Patent Application
20070096371
The present invention generally relates to ceramic matrix composite (CMC) articles. More particularly, this invention is directed to a process of producing melt-infiltrated (MI) CMC articles that includes processing steps capable of yielding prepreg tapes with improved physical and mechanical properties, thereby enabling the production therefrom of CMC articles that exhibit improved physical, mechanical, and microstructural properties.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. Though significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been investigated. Notable examples include ceramic matrix composite (CMC) materials, whose high temperature capabilities are capable of significantly reducing cooling air requirements. CMC materials, particularly continuous fiber ceramic composite (CFCC) materials, are currently being considered for shrouds, combustor liners, nozzles, and other high-temperature components of gas turbine engines. Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material.
CMC materials generally comprise a fibrous or filamentary reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains fiber orientation, and serves to dissipate loads to the reinforcement material. CMC's are frequently fabricated from multiple layers of “prepreg,” which is typically a tape-like structure comprising the reinforcement material impregnated with a slurry that contains a precursor of the matrix material and one or more organic binders. The prepreg tape must undergo processing (including firing) to convert the precursor to the desired ceramic. Prepregs for CFCC materials frequently comprise a two-dimensional fiber array comprising a single layer of unidirectionally-aligned tows (bundles of individual filaments) impregnated with a matrix precursor to create a generally two-dimensional laminate. Multiple plies of the resulting prepregs are then stacked and debulked to form a laminate preform, a process referred to as “lay-up.” The prepregs are typically but not necessarily arranged so that tows of adjacent prepregs are oriented transverse (e.g., perpendicular) to each other, providing greater strength in the laminar plane of the preform (corresponding to the principal (load-bearing) directions of the final CMC component).
Following lay-up, the laminate preform will typically undergo debulking and curing while subjected to applied pressure and an elevated temperature, such as in an autoclave. In the case of melt-infiltrated (MI) CMC articles, the debulked and cured preform undergoes additional processing. First the preform is heated in vacuum or in an inert atmosphere in order to decompose the organic binders, at least one of which pyrolyzes during this heat treatment to form a carbon char, and produces a porous preform for melt infiltration. Further heating, either as part of the same heat cycle as the binder burn-out step or in an independent subsequent heating step, the preform is melt infiltrated, such as with molten silicon supplied externally. The molten silicon infiltrates into the porosity, reacts with the carbon constituent of the matrix to form silicon carbide, and fills the porosity to yield the desired CMC component. As an example,
A step in the typical method for making prepreg MI-CMC preforms is to use a wet drum winding technique to form the fiber tows into a unidirectional prepreg tape, which is then used for lay-up of the composite preform. As represented in
Prepreg tapes produced by wet drum winding processes typically have a surface roughness, or waviness, corresponding to the pitch of the fiber tow 16 on the drum 22. There is also variability in the distribution of fiber and matrix across the tape because of the tow pitch. Furthermore, because the tow 16 is under tension during the winding process, the tow 16 tends to be pulled down onto the drum surface, yielding a prepreg tape that has proportionally more tow 16 at the surface of the tape contacting the drum 22 and proportionally more matrix precursor at the surface of the tape facing away from the drum 22. Because the pliability, or drapability, of the resulting prepreg tape is highly dependent on its residual solvent content, there is a limited time window from when a tape is wound to when the tape must be utilized in a composite preform lay-up. For example, after about twelve hours at ambient conditions, tapes can become too stiff and lose tackiness to allow for a consistent lay-up of the composite plies.
Typical wet drum winding processes can also suffer from a significant amount of broken tow fibers and loosely adhering fibers (i.e., “fuzz”) that can break off and cause blockage of the orifice 24. When blockage occurs, the amount of slurry remaining on the tow 16 downstream of the orifice 24 is diminished, leading to a region on the resulting prepreg tape with lower than optimum matrix content. In severe cases the blockage of the orifice 24 can continue to accumulate broken fiber from the tow 16 until it eventually causes the tow 16 to break. In order to prevent such problems from blockage, the orifice 24 is typically sufficiently sized to allow a majority of tow 16, even those with moderate amounts of damaged fiber and adhering loose fiber, to readily pass. Consequently the amount of matrix picked up by the tow 16 may be higher than would be optimum, thus leading to lower than desired fiber volume fraction in the finished composite. Even with the use of a large orifice 24, orifice blockage can still occur. Consequently, drum winding operations require constant operator supervision so that such blockages can be removed as they occur.
Another complication of conventional drum winding processes is that the tow 16 must be completely impregnated (i.e., wet out) with slurry during the winding process, which requires that the tow 16 spend a sufficient amount of time submersed in the slurry to allow for complete wet out. This submersion time, which can be about five seconds for certain processes, places a limit on the speed with which the tow 16 can be drawn through the slurry bath. Consequently the time necessary to drum wind a 100 meter tow can be on the order of about thirty minutes.
In view of the above, improvements in drum winding processes for producing CMC products would be highly desirable.
The present invention provides a process for use in the production of CMC articles, such as components of gas turbine engines. The invention is particularly directed to a drum winding process capable of promoting the physical and mechanical properties of a prepreg tape, and thereby enable the production therefrom of CMC articles that exhibit improved physical, mechanical, and microstructural properties.
According to one aspect of the invention, steps carried out to produce a CMC article entail applying a first release sheet to a drum and then forming on the drum an impregnated tape that overlies the first release sheet. The impregnated tape comprises a filamentary material unidirectionally oriented within the impregnated tape and a slurry mixture consisting of a ceramic precursor material and additional constituents that are nonreactive to the first release sheet. A second release sheet is then applied to overlie the impregnated tape, with the result that the impregnated tape is between the first and second sheets to yield a laminate structure. As with the first release sheet, the second release sheet is also nonreactive to the constituents of the slurry mixture carried by the filamentary material. The laminate structure is then passed through an apparatus that flattens the impregnated tape and redistributes the slurry mixture within the impregnated tape.
According to another aspect of the invention, the filamentary material can be wound onto the drum after or before being impregnated with the slurry mixture. If the former, the impregnated tape is formed in situ on the drum. If the latter, the impregnated tape can be formed in situ on the drum by impregnating the filamentary material with the slurry mixture while still on the drum or after the filamentary material has been removed from the drum in the form of a filamentary mat, in which case the first release sheet carries an adhesive that adheres the filamentary material together on the drum so that the filamentary mat can be removed and impregnated with the slurry mixture without disrupting the filamentary material.
According to the first aspect of the invention, the operation of flattening the tape and redistributing the slurry mixture within the tape is able to improve the uniformity of the matrix and fiber distribution in the tape, improve tape thickness uniformity, and improve the shelf life of the tape. More particularly, this operation decreases tape surface roughness yielding a more uniform tape thickness that aids in the lay-up process, compacts the tape to force out air entrapped within the tape and thereby removes a potential source of preform porosity, and redistributes the ceramic precursor material more evenly to the two surfaces of the tape thereby improving the matrix-to-fiber uniformity between the two surfaces which aids in the subsequent lamination process and yields a composite preform with a more uniform distribution of fiber throughout the matrix. The operation performed by the apparatus also effectively traps the tape between the two release sheets, which protects the tape from contamination and reduces the evaporation rate of solvents within the tape to increase the shelf life of the tape.
According to the second aspect of the invention, by winding the filamentary material onto the drum in a dry condition, fuzzy or damaged tows can be used that would otherwise not be suitable if pre-impregnated with slurry before winding on the drum. More particularly, by winding tow in the dry state, broken and fuzzy filaments have been found to be less likely to plug an orifice used to control the position of the tow during winding. Furthermore, the speed of the tow is not limited by the requirement to completely wet-out the tow with the slurry upstream of the winding operation. Impregnating the tow following winding also offers the ability of more accurately controlling the amount of slurry impregnated into the tow, such as impregnating the tow with the minimum amount of slurry necessary to fully wet-out and cover the tow, yielding a prepreg tape of minimal thickness and a high fiber volume fraction.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
The present invention is generally applicable to CMC components, and particularly CFCC components. Notable examples of such components include combustor components, high pressure turbine vanes, and other hot section airfoils and components of gas turbine engines, though the invention has application to other components, including advanced power generation steam turbines and various other equipment that can make use of CMC components. Examples of CMC materials to which the invention pertains include those with a silicon carbide, silicon nitride and/or silicon reinforcement material in a ceramic matrix of silicon carbide, silicon nitride and/or silicon, e.g., a SiC/SiC CMC, though the invention also applies to other types of CMC materials. Examples of such materials and components are disclosed in commonly-assigned U.S. Pat. Nos. 6,024,898, 6,258,737, and 6,503,441, the contents of which relating to CMC materials and processing steps are incorporated herein by reference.
In the fabrication of the component 10, a desired number of prepreg tapes are laid-up to form a preform (not shown) that undergoes further processing to yield the component 10. Each tape is formed to contain a reinforcement architecture (formed by the fibers 17) with a precursor of the desired matrix material 14, e.g., SiC, C, and/or other carbon-containing particulate material if the desired matrix material is SiC. According to conventional practice, such prepreg tapes are formed in a single operation by applying the precursor-containing slurry during winding of a continuous strand of tow 16 onto a drum 22, as previously described in reference to
The present invention is primarily concerned with the winding operation, and therefore generally diverges from the process described above with respect to the manner in which the prepreg tape is processed prior to composite lay-up. According to a first aspect of the invention, after impregnation of a dry tow 16 with the desired slurry to yield an impregnated tow 20 (as described in reference to
The laminate structure 32 is preferably allowed to stabilize, such as for about fifteen minutes or so, after which the entire structure 32 is passed through a rolling mill (as represented in
Before being laid-up with other tapes, the tape 30 is typically cut to the desired ply shape for the intended component 10 while still captured between the sheets 26 and 36. The structure 32 can then be cooled by placing in a freezer or dipping in a cooling liquid such as liquid nitrogen until the tape 30 becomes hard and sufficiently loses its tackiness to permit the sheets 26 and 28 to be easily removed. Thereafter, the tape 30 can be permitted to warm to room temperature, restoring some of its tackiness to facilitate the lay-up process. According to a preferred embodiment of the invention, the pliability and tackiness of the tape 30 can be further increased by applying to the surfaces of the tape 30 a thin coating of a mixture containing a solvent and binder, the latter of which is preferably an organic binder of the slurry 34 used to form the tape 30. An example of such a mixture contains about 50% by volume of both acetone and a resin commercially available under the name Cotronics 980. The mixture can be applied to the tape surface and any excess wiped off, for example, with a squeegee.
Absorption of the binder-solvent mixture into the tape 30 replaces some of the binder and solvent that are inherently lost during exposure and storage of the tape 30, particularly along the edges of the tape 30. The increased pliability and tackiness resulting from this re-wetting step further aid in the lay-up of the tape 30 with other similarly processed tapes to form a composite preform, particularly if the preform is to have curved surfaces or complex features. If other tapes have been stored for different durations or under different conditions that result in different levels of dryness, the application of the binder-solvent mixture increases the consistency between tapes and reduces thickness variations that could otherwise lead to inconsistencies in the thickness of the composite preform following lamination. Re-wetting of the tape 30 in this manner is also believed to promote more uniform shrinkage of the tape 30 during subsequent processing by filling in any regions of the tape 30 containing relatively low levels of precursor, thereby yielding a more consistent preform.
The above process was reduced to practice during experiments performed to assess the effects of tape rolling on the performance of composites.
In addition to the improvements noted above, further processing improvements are believed to be attainable if the tow 16 is wound onto the drum 22 in a dry state (i.e., without impregnation with the slurry or other precursor-containing composition). By winding only the tow 16 onto the drum 22, the result is a mat comprising unidirectionally-aligned dry tows 16 that can be impregnated with the slurry while on the drum 22 or after the mat is removed from the drum 22. In the latter case, the release sheet 26 preferably carries an adhesive on its surface facing the tow 16 so that the individual strands of tow 16 are held together and the mat can be removed from the drum 22 without disturbing the tow 16. Similar to the re-wetting step described above, impregnation of the dry mat can be achieved by applying the slurry to the mat using a squeegee, a doctor blading operation, etc. Thereafter, the resulting tape can undergo flattening and compaction as described previously in accordance with the present invention.
The dry winding technique of this invention described above was investigated with a sample of HI-NICALON® tow that had been extensively damaged to the extent that conventional wet drum winding was found to be extremely difficult due to plugging of the slurry orifice. The tow was wound dry over a TEFLON® release sheet onto a drum at a pitch of about 0.045 inch (about 1.1 mm) with no orifice plugging problems. The wrapped tow was then adhesively attached to the release sheet at regular intervals to permit the sheet and tow to be removed as a unitary mat from the drum. The mat was then flattened, and a matrix slurry having the same composition as that of the previous investigation was then poured onto the mat and worked into the unidirectional tow by hand with a plastic squeegee. After drying, the now-impregnated tape was removed from the release sheet and used to lay-up an 8-ply 0-90 composite, which was subsequently autoclave laminated, burned-out and infiltrated using standard practices. Another sample of the same HI-NICALON® tow (with similar damage) was used to produce additional composites in the same manner, but using a conventional wet winding process to apply the slurry directly to the tow prior to winding. The experimental composite made using the dry winding process was thinner than the composite produced by the wet winding process, with the result that the experimental composite had a higher fiber volume fraction. The resulting composites were then tensile tested at room temperature, with the resulting stress-strain curves shown in
While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.
This invention was made with Government support under Agreement No. DE-FC02-92CE41000 awarded by the Department of Energy. The Government has certain rights in the invention.
