Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
2001-09-13
2004-06-29
Cole, Elizabeth M. (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S179000, C427S288000, C427S372200, C427S384000, C427S394000
Reexamination Certificate
active
06756112
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to high-performance, high-temperature fiber-reinforced structural composites with carbon matrices.
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.”
For elevated temperature applications, high-temperature fiber-reinforced composites (HTFRC's) are employed. These composites have excellent high-temperature strength retention, high strength-to-density ratio, and possess fracture toughness. In addition, the carbon-carbon (C—C) composites have high specific modulus, good thermal conductivity and thermal shock resistance. 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.
Two categories of commercial processes have been developed to manufacture High-temperature Fiber-reinforced Composites (HTFRC) such as 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 vapor or liquid hydrocarbon precursor, is to hold the reinforcing fibers in the shape desired for the final product. The ensemble of cemented fibers is called the rigidzed-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 processing. This subsequent processing to deposit material between the fibers in the preform is usually referred to as “densification,” and is usually repeated many times.
As mentioned above, densification using existing technology takes one of two forms. The first is vapor-phase-based and involves placing the rigid-preform in an oven containing gases which decompose at high temperatures inside the preform to form carbon matrices. This process is referred to as chemical-vapor infiltration (CVI). The decomposition reaction is usually referred to as “cracking”, since the splitting-apart of gas molecules is involved. However, it is also sometimes called pyrolysis, the same term used previously to describe similar thermal decomposition reactions occurring in solids and liquids. CVI has a number of problems associated with its use, the two most critical being pore closure at the surface leading to non-uniform densification, and poor matrix quality due to existence of multiple decomposition reaction-pathways leading simultaneously to multiple phases. Pore closure is detrimental because it denies access of infiltration gases to the preform interior. It occurs because cracking occurs more easily at solid surfaces. Thus, as gases attempt to enter rigid-preforms, decomposition takes place almost immediately on or near the hot exterior surfaces. This results in a density gradient through the sample, with a higher density matrix near the surface. This density gradient also limits the thickness of a high-performance part to less than 2″. The preferential deposition on or near the surface ultimately leads to the sealing off of the surface pore entrances in a relatively short period of time. Multiple phases are also harmful in most instances because they do not join together or consolidate well, making the matrix weak. These problems are both minimized to some extent by slowing down the CVI process. Also, partially-densified composites can be periodically removed from the CVI oven and have their surfaces machined away enough to reopen sealed pore entrances. This is, of course, very time consuming and adds expense. For carbon matrix composites, prior to or subsequent to machining, the partially-densified preform is heated to about 2400° C. for long periods of time to convert the carbon matrix to a graphitic matrix. This process takes days to weeks and has associated high energy costs. The result of the steps described is processing times of many months, severe quality control problems, and associated high costs in both labor and energy.
The second densification process using existing commercial technology is liquid-phase-based. It involves impregnating rigid-preforms with liquid matrix-precursors and subsequently heating to high temperatures to initiate pyrolysis. It is in many ways similar to formation of rigid preform themselves, and suffers from the same drawbacks. Ease of impregnation and gentleness of
Hoffman Wesley P.
Jones Steven P.
Wapner Phillip G.
Bricker Charles E.
Cole Elizabeth M.
Kundert Thomas L.
The United States of America as represented by the Secretary of
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