A thermal protective metal enamel composite coating with low infrared emissivity and a preparation method thereof

Abstract

The present invention relates to the technical field of functional coatings and composite materials thereof, and specifically to a thermally protective metal enamel composite coating with low infrared emissivity and a preparation method thereof. The composite coating is composed of a high-temperature resistant enamel layer, a platinum-doped enamel layer, and a metal layer, and is prepared by cold spraying and then sintering. The enamel layer has a thickness of 5 to 10 μm, the platinum-doped enamel layer has a thickness of 5 to 10 μm, and the metal layer has a thickness of 10 to 15 μm. The present invention promotes seamless connection between the enamel and the metal layer through high-temperature creep during enamel sintering, thereby reducing internal stress in the coating; the high chemical inertness of the enamel ensures excellent corrosion resistance, high-temperature resistance, and oxidation resistance of the coating, while the incorporation of platinum improves the toughness of the enamel; the metal layer has low infrared emissivity and high-temperature stability. The present invention enables the coating to have the characteristics of high-temperature corrosion resistance, oxidation resistance, thermal shock resistance, high bonding strength, and low infrared emissivity, and can be used to reduce the surface emissivity of various high-temperature alloy parts and provide thermal protection.

Description

Technical Field

[0001] The present invention relates to the technical field of functional coatings and composite materials thereof, and in particular to a thermal protective metal enamel composite coating with low infrared emissivity and a preparation method thereof. The coating can be used to reduce the surface emissivity of various high-temperature alloy parts and provide a thermal protection effect. Background Art

[0002] During high-speed flight, the aerodynamic heat released by fuel combustion and the intense friction between the aircraft's surfaces and the atmosphere can cause certain components, such as the nozzle and fairing cone, to be subjected to high temperatures (up to 900°C). The infrared radiation emitted by these thermal components in the 3-5μm and 8-14μm bands will be significantly higher than the surrounding environment, attracting attention from infrared detectors. Therefore, reducing the infrared radiation emitted by thermal components is a top priority for improving aircraft survivability.

[0003] According to the Stephen-Boltzmann law, infrared radiation intensity is proportional to the material's infrared emissivity and surface temperature. Therefore, reducing an aircraft's surface temperature and infrared emissivity are two effective ways to reduce infrared radiation. However, the surface temperature of hot components is primarily contributed by the heat released by fuel combustion. Therefore, directly reducing surface temperature may compromise aircraft power. Clearly, coating hot components with low-infrared emissivity materials is a more effective and convenient method for reducing infrared radiation.

[0004] Based on the characteristics of infrared low-emissivity materials, infrared low-emissivity coatings with potential for medium and high-temperature applications are roughly divided into three categories: metal micropowder coatings, metal films, and inorganic low-emissivity coatings. However, there are still many problems with low-emissivity coatings in my country: (1) The temperature resistance level of the coating material is not high enough, and a new infrared low-emissivity coating system is needed. (2) The high-temperature service life is short and the stability is poor, mainly manifested in: the coating is prone to cracking and peeling under harsh conditions such as high and low temperature thermal shock, high-temperature salt corrosion, and gas thermal corrosion. (3) The process is complex: it has high requirements for production equipment and environment. (4) The overall emissivity has not reached the level of actual application, etc. Summary of the Invention

[0005] The purpose of the present invention is to provide a thermal protective metal enamel composite coating with low infrared emissivity and a preparation method thereof, so that the coating has the characteristics of high temperature corrosion resistance, oxidation resistance, thermal shock resistance, high bonding strength and low infrared emissivity.

[0006] The technical solution of the present invention is:

[0007] A thermal protective metal enamel composite coating with low infrared emissivity, the composite coating comprising, from the inside out, an enamel layer, a platinum-doped enamel layer, and a metal layer, wherein the metal layer is a pure platinum layer or a platinum-silver alloy layer, the enamel layer is 5 to 10 μm thick, the platinum-doped enamel layer is 5 to 10 μm thick, and the metal layer is 10 to 15 μm thick; the components of each layer, calculated by weight percentage, are as follows:

[0008] Enamel layer: silicon dioxide 50-65%, zirconium dioxide 6-11%, aluminum oxide 3-10%, boron trioxide 2-6%, calcium oxide 3-7%, strontium oxide 7-13%, potassium oxide 2-6%, the total content of calcium oxide, strontium oxide and potassium oxide is 16-22%;

[0009] Platinum-doped enamel layer: enamel 80-90%, platinum 10-20%; wherein the enamel composition is the same as that of the enamel layer: silicon dioxide 50-65%, zirconium dioxide 6-11%, aluminum oxide 3-10%, boron trioxide 2-6%, calcium oxide 3-7%, strontium oxide 7-13%, potassium oxide 2-6%, and the total content of calcium oxide, strontium oxide and potassium oxide is 16-22%;

[0010] Metal layer: silver 0-50%, platinum 50-100%.

[0011] The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity comprises the following steps: 1) melting enamel frit; 2) preparing enamel micropowder; 3) preparing a composite enamel suspension; 4) spraying the composite enamel suspension; 5) preparing a platinum-doped composite enamel suspension; 6) spraying the platinum-doped composite enamel suspension; 7) preparing a metal powder suspension; 8) spraying the metal powder suspension; and 9) firing the composite coating and polishing it after cooling.

[0012] The method for preparing the thermally protective metal enamel composite coating with low infrared emissivity, steps 1) to 4) of preparing the enamel layer, specifically:

[0013] Step 1): Various oxides are ball-milled and mixed according to the enamel layer formula at a speed of 300-350 rpm for 20-30 hours. After uniform mixing, they are heated and smelted. The smelting process is as follows: RT → 300-600°C, uniform heating for 0.5-1.5 hours; 300-600°C → 800-1100°C, uniform heating for 25-35 minutes; 800-1100°C → 1300-1550°C, uniform heating for 0.5-1.5 hours; after maintaining a constant temperature of 1300-1550°C for 1-2 hours, water quenching is performed to obtain enamel glaze particles.

[0014] Step 2): ball milling the enamel glaze particles obtained after water quenching to obtain enamel micropowder with a particle size of ≤5 μm;

[0015] Step 3): Using anhydrous ethanol or purified water as a dispersant, 10 to 30 ml of anhydrous ethanol or purified water is mixed with 1 gram of enamel micropowder, and the mixture is subjected to magnetic stirring and ultrasonic oscillation for 15 to 30 minutes to obtain a uniformly dispersed enamel suspension;

[0016] Step 4): Use an air compressor to evenly spray the enamel suspension onto the surface of the high-temperature alloy component at a spraying pressure of 0.2 to 0.4 MPa and a spraying distance of 15 to 40 cm.

[0017] The preparation method of the heat-protective metal enamel composite coating with low infrared emissivity comprises the following steps: spraying and then drying in an oven at 200-300° C. for 20-30 minutes to obtain an initial enamel layer.

[0018] The method for preparing the thermally protective metal enamel composite coating with low infrared emissivity, steps 5) to 6) of preparing the platinum-doped enamel layer, specifically:

[0019] Step 5): preparing a platinum-doped composite enamel powder from enamel micropowder and platinum powder, using anhydrous ethanol or purified water as a dispersant, mixing the mixture in an amount of 10 to 30 ml of anhydrous ethanol or purified water per gram of the platinum-doped composite enamel powder, and subjecting the mixture to magnetic stirring and ultrasonic oscillation for 15 to 30 minutes to obtain a uniformly dispersed platinum-doped composite enamel suspension;

[0020] Step 6): Use an air compressor to evenly spray the platinum-doped composite enamel suspension onto the enamel layer on the surface of the high-temperature alloy component. The spraying pressure is 0.2 to 0.4 MPa and the spraying distance is 15 to 40 cm.

[0021] The method for preparing the heat-protective metal enamel composite coating with low infrared emissivity comprises the following steps: spraying the coating and then drying it in an oven at 200-300° C. for 20-30 minutes to obtain a platinum-doped enamel layer.

[0022] The method for preparing the thermal protective metal enamel composite coating with low infrared emissivity, steps 7) to 8) of preparing the metal layer, specifically:

[0023] Step 7): silver powder and platinum powder are prepared in a certain proportion or platinum powder is directly used as the metal powder. The original powder particle size of the silver powder and platinum powder is ≤10 μm. Anhydrous ethanol or purified water is used as a dispersant. 30-35 ml of anhydrous ethanol or purified water is required for 1 gram of metal powder. The mixture is magnetically stirred and ultrasonically vibrated for 3-5 minutes to obtain a uniformly dispersed metal powder suspension.

[0024] Step 8): Use an air compressor to evenly spray the metal powder suspension onto the platinum-doped enamel layer on the surface of the high-temperature alloy component. The spraying pressure is 0.2 to 0.4 MPa and the spraying distance is 15 to 40 cm.

[0025] The preparation method of the heat-protective metal enamel composite coating with low infrared emissivity comprises the following steps: spraying and then drying in an oven at 200-300° C. for 20-30 minutes to obtain a metal layer.

[0026] The preparation method of the thermal protective metal enamel composite coating with low infrared emissivity, step 9) is specifically as follows: after spraying and drying in sequence, the original blank of the composite coating of enamel layer + platinum-doped enamel layer + metal layer is obtained, and then sintering treatment is carried out. There are two sintering schemes: (1) the composite coating original blank after drying is sintered at a high temperature of 1000-1100 ° C for 10-20 minutes, then taken out and cooled in the atmosphere; after cooling, it is first polished with 2000 mesh sandpaper and then with 1 micron diamond grinding paste to the coating surface. The surface roughness is Ra≤2μm, and a thermal protective metal enamel composite coating is obtained; (2) the original blank of the composite coating after drying is kept at 800-950℃ for 1-3h, then the temperature is raised to 1000-1100℃ in the furnace and sintered at high temperature for 10-20min, and finally the temperature is lowered to 600-800℃ in the furnace, taken out and cooled in the atmosphere; after cooling, it is first polished with 2000 mesh sandpaper and then with 1 micron diamond grinding paste until the surface roughness of the coating is Ra≤2μm, and a thermal protective metal enamel composite coating is obtained.

[0027] The method for preparing the thermally protective metal enamel composite coating with low infrared emissivity has the following performance indicators:

[0028] Thickness ≤ 25μm, surface roughness: Ra ≤ 2μm, bonding strength ≥ 15MPa, no cracks after 1000 times of air cooling at 900℃, insulation temperature ≥ 100℃, seawater immersion corrosion resistance life ≥ 150h, salt spray corrosion resistance life ≥ 1000h, gas thermal corrosion rate ≤ 0.5g / m 2 h, 900℃ fully anti-oxidation grade; in the 3-5μm and 8-14μm bands, the infrared emissivity at 50℃ and 900℃ is less than 0.15.

[0029] The design idea of the present invention is:

[0030] The composite coating designed in the present invention is mainly sprayed on a high-temperature alloy substrate. The composite coating consists of three layers from the inside out, namely a high-temperature resistant enamel layer, a platinum-doped enamel layer and a metal layer (platinum or platinum-silver alloy), which are prepared by cold spraying and sintering in sequence. Based on the good interface bonding between the enamel and the high-temperature alloy and the chemical inertness in the range from room temperature to 900°C, the coating is guaranteed to have excellent interface bonding, high-temperature oxidation resistance and corrosion resistance. In addition, the high-temperature creep during sintering of the enamel fully ensures the seamless connection between the enamel and the metal layer, reducing the internal stress of the coating. The enamel layer and the platinum-doped enamel layer ensure the density and smooth surface of the outermost metal layer. At the same time, the enamel coating incorporating high-toughness heat-resistant metal platinum particles greatly improves the toughness. The high-temperature oxidation resistance of the platinum or platinum-silver alloy itself enables the coating to have both low emissivity and high-temperature stability.

[0031] The composition of the enamel layer designed by the present invention strictly requires the following: calcium oxide 3-7%, strontium oxide 7-13%, potassium oxide 2-6%, and the total content of calcium oxide, strontium oxide and potassium oxide is controlled to 16-22% to ensure interfacial chemical bonding between the enamel layer and the alloy substrate, high-temperature structural stability and excellent antioxidant effect. In addition, the enamel composition ensures high-temperature fluidity of the enamel and its compatibility with platinum particles, ensuring good matching with the high-temperature alloy substrate and the surface alloy layer.

[0032] The advantages and beneficial effects of the present invention are:

[0033] (1) The enamel layer of the present invention has a high content of network former components and good high-temperature stability. In addition, highly stable strontium oxide is used to replace highly active solvent components such as zinc oxide and sodium oxide in traditional enamels to increase the firing temperature of the enamel coating to the sintering temperature of the metal layer, while suppressing excessive interface reaction between the coating and the base alloy, thereby obtaining a controllable interface structure between the coating and the alloy.

[0034] (2) The platinum-doped enamel layer of the present invention improves the toughness of the enamel, reduces the sintering stress of the enamel, and improves the thermal shock peeling resistance by adding heat-resistant metal platinum particles.

[0035] (3) The metal layer of the present invention has low infrared emissivity and high-temperature stability. It has a good mechanical fit with the platinum-doped enamel layer by spraying. The smooth surface of platinum or platinum-silver alloy can reduce the emissivity of the entire component to below 0.15. The high-temperature stability of platinum or platinum-silver alloy enables it to serve for a long time at 900°C, which can significantly improve the service life of high-temperature components while reducing the emissivity.

[0036] (4) The thermal protective metal enamel composite coating with low infrared emissivity of the present invention does not require any expensive equipment and has a simple preparation process and can be sprayed on the surface of high-temperature alloy parts of various shapes. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Macroscopic morphology of thermal protective metal enamel composite coating with low infrared emissivity prepared on high-temperature alloy GH536 substrate.

[0038] Figure 2 The average value of the kinetic curves of the oxidation weight gain of three parallel groups of low infrared emissivity thermal protective metal enamel composite coating samples and GH536 alloy substrate samples at 900℃. In the figure, the horizontal axis Time is time (h), and the vertical axis Weight gain is weight gain (mg·cm -2 ); Bare alloy refers to the GH536 alloy substrate sample, and Composite coating refers to the thermal protective metal enamel composite coating sample.

[0039] Figure 3 This is the SEM image of the microstructure of the thermal protective metal enamel composite coating with low infrared emissivity prepared on the high-temperature alloy GH536 substrate.

[0040] Figure 4 This is the SEM image of the enamel layer morphology of the cross section of the thermal protective metal enamel composite coating with low infrared emissivity after tensile test after being kept at 900℃ for 500 hours.

[0041] Figure 5 This is the SEM image of the metal layer morphology of the cross section of the thermal protective metal enamel composite coating with low infrared emissivity after tensile test after being kept at 900℃ for 500 hours. DETAILED DESCRIPTION

[0042] The following examples are further detailed descriptions of the present invention. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0043] Example 1:

[0044] In this embodiment, the spraying object is a GH536 high-temperature alloy sample, and the thermal protective metal enamel composite coating with low infrared emissivity is prepared by the following process:

[0045] (1) Enamel glaze melting:

[0046] Calculated by weight percentage, the formula of enamel glaze is: 64% silicon dioxide, 9% zirconium dioxide, 3% aluminum oxide, 6% boron trioxide, 4% calcium oxide, 10% strontium oxide, and 4% potassium oxide.

[0047] The above oxides were ball milled and mixed at a speed of 320 rpm for 24 hours. After mixing evenly, they were heated and smelted. The smelting process was as follows:

[0048] Raise the temperature from RT (room temperature) to 500°C and heat at a constant rate for 1 hour;

[0049] Heat from 500°C to 1000°C at a constant speed for 30 minutes;

[0050] Heat from 1000℃ to 1425℃, heating at a constant speed for 1 hour;

[0051] The mixture was kept at a constant temperature of 1425°C for 1.5 hours and then quenched in water to obtain enamel glaze particles.

[0052] (2) Preparation of enamel micropowder: The enamel glaze particles obtained after water quenching were subjected to planetary ball milling (speed 320 rpm, time 100 hours) to prepare enamel micropowder with a particle size of less than 5 μm.

[0053] (3) Preparation of enamel suspension: A composite enamel suspension was prepared according to the ratio of 1 g enamel powder to 16 ml anhydrous alcohol, and magnetic stirring and ultrasonic oscillation were performed for 20 min to obtain a uniformly dispersed composite enamel suspension.

[0054] (4) Enamel layer spraying: The composite enamel suspension was sprayed onto the GH536 high-temperature alloy sample at an atmospheric pressure of 0.3 MPa using an air compressor. The spraying distance was 20 cm. The initial enamel layer was obtained after drying in an oven at 250 °C for 25 min. The enamel layer thickness was 6 μm.

[0055] (5) Preparation of platinum-doped composite enamel suspension:

[0056] Calculated by weight percentage, the enamel glaze is 80% and the platinum is 20% (the composition of the enamel glaze in the platinum-doped enamel layer is the same as that of the above-mentioned enamel layer).

[0057] Enamel micropowder and platinum powder are used to prepare platinum-doped composite enamel powder. A platinum-doped composite enamel suspension is prepared according to the ratio of 1 g of platinum-doped composite enamel powder to 16 ml of anhydrous alcohol. The suspension is subjected to magnetic stirring and ultrasonic oscillation for 20 minutes to obtain a uniformly dispersed platinum-doped composite enamel suspension.

[0058] (6) Spraying of platinum-doped composite enamel suspension: The platinum-doped composite enamel suspension was sprayed onto the sample at an atmospheric pressure of 0.3 MPa using an air compressor. The spraying distance was 20 cm. The sample was dried in an oven at 250°C for 25 min to obtain a platinum-doped enamel layer with a thickness of 8 μm.

[0059] (7) Preparation of metal powder suspension: The metal powder was prepared in a weight ratio of silver powder to platinum powder of 1:1, and anhydrous ethanol was used as a dispersant. 32 ml of anhydrous ethanol was required for 1 g of metal powder. After magnetic stirring and ultrasonic oscillation for 4 minutes, a uniformly dispersed metal powder suspension was obtained.

[0060] (8) Spraying of metal powder suspension: The metal powder suspension was sprayed onto the sample at an atmospheric pressure of 0.3 MPa using an air compressor. The spraying distance was 20 cm. The metal layer was obtained after drying in an oven at 250°C for 25 min. The thickness of the metal layer was 10 μm.

[0061] (9) Firing the composite coating and polishing after cooling: After spraying, the coating was dried at 250°C for 30 minutes to obtain a composite coating blank. The dried composite coating blank was sintered at 1050°C for 15 minutes, then removed and cooled to room temperature in the atmosphere. After cooling, it was first polished with 2000 grit sandpaper and then with 1 micron diamond paste to a coating surface roughness of Ra = 2 μm.

[0062] The macroscopic morphology of the prepared coating is as follows Figure 1 As shown, the coating is dense and has no surface defects such as bubbles and cracks. The coating sample prepared in this way was oxidized at 900℃ for 500h, and the oxidation rate was 0.007g / m 2 h(less than 0.1g / m 2 h), reaching the complete oxidation resistance level. The kinetic curves of the average weight gain of three parallel groups of composite coating samples and GH536 alloy at 900℃ are shown in Figure 2. Figure 2 As shown. The coating emissivity was measured at 50°C to be 0.0937 (3-5μm band) and 0.0564 (8-14μm band); at 950°C, the coating emissivity was 0.0752 (3-5μm band) and 0.0742 (8-14μm band). The shrinkage porosity was measured to be ≤1.33%. The initial sample had a tensile bond strength of 23.0 MPa, and after holding at 900°C for 500 hours, the tensile bond strength was 18.2 MPa.

[0063] Comparative Example 1

[0064] The difference from Example 1 is that in step 7, when preparing the metal powder suspension, the weight ratio of silver powder to platinum powder is 17:3.

[0065] After sintering, the surface of the composite coating is vitrified and the bottom of the glass is porous.

[0066] Comparative Example 2

[0067] The difference from Example 1 is that steps 5 and 6 are omitted.

[0068] After sintering and polishing, the composite coating surface is smooth and dense. The cross-sectional micromorphology of the coating is as follows: Figure 3 The measured roughness is Ra=1.8μm, and the coating emissivity at 50°C is 0.0966 (3-5μm band) and 0.0646 (8-14μm band).

[0069] Comparative Example 3

[0070] The difference from Example 1 is that: after spraying the enamel layer in step 4, the blank is placed at a high temperature of 1050°C for 15 minutes, and after cooling, the coating surface is sandblasted (pressure 0.1 MPa) and washed; after spraying the platinum-doped enamel layer in step 6, the blank is placed at a high temperature of 1050°C for 15 minutes, and after cooling, the coating surface is sandblasted (pressure 0.1 MPa) and washed.

[0071] The rest of the steps in Example 1 were followed, and the composite coating was placed at 900° C. for 500 hours. After air cooling, the metal layer cracked and fell off.

[0072] Comparative Example 4

[0073] The difference from Example 1 is that in step 9, the surface is polished to a roughness of Ra=7 μm.

[0074] The emissivity of the prepared coating at 50°C is 0.1430 (3-5 μm band) and 0.0877 (8-14 μm band).

[0075] Example 2

[0076] In this embodiment, the spraying object is changed to high-temperature alloy IN738, and the thermal protective metal enamel composite coating with low infrared emissivity is prepared. The process is the same as that of Example 1.

[0077] The prepared coating is dense, with no appearance defects such as bubbles and cracks on the surface. The coating sample prepared in this way reaches a complete oxidation resistance level after high-temperature oxidation at 900°C for 500 hours.

[0078] Example 3

[0079] In this embodiment, the spraying object is changed to high-temperature alloy K444 to prepare the thermal protective metal enamel composite coating with low infrared emissivity, and the process is the same as that of Example 1.

[0080] The prepared coating is dense, with no appearance defects such as bubbles and cracks on the surface. The coating sample prepared in this way reaches a complete oxidation resistance level after high-temperature oxidation at 900°C for 500 hours.

[0081] Example 4

[0082] In this embodiment, the spraying object is changed to high-temperature alloy K438, and the thermal protective metal enamel composite coating with low infrared emissivity is prepared. The process is the same as that of Example 1.

[0083] The prepared coating is dense, with no appearance defects such as bubbles and cracks on the surface. The coating sample prepared in this way reaches a complete oxidation resistance level after high-temperature oxidation at 900°C for 500 hours.

[0084] Example 5

[0085] In this embodiment, step 9 in Example 1 was modified to: After spraying the coating, the composite coating blank was dried at 250°C for 30 minutes to obtain a composite coating blank. The baked composite coating blank was then held at 800°C for 2 hours, then heated from 800°C to 1050°C over 15 minutes and sintered for 15 minutes. Finally, the blank was cooled to 600°C in the furnace and then removed from the furnace and cooled to room temperature in the atmosphere. After cooling, the blank was polished with 2000-grit sandpaper and then a 1-micron diamond paste to a coating surface roughness of Ra = 2 μm.

[0086] The composite coating after preparation is dense, and there are no appearance defects such as bubbles and cracks on the surface. The coating sample prepared in this way was kept at 900℃ for 500h and the tensile bonding strength was measured to be 19.4MPa. The micromorphology of the cross section of the sample after stretching is as follows: Figure 4 、 Figure 5 shown.

[0087] Comparative Example 5

[0088] The difference from Example 5 is that the diameter of the platinum powder particles used in the process is 50-100 μm.

[0089] The prepared composite coating is dense, with no appearance defects such as bubbles and cracks on the surface. The platinum-silver alloy phase on the surface of the coating sample prepared in this way is unevenly distributed.

[0090] Comparative Example 6

[0091] The difference from Example 5 is that the strontium oxide in the enamel component is replaced by an equal weight of sodium oxide.

[0092] The surface of the prepared composite coating is uneven, with local defects such as bubbles and cracks.

[0093] Comparative Example 7

[0094] The difference from Example 5 is that the strontium oxide in the enamel component is replaced by zinc oxide of equal weight.

[0095] The surface of the prepared composite coating is uneven, with local defects such as bubbles and cracks.

[0096] The results of the examples and comparative examples show that:

[0097] (1) The present invention is applicable to a variety of high-temperature alloy substrates;

[0098] (2) Enamel has excellent interfacial bonding with high-temperature alloys, and its chemical inertness from room temperature to 900°C gives the coating excellent resistance to high-temperature oxidation and corrosion. The high-temperature creep during sintering of the enamel fully ensures the seamless connection between the enamel and the metal layer, reduces the internal stress of the coating, and also ensures the density and smooth surface of the outermost metal layer.

[0099] (3) The addition of high-toughness heat-resistant metal particles into the enamel improves the toughness of the coating.

[0100] (4) The uniform distribution of platinum or platinum-silver alloy phase gives the coating both low emissivity and high-temperature stability.

[0101] Applicants declare that while the detailed composition and preparation methods of the present invention have been shown and described, it will be apparent to those skilled in the art that the present invention is not limited to the aforementioned detailed composition and preparation methods. Any improvements to the present invention, substitutions of raw materials for the product, and addition of auxiliary ingredients are within the scope of protection and disclosure of the present invention.

Claims

1. A thermally protective metal enamel composite coating with low infrared emissivity, characterized in that: The composite coating comprises, from the inside out, an enamel layer, a platinum-doped enamel layer, and a metal layer. The metal layer is a pure platinum layer or a platinum-silver alloy layer. The enamel layer has a thickness of 5 to 10 μm, the platinum-doped enamel layer has a thickness of 5 to 10 μm, and the metal layer has a thickness of 10 to 15 μm. The composition of each layer, calculated by weight percentage, is as follows: Enamel layer: silicon dioxide 50-65%, zirconium dioxide 6-11%, aluminum oxide 3-10%, boron trioxide 2-6%, calcium oxide 3-7%, strontium oxide 7-13%, potassium oxide 2-6%, the total content of calcium oxide, strontium oxide and potassium oxide is 16-22%; Platinum-doped enamel layer: enamel 80-90%, platinum 10-20%; wherein the enamel composition is the same as that of the enamel layer: silicon dioxide 50-65%, zirconium dioxide 6-11%, aluminum oxide 3-10%, boron trioxide 2-6%, calcium oxide 3-7%, strontium oxide 7-13%, potassium oxide 2-6%, and the total content of calcium oxide, strontium oxide and potassium oxide is 16-22%; Metal layer: silver 0-50%, platinum 50-100%; The composite coating consists of three layers from the inside out: a high-temperature resistant enamel layer, a platinum-doped enamel layer and a metal layer, which are prepared by cold spraying and then sintering. The enamel creeps at high temperature during sintering, fully ensuring the seamless connection between the enamel and the metal layer, reducing the internal stress of the coating; the enamel layer and the platinum-doped enamel layer ensure the density and smooth surface of the outermost metal layer, and the enamel coating with high-toughness heat-resistant metal platinum particles improves the toughness; the metal layer has low infrared emissivity and high-temperature stability, and is mechanically interlocked with the platinum-doped enamel layer by spraying. The smooth surface of platinum or platinum-silver alloy reduces the emissivity of the entire component to below 0.15, and the high-temperature stability of platinum or platinum-silver alloy enables it to serve for a long time at 900°C, thereby reducing the emissivity while increasing the service life of high-temperature components.

2. A method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 1, characterized in that: The method comprises the following steps: 1) melting enamel glaze; 2) preparing enamel micropowder; 3) preparing composite enamel suspension; 4) spraying the composite enamel suspension; 5) preparing a platinum-doped composite enamel suspension; 6) spraying the platinum-doped composite enamel suspension; 7) preparing a metal powder suspension; 8) spraying the metal powder suspension; 9) firing the composite coating and polishing it after cooling.

3. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 2, characterized in that: Steps 1) to 4) are to prepare the enamel layer, specifically: Step 1): Various oxides are ball-milled and mixed according to the enamel layer formula at a speed of 300-350 rpm for 20-30 hours. After uniform mixing, they are heated and smelted. The smelting process is as follows: RT → 300-600°C, uniform heating for 0.5-1.5 hours; 300-600°C → 800-1100°C, uniform heating for 25-35 minutes; 800-1100°C → 1300-1550°C, uniform heating for 0.5-1.5 hours; after maintaining a constant temperature of 1300-1550°C for 1-2 hours, water quenching is performed to obtain enamel glaze particles. Step 2): ball milling the enamel glaze particles obtained after water quenching to obtain enamel micropowder with a particle size of ≤5 μm; Step 3): Using anhydrous ethanol or purified water as a dispersant, 10 to 30 ml of anhydrous ethanol or purified water is mixed with 1 gram of enamel micropowder, and the mixture is subjected to magnetic stirring and ultrasonic oscillation for 15 to 30 minutes to obtain a uniformly dispersed enamel suspension; Step 4): Use an air compressor to evenly spray the enamel suspension onto the surface of the high-temperature alloy component at a spraying pressure of 0.2 to 0.4 MPa and a spraying distance of 15 to 40 cm.

4. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 3, characterized in that: After spraying, the initial enamel layer is obtained by drying in an oven at 200-300°C for 20-30 minutes.

5. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 2 or 4, characterized in that: Steps 5) to 6) are to prepare a platinum-doped enamel layer, specifically: Step 5): preparing a platinum-doped composite enamel powder from enamel micropowder and platinum powder, using anhydrous ethanol or purified water as a dispersant, mixing the mixture in an amount of 10 to 30 ml of anhydrous ethanol or purified water per gram of the platinum-doped composite enamel powder, and subjecting the mixture to magnetic stirring and ultrasonic oscillation for 15 to 30 minutes to obtain a uniformly dispersed platinum-doped composite enamel suspension; Step 6): Use an air compressor to evenly spray the platinum-doped composite enamel suspension onto the enamel layer on the surface of the high-temperature alloy component. The spraying pressure is 0.2 to 0.4 MPa and the spraying distance is 15 to 40 cm.

6. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 5, characterized in that: After spraying, the platinum-doped enamel layer is obtained by drying in an oven at 200-300°C for 20-30 minutes.

7. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 2 or 6, characterized in that: Steps 7) to 8) are to prepare the metal layer, specifically: Step 7): silver powder and platinum powder are prepared in a certain proportion or platinum powder is directly used as the metal powder. The original powder particle size of the silver powder and platinum powder is ≤10 μm. Anhydrous ethanol or purified water is used as a dispersant. 30-35 ml of anhydrous ethanol or purified water is required for 1 gram of metal powder. The mixture is magnetically stirred and ultrasonically vibrated for 3-5 minutes to obtain a uniformly dispersed metal powder suspension. Step 8): Use an air compressor to evenly spray the metal powder suspension onto the platinum-doped enamel layer on the surface of the high-temperature alloy component. The spraying pressure is 0.2 to 0.4 MPa and the spraying distance is 15 to 40 cm.

8. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 7, characterized in that: After spraying, the metal layer is obtained by drying in an oven at 200-300°C for 20-30 minutes.

9. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 2 or 8, characterized in that: Step 9) is specifically as follows: after spraying and drying in sequence, an original blank of the composite coating of enamel layer + platinum-doped enamel layer + metal layer is obtained, and then sintering treatment is carried out. There are two sintering schemes: (1) the composite coating original blank after drying is sintered at a high temperature of 1000-1100°C for 10-20 minutes, then taken out and cooled in the atmosphere; after cooling, it is first polished with 2000 mesh sandpaper and then with 1 micron diamond grinding paste until the coating surface roughness is Ra≤2μm, thereby obtaining a thermal protective metal enamel composite coating; (2) The dried composite coating raw blank is first kept at 800-950°C for 1-3 hours, then heated to 1000-1100°C in a furnace and sintered at high temperature for 10-20 minutes, and finally cooled to 600-800°C in the furnace, taken out and cooled in the atmosphere; after cooling, it is first polished with 2000-grit sandpaper and then with 1-micron diamond grinding paste until the coating surface roughness is Ra ≤ 2μm, thereby obtaining a thermal protective metal enamel composite coating.

10. The method for preparing a thermally protective metal enamel composite coating with low infrared emissivity according to claim 9, characterized in that: The performance indicators of the composite coating are as follows: Thickness ≤ 25μm, surface roughness: Ra ≤ 2μm, bonding strength ≥ 15MPa, no cracks after 1000 times of air cooling at 900℃, insulation temperature ≥ 100℃, seawater immersion corrosion resistance life ≥ 150h, salt spray corrosion resistance life ≥ 1000h, gas thermal corrosion rate ≤ 0.5g / m 2 h; In the 3-5μm and 8-14μm bands, the infrared emissivity at 50℃ and 900℃ is less than 0.15.

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