Method of producing an internal cavity in a ceramic matrix composite and mandrel therefor

Abstract

A process for producing an internal cavity in a CMC article and mandrels used therewith. The process entails incorporating a mandrel made of a material that is substantially absorbed during thermal treatment of a preform to form the CMC article. The mandrel material is preferably reactive with one or more constituents of the CMC preform during the thermal treatment. The material is preferably silicon or a silicon alloy.

Claims

1. A method of forming a CMC article with at least one internal cavity, the method comprising: coating at least one mandrel with a coating that is impervious to constituents of the CMC preform; wrapping ceramic composite precursor prepreg plies over the coating of the at least one mandrel, wherein each mandrel comprises silicon; curing the ceramic composite precursor prepreg plies into a laminate CMC preform, wherein the laminate CMC preform comprises resinous components; after curing, heating the laminate CMC preform and the at least one mandrel to above 500 C. to remove the resinous components to yield a porous CMC preform from the laminate CMC preform; and after heating, subjecting the porous CMC preform to a thermal treatment, wherein the silicon of each mandrel melts to yield molten silicon that wets the CMC preform, and is reacted with and/or absorbed into the CMC porous preform leaving behind at least one internal cavity within the CMC preform.

2. The method according to claim 1, wherein the at least one mandrel consists of silicon or a silicon alloy.

3. The method according to claim 1, wherein the thermal treatment comprises a melt-infiltration step.

4. The method according to claim 1, wherein the at least one internal cavity comprises multiple internal cavities and the at least one mandrel comprises multiple mandrels.

5. The method according to claim 1, wherein the at least one mandrel contains a material that is chemically reactive with a constituent of the CMC preform.

6. The method according to claim 1, wherein the mandrel comprises a powder silicon material.

7. The method according to claim 1, wherein the ceramic composite precursor prepreg plies are based on silicon compounds.

8. The method according to claim 7, wherein the silicon compound is SiC.

9. The method according to claim 1, wherein the at least one mandrel consists of elemental silicon.

10. The method according to claim 1, further comprising: after coating the at least one mandrel with the coating and prior to wrapping the ceramic composite precursor prepreg plies over the at least one mandrel, sintering the at least one mandrel.

11. The method according to claim 1, wherein the coating comprises an acrylic resin.

12. The method according to claim 1, wherein the acrylic resin is methyl methacrylate.

13. The method according to claim 1, wherein subjecting the porous CMC preform to a thermal treatment is performed within a vacuum furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically represents a cross-section of a CMC preform with a non-reactive mandrel.

(2) FIG. 2 schematically a cross-section of a CMC preform with a reactive or otherwise absorbable mandrel.

(3) FIG. 3 schematically represents a cross-section CMC article with a cavity achieved through the use of a reactive or otherwise absorbable mandrel.

DETAILED DESCRIPTION OF THE INVENTION

(4) This invention is directed to the creation of internal cavities within CMC articles, for example, to create cooling channels, achieve weight reduction and/or any other desired purpose. Conventional processes of creating such cavities have utilized mandrels formed of materials such as fugitive resins or non-reactive metals. Both of these methods have several limitations and disadvantages as described previously. The current invention addresses difficulties and disadvantages of the prior art by methods that incorporate use of mandrels made of materials that can be absorbed by and preferably reacted with a CMC preform used in the manufacture of a CMC article. In particular, preferred materials for mandrels employed with the invention are molten at a thermal treatment temperature of the preform, for example, during melt infiltration performed after a curing (firing) step carried out on a laminate preform to form a porous preform.

(5) Preferred characteristics for materials for mandrels that can be advantageously eliminated by absorption and reaction with a CMC preform include the ability to be formed into a suitable shape for a mandrel, wet the CMC preform at melt infiltration temperatures, react with constituents of the CMC preform to form reaction products that are advantageous or at least not detrimental to the final article, and be absorbed nearly completely by the CMC preform either by reaction, by infiltration, or both.

(6) Preferred materials for such mandrels are silicon and alloys of silicon. Sintered silicon-containing mandrels can be manufactured by, for example, damp pressing and sintering a powder material in large lots to minimize costs. Elemental silicon and silicon alloy powder materials are capable of exhibiting nearly zero shrinkage during sintering, yet exhibit sufficient strength to survive handling and autoclave curing pressures. Furthermore, a sintered silicon-containing mandrel can remain within the preform during the entire process sequence leading up to melt infiltration and exhibit thermal expansion characteristics similar to those of the CMC preform. While silicon or silicon-based materials can be well suited as the material of a mandrel used to create a cavity within a SiC-based CMC articles, it is foreseeable that different mandrel materials with different reaction and/or infiltration characteristics may exist or be developed and would be compatible with the chemistry of a CMC article.

(7) A mandrel of this invention may be formed entirely of elemental silicon or a silicon alloy. Alternatively, sintered silicon-containing mandrels may also contain fugitive binders, such as acrylic resins or polyvinyl alcohol. A small amount of water or alcohol may be added to render the initial powder mixture damp and suitable for pressing in a mold. The powder mixture can be pressed under sufficient pressure to yield a desired freestanding shape, herein after called a core. In a preferred, non-limiting method, this core can be dried and then loaded into a vacuum furnace to undergo sintering, for example, at a temperature of about 1385+/10 C. for about ninety minutes, to render it free-standing and able to survive being covered with ceramic composite precursor prepreg plies to yield a laminate CMC preform. Since the core shrinks during the sintering operation, allowance must be made by oversizing the original pressed core shape. The sintered shape is then removed from the vacuum furnace. One could also envision using a 3-dimensional (3-D) printer with silicon powder in a printing ink suitable for use with 3-D printers as a way to make a mandrel with fine features. A core made with silicon ink can be sintered as described above.

(8) The mandrel is preferably coated to yield a substantially impervious surface capable of preventing any resins of the CMC preform from penetrating the sintered silicon mandrel during lay-up and curing of the laminate CMC preform. An impervious surface is particularly desirable if preform resins are of the type that would form a silicon compound, such as silicon carbide, and could therefore react with the silicon in the mandrel and possibly cause the inner cavity dimensions formed by the mandrel to be altered. Suitable coating materials for this purpose include, but are not limited to, acrylic spray resins, such as poly methyl methacrylate.

(9) Ceramic composite precursor prepreg plies may then be wrapped over the sintered mandrel shape and cured in an autoclave or matched-plate die set. After curing, the preform and sintered mandrel may be heated to above 500 C. to remove the resinous components to yield a porous preform. The porous preform plus the sintered silicon core and any additional silicon can then be further heated in another vacuum or atmosphere furnace to cause the silicon to melt and infiltrate the porous preform. FIG. 2 schematically represents a sintered silicon mandrel 40 incorporated into a section 20 of a laminate preform 10. During a melt infiltration process or other thermal treatment of the preform 10, the silicon mandrel 40 melts and the resulting molten material wets the CMC preform 10, infiltrates into the preform 10 and chemically reacts with constituents of the CMC preform to form, for example, SiC or another silicon compound. Any unreacted silicon may simply remain infiltrated. FIG. 3 schematically represents a cavity 50 formed in the section 20 of the fully infiltrated CMC preform 10 indicated in FIG. 3 as the final CMC article 100.

(10) It will be noted by those skilled in the art that use of a reactive mandrel material can result in unconsumed mandrel material. This condition can be avoided by proper tuning of the processing conditions. The presence of unconsumed mandrel material within the CMC article may not have any deleterious effects except in cases wherein the weight of the CMC article has to be closely controlled. In such cases complete consumption of the mandrel material can be ensured through additional processing steps as required. Multiple cavities can be formed in a CMC preform utilizing multiple mandrels and following the methods described herein. A single cavity or multiple cavities formed in a CMC article can be utilized for purposes of weight reduction, and/or as cooling slots/holes.

(11) It is foreseeable that other materials could be used that wet the CMC preform 10 during a melt-infiltration process and are completely absorbed through penetration into the CMC preform 10, but would not necessarily react as silicon does with the CMC preform 10. However, in such cases one needs to ensure that such materials do not contribute to any ill effects either due to thermal expansion characteristics or other physical properties. Preferred embodiments of the invention are thus directed to the utilization of silicon-containing materials such that no extraneous materials are used or formed other than those of conventional CMC melt-infiltration processes, resulting in a SiC-based CMC article. It is to be further noted that the methods disclosed may be used to create internal cavities in CMC articles based on silicon compounds other than SiC, such as SiN as anon-limiting example.

(12) In view of the above, it can be seen that a significant advantage of this invention is that it solves problems associated with forming hollow internal cavities within CMC articles without having to physically remove a mandrel from the resulting cavity after curing, and without introducing potentially deleterious materials into the final CMC article.

(13) While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, it should be understood that the invention is not limited to the specific disclosed embodiments. It should also be understood that the phraseology and terminology employed above are for the purpose of disclosing the invention and the embodiments, and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims.