Forced-flow Thermal-gradient Chemical Vapor Infiltration Research Articles

Microstructure of C<sub>f</sub>/SiC Composite with Precursor of Hexamethyldisilazane by Chemical Vapor Infiltration

A method of waved-thermal field chemical vapor infiltration was introduced. And interphases of silicon carbide layer and carbon layer were processed via the route. The preforms with the interfacial coatings were densified by method of forced-flow thermal-gradient chemical vapor infiltration (FCVI) employing hexamethyldisilazane (HMDS) as precursor material of the matrix. The matrix of the composites annealed at 1400°C consists of nano-polycrystalline silicon carbide. The configuration of fracture surface was observed by scanning electronic microscopy (SEM). The interphases behaved successfully as mechanical fuse for the reinforcing fibers.

Ceramic Composites with Multilayer Interface Coatings

Silicon carbide matrix composites have been fabricated from either ceramic‐grade NicalonTM or Hi‐NicalonTM fibers coated with an interface material consisting of six alternating carbon and silicon carbide layers. Initial efforts involved the use of chemical vapor infiltration to produce minicomposites (single tows of fibers). In subsequent work, forced‐flow thermal‐gradient chemical vapor infiltration was used to produce a single composite plate with a multilayer interface from ceramic‐grade Nicalon fabric and two plates from Hi‐Nicalon fabric, one with a single carbon layer and one with a multilayer interface. Tensile testing of the minicomposites and of specimens cut from the plates revealed typical composite behavior and strengths for the as‐processed samples. Exposure of tensile specimens to 950°C air for 100 h resulted in large losses in strength and strain tolerance regardless of the interface coating. The results demonstrate that forced‐flow thermal‐gradient chemical vapor infiltration can be used to prepare multilayer interface material. The results also verified that relatively thick (>100 nm) single or multiple carbon layers are susceptible to oxidation that causes the loss of composite properties.

Effects of deposition temperature on microstructure of laminated (SiC-C) matrix composites

The microstructure of fiber-reinforced laminated SiC-C matrix composites has been studied using scanning and transmission electron microscopy (SEM and TEM) techniques. Different regions of the composites were found to exhibit microstructural differences due to the temperature gradient imposed during composite fabrication by the forced-flow thermal-gradient chemical vapor infiltration process. The matrix layers of alternating Carbon and SiC were found to be thicker but less clearly defined in the higher-temperature regions than in the lower-temperature regions. This feature was found to be more pronounced with increasing distance from the fiber surface. The medium temperature region was found to represent the intermediate point in the microstructural development of this composite. Possible qualitative explanations for the observed microstructural differences are suggested in the context of the temperature gradient and other process parameters employed.

Characterization of interfaces in C fiber-reinforced laminated C–SiC matrix composites

Abstract Interfacial studies of carbon fiber-reinforced laminated matrix composites (LMCs) of alternating matrix layers of SiC and carbon prepared by forced-flow thermal-gradient chemical vapor infiltration (FCVI) have been undertaken by transmission electron microscopy (TEM). The carbon–carbon interfacial region consistently displayed distinctive features that enabled the microstructures of the fiber carbon, a thin carbon layer deposited as fiber coating, and the matrix carbon to be distinguished. A thin discontinuous layer of relatively lower graphitic degree was observed at the interface of the fiber and its carbon coating, whereas the interface between the fiber coating and the first carbon layer of the matrix displayed a thin, patchy, and partially ordered layer. At the C–SiC interfaces within the matrix, the interface was observed to be well-defined on one side with the c -axis of the pyrocarbon layers nearly perpendicular to the interface, but, at the other end, the interface appeared rough and somewhat diffuse due to the nature of SiC growth during CVI.

Growth and Texture of Polycrystalline SIC on Graphite During Forced flow Thermal Gradient Chemical Vapor Infiltration (FCVI)

Abstract Silicon carbide is a versatile material possessing properties such as a wide energy bandgap, high thermal conductivity, high elastic modulus and high-temperature creep resistance, which enable it to be used in a variety of electronic, optical and structural applications. Chemical vapor infiltration/deposition (CVI/CVD) coupled with the application of a temperature gradient and forced flow of reagents is particularly suited to the production of SiC structural composites due to the benefits of reduced infiltration time and uniform composite density. In this work, the growth and orientation of polycrystalline SiC on graphite during CVI is investigated using TEM and HRTEM. The composites studied possess a laminated matrix of alternating layers of carbon and SiC which were deposited by alternating the reagent streams from one layer to the next. Specimens for TEM examination were obtained by cutting ∽1 mm thickness slices from the bulk sample with a low speed diamond saw.

Model for prediction of matrix microstructure for carbon/carbon composites prepared by forced flow-thermal gradient CVI

A model has been developed to predict the microstructure of the carbon matrix deposited during forced flow-thermal gradient chemical vapor infiltration (FCVI). This method employs previous thermodynamic-microstructure and densification models to determine conditions throughout the preform as a function of time. The model was verified by comparison with samples prepared over a range of deposition temperatures, reagent concentrations and flow rates. The model also showed that it should be possible to deposit a matrix of uniformly high thermal conductivity onto conventional size carbon fibers, as well as small diameter, low cost, high thermal conductivity carbon whiskers.

Carbon/carbon processing by forced flow-thermal gradient chemical vapor infiltration (FCVI) using propane

A 2 3 factorial statistically designed experiment was used to study fabrication of carbon/carbon composites using the forced flow-thermal gradient chemical vapor infiltration process. Propane, diluted with hydrogen, was used as the feed. The independent variables were the temperature of the bottom of the preform holder, the concentration of the reagent, and the total flow rate. The response variables were infiltration time, final porosity, rate of weight gain, and uniformity of infiltration. It was found that the infiltration time was affected only by the temperature of the preform and the concentration of the reagent. Infiltration times as low as 7 hours were achieved. The bulk density of the final composite was found to be independent of the operating conditions, provided the process was continued until a fixed pressure drop was achieved. Density variations with position within a given composite were no more than ± 0.08 g cm −3. Deposition efficiency ranged from 3 to 14% and increased with temperature while decreasing with flow rate. The values of the coating thickness near the hot and cold sides of the composite were used to calculate the rate constant for carbon deposition from propane. The activation energy was found to be from ~23.6 Kcal mol −1 and the rate constant was given by ln k s(cm s −1) = 2.2–23610/RT.

A CVI-processing marker study

Fiber reinforced ceramic composites greatly extend the region of application for ceramic materials because the inclusion of a second phase such as fibers helps to prevent catastrophic failure via the mechanisms of crack branching, deflection, bridging and fiber pull-out. While ceramic composites are excellent candidate materials for high temperature structural applications, many conventional manufacturing processes damage the fibers during fabrication and thereby reduce the strength and toughness from what is theoretically possible. Chemical vapor infiltration (CVI) is an effective and versatile technique for fabrication of ceramic matrix composites which promises to reduce fiber degradation.While CVI is no longer a truly novel processing technique, it is still difficult to predict the best set of operating conditions for what can be a tedious process. Conventional CVI methods may take up to a week to fully infiltrate a small part and even state-of-theart forced flow-thermal gradient CVI (FCVI) processing takes the better part of a day. It is, more over, difficult to monitor the evolution of deposition within a, for example, woven preform.

Fabrication of carbon-carbon composites by forced flow-thermal gradient chemical vapor infiltration

Carbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. The preforms for the infiltration were prepared by stacking 40 layers of carbon cloth in a graphite holder. The preforms were infiltrated with carbon using propylene or methane as a reactant, with hydrogen as a diluent. Composites with porosities as low as 7% have been processed within 8-12 h. The highest deposition rate obtained in the present study was ∼3 μm/h, which is more than an order of magnitude faster than the typical value of 0.1-0.25 μm/h for the isothermal infiltration process.

Forced flow-thermal gradient chemical vapor infiltration (FCVI) for fabrication of carbon/ carbon

Carbon/carbon composites with porosities as low as 7% were fabricated within 8–12 hours using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. Preforms consisting of 40 layers of T-300 plain weave carbon cloth were infiltrated with a feed containing a carbon source and a diluent. The carbon sources studied in the present work included propylene, propane, and methane and the diluent was hydrogen. Shorter processing times were obtained when propylene and propane were used as compared to methane. The highest deposition rate obtained in the present study was ~ 3 μm/h which is more than an order of magnitude faster than the typical value of 0.1–0.25 μm/h for the isothermal infiltration process. In the infiltrated composites it was observed that the tows in a cloth were appreciably infiltrated, independent of their position in the preform. Whereas, the coating thickness between the tows and cloth layers strongly depended on the temperature, i.e. position within the preform.

Rapid Processing of Carbon-Carbon Composites by Forced Flow-Thermal Gradient Chemical Vapor Infiltration (FCVI)

ABSTRACTCarbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. Preforms were prepared by stacking 40 layers of plain weave carbon cloth in a graphite holder. The preforms were infiltrated using propylene, propane, and methane. The present work showed that the FCVI process is well suited for fabricating carboncarbon composites; without optimization of the process, we have achieved uniform and thorough densification. Composites with porosities as low as 7% were fabricated in 8–12 h. The highest deposition rate obtained in the present study was ∼3 μm/h which is more than an order of magnitude faster than the typical value of 0.1–0.25 μm/h for the isothermal process. It was also found that the use of propylene and propane as reagents resulted in faster infiltration compared to methane.

Fiber-Reinforced Tubular Composites by Chemical Vapor Infiltration°

AbstractA forced-flow thermal-gradient chemical vapor infiltration process has been developed to fabricate composites of thick-walled tubular geometry common to many components. Fibrous preforms of different fiber architectures (3-dimensionally braided and filament wound) have been investigated to accommodate components with different mechanical property requirements. This paper will discuss the fabrication of tubular, fiber-reinforced SiC matrix composites and their mechanical properties.