Textures of pyrolytic carbon formed in the chemical vapor infiltration of capillaries

Capillaries, 1.1 mm in diameter and 17.0 or 32.5 mm in length, were infiltrated at a temperature of 1100 °C and methane pressures from 5 to 30 kPa. Layer thickness and carbon texture were determined at cross-sections of 2, 16 and 32 mm from the open end of the capillaries using polarized light microscopy. Average deposition rates, determined from layer thickness and infiltration time, as a function of methane pressure indicate a rate increase up to a saturation adsorption at pressures between 10 and 15 kPa (range 1) and a strong rate increase above these pressures (range 2). This result implies carbon formations based on the growth mechanism in range 1 and the nucleation mechanism in range 2. The carbon texture shows a maximum in range 1 and a minimum in the transition from range 1 to range 2 followed by a clear increase in range 2. The maximum in range 1 corresponds to the particle-filler model describing formation of various textures of carbon by the ratio of aromatic species to C₂ species. Increasing texture degrees in range 2 suggest that the nucleation mechanism may lead to high textured carbon provided that the residence time for intramolecular rearrangments of polycyclic aromatic hydrocarbons is sufficient.     

Fabricating thick-section carbon fiber/silicon carbide composites by machining-aided chemical vapor infiltration

Chemical vapor infiltration (CVI) is a prominent process for fabricating carbon fiber/silicon carbide (C/SiC) composites. However, the preparation of enclosed-structure or thick-section C/SiC composites/components with CVI remains a challenge, since the difficulty of densification increases. Here, machining-aided CVI (MACVI) is designed, in which infiltration-assisting holes are utilized (machined) to increase matrix deposition. To validate the approach, thick-section (10 mm thick) C/SiC composites were fabricated by MACVI. Porosity analysis and microstructure characterization were performed on the fabricated MACVI C/SiC composites and their CVI counterparts, showing a density increase up to 12.7% and a porosity decrease up to 32.1%. The mechanical behavior of the fabricated MACVI C/SiC composites was characterized, showing an increase of flexural strength by a factor of 1.72 at most. Besides, the toughness also largely increases. Both the porosity decrease and the strength and toughness increase brought by MACVI demonstrate its effectiveness for fabricating stronger and tougher enclosed-structure or thick-section ceramic matrix composites/components.     

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.     

Carbon infiltration of carbon-fiber preforms by catalytic CVI

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 1-3 h. 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 0.916 g/ml/h when the sample was formed at 600 °C for 1 h with the catalytic of 0.7 wt.% Ni, and the carbon yield arrived to 90 wt.% . 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.     

Chemical vapor infiltration of C/C composites: Fast densification processes and matrix characterizations

Fast densification processes have been developed to improve the fabrication of C/C composite materials. In this work, a comparison is made between two techniques: the film boiling technique with a liquid reagent and the gas infiltration method. In both methods, the same home-made reactor was used. For the film boiling technique, the preform is either wrapped or not with a porous thermal barrier.Two different substrates have been densified, a carbon felt (RVC-2000^® from Le Carbone-Lorraine), and a 3D carbon cloth (Novoltex^® from Snecma). In situ temperature gradients and their temporal changes during the infiltration process have been recorded together with the delivered power necessary to maintain a constant deposition temperature. From these experiments, we have concluded about the following main points:

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the analysis of in situ parameters, powers and temperatures, and the associated profiles of the pyrocarbon deposits,

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the matrix quality with their associated microstructures as characterized by helium density, optical microscopy and Raman scattering experiments,

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the key role of the evolutive preforms as heat and mass exchangers during the process, and the assisted thermal fluxes inside the reactor.

This paper presents results which should allow to control automatically the process at an industrial scale.     

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.     

Microscopic study of failure mechanisms in infiltrated carbon fiber felts

Failure mechanisms in infiltrated carbon fiber felts have been studied by optical light microscopy, transmission electron microscopy and scanning electron microscopy combined with mechanical testing experiments. A model is presented which describes crack generation and propagation at layer-layer and fiber-matrix interfaces as well as within matrix carbon layers with different textures. Intensive cracking occurs within high- and less frequently in medium- and low-textured pyrolytic carbon layers. In particular, fracture does not occur directly at the fiber-matrix interface but within the low-textured matrix layer deposited on the fiber. Crack deflection in interface regions between layers with different textures, crack deflection along boundaries of columnar grains in high -textured layers and at interfaces between polyhedral nanoparticles, and finally crack bridging within high -textured lamellae are cooperative failure mechanisms contributing to the toughness enhancement.     

Analytical and numerical study of the densification of carbon/carbon composites by a film-boiling chemical vapor infiltration process

The film-boiling chemical vapor infiltration (CVI) process is a fast process developed for composite material fabrication, and especially carbon/carbon composites. In order to help define optimal conditions, a local 1D model has been developed to study the densification front which establishes itself during the processing of a carbon/carbon fibrous preform. The model features heat conduction, precursor gas diffusion, densification reactions and structural evolution of the porous medium. The effects of total mass flux, Thiele modulus, porous medium geometry on front behavior parameters such as width, velocity and residual porosity are presented as semi-analytical correlations. An existence criterion is produced, which involves a minimal heat flux. Comparison between process-scale experiments and simulation is then possible by inserting the semi-analytical results achieved in the local study of the front into a light numerical model involving the entire preform. The model has been validated with respect to previous experimental and numerical data.     

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.     

Pyrocarbons

A review of literature on various kinds of pyrocarbons is given. Their characterization by optical microscopy, TEM imaging (transmission electron microscopy) and diffraction techniques is discussed. Various models are also described critically, as well as the possible mechanisms of deposition.     

Ring compression properties of SiCf/SiC composites prepared by chemical vapor infiltration

SiC fiber reinforced SiC matrix (SiC_f/SiC) composites prepared by chemical vapor infiltration are one of promising materials for nuclear fuel cladding tube due to pronounced low radioactivity and excellent corrosion resistance. As a structure component, mechanical properties of the composites tubes are extremely important. In this study, three kinds of SiC_f preform with 2D fiber wound structure, 2D plain weave structure and 2.5D shallow bend-joint structure were deposited with PyC interlayer of about 150–200?nm, and then densified with SiC matrix by chemical vapor infiltration at 1050?°C or 1100?°C. The influence of preform structure and deposition temperature of SiC matrix on microstructure and ring compression properties of SiC_f/SiC composites tubes were evaluated, and the results showed that these factors have a significant influence on ring compression strength. The compressive strength of SiC_f/SiC composites with 2D plain weave structure and 2.5D shallow bend-joint structure are 377.75?MPa and 482.96?MPa respectively, which are significantly higher than that of the composites with 2D fiber wound structure (92.84?MPa). SiC_f/SiC composites deposited at 1100?°C looks like a more porous structure with SiC whiskers appeared when compared with the composites deposited at 1050?°C. Correspondingly, the ring compression strength of the composites deposited at 1100?°C (566.44?MPa) is higher than that of the composites deposited at 1050?°C (482.96?MPa), with a better fracture behavior. Finally, the fracture mechanism of SiC_f/SiC composites with O-ring shape was discussed in detail.     

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.     

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.     

Structural analysis of pyrolytic carbon deposits on a planar cordierite substrate

Pyrolytic carbon deposits were produced by chemical vapor deposition on a planar substrate of cordierite using methane as a source gas. The structure of the deposits was characterized by light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with electron-energy-loss spectroscopy (EELS). The surface morphology is characterized by a cell structure induced by grains elongated perpendicular to the substrate surface. Energetic shift and intensity fluctuation of plasmon peaks in EELS spectra taken from cell and interface regions between the cells correlate with an alteration of the SEM image contrast observed on freshly fractured surfaces. This correlation suggests the presence of a mixture of two materials exhibiting different crystallization degrees. The material located at the interface is more amorphous-like in comparison to the graphite-like material located within cells.     

Densification of a 2D carbon fiber preform by isothermal, isobaric CVI: Kinetics and carbon microstructure

Chemical vapor infiltration of a 2D carbon fiber preform with a 0/0/90/90° fiber architecture and a fiber volume fraction of 22.5% was investigated as a function of methane pressure at various temperatures as well as a function of infiltration time at constant pressure. Inside-outside densification was obtained at the most attractive temperature of 1095 °C up to 29 kPa resulting in a maximum bulk density of 1.84 g cm⁻³ and a matrix density of 2.17 g cm⁻³, which corresponds to high-textured carbon. Texture formation can be perfectly explained with the earlier proposed particle-filler model. Studies at increasing infiltration times suggest a recrystallization of carbon deposited in the early stages of the infiltration.