Preparation method of aluminum oxide ceramic part surface coating

Abstract

The invention discloses a preparation method of an aluminum oxide ceramic part surface coating, which comprises the following steps: a, completely covering an aluminum oxide ceramic part, and carrying out vibration treatment on a container, so that chromium oxide powder fully covers the aluminum oxide ceramic part; b, the aluminum oxide ceramic part covered with the chromic oxide powder is placed in a furnace and heated to the specific temperature T1, heat preservation is conducted for the specific time t1, and then cooling is conducted; c, after the aluminum oxide ceramic part is cooled in the furnace, taking out the aluminum oxide ceramic part, and removing chromium oxide floating powder covering the surface of the aluminum oxide ceramic part; d, the aluminum oxide ceramic part is polished, and chromic oxide particles attached to the surface of the aluminum oxide ceramic part are removed; e, the aluminum oxide ceramic part is placed in a furnace and heated to a specific temperature T2, heat preservation is conducted for a specific time t2, and then cooling is conducted; and f, taking out the aluminum oxide ceramic part after the aluminum oxide ceramic part is cooled in the furnace. The method is easy and convenient to operate, excellent in quality and capable of being used for preparing the chromium oxide coating on the surface of the complex-structure aluminum oxide ceramic part.

Description

Technical Field

[0001] The invention belongs to the field of ceramic coating preparation, and relates to a method for preparing a surface coating of an alumina ceramic part. Background Art

[0002] Alumina ceramics have the advantages of high mechanical strength, high resistivity, low dielectric loss, high dielectric breakdown strength, high temperature resistance, and high thermal conductivity. They are excellent insulating materials and can be sealed with metals through metallization processes. They are widely used in the manufacture of insulating tube shells for vacuum devices. In vacuum devices, insulating supports are always inevitably introduced to form parallel composite insulation of vacuum-solid dielectrics. It should be pointed out that vacuum itself has a very high dielectric strength, and its critical breakdown field strength can reach 350kV / cm. At the same time, the breakdown field strength of alumina ceramics is as high as 300-400kV / cm. The vacuum alumina composite insulation usually flashes along the surface under an applied electric field of tens of kV / cm (when the vacuum degree is 10-3Pa), resulting in system insulation failure and device failure, that is, the flashover voltage is much lower than the breakdown voltage of the vacuum gap of the same geometric length or the ceramic material itself. Flashover along the surface of the tube shell is an important factor leading to the failure of vacuum devices. Therefore, improving the surface flashover voltage of alumina ceramics is of great significance to improving the reliability of vacuum devices.

[0003] According to the secondary electron emission avalanche (SEEA) theory of 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) by adding chromium oxide to the ceramic raw materials, a certain proportion of chromium oxide is doped into the alumina ceramic matrix to reduce the secondary electron emission coefficient to improve the surface withstand voltage; (2) a chromium oxide coating with a low secondary electron emission coefficient is prepared on the surface of alumina ceramics to improve the surface withstand voltage. When the method of doping chromium oxide is used to improve the surface withstand voltage of alumina ceramics, the surface withstand voltage often increases with the increase of the chromium oxide doping amount. However, when the content of chromium oxide doped in the alumina matrix is ​​high, the chromium oxide doped in the matrix will have a great influence on the properties of alumina ceramics such as resistivity, dielectric constant, dielectric loss, bulk withstand voltage, metallization performance, etc., which seriously affects the use of alumina ceramics. Therefore, the doping amount of chromium oxide in the alumina ceramic matrix is ​​often not too high, which limits the potential of using the method of doping chromium oxide to improve the surface withstand voltage performance of alumina ceramics. Considering that the occurrence process of flashover (surface breakdown) is mainly related to the surface properties of the material, preparing a layer of chromium oxide coating of several microns to tens of microns on the surface of alumina ceramics can effectively reduce the secondary electron generation coefficient of alumina ceramics without changing the performance of the alumina ceramic matrix material, thereby improving the surface withstand voltage performance. Therefore, in recent years, more and more work has been devoted to seeking a cost-effective method for coating chromium oxide coatings on the surface of alumina ceramic parts.

[0004] At present, the methods for preparing chromium oxide coating on the surface of alumina ceramic parts in the industry mainly include dipping, slurry coating, plasma spraying, etc. At present, the dipping method has two main disadvantages: (1) The thickness of the prepared chromium oxide coating is very small, and if you want to use this method to prepare a thicker chromium oxide coating on the surface of alumina ceramic parts, you often need to repeatedly dip, dry and bake; (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 surface of the alumina ceramic, and the bonding between the chromium oxide coating and the alumina ceramic is mainly physical adsorption. If the baking temperature in this method is increased, the chromium oxide will easily diffuse into the alumina, and the chromium oxide coating cannot be formed. There are three main disadvantages of the slurry coating method: (1) It is difficult to operate complex structure ceramic parts, and deep holes, folds and other locations cannot be fully coated with chromium oxide slurry; (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 the alumina ceramic using this method often cracks and peels off during use; (3) The chromium oxide coating prepared on the surface of the alumina ceramic parts using this method is not only thick, but also uneven in thickness, which seriously affects the size qualification rate of the alumina ceramic parts. The disadvantage of plasma spraying is that it cannot be used to prepare surface chromium oxide coatings on alumina ceramic parts with complex structures. In addition, the process flow of the immersion method, slurry coating method, and plasma spraying method are relatively long and the operation is relatively complicated. Therefore, they have not been used on a large scale in production. At present, the manufacture of vacuum device 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, has excellent quality, and can be used to prepare chromium oxide coatings on the surfaces of alumina ceramic parts with complex structures. Summary of the invention

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

[0007] To achieve this object, the present invention adopts the following technical solutions:

[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 the alumina ceramic parts in a high temperature oxidation resistant container such as an alumina ceramic crucible, completely cover the alumina ceramic parts with the chromium oxide powder, and vibrate the container so that the chromium oxide powder covers the alumina ceramic parts fully and tightly;

[0010] b. Placing the alumina ceramic part covered with chromium oxide powder in a furnace, heating it to a specific temperature T1, keeping it at a specific time t1, and then cooling it down;

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

[0012] d. Use vibration finishing to polish the alumina ceramic parts to remove the chromium oxide particles adhering to the surface of the alumina ceramic parts and improve the surface finish;

[0013] e. placing the treated alumina ceramic parts in a furnace, heating them to a specific temperature T2, keeping them at a specific time t2, and then cooling them down;

[0014] f. After the furnace cools down, take out the alumina ceramic parts.

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

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

[0017] Preferably, 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≦1.0 μm, and the thickness is between several microns and tens of microns.

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

[0022] Preferably, in step e, when heating, the heating rate in the temperature range above 1100°C is not less than 100°C / hour; when cooling, the cooling rate in the temperature range above 1300°C is not less than 100°C / hour. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the temperature control curve diagram of step b of Example 1.

[0024] Figure 2 This is the temperature control curve diagram of step b of Example 2. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0026] Since chromium oxide (chromium trioxide) and aluminum oxide have similar crystal structures and both belong to the hexagonal system, and the radius of chromium ions is very close to that of aluminum ions, when aluminum oxide ceramics are buried in chromium oxide powder and heated at high temperature, chromium oxide is very easy to recrystallize on the surface of aluminum oxide ceramics to form a solid chromium oxide coating. The heating temperature and time must be controlled to control the quality and production cost of the chromium oxide coating. First, the temperature should not be too high for two main reasons: (1) Too high a temperature will lead to too high a recrystallization rate, which will reduce the density of the chromium oxide coating; (2) Too high a temperature will cause a large amount of chromium oxide powder to sinter with the aluminum oxide ceramic, increasing the difficulty of handling the floating chromium oxide powder on the surface of the aluminum oxide ceramic. Secondly, the temperature should not be too low, otherwise it will lead to a decrease in the recrystallization rate and increase the production cost. In addition, according to the principle of reaction kinetics, the rate of chromium oxide recrystallization on the surface of aluminum oxide 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 control the quality of the chromium oxide coating. The particle size of the chromium oxide powder should not be too high, otherwise it will cause uneven contact between the chromium oxide powder and the surface of the alumina ceramic, thus affecting the quality of the chromium oxide coating. The particle size of the chromium oxide powder should not be too low, otherwise it will cause the fluidity of the chromium oxide powder to decrease, making the chromium oxide powder insufficiently cover the alumina ceramic parts, thus affecting the quality of the chromium oxide coating. Experiments show that the particle size of the chromium oxide powder is suitable between 100 and 400 mesh.

[0027] After the alumina ceramic parts are buried in chromium oxide powder and heated, not only a solid chromium oxide coating will be formed on their surface, but also floating powder and particles that are difficult to remove will exist outside the chromium oxide coating. Therefore, after the alumina ceramic parts are taken out, the floating chromium oxide powder covering the surface of the alumina ceramic parts is firstly removed by blowing or other methods, and 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 finish.

[0028] The treated alumina ceramic parts need to be heated for a second time, and the heating temperature is equal to the sintering temperature of the alumina ceramic parts. There are two main purposes: (1) to further improve the bonding strength between the chromium oxide coating and the alumina ceramic; (2) to eliminate the phase change of the alumina ceramic caused by the first heating process. The time of the second 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, resulting in a decrease in the mechanical strength, dielectric breakdown strength, metallization strength, etc. of the alumina ceramic.

[0029] According to the analysis of the above principles, the method of the present invention comprises the following steps:

[0030] a. Place the chromium oxide powder and the alumina ceramic parts in a high temperature oxidation resistant container such as an alumina ceramic crucible, completely cover the alumina ceramic parts with the chromium oxide powder, and vibrate the container so that the chromium oxide powder covers the alumina ceramic parts fully and tightly;

[0031] b. Place the alumina ceramic part covered with chromium oxide powder in a furnace, heat it to a specific temperature T1, keep it warm for a specific time t1, and then cool it down. In this step, the larger the specific temperature T1, the smaller the specific time t1. When the specific temperature T1 is high, the specific time t1 should be appropriately reduced. In the present invention, the setting of the specific temperature T1 and the specific time t1 can refer 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 time and cooling time in the temperature range above 1100°C are long, the specific time t1 is appropriately reduced;

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

[0035] d. Use vibration finishing to polish the alumina ceramic parts to remove the chromium oxide particles adhering to the surface of the alumina ceramic parts and improve the surface finish;

[0036] e. Place the treated alumina ceramic parts in the furnace, heat them up to a specific temperature T2. When heating, the heating rate in the temperature range above 1100°C shall not be less than 100°C / hour. Keep them warm for a specific time t2, which shall not be longer than 0.2 hours. Then cool them down. When cooling, the cooling rate cannot be too low, especially in the temperature range above 1300°C, the cooling rate shall not be less than 100°C / hour;

[0037] f. After the furnace is cooled, the alumina ceramic parts are taken out, and the preparation of the chromium oxide coating on the surface of the alumina ceramic parts is completed.

[0038] Example 1

[0039] Alumina ceramics use a 95 alumina ceramic (chemical composition percentage: 94.5% Al2O3, 2.2% SiO2, 3.3% CaO and trace impurities) disc, the size of the alumina ceramic part is 30mm in diameter and 3mm in thickness

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

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

[0042] a. Place the chromium oxide powder and the alumina ceramic parts in a high temperature oxidation resistant container such as an alumina ceramic crucible, completely cover the alumina ceramic parts with the chromium oxide powder, and vibrate the container so that the chromium oxide powder covers the alumina ceramic parts fully and tightly;

[0043] b. Place the alumina ceramic part covered with chromium oxide powder in a furnace, heat it to 1300±10°C, and keep it warm for 12 hours. The temperature control rule of step b of this embodiment is as follows: Figure 1 shown.

[0044] c. After the furnace cools down, take out 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 floating 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 finish; thereby, the alumina ceramic parts having a dense and uniform chromium oxide recrystallized coating formed on the surface are obtained, and the thickness of the chromium oxide coating is between 10 and 20 microns.

[0046] e. Place the treated alumina ceramic parts in a furnace, heat to a specific temperature of 1600°C at a heating rate of 160°C / hour, and keep the temperature for 15 minutes before cooling down. The average cooling rate at 1300°C is about 250°C / hour. Since the long-term heat preservation in step b will cause the alumina ceramic to undergo phase change and crystallization of the intercrystalline glass phase, this embodiment performs a secondary heating treatment in step e until the phase change is eliminated;

[0047] f. After the furnace is cooled, the alumina ceramic parts are taken out, and the preparation of the chromium oxide coating on the surface of the alumina ceramic parts is completed.

[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 withstand voltage properties of the alumina ceramic parts and the alumina ceramic parts prepared with chromium oxide coating 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 withstand voltage properties of the alumina ceramic parts.

[0050] Example 2

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

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

[0053] a. Place the chromium oxide powder and the alumina ceramic parts in a high temperature oxidation resistant container such as an alumina ceramic crucible, completely cover the alumina ceramic parts with the chromium oxide powder, and vibrate the container so that the chromium oxide powder covers the alumina ceramic parts fully and tightly;

[0054] b. Place the alumina ceramic part covered with chromium oxide powder in a furnace, heat it to 1450±10°C, and keep it warm for 4 hours. The temperature control rule of step b of this embodiment is as follows: Figure 2 shown.

[0055] c. After the furnace cools down, take out 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 floating powder covering the surface of the alumina ceramic parts, the alumina ceramic parts are polished using vibration finishing to remove the chromium oxide particles adhering to the surface of the alumina ceramic parts and improve the finish; thereby, an alumina ceramic part having a dense and uniform chromium oxide recrystallized coating formed on the surface is obtained, and the thickness of the chromium oxide coating is between 20 and 30 microns.

[0057] e. Place the treated alumina ceramic parts in a furnace, heat to a specific temperature of 1620°C at a heating rate of 162°C / hour, and keep the temperature for 15 minutes before cooling down. The average cooling rate at 1300°C is about 250°C / hour. Since the long-term heat preservation in step b will cause the alumina ceramic to undergo phase change and crystallization of the intercrystalline glass phase, this embodiment performs a secondary heating treatment in step e until the phase change is eliminated;

[0058] f. After the furnace is cooled, the alumina ceramic parts are taken out, and the preparation of the chromium oxide coating on the surface of the alumina ceramic parts is completed.

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

[0060] The surface withstand voltage performance of the alumina ceramic part and the alumina ceramic part prepared with a chromium oxide coating 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 withstand voltage performance of the alumina ceramic part.

Claims

1. A method for preparing a surface coating of an alumina ceramic part, characterized in that: The method comprises the following steps: a. Place the chromium oxide powder and the alumina ceramic parts in a container resistant to high temperature oxidation, completely cover the alumina ceramic parts with the chromium oxide powder, and vibrate the container to ensure that the chromium oxide powder fully covers the alumina ceramic parts; b. Placing the alumina ceramic part covered with chromium oxide powder in a furnace, heating it to a specific temperature T1, keeping it at a specific time t1, and then cooling it down; c. After the furnace cools down, take out the alumina ceramic parts and remove the chromium oxide powder covering the surface of the alumina ceramic parts; d. Polishing the alumina ceramic parts to remove chromium oxide particles adhering to the surface of the alumina ceramic parts; e. Placing the alumina ceramic part in the furnace, heating it to a specific temperature T2, keeping it at a specific time t2, and then cooling it down; f. After the furnace cools down, take out the alumina ceramic parts.

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

3. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that: In step b, the specific temperature T1 is between 1100 and 1500°C.

4. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that: In step b, the specific time t1 is between 2 and 72 hours.

5. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that: In step e, the specific temperature T2 is between 1600 and 1650°C.

6. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that: In step e, the specific time t2 is not longer than 0.2 hours.

7. The method for preparing the surface coating of an alumina ceramic part according to claim 1, characterized in that: In step d, the roughness of the chromium oxide coating is Ra≦1.0 μm, and the thickness is between several microns and tens of microns.

8. The method for preparing the surface coating of alumina ceramic parts according to claim 1, characterized in that: In the step b, the greater the specific temperature T1 is, the smaller the specific time t1 is.

9. The method for preparing the surface coating of an alumina ceramic part according to claim 1, characterized in that: In the step e, when heating, the heating rate in the temperature range above 1100°C is not less than 100°C / hour; when cooling, the cooling rate in the temperature range above 1300°C is not less than 100°C / hour.

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