Chemical Vapor Infiltration Process Research Articles

Analysis of the chemical vapor infiltration process computational experiments

A model for chemical vapor infiltration is analyzed. Consider a cylindrical pore with a reacting and carrier gas flowing in from the left. The gas reacts with the interior of the pore and the result is a solid matrix. The model assumes that the flux due to binary diffusion is negligible. The model also assumes that the reactions are first order. Numerically, we look at how the void and the concentration of reacting gas change as a function of space and time. We examine the progress of the process as α 2 (proportional to reaction rate/diffusion rate) and the flux of the reactant out of the preform vary. We also compare the asymptotic analysis with the computational results.

Effect of SiC coating and heat treatment on damping behavior of C/SiC composites

Three groups of 2D and 3D C/SiC composites were fabricated by chemical vapor infiltration (CVI) process: the first group was as received, the second group was treated at 1500 °C in vacuum atmosphere for 2 h, and the third group was deposited with a chemical-vapor-deposited (CVD) SiC coating. Damping properties of these composites were measured by dynamical mechanical analyzer (DMA) at different frequencies from room temperature to 400 °C in air atmosphere. The results show that SiC coating and heat treatment decrease damping capacity of C/SiC composites and the damping peak disappears or decreases in the testing temperature range. The effect of CVD SiC coating on damping behavior of 2D and 3D C/SiC composites is mainly related to the change of porosity and is independent of fiber preform architecture. However, the effect of heat treatment on damping behavior of 2D and 3D C/SiC composites is mainly attributed to the change in the SiC matrix and interphase bonding, and it is dependent on fiber preform architecture. Both of CVD SiC coating and heat treatment studied in this paper have no influence on relationship between damping behavior of C/SiC composites and frequency.

Analytical stability study of the densification front in carbon- or ceramic-matrix composites processing by TG-CVI

Thermal-gradient chemical vapor infiltration (TG-CVI) is a group of alternatives to classical CVI, involving a strong thermal gradient. It allows one to fabricate, e.g. carbon-matrix or ceramic-matrix composite materials starting from a fibrous preform and a gaseous precursor, the cracking of which results in a solid deposit constituting the composite matrix. The main interest of these processes short fabrication time; however, their control is difficult. Past modelling works [Vignoles, G.L., Goyhénèche, J.-M., Sébastian, P., Puiggali, J.-R., Lines, J.-F., Lachaud, J., Delhaès, P., Trinquecoste, M., 2006. The film-boiling densification process for C C composite fabrication: from local scale to overall optimization. Chemical Engineering Science 61, 5336–5353; Nadeau, N., Vignoles, G.L., Brauner, C.-M., 2006. Analytical and numerical study of the densification of carbon/carbon composites by a film-boiling chemical vapor infiltration process. Chemical Engineering Science 61, 7509–7527] have shown that process control and optimization are possible and are based on the notion of densification front. In this paper, a 2D transverse stability study is presented for this front. A condition for stability is worked out; the influence of processing parameters is discussed. It appears that in usual processing cases, the stability criterion is fulfilled, but that it could be violated if some careless process up-scaling is performed.

Helium‐Permselective Amorphous SiC Membrane Modified by Chemical Vapor Infiltration

Helium‐permselective silicon carbide (SiC) membrane was successfully synthesized on an outer surface of γ‐alumina‐coated α‐alumina porous support by both pyrolysis of polycarbosilane without oxygen‐curing process and chemical vapor infiltration (CVI). An amorphous SiC membrane possessed He permeance of 7.7×10−8 mol · m−2 · s−1 · Pa−1, He/CO2 permselectivity of 64, and He/H2 permselectivity of 4.4 at 873 K. CVI process is an effective way to increase permselectivity of gases with small kinetic diameter.

Mechanical properties of SiC/SiC composite with magnesium–silicon oxide interphase

A SiC fiber reinforced SiC composite with magnesium–silicon-based oxide interphase was fabricated by the chemical vapor infiltration process and its mechanical properties were evaluated. An interphase was prepared on the advanced SiC fiber, Tyranno SA, using the alkoxide method. The tensile strengths of SiC fiber with the Mg–Si–O coating retained at 85% or higher compared to uncoated-unheated fiber after heating below the 1300 °C, while strength were slightly degraded to 80% after heating at 1400 °C. The composite showed ductile failure behavior and the fiber pull-out effect was observed in the fracture surface of the composite.

Mechanical properties of advanced SiC/SiC composites after neutron irradiation

The effect of neutron irradiation on tensile properties in advanced 2D-SiC/SiC composites was evaluated. The composites used were composed of a SiC matrix obtained by the forced-flow chemical vapor infiltration (FCVI) process and either Tyranno™-SA Grade-3 or Hi-Nicalon™ Type-S fibers with single-layered PyC coating as the interphase. Neutron irradiation fluence and temperature were 3.1 × 10 25 n/m 2 ( E > 0.1 MeV) and 1.2 × 10 26 n/m 2 at 740–750 °C. Tensile properties were evaluated by cyclic tensile test, and hysteresis loop analysis was applied in order to evaluate interfacial properties. Both composites exhibited excellent irradiation resistance in ultimate and proportional limit tensile stresses. From the hysteresis loop analysis, the level of interfacial sliding stress decreased significantly after irradiation to 1.5 × 10 26 n/m 2 at 750 °C.

Growth of SiC nanowires within stacked SiC fiber fabrics by a noncatalytic chemical vapor infiltration technique

An atmospheric pressure chemical vapor infiltration (CVI) process without metallic catalysts was applied for the growth of SiC nanowires within stacked SiC fiber fabrics. We investigated the effect of the concentration of a reactant gas (CH 3SiCl 3, MTS) on the growth behavior and microstructure of the SiC nanowires. At high concentration of MTS in a H 2+MTS mixture gas, one-dimensional (1D) SiC deposits with diameters of several hundreds of nanometers were formed. Microstructures of the 1D SiC deposits exhibited a strong positional dependency throughout the thickness direction of the stacked fabric due to a depletion of the MTS gas. On the other hand, single-crystalline SiC nanowires with average diameters of 50–60 nm could be obtained at a low concentration of MTS. The SiC nanowires also exhibited a homogeneous growth both in the plane of each fabric layer and throughout the thickness of the sample.

Colloidal Crystals as Templates for Macroporous Carbon Electrodes of Controlled Thickness

AbstractMacroporous carbon films were synthesized using colloidal crystals as a template and were characterized using scanning electron microscopy (SEM) and Raman spectroscopy. The colloidal crystals were elaborated by the Langmuir‐Blodgett technique and were infiltrated with carbon by a controlled chemical vapor infiltration (CVI) process. After removal of the template, thin free‐standing carbon membranes whose thicknesses match perfectly those of the templates were obtained. Their ability to act as electrodes was checked by carrying out cyclic‐voltammetry experiments.

Densified aligned carbon nanotube films via vapor phase infiltration of carbon

We report a simple way to produce fully densified aligned carbon nanotube (ACNT) films. The simultaneous growth of nanotubes and densification of the ACNT films by carbon infiltration in the interstitial spaces between nanotubes are accomplished in a single step by the combination of the chemical vapor deposition and chemical vapor infiltration processes. Scanning electron microscope analysis and microbalance measurements showed that after infiltration, the diameters of nanotubes and bulk density of the ACNT films are increased by an order of magnitude (and hence the porosity of the ACNT films is decreased). Transmission electron microscope and Raman scattering analysis showed that after densification, the nanotubes are conformally coated by partially graphitized pyrolytic carbon. The compressive modulus of the densified ACNT films could be increased by three orders of magnitude compared to the pristine ACNT films. Electrical properties are also measured for the densified films showing marked differences with the ACNT films. The property enhanced densified ACNT films constitute a new form of carbon–carbon nanocomposites and could find applications as multifunctional nanocomposites.

Structural Peculiarities of Mesostructured Carbons Obtained by Nanocasting Ordered Mesoporous Templates via Carbon Chemical Vapor or Liquid Phase Infiltration Routes

Mesostructured carbon materials were obtained by nanocasting MCM-48 and SBA-15 ordered mesoporous silica templates via two carbon infiltration routes: a liquid-phase process (LPI) using sucrose solution or a gas-phase chemical vapor infiltration (CVI) process using propene. The structural characteristics of the carbon replicas were investigated by synchrotron low-angle X-ray diffraction (LAXRD) analysis in combination with transmission electron microscopy (TEM). The materials obtained by the liquid-phase process demonstrated a long-range mesoscopic order at relatively low carbon loading (ca. 35 wt % in SiO2/C composite), with their structural elements being essentially shrunk compared to that of the templates. The CVI replicas at similar low infiltration content were found to be only partly organized within nanodomains, but at bigger carbon loading (ca. 50 wt %) highly ordered mesostructures displaying up to ten nonzero XRD reflections were obtained. Moreover, these materials were shown to have thicker frameworks faithfully replicating their templates. Particular features of the CVI replica of MCM-48 were an amorphous carbon shell on the external particle surface and a gradient of the nanoframework displacement. The carbon replicas of SBA-15 were characterized by a semitubular structure.

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.

Numerical Simulation of Effect of Methyltrichlorosilane Flux on Isothermal Chemical Vapor Infiltration Process of C/SiC Composites

A two‐dimensional axisymmetrical mathematical model for the isothermal chemical vapor infiltration process of C/SiC composites was developed. Transport phenomena of momentum, energy, and mass in conjunction with infiltration‐induced changes of preform structure were taken into account. The integrated model was implemented by the finite‐element method to simulate numerically the isothermal chemical vapor infiltration (ICVI) process of C/SiC composites at different methyltrichlorosilane (MTS) fluxes. The influence of MTS flux on concentration distribution and time‐dependent densification behaviors of C/SiC composites was studied in detail. Calculation results imply that MTS flux has an obvious influence on infiltration in micro‐pores and little influence on infiltration in macro‐pores. Increasing flux will lead to an evident acceleration for infiltration in micro‐pores. Moderate flux is preferable by a combination of both a relatively high infiltration rate and a relatively low fabrication cost. This model is helpful to understand the fundamentals of the ICVI process for the fabrication of C/SiC composites.

A two-dimensional model for densification behaviour of C/SiC composites in isothermal chemical vapour infiltration

A two-dimensional model was developed according to infiltration-induced structural changes in C/SiC composites and physicochemical phenomena involved in isothermal chemical vapour infiltration (ICVI) process. The mathematical model was implemented to simulate the densification behaviour of the C/SiC component of a small-scale thruster liner for rocket engine. The calculated results show that infiltration efficiency is high at first and then decreases dramatically, which is in agreement with the corresponding experimental results. The correspondence between calculated results and experimental data implied that this mathematical model is reasonable and feasible for characterizing the densification behaviour of C/SiC composites in the ICVI process. The dependence of densification behaviour of C/SiC composites on infiltration temperature has also been investigated. Calculation results show that the densification rate increases while density uniformity of overall composites decreases evidently with elevated temperature.

Comparative modeling of gas transport in isothermal chemical vapor infiltration process of C/SiC composites

Two comparative modeis taking into account of momentum, energy and mass transport coupled with chemical reaction kinetics were proposed to simulate gas transport in isothermal CVI reactor for fabrication of C/SiC composites. Convection in preform was neglected in one model where momentum transport in preform is neglected and mass transport in preform is dominated by diffusion. Whereas convection in preform was taken into account in the other model where momentum transport in preform is represented by BRINKMAN equations and mass transport in preform includes both diffusion and convection. The integrated models were solved by finite element method. The calculation results show that convection in preform have negligible effect on both velocity distribution and concentration distribution. The difference between MTS molarities in preform of the two models is less than 5x10 −5, which indicates that ignorance of convection in preform is reasonable and acceptable for numerical simulation of ICVI process of C/SiC composites.

Preparation and mechanical properties of C/SiC composites with carbon fiber woven preform

C/SiC composites reinforced with multilayer carbon fiber woven preforms were fabricated by isothermal chemical vapor infiltration (ICVI) process. To characterize the mechanical properties of the composites, mechanical testing was carried out under various loading conditions, including tension, bending and shear loads. The results indicated that the composites, with superior intrinsic through-the-thickness properties, exhibited high in-plane mechanical properties. Therefore, the composites developed can well meet the demands of the reusable nose cap, i.e. the easiness of near-net shaping and the capability of withstanding multidirectional mechanical and thermal stresses.

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 C 3H 6/Ar/H 2 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.

Improvement of film boiling chemical vapor infiltration process for fabrication of large size C/C composite

An improved film boiling chemical vapor infiltration process was developed to fabricate a large size C/C composite with homogeneous density and microstructure. The C/C composite was prepared by processing a disc-shaped carbon felt preform, whose upper and lower sides were fixed and heated simultaneously by two flat surfaces of two heat sources, with kerosene as a precursor at 1050 °C for 3 h at an atmospheric pressure. The in-situ temperature distribution along the radial direction of the preform upper surface was analyzed to get better information and control of the process. Experimental results show that the average density of the composite of Φ 110×10 mm3 size is about 1.72 g/cm3 and its maximal difference along radial direction is 0.05 g/cm3. Polarized light microscopy (PLM) and scanning electron microscopy (SEM) reveal that the carbon fibers of the composite are surrounded by ring-shaped pyrocarbons with a thickness of ∼20 μm, and that pyrocarbons are delaminated to 4–6 layers. A schematic model is proposed to analyze the process by dividing the reactor into different regions associated with specific functions.

Tribological properties of carbon nanotube-doped carbon/carbon composites

Carbon nanotube (CNT)-doped carbon/carbon (C/C) composites were fabricated by the chemical vapor infiltration (CVI) method to investigate the effect of CNTs on tribological properties of C/C composites. CNTs, which had been synthesized by catalytic pyrolysis of hydrocarbons, were added to carbon fiber formed preforms before CVI process. Ring-on-block-type wear tests were performed to evaluate the frictional properties of CNT-doped C/C composites. Results show that CNTs can not only increase wear resistance of C/C composites but also maintain stable friction coefficients under different loads. Polarized light microscopy, X-ray diffraction, scanning electron microscopy and Raman spectroscopy analyses demonstrate that favorable effects of CNTs on tribological properties of C/C composites have been achieved indirectly by altering microstructure of pyrocarbons and directly by serving as high-strength lubricative frictional media at the same time. Electron dispersive spectroscopy (EDS) analyses verify the existence of adhesive wear mechanism in both pure C/C composites and CNT-doped C/C composites albeit the two-body abrasive mechanism dominates in pure C/C composites.

Thermal Shock Damage of a 3D‐SiC/SiC Composite

Thermal shock of a three‐dimensional (3D) SiC/SiC composite prepared by chemical vapor infiltration (CVI) process was conducted using water quenched method. Thermal shock damage of the composite was assessed by SEM characterization and measurement mechanical properties using three‐point flexure after quenching. After quenched from 1200°C to 25°C water for 100 cycles, the composite retained 80% of the original flexural strength in the longitudinal direction while cracked through the width direction. Thermal shock damage of the composite was analyzed by thermal stress analysis based on the braiding structure of the composite as well as the distribution and shape of flaws referred to residual pores in the matrix. The braided structure and the dimension difference resulted in the anisotropy of mechanical properties and the matrix pores configuration of the composite, which led to the thermal shock damage anisotropy of the composite.

Property enhancements via matrix microstructure modification of carbon–carbon composites prepared by CVI processing

Carbon-carbon (CC) composites have received considerable interest for aerospace applications due to their superior strength retention at high temperatures and high heats of ablation etc. [1]. In the manufacturing of CC composites, carbon fiber preforms can be categorized dependent upon their final applications, e.g. UD, 2-D, 3-D and n-D etc. [2, 3]. Among them, even though they are high labor-dependent, carbon fiber rodnetwork n-D preforms have been used for thick (larger than 100 mm thickness) and high performance CC composites, especially for aerospace applications. In general, rod-network n-D CC composites are employing three routes for densification (1) chemical vapor infiltration (CVI) of hydrocarbon gases (2) liquid process with impregnation and pyrolysis of high carbon yield polymers or pitches (liquid process) (3) combination of CVI and liquid process [4]. In CVI process, under suitable conditions, pyrolytic carbon (PC) matrix has a preferred crystalline orientation which results in considerable anisotropy. In pyrolytic graphite (PG) which is more anisotropic than PC, for instance, the thermal expansion in the thickness direction (c direction in graphite structure) is approximately 20 times that in the plane of deposition (ab plane) [5]. Similarly, mechanical properties measured parallel to the ab plane are considerably higher than those measured perpendicular to it. Even though the degree of anisotropy is lower than that of PG, PC anisotropy creates considerable thermal stresses during CC processing, which lead to delaminations between PC matrix and the carbon fiber rods and micro-cracks in PC matrix itself. These delamination defects may be eliminated or at least greatly reduced by the modification of PC microstructure resulting in increase of CC composites performance [6, 7]. High-strength PAN-based carbon fiber (ACELAN TZ-307, Korea) with 12 K manufactured by Taekwang Industries Co., (Korea) was used in the present study. The fiber has tensile strength of 3800 MPa, tensile modulus 260 GPa, elongation 1.3%, and density of 1800 kg/m3 [8]. Polyvinyl alcohol (PVA 110, Kuraray, Japan) was used as a binder for carbon fiber rods preparation. Carbon fiber rods with 1.0 mm diameter were prepared using a typical pultrusion process. Three-D orthogonal rod-network preforms (W 150 × D150 × H200 mm) having 59% fiber volume percent