Fabrication of Carbon/Silicon Carbide Composites by Isothermal Chemical Vapor Infiltration,Using the In Situ Whisker-Growing and Matrix-Filling Process

C/SiC composites were prepared via isothermal chemical vapor infiltration (ICVI). A novel process of in situ whisker growing and matrix filling during ICVI was devised to reduce the porosity of the C/SiC composites, by alternating the dilute-gas species. C/SiC composites with increased density were prepared successfully using this novel process, in comparison with those obtained from the conventional ICVI process. The whiskers seem to have grown into the large pores and modified the pore structure that is filled by the SiC matrix.     

等温化学气相渗透制备碳化硅复合材料的工艺研究

通过研究等温化学气相渗透(ICVI)工艺的主要影响因素,如渗透温度、前驱气体组分比与流速等,对制件密度的影响规律,为 ICVI 制备碳化硅复合材料提供了优化的工艺参数.实验表明:渗透温度为 1050℃,炉压为6666.2Pa (50乇),反应气体配比Ar/CH3SiCl3(iTS)约为 10,仅渗透 3h 表面的渗透厚度便接近 20μm,密度略有增加,渗透效果较好.     

先进陶瓷基复合材料制备技术-CVI法现状及进展

化学气相渗透法(CVI)是制备先进陶瓷基复合材料最赋潜力的技术.本文概要阐述了CVI法的原理与动力学机制,论述了CVI先进陶瓷基复合材料中纤维、基体、界面的研究现状,对不同类型的CVI工艺及目前的CVI模拟技术作了一定的评价,提出了CVI技术的发展方向和研究课题.     

Time-Dependent Solution to the Tai-Chou Chemical Vapor Infiltration Model

A recently proposed steady-state isothermal model for chemical vapor infiltration in parallel cylindrical pores has been extended to include transient behavior. Availability of a time-dependent solution should prove useful in comparison with experimental data and validation of the assumed boundary conditions. Attention is also drawn to analogous problems in the heat transfer literature which may prove useful in development of enhanced vapor infiltration models of this class.     

Overlap Model for Chemical Vapor Infiltration of Fibrous Yarns

Attention is focused on fabrication of fiber-reinforced ceramic composites by chemical vapor infiltration. A model is proposed for infiltration of a cylindrical fiber bundle, or yarn strand, comprised in turn of small cylindrical fibers. Along any cross section perpendicular to the yarn axis, the centers of the fibers are assumed to be randomly distributed throughout the cross-sectional area without fiber overlap. As infiltration/densification proceeds, growth is assumed to occur via sequential deposition of uniform layers such that the fiber-matrix composite consists of growing cylinders whose edges eventually overlap. Based on the random overlap model, expressions for key time-dependent properties are developed for both the reaction-limited case and for cases with significant diffusional limitations. An analytical solution to the resulting equations is obtained for the reactionlimited case whereas numerical solutions are required for the diffusion-limited case. The effects of geometric, kinetic, and diffusional parameters on the infiltration dynamics are explored.     

Analytical Modeling of Chemical Vapor Infiltration in Fabrication of Ceramic Composites

A model for chemical vapor infiltration is applied to the study of the growth of alumina from the chemical reaction among AlCl₃, H₂, and CO₂ within a SiC-fiber bundle which is situated in an isothermal hot-wall reactor. The pore space between the fibers is simulated by cylindrical capillary tubes. The model considers binary diffusion of CO₂ and H₂, chemical reaction on the inner surface of the tube, and deposition film growth. Furthermore, diffusion-controiled and chemical-reaction-controlled processes are taken into account to determine the dominating process in chemical vapor infiltration. Both molecular diffusion and Knudsen diffusion are considered sequentially in this model during the infiltration process. Based upon this model, the optimum processing conditions required for chemical vapor infiltration to form a SiC/Al₂O₃ composite can be predicted for different fiber preform systems.     

Modeling of Chemical Vapor Infiltration for Ceramic Composites Reinforced with Layered, Woven Fabrics

A homogeneous model is developed for the chemical vapor infiltration by one-dimensional diffusion into a system of layered plies consisting of woven tows containing bundles of filaments. The model predictions of the amount of deposition and the porosity of the sample as a function of time are compared with the predictions of a recent nonhomogeneous model with aligned holes formed by the weave. The nonhomogeneous model allows for diffusion through the aligned holes, into the spaces between plies, and into the gaps around filaments; i.e., three diffusion equations apply. Relative to the nonhomogeneous results, the homogeneous model underestimates the amount of deposition, since the absence of holes and spaces allows earlier occlusion of gaps around filaments and restricts the vapor infiltration.     

Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

Modeling of Chemical Vapor Infiltration Rate Considering a Pore Size Distribution

To analyze chemical vapor infiltration (CVI) rates, a simulation model (PC model) was proposed, which is capable of considering difficulties of densification in the internal regions of porous bodies and the pore closure phenomena. In the model, a fictitious tapered pore is constructed based on the pore size distribution of the preform body, and variation of the pore shape during the process is calculated. For simulation study, fictitious tapered pores were constructed using different pore size distribution functions of Gaussian, δ-function, and flat distribution, and their shapes and characteristics were compared. In the case of the Gaussian flat distribution (large dispersion), the fraction of fine pores was large and these pores became closed near their mouths. Reconversion of temporal fictitious pore shape into the pore distribution function was attempted and the PC model was successfully applied to monitor variations in the pore size distributions during the CVI process.     

Microstructure and Mechanical Properties of Three-Dimensional Textile Hi-Nicalon SiC/SiC Composites by Chemical Vapor Infiltration

Three-dimensional textile Hi-Nicalon SiC-fiber-reinforced SiC composites were fabricated using chemical vapor infiltration. The microstructure and mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g·cm⁻³ after the three-dimensional SiC perform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300°C under vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mismatch of thermal expansion coefficients between fiber and matrix. The fracture toughness was 30.2 MPa·m^1/2. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance, and the dynamic fracture toughness of 36.0 kJ·m⁻² was measured using Charpy impact tests.     

Chemical Vapor Deposited Sic Matrix Composites

Composites of Sic yarnlchemical vapor deposited Sic matrix and multitei chemical vapor deposited Sic matrix were prepared using methyldichloro-silane and applying a thermal gradient in a low-pressure reactor environment. The preparation parameters were selected from an X-ray study where the pressure and temperature were varied. The Sic-reinforced composite had a higher strength (450 versus 120 MPu) and exhibited more fiber pullout than the mullite-reinforced composite.     

Experimental Whisker Growth and Thermodynamic Study of the Hafnium-Carbon System for Chemical Vapor Deposition Applications

Chemical vapor deposition of nonstoichiometric hafnium carbide is studied by computer minimization of the Gibbs free energy of the system. Isostoichiometric curves in the HfC_{1- x} region of the deposition diagram are calculated as a function of HfCI₄, CH₄, and H₂ concentration and reaction temperature. Possible experimental reactions are investigated using thermodynamic efficiency and partial pressure diagrams. A comparison between the calculated solid-phase regions and the HfC phase diagram is given, and experimental results for HfC whiskers are also compared with the calculations.     

Kinetic Analysis of Chemical Vapor Deposition of Boron Nitride

BN was deposited on Al₂O₃ substrates from the BCl₃─NH₃─Ar reagent system. An impinging jet reactor configuration was used to obtain kinetic data in the temperature range of 800° to 1000°C and in the pressure range of 4 to 20 kPa. The BN deposition could be described by a simple kinetic rate expression with an activation energy of about 39 kcal/mol and a first-order dependency on BCl₃ concentration. The BN deposition rate increased with temperature, pressure, and BCl₃ concentration. The microstructure of the BN coatings was not strongly influenced by the process parameters.     

Simultaneous Chemical Vapor Deposition of Boron Nitride and Aluminum Nitride

Two chemically different phases, hexagonal BN and AIN, were simultaneously produced by chemical vapor deposition (CVD) using an impinging jet reactor and the BCl₃─AlCl₃─NH₃─Ar reagent system. The microstructure of the BN + AIN composite coatings was strongly dependent on temperature, pressure, and BCl₃ and AlCl₃ concentrations. The growth characteristics of BN and AIN in the codeposition system were similar to those expected from the single-phase deposition processes (i.e., BN-CVD and AIN-CVD), except the growth of AIN whiskers was accentuated, and competition between BN and AIN deposition in the composites was suspected to be the cause of less-crystalline deposits. In both BN + AIN-CVD and AIN-CVD, the growth of AIN whiskers became more apparent with increasing pressure or temperature. The codeposition behavior observed experimentally was compared with thermodynamic predictions.     

Nanofiber Formation in the Fabrication of Carbon/Silicon Carbide Ceramic Matrix Nanocomposites by Slurry Impregnation and Pulse Chemical Vapor Infiltration

The objectives of this work were to investigate the fabrication of carbon-fiber-reinforced SiC ceramic nanocomposites using the slurry impregnation process and the pulse chemical vapor infiltration (PCVI) process and to study the influences of processing parameters of the PCVI process on the microstructure variation of the nanocomposites. In this work, SiC nanosized powder was added to the matrix precursor (silicon powder mixed with phenolic resin), followed by the impregnation of the slurry into the preform. In the PCVI process, to densify the nanocomposites, tetramethylsilane (TMS) vapor mixed with hydrogen was used as the vapor precursor for matrix deposition. Fabrication parameters, such as reactant concentrations, pulse number, and holding time, were studied. Morphologies obtained from various processes were compared.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

Kinetic and Thermodynamic Analyses of Chemical Vapor Deposition of Aluminum Nitride

AIN coatings were prepared by chemical vapor deposition from the AlCl₃—NH₃—Ar reagent system using an impinging jet reactor in the temperature range of 700° to 1100°C. A mass transfer model and thermodynamic calculations were used to analyze the deposition data. The AIN-CVD process could be approximated by calculating mass transfer—thermodynamic limits at low AlCl₃ concentrations. The AIN deposition rate decreased drastically with increasing temperature above 1000°C in agreement with thermodynamic predictions. At high AlCl₃ concentrations, a surface kinetic mechanism involving AlCl₃ adsorbed on the deposition surface appeared to be the rate-limiting step. The AIN deposition rate decreased on increasing the AlCl₃ concentration or total pressure. The crystalline structure of AIN was strongly influenced by the processing parameters. The AIN coatings became highly crystalline and preferentially oriented with an increase in the AlCl₃ concentration or pressure.     

Chemical Vapor Infiltration Rates of ZrO2 into MoSi2 Porous Bodies and Comparison with Model Calculation

Zirconia (ZrO₂) was chemical vapor infiltrated (CVI) into a partially sintered MoSi₂ body (preform) by using zirconium n -propoxide (Zr(OC₃H₇)₄) as a gas precursor. Infiltration distances at different conditions were compared with the calculated results. Chemical vapor deposition (CVD) film growth rates of ZrO₂ were measured, and the data were incorporated into the model calculations. Two models were used to analyze the observed infiltration distances. Initially a conventional model assuming a pore with constant radius (SP model) was used. With this model, it was possible to predict the approximate infiltration distance. However, the model cannot predict pore closure and the infiltration distances for a variety of CVI conditions. Secondly, a newly proposed model (PC model) from a previous paper was applied to calculate the infiltration distance. Using this model, it was possible to predict the occurrence of pore closure or the formation of the deposition layer on the preform surface.     

Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride

An experimental study has been performed to gain some insight into the correlations between the deposition conditions and the structure of boron nitride (BN) coatings that are used in ceramic-matrix composites. BN has been deposited at 700°C from BCl₃-NH₃-H₂ mixtures on various substrates, by using chemical vapor deposition (CVD) and isothermal-isobaric chemical vapor infiltration (ICVI) processes, simultaneously in the same reactor. A kinetic study has shown that the CVD process is governed either by a combination of mass transfer with chemical kinetics at low flow rates or by the heterogeneous kinetics only at high flow velocities. In contrast, the limiting contribution of mass transfer always is observed for the ICVI process. The influence of diffusion cages that are positioned around the fibrous preforms is reported. The structure of BN deposits has been studied as a function of the various deposition conditions via transmission electron microscopy. The chosen CVD conditions lead to a poor organization of the BN deposits. Fairly well-organized BN coatings are deposited on all fibers of a fibrous preform via ICVI. The results are discussed in terms of supersaturation and deposition yields. The use of diffusion cages and the adjustment of the inlet composition and mass flow rate seem to be very important to obtain the best BN organization and thickness uniformity.     

Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite

A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC composite was fabricated via simple in situ growth of SiC nanowires directly in the fibrous preform before CVI matrix densification; the purpose of the SiC nanowires was to markedly improve strength and toughness. The nanowires consisted of single-crystal β-phase SiC with a uniform ∼5 nm carbon shell; the nanowires had diameters of several tens to one hundred nanometers. The volume fraction of the nanowires in the fabricated composite was ∼5%. However, the composite did not show significant increase in strength and toughness, likely because of strong bonding between the nanowires and the matrix caused by the very thin carbon coating on the nanowires. Little debonding and pullout of SiC nanowires from the matrix were observed at the fracture surfaces of the composite.