Method for preparing aluminum oxide coating on quartz crucible

Abstract

The invention belongs to the technical field of high-temperature materials, and discloses a method for preparing an aluminum oxide coating on a quartz crucible. The method comprises the following steps: preparing a nanometer aluminum oxide dispersion liquid, then spraying the nanometer aluminum oxide dispersion liquid on the surface of a quartz crucible through plasma spraying, and then carrying out heat treatment under vacuum to prepare an aluminum oxide coating; the nanometer aluminum oxide dispersion liquid comprises the following components in percentage by weight: 25%-50% of nanometer aluminum oxide, 0.5%-3% of a metal oxide auxiliary agent, 0.5%-2% of an adhesive and 45%-70% of water; the metal oxide auxiliary agent comprises magnesium oxide and silicon dioxide, and the mass ratio of the magnesium oxide to the silicon dioxide is (3-6): 1. The prepared aluminum oxide coating has the advantages of being high in binding force with a matrix, not prone to peeling off, capable of resisting the high temperature of 1800 DEG C and free of coating damage phenomena such as cracking and peeling off; and the coating is high in compactness and excellent in high-temperature resistance and corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature materials technology, specifically relating to a method for preparing an alumina coating on a quartz crucible. Background Technology

[0002] Barium coating technology has been adopted in over 80% of mainstream semiconductor and photovoltaic quartz crucibles in China, especially in large-size crucibles of 28 inches and above, where it has become standard. The mechanism of barium coating is as follows: (BaCO3 / Ba(OH)2) generates barium silicate (BaSiO3) at high temperatures, increasing the crystal density of cristobalite on the inner wall of the quartz crucible, reducing spalling caused by molten silicon erosion, while simultaneously slowing down the quartz crystallization process, reducing the risk of crystal breakage during growth, and improving the yield of single-crystal silicon growth; furthermore, barium has an extremely low segregation coefficient in silicon (approximately 2.25 × 10⁻⁶). -8 Trace amounts of residue do not affect the electrical properties of single-crystal silicon. However, the traditional barium coating preparation process has many defects, as follows: (1) Toxicity risk: Barium hydroxide and barium carbonate are highly toxic and extremely harmful to the human body. The spraying process requires strict protection and waste gas treatment, and the waste gas treatment cost is high; (2) Complex process: Precision equipment such as nitrogen protection and two-fluid pressure spray system are required to basically ensure the uniformity and stability of the coating; (3) The coating is prone to oxidation failure in the later stage, which leads to a shortened service life of the crucible. The service life of the current barium coating crucible is 300-500 hours; (4) After the coating crucible is scrapped and fails, there is a risk of environmental pollution due to the toxicity of barium.

[0003] To address the problems associated with barium coatings, research has also focused on preparing alumina coatings on crucibles. Alumina coatings leverage their high-temperature stability, corrosion resistance, high hardness, and wear resistance to provide effective protection. Protecting the crucible from erosion and corrosion by molten materials such as monocrystalline silicon during use can extend its lifespan and reduce production costs. However, Al2O3 ceramic coatings still face several challenges: firstly, poor density allows molten metal or corrosive media to penetrate, easily leading to substrate corrosion; secondly, the bonding strength between the Al2O3 ceramic coating and the quartz crucible substrate is weak, causing the coating to fail easily, lose its protective function, and result in a short lifespan.

[0004] Therefore, developing a method to enhance the bonding force between the Al2O3 coating and the quartz crucible substrate and improve its density is of great significance for crucible preparation and repair. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a method for preparing an alumina coating on a quartz crucible. The method provided by the present invention can enhance the adhesion between the Al2O3 coating and the quartz crucible substrate, improve its density, and extend its service life.

[0006] This invention provides a method for preparing an alumina coating on a quartz crucible.

[0007] Specifically, a method for preparing an alumina coating on a quartz crucible includes the following steps: A nano-alumina dispersion was prepared, then plasma-sprayed onto the surface of a quartz crucible, and then heat-treated under vacuum to obtain an alumina coating. The nano-alumina dispersion comprises, by weight percentage: 25%-50% nano-alumina, 0.5%-3% metal oxide additives, 0.5%-2% binder, and 45%-70% water; the metal oxide additives include magnesium oxide and silicon dioxide, wherein the mass ratio of magnesium oxide to silicon dioxide is (3-6):1; The conditions for plasma spraying are as follows: argon is used as the main gas and hydrogen as the auxiliary gas; the argon flow rate is 30-70 L / min and the hydrogen flow rate is 2-20 L / min; the spray gun power is 100-200 kW; and the spraying distance is 80-120 mm.

[0008] In some embodiments of the present invention, the mass ratio of magnesium oxide to silicon dioxide is (3.5-5):1.

[0009] In some embodiments of the present invention, the nano-alumina dispersion comprises, by weight percentage: 30%-45% nano-alumina, 0.5%-3% metal oxide additives, 0.5%-2% binder, and 50%-65% water.

[0010] In some embodiments of the present invention, the adhesive comprises polyacrylic acid and / or polyvinyl alcohol. Preferably, the adhesive comprises polyacrylic acid and polyvinyl alcohol; more preferably, the mass ratio of polyacrylic acid to polyvinyl alcohol is (3-6):1, such as 3:1, 4:1, 5:1, and 6:1. The present invention uses polyvinyl alcohol and polyacrylic acid as adhesives, which allows the two to complement each other, promoting initial bonding between particles during spraying and improving the initial strength of the coating; simultaneously, the combined use of the two can improve the atomization effect of the dispersion.

[0011] In some embodiments of the present invention, the preparation method of the nano-alumina dispersion is as follows: the nano-alumina, the metal oxide auxiliary agent and the binder are added to water under continuous stirring, and after stirring evenly, ammonia water is added to adjust the pH value of the dispersion to 5-8; then ball milling is performed to obtain the nano-alumina dispersion.

[0012] Preferably, the mass fraction of the ammonia solution is 5%-15%; more preferably, the mass fraction of the ammonia solution is 8%-12%. Ammonia solution can partially neutralize the carboxyl groups of polyacrylic acid (PAA) (-COOH → -COONH4), adjusting the pH of the system from the relatively strong acidity of PAA to a slightly alkaline neutral state, thereby improving flexibility and water resistance. By controlling the mass fraction of the ammonia solution, the degree of neutralization can be precisely controlled, while the addition of ammonia solution can be avoided from damaging the stability of the adhesive.

[0013] In some embodiments of the present invention, the ball mill rotation speed is 500-800 rpm, and the ball milling time is 1-4 hours. Following the ball milling step, a sieving step is also included; specifically, the ball-milled liquid is passed through a 200-mesh sieve.

[0014] In some embodiments of the present invention, the quartz crucible further includes a pretreatment step before spraying. Specifically, the pretreatment process is as follows: first, the surface of the quartz crucible is wiped with anhydrous ethanol; then, the surface of the quartz crucible to be processed is roughened by sandblasting with 80-160 mesh alumina particles; and finally, the quartz crucible is ultrasonically cleaned.

[0015] In some embodiments of the present invention, the plasma spraying conditions are as follows: argon is used as the main gas, and hydrogen is used as the auxiliary gas; the argon flow rate is 40-60 L / min, and the hydrogen flow rate is 5-15 L / min; the spray gun power is 120-180 kW; and the spraying distance is 90-110 mm. Using atmospheric supersonic plasma spraying technology to prepare an alumina coating on a quartz crucible reduces the technical difficulty and cost of coating preparation.

[0016] In some embodiments of the present invention, the heat treatment process is as follows: heating to 1000℃-1200℃ at a heating rate of 1-5℃ / min, and holding at that temperature for 2-10 hours.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention uses plasma spraying technology to prepare nano-alumina coating on quartz crucible. By optimizing the formulation of nano-alumina dispersion, an appropriate proportion of silicon dioxide and magnesium oxide are added to nano-alumina. Under the premise of ensuring the high temperature resistance of the coating, the density of Al2O3 coating and the bonding force between Al2O3 coating and quartz crucible substrate can be improved, thereby reducing coating peeling and corrosion and extending service life.

[0018] (2) The addition of magnesium oxide and silicon dioxide in a mass ratio of (3-6):1 in the nano-alumina dispersion can optimize the microstructure of the nano-alumina coating, improve the bonding strength and density of the coating, and effectively improve the coating's resistance to aluminum corrosion, thus greatly extending the service life of the crucible. Magnesium oxide forms spinel (MgAl2O4, melting point 2135℃) with Al2O3, which not only prevents the formation of a glassy phase and does not affect the high-temperature resistance of the coating, but also inhibits excessive growth of Al2O3 grains and refines the coating structure. Silicon dioxide forms a eutectic with Al2O3, which can significantly improve melt flowability and reduce porosity. By controlling the amount of magnesium oxide and silicon dioxide added, and their ratio, the density and bonding strength of the Al2O3 coating can be improved while ensuring the coating's high-temperature resistance.

[0019] (3) The alumina coating prepared by the present invention has the advantages of strong bonding with the substrate, not easy to peel off, and can withstand high temperature of 1800℃ without cracking or peeling. The coating has excellent high temperature resistance and corrosion resistance. Detailed Implementation

[0020] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0021] The quartz crucibles used in the following examples and comparative examples were from the same batch, made of the same material (quartz), and of the same size. The nano-alumina powder used had a D50 ≤ 100 nm and a purity ≥ 99.9%; the magnesium oxide particle size was 30-100 nm, and the silicon dioxide particle size was 100-800 nm. The polyacrylic acid had a molecular weight of 20,000, and the polyvinyl alcohol had a molecular weight of 30,000.

[0022] Unless otherwise specified, all raw materials, reagents or apparatus used are available from conventional commercial sources or can be obtained by existing known methods. Example

[0023] A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 58% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 38% nano alumina powder and 2.5% metal oxide additive (mass ratio of magnesium oxide to silicon dioxide 4:1), then add 1.5% binder (mass ratio of polyacrylic acid to polyvinyl alcohol 4:1) and stir until uniform. Slowly add 10% ammonia water to adjust the pH of the solution to 7.5. Put the alumina solution with adjusted pH into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The spray gun power is 150kW. The spraying distance is 100mm. XRF is used to test the coating thickness, and the coating thickness is controlled to be 2-3 micrometers. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1100℃; the heating rate is 3℃ / min; and the high temperature is held for 6 hours. Example

[0024] A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 54.2% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 42% nano alumina powder and 2.0% metal oxide additive (mass ratio of magnesium oxide to silicon dioxide 3.5:1), then add 1.8% binder (mass ratio of polyacrylic acid to polyvinyl alcohol 5:1) and stir until uniform. Slowly add 10% ammonia water to adjust the pH of the solution to 7.0. Put the alumina solution with adjusted pH into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The spray gun power is 180kW. The spraying distance is 90mm. XRF is used to test the coating thickness, and the coating thickness is controlled to be 2-3 micrometers. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1200℃; the heating rate is 5℃ / min; and the high temperature is held for 5h. Example

[0025] A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 56% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 40% nano alumina powder and 2.2% metal oxide additive (mass ratio of magnesium oxide to silicon dioxide 4:1), then add 1.8% binder (mass ratio of polyacrylic acid to polyvinyl alcohol 1:5), and stir until uniform. Slowly add 10% ammonia water to adjust the pH of the solution to 7.5. Put the alumina solution with adjusted pH into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The spray gun power is 150kW. The spraying distance is 100mm. XRF is used to test the coating thickness, and the coating thickness is controlled to be 2-3 micrometers. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1100℃; the heating rate is 3℃ / min; and the high temperature is held for 6 hours.

[0026] Comparative Example 1 A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 58% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 38% nano alumina powder and 2.5% magnesium oxide metal oxide, and then add 1.5% binder (the mass ratio of polyacrylic acid and polyvinyl alcohol is 4:1). Stir until uniform. Slowly add 10% ammonia water to adjust the pH of the solution to 7.5. Put the alumina solution with adjusted pH into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The power of the spray gun is 160kW and the spraying distance is 95mm. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1100℃; the heating rate is 3℃ / min; and the high temperature is held for 6 hours.

[0027] Comparative Example 2 A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 58% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 38% nano alumina powder and 2.5% metal oxide additive (mass ratio of magnesium oxide to silicon dioxide 1:4), then add 1.5% binder (mass ratio of polyacrylic acid to polyvinyl alcohol 4:1), and stir until uniform. Slowly add 10% ammonia water to adjust the pH of the solution to 7.5. Put the alumina solution with adjusted pH into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The spray gun power is 150kW. The spraying distance is 100mm. XRF is used to test the coating thickness, and the coating thickness is controlled to be 2-3 micrometers. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1100℃; the heating rate is 3℃ / min; and the high temperature is held for 6 hours.

[0028] Comparative Example 3 A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 73% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 23% nano alumina powder and 2.5% metal oxide additive (mass ratio of magnesium oxide to silicon dioxide 4:1), then add 1.5% binder polyacrylic acid and stir until uniform. Slowly add 10% ammonia water to adjust the pH value of the solution to 7.5. Put the alumina solution with adjusted pH value into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The spray gun power is 150kW. The spraying distance is 100mm. XRF is used to test the coating thickness, and the coating thickness is controlled to be 2-3 micrometers. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1100℃; the heating rate is 3℃ / min; and the high temperature is held for 6 hours.

[0029] Comparative Example 4 A method for preparing an alumina coating on a quartz crucible includes the following steps: (1) Crucible cleaning: Wipe the surface of the crucible clean with a lint-free cloth and anhydrous ethanol; use 100-mesh alumina particles to roughen the surface of the crucible to be processed by sandblasting; put the sandblasted crucible into deionized water for ultrasonic cleaning for 20 minutes. (2) Preparation of nano alumina solution: Weigh 58% deionized water by weight and put it into a mixer. While stirring continuously, slowly add 38% nano alumina powder and 2.5% metal oxide additive (mass ratio of magnesium oxide to silicon dioxide 4:1), then add 1.5% binder (mass ratio of polyacrylic acid to polyvinyl alcohol 4:1) and stir until uniform. Slowly add 10% ammonia water to adjust the pH of the solution to 7.5. Put the alumina solution with adjusted pH into a ball mill for ball milling. The ball mill speed is 600 rpm and the ball milling time is 3 hours. The ball-milled solution is passed through a 200-mesh sieve and set aside. (3) Spraying: Argon is used as the main gas and hydrogen is used as the auxiliary gas. The flow rate of argon is 50L / min and the flow rate of hydrogen is 10L / min. The spray gun power is 80kW. The spraying distance is 130mm. XRF is used to test the coating thickness, and the coating thickness is controlled to be 2-3 micrometers. (4) Vacuum heat treatment: Under vacuum conditions, the heat treatment temperature is 1100℃; the heating rate is 3℃ / min; and the high temperature is held for 6 hours.

[0030] Product effectiveness test The performance of the alumina coatings prepared in the examples and comparative examples was tested. Tests included coating adhesion, coating porosity, and long-term high-temperature service testing. The coating adhesion strength was tested according to ASTM C633 standard; the coating porosity was measured by metallographic observation.

[0031] The long-term high-temperature service test process is as follows: The crucible is placed in a high-temperature furnace, heated to 1600℃ and held for 6 hours, then cooled to 1450℃ and kept at a constant temperature (temperature fluctuation ±5℃). The crucible is removed every 72 hours (cooled to room temperature) and observed for coating peeling, cracks, localized coating fissures, or other coating damage. When any of these conditions occur, the total service time is recorded as the "long-term high-temperature life".

[0032] The test results of the examples and comparative examples are shown in Table 1.

[0033] Table 1

[0034] As shown in Table 1, the alumina coating prepared in the embodiments of the present invention exhibits strong adhesion to the substrate, high coating density, and is not easily peeled off under long-term high-temperature operation, resulting in a long service life. Comparative Examples 1 and 2 show that when only one metal oxide additive is added, or when the ratio of magnesium oxide to silicon dioxide is outside the scope of protection of the present invention, the coating adhesion strength and coating density are affected to varying degrees. Comparative Example 3 shows that when the concentration of the nano-alumina dispersion is less than 25%, the porosity of the coating increases significantly, and the coating adhesion and high-temperature resistance are also significantly affected. Comparative Example 4 shows that when the spraying distance increases and the spray gun power decreases, both the coating adhesion and density decrease significantly. Therefore, strictly controlling the conditions of plasma spraying is crucial for preparing alumina coatings with strong adhesion and high density.

[0035] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing an alumina coating on a quartz crucible, characterized in that, Includes the following steps: A nano-alumina dispersion was prepared, then plasma-sprayed onto the surface of a quartz crucible, and then heat-treated under vacuum to obtain an alumina coating. The nano-alumina dispersion comprises, by weight percentage: 25%-50% nano-alumina, 0.5%-3% metal oxide additives, 0.5%-2% binder, and 45%-70% water; the metal oxide additives include magnesium oxide and silicon dioxide, wherein the mass ratio of magnesium oxide to silicon dioxide is (3-6):1; The conditions for plasma spraying are as follows: argon is used as the main gas and hydrogen as the auxiliary gas; the argon flow rate is 30-70 L / min and the hydrogen flow rate is 2-20 L / min; the spray gun power is 100-200 kW; and the spraying distance is 80-120 mm.

2. The method according to claim 1, characterized in that, The mass ratio of magnesium oxide to silicon dioxide is (3.5-5):

1.

3. The method according to claim 1, characterized in that, The nano-alumina dispersion comprises, by weight percentage: 30%-45% nano-alumina, 0.5%-3% metal oxide additives, 0.5%-2% binder, and 50%-65% water.

4. The method according to any one of claims 1-3, characterized in that, The adhesive comprises polyacrylic acid and / or polyvinyl alcohol.

5. The method according to claim 1, characterized in that, The adhesive comprises polyacrylic acid and polyvinyl alcohol; the mass ratio of the polyacrylic acid and polyvinyl alcohol is (3-6):

1.

6. The method according to any one of claims 1, 2, 3 or 5, characterized in that, The preparation method of the nano-alumina dispersion is as follows: the nano-alumina, the metal oxide auxiliary agent and the binder are added to water under continuous stirring, and after stirring evenly, ammonia water is added to adjust the pH value of the dispersion to 5-8; then ball milling is performed to obtain the nano-alumina dispersion.

7. The method according to claim 6, characterized in that, The ball mill rotates at a speed of 500-800 rpm, and the milling time is 1-4 hours.

8. The method according to claim 1, characterized in that, The quartz crucible also includes a pretreatment step before spraying. The pretreatment process is as follows: first, the surface of the quartz crucible is wiped with anhydrous ethanol, then the surface of the quartz crucible to be processed is roughened by sandblasting with 80-160 mesh alumina particles, and finally the quartz crucible is ultrasonically cleaned.

9. The method according to claim 1 or 8, characterized in that, The conditions for plasma spraying are as follows: argon is used as the main gas and hydrogen as the auxiliary gas; the argon flow rate is 40-60 L / min and the hydrogen flow rate is 5-15 L / min; the spray gun power is 120-180 kW; and the spraying distance is 90-110 mm.

10. The method according to claim 9, characterized in that, The heat treatment process is as follows: the temperature is increased to 1000℃-1200℃ at a heating rate of 1-5℃ / min, and held for 2-10 hours.

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