Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-02-17
2003-06-10
Mayes, Curtis (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C264S682000, C264S029100, C264S029500, C264S029600, C264S029700
Reexamination Certificate
active
06576076
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing fiber-reinforced silicon carbide composites. Specifically, the present invention relates to a process for producing fiber-reinforced silicon carbide composites which are suitable for a variety of applications requiring improved toughness, such as aerospace high-temperature structural members, gas turbine members, fusion reactor materials, furnace members, heater materials, and artificial bones.
DESCRIPTION OF THE RELATED ART
Silicon carbide ceramics are light-weight and are excellent in, for example, heat resistance, abrasion resistance, and corrosion resistance; they have recently come into wide use as, for instance, high-temperature corrosion-resistant members, heater members, abrasion resistant members, as well as abrasives and grindstones. Such silicon carbide ceramics are, however, low in fracture toughness, and have not yet been used in practice as structural members for use at high temperatures.
Ceramic composites compounded with fibrous reinforcements to improve the toughness of such ceramics have been intensively investigated in recent years. Such fiber-reinforced silicon carbide composites are generally manufactured by, for example, (1) an organometallic polymer impregnation pyrolysis (PIP) process, (2) a chemical vapor infiltration (CVI) process, and (3) infiltration of molten silicon (reaction sintering process).
However, the organometallic polymer impregnation pyrolysis (PIP) process has the following disadvantages in practice. According to this process, only low density and low strength can be obtained by a single impregnation, and the impregnation and pyrolysis procedure must be repeated about ten times to reduce the open porosity to 10% or less to and thereby improve strength characteristics. The process therefore requires a long time for production. The chemical vapor infiltration process (2) can provide products having complicated shapes at comparatively low temperatures of about 1100° C., but it requires a very long time, of as much as several months, for infiltration, and gases used therefore are toxic. In addition, composites having an open porosity not exceeding 5% cannot be significantly obtained by the single use of the process (1) or (2).
In contrast, the reaction sintering process (3) requires only a short reaction time and can yield dense composites in a short time period. According to a process employed in Deutsche Forschungs-und Versuchsanstalt fur Luft-und Raumfahrt (DLR), a carbon fiber-reinforced carbon-silicon carbide composite is produced by infiltrating molten silicon into cracks of a carbon-fiber-reinforced carbon composite (C/C composite) to convert part of the matrix carbon into silicon carbide. This process utilizes a phenomenon in which glassy carbon does not react significantly with molten silicon to avoid the reaction between the carbon fiber and the silicon. In the process, however, the mechanical properties depend to a large degree on shapes of the cracks, which shapes in turn depend, for example, on the type of the carbon fiber and on the heat treatment temperature. High mechanical properties can therefore only be provided under specific production conditions suitable for the fiber to be used. The reaction sintering process (3) also includes a process of infiltrating molten silicon into a composite of a carbon powder and a fiber, which is performed by General Electric Co., USA, and Toshiba Corporation, Japan. This process is also disadvantageous in that a large quantity of silicon must be infiltrated, which invites free silicon to remain in large amounts of about 15%, or causes a reaction between the fiber and silicon unless the fiber is coated with boron nitride (BN) or the like.
After investigations on similar ceramic composites, the present inventors previously found that a unidirectional carbon-fiber-reinforced silicon carbide composites having a flexural strength of about 200 to 300 MPa even though having a large open porosity of 30% can be produced by compounding a matrix of a mixture of silicon powder and phenol resin with carbon fiber, and firing the composite in an inert atmosphere. The resultant composite contains a matrix of silicon carbide formed by reaction sintering (Japanese Patent No. 2045825). According to the reaction, however, only porous composites having a large open porosity can be obtained, as the volume of the matrix decreases about 38%. They also found that a unidirectionally carbon-fiber-reinforced silicon carbide composite having a flexural strength of about 500 to 600 MPa, even though it has a large open porosity of about 20%, can be produced by reducing the particle of the silicon powder about 5 &mgr;m or below and by adding an organometallic polymer (Japanese Patent No. 2735151), and that a two-dimensional fiber-reinforced composite having a relatively large open porosity of about 15% as a two-dimensional fiber-reinforced silicon-carbide carbon composite, but having a flexural strength of about 300 MPa, can be obtained by heat-treating a green body containing a fibrous woven fabric reinforcement at a temperature at which silicon does not react with carbon, repeating impregnation and carbonization of a phenol resin, and finally forming silicon carbide (Japanese Patent No. 2879675).
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the various problems in the production of fiber-reinforced silicon carbide composites according to the conventional silicon melt infiltration technique, and to provide a process for producing fiber-reinforced silicon carbide composites, which process can easily yield a composite having a high toughness even if they are to have complicated shapes.
Specifically the invention is directed to provide a process for easily producing fiber-reinforced silicon carbide composites even if they are to have complicated shapes, by covering a fiber bundle region with glassy carbon derived from resin, forming a porous region in a specific portion of a matrix by a reaction between silicon powder and carbon derived from the resin to form silicon carbide, which reaction is accompanied by volumetric reduction, and subjecting the porous region to infiltration of molten silicon. The resultant fiber-reinforced silicon carbide composite is high in toughness and its strength is not deteriorated, without coating the fiber surface with, for example, BN.
After intensive investigations on the production of fiber-reinforced silicon carbide composites to achieve the above objects, the inventors found that a dense fiber-reinforced silicon carbide composite showing nonlinear fracture can be obtained by preparing and molding a prepreg including silicon powder, carbon source resin and fiber to yield a green body, or laminating a prepreg containing resin with a prepreg containing silicon powder and resin in alternate order and molding the laminate to yield a green body; carbonizing the molded green body at a temperature of about 900° C. to 1350° C. in an inert atmosphere; preferably, impregnating the carbonized composite with a resin and carbonizing the impregnated composite at a temperature of about 900° C. to 1350° C. in an inert atmosphere, and repeating this impregnation-carbonization procedure; subjecting the composite to reaction sintering at a temperature of about 1300° C. or higher in vacuo or in an inert atmosphere, and finally infiltrating molten silicon into the sintered composite at a temperature of about 1300° C. to 1800° C. in vacuo or in an inert atmosphere. The present invention has been accomplished on the basis of the above findings.
Specifically, in the process for producing a fiber-reinforced silicon carbide composite of the invention, (i) a mixture of silicon powder, a carbon source resin and reinforcement fiber is carbonized in an inert atmosphere, and the resultant carbonized composite is impregnated with resin and is subjected to carbonization or the like, or (ii) a mixture of (a) a matrix containing silicon powder and carbon source resin, and porous fiber as a container, and (b) reinfo
Agency of Industrial Science and Technology
Mayes Curtis
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