Carbon / carbon aircraft brake material high temperature resistant ceramic oxidation resistant coating and preparation method thereof

Abstract

The application relates to a high-temperature-resistant ceramic anti-oxidation coating of carbon / carbon airplane brake material and a preparation method thereof. The preparation method comprises the following steps: cleaning, drying, and cooling the surface of a base material; brushing and drying a bottom layer raw material to obtain a first coating layer; brushing and drying a silica sol raw material on the surface of the first coating layer to obtain a second coating layer; brushing and drying a surface layer raw material on the surface of the second coating layer to obtain a third coating layer; and performing heat treatment on the coated base material to complete sintering of the ceramic coating layer. The application develops a long-term effective anti-oxidation ceramic coating layer under a high-temperature and high-humidity environment. The bottom coating layer of the coating layer is composed of a bottom layer raw material and a silica sol raw material, and the main coating layer is mainly composed of a surface layer raw material. The coating layer has the characteristics of thin thickness, high stable temperature, no reaction with water, and no permeation of water molecules under high temperature, can be used under high temperature, is not damaged under multiple high-low temperature cycles, can be used under a water environment, and can be sintered into a shape at one time.

Description

Technical Field

[0001] This invention belongs to the field of carbon / carbon material coating technology, specifically relating to a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials and its preparation method. Background Technology

[0002] Carbon / carbon composite materials are a type of composite material reinforced with carbon fibers and using deposited carbon, resin carbon, etc. as the matrix. This type of material has a high operating temperature, high strength, and high permeability in non-oxidizing environments. It also has low wear and high specific heat capacity, making it the preferred material for brake discs of modern large aircraft.

[0003] However, this material has an inherent weakness: it is not resistant to oxidation in atmospheric environments and undergoes significant oxidation at temperatures above 500℃. Aircraft brake discs, due to absorbing enormous kinetic energy during landing, generate high temperatures, often exceeding 500℃ and in some cases surpassing 1000℃. Therefore, without a protective surface layer, the carbon / carbon composite brake discs experience accelerated oxidation over time, leading to a significant decrease in material performance and strength, a drastically shortened service life, and even structural damage and failure, posing a significant safety hazard to the aircraft.

[0004] Given this situation, anti-oxidation coating technology was developed from the very beginning of the use of carbon / carbon composite materials in aircraft brake discs. Early technologies primarily used phosphoric acid, phosphates, and boron oxide, which performed well at low temperatures but had a short lifespan and low temperature resistance (below 700℃). When the carbon disc absorbed a large amount of energy and the temperature rose, the effect was not very good. In recent years, to overcome this challenge, many manufacturers have successively developed new anti-oxidation coating technologies, including multi-layer protective composite coatings and the addition of temperature-resistant oxides and carbides to the coating. The introduction of these technologies has improved the temperature resistance and lifespan of the anti-oxidation coating, but some shortcomings have also been revealed in actual use. For example, when aircraft frequently take off and land, and the brake discs are reused before cooling to room temperature, resulting in high temperatures (above 900℃), and when water accumulates on the runway or de-icing agents contaminate the brake discs, the protective effect of these improved coatings will still decrease significantly. Premature oxidation of the carbon / carbon brake discs will still occur, leading to a significant decrease in the performance and strength of the carbon brake discs, ultimately forcing them to be scrapped prematurely.

[0005] In addition, with the increasing takeoff and landing weight of civil aircraft, the braking energy during takeoff and landing is becoming greater, which puts higher demands on coatings, requiring them to operate at increasingly higher temperatures.

[0006] Therefore, in order to adapt to the current harsh operating environment of high-frequency take-off and landing, all-weather take-off and landing, and increasingly heavy aircraft take-off and landing, it is necessary to develop anti-oxidation coatings that are more resistant to high temperatures and water.

[0007] Currently, many anti-oxidation coating technologies and methods for carbon / carbon composite materials have been developed both domestically and internationally. The main directions are as follows: one is protection based on the glass phase, and the other is protection based on phosphate, or a combination of both. These coatings each have their own advantages, but they all reveal some weaknesses in use.

[0008] An existing patent, application number CN201610844409.0, publication number CN106518166A, provides a method in which the main coating is composed of phosphate + boron + low-temperature borosilicate glass powder + refractory metal carbide. This increases the service temperature of the coating. However, due to the high-temperature reaction between boron and phosphate, as well as the high-temperature reaction between boron and oxygen and water, the generated boric acid will directly volatilize when the temperature is increased. Therefore, this type of coating cannot withstand high temperatures or for long periods of time, especially in humid or water-immersed environments.

[0009] An existing patent, application number CN202510192120.4, publication number CN119684828B, provides a method for a medium-temperature anti-oxidation coating. It uses aluminum phosphate as the first layer and medium-temperature glass powder + kaolin + boron carbide as the second layer, improving the coating's temperature and water resistance. However, it is limited by the temperature resistance of the medium-temperature glass phase (which melts above 700℃, altering the coating structure), making it difficult to meet high-temperature (above 900℃) requirements. Furthermore, while the presence of boron carbide helps improve crack healing, the boron oxide formed at high temperatures is easily hydrolyzed. After high-temperature oxidation, it reacts with water to form boric acid, which is lost, significantly reducing its protective properties in water-immersed environments.

[0010] An existing patent, application number CN201510170819.7, publication number CN104973886B, provides a double-layer temperature-resistant and water-resistant coating anti-oxidation technology. It uses phosphate + surface wetting agent as the first layer and phosphate + boron carbide + refractory metal carbide (or boride) as the second layer, significantly improving high-temperature anti-oxidation and waterproof effects. However, because the strength of the aluminum phosphate matrix reaches its maximum at 900℃, further temperature increases will cause a series of decompositions and phase transitions, resulting in a series of performance degradations in the material. Furthermore, this phosphate... Aluminum substrates are porous, resulting in two main weaknesses during use. First, at temperatures above 900°C, various decomposition reactions occur in the aluminum phosphate substrate, leading to powdering of the coating. This powdering, especially after repeated high-temperature treatments, significantly reduces the coating's waterproofing ability, effectively creating a water-absorbing layer and drastically decreasing the overall protective effect. Second, due to the high viscosity of the aluminum phosphate solution, its wetting effect on carbon / carbon substrates is generally poor. When the coating is thin, its coverage is not good, and after sintering, the coating has a high porosity, allowing oxygen or moisture to pass through easily, thus reducing the protective effect.

[0011] Another drawback of the above patents is that they all require two high-temperature sintering processes. Each high-temperature sintering process, from heating to cooling and removal, takes more than two days, making the coating preparation process too complicated and costly. Summary of the Invention

[0012] To address the aforementioned technical problems, this invention provides a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials and its preparation method. By developing a coating that provides long-term effective anti-oxidation ceramic protection in high-temperature, high-humidity, and immersion environments, the coating's base layer consists of a bottom layer of raw materials and silica sol, primarily phosphoric acid and silica sol. The main coating layer is primarily composed of a top layer of raw materials, mainly a high-temperature resistant mullite ceramic layer. This coating features a thin coating thickness, a high stable temperature (greater than 1000℃), and the ability to not react with water or allow water molecules to pass through at high temperatures. It can be used at relatively high temperatures (greater than 900℃), withstands multiple high and low temperature cycles without damage, and can be used in wet environments. Furthermore, this coating can be sintered in one step.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0014] A method for preparing a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials includes the following steps:

[0015] S1: Substrate surface cleaning, drying and cooling;

[0016] The carbon / carbon composite substrate to be coated with the anti-oxidation coating is cleaned in deionized water, then placed in an oven and dried at a temperature above 100°C for 2 hours. After cooling to below 40°C, it is taken out and ready for coating.

[0017] The surface of the processed carbon / carbon composite substrate contains many tiny carbon particles, which, if not removed, will affect the adhesion of subsequent coatings.

[0018] S2: Apply the base material to the surface of the substrate, dry and cool it to obtain the first coating;

[0019] The underlying raw material contains phosphoric acid and magnesium dihydrogen phosphate; by mass percentage, the composition is as follows: 50-70% phosphoric acid, 5-10% magnesium dihydrogen phosphate, and the balance is deionized water.

[0020] Mix the above ingredients thoroughly and set aside.

[0021] The prepared base material is applied to the surface of the hot carbon / carbon composite substrate. The carbon / carbon composite substrate is then placed in an oven and dried at a temperature of 120-200°C for more than 2 hours. After cooling, it is taken out and ready to be coated with the second coating.

[0022] Both phosphoric acid and magnesium dihydrogen phosphate are water-soluble, and the prepared solution has a low viscosity and is easy to diffuse. Therefore, the solution of phosphoric acid + magnesium dihydrogen phosphate has good wettability with the surface of carbon / carbon materials, which is much better than that of aluminum phosphate. It is easier to diffuse to the entire surface and pores of carbon / carbon materials, resulting in a better coating effect.

[0023] Phosphoric acid dehydrates upon heating, successively forming polyphosphoric acid, pyrophosphoric acid, triphosphoric acid, and polymetaphosphoric acid. Upon heating above 300℃, phosphoric acid gradually loses its molecular water, transforming into polymetaphosphoric acid, as shown in the following reaction formula:

[0024] 3H3PO4 = 3H2O + 3HPO3 (polymetaphosphate)

[0025] Cross-linked polyphosphoric acid after high-temperature treatment is a dense inorganic coating that does not react with oxygen and can completely block oxygen from passing through. Therefore, it has a good anti-oxidation effect at 700℃.

[0026] If the first coating uses only phosphoric acid as the raw material, after being subjected to a higher temperature of 750°C, 3HPO3 (polyphosphoric acid) will be converted into P2O5 (phosphorus pentoxide) in the air, which will reduce the anti-oxidation effect. Therefore, another coating with higher temperature protection, magnesium polyphosphate, will be added.

[0027] Magnesium phosphate (Mg3(PO4)2) is a solid with very low solubility in water and phosphoric acid, making it unsuitable for preparing water-soluble coatings. Magnesium dihydrogen phosphate (Mg(H2PO4)2), a magnesium phosphate, is soluble in water and phosphoric acid, with a composition of 11% MgO and 89% H3PO4. During heating, magnesium dihydrogen phosphate undergoes the following dehydration and conversion reactions:

[0028] Mg(H2PO4)2→2MgH2P2O7→Mg2P4O 12 H₂O → 3Mg₂P₄O 12 (Magnesium polyphosphate)

[0029] Magnesium dihydrogen phosphate loses all its water content and becomes magnesium polyphosphate at temperatures above 600°C.

[0030] Magnesium polyphosphate exhibits excellent thermal stability at high temperatures, with a thermal decomposition temperature exceeding 1000℃. This type of magnesium polyphosphate coating does not react with oxygen and does not allow oxygen molecules to pass through, thus providing good anti-oxidation effects. This ensures the anti-oxidation properties of the magnesium polyphosphate coating at high temperatures.

[0031] S3: Apply silica sol material to the surface of the first coating, dry and cool it to obtain the second coating;

[0032] The silica sol raw material comprises silica sol; by mass percentage, the composition is as follows: silica sol 80-90%, the balance being deionized water.

[0033] Mix the above ingredients thoroughly and set aside.

[0034] The prepared silica sol raw material is brushed onto the dried first coating. Then, the carbon / carbon composite material substrate is placed in an oven and dried at a temperature of 120-180℃ for more than 2 hours. After cooling, the third coating is applied.

[0035] Although the first base coating system of phosphoric acid + magnesium phosphate has a good anti-oxidation effect, its overall temperature resistance is still relatively low. Therefore, a second base layer, silica sol coating, needs to be applied on the first base layer after drying. This base layer has higher temperature resistance.

[0036] Silica sol is a colloidal solution, consisting of nano-sized silica particles dispersed in water or a solvent. Because the SiO2 in silica sol contains a large amount of water and hydroxyl groups, silica sol can also be described as SiO2·nH2O.

[0037] This coating generates SiO2 at high temperatures, which can directly participate in the subsequent reaction of the aluminum oxide and silicon dioxide ceramic layers.

[0038] Although the first and second coatings mentioned above also have good anti-oxidation effects, they are still relatively thin coatings overall and not resistant to erosion. Therefore, in practical applications, a harder and more heat-resistant ceramic coating needs to be added on top of these two base layers.

[0039] S4: Apply the topcoat material to the surface of the second coating, dry and cool it to obtain the third coating;

[0040] The surface layer material comprises aluminum oxide, silicon dioxide, silicon carbide, and zirconium dioxide; by mass percentage, the composition is as follows: 25-35% aluminum oxide, 10-20% silicon dioxide, 10-15% silicon carbide, 5-10% zirconium dioxide, 1-3% dispersant, and the balance being deionized water; the dispersant is an alkoxylated polyol.

[0041] Mix the above ingredients thoroughly and set aside.

[0042] The prepared top layer material is brushed onto the dried second layer. Then, the carbon / carbon composite material substrate is placed in an oven and dried at 100-120°C for more than 1 hour. After cooling, it is taken out for sintering.

[0043] Aluminum oxide, also known as aluminum oxide, is a stable oxide of aluminum with the chemical formula Al₂O₃. In mining, ceramics, and materials science, it is also called bauxite. Aluminum oxide has a melting point of 2050℃ and is extremely resistant to oxidation.

[0044] Silicon dioxide is an inorganic compound, an atomic crystal with the chemical formula SiO2, which represents the ratio of silicon to oxygen atoms in silicon dioxide. It has a melting point of 1723 ℃. The silicon-oxygen covalent bond (Si-O) is one of the strongest chemical bonds found in nature. The Si-O covalent bonds connecting silicon and oxygen atoms in silicon dioxide are very strong, so silicon dioxide usually exhibits many excellent physical properties. At the same time, silicon dioxide also has high chemical stability and is extremely resistant to oxidizing substances.

[0045] The main body of this coating is a mullite-structured ceramic layer formed by the reaction of aluminum oxide and silicon dioxide. This is a ceramic layer with a high melting point and excellent thermal stability, and has good temperature resistance and oxidation resistance.

[0046] At this ratio and sintering temperature, aluminum oxide and silicon dioxide can undergo the following reaction:

[0047] 3Al₂O + 2SiO₂ → 3Al₂O·2SiO₂

[0048] Mullite is the only stable binary compound in the Al₂O₃-SiO₂ system, with a composition that can vary between 3Al₂O₃·2SiO₂ and 2Al₂O₃·SiO₂. 3Al₂O₃·2SiO₂ represents the stoichiometric composition and content of mullite. This mullite-structured ceramic layer can be used in air at temperatures below 1200°C for extended periods without structural changes, making it a stable anti-oxidation coating.

[0049] Zirconium dioxide, with the chemical formula ZrO2, has a melting point of 2700℃. It is a white crystalline solid under normal conditions and is sparingly soluble in water. Chemically inert, it possesses a high melting point, high resistivity, high refractive index, and low coefficient of thermal expansion, making it an important high-temperature resistant material and ceramic insulating material.

[0050] Due to its high melting point, oxygen-reactive properties, and other excellent high-temperature characteristics, ZrO2 has a higher service temperature than other refractory materials such as alumina. Zirconia can be used for extended periods in ultra-high temperature oxidizing atmospheres above 1500 ℃, with a maximum service temperature reaching 2200 ℃, and even maintaining its shape up to 2500 ℃. Furthermore, it exhibits high-temperature chemical stability, corrosion resistance, oxidation resistance, thermal shock resistance, non-volatile nature, and no pollution, making it one of the world's leading refractory materials. In this coating system, zirconium dioxide is added as a reinforcing and toughening material for the Al2O3-SiO2 system, while also improving the coating's temperature resistance.

[0051] Silicon carbide (SiC) is a high-temperature resistant material with a melting point of over 1500 ℃ and good water resistance. It can be in contact with water for a long time without being impermeable, absorbing water, or hydrolyzing. Even after reacting with oxygen in the air at temperatures above 800 ℃ to form molten silica, this molten silica does not react with water and still has anti-oxidation function in aquatic environments. Furthermore, the molten silica can fill micro-cracks in ceramic coatings and reduce oxygen permeability. Therefore, silicon carbide is added as a high-temperature resistant, waterproof, and erosion-resistant coating material.

[0052] The dispersant (alkoxylated polyol) is added to ensure that the various ceramic components are fully and evenly mixed, to prevent component segregation, and to ensure that the predetermined structure is achieved during sintering.

[0053] S5: The coated substrate is heat-treated and then cooled to complete the sintering of the ceramic coating;

[0054] The dried carbon / carbon composite substrate is placed in a heat treatment furnace and sintered at 800-1000℃ for 2 hours under nitrogen protection. Then it is naturally cooled to room temperature to complete the sintering of the ceramic coating.

[0055] The present invention also provides a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake material prepared by the above preparation method. The high-temperature resistant ceramic anti-oxidation coating consists of a base layer and a main layer from bottom to top. The base layer is composed of a bottom layer material and a silica sol material, and the main layer is mainly composed of a top layer material.

[0056] Compared with existing technologies, the advantages of this invention are as follows: It employs a high-temperature resistant mullite-structured ceramic coating formed by the reaction of alumina and silica as the main coating, while also incorporating phosphoric acid and silica gel coatings, which perform well at low temperatures. To improve the overall protective effect, the bottom layers utilize phosphoric acid and magnesium dihydrogen phosphate coatings, which offer good low-temperature protection, along with a silica sol coating. This multi-layer approach fully leverages the advantages of each layer; the first two layers are dried without sintering, while the last three coatings are sintered together, achieving a single-stage sintering process. This multi-layer system exhibits higher high-temperature resistance and chemical stability than coatings based on glass or aluminum phosphate refractory materials. The coating does not hydrolyze or pulverize at high temperatures, nor does it react with water or de-icing agents (potassium acetate). Therefore, it is a thinner, more heat-resistant, more oxidation-resistant, and more waterproof coating, with a longer service life under alternating high and low temperature conditions. Attached Figure Description

[0057] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is a schematic diagram of the anti-oxidation coating firing process of the present invention;

[0059] Figure 2 The graphs show the oxidation weight loss rate at 750°C for coated and uncoated samples of the present invention.

[0060] Figure 3 The graph shows the weight loss rate of the coating sample of the present invention after soaking in potassium acetate solution at 900°C. Detailed Implementation

[0061] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0062] The specific embodiments of the present invention will now be described with reference to the accompanying drawings:

[0063] Example 1

[0064] like Figure 1 As shown, a method for preparing a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials includes the following steps:

[0065] S1: Substrate surface cleaning, drying and cooling;

[0066] The carbon / carbon composite substrate to be coated with the anti-oxidation coating is cleaned in deionized water, then placed in an oven and dried at a temperature above 100°C for 2 hours. After cooling to below 40°C, it is taken out and ready for coating.

[0067] The surface of the processed carbon / carbon composite substrate contains many tiny carbon particles, which, if not removed, will affect the adhesion of subsequent coatings.

[0068] S2: Apply the base material to the surface of the substrate, dry and cool it to obtain the first coating;

[0069] The underlying raw material contains phosphoric acid and magnesium dihydrogen phosphate; by mass percentage, the composition is as follows: 50-70% phosphoric acid, 5-10% magnesium dihydrogen phosphate, and the balance is deionized water.

[0070] Mix the above ingredients thoroughly and set aside.

[0071] The prepared base material is applied to the surface of the hot carbon / carbon composite substrate. The carbon / carbon composite substrate is then placed in an oven and dried at a temperature of 120-200°C for more than 2 hours. After cooling, it is taken out and ready to be coated with the second coating.

[0072] Both phosphoric acid and magnesium dihydrogen phosphate are water-soluble, and the prepared solution has a low viscosity and is easy to diffuse. Therefore, the solution of phosphoric acid + magnesium dihydrogen phosphate has good wettability with the surface of carbon / carbon materials, which is much better than that of aluminum phosphate. It is easier to diffuse to the entire surface and pores of carbon / carbon materials, resulting in a better coating effect.

[0073] Phosphoric acid dehydrates upon heating, successively forming polyphosphoric acid, pyrophosphoric acid, triphosphoric acid, and polymetaphosphoric acid. Upon heating above 300℃, phosphoric acid gradually loses its molecular water, transforming into polymetaphosphoric acid, as shown in the following reaction formula:

[0074] 3H3PO4 = 3H2O + 3HPO3 (polymetaphosphate)

[0075] Cross-linked polyphosphoric acid after high-temperature treatment is a dense inorganic coating that does not react with oxygen and can completely block oxygen from passing through. Therefore, it has a good anti-oxidation effect at 700℃.

[0076] If the first coating uses only phosphoric acid as the raw material, after being subjected to a higher temperature of 750°C, 3HPO3 (polyphosphoric acid) will be converted into P2O5 (phosphorus pentoxide) in the air, which will reduce the anti-oxidation effect. Therefore, another coating with higher temperature protection, magnesium polyphosphate, will be added.

[0077] Magnesium phosphate (Mg3(PO4)2) is a solid with very low solubility in water and phosphoric acid, making it unsuitable for preparing water-soluble coatings. Magnesium dihydrogen phosphate (Mg(H2PO4)2), a magnesium phosphate, is soluble in water and phosphoric acid, with a composition of 11% MgO and 89% H3PO4. During heating, magnesium dihydrogen phosphate undergoes the following dehydration and conversion reactions:

[0078] Mg(H2PO4)2→2MgH2P2O7→Mg2P4O 12 H₂O → 3Mg₂P₄O 12 (Magnesium polyphosphate)

[0079] Magnesium dihydrogen phosphate loses all its water content and becomes magnesium polyphosphate at temperatures above 600°C.

[0080] Magnesium polyphosphate exhibits excellent thermal stability at high temperatures, with a thermal decomposition temperature exceeding 1000℃. This type of magnesium polyphosphate coating does not react with oxygen and does not allow oxygen molecules to pass through, thus providing good anti-oxidation effects. This ensures the anti-oxidation properties of the magnesium polyphosphate coating at high temperatures.

[0081] S3: Apply silica sol material to the surface of the first coating, dry and cool it to obtain the second coating;

[0082] The silica sol raw material comprises silica sol; by mass percentage, the composition is as follows: silica sol 80-90%, the balance being deionized water.

[0083] Mix the above ingredients thoroughly and set aside.

[0084] The prepared silica sol raw material is brushed onto the dried first coating. Then, the carbon / carbon composite material substrate is placed in an oven and dried at a temperature of 120-180℃ for more than 2 hours. After cooling, the third coating is applied.

[0085] Although the first base coating system of phosphoric acid + magnesium phosphate has a good anti-oxidation effect, its overall temperature resistance is still relatively low. Therefore, a second base layer, silica sol coating, needs to be applied on the first base layer after drying. This base layer has higher temperature resistance.

[0086] Silica sol is a colloidal solution, consisting of nano-sized silica particles dispersed in water or a solvent. Because the SiO2 in silica sol contains a large amount of water and hydroxyl groups, silica sol can also be described as SiO2·nH2O.

[0087] This coating generates SiO2 at high temperatures, which can directly participate in the subsequent reaction of the aluminum oxide and silicon dioxide ceramic layers.

[0088] Although the first and second coatings mentioned above also have good anti-oxidation effects, they are still relatively thin coatings overall and not resistant to erosion. Therefore, in practical applications, a harder and more heat-resistant ceramic coating needs to be added on top of these two base layers.

[0089] S4: Apply the topcoat material to the surface of the second coating, dry and cool it to obtain the third coating;

[0090] The surface layer material comprises aluminum oxide, silicon dioxide, silicon carbide, and zirconium dioxide; by mass percentage, the composition is as follows: 25-35% aluminum oxide, 10-20% silicon dioxide, 10-15% silicon carbide, 5-10% zirconium dioxide, 1-3% dispersant, and the balance being deionized water; the dispersant is an alkoxylated polyol.

[0091] Mix the above ingredients thoroughly and set aside.

[0092] The prepared top layer material is brushed onto the dried second layer. Then, the carbon / carbon composite material substrate is placed in an oven and dried at 100-120°C for more than 1 hour. After cooling, it is taken out for sintering.

[0093] Aluminum oxide, also known as aluminum oxide, is a stable oxide of aluminum with the chemical formula Al₂O₃. In mining, ceramics, and materials science, it is also called bauxite. Aluminum oxide has a melting point of 2050℃ and is extremely resistant to oxidation.

[0094] Silicon dioxide is an inorganic compound, an atomic crystal with the chemical formula SiO2, which represents the ratio of silicon to oxygen atoms in silicon dioxide. It has a melting point of 1723 ℃. The silicon-oxygen covalent bond (Si-O) is one of the strongest chemical bonds found in nature. The Si-O covalent bonds connecting silicon and oxygen atoms in silicon dioxide are very strong, so silicon dioxide usually exhibits many excellent physical properties. At the same time, silicon dioxide also has high chemical stability and is extremely resistant to oxidizing substances.

[0095] The main body of this coating is a mullite-structured ceramic layer formed by the reaction of aluminum oxide and silicon dioxide. This is a ceramic layer with a high melting point and excellent thermal stability, and has good temperature resistance and oxidation resistance.

[0096] At this ratio and sintering temperature, aluminum oxide and silicon dioxide can undergo the following reaction:

[0097] 3Al₂O + 2SiO₂ → 3Al₂O·2SiO₂

[0098] Mullite is the only stable binary compound in the Al₂O₃-SiO₂ system, with a composition that can vary between 3Al₂O₃·2SiO₂ and 2Al₂O₃·SiO₂. 3Al₂O₃·2SiO₂ represents the stoichiometric composition and content of mullite. This mullite-structured ceramic layer can be used in air at temperatures below 1200°C for extended periods without structural changes, making it a stable anti-oxidation coating.

[0099] Zirconium dioxide, with the chemical formula ZrO2, has a melting point of 2700℃. It is a white crystalline solid under normal conditions and is sparingly soluble in water. Chemically inert, it possesses a high melting point, high resistivity, high refractive index, and low coefficient of thermal expansion, making it an important high-temperature resistant material and ceramic insulating material.

[0100] Due to its high melting point, oxygen-reactive properties, and other excellent high-temperature characteristics, ZrO2 has a higher service temperature than other refractory materials such as alumina. Zirconia can be used for extended periods in ultra-high temperature oxidizing atmospheres above 1500 ℃, with a maximum service temperature reaching 2200 ℃, and even maintaining its shape up to 2500 ℃. Furthermore, it exhibits high-temperature chemical stability, corrosion resistance, oxidation resistance, thermal shock resistance, non-volatile nature, and no pollution, making it one of the world's leading refractory materials. In this coating system, zirconium dioxide is added as a reinforcing and toughening material for the Al2O3-SiO2 system, while also improving the coating's temperature resistance.

[0101] Silicon carbide (SiC) is a high-temperature resistant material with a melting point of over 1500 ℃ and good water resistance. It can be in contact with water for a long time without being impermeable, absorbing water, or hydrolyzing. Even after reacting with oxygen in the air at temperatures above 800 ℃ to form molten silica, this molten silica does not react with water and still has anti-oxidation function in aquatic environments. Furthermore, the molten silica can fill micro-cracks in ceramic coatings and reduce oxygen permeability. Therefore, silicon carbide is added as a high-temperature resistant, waterproof, and erosion-resistant coating material.

[0102] The dispersant (alkoxylated polyol) is added to ensure that the various ceramic components are fully and evenly mixed, to prevent component segregation, and to ensure that the predetermined structure is achieved during sintering.

[0103] S5: The coated substrate is heat-treated and then cooled to complete the sintering of the ceramic coating;

[0104] The dried carbon / carbon composite substrate is placed in a heat treatment furnace and sintered at 800-1000℃ for 2 hours under nitrogen protection. Then it is naturally cooled to room temperature to complete the sintering of the ceramic coating.

[0105] The present invention also provides a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake material prepared by the above preparation method. The high-temperature resistant ceramic anti-oxidation coating consists of a base layer and a main layer from bottom to top. The base layer is composed of a bottom layer material and a silica sol material, and the main layer is mainly composed of a top layer material.

[0106] Depend on Figure 2 It is known that in the 750℃ oxidation weight loss test of coated and uncoated samples, the weight loss rate of coated samples is much lower than that of uncoated samples, indicating that coated samples have much better oxidation resistance. It can be considered that the coating has good oxidation resistance, which means that the sintered carbon / carbon composite substrate can more effectively resist oxidation reaction and maintain its quality stability in high temperature or oxidizing environment.

[0107] Depend on Figure 3 It is known that after soaking the coated sample in potassium acetate solution, the oxidation weight loss rate at 900℃ for 6 hours still did not exceed 5%, indicating that it has good antioxidant properties.

[0108] Example 2

[0109] This embodiment illustrates the preparation method of the underlying raw materials of the present invention.

[0110] Raw material composition: 50%–70% phosphoric acid, 5%–10% magnesium dihydrogen phosphate, and 20%–45% deionized water.

[0111] The main component of this base material is phosphoric acid, with magnesium dihydrogen phosphate as a secondary component. When the proportion of magnesium dihydrogen phosphate is too high, the viscosity of the slurry will increase. The addition of deionized water is to better mix phosphoric acid and magnesium dihydrogen phosphate, and also to reduce the viscosity of the slurry. However, if the amount of deionized water exceeds 45%, the protective effect of the coating will decrease.

[0112] In the above raw material ratios, a typical ratio is:

[0113] Phosphoric acid: magnesium dihydrogen phosphate: deionized water = 60%: 8%: 32%.

[0114] Example 3

[0115] This embodiment illustrates the preparation method of the silica sol raw material of the present invention.

[0116] Raw material composition: 80%–90% silica sol, 10%–20% deionized water.

[0117] The main component of this silica sol raw material is silica sol, while the addition of deionized water is to reduce the viscosity of the raw material. When too much deionized water is added, the protective effect of silica sol will decrease, so it should not exceed 20%.

[0118] In the above raw material ratio, a typical ratio is: silica sol: deionized water = 85%: 15%.

[0119] Example 4

[0120] This embodiment illustrates the preparation method of the surface layer material of the present invention.

[0121] Raw material composition: 25%–35% aluminum oxide, 10%–20% silicon dioxide, 10%–15% silicon carbide, 5%–10% zirconium dioxide, 1%–3% dispersant (alkoxylated polyol), and 17%–49% deionized water.

[0122] The third coating layer of this surface material, primarily the main coating, cannot be too thin, and the amounts of aluminum oxide and silicon dioxide cannot be too small, with a total content not less than 30%, otherwise the protective effect will decrease. However, it also cannot be too high, otherwise the material's consistency will increase too much, making it difficult to apply evenly. The amount of zirconium dioxide cannot be too high, otherwise, although the coating's temperature resistance will improve, it will easily become brittle; its total content cannot exceed 10%. Similarly, the amount of silicon carbide cannot be too high; exceeding 15% will increase the coating's brittleness and make it prone to cracking. The addition of deionized water is only to reduce consistency, making the material easier to mix evenly and apply, so its amount cannot be too high; exceeding 50% will significantly reduce the overall protective effect of the coating.

[0123] In the above raw material ratios, a typical ratio is:

[0124] Aluminum oxide: Silicon dioxide: Silicon carbide: Zirconium dioxide: Deionized water: Dispersant = 30%: 15%: 12%: 8%: 33%: 2%.

[0125] The beneficial effects of this invention are as follows: It employs a high-temperature resistant mullite-structured ceramic coating formed by the reaction of alumina and silica as the main coating, while also incorporating phosphoric acid and silica gel coatings, which perform well at low temperatures. To enhance the overall protective effect, the bottom layers utilize phosphoric acid and magnesium dihydrogen phosphate coatings, which offer good low-temperature protection, along with a silica sol coating. This multi-layer approach fully leverages the advantages of each layer; the first two layers are dried without sintering, while the last three coatings are sintered together, achieving a single-stage sintering process. This multi-layer system exhibits higher high-temperature resistance and chemical stability compared to coatings based on glass or aluminum phosphate refractory materials. The coating does not hydrolyze or pulverize at high temperatures, nor does it react with water or de-icing agents (potassium acetate). Therefore, it is a thinner, more heat-resistant, more oxidation-resistant, and more waterproof coating, with a longer service life under alternating high and low temperatures. It utilizes readily available domestic raw materials, such as alumina and silica, which are traditional ceramic raw materials. The raw materials selected in this invention fully consider the requirements of environmental protection and occupational disease prevention. The selected materials have environmental protection characteristics, do not pollute the environment, and will not cause occupational hazards to operators during the construction process.

[0126] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials, characterized in that: Includes the following steps: S1: Substrate surface cleaning, drying and cooling; S2: Apply the base material to the surface of the substrate, dry and cool it to obtain the first coating; S3: Apply silica sol material to the surface of the first coating, dry and cool it to obtain the second coating; S4: Apply the topcoat material to the surface of the second coating, dry and cool it to obtain the third coating; S5: The coated substrate is heat-treated and then cooled to complete the sintering of the ceramic coating; The underlying raw material contains phosphoric acid and magnesium dihydrogen phosphate. The underlying raw material, by mass percentage, has the following composition: 50-70% phosphoric acid, 5-10% magnesium dihydrogen phosphate, and the balance is deionized water. The silica sol raw material includes silica sol; The surface layer material comprises aluminum oxide, silicon dioxide, silicon carbide, and zirconium dioxide. The surface layer material, by mass percentage, comprises: 25-35% aluminum oxide, 10-20% silicon dioxide, 10-15% silicon carbide, 5-10% zirconium dioxide, 1-3% dispersant, and the balance being deionized water; the dispersant is an alkoxylated polyol. The prepared surface material is brushed onto the dried second coating surface. The coated substrate is then placed in an oven and dried at 100-120°C for more than 1 hour. After cooling, the third coating is obtained and then removed for sintering.

2. The method for preparing the high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials according to claim 1, characterized in that: The substrate is cleaned in deionized water, then placed in an oven and dried at 100°C for 2 hours. After cooling to below 40°C, the substrate is removed and the first coating layer is applied.

3. The method for preparing the high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials according to claim 1, characterized in that: The prepared base material is applied to the surface of the substrate. The coated substrate is then placed in an oven and dried at a temperature of 120-200°C for more than 2 hours. After cooling, the first coating is obtained, and the substrate is then removed to apply the second coating.

4. The method for preparing the high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials according to claim 1, characterized in that: The silica sol raw material, by mass percentage, has the following composition: 80-90% silica sol, with the remainder being deionized water.

5. The method for preparing the high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials according to claim 4, characterized in that: The prepared silica sol raw material is brushed onto the dried first coating. The coated substrate is then placed in an oven and dried at a temperature of 120-180°C for more than 2 hours. After cooling, the second coating is obtained, and the substrate is then taken out to be coated with the third coating.

6. The method for preparing the high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials according to claim 1, characterized in that: The dried substrate is placed in a heat treatment furnace and sintered at 800-1000°C for 2 hours under nitrogen protection. Then it is naturally cooled to room temperature to complete the sintering of the ceramic coating.

7. A high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials, characterized in that: It is prepared by the method of any one of claims 1 to 6 for preparing high-temperature resistant ceramic anti-oxidation coating for carbon / carbon aircraft brake materials.

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