Chemical vapor infiltration process modeling and optimization

Chemical vapor infiltration is a unique method for preparing continuous fiber ceramic composites that spares the strong but relatively fragile fibers from damaging thermal, mechanical, and chemical degradation. The process is relatively complex and modeling requires detailed phenomenological knowledge of the chemical kinetics and mass and heat transport. An overview of some of the current understanding and modeling of CVI and examples of efforts to optimize the processes is given. Finally, recent efforts to scale-up the process to produce tubular forms are described

Aircraft System and Product Development: Teaching the Conceptual Phase

This paper reports the first offering of a graduate level subject covering the conceptual phase of aircraft product development. The output of the conceptual phase is a system level specification that usually serves as the input for a traditional undergraduate capstone subject on aircraft design. Of critical importance in the conceptual phase is addressing the business case for the candidate product. The conceptual phase spans a much wider range of topics than the technical issues which dominate preliminary design. These include user needs, investment and business requirements, market analysis, operational issues, exogenous constraints (certification, regulation, political, etc.), as well as engineering and manufacturing requirements. Students in the subject were required to Prepare for the Board of Directors of a large aerospace company a compelling business case and specification for a large jet transport product. Three student teams produced original responses to the challenge and have reported their findings in a companion AIAA paper. This paper addresses the pedagogical approaches and outcomes. These encompass the use of distance learning technology and techniques for several off-campus practicing engineering students. Overall, the outcome was very gratifying. The class will be offered in the spring of 2001, focusing on a supersonic business jet

Fabrication and testing of corrosion resistant coatings

The susceptibility of SiC and Si{sub 3}N{sub n} to sodium corrosion mandates that corrosion resistant coatings be developed to protect silicon-based turbine engine components. Materials with good corrosion resistance and thermal expansions that nearly match SiC and Si{sub 3}N{sub 4} have been identified. Corrosion testing of hot-pressed pellets of these compounds has identified the most promising materials. Development of chemical vapor deposition system to apply these materials has been initiated. 20 refs., 3 figs

DEVELOPMENT OF HIGH DENSITY FIBER REINFORCED SILICON CARBIDE FCVI COMPOSITES

Silicon carbide continuous fiber reinforced composites are prepared by Forced flow thermal gradient Chemical Vapor Infiltration. In order to obtain high density and homogeneous ceramic composites the effects of time and thermal gradient across the preform are studied during infiltration. Cross sections of the infiltrated composites are studied with the aid of optical microscopy and image analysis. This manuscript gives an impression of the experimental composite densities and density gradients achieved and critical aspects that play a role in the infiltration process

Partitioning of cesium in hydrofracture grouts

Phase characterization of hydrofracture grouts was accomplished with the use of optical microscopy, scanning electron microscopy, x-ray diffraction, and ..beta..-..gamma.. autoradiography. A laboratory-produced sample containing 1 wt % stable cesium and an actual hydrofracture grout sheet obtained by core dirlling were examined during this work. The phases present in these samples were identified and cesium was found to be absorbed almost entirely by illite clay agglomerates. These clay agglomerates were tightly bound within the grout structure by hydrated calcium silicates. The ..beta..-..gamma.. autoradiography of the core-drilled sample verified that cesium and other radionuclides were trapped within the 20-year-old grout and had not migrated into trapped shale fragments. 14 references, 3 figures, 1 table

Approach to inherently stable interfaces for ceramic matrix composites

Virtually all ceramic matrix composites require and interface coating between the fibers and matrix to achieve the desired mechanical performance. To date, the most effective interface materials for non- oxide matrix composites have been carbon and boron nitride. They are, however, susceptible to oxidation at elevated temperatures, and thus under many envisioned operating environments they will fail, possibly allowing oxidation of the fibers as well, adversely affecting mechanical behavior. Current efforts are directed toward developing stable interface coating, which include oxides and silicon carbide with appropriate thermomechanical properties

A METHOD FOR RAPID CHEMICAL VAPOR INFILTRATION OF CERAMIC COMPOSITES

Des procédés de préparation de composites structuraux ont été développés au laboratoire national d'Oak Ridge (ORNL). Les composites sont préparés par infiltration de préformes fibreuses en céramique, à haute température, à partir de réactants gazeux qui se décomposent pour libérer la matrice céramique entre les fibres et autour. Le procédé ORNL est une nette amélioration des procédés classiques ; les temps de densification classiquement de plusieurs semaines sont réduits à quelques 24h. Des échantillons de haute densité (90 % de la densité théorique) avec des résistances de l'ordre de 400-450MPa ont été obtenus. De plus, la rupture de ces matériaux n'est pas catastrophique, montrant ainsi un réel comportement composite.Processes for the preparation of composite bodies using chemical vapor deposition have been developed at Oak Ridge National Laboratory (ORNL). Composites are prepared by infiltrating ceramic fiber preforms, held at elevated temperatures, with reactant gases that decompose to deposit ceramic matrix material between and around the fibers. The ORNL process is a marked improvement over those commonly in use ; preforms that previously required weeks to densify now require ~24 h. Specimens with densities up to 90 % of theoretical and strengths in the range of 400 to 450 MPa have been produced. Most importantly, the materials fail noncatastrophically, exhibiting typical composite behavior

CVI processing of minicomposites for evaluation of interface coating materials in composites

Continuous fiber ceramic matrix composites require fiber/matrix interfaces which allow load transfer from the matrix to the fibers when the composite materials are stressed. Crack deflection and fiber pullout are also necessary components of the mechanical behavior of composites. Screening interface materials and determining their optimum characteristics is a lengthy and expensive procedure if standard chemical vapor infiltration composite processing is used. A procedure for fabricating minicomposites was developed to address this problem. Minicomposites require very little material and much less labor than is necessary to produce a standard composite. Also, the mechanical property measurements made on minicomposites target the behavior of the interface coatings and their effect on the properties of the composite. Tensile testing of minicomposites was used to optimize the matrix infiltration process and will be utilized in the future to study the mechanical behavior of new materials systems, and specifically, new interface coating materials

Development of sol-gel derived coating for NICALON{sup TM}/SiC composites

Mullite and aluminum titanate precursor polymeric sols were developed for applying as coatings on Nicalon{trademark} fabrics and tows. A Nicalon{trademark}/SiC composite with a mullite interface was fabricated. The mullite precursor interface coatings were applied by a vacuum infiltration method and the SiC matrix was deposited by a forced flow chemical vapor infiltration process. Thin, uniform mullite interface coatings were obtained. However, the Nicalon{trademark}/SiC composite exhibited brittle fracture. Mullite and alumina-titania coatings were applied on Nicalon{trademark} tows and the effect of heat treatment at 1000{degrees}C in air is discussed