A Composite Method of High-Strength Graphite and Si3N4 Ceramics and Its Application

Abstract

The present invention discloses a composite method of high-strength graphite and Si3N4 ceramics and its application. First, high-purity silicon nitride powder and sintering aids are ball-milled to obtain composite powder; then a connection activator containing silicon whiskers is prepared by high-energy ball milling; then the connection activator containing silicon whiskers is evenly sprayed on the surface of the inner cavity of high-strength graphite, and the composite powder is filled into the inner cavity of high-strength graphite; finally, an oscillating pressure is applied to the surface of the composite powder, and sintering is carried out under an inert atmosphere to achieve the composite of high-strength graphite and Si3N4 ceramics.

Description

Technical Field

[0001] The present invention belongs to the technical field of ceramic materials, and particularly relates to a composite method of high-strength graphite and Si3N4 ceramics and its application. Background Art

[0002] Silicon nitride ceramics have high thermal conductivity, low expansion coefficient, and excellent mechanical properties. They are widely used in semiconductor device substrates, and are also used to manufacture sleeves, bearings, etc. in high-temperature gas turbines, and are promoted in the field of automotive engines. In the prior art, agate tanks and ceramic tanks are mainly used to prepare silicon nitride composite powders. However, the costs of agate tanks and ceramic tanks are high, and the tanks are prone to cracking due to their high brittleness. Currently, there is an urgent need for a new material that can replace agate tanks and ceramic tanks.

[0003] High-strength graphite has characteristics such as high strength, high thermal conductivity, high density, high temperature resistance, and corrosion resistance, and also has relatively good machining properties. If high-strength graphite and silicon nitride ceramics can be compounded, the heat dissipation performance of the tank during the preparation of materials can be increased, and the Si3N4 ceramic tank can be effectively protected. How to combine the two and apply them to prepare a container that can replace agate tanks and ceramic tanks is particularly important. Summary of the Invention

[0004] In order to overcome the problems in the prior art, the present invention provides a composite method of high-strength graphite and Si3N4 ceramics and its application, realizing a tight and high-strength combination at the interface between Si3N4 and high-strength graphite, and preparing a composite part with Si3N4 ceramics as the inner lining and high-strength graphite as the outer layer, solving the problems of high price, high brittleness, and easy breakage of existing ceramic or agate tanks.

[0005] In order to solve the above technical problems, the technical solution proposed by the present invention is as follows:

[0006] The present invention provides a composite method of high-strength graphite and Si3N4 ceramics, including the following steps:

[0007] S1. Ball-mill high-purity silicon nitride powder and a sintering aid to obtain a composite powder.

[0008] S2. Prepare the connection activity containing silicon whiskers by high-energy ball milling. The connection activator containing silicon whiskers is a mixture of SiC whiskers and / or Si3N4 whiskers and nano-Si powder and / or nano-SiO2.

[0009] S3. Uniformly spray the connection activator containing silicon whiskers obtained in step S2 on the surface of the inner cavity of high-strength graphite, load the composite powder in step S1 into the inner cavity of high-strength graphite, apply a positive pressure on the surface of the composite powder, and then sinter in a nitrogen atmosphere.

[0010] In the present invention, silicon nitride, a connecting active agent containing silicon whiskers, is brought into contact with high-strength graphite. By means of an interfacial reaction, through the intermediate transition of silicide between Si3N4 and high-strength graphite, a reaction layer is formed. Specifically, Si reacts with C and N2 to form SiC and Si3N4 respectively, and the formation of the reaction layer promotes the connection. Meanwhile, whiskers are added to improve the connection strength, and SiO2 is added to promote the interfacial fusion during the sintering process, ultimately achieving an improvement in the bonding strength of the interface. The silicon nitride powder is sintered at high temperature to become silicon nitride ceramics, realizing the composite of high-strength graphite and Si3N4 ceramics.

[0011] In the present invention, the connecting active agent containing silicon whiskers contains whiskers and other substances, where the whiskers are selected from one or two of SiC whiskers or Si3N4 whiskers, and the other substances are selected from one or two of Si powder or nano-SiO2.

[0012] As an alternative embodiment, in the composite method provided by the present invention, the connecting active agent containing silicon whiskers is a mixture of Si, SiC whiskers, and nano-SiO2.

[0013] In the present invention, strengthening with whiskers can further improve the bonding strength of the interface.

[0014] As an alternative embodiment, in the composite method provided by the present invention, the connecting active agent containing silicon whiskers is a uniform and stable suspension, and the volume fraction of the connecting active agent containing silicon whiskers in the suspension is 10% - 30%.

[0015] In the present invention, the connecting active agent containing silicon whiskers is configured as a uniform and stable suspension, which is beneficial for uniformly spraying it on the roughened graphite surface.

[0016] As an alternative embodiment, in the composite method provided by the present invention, the method for preparing the suspension of the connecting active agent containing silicon whiskers is as follows: The connecting active agent containing silicon whiskers and alcohol are added to a ball mill, and a uniform and stable suspension is obtained after ball milling.

[0017] As an alternative embodiment, in the composite method provided by the present invention, the mass ratio of the high-purity silicon nitride powder to the sintering aid is (8.8 - 9):(1 - 1.2).

[0018] As an alternative embodiment, in the composite method provided by the present invention, the spraying thickness of the connecting active agent containing silicon whiskers is 0.1 - 0.5 mm.

[0019] As an alternative embodiment, in the composite method provided by the present invention, the sintering aid is selected from one or more of Al2O3, Y2O3, and MgO.

[0020] In the present invention, the role of the sintering aid is to promote the densification of Si3N4 ceramics during the sintering process.

[0021] As an alternative embodiment, in the composite method provided by the present invention, the sintering aid is selected as a mixture of Al2O3 and MgO.

[0022] As an alternative embodiment, in the composite method provided by the present invention, the sintering aid is selected as a mixture of Al2O3, Y2O3 and MgO, and the mass ratio of Al2O3, Y2O3 and MgO is 2:3:5.

[0023] In the present invention, controlling the mass ratio of Al2O3, Y2O3 and MgO to be 2:3:5 ensures the densification of the silicon nitride ceramic.

[0024] As an alternative embodiment, in the composite method provided by the present invention, the sintering process is completed in a sintering furnace, the sintering temperature is 1800 - 2000 °C, the sintering time is greater than 3 h, the applied positive pressure is 15 - 30 MPa, and the oscillating pressure is 0.5 - 10 MPa.

[0025] In the present invention, using the oscillating pressure can achieve high densification of high-strength graphite and Si3N4 bulk, and at the same time facilitate the bonding between silicon nitride and high-strength graphite. Controlling the sintering temperature to be 1800 - 2000 °C, a temperature lower than this range will reduce the densification of Si3N4 ceramics, and too high a temperature will result in poor bonding between graphite and Si3N4 ceramics.

[0026] Based on the same technical concept, the present invention also provides an application of the above composite method of high-strength graphite and Si3N4 ceramics in the preparation of graphite and Si3N4 ceramic composite parts.

[0027] As an alternative embodiment, in the application provided by the present invention, the method for preparing a graphite and Si3N4 ceramic composite part includes the following steps:

[0028] (1) Make a cavity from high-strength graphite, with an inwardly concave receiving cavity inside the cavity, and roughen the inner wall of the cavity.

[0029] (2) Spray a silicon-containing connecting active agent on the roughened inner wall of the high-strength graphite cavity in step (1), then load a composite powder composed of high-purity silicon nitride powder and a sintering aid into the inner cavity of the high-strength graphite cavity, and after applying pressure on the top of the composite powder, place the high-strength graphite cavity in a sintering furnace for oscillating hot pressing sintering to obtain a primary composite part.

[0030] (3) Process the excess graphite in the primary composite part to obtain a graphite and Si3N4 ceramic composite part.

[0031] In the present invention, an inwardly recessed accommodating cavity is provided inside the cavity. The specific shape of the accommodating cavity is not limited and may be a triangle, a trapezoid, etc. After the accommodating cavity is provided, the high-strength graphite and the silicon nitride powder are staggered to form a mosaic-like structure, thereby further realizing a close combination of the high-strength graphite and the silicon nitride powder.

[0032] As an optional embodiment, in the application provided by the present invention, in step (1), a mechanical processing method is used to process the graphite inner cavity to prepare an inwardly recessed receiving cavity.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) In the present invention, the graphite surface is roughened and then sprayed with a connecting activator, and then silicon nitride composite powder is added and calcined in an inert atmosphere under pressure to achieve a tight and high-strength interface bonding between Si3N4 ceramics and high-strength graphite.

[0035] (2) In the present invention, the high densification of Si3N4 ceramics is achieved by high-temperature oscillating hot pressing sintering of Si3N4 composite powder. At the same time, through the interface reaction sintering and structural design of Si3N4 ceramics and high-strength graphite, the close and high-strength bonding of Si3N4 and high-strength graphite interfaces is achieved, and a composite component with Si3N4 ceramic as the inner lining and high-strength graphite as the outer layer is prepared, which solves the problems of high price, high brittleness and easy breakage of existing ceramic or agate tanks. The density of Si3N4 ceramics in the prepared composite component is more than 97%, the bending strength of the joint reaches more than 20MPa, and the joint of the component is dense and seamless. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0037] Figure 1 Schematic diagram of the structure of the cross section of the graphite inner cavity;

[0038] Figure 2 It is a structural schematic diagram of the cross section of the graphite inner cavity after the indenter is placed;

[0039] Figure 3 This is a bonding interface diagram of the connection between the graphite and Si3N4 composite parts prepared in Example 1. DETAILED DESCRIPTION

[0040] For the convenience of understanding the present invention, the present invention will be described more comprehensively and in detail below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0041] Unless otherwise defined, all the technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0042] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in the present invention can be obtained through market purchase or can be prepared by existing methods.

[0043] Example 1

[0044] Preparation of a composite part of graphite and Si3N4 ceramic, specifically a powder-making tank body with an outer diameter of 100 mm and an inner diameter of 50 mm. The preparation method is as follows:

[0045] (1) Design and processing of the inner cavity structure of high-strength graphite: Process a graphite cavity with a diameter of 60 mm from high-strength graphite with an outer diameter of 100 mm, and use a grinder to grind 3 alternating trapezoidal structures on the inner cavity, and perform roughening treatment on the inner cavity surface with a grinding wheel.

[0046] (2) Composite of silicon nitride powder: Composite silicon nitride powder with a sintering aid, where Y2O3, Al2O3, and MgO in the sintering aid are combined in a ratio of 2:3:5. Ball-mill the silicon nitride powder and the sintering aid, where the mass ratio of silicon nitride is 90%, and the remainder is the sintering aid.

[0047] (3) Spraying of the graphite inner cavity binder: Place the spray gun into the processed graphite cavity, and spray a silicon-containing connecting active agent on the inner wall of the graphite, with a spraying thickness of 0.2 mm. The silicon-containing connecting active agent is composed of Si, SiC whiskers, and nano-SiO2, where the mass ratio of Si is 80%; the mass ratio of SiC whiskers is 15%, and the mass ratio of nano-SiO2 is 5%. After being ball-milled by a planetary ball mill in an alcohol medium, a uniform and stable suspension with a content of 30% is obtained.

[0048] (4) Load the silicon nitride composite powder prepared in (2) into the inner cavity of high-strength graphite, and then place a pressure head with a diameter of 50 mm. At the same time, fill the periphery of the pressure head with the composite silicon nitride powder.

[0049] (5) Place the above-mentioned mold filled with powder in a furnace, apply an oscillating pressure, and then vacuum pump and oscillating sinter to form a composite part. The sintering temperature is 1850 °C, the sintering time is 6 h, the sintering atmosphere is N2 atmosphere, the applied positive pressure is 20 MPa, and the oscillating pressure is 3 MPa.

[0050] (6) Composite part processing and treatment: Process the protruding part in the graphite mold, and process the stainless steel cover plate and the stress-bearing part to prepare a ceramic powder-making tank.

[0051] The schematic structural diagram of the cross-section of the graphite inner cavity prepared in the present invention is as Figure 1 shown, and the schematic structural diagram after putting in the pressing head is as Figure 2 shown, including a pressing head 1, a silicon-containing connecting activator 2, a composite powder of silicon nitride and a sintering aid 3, and a high-strength graphite cavity 4. In the graphite and Si3N4 composite part of the present invention, the density of the Si3N4 ceramic is more than 98.5%, the bending strength at the joint reaches 25 MPa, the joint of the workpiece is dense and seamless, and the joint interface is as Figure 3 shown.

[0052] Example 2

[0053] (1) Design and processing of the high-strength graphite inner cavity structure: Process a graphite cavity with a diameter of 140 mm from high-strength graphite with an outer diameter of 200 mm, and use a grinder to grind 5 alternating trapezoidal structures on the inner cavity, and perform roughening treatment on the inner cavity surface with a grinding wheel.

[0054] (2) Silicon nitride powder compounding: Compound silicon nitride powder with a sintering aid, in which Y2O3 and Al2O3 in the sintering aid are combined in a ratio of 2:3, and ball-mill the silicon nitride powder with the sintering aid, where the proportion of silicon nitride is 88%, and the rest is the sintering aid.

[0055] (3) Spraying of the graphite inner cavity connector: Place the spray gun into the processed graphite cavity, and spray a silicon-containing connecting activator on the inner wall of the graphite, with a spraying thickness of 0.3 mm. The silicon-containing connecting activator is composed of Si and SiC whiskers, where the mass proportion of Si is 85%; the mass proportion of SiC whiskers is 15%, and it is obtained as a uniform and stable suspension with a content of 20% after being ball-milled by a planetary ball mill in an alcohol medium.

[0056] (4) Load the silicon nitride composite powder prepared in (2) into the high-strength graphite inner cavity, put in the upper pressing head, the diameter of the pressing head is 120 mm, and at the same time, the surrounding of the pressing head is filled with the compounded silicon nitride powder.

[0057] (5) Place the above-mentioned mold filled with powder in the furnace, apply an oscillating pressure, and then make a composite part through vacuum pumping and oscillating sintering. Among them, the sintering temperature is 1880 °C, the sintering time is 7 h, the sintering atmosphere is N2 atmosphere, the applied positive pressure is 25 MPa, and the oscillating pressure is 5 MPa.

[0058] (6) Composite part processing and treatment: Process the protruding part in the graphite mold, and process the stainless steel cover plate and the stress-bearing part to prepare a ceramic powder-making tank.

[0059] The schematic structural diagram of the cross-section of the graphite inner cavity prepared in the present invention is as shown in Figure 1 and the schematic structural diagram after placing the punch is as shown in Figure 2 It includes a punch 1, a silicon-containing connecting activator 2, a composite powder of silicon nitride and a sintering aid 3, and a high-strength graphite cavity 4. For the graphite and Si3N4 composite prepared in the present invention, the density of the Si3N4 ceramic is more than 98%, the bending strength at the joint reaches 23 MPa, and the joint of the manufactured part is dense and seamless.

[0060] Example 3

[0061] (1) Design and processing of the high-strength graphite inner cavity structure: A high-strength graphite with an outer diameter of 200 mm is processed to form a graphite cavity with a diameter of 100 mm, and 5 alternating trapezoidal structures are polished on the inner cavity using a grinder, and the inner cavity surface is coarsened by grinding with a grinding wheel.

[0062] (2) Silicon nitride powder compounding: The silicon nitride powder is compounded with a sintering aid. The ratio of Y2O3, Al2O3, and MgO in the sintering aid is 2:3:5. The silicon nitride powder and the sintering aid are ball-milled, with silicon nitride accounting for 90% and the remaining being the sintering aid.

[0063] (3) Spraying of the graphite inner cavity connector: The spray gun is placed into the processed graphite cavity, and a silicon-containing connecting activator is sprayed on the inner wall of the graphite. The spraying thickness is 0.3 mm, with the Si mass fraction being 80%; the SiC whisker mass fraction is 20%, and it is ball-milled in an alcohol medium by a planetary ball mill to obtain a uniform and stable suspension with a content of 10%.

[0064] (4) The silicon nitride composite powder prepared in (2) is loaded into the high-strength graphite inner cavity, and a punch is placed. The diameter of the punch is 80 mm, and the surrounding of the punch is filled with the composite silicon nitride powder.

[0065] (5) The above-mentioned mold filled with powder is placed in a furnace, an oscillating pressure is applied, and then it is vacuum-pumped and oscillatingly sintered to form a composite part. The sintering temperature is 2000 °C, the sintering time is 4 h, the sintering atmosphere is N2 atmosphere, the applied positive pressure is 28 MPa, and the oscillating pressure is 2 MPa.

[0066] (6) Processing and treatment of the composite part: The excess part in the graphite mold is processed, and a stainless steel cover plate and a stress-bearing part are processed to prepare a ceramic powder-making tank body.

[0067] The schematic structural diagram of the cross-section of the graphite inner cavity prepared in the present invention is as shown in Figure 1 and the schematic structural diagram after placing the punch is as shown in Figure 2As shown in the figure, it includes an indenter 1, a silicon-containing connecting activator 2, a composite powder of silicon nitride and a sintering aid 3, and a high-strength graphite cavity 4. In the graphite and Si3N4 composite prepared by the present invention, the density of the Si3N4 ceramic is more than 98.3%, the bending strength at the joint reaches 22 MPa, and the joint of the workpiece is dense and seamless.

[0068] Comparative Example 1

[0069] (1) Design and processing of the inner cavity structure of high-strength graphite: Process a graphite cavity with a diameter of 60 mm from high-strength graphite with an outer diameter of 100 mm, and use a grinder to grind out 3 alternating trapezoidal structures in the inner cavity, and perform roughening treatment on the inner cavity surface with a grinding wheel.

[0070] (2) Do not use a sintering aid, that is, do not compound the silicon nitride powder, and use pure silicon nitride powder.

[0071] (3) Spraying of the graphite inner cavity connector: Place the spray gun into the processed graphite cavity, and spray the silicon-containing connecting activator on the inner wall of the graphite, with a spraying thickness of 0.2 mm. The silicon-containing connecting activator is composed of Si, SiC whiskers and nano-SiO2, where the mass ratio of Si is 80%; the mass ratio of SiC whiskers is 15%, and the mass ratio of nano-SiO2 is 5%, and it is obtained by ball milling in an alcohol medium to obtain a uniform and stable suspension with a content of 30%.

[0072] (4) Load the silicon nitride powder in (2) into the high-strength graphite inner cavity, and then place an indenter with a diameter of 50 mm. At the same time, fill the periphery of the indenter with the composite silicon nitride powder.

[0073] (5) Place the mold filled with the above powder in the furnace, apply an oscillating pressure, and then make a composite through vacuum pumping and oscillating sintering. Among them, the sintering temperature is 1850 °C, the sintering time is 6 h, the sintering atmosphere is N2 atmosphere, the applied positive pressure is 20 MPa, and the oscillating pressure is 3 MPa.

[0074] (6) Processing and treatment of the composite: Process the protruding part in the graphite mold, and process a stainless steel cover plate and a stress-bearing part to prepare a ceramic powder-making tank body.

[0075] In the composite prepared in the present invention, the density of the Si3N4 ceramic is 91.5%, there are many pores, and at the same time, the bending strength at the joint is 20 MPa. The porosity of the silicon nitride will make the ball milling effect worse.

[0076] Comparative Example 2

[0077] (1) Design and processing of the inner cavity structure of high-strength graphite: Process a graphite cavity with a diameter of 100 mm from high-strength graphite with an outer diameter of 200 mm, and use a grinder to grind out 5 alternating trapezoidal structures in the inner cavity, and perform roughening treatment on the inner cavity surface with a grinding wheel.

[0078] (2) Silicon nitride powder compounding: Compounding silicon nitride powder with sintering aids, where the ratio of Y2O3, Al2O3, and MgO in the sintering aids is 2:3:5. Ball-mill the silicon nitride powder and the sintering aids, with silicon nitride accounting for 90% and the rest being sintering aids.

[0079] (3) Do not spray the bonding agent on the inner cavity of the graphite.

[0080] (4) Load the silicon nitride composite powder prepared in (2) into the high-strength graphite inner cavity, place the pressure head, the diameter of the pressure head is 80 mm, and at the same time fill the periphery of the pressure head with the composite silicon nitride powder.

[0081] (5) Place the mold filled with the above powder in the furnace, apply an oscillating pressure, and then make a composite part through vacuum pumping and oscillating sintering. The sintering temperature is 1850 °C, the sintering time is 6 h, the sintering atmosphere is N2 atmosphere, the applied positive pressure is 28 MPa, and the oscillating pressure is 2 MPa.

[0082] (6) Machining and treatment of the composite part: Machine the protruding part in the graphite mold, and machine the stainless steel cover plate and the stress-bearing part to prepare a ceramic powder-making tank body.

[0083] In the composite part prepared in the present invention, the density of the Si3N4 ceramic is above 98.3%, the flexural strength at the joint reaches 5 MPa, the joint of the part is not tight, and there are multiple cracks.

[0084] In the graphite and Si3N4 composite parts prepared in Examples 1-3 of the present invention, the density of the Si3N4 ceramic is above 98%, the flexural strength at the joint is greater than 22 MPa, and the joint of the part is dense and seamless. Compared with Example 1, in Comparative Example 1, no sintering aids were used, pure silicon nitride powder was adopted, and the density of the Si3N4 ceramic and the flexural strength at the joint in the prepared composite part were both poor. At the same time, the porosity would make the ball-milling effect worse. Compared with Example 1, in Comparative Example 2, the bonding agent was not sprayed on the inner cavity of the graphite, and the density of the Si3N4 ceramic in the prepared composite part was poor, the flexural strength at the joint decreased significantly, and there were multiple cracks.

[0085] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the art to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A composite method of high-strength graphite and Si3N4 ceramics, characterized in that It includes the following steps: S1. Ball-mill high-purity silicon nitride powder and a sintering aid to obtain a composite powder; S2. Prepare a connecting activator containing silicon whiskers by high-energy ball milling. The connecting activator containing silicon whiskers is a mixture of whiskers and nano-Si powder, or the connecting activator containing silicon whiskers is a mixture of whiskers, nano-Si powder and nano-SiO₂; the whiskers are SiC whiskers and / or Si₃N₄ whiskers; S3. Uniformly spray the connecting activator containing silicon whiskers obtained in step S2 on the surface of the inner cavity of high-strength graphite, and load the composite powder in S1 into the inner cavity of high-strength graphite. After applying a positive pressure on the surface of the composite powder, sinter it in a nitrogen atmosphere.

2. The composite method of high-strength graphite and Si3N4 ceramics according to claim 1, wherein The connecting activator containing silicon whiskers is a mixture of nano-Si powder, SiC whiskers and nano-SiO₂.

3. The composite method of high-strength graphite and Si3N4 ceramics according to claim 1, wherein The connecting activator containing silicon whiskers is a uniform and stable suspension, and the volume fraction of the connecting activator containing silicon whiskers in the suspension is 10%-30%.

4. The composite method of high-strength graphite and Si3N4 ceramics according to claim 3, wherein, The preparation method of the suspension of the connecting activator containing silicon whiskers is as follows: Add the connecting activator containing silicon whiskers and alcohol into a ball mill, and obtain a uniform and stable suspension after ball milling.

5. The composite method of high-strength graphite and Si3N4 ceramics according to claim 1, characterized in that, The mass ratio of the high-purity silicon nitride powder to the sintering aid is (8.8-9):(1-1.2), and the spraying thickness of the connecting activator containing silicon whiskers is 0.1-0.5 mm.

6. The composite method of high-strength graphite and Si3N4 ceramics according to claim 1, characterized in that, The sintering aid is selected from one or more of Al₂O₃, Y₂O₃ and MgO.

7. The composite method of high-strength graphite and Si3N4 ceramics according to claim 1, characterized in that The sintering aid is selected as a mixture of Al₂O₃ and MgO, or the sintering aid is a mixture of Al₂O₃, Y₂O₃ and MgO.

8. The composite method of high-strength graphite and Si3N4 ceramics according to claim 1, characterized in that, The sintering process is completed in a sintering furnace, the sintering temperature is 1800-2000 °C, the sintering time is more than 3 h, the applied positive pressure is 15-30 MPa, and the oscillating pressure is 0.5-10 MPa.

9. Application of the composite method of high-strength graphite and Si₃N₄ ceramic according to any one of claims 1-8 in the preparation of a composite part of graphite and Si₃N₄ ceramic.

10. The application according to claim 9, wherein The method for preparing a composite part of graphite and Si₃N₄ ceramic includes the following steps: (1) Make a cavity from high-strength graphite, an inwardly concave receiving cavity is arranged inside the cavity, and the inner wall of the cavity is roughened; (2) Spray a connecting activator containing silicon whiskers on the roughened inner wall of the high-strength graphite cavity in step (1), then load a composite powder composed of high-purity silicon nitride powder and a sintering aid into the high-strength graphite cavity, and after applying pressure on the top of the composite powder, place the high-strength graphite cavity in a sintering furnace for oscillating hot pressing sintering to obtain a primary composite part; (3) Process the excess graphite in the primary composite part to obtain a composite part of graphite and Si₃N₄ ceramic.

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