Patent Application
20100081350
This invention is concerned with composite materials designed for applications requiring materials with smooth surfaces.
Composite materials exhibit a variety of advantages for high performance applications, including high temperature strength, superior creep and corrosion resistance, low density, high toughness, and resistance to environmental stresses such as shock, fatigue and physical damage. Because of these characteristics, composites are ideal for replacing metallic or ceramic materials in many engineering applications involving high loads, high temperatures, and aggressive environments.
A variety of manufacturing techniques, such as chemical vapor infiltration (CVI), polymer impregnation/pyrolysis (PIP), liquid silicon infiltration, and slurry impregnation/hot pressing, are employed in the art to fabricate composites. Where a smooth surfaced composite is required, however, these known processes may not be satisfactory. Some components in turbine engines, for example, need smooth surfaces in order to avoid surface roughness, which causes increased drag losses and heat transfer in a hot gas flow path.
When a composite is manufactured by CVI infiltration of SiC (silicon carbide) into a fiber preform, SiC-based matrices are deposited from gaseous reactants onto a heated substrate of SiC fiber preforms. An interphase coated on the fibers helps to control damage and maintain the mechanical behavior of the composite. The texture of the fiber preform, however, is preserved as surface roughness on the finished SiC composite. For thicker composites, this roughness can be removed by machining and recoating with CVI SiC.
This method of reducing the surface roughness, however, can be prohibitively costly for parts with intricate or complex shapes, such as the vanes in a turbine engine. Moreover, the method may not be viable for thin skin components, since it requires removal of part of the outer layer of fiber and it may be necessary, for some applications, to retain all of the fibers to maximize the structural integrity of the composite part.
Another approach to reducing surface roughness is to fill surface depressions using another processing method, such as PIP (Polymer Impregnation Pyrolysis) or MI (Melt Infiltration). The structural properties of matrices produced by these approaches, however, are inferior to those fabricated with CVI SiC. In addition, if the dimensions of the depressions are large (greater than approximately 100 microns), the matrix material produced by a PIP or MI method tends to be susceptible to cracking and to debonding from the underlying CVI SiC material.
Consequently, a need has developed in the art for a composite fabrication process that yields smooth surfaces while maintaining a mechanically superior composite structure yet avoiding excessive cost.
A method of making a smooth surfaced, fiber reinforced ceramic matrix composite includes the steps of providing a fiber preform, the preform having a surface containing voids; placing fibers into the voids; coating the preform fibers and the void fibers with a coating material to create a weak interface; and infiltrating the coated fibers with a matrix material to infill the voids and preform, and form strongly bonded networks within the voids.
The fiber preform may include interlaced bundles of fiber tows, with the voids between the interlaced bundles. The preform may be a woven preform, a braided preform, or a sewn preform. The preform fibers may be selected to be chemically compatible with the coating material and the matrix material; in particular, the preform fibers may be selected from carbon, silicon carbide, aluminum oxide, and mullite.
The dimensions of the void fibers may be selected to divide the voids into volumes sufficiently small to inhibit cracking and debonding within the composite. This may be accomplished using void fibers that are chopped fibers or whiskers, or by growing void fibers directly on the fiber preform. The void fibers may advantageously be selected to be chemically compatible with the coating material and the matrix material by choosing, for example, fibers of carbon, silicon carbide, aluminum oxide, or mullite.
The coating material may be a weak coating material, as by selecting a material to weakly bond with the preform fibers, with the void fibers, and/or with the infiltration material. The coating material may also be selected to avoid reacting with the preform fibers, with the void fibers, and with the matrix material. Desirable coating materials may be chosen from pyrolytic carbon, BN, monazites, and xenotime.
The matrix material may be selected from refractory carbides and refractory borides, while the infiltration step may be accomplished by infiltrating the coated fibers via chemical vapor infiltration, by infiltrating the coated fibers via infiltration of slurry particles in a polymer precursor, or by infiltrating the coated fibers via an in situ reaction of molten silicon with carbon to form SiC.
The matrix material may be selected from SiC, carbides, borides, oxides, and silicides. Constituents, such as carbides, B-containing compounds, silicides, and glasses, may be added to the infiltration material to improve oxidation resistance.
The method may further include the step of removing material from the surface of the ceramic matrix composite to smooth the surface, as by grinding the surface or chemically polishing the surface. Surface smoothing may also be accomplished by adding, after the step of placing fibers into the voids, the step of defining the boundaries of the void fibers.
A smooth surfaced, fiber reinforced ceramic matrix composite includes, according to the invention, a fiber preform, the preform having a surface containing voids; void fibers in the voids; a coating material on the preform fibers and the void fibers creating a weak interface; and a matrix material within the coated fibers and the preform to infill the voids and preform, and form strongly bonded networks within the voids.
A smooth surfaced, fiber reinforced ceramic matrix composite includes, according to another embodiment of the invention, a fiber preform, the preform having a surface containing voids; void fibers in the voids; a first coating material on the preform fibers and the void fibers, creating a weak interface; a second coating material on the preform fibers and the void fibers, creating a second coating of substantially uniform thickness on the fibers and forming strongly bonded networks within the voids; and a matrix material within the coated fibers and the preform to infill the voids and preform.
In one embodiment, this invention involves a method of making a smooth surfaced, fiber reinforced ceramic matrix composite that begins by providing a fiber preform, as shown by the fiber preform 100 shown in schematic plan view in
As those skilled in the art will appreciate, the preform 100 is depicted, for purposes of clarity, in schematic form, with straight, smooth and regular rows and columns of fibers, while an actual preform will typically include substantial irregularities in its dimensions and shapes. Moreover, an actual preform will likely include a much higher number of woven fibers than depicted in
Next, as shown in
Finally, as depicted in
In an alternative embodiment, the preform fibers and the void fibers are coated with a first coating material on the preform fibers and the void fibers, creating a weak interface.
Next, a second coating material is applied to the preform fibers and the void fibers, creating a second coating of substantially uniform thickness on the fibers and forming strongly bonded networks within the voids. Finally, the fiber preform is infiltrated with the matrix material that fills the remaining spaces within the networks and in other regions of the fiber preform.
Although the exemplary embodiment of the invention, as illustrated in
In a more particular embodiment, it may be desirable to select preform fibers that are chemically compatible with the coating material and with the infiltration material. The fibers should also have high strength and remain stable at high temperatures, although the exact qualities will vary with the application. Some particular combinations of fiber materials and coating materials, for example, that are known in the art to be desirable for their compatibility are as follows: carbon and silicon carbide fibers with coatings of carbon and boron nitride; aluminum oxide and mullite fibers with coatings of rare-earth phosphate compounds (monazite and xenotime).
In addition, the dimensions of the void fibers may be selected to divide the voids into volumes sufficiently small to inhibit cracking and debonding within the composite. Various approaches to selecting void fiber dimensions, toward this goal, may be pursued, including utilizing chopped fibers, fiber whiskers, or growing void fibers directly on the fiber preform.
The void fibers, like the preform fibers, may be selected to be chemically compatible with the coating material and with the matrix material. As with the preform fibers, materials that are known in the art to be desirable for their compatibility are as follows: carbon and silicon carbide fibers with coatings of carbon and boron nitride; aluminum oxide and mullite fibers with coatings of rare-earth phosphate compounds (monazite and xenotime).
It may be advantageous to select a coating material that is a weak coating material, such as a coating material that weakly bonds with the preform fibers, with the void fibers, and/or with the infiltration material.
Another property of the coating material that may desirable is to select a coating material that is not reactive with either the preform fibers, the void fibers, or the infiltration material. Particular coating materials that may be advantageous include, for non-oxide fibers and matrices, pyrolytic carbon or boron nitride, and, for oxide fibers and matrices, monazites and xenotime.
As those skilled in the art will appreciate, useful methods of infiltrating the coated fibers include infiltrating the coated fibers via chemical vapor infiltration, infiltrating the coated fibers via an in situ reaction of molten silicon with carbon to form SiC, and infiltrating the coated fibers via infiltration of slurry particles in a liquid precursor. Particular liquid precursors that may be advantageous include polycarbosilane polymers that decompose to leave SiC and solutions containing ions that precipitate to form rare-earth phosphates.
Infiltration materials that may be desirable include refractory carbides, in particular SiC, borides, oxides, and silicides. Moreover, constituents, such as carbides (e.g., HfC), boron-containing compounds (such as B4C or HfB2), silicides, and glasses, may be added to the infiltration material to improve oxidation resistance. In the alternative embodiment in which the preform fibers and the void fibers are coated with a first coating material on the preform fibers and the void fibers, then a second coating material is applied to the preform fibers and the void fibers, the infiltration of a second coating material by chemical vapor infiltration produces a thin layer of material on all of the fiber surfaces within the preform and within the voids. This layer is advantageously several times thicker than the diameters of the fibers. In regions where the fibers are touching, the coating forms a continuous layer connecting the fibers, so that the random array of discontinuous fibers/whiskers in the voids forms a rigid three dimensional scaffold that is strongly bonded to the surrounding or underlying fibers tows of the textile preform.
To achieve sufficient surface smoothness for some applications, it may be desirable to further process the composite after the step of infiltrating the fiber preform. Additional surface smoothness, as shown in
The preferred embodiments of this invention have been illustrated and described above. Modifications and additional embodiments, however, will undoubtedly be apparent to those skilled in the art. Furthermore, equivalent elements may be substituted for those illustrated and described herein, parts or connections might be reversed or otherwise interchanged, and certain features of the invention may be utilized independently of other features. Consequently, the exemplary embodiments should be considered illustrative, rather than inclusive, while the appended claims are more indicative of the full scope of the invention.
The United States Government has rights in this invention pursuant to a contract (Integrated High Payoff Rocket Propulsion Technology) awarded by the United States Air Force.
