Patent Description:
Oxidation protection systems for carbon-carbon composites are typically designed to minimize loss of carbon material due to oxidation at operating conditions, which include temperatures of <NUM> (<NUM>°F) or higher. Layers comprising ceramic materials within an oxidation protection system that are deposited onto a substrate by chemical vapor deposition (CVD) may provide desirable oxidation protection. However, CVD processes may be expensive and cost-prohibitive. Thus, a more economical and seamless oxidation protection system having a layer comprising a ceramic material may be desired. <CIT> describes the production of a composite material. <CIT> describes the oxidation inhibition of carbon-carbon composites.

A method for forming an oxidation protection system on a carbon-carbon composite structure is provided. According to the invention, the method comprises applying a ceramic layer slurry to the composite structure, wherein the ceramic layer slurry comprises aluminum and silicon in a solvent or carrier fluid; heating the composite structure to form a ceramic layer on the composite structure, wherein the ceramic layer comprises aluminum nitride; applying a sealing slurry to the composite structure, wherein the sealing slurry comprises a sealing pre-slurry composition and a sealing carrier fluid, wherein the sealing pre-slurry composition comprises a sealing phosphate glass composition; and heating the composite structure to form a sealing layer on the composite structure. In various embodiments, the method may further comprise preparing the ceramic layer slurry by combining aluminum powder and silicon powder in the solvent or carrier fluid. In various embodiments, the method may further comprise preparing the sealing slurry by combining the sealing pre-slurry composition with the sealing carrier fluid. In various embodiments, the aluminum and the silicon, together, may comprise between <NUM>% and <NUM>% by weight of the ceramic layer slurry. In various embodiments, of the aluminum and the silicon together, the aluminum may comprise between <NUM>% and <NUM>% by weight, and the silicon may comprise between <NUM>% and <NUM>% by weight.

In various embodiments, the ceramic layer may be disposed between the sealing layer and the composite structure. In various embodiments, the composite structure may be heated to a temperature between <NUM> and <NUM> to form the ceramic layer. In various embodiments, heating the composite structure to form the ceramic layer may be completed in an environment comprising nitrogen gas, wherein the composite structure may be heated to a temperature of at least <NUM>. The composite structure may be heated to a temperature of at least <NUM>, wherein the ceramic layer may further comprise silicon nitride. In various embodiments, heating the composite structure to form the ceramic layer may be completed in an environment comprising ammonia gas, wherein the composite structure may be heated to a temperature of at least <NUM>. The composite structure may be heated to a temperature of at least <NUM>, wherein the ceramic layer may further comprise silicon nitride.

In various embodiments, the method may further comprise applying a boron compound slurry to the composite structure prior to applying the ceramic layer slurry to the composite structure; and/or allowing the boron compound slurry to dry on the composite structure to form a boron compound layer. In various embodiments, the boron compound slurry may comprise boron carbide, and the ceramic layer may further comprise aluminum boron carbide.

In various embodiments, the ceramic layer slurry may further comprise silicon oxycarbide. The ceramic layer may further comprise alumina and silicon carbide.

In various embodiments, the sealing phosphate glass composition may be represented by the formula a(A'<NUM>O)x(P<NUM>O<NUM>)y1b(GfO)y2c(A"O)z:.

According to the invention, an oxidation protection system disposed on an outer surface of a substrate of a carbon-carbon composite structure comprises a ceramic layer comprising aluminum nitride; and a sealing layer comprising a sealing pre-slurry composition comprising a sealing phosphate glass composition. In various embodiments, the ceramic layer may further comprise silicon oxycarbide, alumina, and/or aluminum borocarbide.

According to the invention, an aircraft brake disk comprises a carbon-carbon composite structure comprising a non-friction surface; and an oxidation protection system disposed on the non-friction surface. The oxidation protection system comprises a ceramic layer comprising aluminum nitride; and a sealing layer comprising a sealing pre-slurry composition comprising a sealing phosphate glass composition. In various embodiments, the ceramic layer may further comprise silicon oxycarbide, alumina, and aluminum borocarbide.

The detailed description of embodiments herein makes reference to the accompanying drawings, which show embodiments by way of illustration. While these embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that logical, chemical, and mechanical changes may be made without departing from the scope of the disclosure. Thus, the detailed description herein is presented for purposes of illustration only and not for limitation. For example, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option. Further, any steps in a method discussed herein may be performed in any suitable order or combination.

With initial reference to <FIG> and <FIG>, aircraft wheel braking assembly <NUM> such as may be found on an aircraft, in accordance with various embodiments is illustrated. Aircraft wheel braking assembly may, for example, comprise a bogie axle <NUM>, a wheel <NUM> including a hub <NUM> and a wheel well <NUM>, a web <NUM>, a torque take-out assembly <NUM>, one or more torque bars <NUM>, a wheel rotational axis <NUM>, a wheel well recess <NUM>, an actuator <NUM>, multiple brake rotors <NUM>, multiple brake stators <NUM>, a pressure plate <NUM>, an end plate <NUM>, a heat shield <NUM>, multiple heat shield segments <NUM>, multiple heat shield carriers <NUM>, an air gap <NUM>, multiple torque bar bolts <NUM>, a torque bar pin <NUM>, a wheel web hole <NUM>, multiple heat shield fasteners <NUM>, multiple rotor lugs <NUM>, and multiple stator slots <NUM>. <FIG> illustrates a portion of aircraft wheel braking assembly <NUM> as viewed into wheel well <NUM> and wheel well recess <NUM>.

In various embodiments, the various components of aircraft wheel braking assembly <NUM> may be subjected to the application of compositions and methods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors <NUM> and stators <NUM>) are disposed in wheel well recess <NUM> of wheel well <NUM>. Rotors <NUM> are secured to torque bars <NUM> for rotation with wheel <NUM>, while stators <NUM> are engaged with torque take-out assembly <NUM>. At least one actuator <NUM> is operable to compress interleaved rotors <NUM> and stators <NUM> for stopping the aircraft. In this example, actuator <NUM> is shown as a hydraulically actuated piston, but many types of actuators are suitable, such as an electromechanical actuator. Pressure plate <NUM> and end plate <NUM> are disposed at opposite ends of the interleaved rotors <NUM> and stators <NUM>. Rotors <NUM> and stators <NUM> can comprise any material suitable for friction disks, including ceramics or carbon materials, such as a carbon/carbon composite.

Through compression of interleaved rotors <NUM> and stators <NUM> between pressure plates <NUM> and end plate <NUM>, the resulting frictional contact slows rotation of wheel <NUM>. Torque take-out assembly <NUM> is secured to a stationary portion of the landing gear truck such as a bogie beam or other landing gear strut, such that torque take-out assembly <NUM> and stators <NUM> are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as composite structures, composite substrates, and carbon-carbon composite structures, interchangeably) in the friction disks may operate as a heat sink to absorb large amounts of kinetic energy converted to heat during slowing of the aircraft. Heat shield <NUM> may reflect thermal energy away from wheel well <NUM> and back toward rotors <NUM> and stators <NUM>. With reference to <FIG>, a portion of wheel well <NUM> and torque bar <NUM> is removed to better illustrate heat shield <NUM> and heat shield segments <NUM>. With reference to <FIG>, heat shield <NUM> is attached to wheel <NUM> and is concentric with wheel well <NUM>. Individual heat shield segments <NUM> may be secured in place between wheel well <NUM> and rotors <NUM> by respective heat shield carriers <NUM> fixed to wheel well <NUM>. Air gap <NUM> is defined annularly between heat shield segments <NUM> and wheel well <NUM>.

Torque bars <NUM> and heat shield carriers <NUM> can be secured to wheel <NUM> using bolts or other fasteners. Torque bar bolts <NUM> can extend through a hole formed in a flange or other mounting surface on wheel <NUM>. Each torque bar <NUM> can optionally include at least one torque bar pin <NUM> at an end opposite torque bar bolts <NUM>, such that torque bar pin <NUM> can be received through wheel web hole <NUM> in web <NUM>. Heat shield segments <NUM> and respective heat shield carriers <NUM> can then be fastened to wheel well <NUM> by heat shield fasteners <NUM>.

Under the operating conditions (e.g., high temperature) of aircraft wheel braking assembly <NUM>, carbon-carbon composites may be prone to material loss from oxidation of the carbon. For example, various carbon-carbon composite components of aircraft wheel braking assembly <NUM> may experience both catalytic oxidation and inherent thermal oxidation caused by heating the composite during operation. In various embodiments, composite rotors <NUM> and stators <NUM> may be heated to sufficiently high temperatures that may oxidize the carbon surfaces exposed to air. At elevated temperatures, infiltration of air and contaminants may cause internal oxidation and weakening, especially in and around rotor lugs <NUM> or stator slots <NUM> securing the friction disks to the respective torque bar <NUM> and torque take-out assembly <NUM>. Because carbon-carbon composite components of aircraft wheel braking assembly <NUM> may retain heat for a substantial time period after slowing the aircraft, oxygen from the ambient atmosphere may react with the carbon matrix and/or carbon fibers to accelerate material loss. Further, damage to brake components may be caused by the oxidation enlargement of cracks around fibers or enlargement of cracks in a reaction-formed porous barrier coating (e.g., a silicon-based barrier coating) applied to the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carbon composite brake components include potassium (K) and sodium (Na). These alkali contaminants may come into contact with aircraft brakes as part of cleaning or de-icing materials. Other sources include salt (e.g., NaCl) deposits left from seawater or sea spray. These and other contaminants (e.g. Ca ions, Fe ions, oxides and salts containing Fe ions and/or Ca ions, etc.) can penetrate and leave deposits in the pores of carbon-carbon composite aircraft brakes, including the substrate and any reaction-formed porous barrier coating. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude.

In various embodiments, components of aircraft wheel braking assembly <NUM> may reach operating temperatures in the range from about <NUM> (<NUM>°F) up to about <NUM> (<NUM>°F), or higher (e.g., <NUM> (<NUM>°F) on a wear or friction surface of a brake disk). However, it will be recognized that the oxidation protection systems compositions and methods of the present disclosure may be readily adapted to many parts in this and other braking assemblies, as well as to other carbon-carbon composite structures susceptible to oxidation losses from infiltration of atmospheric oxygen and/or catalytic contaminants.

According to the invention a method for limiting an oxidation reaction in a substrate of a carbon-carbon composite structure comprises forming an oxidation protection system on the composite structure. Forming the oxidation protection system may comprise forming a ceramic layer slurry by combining aluminum and silicon in a solvent or carrier fluid, applying the ceramic slurry to a composite structure, and heating the composite structure to a temperature sufficient to form a ceramic layer on the composite structure. In various embodiments, forming the oxidation protection system may further comprise forming a sealing pre-slurry composition, comprising a sealing phosphate glass composition (in the form of a glass frit, powder, or other suitable pulverized form), with a carrier fluid (such as, for example, water), applying the sealing slurry to a composite structure, and heating the composite structure to a temperature sufficient to dry the carrier fluid and form an oxidation protection coating on the composite structure, which is referred to a sealing layer. In various embodiments, the sealing pre-slurry composition of the sealing slurry may comprise additives, such as, for example, ammonium dihydrogen phosphate and/or aluminum orthophosphate, among others, to improve hydrolytic stability and/or to increase the composite structure's resistance to oxidation, thereby tending to reduce mass loss of composite structure.

With initial reference to <FIG> and <FIG>, a method <NUM> for coating a composite structure in accordance with various embodiments is illustrated. Method <NUM> may, for example, comprise applying an oxidation protection system to non-wearing surfaces of carbon-carbon composite brake components, such as non-wear surfaces <NUM> and/or rotor lugs <NUM>. Non-wear surfaces (e.g., non-wear surface <NUM>, as labeled in <FIG>) simply reference an exemplary non-wear surface on a brake disk (i.e., a non-friction surface that does not contribute to brake function by having friction with another component, such as another brake disk), but non-wear surfaces similar to non-wear surface <NUM> may be present on any brake disks (e.g., rotors <NUM>, stators <NUM>, pressure plate <NUM>, end plate <NUM>, or the like). In various embodiments, method <NUM> may be used on the back face of pressure plate <NUM> and/or end plate <NUM>, an inner diameter (ID) surface of stators <NUM> including stator slots <NUM>, as well as outer diameter (OD) surfaces of rotors <NUM> including lugs <NUM>. The oxidation inhibiting composition of method <NUM> may be applied to preselected regions of a carbon-carbon composite structure that may be otherwise susceptible to oxidation. For example, aircraft brake disks may have the oxidation inhibiting composition applied on or proximate stator slots <NUM>, rotor lugs <NUM>, and/or non-wear surface <NUM>.

In various embodiments, method <NUM> may comprise forming a ceramic layer slurry (step <NUM>). The ceramic layer slurry comprises aluminum and silicon. The aluminum and silicon may be elemental aluminum and silicon and/or alloys or compounds comprising aluminum and silicon atoms or ions. The aluminum and/or silicon may be in powder form. The aluminum and silicon may be added to a solvent or carrier fluid. The solvent or carrier fluid may comprise any suitable fluid, such as water, methyltrimethoxysilane (MTMS), hexane, cyclohexane, isopropyl alcohol (or other alcohol), any combination of the foregoing, and/or the like. In various embodiments, the aluminum and silicon may be mixed into the solvent or carrier fluid via any suitable method, such as stirring or tumbling. The aluminum and silicon may be mixed into the solvent or carrier fluid for any suitable duration. For example, such mixing may take place for under an hour, over an hour, or for multiple hours. In various embodiments the aluminum and silicon may be mixed into the solvent or carrier fluid for between two and four hours, for about two hours, or about three hours (the term "about" as used in this context means plus or minus <NUM> minutes).

In various embodiments, of the aluminum and silicon (collectively, the "solids"), the aluminum may comprise between <NUM>% and <NUM>% by weight, between <NUM>% and <NUM>% by weight, or between <NUM>% and <NUM>% by weight, and the silicon may comprise between <NUM>% and <NUM>% by weight, between <NUM>% and <NUM>% by weight, or between <NUM>% and <NUM>% by weight. In various embodiments, of the aluminum and silicon, the aluminum may comprise about <NUM>% by weight, and the silicon may comprise about <NUM>% by weight (the term "about" in this context means plus or minus <NUM> weight percent).

In various embodiments, regarding the compositional make up the ceramic layer slurry, the aluminum/silicon solids may comprise between <NUM>% and <NUM>% by weight of the ceramic layer slurry, between <NUM>% and <NUM>% by weight of the ceramic layer slurry, between <NUM>% and <NUM>% by weight of the ceramic layer slurry, or between <NUM>% and <NUM>% by weight of the ceramic layer slurry. In various embodiments, the aluminum/silicon solids may comprise about <NUM>% or about <NUM>% by weight of the ceramic layer slurry (the term "about" in this context means plus or minus <NUM> weight percent). In various embodiments, the solvent(s) and/or carrier fluid(s) may comprise between <NUM>% and <NUM>% by weight, or between <NUM>% and <NUM>% by weight of the ceramic layer slurry, or about <NUM>% by weight of the ceramic layer slurry (the term "about" in this context means plus or minus <NUM> weight percent).

In various embodiments, the ceramic layer slurry may further comprise a binder (but in further embodiments, the ceramic layer slurry may not comprise a binder). The binder may comprise any suitable compound, such as silicon oxycarbide, silicon oxycarbide generating sol, silicon oxycarbide pre-ceramic polymer, and/or polyvinyl alcohol. In various embodiments, the binder may comprise between <NUM>% and <NUM>% by weight of the ceramic layer slurry, between <NUM>% and <NUM>% by weight of the ceramic layer slurry, or about <NUM>% or about <NUM>% by weight of the ceramic layer slurry (the term "about" in this context means plus or minus <NUM> weight percent).

Method <NUM> further comprises applying the ceramic layer slurry to a composite structure (step <NUM>). Applying the ceramic layer slurry may comprise, for example, spraying or brushing the ceramic layer slurry to an outer surface of the composite structure (e.g., a non-wear or non-friction surface). Any suitable manner of applying the ceramic layer slurry to the composite structure is within the scope of the present disclosure. As referenced herein, the composite structure may refer to a carbon-carbon composite structure.

Method <NUM> may further comprise a step <NUM> of heating the composite structure to form a ceramic layer. The composite structure may be heated (e.g., dried or baked) at a temperature in the range from about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F). In various embodiments, the composite structure may be heated to a temperature in a range from about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F), or between about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F). In various embodiments, the composite structure may be heated to, or at least to, about <NUM> (<NUM>°F), or to, or at least to, about <NUM> (<NUM>°F), which may depend on the environment in which the composite structure is being heated (the term "about" in this context means plus or minus <NUM> (<NUM>°F)). Step <NUM> may, for example, comprise heating the composite structure for a period between about <NUM> hour and about <NUM> hours, or about three hours or about four hours (the term "about" in this context means plus or minus one hour). The ceramic layer may also be referred to as a coating. The temperature rise may be controlled at a rate that removes water without boiling, and provides temperature uniformity throughout the composite structure.

In various embodiments, step <NUM> may be performed in an environment comprising nitrogen. The nitrogen may be comprised in a gas, such as nitrogen gas (N<NUM>) or ammonia gas (NH<NUM>). During the heating, the nitrogen in the surrounding environment may react with the aluminum in the ceramic layer slurry to form aluminum nitride. Therefore, the ceramic layer may comprise aluminum nitride. At elevated temperatures, the silicon may also react with the nitrogen in the surrounding environment, forming silicon nitride. Therefore, the ceramic layer may comprise silicon nitride. Also, in embodiments including a silicon oxycarbide binder in the ceramic layer slurry, the aluminum may react with the silicon oxycarbide to form alumina (Al<NUM>O<NUM>) and silicon carbide. The reaction between the aluminum and silicon oxycarbide within the ceramic layer slurry (to form alumina) aids in binding the compounds of the alumina ceramic layer slurry and the resulting ceramic layer to itself, thus strengthening the ceramic layer. Also, the reaction between the aluminum, silicon, silicon oxycarbide, and/or the carbon of the composite structure (forming silicon carbide) aids in binding the ceramic layer to the composite structure.

In various embodiments, in response to being heated in an environment comprising ammonia gas, the substrate may be heated to a temperature of at least <NUM> (<NUM>°F) or about <NUM> (<NUM>°F) (the term "about" in this context means plus or minus <NUM> (<NUM>°F)). At such temperatures, the ammonia gas may react with the aluminum in the ceramic layer slurry to form aluminum nitride comprised in the resulting ceramic layer. In such an environment comprising ammonia, silicon may react with ammonia to form silicon nitride at about <NUM> (<NUM>°F). Thus, in various embodiments, in an ammonia environment, the substrate may be heated to at least <NUM> (<NUM>°F), or to about <NUM> (<NUM>°F), to form aluminum nitride and silicon nitride.

In various embodiments, in response to being heated in an environment comprising nitrogen gas (which is less reactive than ammonia), the substrate may be heated to a temperature of at least <NUM> (<NUM>°F) or about <NUM> (<NUM>°F) (the term "about" in this context means plus or minus <NUM> (<NUM>°F)). At such temperatures, the nitrogen gas may react with the aluminum in the ceramic layer slurry to form aluminum nitride comprised in the resulting ceramic layer. In such an environment comprising nitrogen gas, silicon may react with nitrogen to form silicon nitride at about <NUM> (<NUM>°F). Thus, in various embodiments, in a nitrogen gas environment, the substrate may be heated to at least <NUM> (<NUM>°F), or to about <NUM> (<NUM>°F), to form aluminum nitride and silicon nitride.

The pressure during heating of the substrate may be atmospheric pressure, or between <NUM> pascals and <NUM>,<NUM> pascals.

In various embodiments and with reference now to <FIG>, method <NUM>, which comprises steps also found in method <NUM>, may further comprise applying at least one of a pretreating composition or a barrier coating (step <NUM>) prior to applying the ceramic layer slurry. Step <NUM> may, for example, comprise applying a first pretreating composition to an outer surface of a composite structure, such as a component of aircraft wheel braking assembly <NUM>. In various embodiments, the first pretreating composition comprises an aluminum oxide in water. For example, the aluminum oxide may comprise an additive, such as a nanoparticle dispersion of aluminum oxide (for example, NanoBYK-<NUM>®, sold by BYK Additives & Instruments). The first pretreating composition may further comprise a surfactant or a wetting agent. The composite structure may be porous, allowing the pretreating composition to penetrate at least a portion of the pores of the composite structure.

In various embodiments, after applying the first pretreating composition, the component may be heated to remove water and fix the aluminum oxide in place. For example, the component may be heated between about <NUM> (<NUM>°F) and <NUM> (<NUM>°F), and further, between <NUM> (<NUM>°F) and <NUM> (<NUM>°F).

Step <NUM> may further comprise applying a second pretreating composition. In various embodiments, the second pretreating composition may comprise a phosphoric acid and an aluminum phosphate, aluminum hydroxide, and/or aluminum oxide. The second pretreating composition may further comprise, for example, a second metal salt such as a magnesium salt. In various embodiments, the aluminum to phosphorus molar ratio of the aluminum phosphate is <NUM> to <NUM> or less. Further, the second pretreating composition may also comprise a surfactant or a wetting agent. In various embodiments, the second pretreating composition is applied to the composite structure atop the first pretreating composition. The composite structure may then, for example, be heated. In various embodiments, the composite structure may be heated between about <NUM> (<NUM>°F) and about <NUM> (<NUM>°F), and further, between about <NUM> (<NUM>°F) and <NUM> (<NUM>°F).

In various embodiments, step <NUM> may comprise applying a barrier coating to an outer surface of a composite structure (alternatively or in addition to the pretreating compositions), such as a component of aircraft wheel braking assembly <NUM>, prior to application of the ceramic layer slurry. In various embodiments, the barrier coating composition may comprise carbides or nitrides, including at least one of a boron nitride, silicon carbide, titanium carbide, boron carbide, silicon oxycarbide, and silicon nitride. In various embodiments, the barrier coating may be a boron compound layer formed from a boron compound slurry. The boron compound slurry may comprise a boron compound comprising boron, such as boron, boron carbide, boron nitride, titanium boride, and a solvent and/or carrier fluid (e.g., water, isopropyl alcohol or another alcohol, hexanes, cyclohexane, and/or the like). In embodiments in which the boron compound comprises boron carbide, the boron carbide may comprise a powder comprising granules having sizes ranging from <NUM> micrometer (<NUM>×<NUM>-<NUM> inch) to <NUM> micrometers (<NUM>×<NUM>-<NUM> inch) in size. A boron compound slurry may comprise between <NUM>% and <NUM>% by weight boron compound (e.g., boron carbide), and the remainder comprising solvent and/or carrier fluid. A boron compound slurry may further comprise a dispersant, which may be any suitable dispersant. For example, the dispersant may comprise aluminum oxide (for example, NanoBYK-<NUM>®, sold by BYK Additives & Instruments). A boron compound slurry may comprise less than <NUM>% by weight dispersant.

The boron compound slurry may be applied to the composite structure in any suitable manner (spraying, brushing, etc.) and then dried to form a boron compound layer. In various embodiments, the ceramic layer slurry may be applied to the boron compound layer. In response to heating the composite structure to form the ceramic layer (step <NUM>), the boron compound from the boron compound layer (e.g., boron carbide) may react with the aluminum comprised in the ceramic layer slurry to form aluminum boron carbide. In various embodiments, the ceramic layer resulting from step <NUM> may comprise aluminum boron carbide. In various embodiments, the ceramic layer may at least partially comprise the boron compound layer, or there may be overlap between the ceramic layer and the boron compound layer.

In various embodiments and with reference now to <FIG>, method <NUM> may further comprise a step <NUM>, of forming a sealing slurry by combining a sealing pre-slurry composition, which may comprise a sealing phosphate glass composition in glass frit or powder form, with a carrier fluid (such as, for example, water). In various embodiments, the sealing pre-slurry composition may further comprise ammonium dihydrogen phosphate (ADHP) and/or aluminum orthophosphate. The sealing slurry may be applied to the composite structure (step <NUM>), for example, by spraying or brushing the sealing slurry on to an outer surface of the ceramic layer. Any suitable manner of applying the sealing slurry to the ceramic layer and/or composite structure is within the scope of the present disclosure (e.g., the application methods described in relation to step <NUM>). In various embodiments, the sealing slurry may be substantially free of boron nitride. In this case, "substantially free" means less than <NUM> percent by weight.

In various embodiments, the sealing phosphate glass composition may comprise phosphate glass in the form of a glass frit, powder, or other suitable pulverized and/or ground form, with a carrier fluid (such as, for example, water). The sealing phosphate glass composition may comprise and/or be combined with one or more alkali metal glass modifiers, one or more glass network modifiers and/or one or more additional glass formers. In various embodiments, boron oxide or a precursor may optionally be combined with the P<NUM>O<NUM> mixture to form a borophosphate glass, which has improved self-healing properties at the operating temperatures typically seen in aircraft braking assemblies. In various embodiments, the phosphate glass and/or borophosphate glass may be characterized by the absence of an oxide of silicon. Further, the ratio of P<NUM>O<NUM> to metal oxide in the fused glass may be in the range from about <NUM> to about <NUM> by weight.

Potential alkali metal glass modifiers may be selected from oxides of lithium, sodium, potassium, rubidium, cesium, and mixtures thereof. In various embodiments, the glass modifier may be an oxide of lithium, sodium, potassium, or mixtures thereof. These or other glass modifiers may function as fluxing agents. Additional glass formers can include oxides of boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof.

Suitable glass network modifiers include oxides of vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof.

The sealing phosphate glass composition may be prepared by combining the above ingredients and heating them to a fusion temperature. In various embodiments, depending on the particular combination of elements, the fusion temperature may be in the range from about <NUM>° C (<NUM>°F) to about <NUM>° C (<NUM>°F). The resultant melt may then be cooled and pulverized and/or ground to form a glass frit or powder. In various embodiments, the sealing phosphate glass composition may be annealed to a rigid, friable state prior to being pulverized. Glass transition temperature (Tg), glass softening temperature (Ts) and glass melting temperature (Tm) may be increased by increasing refinement time and/or temperature. Before fusion, the sealing phosphate glass composition comprises from about <NUM> mol% to about <NUM> mol% of P<NUM>O<NUM>. In various embodiments, the sealing phosphate glass composition comprises from about <NUM> mol% to about <NUM> mol% P<NUM>O<NUM>, or precursor thereof. In various embodiments, the sealing phosphate glass composition comprises from about <NUM> to about <NUM> mol% of P<NUM>O<NUM>. In this context, the term "about" means plus or minus <NUM> mol%.

The sealing phosphate glass composition may comprise, or be combined with, from about <NUM> mol% to about <NUM> mol% of the alkali metal oxide. In various embodiments, the sealing phosphate glass composition may comprise, or be combined with, from about <NUM> mol% to about <NUM> mol% of the alkali metal oxide. Further, the sealing phosphate glass composition may comprise, or be combined with, from about <NUM> to about <NUM> mol% of the alkali metal oxide or one or more precursors thereof. In various embodiments, the sealing phosphate glass composition may comprise, or be combined with, from about <NUM> mol% to about <NUM> mol% of one or more of the above-indicated glass formers. The sealing phosphate glass composition may comprise, or be combined with, about <NUM> to about <NUM> mol% of one or more of the above-indicated glass formers. As used herein, mol% is defined as the number of moles of a constituent per the total moles of the solution.

In various embodiments, the sealing phosphate glass composition may comprise, or be combined with, from about <NUM> mol% to about <NUM> mol% of one or more of the above-indicated glass network modifiers. The sealing phosphate glass composition may comprise, or be combined with, from about <NUM> mol% to about <NUM> mol% of one or more of the above-indicated glass network modifiers.

In various embodiments, the sealing phosphate glass composition may be represented by the formula:.

a(A'<NUM>O)x(P<NUM>O<NUM>)y1b(GfO)y2c(A"O)z     [<NUM>].

In Formula <NUM>, A' is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; Gf is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, bismuth, and mixtures thereof; A" is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from <NUM> to about <NUM>; b is a number in the range from <NUM> to about <NUM>; c is a number in the range from <NUM> to about <NUM>; x is a number in the range from about <NUM> to about <NUM>; y<NUM> is a number in the range from about <NUM> to about <NUM>; y<NUM> is a number in the range from <NUM> to about <NUM>; and z is a number in the range from about <NUM> to about <NUM>; (x + y<NUM> + y<NUM> + z)=<NUM>; and x < (y<NUM> + y<NUM>). The sealing phosphate glass composition may be formulated to balance the reactivity, durability and flow of the resulting glass base layer for optimal performance. As used in this context, the term "about" means plus or minus ten percent of the respective value.

The sealing slurry may comprise any suitable weight percentage sealing phosphate glass composition. For example, the sealing slurry may comprise between <NUM>% and <NUM>% by weight sealing phosphate glass composition, between <NUM>% and <NUM>% by weight sealing phosphate glass composition, between <NUM>% and <NUM>% by weight sealing phosphate glass composition, and/or between <NUM>% and <NUM>% by weight sealing phosphate glass composition. The sealing pre-slurry composition (and/or the resulting sealing layer, discussed in association with step <NUM>) may comprise any suitable weight percentage sealing phosphate glass composition. For example, the sealing pre-slurry composition may comprise between <NUM>% and <NUM>% by weight sealing phosphate glass composition, between <NUM>% and <NUM>% by weight sealing phosphate glass composition, and/or between <NUM>% and <NUM>% by weight sealing phosphate glass composition.

Method <NUM> may further comprise a step <NUM> of heating the composite structure to form a sealing layer comprising phosphate glass over the ceramic layer. The composite structure may be heated (e.g., dried or baked) at a temperature in the range from about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F). In various embodiments, the composite structure is heated to a temperature in a range from about <NUM> (<NUM>°F) to about <NUM>° C (<NUM>°F), or between about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F), or further, between about <NUM>° C (<NUM>°F) to about <NUM> (<NUM>°F). Step <NUM> may, for example, comprise heating the composite structure for a period between about <NUM> hour and about <NUM> hours, wherein the term "about" in this context only means plus or minus <NUM> hours. The sealing layer may also be referred to as a coating.

In various embodiments, the composite structure may be heated to a first, lower temperature (for example, about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F)) to bake or dry the sealing layer at a controlled depth. A second, higher temperature (for example, about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F)) may then be used to melt the phosphate glass composition, creating a substantially uniform sealing layer over the ceramic layer. The duration of each heating step can be determined as a fraction of the overall heating time and can range from about <NUM>% to about <NUM>%, wherein the term "about" in this context only means plus or minus <NUM>%. In various embodiments, the duration of the lower temperature heating step(s) can range from about <NUM>% to about <NUM>% of the overall heating time, wherein the term "about" in this context only means plus or minus <NUM>%. The lower temperature step(s) may occupy a larger fraction of the overall heating time, for example, to provide relatively slow heating up to and through the first lower temperature. The exact heating profile will depend on a combination of the first temperature and desired depth of the drying portion.

Step <NUM> may be performed in an inert environment, such as under a blanket of inert gas or less reactive gas (e.g., nitrogen, argon, other noble gases, and the like). For example, a composite structure may be pretreated or warmed prior to application of the sealing slurry to aid in the penetration of the sealing slurry. Step <NUM> may be for a period of about <NUM> hours at a temperature of about <NUM> (<NUM>°F) to about <NUM> (<NUM>°F), wherein the term "about" in this context only means plus or minus <NUM>. The composite structure and the sealing slurry may then be dried or baked in a non-oxidizing, inert or less reactive atmosphere, e.g., noble gasses and/or nitrogen (N<NUM>), to optimize the retention of the sealing pre-slurry composition of the sealing slurry and resulting sealing layer filling any pores or cracks in the ceramic layer. This retention may, for example, be improved by heating the composite structure to about <NUM> (<NUM>°F) and maintaining the temperature for about <NUM> hour before heating the carbon-carbon composite to a temperature in the range described above. The temperature rise may be controlled at a rate that removes water without boiling, and provides temperature uniformity throughout the composite structure.

At elevated operation temperatures of aircraft brake disks (e.g., <NUM> (<NUM>°F) and above), the ceramic layer may crack, which causes a risk of oxygen penetrating through the oxidation protection system and allowing oxidation of the composite structure. At such elevated temperatures, the sealing layer may melt and/or flow, allowing the sealing layer to fill any cracks that may form in the ceramic layer, thus, sealing the ceramic layer and the oxidation protection system.

In various embodiments, the sealing slurry may comprise an additional metal salt. The cation of the additional metal salt may be multivalent. The metal may be an alkaline earth metal or a transition metal. In various embodiments, the metal may be an alkali metal. The multivalent cation may be derived from a non-metallic element such as boron. The term "metal" is used herein to include multivalent elements such as boron that are technically non-metallic. The metal of the additional metal salt may be an alkaline earth metal such as calcium, magnesium, strontium, barium, or a mixture of two or more thereof. The metal for the additional metal salt may be iron, manganese, tin, zinc, or a mixture of two or more thereof. The anion for the additional metal salt may be an inorganic anion such as a phosphate, halide, sulfate or nitrate, or an organic anion such as acetate. In various embodiments, the additional metal salt may be an alkaline earth metal salt such as an alkaline earth metal phosphate. In various embodiments, the additional metal salt may be a magnesium salt such as magnesium phosphate. In various embodiments, the additional metal salt may be an alkaline earth metal nitrate, an alkaline earth metal halide, an alkaline earth metal sulfate, an alkaline earth metal acetate, or a mixture of two or more thereof. In various embodiments, the additional metal salt may be magnesium nitrate, magnesium halide, magnesium sulfate, or a mixture of two or more thereof. In various embodiments, the additional metal salt may comprise: (i) magnesium phosphate; and (ii) a magnesium nitrate, magnesium halide, magnesium sulfate, or a mixture of two or more thereof.

The additional metal salt may be selected with reference to its compatibility with other ingredients in the sealing slurry. Compatibility may include metal phosphates that do not precipitate, flocculate, agglomerate, react to form undesirable species, or settle out prior to application of the sealing slurry to the carbon-carbon composite. The phosphates may be monobasic (H<NUM>PO<NUM>-), dibasic (HPO<NUM>-<NUM>), or tribasic (PO<NUM>-<NUM>). The phosphates may be hydrated. Examples of alkaline earth metal phosphates that may be used include calcium hydrogen phosphate (calcium phosphate, dibasic), calcium phosphate tribasic octahydrate, magnesium hydrogen phosphate (magnesium phosphate, dibasic), magnesium phosphate tribasic octahydrate, strontium hydrogen phosphate (strontium phosphate, dibasic), strontium phosphate tribasic octahydrate and barium phosphate.

In one embodiment, a chemical equivalent of the additional metal salt may be used as the additional metal salt. Chemical equivalents include compounds that yield an equivalent (in this instance, an equivalent of the additional metal salt) in response to an outside stimulus such as, temperature, hydration, or dehydration. For example, equivalents of alkaline earth metal phosphates may include alkaline earth metal pyrophosphates, hypophosphates, hypophosphites and orthophosphites. Equivalent compounds include magnesium and barium pyrophosphate, magnesium and barium orthophosphate, magnesium and barium hypophosphate, magnesium and barium hypophosphite, and magnesium and barium orthophosphite.

While not wishing to be bound by theory, it is believed that the addition of multivalent cations, such as alkaline earth metals, transition metals and nonmetallic elements such as boron, to the sealing slurry enhances the hydrolytic stability of the metal-phosphate network. In general, the hydrolytic stability of the metal-phosphate network increases as the metal content increases, however a change from one metallic element to another may influence oxidation inhibition to a greater extent than a variation in the metal-phosphate ratio. The solubility of the phosphate compounds may be influenced by the nature of the cation associated with the phosphate anion. For example, phosphates incorporating monovalent cations such as sodium orthophosphate or phosphoric acid (hydrogen cations) are very soluble in water, while (tri)barium orthophosphate is insoluble. Phosphoric acids can be condensed to form networks but such compounds tend to remain hydrolytically unstable. Generally, it is believed that the multivalent cations link phosphate anions creating a phosphate network with reduced solubility. Another factor that may influence hydrolytic stability is the presence of -P-O-H groups in the condensed phosphate product formed from the sealing slurry during thermal treatment. The sealing slurry may be formulated to minimize concentration of these species and any subsequent hydrolytic instability. Whereas increasing the metal content may enhance the hydrolytic stability of the sealing slurry, it may be desirable to strike a balance between composition stability and effectiveness as an oxidation inhibitor.

In various embodiments, the additional metal salt may be present in the sealing slurry at a concentration in the range from about <NUM> weight percent to about <NUM> weight percent, and in various embodiments from about <NUM> weight percent to about <NUM> weight percent, and in various embodiments from about <NUM> weight percent to about <NUM> weight percent. In various embodiments, a combination of two or more additional metal salts may be present at a concentration in the range from about <NUM> weight percent to about <NUM> weight percent, and in various embodiments from about <NUM> weight percent to about <NUM> weight percent.

<FIG> depicts a substrate <NUM> of a carbon-carbon composite structure. , with an oxidation protection system <NUM> disposed thereon (e.g., on a non-wear or non-friction surface). The oxidation protection system on a substrate comprises a ceramic layer (e.g., layer <NUM>). The ceramic layer may be directly disposed on and in contact with the substrate. In various embodiments, an oxidation protection system may comprise a layer(s) between the substrate <NUM> and the ceramic layer, for example, layer <NUM>. Layer <NUM> may comprise a pretreating layer(s) or a barrier coating (e.g., a boron compound layer), as discussed herein. The oxidation protection system comprises a sealing layer (e.g., layer <NUM>). The sealing layer may be disposed on, and directly contact, the ceramic layer, such that the ceramic layer is disposed between the sealing slurry and the substrate.

TABLE <NUM> illustrates a sealing slurry prepared in accordance with the embodiments discussed herein. Each numerical value in TABLE <NUM> is the number of grams of the particular substance added to the slurry.

As illustrated in TABLE <NUM>, an oxidation protection system sealing slurry (slurry A) comprises a pre-slurry composition, comprising phosphate glass composition glass frit and various additives such as aluminum orthophosphate, and/or ammonium dihydrogen phosphate, in a carrier fluid (i.e., water), was prepared. Slurry A may be a suitable sealing slurry which will serve as a sealing layer after heating (such as during step <NUM>), which is substantially free of boron nitride. In this case, "substantially free" means less than <NUM> percent by weight.

With combined reference to TABLE <NUM> and <FIG>, TABLE <NUM> and plot <NUM> in <FIG> may allow evaluation of oxidation protection systems comprising a ceramic layer and a sealing layer versus oxidation protection systems comprising and a sealing layer without a ceramic layer. Data sets <NUM> and <NUM> represent oxidation protection systems comprising a ceramic layer and a sealing layer, while data sets <NUM> and <NUM> represent oxidation protection systems comprising a sealing layer (resulting from slurry A) without a ceramic layer. Percent weight loss is shown on the y-axis and exposure time is shown on the x-axis of the graph depicted in <FIG>.

The oxidation protection systems represented by data sets <NUM> and <NUM> were prepared by applying slurry A to carbon-carbon composite coupons and heated to form a sealing slurry, in accordance with embodiments discussed herein. Slurry A for data set <NUM> is hydrated, while the slurry A for data set <NUM> is not (i.e., dry). Example <NUM>, represented by data set <NUM>, was prepared by: (<NUM>) applying a boron compound slurry comprising <NUM> grams of boron carbide, <NUM> grams of water, and <NUM> grams of dispersant to a carbon-carbon composite coupon, by dipping the coupon in the boron compound slurry; (<NUM>) air-drying the boron compound slurry to form a boron compound layer on the coupon; (<NUM>) preparing a ceramic layer slurry comprising <NUM> grams of MTMS, <NUM> grams of water, <NUM> grams of sub-micron silicon powder, and <NUM> grams of <NUM>-micron aluminum powder; (<NUM>) applying the ceramic layer slurry to the boron compound layer by dipping the coupon in the ceramic layer slurry; (<NUM>) air-drying the ceramic layer slurry; (<NUM>) heating the coupon at <NUM> (<NUM>°F) for about three hours under an environment comprising nitrogen gas to form the ceramic layer comprising at least aluminum nitride; (<NUM>) applying slurry A to the ceramic layer; and (<NUM>) heating the coupon at <NUM> (<NUM>°F) to melt the phosphate glass in the sealing slurry and form the sealing layer. Example <NUM>, represented by data set <NUM>, was prepared by the same method as Example <NUM> (data set <NUM>), except to form the ceramic layer, the coupon was heated at <NUM> (<NUM>°F) rather than at <NUM> (<NUM>°F), as in Example <NUM>. The coupons were then heated in accordance with the time and temperature milestones shown on the x-axis of <FIG>. The milestone (<NUM>-<NUM>) along the x-axis of <FIG> are sequential (i.e., they happen one after the other).

As can be seen in <FIG>, the oxidation protection systems having the ceramic layer and the sealing layer (data sets <NUM> and <NUM>) resulted in drastically less weight loss of the composite structure than the oxidation protection systems having a sealing layer with no ceramic layer (data sets <NUM> and <NUM>). That is, oxidation protection systems having the ceramic layer and the sealing layer (data sets <NUM> and <NUM>) resulted in about <NUM>% weight loss of the substrate at the end of the test, while the oxidation protection systems having a sealing layer with no ceramic layer (data sets <NUM> and <NUM>) resulted in about <NUM>% and <NUM>% weight loss, respectively. Thus, oxidation protection systems having the ceramic layer and the sealing layer (data sets <NUM> and <NUM>) provided about (or at least) ten times the oxidation protection than the oxidation protection systems having the ceramic layer and the sealing layer (data sets <NUM> and <NUM>). These results indicate that the oxidation protection systems comprising the ceramic layer (comprising aluminum nitride, silicon, silicon nitride, aluminum boron carbide, and/or silicon carbide), along with the sealing layer disposed thereon, creates a strong barrier which stops oxygen from passing therethrough and causing oxidation of the underlying substrate (especially at elevated temperatures of <NUM>°F). The economical preparation and application of the involved slurries, along with the positive and improved oxidation protection results, indicate that the systems and methods discussed herein provide effective oxidation protection.

Benefits and other advantages have been described herein with regard to specific embodiments.

Claim 1:
A method (<NUM>) for forming an oxidation protection system on a carbon-carbon composite structure, comprising:
applying (<NUM>) a ceramic layer slurry to the carbon-carbon composite structure, wherein the ceramic layer slurry comprises aluminum and silicon in a solvent or carrier fluid;
heating (<NUM>) the carbon-carbon composite structure to form a ceramic layer on the carbon-carbon composite structure, wherein the ceramic layer comprises aluminum nitride;
applying (<NUM>) a sealing slurry to the carbon-carbon composite structure, wherein the sealing slurry comprises a sealing pre-slurry composition and a sealing carrier fluid, wherein the sealing pre-slurry composition comprises a sealing phosphate glass composition; and
heating (<NUM>) the carbon-carbon composite structure to form a sealing layer on the carbon-carbon composite structure.