Thermocouple with an oxidation-resistant corrosion-resistant coating and method of making same

Abstract

The application provides a thermocouple with an oxidation-resistant and corrosion-resistant coating and a preparation method thereof, and the preparation method comprises the following steps: S1, the thermocouple is ultrasonically cleaned in a solvent, is cleaned in deionized water, and is taken out and dried to obtain a pretreated thermocouple; S2, the surface of the pretreated thermocouple is subjected to sand blasting roughening treatment to obtain a roughened thermocouple; S3, a metal transition layer material is sprayed on the surface of the roughened thermocouple by using a plasma spraying process to obtain a thermocouple A; S4, a gradient transition layer metal phase material and a gradient transition layer ceramic phase material are sprayed on the surface of the thermocouple A by using a plasma spraying process to obtain a thermocouple B; and S5, a ceramic functional layer material is sprayed on the surface of the thermocouple B by using a plasma spraying process to obtain the thermocouple with the oxidation-resistant and corrosion-resistant coating. The prepared thermocouple has good oxidation resistance, corrosion resistance, thermoelectric performance and a long service life.

Description

Technical Field

[0001] This invention relates to a thermocouple, and more particularly to a thermocouple with an antioxidant and corrosion resistant coating and its preparation method. Background Technology

[0002] A thermocouple is a commonly used temperature-sensing element in temperature measuring instruments. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium by an electrical instrument. Its basic structure consists of thermocouples, insulating sheaths, protective tubes, and a junction box. It is often used in conjunction with display instruments, recording instruments, and electronic controllers, and is widely applied in industrial production and scientific experiments. Under harsh conditions such as high temperature, corrosion, and oxidation (e.g., in aerospace engines and petrochemical heating furnaces), the surface of a thermocouple is highly susceptible to oxidation, corrosion, and grain growth. • Oxidation failure: Under long-term high temperature, the oxidation rate of traditional thermocouple matrix materials is fast, which leads to thermoelectric potential drift and increased measurement error.

[0003] • Corrosion and peeling: In strong acid, strong alkali or molten salt environments, ordinary coatings are difficult to resist the penetration of corrosive media and are prone to peeling.

[0004] • Short lifespan: Due to the reasons mentioned above, existing thermocouples have a short lifespan (usually about 40 hours) under high temperature and high pressure conditions, and the replacement cost is high.

[0005] In summary, existing thermocouples suffer from problems such as poor oxidation resistance, high-temperature thermal corrosion, and unstable thermoelectric performance under ultra-high temperature and high pressure composite environments. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing a thermocouple with an antioxidant and corrosion resistant coating. The thermocouple prepared by this method has good antioxidant properties, corrosion resistance, thermoelectric properties and long service life.

[0007] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: A method for preparing a thermocouple with an antioxidant and corrosion-resistant coating includes the following steps: S1. Add the thermocouple to the solvent for ultrasonic cleaning, then transfer it to deionized water for cleaning, and take it out and air dry to obtain the pretreated thermocouple; S2. The surface of the pretreated thermocouple obtained in step S1 is roughened by sandblasting to obtain a roughened thermocouple; S3. Place the spherical hafnium powder in a vacuum drying oven to dry it to obtain a metal transition layer material. Use a plasma spraying process to spray the metal transition layer material onto the surface of the roughened thermocouple obtained in step S2 to form a metal transition layer and obtain thermocouple A. S4. Place pure hafnium powder and hafnium dioxide powder in a vacuum drying oven to dry them to obtain gradient transition layer metal phase material and gradient transition layer ceramic phase material respectively. Load the gradient transition layer metal phase material and gradient transition layer ceramic phase material into powder feeder A and powder feeder B of a plasma spraying equipment respectively. Then, use plasma spraying process to spray the surface of thermocouple A obtained in step S3 to form a gradient transition layer to obtain thermocouple B. S5. After mixing hafnium dioxide powder and alumina powder evenly, dry them in a vacuum drying oven to obtain a ceramic functional layer material. Then, use a plasma spraying process to spray the ceramic functional layer material onto the surface of thermocouple B obtained in step S4 to form a ceramic functional layer, thereby obtaining a thermocouple with an anti-oxidation and corrosion-resistant coating.

[0008] Further, in step S1 of this invention, the thermocouple is an N-grade armored thermocouple, and the solvent is acetone or anhydrous ethanol; the ultrasonic cleaning power is 200-300W, the ultrasonic cleaning time is 5-15 minutes, and the deionized water cleaning time is 5-15 minutes; the surface cleanliness of the pretreated thermocouple reaches Sa 3.0 level. Step S1 can remove oil, dust, and oxides from the surface of the thermocouple.

[0009] Furthermore, in step S2 of this invention, the sandblasting roughening treatment uses glass beads with a mesh size of 60-100, a sandblasting pressure of 0.4-0.7 MPa, a sandblasting distance of 100-300 mm, and a sandblasting angle of 45-75°. After sandblasting roughening, the coating process must be completed within 2 hours to prevent further surface oxidation.

[0010] Further, in step S3 of this invention, the particle size of the spherical hafnium powder is 1-5 μm, the drying temperature is 100-120℃, and the drying time is 2 hours; the plasma spraying process uses argon gas with a flow rate of 40-50 L / min as the main gas and hydrogen gas with a flow rate of 8-12 L / min as the auxiliary gas, the spraying current is 500-600 A, the spraying voltage is 60-70 V, the spraying distance is 80-120 mm, and the thickness of the metal transition layer is 10-20 μm. The drying process in step S3 can remove the moisture adsorbed by the spherical hafnium powder.

[0011] Further, in step S4 of this invention, the particle size of the pure hafnium powder is 15-45 μm, the particle size of the hafnium dioxide powder is 15-45 μm, the drying temperature is 100-120℃, and the drying time is 2 hours; the plasma spraying process uses argon gas with a flow rate of 45-55 L / min as the main gas and hydrogen gas with a flow rate of 10-14 L / min as the auxiliary gas, the spraying current is 600-650 A, the spraying voltage is 65-75 V, the spraying distance is 100-120 mm, and the thickness of the gradient transition layer is 30-45 μm. The drying process in step S4 can remove the moisture adsorbed by the pure hafnium powder.

[0012] Further, in step S4 of this invention, the gradient transition layer consists of a first sublayer, a second sublayer, and a third sublayer arranged sequentially from the inside out. The first sublayer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 3:1; the second sublayer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:1; and the third sublayer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:3. The thickness of the first, second, and third sublayers is 10-15 μm. The first sublayer serves to support the metal transition layer and initiate the introduction of the ceramic phase; the second sublayer is a stress-buffering core region; and the third sublayer serves to transition to the ceramic functional layer.

[0013] Further, in step S5 of this invention, the hafnium dioxide powder has a particle size of 1-5 μm, the alumina powder has a particle size of 1-5 μm, the mass ratio of hafnium dioxide powder to alumina powder is 19:1, the drying temperature is 100-120℃, and the drying time is 2 hours; the plasma spraying process uses argon gas with a flow rate of 50 L / min as the main gas and hydrogen gas with a flow rate of 12-15 L / min as the auxiliary gas, the spraying current is 650-670 A, the spraying voltage is 75-85 V, the spraying distance is 110-130 mm, and the thickness of the ceramic functional layer is 30-50 μm. The drying process in step S5 can remove the moisture adsorbed by the hafnium dioxide powder and alumina powder.

[0014] Another technical problem to be solved by the present invention is to provide a thermocouple with an antioxidant and corrosion resistant coating prepared by the above preparation method.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1) This invention innovatively designs a gradient coating structure of "metal transition layer - gradient transition layer - ceramic functional layer": the innermost metal transition layer is metallurgically bonded to the thermocouple substrate, which can eliminate the risk of coating peeling; the middle gradient transition layer can alleviate thermal stress and improve the reliability in dynamic high-temperature environments; the outer ceramic functional layer forms a hafnium dioxide protective film at high temperatures, which can block oxygen and corrosive media, inhibit corrosion and grain coarsening, and the coating has a "self-healing" ability, which extends the service life of the thermocouple in extreme environments. Moreover, compared with traditional metallurgical smelting or electroplating processes, the plasma spraying used in this invention can avoid high-temperature damage to the microstructure of the substrate material and precisely control the thickness and density of the coating. 2) Significantly improved antioxidant performance: Compared with the traditional ASTM G54 specification of ≤5mg, the weight gain of the product of this invention after oxidizing in air at 1400℃ for 100 hours is only about 0.8mg / cm² (the weight gain of an undoped thermocouple is about 2.5mg / cm²), and the antioxidant performance is improved by about 68%.

[0016] 3) Enhanced corrosion resistance: The corrosion resistance of the product of this invention was verified in molten salt at 800℃ for 50 hours. The corrosion weight loss decreased from <0.02g / cm²·h to <0.015g / cm²·h, which is only about 30% of that of the undoped coated thermocouple. 4) Stable thermoelectric performance: According to the ASTM E230 thermoelectric potential standard, the accuracy of an undoped thermocouple at 1200℃ is ≤±192μV. After working at 1200℃ for 100 hours, the thermoelectric potential drift of the product of this invention is reduced from 15μV to about 5μV, and the stability is improved by about 67%.

[0017] 5) Extended service life: The product of this invention has been verified to have a service life that has been extended from about 40 hours to more than 100 hours, an extension of more than 1.5 times. Detailed Implementation

[0018] The present invention will now be described in detail with reference to specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention. Example

[0019] Prepare a thermocouple with an antioxidant and corrosion-resistant coating according to the following steps: S1. Add the armored thermocouple with N graduations to acetone and ultrasonically clean it at 200W power for 10 minutes, then transfer it to deionized water for 10 minutes, take it out and air dry to obtain a pre-treated thermocouple with a surface cleanliness of Sa 3.0 level. S2. The surface of the pretreated thermocouple obtained in step S1 is roughened by sandblasting to obtain a roughened thermocouple. The sandblasting medium used for sandblasting is glass beads with a specification of 80 mesh, the sandblasting pressure is 0.6MPa, the sandblasting distance is 200mm, and the sandblasting angle is 60°. S3. Spherical hafnium powder with a particle size of 1-5 μm is placed in a vacuum drying oven and dried at 110°C for 2 hours to obtain a metal transition layer material. The surface of the roughened thermocouple obtained in step S2 is coated with the metal transition layer material using a plasma spraying process to form a metal transition layer and obtain thermocouple A. The main gas used in the plasma spraying process is argon with a flow rate of 45 L / min, the auxiliary gas is hydrogen with a flow rate of 10 L / min, the spraying current is 550 A, the spraying voltage is 65 V, the spraying distance is 100 mm, and the thickness of the metal transition layer is 15 μm. S4. Pure hafnium powder with a particle size of 15-45 μm and hafnium dioxide powder with a particle size of 15-45 μm are respectively placed in a vacuum drying oven and dried at 110°C for 2 hours to obtain gradient transition layer metallic phase material and gradient transition layer ceramic phase material, respectively. The gradient transition layer metallic phase material and gradient transition layer ceramic phase material are respectively loaded into powder feeder A and powder feeder B of a plasma spraying equipment. Then, the surface of thermocouple A obtained in step S3 is sprayed with plasma spraying process to form gradient transition layer to obtain thermocouple B. The main gas used in the plasma spraying process is argon gas with a flow rate of 50 L / min, and the auxiliary gas is argon gas with a flow rate of 1 L / min. The spraying process used 2 L / min of hydrogen gas, a spraying current of 620 A, a spraying voltage of 70 V, a spraying distance of 110 mm, and a gradient transition layer thickness of 36 μm. The gradient transition layer consisted of a first sub-layer, a second sub-layer, and a third sub-layer arranged sequentially from the inside out. The first sub-layer was formed by plasma spraying pure hafnium powder and hafnium dioxide powder in a mass ratio of 3:1, the second sub-layer was formed by plasma spraying pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:1, and the third sub-layer was formed by plasma spraying pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:3. The thickness of the first, second, and third sub-layers was 12 μm. S5. Hafnium dioxide powder with a particle size of 1-5 μm and alumina powder with a particle size of 1-5 μm are mixed evenly at a mass ratio of 19:1 and dried in a vacuum drying oven at 110°C for 2 hours to obtain a ceramic functional layer material. The ceramic functional layer material is then sprayed onto the surface of thermocouple B obtained in step S4 using a plasma spraying process to form a ceramic functional layer, resulting in a thermocouple with an anti-oxidation and corrosion-resistant coating. The main gas used in the plasma spraying process is argon gas with a flow rate of 50 L / min, the auxiliary gas is hydrogen gas with a flow rate of 14 L / min, the spraying current is 660 A, the spraying voltage is 80 V, the spraying distance is 120 mm, and the thickness of the ceramic functional layer is 40 μm.

[0020] Comparative Example The difference from Example 1 is that steps S3-S5 are not included, that is, the comparative example is an armored thermocouple that has only been cleaned and roughened and has not been coated.

[0021] Experimental Example 1: Antioxidant Performance Test The oxidation weight gain of the thermocouples prepared in Example 1 and the comparative example was measured according to ASTM G54 after being oxidized in air at 1400°C for 1000 hours. The smaller the oxidation weight gain, the better the antioxidant performance. The test results are shown in Table 1.

[0022]

[0023] As can be seen from Table 1, the oxidative weight gain of Example 1 of the present invention is significantly less than that of the comparative example, indicating that the thermocouple prepared by the present invention has better antioxidant properties.

[0024] Experiment Example 2: Corrosion Resistance Test The thermocouples prepared in Example 1 and the comparative example were placed in molten salt at 800°C and left to stand for 50 hours. After removal, the corrosion weight loss was measured. The smaller the corrosion weight loss, the better the corrosion resistance. The test results are shown in Table 2.

[0025]

[0026] As can be seen from Table 2, the corrosion weight loss of Example 1 of the present invention is less than that of the comparative example, indicating that the thermocouple prepared by the present invention has better corrosion resistance.

[0027] Experiment Example 3: Thermoelectric Performance Test The thermoelectric potential drift of the thermocouples prepared in Example 1 and the comparative example was measured at 1200°C for 100 hours according to ASTM E230. The smaller the thermoelectric potential drift, the better the thermoelectric performance. The test results are shown in Table 3.

[0028]

[0029] As can be seen from Table 3, the thermoelectric potential shift of Example 1 of the present invention is significantly smaller than that of the comparative example, indicating that the thermocouple prepared by the present invention has stable thermoelectric performance.

[0030] Experiment Example 4: Service Life Test The service life of the thermocouples prepared in Example 1 and the comparative example was measured respectively, and the test results are shown in Table 4.

[0031]

[0032] As can be seen from Table 4, the service life of Example 1 of the present invention is significantly longer than that of the comparative example, indicating that the thermocouple prepared by the present invention has a longer service life. Example

[0033] Prepare a thermocouple with an antioxidant and corrosion-resistant coating according to the following steps: S1. Add the armored thermocouple with N graduations to anhydrous ethanol and ultrasonically clean it at 300W power for 5 minutes. Then, transfer it to deionized water for 5 minutes. After taking it out, air dry it to obtain a pretreated thermocouple with a surface cleanliness of Sa 3.0 level. S2. The surface of the pretreated thermocouple obtained in step S1 is roughened by sandblasting to obtain a roughened thermocouple. The sandblasting medium used for the roughening treatment is 60-mesh glass beads, the sandblasting pressure is 0.4MPa, the sandblasting distance is 100mm, and the sandblasting angle is 45°. S3. Spherical hafnium powder with a particle size of 1-5 μm is placed in a vacuum drying oven and dried at 120°C for 2 hours to obtain a metal transition layer material. The surface of the roughened thermocouple obtained in step S2 is coated with the metal transition layer material using a plasma spraying process to form a metal transition layer and obtain thermocouple A. The main gas used in the plasma spraying process is argon with a flow rate of 40 L / min, the auxiliary gas is hydrogen with a flow rate of 8 L / min, the spraying current is 500 A, the spraying voltage is 60 V, the spraying distance is 80 mm, and the thickness of the metal transition layer is 10 μm. S4. Pure hafnium powder with a particle size of 15-45 μm and hafnium dioxide powder with a particle size of 15-45 μm are respectively placed in a vacuum drying oven and dried at 120°C for 2 hours to obtain gradient transition layer metallic phase material and gradient transition layer ceramic phase material, respectively. The gradient transition layer metallic phase material and gradient transition layer ceramic phase material are respectively loaded into powder feeder A and powder feeder B of a plasma spraying equipment. Then, the surface of thermocouple A obtained in step S3 is sprayed with plasma spraying process to form gradient transition layer to obtain thermocouple B. The main gas used in the plasma spraying process is argon gas with a flow rate of 45 L / min, and the auxiliary gas is argon gas with a flow rate of 1 L / min. The hydrogen flow rate is 0 L / min, the spraying current is 600 A, the spraying voltage is 65 V, the spraying distance is 100 mm, and the thickness of the gradient transition layer is 30 μm. The gradient transition layer consists of a first sub-layer, a second sub-layer, and a third sub-layer arranged sequentially from the inside out. The first sub-layer is formed by plasma spraying pure hafnium powder and hafnium dioxide powder in a mass ratio of 3:1, the second sub-layer is formed by plasma spraying pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:1, and the third sub-layer is formed by plasma spraying pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:3. The thickness of the first, second, and third sub-layers is 10 μm. S5. Hafnium dioxide powder with a particle size of 1-5 μm and alumina powder with a particle size of 1-5 μm are mixed evenly at a mass ratio of 19:1 and dried in a vacuum drying oven at 120°C for 2 hours to obtain a ceramic functional layer material. The ceramic functional layer material is then sprayed onto the surface of thermocouple B obtained in step S4 using a plasma spraying process to form a ceramic functional layer, resulting in a thermocouple with an anti-oxidation and corrosion-resistant coating. The main gas used in the plasma spraying process is argon gas with a flow rate of 50 L / min, the auxiliary gas is hydrogen gas with a flow rate of 12 L / min, the spraying current is 650 A, the spraying voltage is 75 V, the spraying distance is 110 mm, and the thickness of the ceramic functional layer is 30 μm. Example

[0034] Prepare a thermocouple with an antioxidant and corrosion-resistant coating according to the following steps: S1. Add the armored thermocouple with N graduations to acetone and ultrasonically clean it at 300W power for 15 minutes, then transfer it to deionized water for 15 minutes, take it out and air dry to obtain a pretreated thermocouple with a surface cleanliness of Sa 3.0 level. S2. The surface of the pretreated thermocouple obtained in step S1 is roughened by sandblasting to obtain a roughened thermocouple. The sandblasting medium used for the roughening treatment is 100-mesh glass beads, the sandblasting pressure is 0.7MPa, the sandblasting distance is 300mm, and the sandblasting angle is 75°. S3. Spherical hafnium powder with a particle size of 1-5 μm is placed in a vacuum drying oven and dried at 100°C for 2 hours to obtain a metal transition layer material. The surface of the roughened thermocouple obtained in step S2 is coated with the metal transition layer material using a plasma spraying process to form a metal transition layer and obtain thermocouple A. The main gas used in the plasma spraying process is argon with a flow rate of 50 L / min, the auxiliary gas is hydrogen with a flow rate of 12 L / min, the spraying current is 600 A, the spraying voltage is 70 V, the spraying distance is 120 mm, and the thickness of the metal transition layer is 20 μm. S4. Pure hafnium powder with a particle size of 15-45 μm and hafnium dioxide powder with a particle size of 15-45 μm are respectively placed in a vacuum drying oven and dried at 100°C for 2 hours to obtain gradient transition layer metallic phase material and gradient transition layer ceramic phase material, respectively. The gradient transition layer metallic phase material and gradient transition layer ceramic phase material are respectively loaded into powder feeder A and powder feeder B of a plasma spraying equipment. Then, the surface of thermocouple A obtained in step S3 is sprayed with plasma spraying process to form gradient transition layer to obtain thermocouple B. The main gas used in the plasma spraying process is argon gas with a flow rate of 55 L / min, and the auxiliary gas is argon gas with a flow rate of 1 L / min. The spraying process uses 4 L / min of hydrogen gas, a spraying current of 650 A, a spraying voltage of 75 V, a spraying distance of 120 mm, and a gradient transition layer thickness of 45 μm. The gradient transition layer consists of a first sub-layer, a second sub-layer, and a third sub-layer arranged sequentially from the inside out. The first sub-layer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 3:1, the second sub-layer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:1, and the third sub-layer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:3. The thickness of the first, second, and third sub-layers is 15 μm. S5. Hafnium dioxide powder with a particle size of 1-5 μm and alumina powder with a particle size of 1-5 μm are mixed evenly at a mass ratio of 19:1 and dried in a vacuum drying oven at 100°C for 2 hours to obtain a ceramic functional layer material. The ceramic functional layer material is then sprayed onto the surface of thermocouple B obtained in step S4 using a plasma spraying process to form a ceramic functional layer, resulting in a thermocouple with an anti-oxidation and corrosion-resistant coating. The main gas used in the plasma spraying process is argon gas with a flow rate of 50 L / min, the auxiliary gas is hydrogen gas with a flow rate of 15 L / min, the spraying current is 670 A, the spraying voltage is 85 V, the spraying distance is 130 mm, and the thickness of the ceramic functional layer is 50 μm.

[0035] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing a thermocouple with an antioxidant and corrosion-resistant coating, characterized in that: Includes the following steps: S1. Add the thermocouple to the solvent for ultrasonic cleaning, then transfer it to deionized water for cleaning, and take it out and air dry to obtain the pretreated thermocouple; S2. The surface of the pretreated thermocouple obtained in step S1 is roughened by sandblasting to obtain a roughened thermocouple; S3. Place the spherical hafnium powder in a vacuum drying oven to dry it to obtain a metal transition layer material. Use a plasma spraying process to spray the metal transition layer material onto the surface of the roughened thermocouple obtained in step S2 to form a metal transition layer and obtain thermocouple A. S4. Place pure hafnium powder and hafnium dioxide powder in a vacuum drying oven to dry them to obtain gradient transition layer metal phase material and gradient transition layer ceramic phase material respectively. Load the gradient transition layer metal phase material and gradient transition layer ceramic phase material into powder feeder A and powder feeder B of a plasma spraying equipment respectively. Then, use plasma spraying process to spray the surface of thermocouple A obtained in step S3 to form a gradient transition layer to obtain thermocouple B. S5. After mixing hafnium dioxide powder and alumina powder evenly, dry them in a vacuum drying oven to obtain a ceramic functional layer material. Then, use a plasma spraying process to spray the ceramic functional layer material onto the surface of thermocouple B obtained in step S4 to form a ceramic functional layer, thereby obtaining a thermocouple with an anti-oxidation and corrosion-resistant coating.

2. The method for preparing a thermocouple with an antioxidant and corrosion-resistant coating according to claim 1, characterized in that: In step S1, the thermocouple is an armored thermocouple with N graduations, and the solvent is acetone or anhydrous ethanol; the ultrasonic cleaning power is 200-300W, the ultrasonic cleaning time is 5-15min, and the deionized water cleaning time is 5-15min; the surface cleanliness of the pretreated thermocouple reaches Sa 3.0 level.

3. The method for preparing a thermocouple with an antioxidant and corrosion-resistant coating according to claim 1, characterized in that: In step S2, the sandblasting roughening treatment uses glass beads with a mesh size of 60-100 as the sandblasting medium, a sandblasting pressure of 0.4-0.7MPa, a sandblasting distance of 100-300mm, and a sandblasting angle of 45-75°.

4. The method for preparing a thermocouple with an antioxidant and corrosion-resistant coating according to claim 1, characterized in that: In step S3, the particle size of the spherical hafnium powder is 1-5 μm, the drying temperature is 100-120℃, and the drying time is 2 hours; the main gas used in the plasma spraying process is argon with a flow rate of 40-50 L / min, the auxiliary gas is hydrogen with a flow rate of 8-12 L / min, the spraying current is 500-600 A, the spraying voltage is 60-70 V, the spraying distance is 80-120 mm, and the thickness of the metal transition layer is 10-20 μm.

5. The method for preparing a thermocouple with an antioxidant and corrosion-resistant coating according to claim 1, characterized in that: In step S4, the particle size of pure hafnium powder is 15-45 μm, the particle size of hafnium dioxide powder is 15-45 μm, the drying temperature is 100-120℃, and the drying time is 2 hours. The plasma spraying process uses argon gas with a flow rate of 45-55 L / min as the main gas and hydrogen gas with a flow rate of 10-14 L / min as the auxiliary gas. The spraying current is 600-650 A, the spraying voltage is 65-75 V, the spraying distance is 100-120 mm, and the thickness of the gradient transition layer is 30-45 μm.

6. The method for preparing a thermocouple with an antioxidant and corrosion-resistant coating according to claim 5, characterized in that: In step S4, the gradient transition layer consists of a first sublayer, a second sublayer, and a third sublayer arranged sequentially from the inside out. The first sublayer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 3:

1. The second sublayer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:

1. The third sublayer is formed by plasma spraying of pure hafnium powder and hafnium dioxide powder in a mass ratio of 1:

3. The thickness of the first, second, and third sublayers is 10-15 μm.

7. The method for preparing a thermocouple with an antioxidant and corrosion-resistant coating according to claim 1, characterized in that: In step S5, the particle size of hafnium dioxide powder is 1-5 μm, the particle size of alumina powder is 1-5 μm, the mass ratio of hafnium dioxide powder to alumina powder is 19:1, the drying temperature is 100-120℃, and the drying time is 2 hours. The plasma spraying process uses argon gas with a flow rate of 50 L / min as the main gas and hydrogen gas with a flow rate of 12-15 L / min as the auxiliary gas. The spraying current is 650-670 A, the spraying voltage is 75-85 V, the spraying distance is 110-130 mm, and the thickness of the ceramic functional layer is 30-50 μm.

8. Thermocouples with antioxidant and corrosion-resistant coatings prepared by the preparation method according to claims 1 to 7.

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