A method for preparing a surface coating of an alumina ceramic part

Abstract

The application discloses a preparation method of an alumina ceramic part surface coating, and comprises the following steps: a. completely covering the alumina ceramic part, performing vibration treatment on a container, and making the coverage of the chromium oxide powder on the alumina ceramic part sufficient; b. placing the alumina ceramic part covered by the chromium oxide powder in a furnace, heating to a specific temperature T1, keeping the temperature for a specific time t1, and then cooling; c. after the furnace is cooled, taking out the alumina ceramic part, and removing the chromium oxide floating powder on the surface of the alumina ceramic part; d. performing polishing treatment on the alumina ceramic part, and removing the chromium oxide particles adhered to the surface of the alumina ceramic part; e. placing the alumina ceramic part in the furnace, heating, heating to a specific temperature T2, keeping the temperature for a specific time t2, and then cooling; and f. after the furnace is cooled, taking out the alumina ceramic part. The method is simple in operation, excellent in quality, and can be used for preparing the chromium oxide coating on the surface of the alumina ceramic part with a complex structure.

Description

Technical Field

[0001] This invention belongs to the field of ceramic coating preparation, and relates to a method for preparing a surface coating for alumina ceramic parts. Background Technology

[0002] Alumina ceramics possess advantages such as high mechanical strength, high resistivity, low dielectric loss, high dielectric breakdown strength, high temperature resistance, and high thermal conductivity, making them an excellent insulating material. Furthermore, they can be sealed to metals through metallization processes and are widely used in the manufacture of insulating shells for vacuum electronic devices. In vacuum electronic devices, insulating supports are inevitably introduced to form a parallel composite insulation of vacuum and solid dielectric. It should be noted that vacuum itself has a very high dielectric strength, with a critical breakdown field strength reaching 350 kV / cm. Meanwhile, the breakdown field strength of alumina ceramics is as high as 300–400 kV / cm. Vacuum alumina composite insulation typically experiences surface flashover (at a vacuum level of 10⁻³ Pa) under an applied electric field of tens of kV / cm, leading to system insulation failure and device malfunction. This is because the flashover voltage is far lower than the breakdown voltage of a vacuum gap of the same geometric length or the ceramic material itself. Flashover along the shell surface is a significant factor leading to the failure of vacuum electronic devices. Therefore, improving the surface flashover voltage of alumina ceramics is of great importance for improving the reliability of vacuum electronic devices.

[0003] According to the theory of secondary electron emission avalanche (SEEA) along surface flashover, the high secondary electron emission coefficient of alumina ceramics is one of the key factors leading to its low surface flashover voltage. Therefore, there are currently two main methods in the industry to improve the surface withstand voltage of alumina ceramics: (1) doping the ceramic raw materials with chromium oxide, thereby reducing the secondary electron emission coefficient and improving the surface withstand voltage; (2) preparing a chromium oxide coating with a low secondary electron emission coefficient on the surface of alumina ceramics to improve the surface withstand voltage. When using the method of doping with chromium oxide to improve the surface withstand voltage of alumina ceramics, the surface withstand voltage often increases with the increase of chromium oxide doping amount. However, when the chromium oxide content in the alumina matrix is ​​high, the chromium oxide doping significantly affects the properties of alumina ceramics, such as resistivity, dielectric constant, dielectric loss, bulk withstand voltage, and metallization performance, severely impacting their usability. Therefore, the chromium oxide doping level in the alumina ceramic matrix is ​​often limited, which restricts the potential of using chromium oxide doping to improve the surface withstand voltage of alumina ceramics. Considering that flashover (surface breakdown) is mainly related to the surface properties of the material, preparing a chromium oxide coating of a few micrometers to tens of micrometers on the surface of alumina ceramics can effectively reduce the secondary electron generation coefficient of alumina ceramics and improve surface withstand voltage without changing the properties of the alumina ceramic matrix. Therefore, in recent years, more and more work has focused on finding a cost-effective method for coating alumina ceramic parts with chromium oxide.

[0004] Currently, the main methods for preparing chromium oxide coatings on the surface of alumina ceramic parts in the industry include immersion method, slurry coating method, and plasma spraying method. The immersion method currently has two main drawbacks: (1) the prepared chromium oxide coating is very thin, and if this method is to be used to prepare a thicker chromium oxide coating on the surface of alumina ceramic parts, repeated immersion, drying, and baking are often required; (2) the bonding strength between the prepared chromium oxide coating and the alumina ceramic parts is low. Due to the low baking temperature, the chromium oxide coating does not react with the alumina ceramic surface, and the bonding between the chromium oxide coating and the alumina ceramic is mainly physical adsorption. Furthermore, if the baking temperature in this method is increased, chromium oxide easily diffuses into the alumina, making it impossible to form a chromium oxide coating. The main disadvantages of the slurry coating method are threefold: (1) It is difficult to operate on complex ceramic parts, and the chromium oxide slurry cannot be fully coated on deep holes, wrinkles and other locations; (2) Since the chromium oxide slurry contains organic binders, the organic matter volatilizes during the high-temperature baking process, forming a large number of pores on the surface of the chromium oxide coating. Therefore, the chromium oxide coating prepared on the surface of alumina ceramics using this method often cracks and peels off during use; (3) The chromium oxide coating prepared on the surface of alumina ceramic parts using this method is not only thick, but also uneven in thickness, which seriously affects the dimensional qualification rate of alumina ceramic parts. The disadvantage of plasma spraying is that it cannot be used to prepare surface chromium oxide coatings on complex alumina ceramic parts. In addition, the process flow of the immersion method, slurry coating method and plasma spraying method is relatively long and the operation is relatively complicated. Therefore, they have not been widely used in production. At present, the manufacturing of vacuum tube shells in the industry still mainly uses unmodified alumina ceramics and chromium oxide-doped alumina ceramics.

[0005] In summary, there is an urgent need to develop a method that is simple to operate, produces high-quality coatings, and can be used to prepare chromium oxide coatings on the surface of complex alumina ceramic parts. Summary of the Invention

[0006] In view of this, the present invention provides a method for preparing a surface coating of alumina ceramic parts. The method of the present invention is simple to operate, produces excellent quality, and can be used to prepare chromium oxide coatings on the surface of alumina ceramic parts with complex structures.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] A method for preparing a chromium oxide coating on the surface of an alumina ceramic part, the method comprising the following steps:

[0009] a. Place the chromium oxide powder and alumina ceramic parts in a high-temperature oxidation-resistant container, such as an alumina ceramic crucible. Completely cover the alumina ceramic parts with chromium oxide powder. Vibrate the container to ensure that the chromium oxide powder fully and compactly covers the alumina ceramic parts.

[0010] b. Place the alumina ceramic parts covered with chromium oxide powder in a furnace, heat to a specific temperature T1, hold at that temperature for a specific time t1, and then cool down;

[0011] c. After the furnace has cooled down, remove the alumina ceramic parts and remove the chromium oxide powder covering the surface of the alumina ceramic parts;

[0012] d. Vibration polishing is used to polish alumina ceramic parts to remove chromium oxide particles adhering to the surface of alumina ceramic parts and improve surface smoothness;

[0013] e. Place the treated alumina ceramic parts in the furnace, heat them to a specific temperature T2, hold them at that temperature for a specific time t2, and then cool them down;

[0014] f. After the furnace has cooled down, remove the alumina ceramic parts.

[0015] Preferably, in step a, the particle size of the chromium oxide powder is 100-400 mesh, and the mass percentage of alumina in the alumina ceramic parts is 85%-99%.

[0016] Preferably, in step b, the specific temperature T1 is between 1100 and 1500°C.

[0017] Prioritized, in step b, the specific time t1 is between 2 and 72 hours.

[0018] Preferably, in step e, the specific temperature T2 is between 1600 and 1650°C.

[0019] Preferably, in step e, the specific time t2 is no longer than 0.2 hours.

[0020] Preferably, in step d, the roughness of the prepared chromium oxide coating is Ra≦1.0μm and the thickness is between a few micrometers and tens of micrometers.

[0021] Preferably, in step b, the larger the specific temperature T1, the smaller the specific time t1.

[0022] Preferably, in step e, during heating, the heating rate within the temperature range above 1100℃ is not less than 100℃ / hour; during cooling, the cooling rate within the temperature range above 1300℃ is not less than 100℃ / hour. Attached Figure Description

[0023] Figure 1 This is a temperature control curve for step b of Example 1.

[0024] Figure 2 This is a temperature control curve for step b of Example 2. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] Because chromium oxide (chromium trioxide) and alumina have similar crystal structures, both belonging to the hexagonal crystal system, and the radii of chromium ions and aluminum ions are very close, when alumina ceramics are embedded in chromium oxide powder and heated at high temperatures, chromium oxide easily recrystallizes on the surface of the alumina ceramics to form a solid chromium oxide coating. The heating temperature and time must be controlled to control the quality of the chromium oxide coating and production costs. First, the temperature should not be too high, mainly for two reasons: (1) too high a temperature will lead to an excessively high recrystallization rate, which will reduce the density of the chromium oxide coating; (2) too high a temperature will lead to a large amount of chromium oxide powder sintering with the alumina ceramics, increasing the difficulty of treating the chromium oxide powder floating on the surface of the alumina ceramics. Second, the temperature should not be too low either, otherwise it will lead to a reduced recrystallization rate and increase production costs. In addition, according to the principle of reaction kinetics, the recrystallization rate of chromium oxide on the surface of alumina ceramics increases with the increase of temperature. Therefore, when the temperature is high, the holding time can be appropriately reduced, otherwise the thickness of the chromium oxide coating will be too large, affecting the dimensional accuracy of the ceramic parts. The particle size of the chromium oxide powder used must also be controlled to ensure the quality of the chromium oxide coating. The particle size should not be too large, otherwise it will lead to uneven contact between the chromium oxide powder and the alumina ceramic surface, thus affecting the quality of the chromium oxide coating. The particle size should not be too small, otherwise it will reduce the flowability of the chromium oxide powder, resulting in insufficient coverage of the alumina ceramic parts, thus affecting the quality of the chromium oxide coating. Experiments show that a particle size of 100–400 mesh for chromium oxide powder is suitable.

[0027] After the alumina ceramic parts are heat-treated by being embedded in chromium oxide powder, a solid chromium oxide coating will form on their surface. However, floating powder and particles that are difficult to remove will remain on the outside of the chromium oxide coating. Therefore, after the alumina ceramic parts are removed, the floating chromium oxide powder covering the surface of the alumina ceramic parts is first removed by blowing or other methods. Then, the alumina ceramic parts are polished by vibration finishing to remove the chromium oxide particles adhering to the surface of the alumina ceramic parts and improve the surface smoothness.

[0028] The treated alumina ceramic parts still need to undergo a secondary heating treatment at a temperature equal to the sintering temperature of the alumina ceramic parts. The main purposes are twofold: (1) to further improve the bonding strength between the chromium oxide coating and the alumina ceramic; and (2) to eliminate the phase transformation of the alumina ceramic caused by the first heating process. The secondary heating treatment should not be too long, otherwise it will have a significant impact on the alumina matrix of the alumina ceramic, such as grain growth, leading to a decrease in the mechanical strength, dielectric breakdown strength, and metallization strength of the alumina ceramic.

[0029] Based on the above analysis of principles, the method described in this invention includes the following steps:

[0030] a. Place the chromium oxide powder and alumina ceramic parts in a high-temperature oxidation-resistant container, such as an alumina ceramic crucible. Completely cover the alumina ceramic parts with chromium oxide powder. Vibrate the container to ensure that the chromium oxide powder fully and compactly covers the alumina ceramic parts.

[0031] b. The alumina ceramic part to be covered with chromium oxide powder is placed in a furnace, heated to a specific temperature T1, and held at that temperature for a specific time t1, then cooled. In this step, the higher the specific temperature T1, the shorter the specific time t1. When the specific temperature T1 is high, the specific time t1 should be appropriately reduced. In this invention, the settings of the specific temperature T1 and the specific time t1 can be referred to the following table:

[0032] <![CDATA[T1(℃)]]> 1100 1150 1200 1250 1300 <![CDATA[Range of t1 (hours)]]> 36～72 24～48 15～30 10～20 7～15 <![CDATA[T1(℃)]]> 1350 1400 1450 1500 — <![CDATA[Range of t1 (hours)]]> 5～10 3～7 2～5 2～4 —

[0033] In addition, when the heating and cooling times are long in the temperature range above 1100℃, the specific time t1 should be appropriately reduced.

[0034] c. After the furnace has cooled down, remove the alumina ceramic parts and remove the chromium oxide powder covering the surface of the alumina ceramic parts;

[0035] d. Vibration polishing is used to polish alumina ceramic parts to remove chromium oxide particles adhering to the surface of alumina ceramic parts and improve surface smoothness;

[0036] e. Place the treated alumina ceramic parts in a furnace and heat them to a specific temperature T2. During heating, the heating rate in the temperature range above 1100℃ should not be less than 100℃ / hour. Hold the temperature for a specific time t2, which should not exceed 0.2 hours. Then cool down, and the cooling rate should not be too low, especially in the temperature range above 1300℃, the cooling rate should not be less than 100℃ / hour.

[0037] f. After the furnace cools down, the alumina ceramic parts are removed, completing the preparation of the chromium oxide coating on the surface of the alumina ceramic parts.

[0038] Example 1

[0039] The alumina ceramic is a 95% alumina ceramic (chemical composition percentage: 94.5% Al2O3, 2.2% SiO2, 3.3% CaO and trace impurities) disc. The alumina ceramic part has a diameter of 30mm and a thickness of 3mm.

[0040] The particle size of the chromium oxide powder is 200 mesh.

[0041] The method in this embodiment includes the following steps:

[0042] a. Place the chromium oxide powder and alumina ceramic parts in a high-temperature oxidation-resistant container, such as an alumina ceramic crucible. Completely cover the alumina ceramic parts with chromium oxide powder. Vibrate the container to ensure that the chromium oxide powder fully and compactly covers the alumina ceramic parts.

[0043] b. Place the alumina ceramic parts covered with chromium oxide powder into a furnace, heat to 1300±10℃, and hold at that temperature for 12 hours. The specific temperature control principle in step b of this embodiment is as follows: Figure 1 As shown.

[0044] c. After the furnace has cooled down, remove the alumina ceramic parts and remove the chromium oxide powder covering the surface of the alumina ceramic parts;

[0045] d. After removing the chromium oxide powder covering the surface of the alumina ceramic parts, the alumina ceramic parts are polished by vibration finishing to remove the chromium oxide particles adhering to the surface of the alumina ceramic parts and improve the smoothness; thus, an alumina ceramic part with a dense and uniform chromium oxide recrystallized coating is obtained, and the thickness of the chromium oxide coating is between 10 and 20 micrometers.

[0046] e. Place the treated alumina ceramic parts in a furnace and heat them to a specific temperature of 1600°C at a heating rate of 160°C / hour, hold at that temperature for 15 minutes, and then begin cooling. The average cooling rate is approximately 250°C / hour, starting at 1300°C. Since the prolonged holding time in step b can cause a phase transformation in the alumina ceramic, resulting in the crystallization of the intergranular glass phase, this embodiment employs a secondary heating treatment in step e until this phase transformation is eliminated.

[0047] f. After the furnace cools down, the alumina ceramic parts are removed, completing the preparation of the chromium oxide coating on the surface of the alumina ceramic parts.

[0048] In this embodiment, the surface roughness of the alumina ceramic part and the alumina ceramic part with chromium oxide coating prepared according to the method of the present invention are Ra = 1.125 μm and Ra = 0.937 μm, respectively.

[0049] The surface pressure resistance of the alumina ceramic parts and the alumina ceramic parts with chromium oxide coating prepared according to the present invention are 5.2 kV / mm and 10.3 kV / mm, respectively. It can be seen that the chromium oxide coating improves the surface pressure resistance of the alumina ceramic parts.

[0050] Example 2

[0051] The alumina ceramic is a 99% alumina ceramic (chemical composition percentage: 99.5% Al2O3, a small amount of Y2O3 and trace impurities) disc. The alumina ceramic part has a diameter of 30mm and a thickness of 3mm.

[0052] The particle size of the chromium oxide powder is 300 mesh.

[0053] a. Place the chromium oxide powder and alumina ceramic parts in a high-temperature oxidation-resistant container, such as an alumina ceramic crucible. Completely cover the alumina ceramic parts with chromium oxide powder. Vibrate the container to ensure that the chromium oxide powder fully and compactly covers the alumina ceramic parts.

[0054] b. Place the alumina ceramic parts covered with chromium oxide powder into a furnace, heat to 1450±10℃, and hold at that temperature for 4 hours. The specific temperature control principle in step b of this embodiment is as follows: Figure 2 As shown.

[0055] c. After the furnace has cooled down, remove the alumina ceramic parts and remove the chromium oxide powder covering the surface of the alumina ceramic parts;

[0056] d. After removing the chromium oxide powder covering the surface of the alumina ceramic parts, the alumina ceramic parts are polished by vibration finishing to remove the chromium oxide particles adhering to the surface of the alumina ceramic parts and improve the smoothness; thus, alumina ceramic parts with a dense and uniform chromium oxide recrystallized coating on the surface are obtained, and the thickness of the chromium oxide coating is between 20 and 30 micrometers.

[0057] e. Place the treated alumina ceramic parts in a furnace and heat them to a specific temperature of 1620°C at a heating rate of 162°C / hour, hold at that temperature for 15 minutes, and then begin cooling. The average cooling rate is approximately 250°C / hour. Since the prolonged holding time in step b can cause a phase transformation in the alumina ceramic, resulting in the crystallization of the intergranular glass phase, this embodiment employs a secondary heating treatment in step e until this phase transformation is eliminated.

[0058] f. After the furnace cools down, the alumina ceramic parts are removed, completing the preparation of the chromium oxide coating on the surface of the alumina ceramic parts.

[0059] The surface roughnesses of the alumina ceramic parts and the alumina ceramic parts with chromium oxide coating prepared according to the present invention are Ra = 1.357 μm and Ra = 0.894 μm, respectively.

[0060] The surface pressure resistance of the alumina ceramic parts and the alumina ceramic parts with chromium oxide coating prepared according to the present invention are 4.3 kV / mm and 10.7 kV / mm, respectively. It can be seen that the chromium oxide coating improves the surface pressure resistance of the alumina ceramic parts.

Claims

1. A method for preparing a surface coating on an alumina ceramic part, characterized in that, The method includes the following steps: a. Place chromium oxide powder and alumina ceramic parts in a high-temperature oxidation resistant container, completely cover the alumina ceramic parts with chromium oxide powder, and vibrate the container to ensure that the chromium oxide powder fully covers the alumina ceramic parts. b. Place the alumina ceramic parts covered with chromium oxide powder in a furnace, heat to a specific temperature T1, hold at that temperature for a specific time t1, and then cool down; the specific temperature T1 is between 1100 and 1500℃; the specific time t1 is between 2 and 72 hours. c. After the furnace has cooled down, remove the alumina ceramic parts and remove the chromium oxide powder covering the surface of the alumina ceramic parts; d. Polish the alumina ceramic parts to remove chromium oxide particles adhering to the surface of the alumina ceramic parts; e. Place the alumina ceramic parts in the furnace, heat them to a specific temperature T2, hold them at that temperature for a specific time t2, and then cool them down; the specific temperature T2 is between 1600 and 1650℃; the specific time t2 is no longer than 0.2 hours; f. After the furnace has cooled down, remove the alumina ceramic parts.

2. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that, In step a, the particle size of the chromium oxide powder is 100~400 mesh, and the mass percentage of alumina in the alumina ceramic parts is 85%~99%.

3. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that, In step d, the roughness of the prepared chromium oxide coating is Ra≦1.0μm and the thickness is between a few micrometers and tens of micrometers.

4. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that, In step b, the higher the specific temperature T1, the lower the specific time t1.

5. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that, In step e, when heating, the heating rate in the temperature range above 1100℃ shall not be less than 100℃ / hour; when cooling, the cooling rate in the temperature range above 1300℃ shall not be less than 100℃ / hour.

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