The present invention generally relates to composite articles and processes for their production. More particularly, this invention is directed to a process of fabricating composite articles containing multiple plies of continuous fiber reinforcement material, by which delamination of the plies within bends of such articles is inhibited.
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. Ceramic materials and ceramic matrix composite (CMC) materials are notable examples because their high temperature capabilities can significantly reduce cooling air requirements. CMC materials are generally characterized by a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material may be discontinuous short fibers dispersed in the matrix material or continuous fibers or fiber bundles oriented within the matrix material, and serves as the load-bearing constituent of the CMC in the event of a matrix crack. In turn, the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material.
Continuous fiber reinforced ceramic composites (CFCC) are a type of CMC that offers light weight, high strength, and high stiffness for a variety of high temperature load-bearing applications, including shrouds, combustor liners, nozzles, and other high-temperature components of gas turbine engines. Continuous fibers (filaments) of a CFCC material may be arranged to form a unidirectional array of individual fibers that are roughly parallel. Alternatively, fibers may be bundled in tows that are arranged to form a unidirectional array of tows, or bundled in tows that are woven to form a two-dimensional fabric or woven or braided to form a three-dimensional fabric. For three-dimensional fabrics, sets of unidirectional tows may, for example, be interwoven transverse to each other. In CFCC materials with unidirectional arrays of individual fibers, the fibers are typically arranged in alternating layers, each containing an array of unidirectional (parallel) fibers, with fibers in any given layer being transverse to the unidirectional fibers of each adjacent layer.
Of particular interest to high-temperature applications are silicon-based ceramic materials, and particularly CMCs that contain silicon carbide (SiC) as the matrix and/or reinforcement material. A notable example of a CFCC has been developed by the General Electric Company under the name HiPerComp®, and contains continuous silicon carbide fibers in a matrix of silicon carbide and elemental silicon. Notable silicon carbide fiber materials include NICALON®, HI-NICALON®, and HI-NICALON® Type S fibers commercially available from Nippon Carbon Co., Ltd. Examples of SiC/Si—SiC (fiber/matrix) CMC and CFCC materials and processes are disclosed in commonly-assigned U.S. Pat. Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, 6,024,898, 6,258,737, 6,403,158, and 6,503,441, and commonly-assigned U.S. Patent Application Publication No. 2004/0067316. One such process is known as “prepreg” melt-infiltration, which in general terms entails the fabrication of CMCs using multiple prepreg layers, each in the form of a tape-like structure comprising the desired reinforcement material, a precursor of the ceramic matrix material, and one or more binders. A laminate preform is formed by laying up multiple prepreg tapes, and undergoes processing (including firing) to burn out the binders and convert the precursor to the desired ceramic matrix material. The resulting CMC contains multiple laminae, each derived from an individual prepreg tape. As a result of curing and firing the laminate preform, each lamina contains the reinforcement material encased in the ceramic matrix formed by conversion of the precursor during firing, and the laminae are bonded to each other by the ceramic matrix. The resulting CMC article then undergoes infiltration within molten silicon or another suitable material to fill porosity that formed as a result of removing the binder during burn-out.
Delamination defects occur as a result of separation of the laminae within the CMC structure.
During lay-up and compaction, as occurs, for instance, during the heating of the laminate preform 10 under pressure, for example, during autoclaving, in-plane compressive strains are created in the outer prepreg layers 12 in response to through-thickness strains induced during compaction and by the geometry of the bend. The continuous fibers that are oriented directly around the bend radius R and lie in a plan perpendicular to the axis 16 of curvature of the bend will subsequently be subjected to high compressive stress along their axes. This compressive stress within the fibers is believed to cause defects in the composite in at least two ways. First, the fibers can buckle under the compressive stress, leading to the formation of wrinkles within the composite plies. Secondly, during subsequent burnout and infiltration process steps, the strain in the fibers is elastically released, which may cause delaminations 24 of the laminae 22 within the resulting CFCC article 20, as represented in
In practice, interlaminar delaminations 24 of the type described above for CMC articles containing unidirectional fiber structures are not observed in CMC articles containing woven fiber plies, such as cloth and woven structures that tend to have better drapability and are thus less prone to forming interlaminar defects around bends. On the other hand, because interlaminar delaminations are a recognized problem for CFCC articles with unidirectional fiber structures, solutions have been investigated and proposed, as evident from U.S. Pat. No. 5,348,602. However, solutions that do not entail special fixturing and steps would be desirable.
The present invention provides a process for inhibiting delamination in a bend of a component formed of a continuous fiber-reinforced composite material comprising layers (laminae) that are prone to delamination during thermal processing of the composite material.
According to a first aspect of the invention, each layer of the composite material contains an array of unidirectional fibrous elements in a matrix material, and at least a portion of the bend is defined by an axis of curvature. The process entails forming prepreg tapes, each containing a matrix precursor, a binder, and one of the arrays of the unidirectional fibrous elements so that each prepreg tape will form one of the layers of the composite when sufficiently heated to burn out the binder and convert the matrix precursor to the matrix material of the component. The prepreg tapes are then laid-up to form a preform of the component and define the bend thereof, and so that the unidirectional fibrous elements of at least a first of the tapes traverse the bend and the unidirectional fibrous elements of all the tapes lie in planes that are not perpendicular to the axis of curvature of the bend. The preform is then heated under external pressure to consolidate the prepreg tapes. According to a preferred aspect of the invention, delamination of the prepreg tapes in the bend, and therefore delamination of the resulting composite layers in the bend, is inhibited as a result of none of the unidirectional fibrous elements lying in a plane that is perpendicular to the axis of curvature of the bend, as opposed to orientations perpendicular to the axis.
Another aspect of the invention are the components produced by the process described above. A nonlimiting example is a turbine shroud of a gas turbine engine.
A significant advantage of this invention is the ability to produce continuous fiber-reinforced composite components, and particularly continuous fiber-reinforced ceramic composite (CFCC) components having shapes with bends that are prone to delamination during thermal processing of the composite material. By orienting the individual fibers within unidirectional arrays to be oriented at an angle other than perpendicular to the bend axis, interlaminar defects caused by delamination can be greatly inhibited. While +/−45 degree fiber architectures have been shown to be effective in inhibiting delamination, other angles are also within the scope of the invention, particularly in a range of about ten to ninety degrees from a plane perpendicular to the bend axis of curvature. Particular nonlimiting examples include biaxial fiber architectures such as +80/−10, +70/−20, +60/−30, etc., as well as multi-axial fiber architectures such as 90/+30/−30, etc.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
The present invention will be described in terms of processes for producing composite articles containing continuous fibers arranged to form unidirectional arrays of fibrous elements, and particularly processes for producing CFCC articles. Various composite materials are encompassed by the invention, a preferred but nonlimiting example being CFCC materials that contain silicon carbide reinforcement fibers in a ceramic matrix that contains silicon carbide. While various applications are foreseeable, particular applications include components of gas turbine engines, nonlimiting examples of which include turbine shrouds used in the hot gas paths of gas turbine engines.
For the purpose of facilitating the present disclosure of the invention, the preform 30 of
To avoid or at least greatly inhibit delamination defects that occur during thermal processing of prior art preforms of the type shown in
In investigations carried out in furtherance of this invention, the +/−45 degree fiber architecture has been shown to be particularly effective in inhibiting delamination of CFCC articles produced in the manner described above, namely, thermal processing that includes tape consolidation, binder burn-out, and melt infiltration, during which delamination is likely to occur in prior art preforms of the type represented in
While not wishing to be held to any particular theory, the ability of the fiber architecture of this invention to avoid or at least greatly inhibit the formation of delamination defects during thermal processing of the preform 30 may be related to the occurrence of fiber shifts that may occur during thermal processing, particular during consolidation of the prepreg tapes. Such a shift phenomena is represented in
Defect-free CFCC shrouds that have been produced in accordance with this invention include the shroud whose IR thermographic image is shown in
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. Therefore, 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-FC02-00CH11047 awarded by the Department of Energy. The Government may have certain rights in the invention.