Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
2001-09-10
2004-03-16
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C428S294100, C428S107000, C428S907000, C428S698000, C264S137000, C264S257000, C264S299000, C264S447000, C264S494000, C264S463000
Reexamination Certificate
active
06706401
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to high-performance, high-temperature multi-dimensionally fiber-reinforced structural composites with carbon or ceramic matrices. These composites, which may have a complex shape, possess a uniform density even with thicknesses as great as 5 inches. The present invention also relates to a rapid, low-cost method of manufacture of these composites utilizing a wetting monomer, which is polymerized into the matrix precursor inside the preform.
Fiber-reinforced composites consist of two distinct components, fibers and matrix. Fibers, either continuous or in the form of short segments, are normally oriented in preferred directions in composites to utilize as much as possible the fiber's great strength and stiffness properties. However, for low performance applications the fibers can be randomly placed to lower the cost of fabrication. Because fibers are heavily drawn and stretched during manufacture, they have properties superior to those of the same material in its undrawn and unstretched state; i.e., their bulk properties.
The matrix consists of material surrounding the fibers and has two purposes. The first is to fill the space between the fibers, which increases density and physically holds the reinforcing fibers in the preferred direction. The second is to transmit forces applied to the overall composite structure to individual fibers in such a manner as to distribute any applied forces, or loads, as nearly as possible to all fibers simultaneously. In this fashion, the high-performance fiber properties are retained by the composite since fibers bear more-or-less equal loads and hence do not break individually. This is accomplished with greatest success when all void spaces around fibers are filled in with matrix material. The void spaces are usually referred to as “porosity.”
An example of a naturally occurring composite is wood. High volume man-made composites are exemplified by polymer matrix composites, which are used principally for ambient temperature applications. The best known example of this family of composites is the fiberglass-epoxy composites used for ladders, boats and for body panels in Corvette automobiles.
For elevated temperature applications, high-temperature fiber-reinforced composites (HTFRC's) are employed. These composites consist of ceramic matrix and carbon-matrix composites reinforced with either carbon or ceramic fibers. These composites have excellent high-temperature strength retention, high strength-to-density ratio, good thermal conductivity, and possess fracture toughness. In addition, the carbon-carbon composites have high specific modulus and thermal shock resistance while the ceramic-ceramic composites have resistance to oxidation. High-performance HTFRC's are used for structural applications in aerospace and rocket propulsion, such as, heat shields, leading edges and nozzles. To fabricate a high-performance HTFRC it is necessary to employ a high volume fraction (volume occupied by fibers/volume of composite) of the proper type and orientation of high performance reinforcement fibers that are held together in the composite by a high-density, high-quality matrix material.
To make low-cost fiberglass composites, which are simply glass fiber-reinforced plastics, the process is rather straightforward. One simply fixes the fibers in the position desired in the final product and then places the fluid matrix material around the fibers. When the polymer matrix sets, the composite is ready for use. In contrast, the manufacture of high-temperature fiber-reinforced composites (HTFRC's) such as fiber-reinforced ceramic matrix composites (FR/CMC's) and carbon matrix composites, such as carbon-carbon (C—C) composites is a much more difficult and expensive process, for a number of reasons. First, there is the much higher cost of the heat-resistant reinforcing fibers themselves. Many of these high-performance filamentous materials, such as silicon-carbide fibers or graphite fibers, are extremely brittle and difficult to handle. Forming them into fibers is therefore a very laborious and time-consuming process. Secondly, depending on the technique employed, positioning of fibers within the composite component being fabricated can also be an elaborate and expensive process. And lastly, surrounding the reinforcing fibers completely with an appropriate matrix is also a labor-intensive and very time-consuming process using existing technology. This is due principally to two factors. The first is a result of the extreme difficulties associated with physically handling high-performance fibers. Because of their inherent brittleness, placing matrix material around fibers (that have been oriented in such a fashion as to maximize the resulting composite's physical properties) must be undertaken with great care. Otherwise, fiber damage will more than offset the performance potential of FR/CMC's and carbon matrix composites. The second is a result of the difficulty of getting the matrix material to fill the void space in the preform as uniformly as possible. This requires many processing cycles involving many months and high energy costs as will be described in detail below.
Two categories of commercial processes have been developed to manufacture High-temperature Fiber-reinforced Composites (HTFRC) such as, fiber-reinforced ceramic matrix and carbon matrix composites. These processes differ principally in the techniques used for the deposition of matrix materials around reinforcing fibers that have already been oriented and positioned into the locations they will occupy in finished products. One technique is vapor-phase in nature and is called “infiltration.” The other is liquid-phase in nature, and is called “impregnation.” Both of these existing techniques share a common initial step. That is the formation of a “rigid-preform”. This process can involve the holding of the fibers in the desired orientation and position in a mechanical frame and coating them with a suitable binder material, but usually involves the more simple steps of coating reinforcing fibers with a binder, which may be the same material as used to form the matrix, and then forming them into the desired shape by filament winding, hand lay-up, weaving, braiding, or some other means. This coated fiber preform is then heated to high-temperatures, with appropriate means taken to prevent loss of preform shape. The result of the heat treatment is the conversion of the binder to an inorganic cement. At this point any mechanical means of holding the fibers may be removed. The purpose of this cement, which can be produced from either a ceramic or hydrocarbon precursor, is to hold the reinforcing fibers in the shape desired for the final product. The ensemble of cemented fibers is called the rigidized-preform which is then subjected to subsequent processing. The task of heating the binder, or other materials used in HTFRC fabrication, to high temperatures to effect a change in chemical composition is usually referred to as “pyrolysis.” In most cases, this modification of the binder is from an organic to an inorganic substance. The cement formed by pyrolysis of the binder is very porous because of the relationship between pyrolysis efficiency and binder physical-property requirements. As mentioned previously, great care must be taken when handling high-performance fibers or the resulting damage will greatly diminish composite properties. This means that forces encountered by the reinforcing fibers during coating and positioning must be minimal. This can only occur if binder viscosity is low and care is taken in handling. Unfortunately, pyrolysis efficiency (the weight percent of binder remaining after pyrolysis) is usually found experimentally to increase only when binder viscosity is high. One solution to this dichotomy is the following current manufacturing methodology: keep rigid-preform performance potential high by utilizing low viscosity binders, and adjust for the resulting high initial porosity with subsequent pro
Hoffman Wesley P.
Jones Steven
Wapner Phillip G.
Bricker Charles E.
Dixon Merrick
Kundert Thomas L.
The United States of America as represented by the Secretary of
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