Fabric (woven – knitted – or nonwoven textile or cloth – etc.) – Coated or impregnated woven – knit – or nonwoven fabric which... – Coated or impregnated inorganic fiber fabric
Reexamination Certificate
1998-10-19
2001-07-10
Weisberger, Richard (Department: 1774)
Fabric (woven, knitted, or nonwoven textile or cloth, etc.)
Coated or impregnated woven, knit, or nonwoven fabric which...
Coated or impregnated inorganic fiber fabric
C442S060000, C442S074000, C442S136000, C428S113000, C428S408000, C428S446000, C428S697000, C428S698000, C427S383300, C427S387000, C521S095000, C521S095000, C521S154000
Reexamination Certificate
active
06258737
ABSTRACT:
FIELD OF THE INVENTION
Generally, this invention relates to novel processing techniques and slurry modifications incorporating small diameter fibers in complex preform shapes. The invention further relates to making silicon carbide composites using melt infiltration processing. In particular, this invention relates to using bundles of continuous silicon carbide-containing fibers having diameters in the range of about 5 to 40 micrometers, that are impregnated with a high char yielding resin slurry containing fine carbon and silicon carbide particles.
BACKGROUND OF THE INVENTION
Fiber reinforcement of a ceramic composite offers significant opportunities for toughening the brittle matrix of the composite. There is an interest in fiber reinforced composites for use in high temperature applications, such as combustors and engines. Structures with carbon fiber reinforced carbon matrices have been tried but they have the disadvantage of poor oxidation resistance at high temperatures, such as 1200° C. or above. High strength carbon fibers have also been tape cast in a slurry and formed into a preform and then infiltrated with molten silicon with the hope that the silicon matrix would protect the carbon filaments in high temperature, oxidative environments. However, in this process the carbon filaments tend to convert into relatively weak, irregular columns of silicon carbide crystals resulting in composites with low toughness and relatively modest strength.
To alleviate the problems with carbon fibers in a carbon matrix, large diameter (about 140 micrometers or greater) silicon carbide-containing fibers provided with a coating of boron nitride have been tried with molten silicon infiltration to form a silicon-silicon carbide composite. The silicon melt approach in combination with the silicon carbide material has proven to be effective in preventing damage to the fiber. The spacing of the fibers in the preform due to the large diameter size allows the molten silicon to react with the carbonaceous material in the preform to provide a silicon-silicon carbide matrix. However, the size of the fibers prevents the formation of complex shapes due to limited bend radii. When smaller discontinuous fibers are used (about 0.3 to 50 micrometers in diameter), a silicon-wetting coating is required over the boron nitride coating. Even then, if the small diameter fibers are bundled, they often do not wet with molten silicon and as a result, pockets of elemental silicon form in the silicon carbide matrix. This results in a weaker silicon carbide composite.
U.S. Pat. Nos. 5,015,540; 5,330,854; and 5,336,350; incorporated herein by reference, relate to the production of silicon carbide matrix composites containing fibrous material that is infiltrated with molten silicon, herein referred to as the Silcomp process. The fibers generally have diameters of about 140 micrometers or greater, which prevents intricate, complex shapes to be manufactured during formation of the preform. Limited bend radii of these fibers restrict their use to structures with radii of curvature between 1-2 inches in diameter. At smaller radii of curvature, the large diameter fibers will fracture at elevated temperatures of about 2000° F. or greater due to the high residual stresses imposed in the fiber. Thus, there is a need for a method to make complex shaped preforms that incorporate continuous bundles of smaller diameter fibers in the preform that will subsequently provide tougher, stronger high temperature composites.
Presently, in the manufacture of silicon-silicon carbide composites, the fibers in the preform are coated with a low char yield slurry composition containing polymers that decompose upon heating. The polymers produce little or no char after decomposition, which means that there is little or no solid material after burnout. As a result, a low char yield slurry composition used to coat the fiber provides a low strength preform after burn-out processing. This is not desirable if the preform has to be moved or transported from one furnace to another. A need is created for an improved preform with high strength after burn-out.
The Silcomp process for making silicon-silicon carbide composite, uses coarse carbon and silicon carbide powders as filler materials in the slurry composition that is coated on the fibers or in the preform itself. The coarse powders do not completely react during the molten silicon infiltration to convert all of the available carbon to silicon carbide, thus yielding a high residual carbon content in the matrix (about 10-20 volume percent silicon). This creates a need for a lower elemental carbon in the silicon-silicon carbide composite.
Although operable, the above-mentioned Silcomp process provides an opportunity for process and product improvement. There is a need for complex shaped preforms that accommodate small diameter silicon carbide-containing fibers that are bundled and gathered in tows. There is also a need for a novel slurry composition to coat the small diameter fibers with high char yield resins to provide a stronger and tougher preform after burnout and containing fine particles of carbon and silicon carbide that can penetrate between the fibers that are bundled in tows. There is also a need for a method of making complex shaped preforms and composites by molten silicon infiltration that provides an improved silicon-silicon carbide composite.
SUMMARY OF THE INVENTION
The present invention provides process, preform, and composite improvements that meet the above needs. In accordance with one aspect of the invention, there is provided a method for preparing a complex-shaped preform comprising: providing a mass of continuous fibers, each fiber having a diameter less than about 40 micrometers; impregnating the mass of fibers with a slurry composition containing at least one high char yield resin selected from a carbon forming resin or a ceramic forming resin or a mixture of both; forming the impregnated mass of fibers into a preselected preform; and firing the preform at a temperature and time sufficient to cure the preform. Silicon carbide-containing fibers are contemplated as fibers. Generally, the mass of fibers are bundled in tows and continuous lengths. By the term “continuous lengths” is meant fiber lengths greater than or equal to 1 centimeter (>1 cm). The bundles of fibers are often formed into complex shapes to form the fiber preform which is subsequently infiltrated with molten silicon.
In accordance with a further aspect of the invention, a method is provided for preparing a silicon-silicon carbide composite comprising the steps of: providing a mass of silicon carbide-containing fibers bundled in tows, each fiber having a diameter less than about 40 micrometers; impregnating the mass of silicon carbide-containing fibers with a slurry composition containing at least one high char yield resin selected from the group consisting of a carbon forming resin, a ceramic forming resin, and mixtures thereof; forming the impregnated mass of fibers into a complex shape of a preselected preform; curing the complex shape to form a silicon carbide-containing fiber preform; providing a necessary amount of a carbonaceous material in the preform to react with molten silicon to form silicon carbide; and then infiltrating the preform with molten silicon. Substantially all of the carbonaceous material in the preform reacts with the molten silicon to form a substantially dense silicon-silicon carbide composite.
Yet another aspect of the invention is an improved silicon carbide-containing fiber preform comprising 1-dimensional, 2-dimensional, or 3-dimensional aligned tows of silicon carbide-containing fibers, each fiber having a diameter up to and including 40 micrometers and coated with a slurry composition comprising at least one high char yield resin, and particulate material comprising carbon particles, or silicon carbide particles, or mixtures thereof, said particles having a mean particle size of about 0.1 to 20 micrometers.
Still a further aspect of the invention is a silicon carbide matr
Corman Gregory S.
Schikner Robert C.
Steibel James D.
Szweda Andrew
General Electric Company
Johnson Noreen C.
Stoner Douglas E.
Weisberger Richard
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