Ceramic bond and method for preparing diamond grinding wheel by using same

Abstract

The invention discloses a ceramic bond which comprises the following raw materials in parts by weight: 30-50 parts of silicon dioxide, 15-30 parts of boric acid, 4-6 parts of aluminum oxide, 4-8 parts of barium oxide, 5-10 parts of aluminum oxide, 5-10 parts of zinc oxide, 4-8 parts of sodium carbonate, 1-5 parts of calcium carbonate, 1-5 parts of magnesium oxide and 1-10 parts of lithium carbonate. The invention further discloses a diamond grinding wheel prepared from the ceramic bond. The diamond grinding wheel is prepared from the following raw materials in percentage by weight: 10-20% of the ceramic bond, 50-60% of diamond micro powder, 5-10% of white corundum, 10-20% of a pore forming agent and 5-15% of resin powder. The ceramic bond is low in firing temperature, tightly combined with diamond micro-powder and high in holding force, the porosity of the prepared diamond grinding wheel reaches 70%, the abrasive ratio is low, and meanwhile the stability is high in the using process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of grinding wheel preparation, in particular to a ceramic binder and a method for preparing a diamond grinding wheel using the same. BACKGROUND

[0002] In recent years, silicon carbide substrates have been increasingly widely used in the semiconductor industry due to their advantages of wider band gap, higher breakdown field strength, greater thermal conductivity, and high-voltage resistance.

[0003] The extremely high hardness and brittleness of silicon carbide substrates make it necessary to rely on ultra-precision grinding of diamond grinding wheels for thinning processing. Currently, diamond grinding wheels for thinning silicon carbide substrates are mainly prepared by mixing ceramic binders, diamond and auxiliary materials, cold pressing, low-temperature curing, and then sintering. Among them, sintering is performed at a temperature of 700-800 degrees for 1-3 hours. However, the diamond grit used for silicon carbide thinning grinding wheels is relatively fine, usually 2000 to 8000 mesh diamond powder, and the carbonization rate of diamond powder above 700 degrees rises sharply, which will seriously affect the performance and stability of the grinding wheel. SUMMARY

[0004] In view of the above-mentioned deficiencies in the prior art, the present application provides a ceramic binder and a method for preparing a diamond grinding wheel using the same. The ceramic binder has a low sintering temperature, is tightly combined with diamond powder, has strong holding force, and the prepared diamond grinding wheel has a porosity of 70%, low wear ratio, and high stability during use.

[0005] To achieve the above-mentioned purposes, the embodiments of the present application adopt the following technical solutions:

[0006] A ceramic binder comprises the following raw materials in parts by weight: 30-50 parts of silicon dioxide, 15-30 parts of boric acid, 4-6 parts of aluminum oxide, 4-8 parts of barium oxide, 5-10 parts of aluminum oxide, 5-10 parts of zinc oxide, 4-8 parts of sodium carbonate, 1-5 parts of calcium carbonate, 1-5 parts of magnesium oxide, and 1-10 parts of lithium carbonate.

[0007] The preparation method of the above-mentioned ceramic binder comprises:

[0008] The raw materials are proportioned and mixed uniformly to obtain a mixture;

[0009] The mixture is melted and cold extracted to obtain a vitrified material;

[0010] The vitrified material is ball milled to obtain a slurry;

[0011] The slurry is dried, sieved, and crushed to obtain a ceramic binder.

[0012] According to one aspect of the present application, the ceramic binder has a particle size of 1-5 microns.

[0013] According to one aspect of the present application, the melting is performed at a temperature of 1000-1300 degrees Celsius.

[0014] According to one aspect of the present application, the ball milling of the vitrified material comprises: ball milling the vitrified material with water, the mass ratio of the vitrified material to water being 1:1.

[0015] A diamond grinding wheel made of the ceramic binder, which is made of raw materials in the following weight percentages: ceramic binder 10-20%, diamond micro-powder 50-60%, white corundum 5-10%, pore-forming agent 10-20%, and resin powder 5-15%.

[0016] According to one aspect of the present application, the pore-forming agent is polymethyl methacrylate, and the particle size of the pore-forming agent is 10-50 microns.

[0017] According to one aspect of the present application, the resin powder is a low-temperature thermosetting resin powder, and the low-temperature thermosetting resin powder is one of phenolic resin, polyimide resin, or epoxy resin.

[0018] According to one aspect of the present application, the diamond micro-powder is 1000-5000 mesh.

[0019] A method for preparing a diamond grinding wheel, comprising the following steps:

[0020] Mixing the raw materials in proportion, uniformly, and sieving, to obtain a mixture;

[0021] Placing the mixture into a mold for hot-pressing molding and sintering, to obtain a diamond grinding wheel.

[0022] According to one aspect of the present application, the hot-pressing temperature is 168-172 degrees Celsius, and the holding time is 20-60 minutes.

[0023] According to one aspect of the present application, the sintering comprises: heating the blank to 350 degrees Celsius at a heating rate of 4-5 degrees Celsius per minute under inert gas protection, and holding for 2 hours; heating the blank after holding to 500 degrees Celsius at a heating rate of 4-5 degrees Celsius per minute, and holding for 2 hours; and heating the blank after holding to 650 degrees Celsius at a heating rate of 4-5 degrees Celsius per minute, and holding for 1 hour.

[0024] According to one aspect of the present application, the inert gas is nitrogen.

[0025] Advantages of this invention: The ceramic binder is formed by compounding silicon dioxide, boric acid, aluminum oxide, barium oxide, zinc oxide, sodium carbonate, calcium carbonate, magnesium oxide, and lithium carbonate. Silicon dioxide, as the main raw material, provides structural strength and high-temperature resistance. Boric acid improves the mechanical properties of the binder. Aluminum oxide enhances high-temperature viscosity, hardness, and acid and alkali resistance. Sodium carbonate and lithium carbonate act as fluxes, lowering the sintering temperature. The introduction of zinc oxide, barium oxide, and calcium carbonate can regulate sintering characteristics and enhance the overall strength of the binder. Each component has a synergistic effect, ensuring the holding force of the binder and reducing the sintering temperature to 600-700℃. The ceramic binder, together with polymethyl methacrylate pore-forming agent, diamond micron powder, and resin powder, is used to prepare a diamond grinding wheel. A hot-pressing process is employed, resulting in a grinding wheel porosity of over 70%, which is beneficial to macroscopic structural strength. During silicon carbide substrate thinning, the grinding wheel wear ratio is reduced by 10.43%, and the grinding wheel maintains stable current during use, meeting customer requirements. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 Microscopic images of the structure of the diamond grinding wheel prepared in Example 1.

[0028] Figure 2 The image shows the finished product of the diamond grinding wheel prepared in Example 1. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] A ceramic binder comprises the following raw materials in parts by weight: 30-50 parts silicon dioxide, 15-30 parts boric acid, 4-6 parts aluminum oxide, 4-8 parts barium oxide, 5-10 parts aluminum oxide, 5-10 parts zinc oxide, 4-8 parts sodium carbonate, 1-5 parts calcium carbonate, 1-5 parts magnesium oxide, and 1-10 parts lithium carbonate. The preparation method of the ceramic binder includes:

[0031] Prepare the raw materials according to the specified proportions and mix them evenly to obtain a mixture.

[0032] The mixture is melted and cold-extracted to obtain vitrified material;

[0033] The vitrified material is wet-ball-milled to obtain a slurry;

[0034] The slurry is dried, sieved, and crushed to obtain ceramic binder powder.

[0035] In practical applications, the particle size of the ceramic binder is 1-5 micrometers.

[0036] In practical applications, the melting is carried out at a temperature of 1000-1300℃.

[0037] A diamond grinding wheel comprises the following raw materials in weight percentages: 10-20% ceramic binder, 50-60% diamond, 5-10% white corundum, 10-20% pore-forming agent, and 5-15% resin powder. The pore-forming agent is polymethyl methacrylate, and the particle size of the pore-forming agent is 10-50 μm. The resin powder is a low-temperature thermosetting resin powder, specifically phenolic resin, polyimide resin, or epoxy resin, without limitation. The diamond is 1000-5000 mesh diamond powder, preferably 2000 mesh diamond powder.

[0038] A method for preparing a diamond grinding wheel includes the following steps:

[0039] Mix the raw materials evenly according to the proportions, sieve them, put them into a mold, hot press them into shape, and sinter them to obtain a diamond grinding wheel.

[0040] In practical applications, the hot pressing temperature is 168-172 degrees Celsius, and the holding time is 20-60 minutes.

[0041] In practical applications, the sintering process includes: heating the billet to 350°C at a heating rate of 4-5°C / min under inert gas protection and holding it at that temperature for 2 hours; heating the billet to 500°C at a heating rate of 4-5°C / min and holding it at that temperature for 2 hours; and then heating the billet to 650°C at a heating rate of 4-5°C / min and holding it at that temperature for 1 hour.

[0042] In practical applications, the inert gas is one or more of nitrogen, helium, or argon.

[0043] In the following examples, unless otherwise specified, the raw materials used are analytically pure compounds, which are all commercially available products that can be purchased directly. Operations and procedures not mentioned in detail can be performed using conventional techniques in the field.

[0044] Example 1

[0045] 1. Preparation of ceramic binders:

[0046] 30 parts silicon dioxide, 15 parts boric acid, 4 parts aluminum oxide, 4 parts barium oxide, 5 parts aluminum oxide, 5 parts zinc oxide, 4 parts sodium carbonate, 1 part calcium carbonate, 1 part magnesium oxide, and 1 part lithium carbonate are mixed evenly to obtain a mixture. This mixture is then completely melted at 1000-1300 degrees Celsius and cold-extracted with cooling water to form a vitrified material. The vitrified material is then wet-milled into a slurry with water added, wherein the mass ratio of vitrified material to water is 1:1, the grinding media are zirconia balls with a diameter of 5-7 mm, and the ball-to-material ratio is 3:1. The water in the slurry is then dried at 140-160 degrees Celsius for 9-10 hours. The slurry is then crushed using a ball mill and passed through a 200-mesh sieve to obtain the undersize. This undersize is then passed through an air jet crusher to obtain a ceramic binder with a particle size of 1-5 micrometers.

[0047] 2. Preparation of ceramic diamond grinding wheels:

[0048] The following ingredients, by weight percentage: 15% ceramic binder, 55% 2000-mesh diamond powder, 5% white corundum, 15% polymethyl methacrylate pore-forming agent with a particle size of 10-50 μm, and 10% resin powder, are weighed and dry-mixed for 3-5 hours using a three-dimensional mixer, then sieved through a 100-mesh sieve. The mixed material is placed in a mold, and the mold is then placed on a hot press table with a nominal pressure of 2000 kN for hot pressing at a temperature of 168-172 degrees Celsius for 30 minutes. After demolding, a green body is obtained, which is then placed in a protective atmosphere furnace for sintering. The protective gas in the protective atmosphere furnace is nitrogen. During the sintering process, the temperature is first raised to 350°C at a rate of 4-5°C / min and held for 2 hours. After holding, the billet is raised to 500°C at a rate of 4-5°C / min and held for 2 hours. Then, the billet is raised to 650°C at a rate of 4-5°C / min and held for 1 hour to obtain diamond grinding wheel teeth. These diamond grinding wheel teeth are then bonded to grooves in an aluminum substrate at 2mm intervals. The groove depth in the aluminum substrate is 1-2mm, and the groove width is 3-6mm. After bonding, the inner and outer circular end faces are trimmed to obtain the diamond grinding wheel. A microscopic photograph of the microstructure of the diamond grinding wheel is shown below. Figure 1 As shown, the finished product image is as follows. Figure 2 As shown.

[0049] Example 2

[0050] 1. Preparation of ceramic binders:

[0051] 50 parts silica, 30 parts boric acid, 6 parts aluminum oxide, 8 parts barium oxide, 10 parts aluminum oxide, 10 parts zinc oxide, 8 parts sodium carbonate, 5 parts calcium carbonate, 5 parts magnesium oxide, and 10 parts lithium carbonate are mixed to obtain a mixture. This mixture is then completely melted at 1000-1300 degrees Celsius and cold-extracted with cooling water to form a vitrified material. The vitrified material is then wet-milled into a slurry with water added, wherein the mass ratio of vitrified material to water is 1:1, the grinding media are zirconia balls with a diameter of 5-7 mm, and the ball-to-material ratio is 3:1. The water in the slurry is then dried at 140-160 degrees Celsius for 9-10 hours. The slurry is then crushed using a ball mill and passed through a 200-mesh sieve to obtain the undersize. This undersize is then passed through an air jet crusher, and after further crushing, a ceramic binder is obtained with a particle size of 1-5 micrometers.

[0052] 2. Preparation of ceramic diamond grinding wheels:

[0053] The following ingredients, by weight percentage: 10% ceramic binder, 60% 2000-mesh diamond powder, 5% white corundum, 20% polymethyl methacrylate pore-forming agent with a particle size of 10-50 μm, and 5% resin powder, are weighed and dry-mixed for 3-5 hours using a three-dimensional mixer, then sieved through a 100-mesh sieve. The mixed material is placed in a mold, and the mold is then placed on a hot press table with a nominal pressure of 2000 kN for hot pressing at 168-172 degrees Celsius for 30 minutes. After demolding, a green body is obtained, which is then placed in a protective atmosphere furnace for sintering. The protective gas in the protective atmosphere furnace is nitrogen. During the sintering process, the temperature is first raised to 350°C at a rate of 4-5°C / min and held for 2 hours. After holding, the billet is raised to 500°C at a rate of 4-5°C / min and held for 2 hours. Then, the billet is raised to 650°C at a rate of 4-5°C / min and held for 1 hour to obtain diamond grinding wheel teeth. These diamond grinding wheel teeth are then bonded to grooves in an aluminum substrate at 2mm intervals. The grooves in the aluminum substrate are 1-2mm deep and 3-6mm wide. After bonding, the inner and outer circular end faces are trimmed to obtain the diamond grinding wheel.

[0054] Example 3

[0055] 1. Preparation of ceramic binders:

[0056] A mixture of 40 parts silica, 20 parts boric acid, 5 parts aluminum oxide, 6 parts barium oxide, 10 parts aluminum oxide, 8 parts zinc oxide, 7 parts sodium carbonate, 3 parts calcium carbonate, magnesium oxide, and 6 parts lithium carbonate is prepared. This mixture is then completely melted at 1000-1300 degrees Celsius and cold-extracted with cooling water to form a vitrified material. The vitrified material is then wet-milled into a slurry with water added, where the mass ratio of vitrified material to water is 1:1. The grinding media are zirconia balls with a diameter of 5-7 mm, and the ball-to-material ratio is 3:1. The water in the slurry is then dried at 140-160 degrees Celsius for 9-10 hours. The slurry is then crushed using a ball mill and passed through a 200-mesh sieve to obtain the undersize. This undersize is then passed through an air jet crusher to obtain a ceramic binder with a particle size of 1-5 micrometers.

[0057] 2. Preparation of ceramic diamond grinding wheels:

[0058] The following ingredients, by weight percentage: 20% ceramic binder, 50% 2000-mesh diamond powder, 10% white corundum, 10% polymethyl methacrylate pore-forming agent with a particle size of 10-50 μm, and 10% resin powder, are weighed and dry-mixed for 3-5 hours using a three-dimensional mixer, then sieved through a 100-mesh sieve. The mixed material is placed in a mold, and the mold is then placed on a hot press table with a nominal pressure of 2000 kN for hot pressing at 168-172 degrees Celsius for 30 minutes. After demolding, a preform is obtained, which is then placed in a protective atmosphere furnace for sintering. The protective gas in the protective atmosphere furnace is nitrogen. During the sintering process, the temperature is first raised to 350°C at a rate of 4-5°C / min and held for 2 hours. After holding, the billet is raised to 500°C at a rate of 4-5°C / min and held for 2 hours. Then, the billet is raised to 650°C at a rate of 4-5°C / min and held for 1 hour to obtain diamond grinding wheel teeth. These diamond grinding wheel teeth are then bonded to grooves in an aluminum substrate at 2mm intervals. The grooves in the aluminum substrate are 1-2mm deep and 3-6mm wide. After bonding, the inner and outer circular end faces are trimmed to obtain the diamond grinding wheel.

[0059] Comparative Example 1

[0060] The difference from Example 1 is that in the preparation of the ceramic binder, potassium carbonate is replaced with lithium carbonate, and the rest is the same as in Example 1.

[0061] Comparative Example 2

[0062] The difference from Example 1 is as follows: 2. The preparation of the ceramic diamond grinding wheel differs; all other aspects are the same as in Example 1. The specific preparation of the ceramic diamond grinding wheel is as follows:

[0063] The following ingredients, by weight percentage: 15% ceramic binder, 55% 2000-mesh diamond powder, 5% white corundum, 15% polymethyl methacrylate pore-forming agent with a particle size of 10-50 μm, and 10% resin powder, are weighed and dry-mixed for 3-5 hours using a three-dimensional mixer, then sieved through a 100-mesh sieve. The mixed material is placed in a mold, cold-pressed at a pressure of 15 MPa. After demolding, a green body is obtained, which is then placed in a protective atmosphere furnace for sintering. The protective gas in the protective atmosphere furnace is nitrogen. During the sintering process, the temperature is first raised to 350°C at a rate of 4-5°C / min and held for 2 hours. After holding, the billet is raised to 500°C at a rate of 4-5°C / min and held for 2 hours. Then, the billet is raised to 650°C at a rate of 4-5°C / min and held for 1 hour to obtain diamond grinding wheel teeth. These diamond grinding wheel teeth are then bonded to grooves in an aluminum substrate at 2mm intervals. The grooves in the aluminum substrate are 1-2mm deep and 3-6mm wide. After bonding, the inner and outer circular end faces are trimmed to obtain the diamond grinding wheel.

[0064] Comparative Example 3

[0065] The difference from Example 1 is that the sodium carbonate and lithium carbonate content in the ceramic binder exceeds the specified range; otherwise, they are the same as in Example 1.

[0066] Mix 30 parts silicon dioxide, 15 parts boric acid, 4 parts aluminum oxide, 4 parts barium oxide, 5 parts aluminum oxide, 5 parts zinc oxide, 10 parts sodium carbonate, 1 part calcium carbonate, 1 part magnesium oxide, and 11 parts lithium carbonate evenly to obtain a mixture.

[0067] Comparative Example 4

[0068] The difference from Example 1 is that barium oxide is replaced with zinc oxide in the preparation of the ceramic binder; all other aspects are the same as in Example 1.

[0069] Mix 30 parts silicon dioxide, 15 parts boric acid, 4 parts aluminum oxide, 5 parts aluminum oxide, 9 parts zinc oxide, 4 parts sodium carbonate, 1 part calcium carbonate, 1 part magnesium oxide, and 1 part lithium carbonate evenly to obtain a mixture.

[0070] Comparative Example 5

[0071] The difference from Example 1 is that sodium carbonate is replaced with lithium carbonate in the preparation of the ceramic binder; all other aspects are the same as in Example 1.

[0072] Mix 30 parts silicon dioxide, 15 parts boric acid, 4 parts aluminum oxide, 4 parts barium oxide, 5 parts aluminum oxide, 5 parts zinc oxide, 1 part calcium carbonate, 1 part magnesium oxide, and 5 parts lithium carbonate evenly to obtain a mixture.

[0073] Performance testing:

[0074] The ceramic-bonded diamond grinding wheels prepared in Examples 1, 2, and 3, and Comparative Examples 1, 2, 3, 4, and 5 were subjected to porosity testing and grinding performance tests. Specifically, a porosity analyzer was used to detect the porosity of the diamond grinding wheels.

[0075] The specific testing method for grinding performance is as follows: The grinding wheel is mounted on a single-station thinning machine. Taking the rough grinding of a 6-inch conductive silicon carbide wafer as an example, the spindle speed is set to 1750 rpm, the feed rate to 0.4 μm / s, and the feed depth to 40 μm. The silicon carbide wafer is then ground using the grinding wheel. A contact measuring instrument is used to detect the wear of the grinding wheel after grinding. The corresponding test results are shown in Table 1 below.

[0076] Table 1

[0077] Grinding wheel porosity (%) Wear ratio Current fluctuation Example 1 72％ 40％ 15A-17A Example 2 70％ 46％ 18A-20A Example 3 72％ 48％ 15A-17A Comparative Example 1 70％ 70％ 15A-17A Comparative Example 2 70％ 55％ 14A-20A Comparative Example 3 69％ 60％ 15A-17A Comparative Example 4 70％ 70％ 15A-17A Comparative Example 5 70％ 80％ 15A-17A

[0078] Advantages of this invention: The ceramic binder is formed by compounding silicon dioxide, boric acid, aluminum oxide, barium oxide, zinc oxide, sodium carbonate, calcium carbonate, magnesium oxide, and lithium carbonate. Silicon dioxide, as the main raw material, provides structural strength and high-temperature resistance. Boric acid improves the mechanical properties of the binder. Aluminum oxide enhances high-temperature viscosity, hardness, and acid and alkali resistance. Sodium carbonate and lithium carbonate act as fluxes, lowering the sintering temperature. The introduction of zinc oxide, barium oxide, and calcium carbonate can regulate sintering characteristics and enhance the overall strength of the binder. Each component has a synergistic effect, ensuring the holding force of the binder and reducing the sintering temperature to 600-700℃. The ceramic binder, together with polymethyl methacrylate pore-forming agent, diamond micron powder, and resin powder, is used to prepare a diamond grinding wheel. A hot-pressing process is employed, resulting in a grinding wheel porosity of over 70%, which is beneficial to macroscopic structural strength. During silicon carbide substrate thinning, the grinding wheel wear ratio is reduced by 10.43%, and the grinding wheel maintains stable current during use, meeting customer requirements.

[0079] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A ceramic bond characterized in that, The raw materials include the following components in parts by weight: 30-50 parts of silicon dioxide, 15-30 parts of boric acid, 4-6 parts of aluminum trioxide, 4-8 parts of barium oxide, 5-10 parts of aluminum oxide, 5-10 parts of zinc oxide, 4-8 parts of sodium carbonate, 1-5 parts of calcium carbonate, 1-5 parts of magnesium oxide, and 1-10 parts of lithium carbonate.

2. The method of claim 1, wherein the ceramic bond is prepared by a method comprising the steps of: The method comprises the following steps: ​ The raw materials are proportioned and mixed to obtain a mixture; The mixture is melted and cold extracted to obtain a vitrified material; The vitrified material is ball milled to obtain a slurry; The slurry is dried, sieved and crushed to obtain a ceramic binder.

3. The method of claim 2, wherein the ceramic bond is prepared by mixing the ceramic powder and the organic binder, and then drying and sintering the mixture. The ceramic binder has a particle size of 1-5 microns.

4. A diamond wheel made of the ceramic bond of claim 1, wherein The raw materials include the following components in percentage by weight: 10-20% of a ceramic binder, 50-60% of diamond, 5-10% of white corundum, 10-20% of a pore-forming agent, and 5-15% of a resin powder.

5. A diamond grinding wheel according to claim 4, wherein The pore-forming agent is polymethyl methacrylate, and the particle size of the pore-forming agent is 10-50 microns.

6. The diamond wheel of claim 4, wherein The resin powder is a low-temperature thermosetting resin powder.

7. A diamond grinding wheel according to claim 6, wherein The low-temperature thermosetting resin powder is one of phenolic resin, polyimide resin or epoxy resin.

8. A method of producing a diamond grinding wheel according to any one of claims 4 to 7, characterized in that, The method comprises the following steps: The raw materials are proportioned and mixed to obtain a mixture; The mixture is hot-pressed and sintered in a mold to obtain a diamond grinding wheel.

9. The method of claim 8, wherein the diamond wheel is prepared by the steps of: The hot-pressing temperature is 168-172 degrees.

10. The method of claim 8, wherein the diamond wheel is prepared by the steps of: The sintering comprises the following steps: under the protection of inert gas, the sintering is performed in stages, the first stage comprises heating to 350 degrees at a heating rate of 4-5 degrees per minute and keeping for 2 hours, the second stage comprises heating to 500 degrees at a heating rate of 4-5 degrees per minute and keeping for 2 hours, and the third stage comprises heating to 650 degrees at a heating rate of 4-5 degrees per minute and keeping for 1 hour.

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