Patent Application
20240279129
The disclosure relates to high temperature coatings.
Composites, such as carbon-carbon (C/C) composites, may be used in high temperature applications. For example, the aerospace industry employs C/C composite components as friction materials for commercial and military aircraft, such as brake friction materials. In high temperature applications, C/C composites may be susceptible to oxidation, which may lead to deterioration of physio-mechanical properties.
The disclosure describes high temperature coatings for carbon-based composite substrates that protect against oxidation at relatively high temperatures, and techniques for making the same. In some examples, the high temperature coatings include an antioxidant layer of a metal carbide on a surface of a composite articles that includes a composite substrate including a carbon matrix. The surface of the composite article may include surface voids that result from fabrication or machining of the composite substrate. A relatively thin carbon layer may be deposited onto the surface of the composite article using chemical vapor deposition to smooth the surface voids, seal small surface pores, and enhance the formation of a continuous coating. To form the metal carbide, at least a portion of the deposited carbon layer may be reacted with a metal, such as silicon, titanium, or tungsten, in stoichiometric excess. The resulting metal carbide may form a dense antioxidant coating having a high quality, uniform, and continuous crystalline metal carbide that seals the surface defects and is substantially free from surface voids.
In one example, a method for forming a high temperature coating includes depositing a carbon layer on to a surface of a composite article using chemical vapor deposition. The composite article includes a composite substrate that includes a carbon matrix. The surface of the composite article includes one or more surface voids. The method further includes applying a metal slurry to the surface of the composite article following the deposition of the carbon layer and reacting a metal of the metal slurry with carbon of the carbon layer to form an antioxidant layer of a metal carbide on the composite article.
In another example, a high temperature article includes a composite article and a high temperature coating on a surface of the composite article. The composite article includes a composite substrate including a carbon matrix. The high temperature coating comprises an antioxidant layer of a metal carbide on the surface of the composite article. The metal carbide of the antioxidant layer is formed from carbon of a carbon layer deposited using chemical vapor deposition. The antioxidant layer extends across one or more surface voids of the surface of the composite article.
The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
The disclosure describes high temperature coatings for composite articles, the composite articles including a carbon-based composite substrate and an antioxidant layer made from a metal carbide for relatively high temperature (e.g., greater than 1500 degrees Celsius (° C.)) applications on the composite substrate.
Carbon-based (e.g., including a carbon matrix) composite components, such as carbon-carbon (C/C) composite components, may provide good mechanical properties and have low mass density relative to other materials, such as metal alloys. However, at relatively high temperatures, carbon-carbon composite components may be susceptible to oxidation, environmental attack, and degradation of physio-mechanical properties. Metal carbide-based antioxidant coatings may improve resistance to oxidation and/or environmental attack at high temperatures experienced in aerospace applications, such as aircraft brakes (e.g., temperatures as high as 1600° C.), and hypersonic applications, such as leading edges and rocket nozzles.
Prior to use in an oxidizing atmosphere, high temperature carbon-based composite substrates may be coated with a metal carbide-based antioxidant coating that may reduce oxidation of an external surface of the substrate. However, a surface of the composite substrate may include one or more surface voids, such as defects, cracks, pores, ripples, or machining textures, that extend into the substrate. If a metal carbide coating is applied to a surface portion of a composite substrate that includes surface voids, the metal carbide coating may not adequately conform to the surface. Alternatively, if a surface portion of a composite substrate that includes surface voids is reacted with a metal to form a metal carbide coating, the resulting metal carbide coating may not be continuous on the composite substrate. In either case, the underlying composite substrate may be susceptible to oxidation during use in high temperature applications.
In examples described herein, a high temperature coating includes an antioxidant layer that extends across the surface voids of the composite substrate and forms a dense barrier against oxidizing species. An antioxidant layer of the present coatings may include a metal carbide phase formed from a metal, such as silicon, titanium, or tungsten, and reactive carbon from a carbon layer on a surface of the composite substrate. In some examples, prior to reaction with the metal, the reactive carbon layer is deposited onto the surface, including into the surface voids, by chemical vapor deposition (CVD). For example, powders may not bind with the composite substrates, and may only be useful for filling and packing powders into defects until the powders may be converted to a metal carbide. In contrast, CVD processes may form a relatively uniform reactive carbon layer compared to slurry-based methods for depositing carbon powder, and may reduce an amount of labor and/or be more conducive to automation than such slurry-based methods, which may involve carbon powder abrasion or compaction to adequately coat the surface of the composite substrate. Additionally or alternatively, carbon deposited by CVD may inherently bond with the composite substrate very strongly, which may functionally correspond to growing and/or healing the carbon matrix of the composite substrate. The carbon layer may fill small pores (e.g., pores with sizes comparable to the CVD carbon thickness) and smooth out larger pores that may have jagged (high angle and sharp edges) sides. As a result, during reaction with the metal, the carbon of the carbon layer reacts with the metal to form a uniform antioxidant coating that bridges the metal carbide across the surface voids to form a continuous antioxidant coating.
High temperature coatings described herein may be used in a variety of high temperature applications. Due to high temperatures experienced by components in high speed, friction, or combustion environments, the high temperature coatings may be particularly suited to aerospace applications.
In the example of
Wheel and brake assembly 10 includes wheel 12, which in the example of
Wheel and brake assembly 10 may be mounted to a vehicle via torque tube 42 and axle 18. In the example of
During operation of the vehicle, braking may be necessary from time to time, such as during landing and taxiing procedures of an aircraft. Wheel and brake assembly 10 is configured to provide a braking function to the vehicle via actuator assembly 14 and brake stack 16. Actuator assembly 14 includes actuator housing 30 and ram 34. Actuator assembly 14 may include different types of actuators such as one or more of, e.g., an electrical-mechanical actuator, a hydraulic actuator, a pneumatic actuator, or the like. During operation, ram 34 may extend away from actuator housing 30 to axially compress brake stack 16 against a compression point for braking.
Brake stack 16 includes alternating rotor brake discs 36 and stator brake discs 38. Rotor brake discs 36 are mounted to wheel hub 20 for common rotation by beam keys 40. Stator brake discs 38 are mounted to torque tube 42 by splines 44. In the example of
In some examples, rotor brake discs 36 and stator brake discs 38 may be mounted in wheel and brake assembly 10 by beam keys 40 and splines 44, respectively. In some examples, beam keys 40 may be circumferentially spaced about an inner portion of wheel hub 20. Beam keys 40 may, for example, be shaped with opposing ends (e.g., opposite sides of a rectangular) and may have one end mechanically affixed to an inner portion of wheel hub 20 and an opposite end mechanically affixed to an outer portion of wheel hub 20. Beam keys 40 may be integrally formed with wheel hub 20 or may be separate from and mechanically affixed to wheel hub 20, e.g., to provide a thermal barrier between rotor brake discs 36 and wheel hub 20. Toward that end, in different examples, wheel and brake assembly 10 may include a heat shield (not shown) that extends out radially and outwardly surrounds brake stack 16, e.g., to limit thermal transfer between brake stack 16 and wheel 12.
In some examples, splines 44 may be circumferentially spaced about an outer portion of torque tube 42. As such, stator brake discs 38 may include a plurality of radially inwardly disposed lug notches along an inner diameter of the brake disc configured to engage with splines 44. Similarly, rotor brake discs 36 may include a plurality of radially inwardly disposed lug notches along an outer diameter of the brake disc configured to engage with beam keys 40. As such rotor brake discs 36 will rotate with the motion of the wheel while stator brake discs 38 remain stationary allowing the friction surfaces of an adjacent stator brake disc 38 and rotor brake disc 36 to engage with one another to deaccelerate the rotation of wheel 12.
Rotor brake discs 36 and stator brake discs 38 may provide opposing friction surfaces for braking an aircraft. As kinetic energy of a moving aircraft is transferred into thermal energy in brake stack 16, temperatures may rapidly increase in brake stack 16. As such, rotor brake discs 36 and stator brake discs 38 that form brake stack 16 may include coatings capable of operating at very high temperatures and blocking various oxidizing species.
In some examples, articles or components, such as brake discs 36 and/or 38 of
High temperature article 50 includes a composite article 51 that includes a carbon-based composite substrate 52. In the example of
Substrate 52 may include reinforcement fibers and a carbon-based matrix (or “carbon matrix”) material at least partially surrounding the reinforcement fibers. In some examples, substrate 52 may be a carbon-carbon (C/C) composite substrate that includes carbon-based reinforcement fibers and a carbon-based matrix material at least partially surrounding the carbon-based reinforcement fibers. In such examples, substrate 52 may be formed form a porous preform that includes carbon-based reinforcement fibers or carbon-precursor reinforcement fibers. Examples of porous preforms that may be used to produce substrate 52 include, but are not limited to: a fibrous preform, such as a woven fiber preform, a nonwoven fiber preform, a chopped-fiber and binder preform, a binder-treated random fiber preform, a carbon fiber preform, or a ceramic fiber preform; a foam preform; a porous carbon body preform; or a porous ceramic body preforms.
In some examples, the porous preform includes a plurality of mechanically bound layers, which can be, for example, a plurality of fibrous layers, such as a plurality of woven or nonwoven fabric layers, connected together, e.g., bound by a binder, such as a resin binder, or via needle-punching of the plurality of layers. In some examples, the layers include one or more tow layers, one or more web layers, or combinations thereof. Tow layers may include one or more tows of fibers. Tows of fibers may be arranged in any suitable arrangement including, for example, linear, radial, chordal, or the like. Web layers may include web fibers, which may include relatively short, chopped, and entangled fibers of fibers. In other examples, the porous preform may not include predefined layers, but, rather, may be formed from a bundle of fibers that are mechanically bound together, e.g., via needling. In other examples, a combination of any of the aforementioned types of porous preforms can be used.
Substrate 52 includes a carbon matrix material that at least partially encapsulates the reinforcement fibers. The matrix material may be introduced into the reinforcement fibers, such as a porous preform, using one or more of a variety of techniques, including, for example, chemical vapor deposition/chemical vapor infiltration (CVD/CVI), resin transfer molding (RTM), vacuum/pressure infiltration (VPI), high pressure impregnation/carbonization (PIC), or the like. The carbon matrix may have a microstructure that reflects the particular conditions of formation, such as deposition conditions used to form the carbon matrix and composition and/or orientation of the reinforcement fibers. For example, different growth conditions between two carbon materials may result in discontinuities between different microstructures, such that the presence of the discontinuities may indicate the presence of different growth conditions. For example, a carbon matrix that forms via CVD/CVI around fibers of a composite substrate may form relatively slow and infiltrate the porosity of the fibers, while a carbon surface layer that forms via CVD on a surface may form relatively quickly and fill only surface pores before forming a dense outer layer. As a result, the crystallinity, density, microstructure, and other characterization signatures, like Raman spectra, may be different between the carbon matrix of the composite substrate and the carbon matrix of the surface layer. The microstructure of a carbon matrix formed from a particular fabrication method under particular fabrication conditions may have particular structural properties including, but not limited to, porosity; matrix phase composition, such as interbundle and/or interfilament phases; microcracking; interface presence and orientation, such as grains; or other structural properties that result from a mechanism of formation within and/or around the reinforcement fibers.
A surface of composite article 51 may include one or more surface voids. In examples in which substrate 52 is an outermost portion of article 51, such as illustrated in
Substrate 52 may be subject to high temperatures during operation. As one example, carbon-carbon composite brake discs may be subject to temperatures as high as about 3,000 degrees Fahrenheit (° F.) (about 1,649° C.) during braking events. About may be, for example, exactly or nearly exactly to the extent permitted by manufacturing, measurement, and/or operational tolerances, or within 1%, 2%, 3%, 4%, or 5% of the value. To protect substrate 52 from oxidation, article 50 includes a high temperature coating 54 on one or more surfaces of substrate 52. Coating 54 may be stable at temperatures of up to about 3600° F. (about 2000° C.). In this context, “stable” may mean that coating 54 does not degrade into its constituent elements, does not react with carbon, and/or does not react with other elements or compounds present in the environment in which coating 54 is used including, but not limited to, oxidation, for a period of time (e.g., minutes or hours). Coating 54 may have any suitable thickness. In some examples, a thickness of coating 54 may be about 1 micrometer (m) to about 100 m. In some examples, a thickness of coating 54 may be self-terminating and determined by diffusion properties of the metal carbon system and/or deposition properties of the carbon layer.
High temperature coating 54 includes an antioxidant layer 56 on a surface of substrate 52. Antioxidant layer 56 includes a metal carbide. Metal carbides may have high strength, wear-resistance, and temperature resistance, and may be chemically compatible with underlying substrate 52. In some examples, the metal carbide includes at least one of silicon carbide, titanium carbide, or tungsten carbide.
Surface voids, such as defects or pores, on the surface of substrate 52 may cause discontinuities to form in a metal carbide layer that is either applied to a surface of substrate 52 or formed from a surface portion of substrate 52, and thereby permit oxidizing species to react with the underlying substrate 52. As will be explained further in
In some examples, antioxidant layer 56 is formed by reacting a metal, such as in a metal slurry, with a carbon layer that extends across the surface of substrate 52, including into the surface voids. As a result, portions of antioxidant layer 56 may extend across and substantially seal surface voids (e.g., seal all surface voids or nearly all surface voids, such as 90% or more, or 95% or more, or 99% or more of the surface voids) in the surface of substrate 52 to form a substantially continuous coating.
Carbon layer 66 is formed by chemical vapor deposition (CVD). Chemical vapor deposition processes for forming carbon layer 66 may include any process in which gaseous carbon precursors decompose to form solid carbon. In contrast to slurry-based processes for forming carbon deposits, which may cause significant variations in layer thickness and composition, or physical vapor deposition (PVD) processes, which may be limited by line-of-sight, CVD processes may enable greater control of thickness and composition of carbon layer 66 and greater infiltration of carbon layer 66 into surface voids. For example, a thickness and composition of carbon layer 66 may be controlled by controlling a rate of decomposition of the carbon precursors, and infiltration of carbon layer 66 into surface voids may be driven by diffusion or mixing. As a result, carbon layer 66 may provide a uniform reactive portion of carbon for subsequent reaction with a metal to form a metal carbide. A variety of carbon precursors may be used to form carbon layer 66 including, but not limited to, hydrocarbons, such as methane, acetylene, ethylene, propylene, benzene, toluene, and xylene; and any other carbon compound that may decompose from a gas to form solid carbon. Carbon precursors may be selected based on various properties of carbon layer 66 and conditions of the CVD process.
Carbon layer 66 may have a different microstructure than the carbon matrix of substrate 52. For example, carbon layer 66 may be formed under particular CVD conditions, such as temperature and pressure, that result in different structural properties of a surface portion of the carbon matrix of substrate 52, including a carbon matrix formed by CVI. As such, carbon layer 66 may be structurally differentiated from the carbon matrix of substrate 52.
In some examples, carbon layer 66 may have a relatively smooth surface compared to an underlying surface of article 51. Surface voids in the surface of article 51 may define a roughness of the surface, such that an applied silicon carbide coating (e.g., via slurry) may not adhere to all or part of the surface or may have a low thickness in areas of peaks and high thickness or voids in areas of troughs. While abrasive techniques, such as polishing, may reduce the roughness of the surface, such abrasive techniques may be labor intensive and difficult to automate. By using CVD processes to deposit carbon layer 66, carbon forming carbon layer 66 may deposit into surface voids and form a smoother surface for reaction with the metal without mechanical additive (e.g., compaction) or subtractive (e.g., polishing) processes. A rough surface may be characterized by displacement or angular deviations from a general surface plane. Surface roughness may be characterized in any of a variety of ways including, but not limited to, average profile height deviation, average trough depth, average peak height, average peak to trough height, average profile angle, or any other measure of surface roughness. In some examples, a surface roughness of the surface of carbon layer 66 may be less than half the surface roughness of the surface of substrate 51.
Carbon layer 66 may have a thickness that is sufficiently high to provide a smoother surface than article 51 and/or provide enough reactive carbon to form a robust antioxidant layer 56 of silicon carbide. For example, carbon layer 66 may have a thickness such that carbon fills and/or seals defects in the surface of substrate 51 and may be reacted with the metal to form a metal carbide having a minimum thickness. Carbon layer 66 may have a thickness that is sufficiently low to reduce an amount of unreacted carbon in carbon layer 66 (e.g., less than about 50 percent by volume of the total volume of carbon layer 66). For example, as will be explained further below, a thickness of the metal carbide formed from carbon layer 66 may be limited by formation of the metal carbide, which may prevent the metal from further infiltrating into carbon layer 66. In some examples, a thickness of carbon layer 66 may be greater than or equal to about 2 micrometers and/or less than or equal to about 100 micrometers, such as from about 5 micrometers to about 10 micrometers.
In some examples, carbon layer 66 has a minimum thickness. For example, CVD processes for forming carbon layer 66 may deposit carbon at a relatively uniform rate across the surface of article 51, such that carbon layer 66 may have a controllable minimum thickness across the surface of article 51. A thickness of carbon layer 66 may vary depending on a location on substrate 52, how CVD gases are introduced into a reactor, and other factors or operating parameters of CVD. A desired minimum thickness may correspond to a thickness that enables a smooth shell that cover surface defects, such that the minimum thickness may correspond to an underlying surface roughness and a depth of various defects on substrate 52. In contrast, a thickness of a resulting metal carbide formed from carbon layer 66 may be thermodynamically determined and self-terminating, and may be independent of a thickness of carbon layer 66 or various operating parameters of CVD. In some examples, a minimum thickness of carbon layer 66 may be at least 5 micrometers.
In some examples, carbon layer 66 may have a substantially uniform composition and morphology. For example, prior to reaction with a metal, at least a surface portion of carbon layer 66 (e.g., outer-most 10-20 microns) includes carbon that is capable of reacting with the metal to form a metal carbide. The carbon of the surface portion of carbon layer 66 may have a substantially uniform composition and/or morphology, such as microstructure, phase composition, geometry of component phases, morphology of components phases, crystal structure, presence and type of impurities, particle morphology shape and size, crystal surface terminations (e.g., active facets), crystal defects, and/or surface functionalization.
The particular composition and/or morphology may result in a reaction with the metal according to particular reaction thermodynamics and kinetics, such as a temperature of reaction and a rate of reaction. As a result of a substantially uniform composition and/or morphology, carbon layer 66 may have a substantially uniform reaction thermodynamics and kinetics across carbon layer 66. By forming a metal carbide formed from a deposited carbon layer having a substantially uniform composition and/or morphology, antioxidant layer 56 may be formed at substantially the same time and rate, thereby bridging the resultant metal carbide across the surface voids.
In some examples, additional antioxidant layers may be formed on antioxidant layer 56 to increase an overall thickness of antioxidant layers on substrate 52. For example, powder-based carbon applications may not include multiple antioxidant layers, as carbon powder may only be provided to a surface prior to forming an outermost antioxidant layer. In contrast, carbon layer 66 formed by CVD may provide a solid layer of carbon, such that additional carbon layers 66 be added on antioxidant layer 56 and subsequently converted to metal carbide to overcome a self-terminating thickness limit and make thicker coatings.
As will be described further in
In some examples, antioxidant layer 56 is metal-rich, such that antioxidant layer 56 may include a metal carbide having metal in stoichiometric excess. For example, during formation of the metal carbide from the carbon of carbon layer 66, a portion of excess metal may remain in antioxidant layer 56. During operation of article 50, the metal may form a metal oxide that may migrate to form a passivation layer, such as an outer layer 58 described below, or may fill small cracks or pores in antioxidant layer 56 caused by a difference in coefficient of thermal expansion between substrate 52 and antioxidant layer 56, such as by expanding through oxidation of the excess metal at high temperatures in the presence of oxidation during operation. As a result, the metal of antioxidant layer 56 may perform a passivation and/or self-healing function to further protect substrate 52.
In some examples, coating 54 includes an outer layer 58 of a metal oxide on antioxidant layer 56. For example, metal that reacts with the carbon of carbon layer 66 during formation of antioxidant layer 56 may be applied to the surface of carbon layer 66 as metal powder or particles. This metal powder may include metal oxide surface layers that form in an oxidizing atmosphere, such as an oxide of the underlying metal or an oxide of a different element. For example, the metal oxide surface layers may have a thickness of about 1 nanometer to about 1 micrometer. During formation of antioxidant layer 56, the metal oxides may migrate to a surface of antioxidant layer 56 and form outer layer 58. Outer layer 58 may have a relatively high temperature resistance, such as greater than about 1500° C. In this way, metal oxides that may otherwise be present in the metal powder as impurities may form a further protective layer to protect substrate 52 from oxidation.
While
First antioxidant layer 69 may include a metal carbide that is formed from the carbon matrix of a surface portion of composite substrate 52. For example, prior to reaction with a metal, substrate 52 may include a surface portion (e.g., outer-most 10-20 microns) that includes carbon matrix capable of reacting with a metal to form a metal carbide. First antioxidant layer 69 may include one or more substantially crystallized (e.g., greater than 95% by volume crystalline phase) metal carbide layers. In some examples, first antioxidant layer 69 may include at least one of silicon carbide (SiC), titanium carbide (TiC), tungsten carbide (WC), zirconium carbide (ZrC), combinations thereof, or any carbide layer formed using the principles of the process described in U.S. Pat. No. 6,555,173 and/or 4,837,073, which are incorporated by reference herein in their entirety.
In some examples, first antioxidant layer 69 may be configured to increase adhesion between second antioxidant layer 56 and substrate 52. For example, first antioxidant layer 69 may be formed from an in situ process that involves reaction between reactive carbon in the carbon matrix of a surface portion of substrate 52 and metal. In some examples, inclusion of first antioxidant layer 69 and second antioxidant layer 56 may provide multiple stages of reduction of surface defects in substrate 52. For example, first antioxidant layer 69 may reduce a number and/or extent of surface defects in substrate 52, such that second antioxidant layer 56 may further reduce the remaining surface defects.
In some examples, the metal may be in stoichiometric excess, and may form a metal oxide that may migrate into microcracks of substrate 52, first antioxidant layer 69, and/or second antioxidant layer 56 to provide a self-healing functionality. The metal oxide may more strongly adhere to the silicon carbide matrix of second antioxidant layer 56, and/or may be at least partially impregnated into open pores of substrate 52. Additionally or alternatively, second antioxidant layer 56 may have a relatively low thickness, such as less than about 20 micrometers, and/or consistent thickness, such as within about 10 micrometers, that is controlled by an amount of reactive carbon present on substrate 52.
Second antioxidant layer 56 may be formed on first antioxidant layer 69 via reactive carbon layer 66. As described in
In some examples, carbon layer 66 may have a lower porosity than the carbon matrix of substrate 52, such that second antioxidant layer 56 may form a more protective barrier than first antioxidant layer 69. For example, carbon deposited by CVD on a surface may be denser than carbon deposited by chemical vapor infiltration or slurry infiltration on a porous preform. As a result, a silicon carbide formed from carbon layer 66 may have a lower porosity than a silicon carbide formed from the carbon matrix of a surface portion of composite substrate 52.
High temperature antioxidant coatings described herein, such as coating 54 of
The example technique of
In some examples, prior to depositing a carbon layer on the composite article, an antioxidant layer may be formed on the composite substrate. In such examples, the example technique of
Regardless of whether substrate 52 is machined or whether article 51 includes substrate 52 alone or additional surface layers, article 51 may include various surface voids that, if left unsealed or partially sealed, may permit oxidizing species to penetrate into and react with substrate 52.
Referring back to
In some examples, depositing the carbon of carbon layer 66 includes introducing a gaseous carbon precursor, optionally including a carrier gas, to an environment near surface 62A and maintaining deposition conditions of the gaseous carbon precursor to cause the gaseous carbon precursor to decompose and deposit on surface 62A and/or a new or intermediate surface 62B of carbon layer 66. Maintaining deposition conditions of the gaseous carbon precursor may include maintaining a temperature of the gaseous carbon precursor above a decomposition temperature of the gaseous carbon precursor to decompose the gaseous carbon precursor into carbon and a byproduct gas, such as hydrogen. Maintaining deposition conditions of the gaseous carbon precursor may further include controlling the temperature and pressure of the gaseous carbon precursor to achieve a particular deposition rate of carbon. For example, a higher temperature and lower pressure may increase a rate of decomposition of the gaseous carbon precursor and subsequent deposition of the carbon, but may result in carbon layer 66 having a higher amorphous phase than deposition conditions that include a lower temperature and/or higher pressure.
A variety of parameters, such as a temperature of gaseous carbon precursor, a pressure of gaseous carbon precursor, a concentration of gaseous carbon precursor, and a time of deposition, may be controlled to maintain the gaseous carbon precursor above deposition conditions, and optionally encourage controlled deposition of the carbon into carbon layer 66. As one example, for a carbon precursor gas of methane, the temperature may be maintained greater than or equal to about 800° C., the pressure may be maintained at about 0.005 Torr to about 20 Torr, and the temperature and pressure may be maintained for greater than or equal to about one hour.
Carbon layer 66 may form new surface 62B of portion 60. After deposition of carbon layer 66, surface 62B of carbon layer 66 may be smoother than surface 62A of article 51. For example, as shown in
In some examples, carbon layer 66 has a substantially uniform composition and morphology across surface 62B. For example, the microstructure and/or crystallinity of carbon layer 66 may be substantially the same at various portions of carbon layer 66, and so may have a substantially same reaction thermodynamics and kinetics across carbon layer 66, such that reaction of an infiltrated metal with carbon layer 66 may occur at substantially the same temperature and substantially the same rate across carbon layer 66. As explained above, reaction thermodynamics and kinetics of carbon layer 66 may be a product of a respective type, feedstock source, processing history, and other properties and conditions of carbon layer 66 that influence a temperature and rate at which carbon layer 66 may react with an infiltrated metal. The resulting carbon layer 66 may have a uniform composition and morphology, may be generated proximate to voids 64, and may penetrate into voids 64 without the use of a carrier medium.
Referring to
Referring back to
Metal slurry 68 may include metal particles in an application medium. In some examples, the metal particles of metal slurry 68 include at least one of silicon, titanium, or tungsten. The metal particles may be coated by a thin layer of a metal oxide, such as may be formed in an oxidizing atmosphere during formation or shelf-life of the metal particles. For example, a relatively pure feedstock of metal particles may be prohibitively expensive due to inert storage, such that use of metal particles that include a metal oxide film may broaden available feedstocks of material for the metal particles and/or reduce a cost of the metal particles.
Referring to
Referring back to
In some instances, this reaction may be limited by diffusion of the metal into the carbon layer 66. As the metal reacts with the carbon of carbon layer 66 and forms a metal carbide, the newly formed metal carbide may form a diffusion barrier separating the reactants (e.g., carbon and metal), which may stop the thickening and further creation to form thicker metal carbides (e.g., by preventing metal from further penetrating into a depth of carbon layer 66 and/or preventing diffusion of carbon out of carbon layer 66 to react with the metal). The metal may be applied to surface 62B of carbon layer 66 until the reaction ends either by diffusion limitation, metal evaporation or exhaustion, or both. For example, any remaining metal on surface 62 may be removed, such as through evaporation. The resulting layer 56 may be a relatively homogeneous metal carbide having a relatively uniform thickness that may include some deviations to fill voids 64. In some examples, a thickness of antioxidant layer 56 at a surface of article 51 is less than about 50 microns, such as about 10 microns to about 20 microns.
Reaction of the metal of metal slurry 68 with carbon of carbon layer 66 may be performed under stoichiometric excess of the metal, such that the resulting metal carbide antioxidant layer 56 is metal-rich. Metal-rich may include a metal carbide phase that includes excess free metal. For example, a metal-rich metal carbide phase may include a stoichiometric ratio of the metal to the carbon of carbon layer 66 that is greater than 1.1, such as greater than about 1.001:1. By performing the reaction at stoichiometric excess of the metal, the resulting antioxidant layer 56 may include excess metal. During formation of antioxidant layer 56 or during operation of article 51 (e.g., as a component), the excess metal may form a metal oxide. In some instances, such as will be described in
In some examples, reacting the metal of metal slurry 68 with the carbon of carbon layer 66 includes heating surface 62A of article 51 and/or surface 62B of carbon layer 66 above a melting point of the metal and maintaining a vapor pressure of the metal at surface 62A of article 51 and/or surface 62B of carbon layer 66 in stoichiometric excess. A variety of parameters, such as a temperature at surface 62A and/or surface 62B, a concentration (e.g., as indicated by pressure) of the metal at surface 62A and/or surface 62B, and a time of reaction, may be controlled to maintain the metal at stoichiometric excess and encourage migration of the metal into, and reaction with, carbon layer 66. As one example, for a metal of silicon, the temperature may be maintained at greater than about 1400° C., the pressure may be maintained at about 0.1 mTorr to about 300 mTorr, and the temperature and pressure may be maintained for greater than about one hour.
Referring to
Referring back to
Example 1: A method for forming a coating includes depositing a carbon layer on to a surface of a composite article using chemical vapor deposition (CVD), wherein the composite article includes a composite substrate that includes a carbon matrix, and wherein the surface of the composite article includes one or more surface voids; applying a metal slurry to a surface of the carbon layer following the deposition of the carbon layer; and reacting a metal of the metal slurry with carbon of the carbon layer to form an antioxidant layer of a metal carbide on the composite article.
Example 2: The method of example 1, wherein the composite substrate comprises a carbon/carbon (C/C) composite substrate.
Example 3: The method of any of examples 1 and 2, wherein a thickness of the carbon layer is less than about 20 microns.
Example 4: The method of any of examples 1 through 3, wherein the carbon of the carbon layer has a different microstructure than carbon of the carbon matrix of a surface portion of the composite article.
Example 5: The method of any of examples 1 through 4, wherein, after reacting the metal of the metal slurry with carbon of the carbon layer, at least a portion of the carbon layer is unreacted.
Example 6: The method of any of examples 1 through 5, further comprising machining the surface of the composite substrate prior to depositing the carbon layer.
Example 7: The method of any of examples 1 through 6, wherein the antioxidant layer is a metal-rich antioxidant layer, and wherein the metal of the metal slurry is maintained at stoichiometric excess during the reaction of the metal with the carbon to form the metal-rich antioxidant layer.
Example 8: The method of any of examples 1 through 7, wherein the metal slurry comprises metal particles coated by a layer of a metal oxide, and wherein the method further comprises forming an outer layer of the metal oxide on the metal-rich antioxidant layer using at least a portion of the metal oxide from the metal slurry.
Example 9: The method of any of examples 1 through 8, wherein the carbon layer is deposited on a surface of the composite substrate.
Example 10: The method of any of examples 1 through 9, wherein the antioxidant layer comprises a first antioxidant layer that includes a first metal carbide, wherein the composite article further comprises a second antioxidant layer overlying a surface of the composite substrate, and wherein the second antioxidant layer includes a second metal carbide.
Example 11: The method of example 10, further comprising forming, prior to depositing the carbon layer, the second antioxidant layer from a surface portion of the carbon matrix of the composite substrate.
Example 12: A high temperature article includes a composite substrate that includes a carbon matrix; and a coating on a surface of the composite substrate, wherein the coating comprises a metal-rich antioxidant layer of a metal carbide on the surface of the composite substrate, wherein the metal carbide of the metal-rich antioxidant layer is formed from carbon of a carbon layer deposited using chemical vapor deposition, and wherein the metal-rich antioxidant layer extends into one or more surface voids of the surface of the composite substrate.
Example 13: The high temperature article of example 12, wherein the composite substrate comprises a carbon/carbon (C/C) composite substrate.
Example 14: The high temperature article of any of examples 12 and 13, wherein the carbon of the carbon layer has a different microstructure than carbon of the carbon matrix of a surface portion of the composite substrate.
Example 15: The high temperature article of any of examples 12 through 14, wherein the coating further comprises an unreacted portion of the carbon layer between the metal carbide and the surface of the composite substrate.
Example 16: The high temperature article of any of examples 12 through 15, wherein the metal carbide comprises at least one of silicon carbide, titanium carbide, or tungsten carbide.
Example 17: The high temperature article of any of examples 12 through 16, further comprising an outer layer of a metal oxide on the coating.
Example 18: The high temperature article of any of examples 12 through 17, wherein the metal carbide includes a stoichiometric ratio of a metal carbide phase of the metal to the carbon of the carbon layer that is greater than 1:1.
Example 19: The high temperature article of any of examples 12 through 18, wherein the coating is directly overlying a surface of the composite substrate.
Example 20: The high temperature article of any of examples 12 through 19, wherein the antioxidant layer comprises a first antioxidant layer that includes a first metal carbide, wherein the composite article further comprises a second antioxidant layer overlying a surface of the composite substrate, and wherein the second antioxidant layer includes a second metal carbide.
Various examples have been described. These and other examples are within the scope of the following claims.
