Preparation method of graphene composite toughened high-entropy nitride ceramic

Abstract

The application discloses a preparation method of graphene composite high-entropy nitride ceramic, which comprises the steps of dispersing graphene, synthesizing high-entropy powder, mixing graphene, sintering ceramic and the like. The graphene is dispersed in a solution formed by PVP and anhydrous ethanol by using a strong stirring method, high-efficiency dispersion of the graphene is realized, and finally the composite ceramic with uniform composition is obtained. The intrinsic toughening mode (i.e. sliding, bending and wrapping of the graphene) and the non-intrinsic toughening mode (i.e. pulling out of the graphene and crack deflection, bridging and branching caused by the graphene) introduced by the graphene can effectively dissipate energy, and improve the ceramic fracture toughness and bending strength. When the graphene addition amount is 2.5wt.%, the Vickers hardness, fracture toughness and bending strength of the composite ceramic are 18.36 GPa, 383 MPa and 4.78 MPa.m 1 / 2 , respectively, and the fracture toughness is increased by 63.1%.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-high temperature toughness ceramic materials, and in particular to a method for preparing graphene composite toughened high-entropy nitride ceramics. Background Art

[0002] High-entropy nitride ceramics possess properties such as high hardness, high strength, good oxidation resistance, and low thermal conductivity, and have broad application prospects in high-temperature fields and extreme environments. However, their intrinsic brittleness and mechanical unreliability significantly limit their applications. Therefore, a method to improve the fracture toughness of high-entropy nitrides is urgently needed.

[0003] In recent years, people have achieved the toughening effect of ceramics by adding a second phase (such as particles, fibers or whiskers and two-dimensional materials). Traditional second-phase additives include particles, whiskers and fibers, which can improve the structure and properties of ceramics, but it is still difficult to effectively expand the application field of ceramics. Therefore, adding high-performance filler materials to the ceramic matrix is ​​the key to improving its performance. So researchers discovered graphene, a material with excellent performance, which can greatly improve the fracture toughness of the material. Compared with traditional second-phase additives, graphene has a larger specific surface area and a higher aspect ratio, which is conducive to the formation of more grain boundaries in ceramic materials, thereby increasing the interaction between grain boundaries and dislocation slip, and greatly improving its mechanical properties. However, the existing graphene dispersion methods have the problems of poor dispersion effect and low dispersion efficiency, which limits the toughening effect of graphene.

[0004] In addition, most of the reports so far are on composite ceramics of graphene and binary oxides or non-oxides, and there is little research on the toughening effect of adding graphene to high-entropy nitride ceramics.

[0005] Therefore, in order to further improve the fracture toughness of high-entropy nitride ceramics, it is necessary to add an appropriate amount of graphene to improve the brittleness of ceramics and increase the bending strength of ceramics. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a preparation method of graphene composite toughened high-entropy nitride ceramics, by improving the dispersed state of graphene, preparing a well-dispersed graphene suspension, achieving uniform dispersion of graphene in high-entropy nitride, and improving the fracture toughness and flexural strength of high-entropy nitride ceramics.

[0007] The present invention provides a method for preparing a graphene composite toughened high entropy nitride ceramic, comprising the following steps:

[0008] (1) Dispersion of graphene: adding graphene, PVP and anhydrous ethanol to a wall breaking machine at a mass ratio of graphene: PVP: anhydrous ethanol = 1: (0.7-0.9): (400-667), starting the wall breaking machine to disperse the graphene at a speed of 17000-19000 r / min for 20-60 min, using running water for cooling during the dispersion period to obtain a graphene suspension;

[0009] (2) Synthesis of high entropy powder: Weigh the above raw material powders according to the molar ratio of HfO2: ZrO2: TiO2: Nb2O5: Ta2O5: Si3N4 = 2: 2: 2: 1: 1: 10, and place the obtained mixed raw material powder in a high energy density stirred bead mill, add appropriate amount of deionized water and grinding balls, and mix and finely grind the mixed raw material powder at a speed of 1200 r / min for 8 to 12 hours. After the mixing is completed, the obtained slurry is separated from the grinding balls, and then the slurry is dried, ground and passed through a 60-mesh sieve to obtain a precursor powder. The precursor powder is then pressed into several small round blocks with a single weight of 3 g and a diameter of 20 mm and placed in a pressureless furnace and kept warm at 1750 to 1850 ° C in an argon atmosphere for 0.5 to 1.5 hours to synthesize (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramic powder, grinding the obtained high entropy ceramic powder through a 100 mesh sieve;

[0010] (3) Mixing of graphene: adding the high entropy ceramic powder and the graphene suspension to a high energy density stirred bead mill according to the addition amount of graphene in the high entropy ceramic powder being 0 to 5.0 wt.%, adding grinding balls, and ball milling the two at a speed of 1100 to 1300 r / min for 4 to 6 h to obtain a mixed slurry. After separating the grinding balls, the mixed slurry is dried for 3 to 4 h using a rotary evaporator at a temperature of 35 to 45 ° C and a vacuum condition of -(0.08 to 0.1) MPa, and then ground and passed through a 100 mesh sieve to obtain a composite powder;

[0011] (4) Sintering of ceramics: The composite powder is loaded into an SPS graphite mold, which is surrounded by graphite paper. The graphite mold is placed in an SPS sintering furnace. After the hydraulic station is started to compact the powder, the temperature is raised to 2050-2150°C in a vacuum environment. The temperature is kept at 35-45 MPa for 5-15 minutes to sinter the ceramics. The heating process is divided into three stages:

[0012] The first stage: 0-650℃, heating rate is 92℃ / min;

[0013] The second stage: 1600-1900℃, heating rate of 65℃ / min;

[0014] The third stage: above 1900℃, heating rate is 29℃ / min.

[0015] Preferably, in step (1), the mass ratio of graphene:PVP:anhydrous ethanol is 1:0.8:588, the rotation speed of the wall breaking machine blade is 18000r / min, and the dispersion time is 40min.

[0016] Preferably, the grinding balls in step (2) and step (3) are ZrO2 balls with a diameter of 1 mm.

[0017] Preferably, the mass ratio of materials in step (2) is raw material powder: grinding balls: deionized water = 100:300:750.

[0018] Preferably, in step (2), the rotation speed of the high energy density stirring bead mill is 1200 r / min, and the mixing time is 10 h.

[0019] Preferably, in step (2), the sintering temperature in the pressureless furnace is 1800° C., and the holding time is 1 hour.

[0020] Preferably, in step (3), the rotation speed of the high energy density stirring bead mill is 1200 r / min, and the mixing time is 5 h.

[0021] Preferably, in step (3), the temperature of the rotary evaporator is 40° C., the vacuum degree is -0.09 MPa, and the drying time is 3.5 h.

[0022] Preferably, in step (4), the sintering temperature is 2100° C., the pressure is 40 MPa, and the sintering time is 10 min.

[0023] Working principle of the present invention: The present invention uses polyvinyl pyrrolidone (PVP) as a dispersant and anhydrous ethanol as a dispersion medium, and disperses graphene by the rapid rotation of the wall-breaking machine blade. A strong stirring method is used to disperse the graphene, which further shortens the dispersion time and improves the dispersion efficiency of the graphene. The dispersion effect of the graphene in the obtained suspension is better, basically wrinkle-free, and the flakes are stretched. Then, the graphene suspension is mixed with the high-entropy ceramic powder through a high-energy density stirring bead mill to achieve uniform dispersion of graphene in the high-entropy nitride ceramic. The mutual high-speed friction between the grinding balls is utilized to evenly and stretch the graphene flakes between the particles of the high-entropy ceramic powder. In addition, the slurry is dried under vacuum conditions using a rotary evaporator to prevent graphene from agglomerating during the slurry drying process. In the graphite mold used for sintering in the spark plasma sintering furnace (SPS), graphite paper is surrounded around the mold for subsequent demolding. Finally, when sintered under the high temperature of about 2100 ° C, the self-lubricating effect of graphene is conducive to particle rearrangement, and heat transfer can also be improved to play a sintering effect, so that graphene and high-entropy powder materials are closely combined with each other and produce good interface compatibility, and the embedding of an appropriate amount of graphene in the ceramic matrix does not cause holes and slits, thereby improving the density of the composite ceramic. The graphene composite toughened high-entropy nitride ceramic prepared by the present invention, when subjected to external forces, the addition of graphene can introduce its intrinsic toughening mode (i.e., the sliding, bending and encapsulation of graphene to grains) and non-intrinsic toughening mode (the extraction of graphene and the crack deflection, bridging and bifurcation caused by graphene). The emergence of these phenomena can dissipate extra energy to eliminate stress concentration phenomena, reduce the penetration of cracks, enhance the bearing capacity and crack tolerance of the ceramic matrix, thereby improving the fracture toughness of the ceramic matrix.

[0024] Beneficial effects of the present invention: The present invention uses a strong stirring method to disperse graphene in a solution formed by polyvinyl pyrrolidone (PVP) and anhydrous ethanol, further shortening the dispersion time and achieving efficient dispersion of graphene. The graphene in the resulting suspension has a better dispersion effect, is basically wrinkle-free, and has stretched sheets that can be evenly and stretchily distributed in the material. The well-dispersed graphene suspension is compounded in a high-entropy nitride ceramic to ultimately obtain a composite ceramic with uniform composition. The graphene further improves the fracture toughness and flexural strength of the ceramic matrix. When the graphene addition amount is 2.5wt.%, the Vickers hardness, fracture toughness, and flexural strength of the composite ceramic reach 18.36GPa, 383MPa, and 4.78MPa·m, respectively. 1 / 2 , the fracture toughness increased by 63.1%. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 For different doping amounts of graphene / (Hf 0.2 Zr0.2 Ti 0.2 Nb 0.2 Ta 0.2 ) Distribution of graphene in N composite high-entropy ceramics: (a) 0.5 wt.%, (b) 2.5 wt.%, (c) 5.0 wt.%;

[0026] Figure 2 For different doping amounts of graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 ) Dispersion degree of graphene in N composite high entropy ceramics: (a) 0.5 wt.%, (b) 2.5 wt.%, (c) 5.0 wt.%;

[0027] Figure 3 For different graphene doping amounts (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N ceramic density;

[0028] Figure 4 is the amount of graphene added for graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )Effect of N on the mechanical properties of composite high entropy ceramics: (a) Vickers hardness, (b) fracture toughness, (c) flexural strength;

[0029] Figure 5 For graphene and (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )Bonding interface of N high entropy ceramics (a) 0.5wt.%, (b) 2.5wt.%, (c) 5.0wt.%;

[0030] Figure 6 The doping amount is 2.5wt.% graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 ) IFFT image of (a) region 3 of N composite high entropy ceramic, (b) lattice fringes in the transition region, and (c) IFFT image of region 4;

[0031] Figure 7Intrinsic toughening modes of graphene: (a) sliding, (b) bending and wrapping of grains, (c) U-shaped bending;

[0032] Figure 8 It is a non-intrinsic toughening method of graphene. DETAILED DESCRIPTION

[0033] In order to make the technical solution of the present invention easier to understand, the technical solution of the present invention is now clearly and completely described in the form of specific embodiments in conjunction with the accompanying drawings.

[0034] 1.1 Preparation of graphene composite toughened high entropy nitride ceramics

[0035] Comparative Example:

[0036] The preparation method of the graphene composite toughened high entropy nitride ceramic of this comparative example comprises the following steps:

[0037] (1) Graphene dispersion: Graphene, PVP, and anhydrous ethanol were added to a wall breaking machine at a mass ratio of graphene: PVP: anhydrous ethanol = 0:0.8:588, and the wall breaking machine was started to disperse at a speed of 18,000 r / min for 40 min. During the dispersion, running water was used for cooling to obtain a graphene suspension comparison solution;

[0038] (2) Synthesis of high entropy powder: Weigh the above raw material powders according to the molar ratio of HfO2: ZrO2: TiO2: Nb2O5: Ta2O5: Si3N4 = 2: 2: 2: 1: 1: 10, and place the obtained mixed raw material powder in a high energy density stirred bead mill, add appropriate amount of deionized water and Φ1mm ZrO2 grinding balls, and mix and finely grind the mixed raw material powder at a speed of 1200r / min for 10h. After mixing, separate the obtained slurry from the grinding balls, dry, grind and pass through a 60-mesh sieve to obtain a precursor powder, and then press the precursor powder into several small round blocks with a single weight of 3g and Φ20mm and place them in a pressureless furnace and keep them warm at 1800℃ in an argon atmosphere for 1h to synthesize (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramic powder, grinding the obtained high entropy ceramic powder through a 100 mesh sieve;

[0039] (3) Graphene mixing: 150 mL of graphene suspension reference solution and 150 mL of anhydrous ethanol were added to 10 g of high entropy ceramic powder to form a slurry. The slurry was added to a high energy density stirred bead mill and ZrO2 grinding balls were added. The slurry was ball milled at a speed of 1200 r / min for 5 h to obtain a slurry. The grinding balls were separated and the slurry was dried on a rotary evaporator at a temperature of 40 ° C and a vacuum condition of -0.09 MPa for 3.5 h. The slurry was then ground and passed through a 100 mesh sieve to obtain a composite powder reference sample.

[0040] (4) Sintering of ceramics: The composite powder is loaded into an SPS graphite mold, which is surrounded by graphite paper. The graphite mold is placed in an SPS sintering furnace. After the hydraulic station is started to compact the powder, the temperature is raised to 2100°C in a vacuum environment. The temperature is maintained at 40 MPa for 10 minutes to sinter the ceramics. The heating process is divided into three stages:

[0041] The first stage: 0-650℃, heating rate is 92℃ / min;

[0042] The second stage: 1600-1900℃, heating rate of 65℃ / min;

[0043] The third stage: above 1900℃, heating rate is 29℃ / min;

[0044] In this comparative example, the amount of graphene added is 0, and the obtained ceramic sample is numbered G0.

[0045] Example 1:

[0046] The preparation method of the graphene composite toughened high entropy nitride ceramic of this embodiment comprises the following steps:

[0047] (1) Graphene dispersion: Graphene, PVP, and anhydrous ethanol were added to a wall breaking machine at a mass ratio of graphene: PVP: anhydrous ethanol = 1:0.8:588, and the wall breaking machine was started to disperse the graphene at a speed of 18000 r / min for 40 min. During the dispersion, running water was used for cooling to obtain a graphene suspension;

[0048] (1) Synthesis of high entropy powder: Weigh the above raw material powders according to the molar ratio of HfO2: ZrO2: TiO2: Nb2O5: Ta2O5: Si3N4 = 2: 2: 2: 1: 1: 10, and place the obtained mixed raw material powder in a high energy density stirred bead mill, add appropriate amount of deionized water and Φ1mm ZrO2 grinding balls, and mix and finely grind the mixed raw material powder at a speed of 1200r / min for 10h. After mixing, separate the obtained slurry from the grinding balls, dry, grind and pass through a 60-mesh sieve to obtain a precursor powder, and then press the precursor powder into several small round blocks with a single weight of 3g and a Φ20mm and place them in a pressureless furnace and keep them warm at 1800℃ in an argon atmosphere for 1h to synthesize (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramic powder, grinding the obtained high entropy ceramic powder through a 100 mesh sieve;

[0049] (3) Graphene mixing: 30 mL of graphene suspension and 270 mL of anhydrous ethanol were added to 10 g of high-entropy ceramic powder to form a slurry. The slurry was added to a high-energy density stirred bead mill, and ZrO2 grinding balls were added. The two were ball-milled at a speed of 1200 r / min for 5 h to obtain a mixed slurry. After separating the grinding balls, the mixed slurry was dried on a rotary evaporator at a temperature of 40°C and a vacuum condition of -0.09 MPa for 3.5 h, and then ground and passed through a 100-mesh sieve to obtain a composite powder.

[0050] (4) Sintering of ceramics: The composite powder is loaded into an SPS graphite mold, which is surrounded by graphite paper. The graphite mold is placed in an SPS sintering furnace. After the hydraulic station is started to compact the powder, the temperature is raised to 2100°C in a vacuum environment. The temperature is maintained at 40 MPa for 10 minutes to sinter the ceramics. The heating process is divided into three stages:

[0051] The first stage: 0-650℃, heating rate is 92℃ / min;

[0052] The second stage: 1600-1900℃, heating rate of 65℃ / min;

[0053] The third stage: above 1900℃, heating rate is 29℃ / min;

[0054] In this embodiment, the addition amount of graphene is 0.5 wt.%, and the prepared ceramic sample is numbered G 0.5 .

[0055] Example 2:

[0056] The preparation method of the graphene composite toughened high entropy nitride ceramic of this embodiment comprises the following steps:

[0057] (1) Graphene dispersion: Graphene, PVP, and anhydrous ethanol were added to a wall breaking machine at a mass ratio of graphene: PVP: anhydrous ethanol = 1:0.8:588, and the wall breaking machine was started to disperse the graphene at a speed of 18000 r / min for 40 min. During the dispersion, running water was used for cooling to obtain a graphene suspension;

[0058] (1) Synthesis of high entropy powder: Weigh the above raw material powders according to the molar ratio of HfO2: ZrO2: TiO2: Nb2O5: Ta2O5: Si3N4 = 2: 2: 2: 1: 1: 10, and place the obtained mixed raw material powder in a high energy density stirred bead mill, add appropriate amount of deionized water and Φ1mm ZrO2 grinding balls, and mix and finely grind the mixed raw material powder at a speed of 1200r / min for 10h. After mixing, separate the obtained slurry from the grinding balls, dry, grind and pass through a 60-mesh sieve to obtain a precursor powder, and then press the precursor powder into several small round blocks with a single weight of 3g and a Φ20mm and place them in a pressureless furnace and keep them warm at 1800℃ in an argon atmosphere for 1h to synthesize (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramic powder, grinding the obtained high entropy ceramic powder through a 100 mesh sieve;

[0059] (3) Graphene mixing: 150 mL of graphene suspension and 150 mL of anhydrous ethanol were added to 10 g of high entropy ceramic powder to form a slurry. The slurry was added to a high energy density stirred bead mill, and ZrO2 grinding balls were added. The two were ball milled at a speed of 1200 r / min for 5 h to obtain a mixed slurry. After separating the grinding balls, the mixed slurry was dried on a rotary evaporator at a temperature of 40°C and a vacuum condition of -0.09 MPa for 3.5 h, and then ground and passed through a 100 mesh sieve to obtain a composite powder.

[0060] (4) Sintering of ceramics: The composite powder is loaded into an SPS graphite mold, which is surrounded by graphite paper. The graphite mold is placed in an SPS sintering furnace. After the hydraulic station is started to compact the powder, the temperature is raised to 2100°C in a vacuum environment. The temperature is maintained at 40 MPa for 10 minutes to sinter the ceramics. The heating process is divided into three stages:

[0061] The first stage: 0-650℃, heating rate is 92℃ / min;

[0062] The second stage: 1600-1900℃, heating rate of 65℃ / min;

[0063] The third stage: above 1900℃, heating rate is 29℃ / min;

[0064] In this embodiment, the addition amount of graphene is 2.5 wt.%, and the prepared ceramic sample is numbered G 2.5 .

[0065] Example 3:

[0066] (1) Graphene dispersion: Graphene, PVP, and anhydrous ethanol were added to a wall breaking machine at a mass ratio of graphene: PVP: anhydrous ethanol = 1:0.8:588, and the wall breaking machine was started to disperse the graphene at a speed of 18000 r / min for 40 min. During the dispersion, running water was used for cooling to obtain a graphene suspension;

[0067] (1) Synthesis of high entropy powder: Weigh the above raw material powders according to the molar ratio of HfO2: ZrO2: TiO2: Nb2O5: Ta2O5: Si3N4 = 2: 2: 2: 1: 1: 10, and place the obtained mixed raw material powder in a high energy density stirred bead mill, add appropriate amount of deionized water and Φ1mm ZrO2 grinding balls, and mix and finely grind the mixed raw material powder at a speed of 1200r / min for 10h. After mixing, separate the obtained slurry from the grinding balls, dry, grind and pass through a 60-mesh sieve to obtain a precursor powder, and then press the precursor powder into several small round blocks with a single weight of 3g and a Φ20mm and place them in a pressureless furnace and keep them warm at 1800℃ in an argon atmosphere for 1h to synthesize (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramic powder, grinding the obtained high entropy ceramic powder through a 100 mesh sieve;

[0068] (3) Graphene mixing: 300 mL of graphene suspension was added to 10 g of high entropy ceramic powder to form a slurry, the slurry was added to a high energy density stirred bead mill, and ZrO2 grinding balls were added. The two were ball milled at a speed of 1200 r / min for 5 h to obtain a mixed slurry. After separating the grinding balls, the mixed slurry was dried on a rotary evaporator at a temperature of 40°C and a vacuum condition of -0.09 MPa for 3.5 h, and then ground and passed through a 100 mesh sieve to obtain a composite powder;

[0069] (4) Sintering of ceramics: The composite powder is loaded into an SPS graphite mold, which is surrounded by graphite paper. The graphite mold is placed in an SPS sintering furnace. After the hydraulic station is started to compact the powder, the temperature is raised to 2100°C in a vacuum environment. The temperature is maintained at 40 MPa for 10 minutes to sinter the ceramics. The heating process is divided into three stages:

[0070] The first stage: 0-650℃, heating rate is 92℃ / min;

[0071] The second stage: 1600-1900℃, heating rate of 65℃ / min;

[0072] The third stage: above 1900℃, heating rate is 29℃ / min;

[0073] In this embodiment, the addition amount of graphene is 5.0 wt.%, and the prepared ceramic sample is numbered G5.

[0074] The performance characterization and analysis of graphene composite toughened high entropy nitride ceramic materials are as follows:

[0075] Several groups of samples were analyzed. Figure 1 and Figure 2 Represents different addition amounts of graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N composite ceramics. Figure 1 It can be seen that graphene is mostly distributed at the grain boundaries in the composite ceramics and is oriented perpendicular to the applied uniaxial pressure. 0.5 and G 2.5 (see, Figure 1 (a), (b), Figure 2 (a), (b)), graphene is evenly distributed in the matrix and embedded in the ceramic block in a thin flat state, with a layer thickness of 0.25μm-0.4μm. However, in sample G5 ( Figure 1 (c) Figure 2 (c) Graphene agglomerates and thin graphene layers are stacked up again, making the thickness of graphene embedded in the ceramic cross section reach 0.9 μm. This is because the van der Waals force between thin graphene molecules is very strong. Sample G 0.5 and G 2.5 In (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N ceramic matrix can withstand the lower graphene filler density at this time, (Hf 0.2 Zr0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N ceramic particles completely isolate the added graphene, and graphene can be well dispersed. However, at a higher loading of graphene filler (e.g., ≧5.0wt.%), the graphene density increases, and it is difficult for the matrix to separate more graphene. Graphene will reduce the van der Waals force by increasing the number of layers to achieve kinetic stability of the composite ceramic, so the graphene in sample G5 agglomerates. Figure 3 For different doping amounts of graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N composite high entropy ceramics density. Figure 3 It can be seen that samples G0 and G 0.5、 , G 2.5 The density of G5 and G6 are 92.6%, 93.3%, 93.9% and 90.4% respectively. Obviously, with the increase of graphene content, the density increases first and then decreases. This is because when the appropriate amount of graphene (e.g., ≤2.5wt.%) is embedded in the ceramic matrix, it does not cause pores and cracks, and has a close bond and good interface compatibility (see Figure 1 (a), (b)), and the self-lubricating effect of graphene during sintering is conducive to particle rearrangement, and can also improve heat transfer and play a role in sintering, thereby increasing the density of the composite ceramics. However, when excessive graphene (e.g., ≥5.0 wt.%) is added to (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N ceramic matrix, graphene is difficult to disperse effectively and agglomerates, making the system uneven and generating holes (see Figure 1 (c)), resulting in a decrease in the density of the composite ceramics. Figure 4 Shows the different doping amounts of graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N composite high entropy ceramics hardness, fracture toughness, flexural strength changes. Figure 4 It can be seen that with the increase of graphene addition, the hardness of the composite ceramic decreases from 20.15GPa to 16.78GPa. This is because although graphene has high hardness and stable structure, it is a flexible material. This flexibility makes the composite ceramic more likely to deform when subjected to external force, thereby reducing the hardness and offsetting the effect of increasing density. However, with the increase of graphene addition, the fracture toughness and flexural strength of the composite ceramic first increase and then decrease. Sample G 0.5, G 2.5 The fracture toughness is 3.45MPa·m 1 / 2 , 4.78MPa·m 1 / 2 , which are higher than the value of sample G0 (i.e., 2.93 MPa·m 1 / 2 ). In addition, sample G 0.5 and G 2.5 The flexural strength of the sample G0 is also higher than that of the sample G0. The improvement in flexural strength is attributed to the high tensile strength of graphene and the high tensile strength of graphene in (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N pull-out phenomenon in high entropy ceramic matrix (see, Figure 8 (e) and (f) show that graphene overcomes friction in the ceramic matrix during pull-out, delaying the fracture of the flexurally resistant sample. However, when graphene is added in excess (e.g., ≥5.0 wt.%), it agglomerates. The resulting agglomeration defects lead to weak bonding interfaces, resulting in poor mechanical properties of the ceramic.

[0076] Figure 5 , Figure 6 shows graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N composite high entropy ceramic bonding interface, where the black area is (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramics, the white area is graphene, the interplanar spacing is 0.244nm and 0.343nm respectively, it can be observed that there are two-phase interface transition zones in the composite ceramics with different doping amounts, (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N region has clear lattice fringes and regular spacing, but in the area in contact with graphene, part of the graphene enters the (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N high entropy ceramics produce lattice distortion (see Figure 6 (a)), which makes the lattice fringes become curved and disordered. This lattice distortion is more obvious in the transition region (see Figure 6 (c)), and make (Hf 0.2 Zr 0.2 Ti0.2 Nb 0.2 Ta 0.2 )N further increases in interplanar spacing (d = 0.251 nm), and with the increase in graphene content, the transition layer caused by lattice distortion becomes more and more obvious (see Figure 5 ). The lattice distortion caused by graphene entering the lattice can improve the fracture toughness of the matrix ceramic. 0.5 , G 2.5 Graphene and (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N composite high entropy ceramics have good interface bonding, and no cracks or pores are found at the bonding point (see Figure 5 (a), (b)), in sample G5, due to the agglomeration of graphene, the interface bonding ability between the two phases is weak, there is a crack in the bonding interface, and more carbon will enter (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N lattice, destroying the original lattice structure and reducing the density.

[0077] Figure 7 shows graphene / (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N composite high entropy ceramics, including the intrinsic toughening mode of graphene in the ceramic matrix ( Figure 7 (a)), bending and wrapping of grains ( Figure 7 (b), (c)). The sliding of graphene in the ceramic matrix is ​​conducive to the extraction of graphene after cracks are generated, absorbing more energy. At the same time, due to the flexibility of graphene, a large-angle U-shaped bend can be formed in the ceramic ( Figure 7 (c)) increases the bonding area between graphene and the ceramic matrix. Graphene bending at different locations and in different directions within the ceramic matrix can dissipate energy during coordinated deformation. Furthermore, graphene bending is often accompanied by wrapping around grains. This wrapping along grain boundaries enhances interfacial adhesion, forming a certain degree of mechanical interlocking, improving load transfer efficiency and generating more energy dissipation when the matrix is ​​subjected to external forces.

[0078] In addition to the intrinsic toughening mode of graphene, graphene and (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta0.2 )N ceramics also produce an extrinsic toughening mode, Figure 8 (a)-(f) are the non-intrinsic toughening modes of graphene, Figure 8 (a)-(f) show that due to the presence of graphene, the deflection, bridging, and bifurcation of the cracks and the pulling out of the graphene occurred when the ceramic was subjected to stress. Figure 8 It can be found in (a) and (b) that when the crack encounters graphene during the propagation process, the crack will deflect and extend along the direction of the graphene sheet structure, resulting in an increase in the crack deflection, thereby extending the crack propagation path, or causing bifurcation, such as Figure 8 As shown in (c), multiple sub-cracks are generated to disperse the energy of the main crack tip, increase the tortuosity of the crack, and prevent crack propagation. Both mechanisms increase toughness by consuming the concentrated stress at the crack tip. In the debonding area of ​​the matrix, due to the large surface area and high toughness of graphene, it can bond the cracks in the fracture process and provide stress to bring the two ends of the crack closer to each other ( Figure 8 (b), (d)), thus effectively preventing the further expansion of the crack; in addition, when the main crack in the ceramic extends to the graphene, the graphene is pulled out from the matrix due to the effect of the interfacial bonding force ( Figure 8 (e)-(f)), due to the two-dimensional layer structure of graphene, the extraction of graphene will cause interlayer slip and complex interlayer friction, absorbing a large amount of fracture energy. Compared with one-dimensional carbon nanotubes, the extraction of graphene with a two-dimensional wrinkled structure can cause more energy dissipation.

[0079] Sample G 2.5 The best mechanical properties (Vickers hardness 18.36GPa, fracture toughness 4.78MPa·m 1 / 2 383 MPa, flexural strength). This is due to the thin graphene sheets and moderate content. This perfect balance allows for ideal dispersion and strong interfacial bonding with relatively low graphene additions, while also ensuring sufficient graphene in the matrix to provide toughening. This compromise allows the material to maintain the effectiveness of various toughening mechanisms while also increasing the frequency of their occurrence.

[0080] It should be noted that the embodiments described herein are only some embodiments of the present invention, not all implementations of the present invention. The embodiments are merely illustrative and serve only to provide a more intuitive and clear way to understand the content of the present invention, rather than to limit the technical solutions described in the present invention. Without departing from the concept of the present invention, all other implementations that can be thought of by ordinary technicians in this field without making creative efforts, as well as other simple replacements and various variations of the technical solutions of the present invention, are within the scope of protection of the present invention.

Claims

1. A method for preparing graphene composite toughened high entropy nitride ceramics, characterized in that: The following steps are involved: (1) Dispersion of graphene: Graphene, PVP, and anhydrous ethanol were added to a wall-breaking machine at a mass ratio of graphene: PVP: anhydrous ethanol = 1: (0.7-0.9): (400-667). The wall-breaking machine was started to disperse the graphene at a speed of 17,000-19,000 r / min for 20-60 min. During the dispersion, running water was used for cooling to obtain a graphene suspension. (2) Synthesis of high entropy powder: Weigh the above raw material powders according to the molar ratio of HfO2: ZrO2: TiO2: Nb2O5: Ta2O5: Si3N4 = 2: 2: 2: 1: 1: 10, and place the obtained mixed raw material powder in a high energy density stirring bead mill, add appropriate amount of deionized water and grinding balls, and mix and finely grind the mixed raw material powder at a speed of 1200 r / min for 8~12 hours. After the mixing is completed, the obtained slurry is The precursor powder is separated from the grinding balls, and the slurry is dried, ground, and passed through a 60-mesh sieve to obtain a precursor powder. The precursor powder is then pressed into several small round blocks weighing 3 g and Φ20 mm and placed in a pressureless furnace. The blocks are kept at 1750-1850°C in an argon atmosphere for 0.5-1.5 hours to synthesize (Hf0.2Zr0.2Ti0.2Nb0.2Ta0.2)N high-entropy ceramic powder. The obtained high-entropy ceramic powder is ground and passed through a 100-mesh sieve. (3) Graphene mixing: According to the addition amount of graphene in high entropy ceramic powder of 0.5~2.5wt.%, high entropy ceramic powder and graphene suspension are added to a high energy density stirred bead mill, grinding balls are added, and the two are ball-milled at a speed of 1100~1300r / min for 4~6h to obtain a mixed slurry. The high-speed friction between the grinding balls is used to evenly and stretch the graphene sheets between the particles of the high entropy ceramic powder. After separating the grinding balls, the mixed slurry is dried for 3~4h using a rotary evaporator at a temperature of 35~45℃ and a vacuum condition of -(0.08~0.1) MPa, and then ground and passed through a 100-mesh sieve to obtain a composite powder; (4) Sintering of ceramics: The composite powder is loaded into an SPS graphite mold, which is surrounded by graphite paper. The graphite mold is placed in an SPS sintering furnace. After the hydraulic station is started to compact the powder, the temperature is raised to 2050~2150℃ in a vacuum environment. The temperature is kept at 35~45MPa pressure for 5~15min to sinter the ceramics. The heating process is divided into three stages: The first stage: 0~650℃, heating rate is 92℃ / min; The second stage: 1600~1900℃, heating rate is 65℃ / min; The third stage: above 1900℃, heating rate is 29℃ / min.

2. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: In the step (1), the mass ratio of graphene:PVP:anhydrous ethanol is 1:0.8:588, the rotation speed of the wall breaking machine blade is 18000 r / min, and the dispersion time is 40 min.

3. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: The grinding balls in step (2) and step (3) are ZrO2 balls with a diameter of 1 mm.

4. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: The mass ratio of materials in step (2) is raw material powder: grinding balls: deionized water = 100:300:

750.

5. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: In step (2), the rotation speed of the high energy density stirring bead mill is 1200 r / min, and the mixing time is 10 h.

6. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: In step (2), the sintering temperature in the pressureless furnace is 1800° C., and the holding time is 1 h.

7. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: In step (3), the rotation speed of the high energy density stirring bead mill is 1200 r / min, and the mixing time is 5 h.

8. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: In step (3), the temperature of the rotary evaporator is 40° C., the vacuum degree is -0.09 MPa, and the drying time is 3.5 h.

9. The method for preparing the graphene composite toughened high entropy nitride ceramic according to claim 1, wherein: In the step (4), the sintering temperature is 2100° C., the pressure is 40 MPa, and the sintering time is 10 min.

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