The subject matter of the present disclosure relates generally to ceramic matrix composites (CMC) and methods for making the same.
Ceramic matrix composites (CMCs) generally include a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications, such as in gas turbine engines or hypersonic applications, are silicon-based composites, which include silicon carbide (SiC) as the matrix and the reinforcement material. CMCs, particularly continuous fiber ceramic composite (CFCC) materials, are currently being utilized for shrouds, combustor liners, nozzles, and other high-temperature components of gas turbine engines.
Different infiltration methods have been employed in forming CMCs. For example, one approach includes chemical vapor infiltration (CVI). CVI is a process whereby a matrix material is infiltrated into a fibrous preform by the use of reactive gases at elevated temperature to form the fiber-reinforced composite. CVI composite matrices typically have no free silicon phase, and thus have good creep resistance and the potential to operate at temperatures above 1400° C., or about the melting point of silicon depending on the impurities therein. One drawback to CVI is the excess residual porosity that occurs when the pores become closed off. The closed off pores prevent the reactive vapor infiltrant from penetrating into the interior of the preform. This reduces matrix dominated properties such as the interlaminar tensile strength.
Another infiltration approach includes melt infiltration (MI), which employs molten metal to infiltrate into a fiber-containing preform. While the MI process leaves no or minimal residual porosity, some of the molten metal remains unreacted within the preform. Accordingly, the matrix of MI composites typically contains an amount of a free metal phase (e.g., elemental silicon or silicon alloy for silicon melt infiltration) that limits use of the CMC to below that of the melting point of the silicon or silicon alloy, or about 1400° C. Moreover, the free metal phase causes the MI SiC matrix to have relatively poor creep resistance.
To realize the advantages and minimize the drawbacks of the CVI and MI infiltration processes, attempts at forming hybrid CMC articles that include CVI and MI infiltrated substrates have been made. However, forming such hybrid articles has proven to be difficult, namely due to the conflicting processing requirements of CVI and MI. For instance, in one approach, an MI substrate is laid up and is processed through MI. Then, additional plies are laid up onto the MI substrate and the article is then processed through CVI. The drawback of this approach is that the process temperature of the article is limited by the free silicon of the MI substrate, leading to an article with inferior mechanical properties. In another approach, a CVI substrate is laid up and is processed through CVI. Then, additional plies are laid up onto the CVI substrate and the article is then processed through MI. The drawback to this approach is that the MI substrate (or additional plies added to the CVI substrate) can be difficult to access due to the geometry of the article.
Accordingly, improved CMC articles and methods for forming CMC articles that address one or more of the challenges noted above would be useful.
Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
In one aspect, the present disclosure is directed to a method for forming a CMC article. The method includes forming a CMC preform defining a first section and a second section, the first section comprising a slurry, reinforcing fibers, and sacrificial fibers and the second section comprising a slurry and reinforcing fibers. Further, the method includes removing the sacrificial fibers to define channels in the first section of the CMC preform. In addition, the method includes subjecting the CMC preform to chemical vapor infiltration to densify the CMC preform with an infiltrant. Further, the method includes subjecting the densified CMC preform to melt infiltration to backfill the channels with a liquid infiltrant.
In another aspect, the present disclosure is directed to a CMC article defining a first section and a second section. The CMC article includes a ceramic matrix and a plurality of ceramic reinforcing fibers disposed throughout the ceramic matrix. Further, the CMC article includes one or more infiltrant veins traversing the first section of the CMC article, wherein the second section has a thickness greater than about 0.75 mm.
In another aspect, the present disclosure is directed to a method for forming a CMC article. The method includes laying up a preform having a first section and a second section, the first section having a plurality of plies comprising a slurry and reinforcing fibers and the second section having a plurality of plies comprising a slurry and reinforcing fibers, and wherein one or more of the plurality of plies of the first section comprise sacrificial fibers. Further, the method includes consolidating the preform at elevated temperatures and pressures to form a pre-green state article. The method also includes firing the pre-green state article to form a green state article, wherein during firing, the sacrificial fibers are burned out such that a plurality of elongated channels are defined by the first section of the green state article. In addition, the method includes subjecting the green state article to chemical vapor infiltration to densify the green state article with an infiltrant to form a CVI-densified article. The method further includes subjecting the CVI-densified article to melt infiltration to backfill the plurality of elongated channels with an infiltrant.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:
Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment.
As used herein, the terms “first,” “second,” and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “upstream” and “downstream” refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows and “downstream” refers to the direction to which the fluid flows. As used herein, the “average particle diameter” or “average fiber diameter” refers to the diameter of a particle or fiber such that about 50% of the particles or fibers have a diameter that is greater than that diameter, and about 50% of the particles or fibers have a diameter that is less than that diameter. As used herein, “substantially” refers to at least about 90% or more of the described group. For instance, as used herein, “substantially all” indicates that at least about 90% or more of the respective group has the applicable trait and “substantially no” or “substantially none” indicates that at least about 90% or more of the respective group does not have the applicable trait. As used herein, the “majority” refers to at least about 50% or more of the described group. For instance, as used herein, “the majority of” indicates that at least about 50% or more of the respective group have the applicable trait.
In the present disclosure, when a layer is being described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers, unless expressly stated to the contrary. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer. Furthermore, chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth.
A method for manufacturing a CMC article is provided. The method includes forming a CMC preform defining a first section and a second section. The first section and the second section are formed of a plurality of plies derived from one or more prepreg tapes. The first section and the second section may be laid up in a stacked arrangement, e.g., before thermally processing the CMC preform. For instance, the second section can be stacked on the first section to form the CMC preform. One or more plies of the first section comprise a slurry, reinforcement fibers, and sacrificial fibers. The plies containing the sacrificial fibers can be interspersed with plies comprising a slurry and reinforcement fibers. The second section is formed of plies comprising a slurry and reinforcement fibers. Notably, none of the plies of the second section comprise sacrificial fibers. The sacrificial fibers can be introduced in the tape making and/or layup process of the manufacturing process and can be generally cylindrical bodies or have other shapes.
The sacrificial fibers can be disposed as single strands, woven or nonwoven mats, continuous grids (e.g., continuous in two dimensions and a single layer), or various other configurations as well as combinations thereof. The sacrificial fibers are generally resistant to solvents present in the tape making process and have enough thermal integrity to resist flow during the autoclave process. The sacrificial fibers also generally do not decompose at temperatures present in the autoclave process; however, the sacrificial fibers do decompose during the burnout process. The composition of the sacrificial fibers may be chosen to target a specific char yield to provide the desired structure of the elongate channels. For example, in some embodiments, it may be desired to have some degree of scaffolding in the elongate channels, thus, a polymer with a higher char yield may be used to form the sacrificial fibers. In other embodiments, it may be desired to have uniform elongate channels, thus, a polymer with a lower char yield may be used to form the sacrificial fibers. In yet other embodiments, the sacrificial fibers can be composed of metal and mechanically removed from the preform.
Once the CMC preform is formed, the CMC preform can undergo thermal processing. For instance, the CMC preform can be consolidated, e.g., at elevated temperatures and pressures in an autoclave, fired or burned out to pyrolyze the matrix precursor of the slurry, and infiltrated to densify the porous fired CMC preform. In some implementations, the sacrificial fibers can be removed mechanically, thermally (e.g., melting, vaporizing, and/or decomposing), and/or chemically (e.g., dissolving into a solvent and/or chemical etching). In some implementations, for example, firing the CMC preform decomposes or otherwise removes the sacrificial fibers, resulting in formation of channels. In certain embodiments, the sacrificial fibers are resistant to any solvent present in the tape making process and are able to survive autoclave conditions (for example, temperatures of about 200° C. or less, such as about 50° C. to about 200° C.). In some embodiments, the sacrificial fibers decompose or pyrolyze to form porous elongated channels within the CMC preform, such as under decomposition conditions at temperatures such as about 200° C. to about 650° C.
Notably, the formed channels are arranged in a gradient along the thickness of the part. That is, as only one or more plies of the second section included sacrificial fibers, channels are formed only within the first section of the CMC preform and not within the second section. Further, the diameter, position, volume fraction, and length of the sacrificial fibers disclosed herein can provide the desired size, shape, and distribution of the channels within the first section of the CMC preform. One or more sacrificial fibers can be used. The channels can be elongated channels. As used herein, “elongate” or “elongated” refers to a body with an aspect ratio (length/width) of greater than 1.
Densification of the green state or fired CMC preform is performed via chemical vapor infiltration (CVI) and then via melt infiltration (MI). During CVI, a gaseous infiltrant infiltrates into the porous CMC preform to densify the CMC preform. The channels formed from the sacrificial fibers increase permeability, in a controlled manner, to improve infiltration into the CMC preform. Particularly, the channels facilitate infiltration into the porous, green state preform by providing gas transport paths for the gaseous infiltrant. The size or diameter of the channels prevent them from being plugged or closed off, thus allowing for infiltration into the interior portions of the CMC preform. This may, for example, reduce the residual porosity of the final CMC article. The use of the sacrificial fibers to form channels can be particularly beneficial for preforms requiring long infiltration distances to ensure complete infiltration. Further, the channels formed from the sacrificial fibers may also provide a pathway for gas to escape during CVI. Gas may evolve from preforms at infiltration temperatures, and if the gas does not have a way to escape, pressure can build in the preform. This may result in bubbles or other voids/pockets in the resulting CMC. The channels formed from the sacrificial fibers of the present disclosure may prevent the increase in pressure by providing a path for gas to escape the preform. In some implementations, the channels can be treated with a polymer solution prior to MI, e.g., to provide better wetting for improved capillary action of the infiltrant into the CMC preform.
After CVI or after treating the CVI-infiltrated CMC preform with a polymer solution, the CVI-densified CMC preform is subjected to MI to backfill the partially infiltrated channels. During the CVI process, the channels may only be partially filled and residual porosity may still be present in and along the channels. Accordingly, the CVI-densified CMC preform is melt infiltrated to backfill the elongated channels with an infiltrant (e.g., a liquid molten silicon) to further densify the article and minimize the residual porosity of the final CMC article. Due to the nature of the MI process, some of the liquid molten infiltrant may remain unreacted, and thus, infiltrant veins comprising unreacted infiltrant may be formed. For instance, in the case of silicon as the infiltrant, infiltrant veins comprising unreacted silicon may be present in the first section of the final CMC article. The second section of the CMC article does not include infiltrant veins because, as noted above, the second section does not include channels formed by sacrificial fibers.
The final CMC article having improved density and mechanical/thermal properties may thus be formed. The final CMC article can be thermally processed without need to layup additional tapes or plies. Notably, the final CMC article has a second section that is thermally capable of being exposed to environments having high temperatures, e.g. temperatures above the melting temperature of the unreacted infiltrant within the infiltrant veins of the first section. In short, the second section creates a thermal gradient between the high temperature environment and the first section of the CMC article that may, as noted above, contain unreacted infiltrant. Preferably, the second section of the CMC article has a thickness that creates a thermal gradient such that the first section of the CMC article is not exposed to temperatures above the melting temperature of unreacted infiltrant.
The CMC article can be utilized in a wide variety of applications and industries. For instance, the CMC article can be utilized in high pressure compressors (HPC), fans, boosters, high pressure turbines (HPT), and low pressure turbines (LPT) of both airborne and land-based gas turbine engines. For instance, the CMC article can be used for a turbofan engine or turbomachinery in general, including turbojet, turboprop and turboshaft gas turbine engines, including industrial and marine gas turbine engines and auxiliary power units. For example, the CMC article can be components such as combustion liners, shrouds, nozzles, blades, etc. The CMC article could also be used in other applications, such as a structural component in a hypersonic vehicle. A hypersonic vehicle can be a vehicle that travels at least 4 times faster than the speed of sound, or greater than Mach 4. Example hypersonic vehicles include, without limitation, airplanes, missiles, and spacecraft.
CMC materials of particular interest to the invention are silicon-containing, carbon containing, or oxide containing matrix and reinforcing materials. Some examples of CMCs for use herein can include, but are not limited to, materials having a matrix and reinforcing fibers comprising non-oxide based materials such as silicon carbide, silicon nitride, silicon oxycarbides, silicon oxynitrides, silicides, carbon, and mixtures thereof. Examples include, but are not limited to, CMCs with a silicon carbide matrix and silicon carbide fiber; silicon nitride matrix and silicon carbide fiber; silicon carbide matrix and carbon fiber; and silicon carbide/silicon nitride matrix mixture and silicon carbide fiber. Furthermore, CMCs can have a matrix and reinforcing fibers comprised of oxide ceramics. Specifically, the oxide-oxide CMCs may be comprised of a matrix and reinforcing fibers comprising oxide-based materials such as aluminum oxide (Al2O3), silicon dioxide (SiO2), yttrium aluminum garnet (YAG), aluminosilicates, and mixtures thereof. Aluminosilicates can include crystalline materials such as mullite (3Al2O3 2SiO2), as well as glassy aluminosilicates. Other ceramic composite materials that are not comprised of either silicon or oxygen may be used, including zirconium carbide, hafnium carbide, boron carbide, or other ceramic materials, alone or in combination with the materials noted above.
At (202), the method (200) includes forming plies. The plies can be derived from one or more prepreg tapes. In some implementations, the plies can be derived from a first prepreg tape and a second prepreg tape. More particularly, a plurality of first plies can be derived from a first prepreg tape and a plurality of second plies can be derived from a second prepreg tape. Accordingly, the method (200) can include forming a plurality of first plies derived from a first prepreg tape and forming a plurality of second plies derived from a second prepreg tape.
The reinforcing fibers 112 may be any suitable fibers that provide reinforcement for the resulting CMC article and may comprise any of the CMC materials set forth herein. The reinforcing fibers 112 may be more specifically referred to as ceramic reinforcing fibers 112. While in the embodiment illustrated in
In some embodiments, the reinforcing fibers 112 may have at least one coating thereon. For instance, in particular embodiments, the at least one coating can have a layer selected from the group consisting of a nitride layer (e.g., a silicon nitride layer), a carbide layer (e.g., a silicon carbide layer), a boron layer (e.g., a boron nitride layer), a carbon layer, and combinations thereof. For example, the at least one coating can be deposited as a coating system selected from the group consisting of a nitride coating and a silicon carbide coating; a boron nitride, a carbide, and a silicon nitride coating system; a boron nitride, a silicon carbide, a carbide, and a silicon nitride coating system; a boron nitride, a carbon, a silicon nitride and a carbon coating system; and a carbon, a boron nitride, a carbon, a silicon nitride, and a carbon coating system; and mixtures thereof. If present, the coating thickness can be about 0.1 micrometer (μm) to about 4.0 μm. In some embodiments, the reinforcing fibers 112 may coated with a silicon-doped boron nitride coating (B(Si)N).
The reinforcing fibers 112 are generally continuous in a single ply. That is, each reinforcing fiber 112 is generally a continuous strand across the ply as opposed to fragments of fibrous material. The reinforcing fibers 112 may have any suitable diameter or length to provide the desired ceramic product. In some embodiments, the reinforcing fibers 112 may have a diameter of about 5 μm to about 20 μm, such as about 7 μm to about 14 μm. In some embodiments, the reinforcing fibers 112 may be considered monofilaments and have an average diameter of about 125 μm to about 175 μm, such as about 140 μm to about 160 μm.
The slurry 114 can include various components such as a resin, a suitable curing agent, a binder, carbonaceous solids, particulates (e.g., silicon, polymers), a suitable solvent, a combination of the foregoing, and/or other suitable constituents. For instance, the slurry 114 may include various matrix precursor materials of the CMC materials set forth herein. Suitable ceramic precursors or powders for the slurry composition will depend on the composition desired for the ceramic matrix of the CMC article. For SiC—SiC articles, for example, suitable precursors or powders include carbon, and/or one or more other carbon-containing particulate materials. A suitable binder for use in the slurry composition is polyvinyl butyral (PVB), a commercial example of which is available from Eastman Chemicals under the name BUTVAR® B-79. Other potential candidates for the binder include other polymeric materials such as polycarbonate, polyvinyl acetate and polyvinyl alcohol. The selection of a suitable binder will depend in part on its compatibility with the rest of the slurry components. One example solvent can include isopropanol (C3H8O). In some embodiments, it may be beneficial to include surfactants, dispersing agents, and/or other components in the slurry, as well as matrix precursor material for the ceramic matrix.
In some embodiments, the sacrificial fibers 116 can include any suitable fibers that are stable in the slurry 114, can withstand compression and heating, and decompose during the decomposition/pyrolysis stage (e.g., at (208) of method (200)). In some embodiments, the sacrificial fibers 116 have a decomposition temperature or melting point at or lower than the temperature at which decomposition/pyrolysis is performed. For instance, the sacrificial fibers 116 may have a decomposition temperature of about 200° C. to about 700° C., such as about 200° C. to about 600° C., or about 400° C. to about 600° C. Suitable materials for the sacrificial fibers 116 may include polymers such as semi-crystalline polymers, cross-linked polymers, amorphous polymers, or combinations thereof, such as crosslinked phenolic resin, crosslinked poly (vinyl butyral), polyamides, polyesters, and combinations thereof. In certain embodiments, low melting point metals or reactive metals that can be etched via liquid or gases may be used as the sacrificial fibers 116 alone or in combination with any of the aforementioned sacrificial materials. While in the embodiment illustrated in
Generally, the sacrificial fibers 116 act as place holders until the firing or burnout process. As will be explained in detail herein, when the CMC preform is fired or burned out, the sacrificial fibers 116 are burned out or otherwise removed. As a result, a plurality of channels are defined or formed. Advantageously, the channels facilitate infiltration of an infiltrant into the article during CVI. Experimental and microstructural modeling studies have indicated the importance of channels, such as channels about 10 μm to about 300 μm in diameter, in supplying infiltrants, such as silicon, to the reaction front in composite parts, particularly thick composite parts. If there are too many channels or the channels are too large, the resulting infiltrant veins may reduce the mechanical and thermal properties of the part. To maximize the probability of infiltration success, while minimizing any mechanical/thermal property reduction, the size and distribution of the channels can be controlled as described herein.
For example, in some embodiments, a single sacrificial fiber may be used to deliver infiltrant to a particularly difficult to infiltrate area, while in other embodiments, such as larger parts with significant infiltrant delivery issues, more sacrificial fibers may be used. The sacrificial fibers 116 can also have any suitable diameter such as about 5 μm to about 600 μm, such as about 10 μm to about 500 μm, and may have any suitable aspect ratio (length/width), such as about 10 to about 10,000, or about 20 to about 5,000. In yet other embodiments, the sacrificial fibers 116 can have a diameter about 10 μm to about 200 μm. In certain embodiments, the sacrificial fibers 116 have an aspect ratio such that each sacrificial fiber traverses the substantial length or width of a CMC preform as continuous fibers.
As further shown in
The first ply 110 or plies may be prepared in a variety of ways. In some embodiments, the reinforcing fibers 112 and the sacrificial fibers 116 may be introduced into the slurry 114 along with other additional desired components. Once the slurry 114 is combined with the reinforcing fibers 112 and the sacrificial fibers 116, they may be wound on a drum roll to form a tape and then cut into plies. In other embodiments, the slurry can be introduced to the fibers via tape casting, screen printing, or any other suitable method. The slurry 114 and method of introducing the slurry 114 to the reinforcing fibers 112 and the sacrificial fibers 116 may be modified depending on the orientation of the reinforcing fibers 112 and the sacrificial fibers 116.
At (204), returning to
With reference now to
As shown, the first section 101 has a plurality of first plies (denoted as 110a, 110b, and 110c) and a plurality of second plies 120. The first section 101 can be laid up with any suitable number of plies. The first section 101 includes three (3) first plies 110 interspersed with the second plies 120. Stated differently, the first section 101 of the CMC preform 130 is laid up such that the second plies 120 comprising the sacrificial fibers 116 are spaced from one another by one or more second plies 120 that do not comprise the sacrificial fibers. Particularly, for this embodiment, the plies of the first section 101 are laid up such that every third ply is a first ply 110 and the two (2) plies between the first plies 110 are second plies 120. In this way, particularly for the 0-90° lay up arrangement of the first section 101, the sacrificial fibers 116 extend longitudinally in alternating directions. For instance, in this example, the sacrificial fibers 116 of the bottom first ply 110a extend longitudinally into and out of the page, the sacrificial fibers 116 of the middle first ply 110b extend longitudinally from the left to the right of the page, and the sacrificial fibers 116 of the top first ply 110c extend longitudinally into and out of the page. In alternative embodiments, the entire first section 101 of the CMC preform 130 may be formed of first plies 110. In yet other embodiments, the first section 101 of the CMC preform 130 may be formed by alternating first and second plies 110, 120. In some embodiments, two (2) first plies 110 can be laid up consecutively and spaced from one another by a number of first plies 110. This pattern may repeat for the thickness of the first section 101 of the CMC preform 130. In further embodiments, the first plies 110 can be interspersed with the second plies 120 in another suitable fashion. Interspersing second plies 120 with the first plies 110 can minimize the number of channels to backfill via MI.
The second section 102 of the CMC preform 130 has a plurality of second plies 120. Notably, the second section 102 does not include any first plies 110, or plies that include sacrificial fibers. Accordingly, when the sacrificial fibers 116 of the first plies 110 are removed (e.g., burned out during firing of the CMC preform), the resulting channels are arranged in a gradient along a first direction (e.g., the thickness) of the CMC preform 130. That is, a plurality of elongated channels are defined along the first section 101 of the CMC preform 130 and no elongated channels are defined along the second section 102 of the CMC preform 130.
For this embodiment, the second section 102 includes eight (8) second plies 120 each having a thickness of about 0.2 to 0.3 mm. In some embodiments, the second section 102 preferably has between about three (3) and ten (10) plies. In yet other embodiments, the second section 102 preferably has between one (1) and sixteen (16) plies. Further, in some embodiments, the thickness of the second section 102 is between about 0.75 mm and 3 mm. In yet other embodiments, the thickness of the second section 102 is between about 0.2 mm and 6 mm. In some embodiments, the second section 102 of the CMC preform 130 interfaces with a relatively hot environment (e.g., a hot gas path of a turbine engine) and the first section 101 of the CMC preform is spaced from the relatively hot environment (e.g., by the thickness of the second section 102).
In some embodiments, the first section 101 and the second section 102 can be laid up at the same time and then combined together. For example, the second section 102 can be laid up on the first section 101. In yet other embodiments, the first and second sections 101, 102 can be laid up successively with one layer or ply being laid one on top of the other, e.g., on a layup table or mold. Notably, the CMC preform 130 can be laid up as single laminate prior to any thermal processing, e.g., consolidation, firing or burnout, and infiltration, which provides advantages and benefits over conventional practices.
At (206), returning to
At (208), the method (200) includes firing the consolidated CMC preform (i.e., the pre-green state CMC preform). Firing the consolidated CMC preform burns out the binder from the slurry, and notably, burns out, decomposes, or otherwise removes some or all of the sacrificial fibers to define elongated channels in the first section of the fired CMC preform. An example of the defined elongated channels within the first section of the fired CMC preform is provided below.
Referring now to
The elongated channels 118 are generally continuous hollow channels formed in the fired CMC preform 130. Depending on the degree of decomposition or removal of the sacrificial fibers 116, the elongated channels 118 may have various amounts of scaffolding throughout the channels. For instance, with higher char yield polymers, the elongated channels 118 may have more scaffolding while with lower char yield polymers, the elongated channels 118 may have less scaffolding. The elongated channels 118 are sufficiently porous to allow the flow of infiltrant to fill the elongated channels 118, and may generally be considered cylindrical hollow channels with a higher length than diameter/width. When substantially all of the sacrificial fibers 116 decompose, the elongated channels 118 may have substantially the same size and distributions (for example, the same volume % and aspect ratio) as that of the sacrificial fibers 116. After firing the consolidated CMC preform at (208) to remove the sacrificial fibers 116, among other elements, the fired CMC preform is densified as described below.
At (210), the method (200) includes subjecting the fired preform (i.e., a green state article) to chemical vapor infiltration (CVI). Generally, in a chemical vapor infiltration (CVI) process, an infiltrant in the form of reactive gases infiltrates the porous, green state CMC preform and reacts to form a ceramic material, such as silicon carbide. That is, the method may include reacting the infiltrant with the ceramic precursor (e.g., carbon in some form) to form the ceramic matrix (e.g., silicon carbide). The infiltrant, such as e.g., methyltrichlorosilane, fills the pores and elongated channels to form a densified part. Notably, the elongated channels facilitate infiltration into the porous, green state preform by providing gas transport paths for the gaseous infiltrant. The size or diameters of the elongated channels prevent them from being plugged or closed off thus allowing for infiltration into the interior portions of the article. This may, for example, reduce the residual porosity of the final CMC article. An example densified CMC preform is provided below.
At (212), with reference to
One or more additional layers can be added to the composite structure following the CVI, e.g., after (210) of
At (214), in some implementations, the method (200) includes applying a polymer solution to the CVI-densified CMC preform 130. That is, the method (200) can include treating the CVI-densified CMC preform with a polymer containing solution to wet the channels. As one example, the polymer solution can comprise a phenolic resin dissolved in an organic carrier solvent, such as e.g., acetone. The polymer solution can be applied in any suitable fashion. For instance, in some embodiments, the CVI-densified CMC preform 130 can be soaked in a polymer solution bath. In other embodiments, the CVI-densified CMC preform 130 can be sprayed with the polymer solution. Preferably, the polymer solution is applied such that it soaks the interior surfaces of the partially-infiltrated elongated channels 118. In this way, the polymer solution deposited on the surfaces of the channel will decompose as carbon to provide better wetting for a subsequent melt infiltration process (described below). Better wetting facilitates the capillary action of the melted-liquid infiltrant (e.g., silicon) into the partially-infiltrated elongated channels 118, and thus, better backfill infiltration is achieved and in a more efficient manner.
At (216), the method (200) includes subjecting the CVI-densified CMC preform to melt infiltration (MI) to backfill the plurality of elongated channels, e.g., to further densify the CMC preform. As noted above, during the CVI process, the elongated channels may be only partially filled and residual porosity may still be present in and along the elongated channels. Accordingly, the CVI-densified CMC preform is melt infiltrated to backfill the elongated channels with a liquid infiltrant to further densify the article. Examples of suitable infiltrants for melt infiltration include molten material, such as silicon, silicon alloys, silicides, oxides, or combinations thereof. An example CVI-densified CMC preform undergoing a melt infiltration process is provided below.
In some embodiments, as shown in
Generally, the further densification of the CVI-infiltrated CMC preform using melt infiltration may result in a ceramic matrix composite article that is fully dense, e.g., having generally zero, or less than about 7 or less than about 3 percent by volume residual porosity. This very low porosity gives the composite desirable mechanical properties, such as a high proportional limit strength and interlaminar tensile and shear strengths, high thermal conductivity and good oxidation resistance. The matrices may have a free silicon phase (i.e. elemental silicon or silicon alloy) that may limit the use temperature of the ceramic matrix composite articles to below that of the melting point of the silicon or silicon alloy, or about 1400° C. (≈2,550° F.) to 1410° C. (≈2,570° F.). The free silicon phase may result in a lower creep resistance compared to densification solely by chemical vapor infiltration.
At (218), with reference again to
In some embodiments, as noted above, the one or more infiltrant veins 138 comprise an unreacted infiltrant (e.g., silicon). For instance, for SiC—SiC composites, some of the liquid infiltrant backfilled into the CVI-infiltrated CMC preform 130 during melt infiltration at (216) may not react to form a silicon carbide phase; thus, the liquid infiltrant remains in a silicon phase. To prevent the CMC article 100 from being limited in use and application by the melting temperature of the unreacted infiltrant utilized during melt infiltration, the second section 102 has a thickness greater than about 0.25 mm and preferably above 0.75 mm and is the section that faces or is exposed to temperatures above the melting temperature of the infiltrant. Particularly, the second section 102, which is silicon free, is preferably the section of the CMC article 100 that is exposed to high temperatures (i.e., temperatures above the melting point of the unreacted infiltrant) and the first section 101, which may be silicon rich, is preferably not exposed to the high temperatures that would cause the silicon within the infiltrated veins 138 to melt. The second section 102 creates a thermal gradient between the high temperature environment and the silicon rich first section 101 of the CMC article. Preferably, the second section 102 of the CMC article 100 has a thickness that creates a thermal gradient such that the first section 101 of the CMC article 100 is not exposed to temperatures above about 1400° C. (≈2,570° F.), e.g., above about the melting temperature of silicon.
As depicted in
A specimen of vapor infiltrated SIC—SiC fiber composite was first prepared using the methods and procedures described in U.S. Pat. No. 9,850,174 owned by General Electric Company. U.S. Pat. No. 9,850,174 is hereby incorporated by reference in its entirety. A SiC fiber material (Hi-Nicalon-S) was coated with a slurry material containing a mixture of ceramic solid material, organo-silane SiC precursor polymer, organic pore forming material, and an organic solvent as a liquid carrier for the slurry. The slurry material was chosen so that upon treatment at high temperature in an inert atmosphere a mixture of SiC and C is formed. Due to residual oxide impurities in the initial material, some oxygen can be present in the heat-treated mixture, but this amount is typically less than ten percent (10%) by weight of the heat-treated material. During the formation of the uncured ply, the slurry coated fibers were combined with nylon sacrificial fibers that decompose during the high temperature heat treatment. The spacing of the nylon fibers was about one millimeter (1 mm). The 19 plys of the preform were laid down in an alternating fashion, with each successive ply oriented ninety degrees (90°) to an adjacent ply.
Following assembly of the plys, the resulting preform was treated through two successive heat treatments, including a first relatively low temperature debulking step followed by a second much higher temperature heat treatment (>1000° C.) in a chemically non-reactive environment. During the second heat treatment process, the nylon fibers decomposed, resulting in long straight pores with diameters of between 160-200 microns in each ply. Following the high temperature pyrolysis treatment, the porous preform was vapor infiltrated at high temperature (>1000° C.) using a mixture of hydrogen and methyl trichloro silane (MTS). The MTS thermally decomposed to form solid silicon carbide in the internal portions of the preform, and during the vapor infiltration process, the preform exhibited a weight gain by a factor of about 1.87. Analysis of the deposits created under the conditions used in the vapor infiltration reaction indicated that the deposit is largely SiC (>95%). Optical analysis of the preform indicated that the net residual porosity of the preform was about 21% following the treatment with MTS. The pores created by the decomposition of the nylon fibers were clearly discernable due their large area and generally circular profile.
A portion of the CVI densified preform was sliced using a diamond saw from the larger piece and then melt infiltrated with silicon. A machined edge of the infiltrated preform was placed on a woven carbon felt wick with a pellet composed of >90% silicon. The pellet and the CMC piece were not directly in contact. The amount of silicon was about the same as the weight of the sectioned preform. The SiC CMC, woven carbon wick, and silicon were placed into a boron nitride coated graphite crucible and heated under vacuum, heated to a nominal temperature at least 15° C. above the melting point of pure silicon and held at this temperature for about ½ hour and then allowed to cool under vacuum. Following cooling, the crucible was removed. The silicon melted and migrated through the wick and coated the CMC piece. The coated CMC piece was then cut with a diamond saw. In some of the large pores that were created by the decomposition of the nylon fibers, silicon could be observed. There were, however, large pores that were unfilled toward the center of the sample.
Another machined section of the same vapor-infiltrated CMC preform was selected and pretreated with a 2% solution of organic resin (Novolak FRJ-425), which upon heat treatment at high temperatures under vacuum will decompose but leave a carbon residue in the large pores. This carbon residue is believed to promote infiltration of the liquid silicon into the porous structure. The resin treated CMC piece was then infiltrated with silicon using a similar procedure as described in Example 1 except the heat treatment procedure was modified so the hold time at the highest temperature was about one (1) hour. Following heat treatment, the piece was sectioned and silicon was observed to have infiltrated into the large pores.
While the invention has been described in terms of one or more particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. It is to be understood that the use of “comprising” in conjunction with the coating compositions described herein specifically discloses and includes the embodiments wherein the coating compositions “consist essentially of” the named components (i.e., contain the named components and no other components that significantly adversely affect the basic and novel features disclosed), and embodiments wherein the coating compositions “consist of” the named components (i.e., contain only the named components except for contaminants which are naturally and inevitably present in each of the named components).
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.