Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1998-07-10
2001-06-12
Weisberger, Richard (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S373000, C428S375000, C428S378000, C428S408000, C428S446000, C428S698000, C428S704000
Reexamination Certificate
active
06245424
ABSTRACT:
BACKGROUND OF THE INVENTION
Reinforced ceramic matrix composites (“CMC's”) are well suited for structural applications because of their toughness, thermal resistance, high temperature strength and chemical stability. These composites can be produced by adding whiskers, fibers or platelets to a ceramic matrix. In the fabrication of continuous fiber reinforced-ceramic matrix composites (“CFCC's”), the fabrication process usually begins by weaving continuous TM fiber tows (e.g., sintered SiC fibers such as Hi-Nicalon or Dow Corning Sylramic™) into a cloth such as 2-dimension 5HS or 8HS, or 3-dimension cloths. The woven fiber cloth is then formed into a panel or shape called a fiber preform. The porosity within the fiber preform is then filled to produce the dense CFCC. The non-brittle nature of the CFCC provides the much needed reliability that is otherwise lacking in monolithic ceramics.
The enhanced fracture resistance of ceramic matrix composites is achieved through crack deflection, load transfer, and fiber pull-out. Fiber pullout is achieved by having little or no chemical bonding between the fibers and matrix, so that the fibers are able to slide along the matrix. However, it is also known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials during densification. Such reaction or interdiffusion can lead to serious degradation in strength, toughness, temperature stability and oxidation resistance. Accordingly, the proper fiber-matrix interface must be selected in order to prevent or minimize chemical reactions and interdiffusion.
Surface modification of the fibers is an effective means to control reaction at the fiber-matrix interface. This can be accomplished by coating the fibers with a suitable ceramic. Equally important, a suitable ceramic coating also allows the debonding of the fiber's matrix interface and enables the fiber to pull out from the matrix and slide along the matrix, thus increasing the fracture toughness of the composite. Coated silicon carbide fibers and whiskers are known in. The art of composite materials. U.S. Pat. No. 4,929,472 (“Sugihara”) discloses SiC whiskers having a surface coated with either a carbonaceous layer or a silicon nitride layer. These surface coated whiskers are used as a reinforcing material for ceramics such as SiC, TiC, Si
3
N
4
, or Al
2
O
3
. U.S. Pat. No. 4,781,993 to Bhatt discloses a SiC fiber reinforced reaction bonded Si
3
N
4
matrix wherein the SiC fibers are coated with an amorphous carbon layer and an overlayer having a high silicon/carbon ratio covering the amorphous layer. U.S. Pat. No. 4,642,271 to Rice discloses BN coated ceramic fibers embedded in a ceramic matrix. The fibers may be SiC, Al
2
O
3
or graphite, while the matrix may be SiO
2
, SiC, ZrO
2
, ZrO
2
-TiO
2
, cordierite, mullite, or coated carbon matrices. U.S. Pat. No. 4,944,904 to Singh et al. discloses a composite containing boron nitride coated fibrous material. Carbon or SiC fibers are coated with BN and a silicon-wettable material and then admixed with an infiltration-promoting material. This mixture is formed into a preform which is then infiltrated with a molten solution of boron and silicon to produce the composite.
The densification of green CFCC's is more difficult than that of green monolithic ceramics. Conventional sintering of a green ceramic matrix reinforced with sintered fibers is not possible, as the green ceramic matrix has rigid inclusions. Densification of green CFCC's can, however, be achieved by chemical vapor infiltration (“CVI”) or molten silicon infiltration. Molten silicon infiltration is the preferred method because it is less time consuming and more often produces a fully dense body than the CVI process. For high temperature applications, full densification is necessary for good thermal and mechanical properties and for preventing rapid oxidation/degradation of the reinforcements or reinforcement coating. For example, desirable characteristics for CFCC's used in air transport applications include a high thermal conductivity, high tensile strength, high tensile strain and a high cyclic fatigue peak stress. One conventional CFCC fabricated by state-of-the-art chemical vapor infiltration processing has been found to have a thermal conductivity of only about 4.7 BTU/hr.ft.F at 2200° F., and a cyclic fatigue peak stress of only about 15 ksi (about 105 MPa) using a Hi-Nicalon™ fiber. It is believed the low thermal conductivity and cyclic fatigue peak stress of this CVI material is due to the material's relatively high porosity (typically 10-20%) which is common for CVI processes. According, the art has focused upon densification by silicon infiltration.
Densification by silicon infiltration has been practiced for monolithic ceramics, such as reaction-bonded silicon carbide, for many years. This process, as described in U.S. Pat. No. 3,205,043 to Taylor, involves infiltrating molten silicon through the pores of a green body containing alpha silicon carbide and carbon. The silicon reacts with the carbon to form beta-SiC, which then bonds the alpha-SiC grains together. The portion of the infiltrated molten silicon which does not react with the carbon solidifies upon cooling, thereby filling the pores of the Sic bonded Sic body. This phenomenon is known as siliconization, and results in a fully dense end product containing SiC and residual free silicon. Since silicon infiltration does not involve shrinkage of the green body (as is the case with conventional sintering), the final dense product is near net shape. The art has used silicon infiltration to densify fiber-containing ceramic composites as well.
U.S. Pat. No. 5,296,311 (“McMurtry”), the specification of which is incorporated by reference, discloses a silicon infiltrated silicon carbide composite reinforced with coated silicon carbide fibers. McMurtry discloses a process including the steps of:
a) coating SiC fibers with a coating selected from the group consisting of aluminum nitride, boron nitride and titanium diboride;
b) treating the surface of the coated fibers with a mixture of SiC powder, water and a non-ionic surfactant;
c) preparing a slurry comprising SiC powder and water;
d) impregnating the coated fibers with the slurry using a vacuum dewatering process to form a cast;
e) drying the cast to form a green body; and
f) silicon infiltrating the green body to form a dense SiC fiber reinforced reaction bonded matrix composite.
McMurtry reports that providing the disclosed coatings on SiC fibers limited both mechanical and chemical bonding with the matrix, and so improved the strength and toughness of the composite material. However, CFCC's produced in substantial accordance with McMurtry have been found to have a four point flexure strength at room temperature of only about 1 ksi. Since the tensile strength of a ceramic is typically only about 60%-90% of its four point flexure strength, these CFCC's likely have a tensile strength of only about 0.6-0.9 ksi. Further assuming an elastic modulus of about 30 million psi, these CFCC's likely have an ultimate tensile strain of less than 0.003% at room temperature. The reason for these low values is believed to be the low strength of the fiber used in McMurtry, as well as the partial reaction of the debonding coating with the molten silicon. Moreover, simple substitution of higher strength SiC fibers, such as Hi-Nicalon fiber, presents more severe degradation problems because the these higher strength fibers are considered to be more susceptible to degradation by molten silicon than the SiC fibers used by McMurtry. In particular, these higher strength fibers typically degrade in the temperature range of only about 1410-1500° C. while the silicon infiltration step in McMurtry is undertaken at a temperature of about 1500° C.
In addition, one specific problem encountered with SiC reinforced SiC composites fabricated by a silicon infiltration process is that the SiC fiber or coating thereon may react with the m
Calandra Salvatore J.
Lau Sai-Kwing
Ohnsorg Roger W.
DiMauro Thomas M.
Saint-Gobain Industrial Ceramics, Inc.
Weisberger Richard
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