Rough Laminar Pyrocarbon Research Articles

Oxidation kinetics of a rough laminar pyrocarbon in O2-, H2O- and CO2-containing atmospheres

The originality of this work relies on the identification of kinetic parameters for a rough laminar pyrocarbon material under three different oxidizing atmospheres. Thermogravimetric analysis was used to carry out oxidation tests on a ex-CVI porous C/C composite under O2-, H2O- and CO2-containing atmospheres at ≪ low-temperatures ≫. They were proven to be in a chemical reaction-controlled regime in the porous volume of the material and at its external surface through the use of X-rays micro-tomography characterization technique and simple analytical model-based considerations. Activation energies of 203.2 kJ.mol−1, 253.9 kJ.mol−1 and 302.2 kJ.mol−1 were identified under O2, H2O and CO2.

Improvement in mechanical and ablation properties of carbon/carbon composites with nanofilamentous carbon and CeC2

Nanofilamentous carbons (NFCs) and CeC2 NF@NFC core-shell structures reinforced carbon/carbon composites were fabricated using chemical vapor infiltration via adding CeCl3 into preforms. The NFCs and CeC2 NF@NFCs constructed reinforcing bridges across the fiber/matrix interface and matrix with proper CeCl3 contents. The flexural and interlaminar shear properties were remarkably improved due to the synergistic reinforcements of the nanofilaments and CeC2 particles. The inhibition of matrix damage by the nanofilaments together with a thickened rough laminar pyrocarbon and rapid heat dissipation alleviated annular matrix oxidation and denudation. Higher CeCl3 content increased the oxidation channels and active sites and then oxidation ablation.

Microstructure and resistance to thermal shock of SiC coatings that are prepared on C/C-composite pyrocarbon matrices of various textures

SiC coatings were prepared on rough laminar (RL) pyrocarbon and smooth laminar (SL) pyrocarbon matrices of carbon/carbon composites with pack cementation via the same process. The influence of the pyrocarbon texture on the thermal shock properties and, where relevant, the microstructures of the coatings were explored. The results demonstrated that the RL-SiC (RLS) coating consisted of large SiC particles compared with the SL-SiC (SLS) coating. The thicknesses of the RLS coating and SLS coating were 288 μm and 121 μm, respectively. At 1500 °C, the RLS coating had a mass loss of 0.29% after 30 cycles, while the mass loss of the SLS coating was 2.01%. The original cracks, which had widths of approximately 3.0 μm, on the RLS coating and the SLS coating could seal quickly and had no catastrophic consequences on the coating thermal shock property. The thermal shock RLS-coated specimen performed well since the rough laminar pyrocarbon had a similar coefficient of thermal expansion to SiC, which was crucial for decreasing the microcrack production. Moreover, the rough laminar pyrocarbon easily reacted with silicon, which was advantageous for the formation of thick coatings and provided durability under thermal shock conditions.

Densification kinetics and mechanical properties of carbon/carbon composites reinforced with carbon nanotubes produced in situ

Carbon nanotube reinforced carbon/carbon composites were fabricated via in situ growth of carbon nanotubes directly on needle-punched felts following direct resistive heating thermal gradient chemical vapor infiltration. The infiltration kinetics, mechanical properties and microstructure of carbon nanotube reinforced carbon/carbon composites were studied. The results show that rough laminar pyrocarbon is obtained owing to CNTs favoring the formation of high-textured pyrocarbon. When nickel nitrate content is 0.10 ml/L, carbon nanotubes with the diameter about 30 nm are obtained together with continuous network on the carbon fibers. The flexural strength and interlaminar shear strength of the composites obtained with 0.1 mol/L nickel nitrate contents are increased by nearly 44.2% and 40.7%, respectively, compared to that of the composite containing no carbon nanotubes.

Floating catalyst chemical vapor infiltration of nanofilamentous carbon reinforced carbon/carbon composites – Densification behavior and matrix microstructure

Nanofilamentous carbon (NFC) reinforced carbon/carbon composites were prepared by floating catalyst film boiling chemical vapor infiltration from xylene pyrolysis at 1000–1100°C using ferrocene as a catalyst. The influence of the catalyst content on the densification behavior and matrix microstructure of the composites was studied. Results showed that the deposition rate of pyrocarbon (PyC) was enhanced remarkably by the catalyst. The density of the composites deposited at a catalyst content of 0–2.0wt% decreased along both the axial and the negative radial directions. Rough laminar (RL) PyC matrix was formed at 0–0.8wt% catalyst content by heterogeneous nucleation and growth. A hybrid matrix consisting of RL and isotropic (ISO) PyCs appeared at a catalyst content of 1.2–2.0wt%. The reasons for this ISO PyC formation were attributed to the deposition of carbon encapsulated iron particles and homogeneous nucleation. A reinforcing network composed of NFCs and vapor grown carbon fibers was formed on the fiber/matrix interface and within the matrix in this floating catalyst process. The structure of NFC transformed from nanotube to nanofiber when the catalyst content was over 0.5wt%, around which composites of a high density of 1.75g/cm3 and uniform RL PyC matrix were produced rapidly.

Structural features of pyrocarbon atomistic models constructed from transmission electron microscopy images

We report on atomistic models of laminar pyrocarbons constructed using a combination of 2D high resolution transmission electron microscopy (HRTEM) lattice fringe image analysis, 3D image synthesis and atomistic simulated annealing. In a first step, the effectiveness of the method and the convergence of the models with respect to the quench rate are checked on small systems. Then, the nanostructural features of large fully carbonaceous atomistic models obtained from the HRTEM images of a rough laminar pyrocarbon, as-prepared and after partial graphitization, are discussed. Both models show a very pronounced sp2 character (⩾97%), essentially made of hexagonal rings (⩾88%) and pentagonal and heptagonal rings in similar amounts (≈6%). The latter mostly form pentagon-heptagon pairs or networks of line defects between misoriented hexagonal domains. Numerous pairs of screw dislocations, connecting different graphene domains, are also observed while edge dislocations with unsaturated carbon atoms are almost absent. The models are validated with respect to experimental pair distribution functions, showing excellent agreement.

New insights into the microstructure of the friction surface layer of C/C composites

An attempt has been undertaken to obtain a better understanding of the friction and wear mechanisms of C/C composites by microstructural analysis of the friction surface layer and wear debris using scanning electron microscopy and transmission electron microscopy. The friction and wear properties of C/C composites of rough laminar pyrocarbon with added resin-derived carbon have been investigated with a home-made laboratory-scale dynamometer to simulate airplane normal landing. The results have shown that a friction layer with a thickness of several micrometers was formed on the bulk material. It was characterized as consisting mainly of amorphous carbon, but with a few microsized particles as well. This friction layer protected the bulk materials from serious degradation. In addition to the friction layer, a friction film with a thickness of less than 200 nm was observed for the first time. It partly covered the top surface of the friction layer. Transmission electron microscopy analysis has demonstrated that the friction film was characterized by a laminar structure, the belts of which were revealed to consist of highly oriented graphene sheets. The constant friction coefficient was attributed to this lubricant film.

Compressive fracture behavior of 3D needle-punched carbon/carbon composites

Compressive fracture behavior under transverse and longitudinal compressive loading are determined for 3D needle-punched carbon/carbon (C/C) composites with single rough laminar (RL) pyrocarbon matrix or dual matrix of RL pyrocarbon and resin carbon. The results of Weibull statistics analysis indicate that scale parameter σ 0 of transverse and longitudinal compression of the composites with single matrix are 153.41 and 94.26 MPa, and σ 0 of the composites with dual matrix are 205.16 and 105.33 MPa, respectively. The mean compressive strength of both composites is nearly equal to σ 0 under each experimental condition. Failure modes of both composites under transverse and longitudinal compressive loading are shear and extension, respectively. Both composites exhibit quasi-ductile fracture behavior under transverse compression. Many small fragments of fibers and matrix carbon on the fracture surface of the composites are observed for single matrix composites. And the fiber bundle breakage with extensive debonding occurs for dual matrix composites. Under longitudinal loading, the composites with single matrix show quasi-ductile fracture behavior and delamination and splitting of non-woven long carbon fiber cloth layers are observed. The composites with dual matrix exhibit catastrophic failure behavior and crack runs through the composites along compressive loading direction.

Densification behavior and microstructure of carbon/carbon composites prepared by chemical vapor infiltration from xylene at temperatures between 900 and 1250 °C

Carbon/carbon composites were prepared by film boiling chemical vapor infiltration from xylene pyrolysis. Their densification behaviors such as the mass gain, the deposition rate and the density profile were investigated. The microstructure was studied by polarized light microscopy and characterized quantitatively with average extinction angle (Ae). Results showed that, under the particular experimental equipment and process, the initial deposition rate and the average Ae of pyrocarbon (PyC) increased with the increasing deposition temperature ( T d). The structural transition of PyC from rough laminar (RL) to smooth laminar along both the axial and radial directions of the composites was retarded as T d increased from 900 to 1100 °C; PyC was deposited by the heterogeneous nucleation and growth. The homogeneous nucleation was generated producing isotropic PyC at the bottom of the composites for 1200–1250 °C deposition. The matrix produced at 1100–1250 °C was dominated by RL PyC, and the composites with high average density and uniform RL matrix were rapidly produced for T d around 1100 °C.

Raman spectroscopy investigation of structural and textural change in C/C composites during braking

The microstructure and texture of C/C composites with a resin-derived carbon, a rough laminar (RL) pyrocarbon and a smooth laminar pyrocarbon, before and after braking tests, were investigated by Raman spectroscopy. The full width at half maximum (FWHM) of the D-band indicates the amount of defects in the in-plane lattice, while the G-to-D band intensity (peak area) ratios (IG/ID) is used to evaluate the degree of graphitization. The results show that the FWHM of D-band of sample with RL pyrocarbon changes greatly from 36 cm−1 to 168 cm−1 after braking tests, which indicates that a large number of lattice defects are produced on its wear surface. However, the graphitization degree of resin-derived carbon sample rises significantly, because the IG/ID increases from 0.427 to 0.928. Braking tests under normal loading conditions, involving high temperature and high pressure, produce a lot of lattice defects on the wear surface, and induce the graphitization of the surface. Sample with RL pyrocarbon having a low hardness is easy to deform, and has the most lattice defects on the wear surface after braking. While raw materials with resin-derived carbon have the lowest graphitization degree which rises greatly during braking.

Effect of Resin-Derived Carbon on the Friction Behavior of Carbon/Carbon Composites

Three different C/C composites with rough laminar (RL) pyrocarbon, RL pyrocarbon with added resin-derived carbon, and pure resin-derived carbon have been evaluated and tested for friction performance. A laboratory dynamometer was used to simulate different braking speeds utilizing a single stator and rotor pair. The morphologies and microstructures of the raw materials, wear surfaces, and wear debris at different braking levels were observed by polarized light microscopy, scanning electron microscopy, and transmission electron microscopy. The results have shown that the friction coefficients of the three C/C composites display the same characteristics with increasing braking speed. They increased to a maximum value at medium braking speed and thereafter decreased with increasing braking speed, and their mean values under the same braking conditions were similar. The C/C composite with pure resin-derived carbon showed the highest loss due to wear under all conditions, while the C/C composite with the RL pyrocarbon showed the lowest loss. Resin-derived carbon in C/C composites does not have a significant effect on the friction coefficient, but the wear rate increases greatly with increasing resin-derived carbon content. Wear debris is composed of flocculent particles with polycrystalline structure, along with the matrix carbon, which is worn off directly from the composites.

Carbon/carbon composites prepared by ethanol pyrolysis

Ethanol,which is a renewable resource,was used as a precursor to prepare carbon/carbon (C/C) composites by chemical vapor infiltration (CVI),and the relationship between density and infiltration time was investigated. The flexural strength was determined by a three-point bend test. The microstructure and fracture-surface morphology of the composites were observed by polarized light microscopy and scanning electron microscopy respectively. Results show that the bulk density reaches 1.47 g/cm3 for a 96 h densification,a significant improvement in densification efficiency compared with other precursors. A high-textured carbon matrix,consisting of rough laminar pyrocarbon,can be easily obtained. The C/C composites exhibit a pseudo-plastic failure behavior. Ethanol is a new promising precursor for the densification of C/C composites.

Fabrication and Characteristics of Carbon Nanofiber‐Reinforced Carbon/Carbon Composites by Fast Catalytic Infiltration Processes

AbstractThe simultaneous in‐situ growth of carbon nanofibers (CNFs) and densification of a CNFs/CF hybrid multiscale felt are accomplished in a single step by thermal gradient chemical vapor infiltration using Fe as the catalyst and vaporized kerosene under atmospheric pressure. A three‐dimensional CNF network which could bridge dissimilar components of composites is formed on carbon fibers (CFs). The length of CNFs can reach several micrometers and the diameters are about 80 nm. Smooth and rough surface densified CNFs can be produced after further higher temperature infiltration. CNFs, anchoring to CFs by the adherence of the catalyst nanoparticles, enhance the bonding between CFs and pyrocarbon as well as promoting the formation of a rough laminar pyrocarbon matrix. The deposition mechanisms and physical model are also discussed. This fast catalytic infiltration process can be applied to other ceramic materials and has significant enlargement potential.

Study of the Texture of C/C Composites Prepared by Improved ICVI Process

The texture and nanostructure of C/C composites infiltrated by improved ICVI procesc was investigated by polarized light microscopy and high spatial resolution transmission electron microscope, respectively.Results show that there are three layers of pyrocarbon with different texture around the fiber.They are smooth laminar,rough laminar and dark laminar pyrocarbon from fiber to exterior, respectively.The interface bonding between the matrix and the fiber as well as that among different texture are well combined.The microcracks generate and propagate in rough laminar pyrocarbon. They do not straight extend through the interface into the lower texture,but propagate by bridged-link through the interface into the lower texture.The preferential orientation of the crystal lattice for the different texture pyrocarbon is quite different in short ranges and long ranges.

Microstructure and ablation performances of dual-matrix carbon/carbon composites

The ablation performances of dual-matrix carbon/carbon composites were tested on a DJ-21 arc heater, and the morphologies of specimens were investigated by scanning electron microscopy (SEM). It is found that the composites have typical rough laminar pyrocarbon structure and ablation always tends to start at interfaces, defects and pores. In addition, the work indicates that ablation performances of parallel ablation are better than vertical ablation and parallel ablation is mainly controlled by block denudation while vertical ablation is more likely to be controlled by particle denudation.

Microstructure of the pyrocarbon matrix prepared by the CLVI process

The chemical liquid-vaporized infiltration (CLVI), a rapid densification technology that has received increased interest in the C/C composite fabrication, was applied to manufacture the two-dimensional carbon/carbon laminates. In this paper, the microstructures and textural characters of the pyrolytic carbon were examined by means of the polarized light microscope (PLM) and the scanning electron microscope (SEM). Nucleation and growth of rough laminar (RL) pyrocarbon, which resulted from the special densification situation and deposition condition of CLVI, was studied. Particular emphasis was put on the laminar-type morphology of the cone-like RL pyrocarbon. Moreover, both the nucleation on the substrate surface and the laminated growth with curved layers were deduced to contribute to this particular pyrocarbon microstructure.

Regenerative laminar pyrocarbon

This work reports on a dense laminar pyrocarbon infiltrated at low temperature, not previously described. By using pulse-CVI and/or toluene for example, a pyrocarbon can be developed with a density as high as that of rough laminar pyrocarbon (RL) but with a regenerative texture. It will be referred to as regenerative laminar (ReL). This pyrocarbon is characterized by a high density ( d=2.11), a high bireflectance and the maximum value for the extinction angle: A e=22°, the latter being related to its high anisotropy. Contrary to rough laminar, regenerative laminar exhibits a smooth extinction of the Maltese-cross in polarized light. This is related, by means of TEM, to the regeneration of thin cones all along the growth. Rough laminar is not regenerative: it contains only primary cones. The regeneration of cones is found to be due to the lateral extent of the layers; on the contrary, rough laminar is grown from small layers which cannot propagate lattice defects (no regeneration). The experimental evidence is based on the infiltration of preforms by using toluene as precursor and pressure-pulsed chemical vapor infiltation (P-CVI). A reference carbon/carbon, RL, is also obtained with the same preform (3D PAN-based needled preform) by using the classical isobaric–isothermal chemical vapor infiltration with a methane–propane mix (I-CVI).

Correlation Between Homogeneous Propane Pyrolysis and Pyrocarbon Deposition

Pyrocarbon deposition through propane pyrolysis is studied in a 1-D hot-wall CVD furnace. The gas-phase pyrolysis is modeled with a partially reduced kinetic mechanism leading to polycyclic aromatic compounds (PAHs). The C2-C4 and C3 reaction paths are in competition for benzene formation. There is also an independent C3-C5 path leading to naphthalene. The gas-phase concentrations are correlated with experimental data including in situ Fourier transform infrared spectra intensities, pyrocarbon deposition rates, and pyrocarbon nanotextures. Rough laminar pyrocarbon deposition appears to be more related to PAHs than smooth laminar pyrocarbon. © 2001 The Electrochemical Society. All rights reserved.

Deposition process of laminar pyrocarbon from propane

The deposition mechanisms of smooth laminar (SL) and rough laminar (RL) pyrocarbons from propane are found to be very different, as evidenced by (i) gas phase analysis, (ii) nucleation experiments and (iii) analysis of the deposit composition and nanostructure. SL pyrocarbon results mainly from aliphatic species; its nucleation needs surface reactive sites where chemisorption and hydrogen abstraction occur; its growth leads to extended but highly distorted graphene layers which can enclose internal nanoporosity. RL pyrocarbon results mainly from polycyclic aromatic hydrocarbons (PAH); its nucleation, which does not depend on the substrate, occurs widely by physisorption of PAH; the corresponding graphene layers are not very extended but parallel to each other and parallel to the anisotropy plane.

Structure and oxidation of C/C composites: role of the interface

The oxidation of N ex-PAN carbon fibre preform as well as the oxidation of the related carbon/carbon (C/C) composites are compared with the oxidation of their counterparts based on M ex-PAN carbon fibres. The oxidation test is conducted on polished specimens by thermogravimetry. In order to localise and quantify the oxidation, SEM observation was performed on the specimens before and after the test. Whatever the matrix, oxidation of the composite is controlled by the oxidation of the preform itself. The N preform oxidizes more slowly than the M reference. The oxidation features of M-based materials are related to catalytic effects because of the presence of pitting at the interface. Oxidation of N-based composites is localised on the external surface and at the fibre/matrix interface. In the matrix, oxidation is controlled by the anisotropy of the pyrocarbon: smooth laminar pyrocarbon (C/C1) oxidising more slowly at the surface than rough laminar pyrocarbon (C/C2), but deeper in the bulk at the fibre/matrix interface.