Large-size boron carbide-graphene composite ceramic and preparation method and application thereof

Abstract

The application discloses large-size boron carbide-graphene composite ceramics, a preparation method and application thereof. The large-size boron carbide-graphene composite ceramics have high compactness and uniform microstructure, the size of the large-size boron carbide-graphene composite ceramics is 17-22 cm, the density is 2.502-2.518 g / cm 3 , the Vickers hardness is 31.5-33.8 GPa, the bending strength is 462-536 MPa, and the fracture toughness is 3.86-4.73 MPa.m 1 / 2 . The boron carbide-graphene composite ceramics prepared by adopting the HP and SPS double-mode sintering mode have fine grains and complete graphene structure, and are beneficial to the improvement of the mechanical properties of the material. Meanwhile, the application has simple process, good repeatability, high stability, and is easy to realize industrialization and batch stable production.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of ceramic materials, and particularly relates to a large-size boron carbide-graphene composite ceramic as well as a preparation method and application thereof. BACKGROUND

[0002] The boron carbide ceramic has the remarkable characteristics of low density and high hardness, and has important application value in the fields of bulletproof equipment, aerospace and national defense construction, and is an important military and civilian impact protection material. However, due to the strong covalent bond of boron carbide and the low self-diffusion coefficient, the densification temperature of the material is relatively high (hot-pressing sintering temperature ≥ 2100 ℃), and the fracture toughness is relatively low (2.2 MPa·m 1 / 2 The above shortcomings seriously limit the wide application of the boron carbide ceramic material in the engineering field.

[0003] As a new type of carbon material that is currently the most popular research, graphene has been proved to be an ideal reinforcing material for the boron carbide ceramic, and can improve the mechanical properties and electrical properties of the boron carbide ceramic, and further realize the processing of a high-performance special-shaped boron carbide ceramic component. However, the traditional sintering method is difficult to realize the preparation of the microstructure uniformity and stability of the large-size high-performance boron carbide-graphene composite ceramic. The hot-pressing sintering (HP) has the advantage of uniform temperature field distribution, but the high-temperature long-time hot-pressing sintering will not only cause the grain coarsening of the boron carbide ceramic, but also cause the graphitization of the graphene located on the grain boundary of the boron carbide ceramic, both of which are not conducive to the improvement of the mechanical properties of the boron carbide-graphene composite ceramic. The spark plasma sintering (SPS) has the advantages of fast heating rate and short holding time, and can effectively avoid the damage of high-temperature long-time heat treatment to the graphene, but due to the non-uniform temperature field distribution of the SPS sintering, a large temperature gradient is caused in the radial direction, which limits the uniformity and stability regulation of the material microstructure. SUMMARY

[0004] In view of the deficiencies in the prior art, the present application aims to provide a large-size boron carbide-graphene composite ceramic as well as a preparation method and application thereof. The present application uses graphene nanosheets and boron carbide powder as raw materials, and uses the current sintering system with a radiation heating device disclosed in Chinese patent CN101050121A to realize vacuum induction and pulse double-mode heating. The large-size boron carbide-graphene composite ceramic is successfully obtained under the sintering temperature of 1800-2000 ℃ and the holding time of 5-10 min. The composite ceramic prepared by the present application has small grain size, high density and uniform microstructure, and has high bending strength and fracture toughness.

[0005] The purpose of the present application is achieved by the following technical solutions:

[0006] A large-size boron carbide-graphene composite ceramic has high compactness and uniform microstructure, and has a size of 17-22 cm, a density of 2.502-2.518 g / cm 3 , a Vickers hardness of 31.5-33.8 GPa, a bending strength of 462-536 MPa, and a fracture toughness of 3.86-4.73 MPa·m 1 / 2 .

[0007] Preferably, the Vickers hardness is 33.5-33.8 GPa, the bending strength is 515-536 MPa, and the fracture toughness is 4.62-4.73 MPa·m 1 / 2 .

[0008] The preparation method of the large-size boron carbide-graphene composite ceramic comprises the following steps:

[0009] (1) uniformly dispersing graphene in ethanol to obtain a graphene dispersion liquid; adding boron carbide into the graphene dispersion liquid, stirring and ultrasonicating, then ball-milling, drying and sieving to obtain a composite ceramic powder;

[0010] (2) compacting the composite ceramic powder of step (1) and placing it in a sintering furnace, introducing a protective atmosphere, first performing hot-pressing sintering and heating to 1200-1500 DEG C and maintaining for 15-45 min, then simultaneously performing discharge plasma sintering, continuing to heat to 1800-2000 DEG C and maintaining for 5-15 min, and finally naturally cooling to obtain the large-size boron carbide-graphene composite ceramic.

[0011] Preferably, in step (1), the graphene accounts for 0.5-4 wt% of the total mass of boron carbide and graphene, and the boron carbide accounts for 96-99.5 wt% of the total mass of boron carbide and graphene.

[0012] Preferably, in step (1), the thickness of the graphene is 1-5 nm, and the flake size is 3-10 μm.

[0013] Preferably, in step (1), the purity of the boron carbide is greater than 97%, and the particle size is 0.5-3 μm.

[0014] Preferably, in step (1), the concentration of the graphene dispersion liquid is 1-5 mg / mL.

[0015] Preferably, in step (1), the ultrasonicating conditions are: an ultrasonic power of 800-1000 W and an ultrasonicating time of 1-3 h.

[0016] Preferably, in step (1), the stirring conditions are: a stirring rate of 200-500 rpm / min and a stirring time of 1-3 h.

[0017] Preferably, in step (1), the ball milling conditions are as follows: a planetary ball mill is used, SiC balls are used as the grinding balls, the ball-to-material ratio is (2-10):1, the ball milling speed is 100-300 rpm / min, and the ball milling time is 12-36 h.

[0018] Preferably, in step (1), the drying conditions are as follows: a vacuum drying oven is used, the drying temperature is 60 DEG C, and the drying time is 24-48 h.

[0019] Preferably, in step (1), the mesh size of the sieving is 100-325 mesh.

[0020] Preferably, in step (2), the protective atmosphere is argon.

[0021] Preferably, in step (2), the hot-pressing sintering conditions are as follows: the temperature is raised to 1200-1500 DEG C at a temperature raising rate of 5-20 DEG C / min, and the pressure is 3-10 MPa.

[0022] Preferably, in step (2), the spark plasma sintering conditions are as follows: the temperature is raised to 1800-2000 DEG C at a temperature raising rate of 50-100 DEG C / min, and the pressure is 20-60 MPa.

[0023] The above large-size boron carbide-graphene composite ceramic is used in the engineering field.

[0024] The principles involved in the present application include the following contents:

[0025] First, the preparation of large-size boron carbide-graphene composite ceramic is very difficult. The mechanism of HP sintering is that heat is radiated and diffused from the outside to the inside of the heating body, so the temperature near the heating body is higher, and the temperature far from the heating body is lower. This temperature gradient distribution usually leads to high density of the peripheral part and low density of the central part of the large-size ceramic member. The mechanism of SPS sintering is that ceramic ions are discharged by a large current to generate Joule heat for heating. The peripheral resistance of the large-size ceramic member is large, so the current passing through is small, and thus the temperature and heat are also low. Therefore, the characteristic of SPS sintering is that the density of the product center is large, and the density of the edge is small. In combination with the sintering characteristics of the two sintering methods, the large-size boron carbide-graphene composite ceramic prepared by double-mode sintering is in a uniform temperature field, the structure is dense, and the rapid heating and short holding time of high-temperature SPS can ensure that the grains of boron carbide ceramic do not abnormally grow, so that the rapid preparation of large-size dense fine-grained boron carbide ceramic is realized.

[0026] Second, graphene with excellent mechanical and electrical properties will generally graphitize under the condition of high temperature and long time holding treatment, which will denature and affect the mechanical properties of the graphene toughened boron carbide ceramic. In the present application, the relatively uniform low-temperature field of HP sintering is used to rise to a certain temperature at a lower heating rate, and this temperature will not cause any damage to the structure of graphene. In the high-temperature stage, HP and SPS sintering modes are used at the same time, which not only ensures the uniformity of the temperature field, but also realizes the densification of boron carbide-graphene ceramic in a short time, while maintaining the integrity of the graphene structure, and realizing the rapid preparation of high-performance boron carbide-graphene composite ceramic.

[0027] Compared with the prior art, the beneficial effects of the present application include:

[0028] (1) The present application solves the problem of preparing large-size boron carbide-graphene composite ceramic by using double-mode sintering furnace.

[0029] (2) The boron carbide-graphene composite ceramic prepared by the double-mode sintering (HP and SPS sintering modes are used at the same time) has fine grain size and complete graphene structure, which is beneficial to the improvement of the mechanical properties of the material.

[0030] (3) In the high-temperature stage of the present application, the SPS has a fast heating rate and a short holding time, which has a small power consumption and a lower cost compared with traditional hot pressing and pressureless long-time sintering.

[0031] (4) The sintering method provided by the present application has simple process, good repeatability and high stability, and is easy to realize industrialization and batch stable production. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The figure is a real photo of the large-size boron carbide-graphene composite ceramic prepared by the present application.

[0033] Figure 2 The figure is a fracture morphology diagram of the boron carbide-graphene composite ceramic prepared in Example 1.

[0034] Figure 3 The figure is a transmission electron microscope diagram of the boron carbide-graphene composite ceramic prepared in Example 1, wherein a and b are transmission electron microscope diagrams with different magnifications.

[0035] Figure 4 The figure is a fracture morphology diagram of the boron carbide-graphene composite ceramic prepared in Example 2.

[0036] Figure 5 The figure is a fracture morphology diagram (left) and a Raman spectrum diagram (right) of the boron carbide-graphene composite ceramic prepared in Comparative Example 1.

[0037] Figure 6The fracture morphology of the boron carbide-graphene composite ceramic prepared in Comparative Example 2. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with examples. It should be understood that the specific examples described herein are only used to explain the present application and not to limit the present application.

[0039] Example 1

[0040] A preparation method of a large-size boron carbide-graphene composite ceramic, the steps are as follows:

[0041] (1) Preparation of graphene dispersion liquid: graphene powder is added into alcohol, and dispersed by ultrasonic and mechanical stirring, the ultrasonic power is 1000 W, the stirring speed is 200 rpm / min, and the dispersion treatment time is 2h, to obtain a uniformly dispersed graphene dispersion liquid;

[0042] (2) Preparation of boron carbide-graphene composite ceramic powder: boron carbide powder is added into the graphene dispersion liquid prepared in step (1), and the mass percentage of boron carbide and graphene is 98%:2%, and the stirring speed is 200 rpm / min, and the stirring time is 2h; then planetary ball milling treatment is carried out, the milling ball is SiC ball, the ball-to-material ratio is 4:1, the high-energy planetary ball milling speed is 200 rpm / min, and the ball milling time is 24h. After ball milling, the sample is dried at 60℃ for 24h, and then sieved through a 200 mesh sieve to obtain a boron carbide-graphene mixed powder with small particles and uniform dispersion.

[0043] (3) The boron carbide-graphene mixed powder obtained in step (2) is loaded into a graphite mold and compacted, the inner wall of the graphite mold and the powder between the pressure head are separated by graphite paper, and then it is placed in an electric current sintering system with a radiation heating device, argon gas is introduced as a protective atmosphere, and the specific sintering system is as follows: low temperature stage, hot-pressing sintering: heating to 1200℃ at a heating rate of 8℃ / min, pressure of 10 MPa, and holding for 30 min; then simultaneously start the SPS sintering mode, heat to the target temperature of 2000℃ at a heating rate of 100℃ / min, pressure of 30 MPa, hold for 10 min, and then naturally cool.

[0044] The phase structure of the large-size boron carbide-graphene composite ceramic prepared in Example 1 is observed by scanning electron microscope, and the observation result is shown in Figure 2 Figure 1. Figure 2We can all know that the double-mode sintering can obtain nearly complete dense boron carbide-graphene composite ceramic material under the sintering condition of 2000℃ high temperature, 30 MPa holding for 10 min, and the grain of boron carbide is more fine compared with the hot-pressing sintering. In addition, Figure 3 The microstructure of the cross section of the large-size boron carbide-graphene composite ceramic prepared in Example 1 is shown in the figure from Figure 3 It can be seen that the graphene nanosheet with a sheet structure is uniformly distributed on the grain boundary of the boron carbide ceramic.

[0045] Further, the ceramic wafer prepared in Example 1 is equally divided into different size sectors along the radial direction of the wafer, and the mechanical properties of the materials in different regions are tested. The results show that the density and mechanical properties of the materials in different positions are very small, and the average performance of the composite ceramic sample prepared in this embodiment is as follows: the density is 2.517 g / cm 3 , the Vickers hardness is 33.5 GPa, the bending strength is 515 MPa, and the fracture toughness is 4.62 MPa·m 1 / 2 .

[0046] Example 2

[0047] A preparation method of a large-size boron carbide-graphene composite ceramic, comprising the following steps:

[0048] (1) preparing a graphene dispersion liquid: adding graphene powder into alcohol, dispersing by ultrasonic and mechanical stirring, the ultrasonic power is 800 W, the stirring speed is 250 rpm / min, and the dispersion treatment time is 2h, to obtain a uniformly dispersed graphene dispersion liquid;

[0049] (2) preparing a boron carbide-graphene composite ceramic powder: adding boron carbide powder into the graphene dispersion liquid prepared in step (1), the mass percentage of boron carbide and graphene is 99%:1%, stirring and ultrasonic, the stirring speed is 300 rpm / min, and the stirring time is 2h; then performing planetary ball milling treatment, the milling ball is SiC ball, the ball-to-material ratio is 4:1, the rotation speed of high-energy planetary ball milling is 300 rpm / min, and the ball milling time is 24h. After ball milling, the sample is dried at 60℃ for 24h, and the boron carbide-graphene mixed powder with small particles and uniform dispersion is obtained after passing through a 200 mesh sieve.

[0050] (3) The boron carbide-graphene mixed powder obtained in step (2) is placed in a graphite mold and compacted. The inner wall of the graphite mold, the pressure head and the powder are separated by graphite paper. Then, it is placed in an electric current sintering system with a radiation heating device, and argon is introduced as a protective atmosphere. The specific sintering system is as follows: low temperature stage, hot pressing sintering: heating to 1500°C at a heating rate of 8°C / min, pressure of 10 MPa, and keeping warm for 30 min; then, the SPS sintering mode is turned on at the same time, heating to the target temperature of 1980°C at a heating rate of 50°C / min, pressure of 30 MPa, keeping warm for 10 min and then cooling naturally.

[0051] The physical structure of the large-scale boron carbide-graphene composite ceramic prepared in Example 2 was observed using a scanning electron microscope. Figure 4 See Figure 4 As we all know, dual-mode sintering at 1980°C and 30 MPa for 10 minutes can produce a nearly fully dense boron carbide-graphene composite ceramic. Unlike Example 1, Example 2 has a lower graphene content, making it difficult to observe the graphene within the boron carbide ceramic substrate using a microscope. This is primarily due to two factors: first, some graphene reacts with small amounts of boron oxide impurities on the surface of the boron carbide ceramic; second, graphene nanosheets are very thin, and at low concentrations, the low resolution of the equipment makes them difficult to observe using a transmission electron microscope at low magnifications.

[0052] Similarly, the boron carbide-graphene composite ceramic disc prepared in Example 2 was divided into sectors of different sizes along the radial direction, and the mechanical properties of the materials in different regions were tested. The results showed that the density and mechanical properties of the materials in different locations were not much different. According to statistics, the average properties of the composite ceramic samples prepared in this example are as follows: density of 2.518 g / cm 3 , Vickers hardness is 33.8 GPa, bending strength is 536 MPa, and fracture toughness is 4.73 MPa·m 1 / 2 .

[0053] Example 3

[0054] A method for preparing large-scale boron carbide-graphene composite ceramics, comprising the following steps:

[0055] (1) Preparation of graphene dispersion: Graphene powder was added to alcohol and dispersed by ultrasonication and mechanical stirring. The ultrasonic power was 1000 W, the stirring speed was 200 rpm / min, and the dispersion treatment time was 2 h to obtain a uniformly dispersed graphene dispersion.

[0056] (2) Preparation of boron carbide-graphene composite ceramic powder: boron carbide powder is added to the graphene dispersion liquid prepared in step (1), and the mass percentage of boron carbide to graphene is 96%:4%. Stirring and ultrasonic treatment are carried out, the stirring speed is 250 rpm / min, the stirring time is 2h; then planetary ball milling treatment is carried out, the milling ball is SiC ball, the ball-to-powder ratio is 4:1, the high-energy planetary ball milling speed is 250 rpm / min, and the ball milling time is 24h. After ball milling, the sample is dried at 60℃ for 24h, and then sieved through a 200 mesh sieve to obtain boron carbide-graphene mixed powder with fine particles and uniform dispersion.

[0057] (3) The boron carbide-graphene mixed powder obtained in step (2) is loaded into a graphite mold, compacted, and the inner wall of the graphite mold and the powder between the pressure head are separated by graphite paper, and then placed in an electric current sintering system with a radiation heating device, and argon is introduced as a protective atmosphere. The specific sintering system is as follows: low temperature stage, hot-pressing sintering: heating to 1350℃ at a rate of 8℃ / min, pressure of 10 MPa, and holding for 30 min; then simultaneously start the SPS sintering mode, heat to the target temperature of 2000℃ at a rate of 100℃ / min, pressure of 30 MPa, hold for 10 min, and then naturally cool.

[0058] The boron carbide-graphene composite ceramic wafer prepared in Example 3 is equally divided into different size sectors in the radial direction, and the mechanical properties of the materials in different regions are tested. According to statistics, the average performance of the composite ceramic sample prepared in this embodiment is as follows: the density is 2.502 g / cm 3 , the Vickers hardness is 31.5GPa, the bending strength is 462 MPa, and the fracture toughness is 3.86MPa·m 1 / 2 .

[0059] Comparative Example 1

[0060] A hot-pressing sintering method for preparing boron carbide-graphene composite ceramics, the steps are as follows:

[0061] (1) Preparation of graphene dispersion liquid: graphene powder is added to alcohol, and ultrasonic and mechanical stirring are carried out for dispersion, the ultrasonic power is 1000 W, the stirring speed is 200 rpm / min, the dispersion treatment time is 2h, and a uniformly dispersed graphene dispersion liquid is obtained;

[0062] (2) Preparation of boron carbide-graphene composite ceramic powder: boron carbide powder was added to the graphene dispersion liquid prepared in step (1), and the mass percentage of boron carbide to graphene was 98.5%:1.5%. The stirring speed was 300 rpm / min, and the stirring time was 2h. Then, planetary ball milling was performed, with SiC balls and a ball-to-powder ratio of 4:1. The high-energy planetary ball milling speed was 200 rpm / min, and the ball milling time was 24h. After ball milling, the sample was dried at 60°C for 24h, and then sieved through a 200-mesh sieve to obtain boron carbide-graphene mixed powder with fine particles and uniform dispersion.

[0063] (3) The boron carbide-graphene mixed powder obtained in step (2) was loaded into a graphite mold, and the inner wall of the graphite mold and the space between the powder and the pressure head were separated by graphite paper. Then, the mixture was placed in a hot-pressing sintering furnace, and the vacuum in the furnace was pumped to 3×10 - 3 When the pressure was below 30 MPa, the equipment was started. The sample was first subjected to an axial pressure of 30 MPa, and then the temperature was increased from room temperature to 1200°C at a rate of 10°C / min. Ar gas was then filled, and the temperature was further increased from 1200°C to 2000°C at a rate of 5°C / min. Finally, the sample was kept at this temperature for 1h.

[0064] The phase structure of the boron carbide-graphene composite ceramic prepared in Comparative Example 1 was observed by scanning electron microscopy, and the observation results are shown in FIG. 2. Figure 5 Figure 5 The fracture morphology and Raman spectrum of the boron carbide-graphene composite ceramic prepared in Comparative Example 1 are shown in FIG. 3. Figure 5 As can be seen, under the sintering conditions of hot-pressing sintering at 2000°C, 30 MPa, and 1h, a nearly fully dense boron carbide-graphene composite ceramic material can be obtained. It can be found that the grains of the boron carbide ceramic are relatively large. In addition, the microstructure of the fracture surface of the boron carbide ceramic shows that there is no obvious presence of graphene nanosheets on the grain boundaries of boron carbide. This phenomenon can also be confirmed from the Raman spectrum. Generally, the D, G, and 2D peaks of GNPs (graphene) are located at 1340 cm -1 , 1586 cm -1 , and 2660 cm -1 , respectively, and the thickness of the graphene nanosheet layer is usually reflected by the ratio of the intensity of the 2D peak to the intensity of the G peak. In the Raman spectrum of this comparative example, the intensity of the 2D peak is weak, and the peak width is large, indicating that the graphene nanosheets have been graphitized under the condition of long-time heat preservation during hot-pressing sintering.

[0065] Through testing, the properties of the composite ceramic sample prepared in Comparative Example 1 were as follows: the density was 2.515 g / cm 3 ​, the Vickers hardness is 32.3 GPa, the bending strength is 428 MPa, and the fracture toughness is 2.76 MPa·m 1 / 2 .

[0066] Comparative Example 2

[0067] A method for preparing a boron carbide-graphene composite ceramic by spark plasma sintering, comprising the following steps:

[0068] (1) Preparing a graphene dispersion liquid: graphene powder is added to alcohol, and dispersed by ultrasonic and mechanical stirring, the ultrasonic power is 1000 W, the stirring speed is 200 rpm / min, and the dispersion treatment time is 1 h, to obtain a uniformly dispersed graphene dispersion liquid;

[0069] (2) Preparing a boron carbide-graphene composite ceramic powder: boron carbide powder is added to the graphene dispersion liquid prepared in step (1), the mass percentage of boron carbide to graphene is 98%:2%, and the stirring speed is 300 rpm / min, and the stirring time is 2 h; then planetary ball milling is performed, the milling balls are SiC balls, the ball-to-material ratio is 4:1, the high-energy planetary ball milling speed is 150 rpm / min, and the ball milling time is 24 h. After ball milling, the sample is dried at 60°C for 24 h, and then sieved through a 200-mesh sieve to obtain a boron carbide-graphene mixed powder with fine particles and uniform dispersion.

[0070] (3) The boron carbide-graphene mixed powder obtained in step (2) is loaded into a graphite mold, and the inner wall of the graphite mold and the space between the powder and the pressure head are separated by graphite paper, and then the graphite mold is placed in a spark plasma sintering furnace, an axial pressure of 50 MPa is applied to the sample, the vacuum in the furnace is pumped to below 3×10-3 Pa to meet the equipment starting conditions, the equipment is heated from room temperature to 600°C within 5 min, and then heated to 1800°C at a heating rate of 100°C / min, and finally kept at this temperature for 10 min.

[0071] The phase structure of the boron carbide-graphene composite ceramic prepared in Comparative Example 2 is observed by scanning electron microscopy, and the observation results are shown in Figure 6 . As shown in Figure 6 , we can know that the boron carbide-graphene composite ceramic material prepared by spark plasma sintering at 1800°C, 50 MPa for 10 min has a non-uniform structure. Specifically, the structure of the center part of the ceramic product is dense, and the graphene nanosheets are uniformly distributed in the boron carbide ceramic matrix, but some pores are found near the edge of the ceramic sheet, which is related to the inherent defects of the spark plasma sintering furnace.

[0072] The ceramic wafer is equally divided into different size sectors along the radial direction of the ceramic wafer, the mechanical properties of the materials in different regions are tested, and through statistics, the average performance of the composite ceramic sample prepared in the comparative example is as follows: the density is 2.42 g / cm 3 , the Vickers hardness is 28.2 GPa, the bending strength is 327 MPa, and the fracture toughness is 2.96 MPa·m 1 / 2 .

[0073] Figure 1 It is a real photo of the large-size boron carbide-graphene composite ceramic prepared by the present application. Figure 1 As can be seen, the boron carbide-graphene composite ceramic prepared by the present application has a size of up to 17 cm and is complete without cracks, which has important practical significance for improving material performance, reducing production cost, expanding application range and promoting industrialization process. Note that the real photo of the boron carbide-graphene composite ceramic prepared in Example 1 is taken as a representative, the sizes of the composite ceramics prepared in Examples 2-3 are basically the same as that of Example 1, and are also complete without cracks.

[0074] The specific embodiments of the present application described above do not constitute a limitation on the protection scope of the present application. Any various other corresponding changes and modifications made according to the technical concept of the present application shall be included in the protection scope of the claims of the present application.

Claims

1. A method for preparing large-scale boron carbide-graphene composite ceramics, characterized in that: The steps include: (1) uniformly dispersing graphene in ethanol to obtain a graphene dispersion; adding boron carbide to the graphene dispersion, stirring and ultrasonicating the dispersion, and then ball milling, drying and sieving to obtain a composite ceramic powder; (2) The composite ceramic powder of step (1) is compacted and placed in a double-mold sintering furnace, a protective atmosphere is introduced, and hot pressing sintering is first performed, the temperature is raised to 1200-1500°C and kept warm for 15-45 minutes, and then spark plasma sintering is performed, the temperature is further raised to 1800-2000°C and kept warm for 5-15 minutes, and finally naturally cooled to obtain the large-sized boron carbide-graphene composite ceramic; In step (1), in terms of mass percentage, graphene accounts for 0.5-4 wt% of the total mass of boron carbide and graphene, and boron carbide accounts for 96-99.5 wt% of the total mass of boron carbide and graphene; The concentration of the graphene dispersion in step (1) is 1-5 mg / mL; The graphene in step (1) has a thickness of 1-5 nm and a sheet diameter of 3-10 μm; The purity of the boron carbide in step (1) is greater than 97%, and the particle size is 0.5-3 μm; The ball milling conditions in step (1) are as follows: a planetary ball mill is used, the grinding balls are SiC balls, the ball-to-material ratio is (2-10):1, the ball milling speed is 100-300 rpm / min, and the ball milling time is 12-36 hours; The drying conditions in step (1) are as follows: drying in a vacuum drying oven at a drying temperature of 60°C for a drying time of 24 to 48 hours; The mesh size of the sieving in step (1) is 100-325 mesh; The protective atmosphere in step (2) is argon; The hot pressing sintering conditions in step (2) are: heating to 1200-1500°C at a heating rate of 5-20°C / min and a pressure of 3-10 MPa; The conditions for the spark plasma sintering in step (2) are: heating to 1800-2000°C at a heating rate of 50-100°C / min and a pressure of 20-60 MPa.

2. The method for preparing the large-size boron carbide-graphene composite ceramic according to claim 1, characterized in that: The ultrasonic conditions in step (1) are as follows: ultrasonic power is 800-1000W, and ultrasonic time is 1-3h; The stirring conditions in step (1) are: a stirring rate of 200-500 rpm / min and a stirring time of 1-3 h.

3. A large-sized boron carbide-graphene composite ceramic prepared by the method according to any one of claims 1-2, characterized in that: It has high density and uniform microstructure, with a size of 17~22cm and a density of 2.502~2.518 g / cm 3 , Vickers hardness is 31.5~33.8 GPa, bending strength is 462~536 ​​MPa, fracture toughness is 3.86~4.73 MPa·m 1 / 2 .

4. The large-sized boron carbide-graphene composite ceramic according to claim 3, characterized in that: The Vickers hardness is 33.5~33.8 GPa, the bending strength is 515~536 MPa, and the fracture toughness is 4.62~4.73 MPa·m 1 / 2 .

5. Application of the large-sized boron carbide-graphene composite ceramic according to any one of claims 3 to 4 in the field of engineering.

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