Method of producing an internal cavity in a ceramic matrix composite

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 fusible material that is melted and drained during a thermal treatment of a CMC preform to form the CMC article. The mandrel material is preferably non-wetting and non-reactive with any constituents of the CMC preform during the thermal treatment. The mandrel is preferably tin or an alloy of tin.

Claims

1. A method of forming a CMC article to have at least one internal cavity, the method comprising: incorporating at least one mandrel into a CMC preform via laying the CMC preform on the at least one mandrel, wherein the CMC preform comprises multiple prepreg layers in the form of a tape-like structure comprising a reinforcement material, a precursor of a CMC matrix material, and binders, wherein the at least one internal cavity comprises multiple cavities and the at least one mandrel comprises multiple mandrels; and subjecting the CMC preform to a thermal treatment wherein the at least one mandrel melts to form a molten material that drains from the CMC preform to leave behind at least one internal cavity within the CMC preform.

2. The method according to claim 1, wherein the mandrel consists of tin or a tin alloy.

3. The method according to claim 1, wherein the thermal treatment is a burn-out process.

4. The method according to claim 1, wherein the at least one mandrel is free of materials that are wetting and/or reactive to constituents of the CMC preform.

5. The method according to claim 1, the method further comprising recovering the molten material and forming another mandrel from the molten material.

6. The method according to claim 1, the method further comprising coating the at least one internal cavity with a slurry comprising a ceramic material.

7. The method according to claim 6, wherein the ceramic material is boron nitride.

8. The method according to claim 7, wherein the average particle size of boron nitride is about 0.5 to 1.0 micrometer.

9. A method of forming a CMC article to have at least one internal cavity, the method comprising: incorporating at least one mandrel into a CMC preform via laying the CMC preform on the at least one mandrel, wherein the CMC preform comprises multiple prepreg layers in the form of a tape-like structure comprising a reinforcement material, a precursor of a CMC matrix material, and binders, the at least one mandrel consisting of a material that is non-wetting and non-reactive with any constituents of the CMC preform, wherein the at least one internal cavity comprises multiple cavities and the at least one mandrel comprises multiple mandrels; subjecting the CMC preform to a burn-out process wherein the at least one mandrel melts to form a molten material that does not wet the CMC preform, does not react with constituents of the CMC preform, and drains from the CMC preform to leave behind at least one internal cavity in the CMC preform; and melt-infiltrating the CMC preform with an infiltrant to form the CMC article which has the at least one internal cavity.

10. The method of claim 9, wherein the material of the at least one mandrel is elemental tin.

11. The method of claim 9, wherein the material of the at least one mandrel is a tin alloy.

12. The method of claim 9, the method further comprising coating the at least one internal cavity with a slurry of a ceramic material prior to the melt infiltration step, the ceramic material not being wettable by the infiltrant.

13. The method of claim 12, wherein the ceramic material is boron nitride.

14. The method of claim 13, wherein the average particle size of boron nitride is about 0.5 to 1.0 micrometer.

15. The method of claim 9, wherein the at least one internal cavity is a cooling slot or hole.

16. The method of claim 9, wherein the CMC article is an airfoil component.

17. The method of claim 9, wherein the CMC article is based on at least one silicon compound.

18. The method of claim 17, wherein the infiltrant is silicon and the at least one silicon compound is SiC.

19. The method of claim 1, wherein the CMC perform in the incorporating step is uncured and is laid on a surface of the at least one mandrel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 2 schematically represents a cross-section of a CMC preform with a mandrel made of a fusible material which is non-wetting and non-reactive to the constituents of the CMC preform.

(3) FIG. 3 schematically represents a cross section of a CMC article with a cavity achieved through the draining and recovery of a mandrel made of a fusible material which is non-wetting and non-reactive to the constituents of the CMC preform from which the article was formed.

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 utilize mandrels made of fusible materials that can be molten and drained off during the burnout process of the CMC preforms and prior to the melt-infiltration stage 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 the 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 from a CMC preform include non-wetting of the CMC preform, low or no reactivity with the constituents of the CMC preform, and being completely fusible and drainable at a temperature of a thermal treatment performed on the CMC preform. In preferred embodiments of the invention a fusible mandrel comprises or consists of elemental tin or a tin alloy. Such a mandrel can be cast into the shape of the desired inner cavity and incorporated into the CMC preform. This mandrel, made of such a low melting metal or alloy as tin or its alloys, can melt and be allowed to drain from the preform during the burnout operation of the CMC preform leaving a hollow cavity in the CMC preform.

(6) A mandrel of this invention many be formed entirely of elemental tin or an alloy of tin. Mandrels of this invention can be cast into the desired shapes for the inner cavities of a CMC article and prepreg tapes can be directly laid up on the mandrels. The low-melting tin or alloy melts during the burn-out stage of the CMC preform and is allowed to drain from the preform leaving behind a hollow cavity of the desired shape. Tin and its alloys have been demonstrated to possess the desired characteristics of a fusible material that is non-wetting and non-reactive with CMC preforms. Furthermore, mandrels made of such fusible and recoverable materials can be removed during the burnout processes without the thermal expansion issues generally associated with using fugitive polymers as mandrel materials.

(7) As tin or tin alloy used for the mandrel is solid when incorporated into the CMC preform and is subsequently molten while being non-wetting and non-reactive with the CMC preform, the dimensions of an inner cavity that is subsequently formed substantially match the dimensions of the fusible mandrel. FIG. 2 schematically represents a cross section of a fusible mandrel 40 incorporated into a section 20 of a laminate preform 10. During the CMC preform burnout process, the fusible mandrel made of tin or tin alloy melts, does not wet or react with the constituents of the CMC preform 10, and is drained away from the CMC preform 10. In a subsequent operation, for example, melt infiltration, a CMC article can be formed from the preform 10. 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. It is possible that tin oxide may form during the burn-out process when the fusible mandrel melts; however, any tin oxide formed will completely evaporate at the subsequent processes used to form the CMC article such as, for example, melt infiltration.

(8) In some processes of forming CMC articles based on silicon compounds, such as, for example, SiC, melt-infiltration may be used utilizing molten silicon as an infiltrant. In such cases, after an internal cavity is formed in a CMC preform, ingress of the infiltrant into the internal cavity during melt infiltration can be prevented or at least inhibited by coating the surface of the internal cavity with a slurry comprising a powder of a ceramic material that is not wettable or at least exhibits low wettability to the infiltrant, for example, boron nitride (BN) if the infiltrant is silicon. The boron nitride slurry can be water-based and a drying step can be used to achieve the coating. Particles of the powder of the ceramic material should be large enough not to go through the openings in the porous preform and small enough to be made into a slurry which can result in uniform coating. The preform openings are typically in the range of 0.1-0.2 micrometer. Accordingly, a preferred average particle size of a boron nitride powder used in a slurry to effect a ceramic coating for the internal cavity when the infiltrant is silicon is about 0.5-1.0 micrometer. This coating advantageously prevents the surfaces of the internal cavity from being wet by the infiltrant during a melt-infiltration step in forming the CMC article, and hence prevents the formation of silicon in the internal cavities of the CMC article.

(9) It is foreseeable that other fusible materials could be used that do not wet the CMC preform or react with the CMC preform during the burn-out process and are completely removable at the CMC preform burn-out temperatures. The preferred embodiment of the invention is thus utilization of any low-melting material such that no materials are formed other than those intended to be created in conventional CMC manufacturing processes, such as melt-infiltration processes that form, for example, SiC. Another feature of this invention is that a fusible material which can be melted during a thermal treatment of the a CMC preform and drained off to form an internal cavity in a laminate preform can be recycled to form another mandrel for use to form an internal cavity in another CMC article. Thus the invention can be described as a method of forming an internal cavity in a CMC preform or a final CMC article using recyclable mandrels.

(10) 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 inner cavity after curing the CMC preform, and without introducing potentially deleterious materials into the final CMC article.

(11) 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 slot/hole or slots/holes.

(12) 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.