directional flow thermal gradient CVI process. The effects of infiltration conditions, including temperature (ranging from 850 to 1050 °C), temperature gradient (5 and 10 °C/mm), pressure (2.5, 5.0, 7.5 and 9.5 kPa) and the type of carrier gas (N₂ or H₂), on the densification behavior of the resultant carbon/carbon composites were investigated. The results showed that lower temperatures (below 900 °C), a larger temperature gradient and higher pressure are favorable for higher average bulk density and homogeneous infiltration. Carbon/carbon composites disks with an average bulk density of 1.78 g/cm³ were achieved in one CVI cycle at a total pressure of 9.5 kPa. It was also found that adding N₂ carrier gas has no pronounced influence on the densification of the preforms. As compared to N₂, H₂ had positive effects on the densification of the preforms for temperatures above 900 °C, but it had negative effects on the densification when the control temperature was as low as 850 °C.     

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.     

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.     

Chemical vapor deposition and infiltration processes of carbon materials

The chemical vapor deposition (CVD) and the chemical vapor infiltration (CVI) processes of carbon materials are reviewed starting from the historical aspects and including the latest developments in the preparation of C/C composites. Our presentation is based on an analysis of the different types of reactors, of the composite materials with different types of pyrocarbon as matrices and a comparison between the different processes. In particular, the classical isothermal-isobaric technique and temperature or pressure gradient reactors, which lead to a higher deposition efficiency, are compared. A complementary aspect is the structural and physical analysis of the deposited pyrocarbons: they are considered as reproducible metastable phases which are obtained under non-equilibrium thermodynamic conditions. The final relevant point concerns the relationship between the process parameters and the type of pyrocarbon. In particular, the so-called rough laminar microstructure, useful for most composite applications, is described.     

Oxidation behavior and protection of carbon/carbon composites prepared using rapid directional diffused CVI techniques

The oxidation kinetics of carbon/carbon (C/C) composites prepared using a rapid directional diffused (RDD) CVI process were studied. The results showed that the Arrhenius curve for the RDD CVI C/C composites consists of two straight lines, the intercept of which is at about 700 °C at the linear oxidation stage. The oxidation rates are controlled by the surface reaction at 600-700 °C, and the corresponding activation energy is 121 kJ/mol. Between 700 and 800 °C, the oxidation rates are dominated by chemical reaction and diffusion, and the relevant activation energy is 80 kJ/mol. SEM investigation showed that the oxidation starts with original pores on the C/C composite surface with the carbon fiber and matrix oxidized simultaneously. An inexpensive and easily pasted coating containing epoxy organic silicon resin, borates, refractory particulates, etc. was developed. After isothermal temperature, thermal cycle and immersion water oxidation tests, the coating was demonstrated to exhibit good oxidation-resistance properties. The oxidation-resistant mechanism of the coating is discussed.     

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.     

The influence of thermal gradient on pyrocarbon deposition in carbon/carbon composites during the CVI process

A thermal gradient CVI process was investigated. A graphite heater in the center of a carbon felt disk preform was heated by Joule heating to a temperature of 900 °C, the carbon felt had a low thermal conductivity, and the rapid natural gas flow cooled the exterior surface of the preform. The rate constant of the chemical vapor deposition reaction increased exponentially with increasing temperatures with pyrocarbon being formed only in the designated deposition zone. Plugging of surface pores in the preforms, which often occurs in the isothermal CVI technology was unusual in this thermal gradient CVI process. As the deposition process went on, the deposited zone moved progressively towards the outside surface of the preform. The electrical resistance between two electrodes decreased gradually while the power of the thermal gradient CVI furnace increased non-linearly with the progressive densification. The temperature distribution in the thermal gradient furnace changed non-linearly with time and position. The relationship between temperature and position in the deposition zone followed the classical Fourier law. The microstructure of pyrocarbon at different positions was discussed.     

Thermal shock behavior of 3-dimensional C/SiC composite

The thermal shock behavior of a three-dimensional carbon fiber reinforced SiC matrix fabricated by chemical vapor infiltration (CVI) technique was studied using the air quenched method. Damage to composites was assessed by a destructive technique of measuring mechanical properties using three-point flexure and SEM characterization. C/SiC composites displayed good resistance to thermal shock, and retained 83% of the original strength after quenching from 1300 to 300°C 100 times. The critical ΔT of C/SiC in combustion environment was 700°C. The critical number of thermal shocks for the C/SiC composite was about 50 times. When the number of thermal shocks was less than 50 times, the residual flexural strength of C/SiC composites decreased with the increase of thermal shock times. When the number of thermal shocks of C/SiC was greater than 50, the strength of C/SiC did not further decrease because the crack density was saturated.     

A reduced reaction model for carbon CVD/CVI processes

Carbon-carbon composites are produced by chemical vapor deposition/chemical vapor infiltration (CVD/CVI) processes. Models of carbon-carbon composite production processes will help reduce production costs. Reliable process models must, however, include details of the gas phase kinetics in order to identify optimal conditions. We have combined detailed gas phase kinetics, surface kinetics, and a pore closure model to predict pore geometry changes with respect to time. To determine the dominant gas phase kinetics, we reduced a large set of reactions to a minimal set using a sensitivity, rate, and dimensional analysis approach. These robust and relatively fast techniques can be used under a variety of conditions, including those within the pores of the composite. The process model shows that the deposition profile depends on the kinetic model chosen. Using the more realistic reaction model, conditions for uniform, or inside-out, densification can be suggested.     

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.     

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.     

Chemical vapor infiltration of carbon fiber felt: optimization of densification and carbon microstructure

A carbon fiber felt with a fiber volume fraction of 7.1% was infiltrated at temperatures of 1070 and 1095 °C and methane pressures from 5 to 30 kPa to confirm the inside-outside densification derived from model studies with capillaries 1 mm in diameter. Bulk densities and residual open porosities were determined as a function of infiltration depth at various heights of the felt. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Inside-outside densification was demonstrated up to the maximum pressure of 30 kPa at 1070 °C and up to 13.5 kPa at 1095 °C, leading to bulk densities above 1.9 g/cm³. A pure, high-textured carbon matrix is formed in the pressure range from 9.5 to 11 kPa at 1095 °C. At lower and higher methane pressures and lower temperature, a less textured carbon is formed. The results are based on the growth mechanism of carbon deposition. They strongly support recent conclusions that high-textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic hydrocarbons to small linear hydrocarbons, preferentially ethine. This model is called the particle-filler model. Aromatic hydrocarbons are the molecular particles and small linear hydrocarbons are the molecular filler, necessary to generate fully condensed planar structures.     

Effect of temperature and layer thickness on these strengths of carbon bonding for carbon/carbon composites

In order to apply carbon/carbon composites (C/Cs) to various hot structures, secondary bonding techniques effective at elevated temperatures are frequently required. In the present study, carbon bonding between lamination type C/Cs was formed by the carbonation of polymer adhesive, and the strength of the bonding was evaluated at temperatures up to 2273 K in a vacuum using the double-notched shear method. The results revealed that bonding strength increased with increasing temperature and became higher than the inter-laminar shear strength of the substrate C/C when the bonding layer was thin. The enhancement of carbon bonding strength with increasing temperature was shown to be caused mainly by the evaporation of absorbed gases, probably water, up to temperatures of 1800 K with a slight additional contribution of thermal residual stress. It was also shown that heat treatment at higher temperatures made the bonding stronger.     

化学气相沉积致密针刺C/C复合材料的结构及力学性能研究

以T300炭纤维无纬布、网胎为原材料,层叠针刺成型炭纤维预制体,并采用化学气相沉积工艺对预制体进行致密,制成密度为1.55 g/cm3的针刺C/C复合材料。对针刺C/C复合材料的微观结构进行了观察分析,并对材料力学性能进行了测试。结果表明:化学气相沉积致密的针刺C/C复合材料呈现出以层间大量垂直纤维束为节点的类钉板状网状结构,这种特殊结构使材料层间结合更好,材料整个结构更加紧密;针刺C/C复合材料内部纤维被沉积形成的热解炭所包裹,热解炭的织构类型为光滑层(SL)和粗糙层(RL)并存;针刺C/C复合材料的各项力学性能均达到了较高水平,并且高温力学性能比常温力学性能有了很大幅度的提高。