Chemical Vapor Infiltration Technique Research Articles

Application of the chemical vapor infiltration and reaction (CVI-R) technique for the preparation of highly porous biomorphic SiC ceramics derived from paper

Chemical vapor infiltration and reaction (CVI-R) is used to produce biomorphic high porous SiC ceramics based on biological structures such as paper. The paper fibers are first converted into a biocarbon (C b) template by a carbonization step. In a second step methyltrichlorosilane (MTS) in excess of hydrogen is infiltrated into the C b-template by CVI technique, depositing a Si/SiC layer around each fiber. The reaction (R) between biocarbon and excess silicon to form additional silicon carbide occurs during a subsequent thermal treatment as a third step of the ceramization process. Due to the mild infiltration conditions (850–900 °C) the initial micro- and macrostructure of the carbon preform is retained in the final ceramics. The applied characterization methods after every step of the ceramization process are X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, Thermal Gravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM). The bending strength of the resulting porous ceramics is measured by the double ring bending test. It is found that a slight excess of free Si in relation to the amount of carbon from the C b-template must be deposited in the Si/SiC layer to achieve a nearly complete conversion of the C b-templates into SiC ceramic. The weight gain after infiltration has to be at least 400 wt.%. Varying the infiltration conditions such as temperature, MTS concentration and infiltration time, ceramics in a wide range of porosity (55–80%) and mechanical properties (5–40 MPa) can be produced. A thermal treatment temperature of 1400 °C is found to be optimal for the reaction between the deposited Si and the biocarbon.

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 Δ T 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.

Preparation of SiC-based cellular substrate by pressure-pulsed chemical vapor infiltration into honeycomb-shaped paper preforms

Using a pressure-pulsed chemical vapor infiltration technique, SiC was infiltrated from a SiCl4 (4%)–CH4 (4%)–H2 gas phase into carbonized paper preforms at 1100°C. SiC-based cellular substrates with cell wall thicknesses of 25, 50 and 100 μm were obtained by using honeycomb-shaped paper preforms as the templates. The reduction of both wall thickness t and cell pitch d of SiC-based honeycomb substrate successfully led to an increase in geometric surface area per unit volume S, keeping pressure drop ΔP at constant, besides; ΔP decreased without lowering S by the reduction of t and the increase in d. The pressure drop in the prepared honeycomb substrates depended on S 2α−3 value, where α was open frontal area fraction. The compressive strength of the honeycomb substrates with t of 25 μm was about 7 MPa. The strength increased in proportion with 1 − α, which corresponded to the volume fraction of the cell wall in the honeycomb substrate.

Damping characteristics of CVI-densified carbon–carbon composites

The damping behavior of various carbon–carbon (C/C) composites fabricated by chemical vapor infiltration (CVI) techniques has been investigated in the low-frequency range by means of a pendulum method. Two groups of samples of C/C composites were employed in this study, one with constant fiber volume fractions but with various densities from 1.02 to 1.69 g cm −3, and the other with the same densities but with volume fractions of carbon fibers varying from 45 to 61%. The internal friction of the C/C composites has been found to decrease with an increase in density and to increase with an increase in the volume fraction of the fibers. Meanwhile, the porosity of the C/C composites with different density was calculated, and the internal friction of the composites was found to increase with increasing porosity. According to the observed experimental results, the damping mechanisms associated with the C/C composites have been analyzed and the porosity damping and interface damping are used to explain the experimental results.

Modeling of chemical vapor infiltration process for fabrication of carbon–carbon composites by finite difference methods

A finite difference (FD)-based method is proposed to describe the chemical vapor infiltration (CVI) processes for fabrication of carbon–carbon composites. The continuous, unsteady-state CVI processing can be divided into many discrete steady-state depositions by this model. Long cylindroid unidirectional carbon–carbon composites are prepared using the isothermal CVI technique to verify the accuracy of the FD methods. Experimental research shows that the modeling values are in good agreement with experimental data. The deviation of infiltration threshold and final bulk density are both minor with error less than 10%. Moreover, computational porosity at different cross-sections of preforms is also consistent with experimental values. The effect of deposition temperature, fiber volume fraction, and size of composites, on densification is analyzed using the FD model, which results in some useful information. Bulk density, infiltration threshold, uniformity, are described quantitatively by regression equations.

Materials design and processing of high temperature ceramic matrix composites: state of the art and future trends

The material design of ceramic matrix composites (CMCs) is discussed in terms of fiber nature and architecture, interphases (pyrocarbon, BN, multilayers), matrices and seal-coating, taking into account lifetime considerations. CMCs are processed by liquid or gas phase routes. Besides the well established processing routes, such as polymer impregnation and pyrolysis (PIP), slurry impregnation/hot pressing (SIHP) and isothermal/isobaric chemical vapor infiltration (I-CVI) techniques, emerging processes allowing densification times of the order of a day, are discussed.

Mechanical properties in compression of CVI-densified porous carbon/carbon composite

Compressive properties of a porous 2-D planar-random carbon/carbon (C/C) composite preform densified by chemical vapour infiltration (CVI) of pyrolytic carbon were determined and compared to those obtained previously for similar C/C composite densified using a non-CVI technique. Stress/strain curves and failure modes of the CVI composite were similar to those seen in the non-CVI C/C composite. For specimens of equivalent density, the CVI technique resulted in improved in-plane properties, but inferior out-of-plane properties, when compared to the non-CVI composite. Compressive property anisotropy ratios for the CVI composite were slightly reduced compared to the uninfiltrated preform state whilst comparison between compressive and (previously determined) flexural properties showed general agreement. A microstructural model of the composite developed by the authors was used to explain the results.

Fabrication of silicon carbide fiber-reinforced silicon carbide composite by hot-pressing

Continuous SiC fiber-reinforced SiC composites (SiC/SiC f) are one of the most attractive structural materials for future fusion reactors. To date, the only successful manufacturing process of SiC/SiC f composites is the chemical vapor infiltration technique, which is a very delicate and expensive process. In this study, therefore, a new method to fabricate the composite with higher density is developed using a conventional hot-press procedure. Matrix SiC sheets and Nicalon or Hi-Nicalon two-dimensional cloths without any coatings were stacked alternately and then this stacked green body was hot-pressed at 1750°C under 40 MPa applied stress in Ar. The matrix SiC sheet consists of fine, β-SiC powders and Al, B and C sintering aids. A sheet of SiC was formed using the doctor blade technique. The relative density of sintered bodies was 66–99%. The load–displacement behavior of the composites with Nicalon cloth showed a slightly ductile load–displacement behavior with low maximum strength, whereas the composite with Hi-Nicalon cloth showed brittle fracture with high maximum strength at room temperature.

The Stability of BN Interfacial Coatings in CFCC Systems During Oxidation and Exposure to Moisture

The mechanical reliability of ceramic matrix composites (CMCs) at elevated temperatures in oxidative environments is primarily dependent upon the chemical and structural stability of the fiber/matrix interface. Graphitic carbon coatings have traditionally been used to control the interfacial properties in CMCs, however, their use is limited in high temperature oxidative environments due to the loss of carbon and subsequent oxidation of the fiber and matrix. Thus, BN is being investigated as an alternative interfacial coating since it has comparable room temperature properties to carbon with improved oxidation resistance. The stability of BN interfaces in SiC/SiC composites is being investigated at elevated temperatures in either flowing oxygen or environments containing water vapor. The effect of several factors on BN stability, including crystallographic structure, extent of BN crystallization, and impurity content, are being evaluated.Nicalon™ fiber preforms were coated with ≈ 0.4 μm of BN by CVD using BCl3, NH3, and H2 at 1373 K. The coated preforms were densified using a forced-flow chemical vapor infiltration (FCVI) technique developed at ORNL.

The Preparation and Economics of Silicon Carbide Matrix Composites by Chemical Vapor Infiltration

ABSTRACTThis paper describes a number of processing techniques that are currently in use for the development and production of continuous fiber reinforced ceramic composite materials. The limited number of available processing routes are compared with respect to the resulting material properties.As it appears, the Chemical Vapor Infiltration technique is one of the most extensively developed methods. During the last decade, at least five different modifications of the isobaric isothermal CVI principle have been developed; each route having its own benefits. CVI techniques have now been developed to the extent that industrial commercialization is being realized. Projected cost aspects of the various CVI manufacturing techniques have been examined and compared.

Thermomechanical durability of CVI-processed two-dimensionally woven and three-dimensionally braided SIC fibre-reinforced SiC composites

The thermomechanical durability of a two-dimensionally woven and a three-dimensionally braided SiC/SiC composite fabricated by chemical vapour infiltration technique were studied. The effects of thermal shock, thermal ageing, and thermal cycling at high temperatures on mechanical properties were determined. The thermal degradation mechanisms were also investigated. The results indicated that the fibre geometry, matrix porosity, interlaminar shear strength and testing conditions affect the thermomechanical durability of the SiC/SiC composites.

Evaluation of matrix and interface properties in SiC/SiC composites

It has become increasingly important to characterize composite matrix materials in terms of processing-microstructure-property relationships. The characteristics of composite matrices can differ significantly from monolithic materials processed using more conventional techniques such as hot-pressing or sintering. Knowledge of the mechanical and thermophysical properties of the matrix as a function of processing parameters is necessary for the modeling and design of composite materials. In this study, the microstucture and mechanical properties of the matrix in the Nicalon/SiC composite system have been characterized as a function of processing temperature.The composites were fabricated by stacking multiple layers of plain-weave Nicalon fabric rotated in a 30°-60°-90° sequence within the cavity of a graphite holder. The preforms were then coated with carbon at 1375 K and 3 kPa. The coated preforms were densified using a forced chemical vapor infiltration (FCVI) technique developed at ORNL. The SiC matrix was produced by the decomposition of methyltrichlorosilane carried in hydrogen at temperatures of 1000, 1100, 1200, 1300, and 1400°C in order to reveal the effect of temperature on the properties of the composite and matrix.

The preparation of C/C-SiC nanomatrix composites by chemical vapour infiltration

C/C composites are widely used as structural materials at high temperature in the aeronautic and space industries [1, 2]. A serious drawback, however, is that carbon in any form will react with oxygen, burning away rapidly at temperatures as low as 500 °C. Since the C/C composites are generally made of several types of carbon, each having its own reactivity to oxygen, the reactivity of carbon matrix would influence the oxidation performance of the C/C composites. In order to improve the oxidation resistance, carbon matrix has been partially or even totally replaced by other refractory materials which can be carbide or nitride. This family of materials were defined as hybrid carbon-ceramic composites, which can be made according to a two-step chemical vapour infiltration (CVI) procedure [3]. One material receiving considerable attention recently is the "functionally graded material" in which the material properties are continuously changed by gradually varying the dispersion-to-matrix ratio from one surface of the material to the other surface. Among these "functionally graded materials", one that consisting of silicon carbide (SIC) and carbon (C) was prepared by the chemical vapour deposition (CVD) technique through a codeposition process using multicomponent gas reactions. This material is called a nanocomposite because of the SiC of nm size dispersed in the pyrocarbon coating. An improvement in the oxidation resistance has been reported when a small amount of SiC (< 5 tool %) is combined with the carbon matrix by CVD [4]. CVD SiC-C containing a trace of carbon was shown to be more resistant to abrasion than CVD SiC. The aim of this study was to prepare carbon fibre-reinforced C-SiC nanomatrix composites, but not nanocomposites, by the CVI technique through the codeposition process. This letter describes the microstructures and properties of the C/C-SiC nanomatrix composite prepared by codeposition. The experimental set-up is illustrated in Fig. 1. CH3SiC13 liquid and C2H2 gas were used as the source, and H2 and N2 gases were used as the carrier and dilutant gases, respectively. The deposition temperature was selected as l l00°C, the total pressure 1000 Pa and CH3SiC13 was carried into the furnace by bubbling H2 carrier gas. The CH3SiC13 vapour flow rate was controlled by the H2 carrier gas flow rate. The gas flow rates were N2 0.1 m 3 h -1 , H 2 0.0018-0.0024 m 3 h -1 and C2H2 0.02 m 3 h -1. The carbon fibre preform was polyacrylonitrile-based carbon felt of density about 0.1 g cm -3. After

Pulsed-power volume-heating chemical vapor infiltration

The dynamic behavior of a novel chemical vapor infiltration (CVI) technique called pulsed-power volume-heating CVI is investigated using a diffusion-reaction model. In this technique, a volume-heating source (e.g., RF or microwave) is used to heat the preform. The source power is modulated in time (e.g., square-wave modulation) with a specific period and duty cycle. During the low-power part of the cycle, the temperature of the composite drops, reducing the reaction rate and thus allowing the precursor gas to diffuse into the composite, essentially “refilling” the composite. This alleviates reactant concentration gradients within the composite minimizing density nonuniformities. The high-power part of the cycle is used to achieve rapid reaction rates, thereby minimizing processing time. CVI of a carbon fiber preform with carbon resulting from methane decomposition is taken as an example to illustrate the technique. The results reveal the dependence of density uniformity and processing time on relevant variables such as pulse period, duty cycle, power density level, and methane mole fraction. It is shown that pulsed-power volume-heating CVI can provide a window of operating conditions leading to rapid and complete densification.

Fiber-Matrix interactions in brittle matrix composites

Brittle matrix composites, including carbon-carbon (C-C) and ceramic matrix, offer a new dimension in the area of high-temperature structural materials. Fiber-matrix interactions determine the mechanism of the load transfer between the fiber and matrix and resulting mechanical properties. Composites studied in this work include a C-C composite densified with a chemical vapor infiltration (CVI) pyrolytic carbon, silicon carbide fiber-silicon carbide matrix composite, and carbon fiber-silicon carbide matrix composites densified by the CVI technique. The type of the interfacial carbon in C-C composites was found to control their mechanical properties. The presence of the compressive stress exerted by the matrix on the carbon fibers was attributed to an increase in flexural strength. The transverse matrix cracking in C/SiC composites was believed to cause a lowering in the flexural strength value. Brittle fracture behavior of SiC/SiC composites was correlated with the presence of an amorphous silica layer at the fiber-matrix interface.

Metallisation and Silicidation of Porous Silicon

ABSTRACTThin films of cobalt, rhenium, or cobalt silicide have been deposited down the pores of porous silicon layers, using HCo(CO)4, HRe(CO)5, and SiH3Co(CO)4as precursors in a Chemical Vapour Infiltration and Decomposition (CVID) technique.

The Effect of Fiber Coatings on Interfacial Shear Strength and the Mechanical Behavior of Ceramic Composites

AbstractThin coatings deposited on ceramic fibers prior to densification employing chemical vapor infiltration techniques have been used to limit fiber-matrix bonding. This has resulted in improvements in strength and toughness at room and elevated temperatures in Nicalon® fiber-reinforced/SiC matrix composites. The properties of the fiber-matrix interface in fiber-reinforced ceramic composites have been examined utilizing an indentation method in which a standard microhardness indentor is used to push on fibers embedded in the ceramic matrix. Compositions and microstructures at the interface have been characterized employing analytical electton microscopy. Correlations between interfacial phenomena and observed mechanical behavior have been made.

PROCESSING OF METAL AND CERAMIC MATRIX COMPOSITES WITH THREE-DIMENSIONAL BRAIDED REINFORCEMENT

ABSTRACT This paper summarizes some of the pioneering work done in developing the processing techniques to consolidate metal and ceramic matrix composites with 3-D braided fiber architecture. The 3-D braided fiber architecture features a fully integrated structure with multidirectional reinforcement and allows for the braiding of complex shapes. For metal matrix composite, a 3-D braided AI2O3 fibers preform reinforced Al-Li matrix composite has been successfully fabricated by liquid vacuum infiltration methods. For ceramic matrix composites, chemical vapor infiltration technique has been used to densify a 3-D braided Nicalon fibers preform with SiC matrix composite. A significant improvement in composite performance can be achieved through the architecture of 3-D integrated reinforcement geometry. The future needs and directions for developing viable methods for fabricating strong, tough high temperature composites with 3-D reinforcements is also discussed.

Alumina Fiber/Alumina Matrix Composites Prepared by a Chemical Vapor Infiltration Technique

Ceramic/ceramic composites made of alumina fibers embedded in an alumina matrix were obtained from fibrous alumina preforms (Vf = 0.12 to 0.40) using a chemical vapor infiltration technique based on gaseous alumina precursors (AICI3-H2-CO2). Low deposition temperatures (950° to 1000°C) and total pressures (2 to 3 kPa) must be used to preferentially deposit alumina within the pores rather than on the external surface of the preform. Different fiber orientations were studied; i.e. randomly oriented short fibers and one- and two-dimensional preforms. Densification was performed down to residual open porosity of the order of 10 to 15%. For a one-dimensional α-alumina fiber composite (residual porosity ≅10%), flexural strengths of 250 MPa and ≅ 100 MPa were obtained at room temperature and 1200°C, respectively. Most composites exhibited brittle failure at room temperature.