Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-12-18
2003-09-30
Mayes, Melvin C. (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S089260, C156S155000, C264S610000, C264S640000, C264S642000
Reexamination Certificate
active
06627019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the manufacture of ceramic matrix composite parts having cooling channels therein.
2. Background Information
Ceramic matrix composites(“CMC”) are well known in the art and may comprise ceramic fibers in a ceramic matrix. CMC are used in high temperature environments, such as in the hot section of a gas turbine engine. For handleability, and to achieve desired mechanical and thermal property orientation in the final product, the ceramic fibers may be woven together to form essentially two-dimensional plies of fiber “cloth”, or they may be woven into a three dimensional preform of the desired thickness. The preform and matrix are then consolidated in a mold by any of several well know processes, such as by melt infiltration, chemical vapor infiltration, or pre-ceramic polymer processing.
One known CMC composition is made using silicon carbide fibers in a silicon carbide matrix (commonly referred to as a SiC/SiC composite). Another well know CMC composition uses SiC fibers in a silicon-nitrogen-carbon matrix (commonly referred to as a SiC/SiNC composite). Although, using current technology, these and other ceramic matrix composites can withstand temperatures as high as 2200° F., they still need to be cooled for today's gas turbine applications, and improvements are being sought to even further increase their temperature capabilities. Usually the walls of gas turbine engine components are made as thin as possible to minimize weight and to decrease the “through thickness” thermal stresses. The wall thickness of hollow CMC turbine vanes is usually no more than about 0.1 inch, and may be considerably less.
A common method for preventing the degradation of components subject to very high temperatures is to flow cooling fluid through channels or passageways within the component. Current CMC fabrication techniques and materials make it difficult to form curved channels and in-plane internal channels having the requisite shapes, locations, tolerances, and dimensions, especially in-plane small diameter channels for thin walled components.
It is well known that straight passages or channels extending from an outer surface of the part to the interior or entirely through the part can be formed by laser machining after consolidating the fiber preform and matrix material; however, laser drilling has its limitations since curved channels cannot be laser drilled, and channels oriented substantially parallel to a wall surface (i.e. in-plane) cannot be made by laser drilling in thin walled parts, unless drilled into an edge. In any event, laser drilling is practical only for straight passages of less than about 0.5 inch long.
Voids or channels of various shapes may also be created within CMC parts by the steps of (a) inserting either carbon rods or graphite paper between the fiber plies as they are being laid-up in the mold, and (b) burning-out (thermal decomposition by oxidation) the carbon after consolidation of the preform and matrix material. The use of carbon rods is not practical for long, narrow or curved channels since they are brittle and mechanically weak (i.e., a 0.02 inch diameter carbon rod has a tensile strength of less then 1.0 ksi), breaking easily during the matrix consolidation step. Graphite paper also has a low mechanical strength and tendency to break during handling and weaving (i.e., a tensile strength of less than about 1.0 ksi). Further, neither the use of carbon rods nor graphite paper is suited to an automated manufacturing process, especially where the channels are long, have small diameters, must be carefully located, may need to interconnect with each other, or may need to have a small radius of curvature. More specifically, in view of manufacturing limitations, it has not been possible to make ceramic matrix composite parts with small diameter (i.e. less than and effective diameter of about 0.10 inch) elongated channels that include a change in direction requiring a radius of curvature smaller than about 6.0 inches. Thus, zig-zag channels could not be formed. Also, it was not possible to form a complex pattern of intersecting elongated channels, such as an intersecting grid of channels within a thin wall. As used herein and in the appended claims, “elongated” means having a length to “effective diameter” ratio of at least about 50. Effective diameter is the diameter of a circle having the same cross sectional area as the channel in question.
BRIEF SUMMARY OF THE INVENTION
In the manufacture of a ceramic matrix composite part, inserts having essentially the size and shape of elongated channels to be formed within the part are disposed at a desired location within a woven ceramic fiber preform. The inserts comprise a plurality of carbon fibers surrounded by a carbonaceous filler. After the inserts are in place, a ceramic matrix material is added and the fiber preform is consolidated with the matrix material. The consolidated part is then heated to thermally decompose the inserts leaving elongated channels within the part. The inserts may be rods of carbon fibers or may be flexible, weaveable carbon fiber tows.
During the consolidation step, the carbonaceous filler material fills the interstices between the carbon fibers of the inserts and acts as a protective shell around individual carbon fibers and around any bundles of those fibers that comprise the insert. This inhibits process gases and matrix material from entering voids between the carbon fibers and from directly contacting the carbon fibers. The matrix material, if able to contact the fibers, would inhibit the oxidation of the carbonaceous inserts by depositing a non-oxidizable coating on the fibers, making successful removal of the carbon fiber difficult, if not impossible. Any matrix material that works its way between the carbon fibers will not be removed during thermal decomposition. Thus, by way of example, in the chemical vapor infiltration (CVI) consolidation of a SiC/SiC composite, the carbonaceous filler inhibits the methyltrichlorosilane gas from contacting the carbon fibers and depositing silicon carbide thereon.
As used herein and in the claims, a “carbonaceous” filler material is a material that produces at least about 10% carbon residue, by weight, upon thermal decomposition in a non-oxidizing environment. Preferably the filler will form a high surface area material upon decomposition, such as a film or closed cell foam that will inhibit infiltration of the matrix material between the carbon fibers. Without intending to limit the same, examples of carbonaceous filler materials usable in the process of the present invention are colloidal graphite (with or without a binder) and polymers containing sufficient carbon molecules to satisfy the definition of “carbonaceous”, such as epoxy, silicone, and polyacrylonitrile.
The process of the present invention is particularly suited for (although not limited to) forming in-plane elongated channels of small effective diameter (i.e. small cross sectional area) between the surfaces of thin walled components, and for closely locating and closely spacing those channels. “Small effective diameter” means a cross section having an effective diameter of less than about 0.10 inch. Parts made by the process of the invention are particularly unique in that they may be made with elongated channels of small effective diameter, wherein the channels may have a radius of curvature of less than 1.0 inch, and even less than 0.02 inch. The weavable braided fibers used in the process of the present invention to form the channels have actually been wrapped around a pin of 0.015 inch diameter without major damage and without significant flattening (which would result in a smaller cross sectional channel area around sharp curves). This makes the process of the present invention particularly suited for making thin walled parts having a plurality of curved, elongated channels of small effective diameter. By a “thin wall” it is meant a wall having a thickness of 0.25 inch or less.
In accordance with one em
Bond Bruce
Burd Steven Wayne
Hu Xiaolan
Jarmon David C.
Jones Christopher Dale
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