Method for preparing carbon nanotube toughened silicon carbide ceramic composite material

Abstract

The invention relates to the technical field of modification of silicon carbide ceramic materials, in particular to a method for preparing a carbon nanotube toughened silicon carbide ceramic composite material. The method comprises the steps of dispersion of the carbon nanotubes, mixing and dispersion of the carbon nanotubes, silicon carbide raw powder and a sintering aid, preparation of mixed powder, compression molding, pyrolysis and glue removal, pressureless sintering and the like. According to the invention, the carbon nanotube with short length-diameter ratio (30-54) and high defect degree (ID / IG = 0.98) is selected as a modified material; the structure with short length-diameter ratio can effectively improve the dispersibility of the carbon nanotube in a matrix; due to the characteristic of high defect degree, interface bonding with a silicon carbide matrix is promoted through reactive sintering in the ultrahigh-temperature sintering process, so that the comprehensive performance of the composite material is remarkably improved. The bending strength and the fracture toughness of the prepared carbon nanotube toughened silicon carbide ceramic material can reach 529.05 Mpa and 4.74 MPa.m < 1 / 2 > respectively, and the method is simple in process, low in production cost, high in production efficiency and suitable for industrial preparation of the high-performance silicon carbide ceramic material.

Description

Technical Field

[0001] The present invention relates to the technical field of modification of silicon carbide ceramic materials, and particularly relates to a method for preparing carbon nanotube toughened silicon carbide ceramic composite materials. Background Art

[0002] Silicon carbide (SiC) ceramics are widely used in high-end fields such as military defense, petrochemical industry, machinery manufacturing, aerospace, and semiconductors due to their excellent high-temperature properties, mechanical strength, wear resistance, oxidation resistance, and low density. However, similar to other ceramic materials, silicon carbide ceramics have high brittleness and low fracture toughness, which become an important bottleneck restricting their further promotion in the integrated application of structural functions.

[0003] In recent years, the use of second-phase reinforcement technology has become an important research direction for improving the toughness of ceramic materials. Carbon nanotubes (CNTs) are considered to be one of the ideal reinforcing phases for improving the mechanical properties of ceramic materials due to their excellent mechanical properties, such as high strength, high modulus, large elastic strain, and good fracture energy dissipation ability. Toughening mechanisms such as bridging, pulling out, and fracture of carbon nanotubes can effectively improve the fracture toughness of the ceramic matrix.

[0004] However, in practical applications, carbon nanotubes as reinforcing phases face the following key problems: (1) There is a strong van der Waals force between carbon nanotubes, which is extremely easy to agglomerate, resulting in difficult uniform dispersion in the ceramic matrix; (2) The interfacial bonding between carbon nanotubes and the ceramic matrix is weak, affecting the load transfer efficiency, thereby reducing the mechanical enhancement effect of the composite material. These problems significantly limit the application of carbon nanotubes in the reinforcement of structural ceramics.

[0005] Previous studies have tried to improve the performance of CNTs / SiC ceramic composites through different methods. For example, Patent CN201610967909.3 discloses a method for in-situ growing carbon nanotubes on the surface of silicon carbide and using a polycarbosilane solution for impregnation molding. Although there is an improvement in thermal conductivity and oxidation resistance, its flexural strength is low, and the hot pressing molding process is complex and inefficient, not suitable for large-scale application. Patent CN201110438186.5 proposes a combined method of hot pressing sintering and pressureless sintering, but it only shows that the excellent mechanical properties of carbon nanotubes play a role in hot pressing sintering silicon carbide materials.

[0006] Therefore, developing a pressureless solid-phase sintering process for carbon nanotube / silicon carbide ceramic composites with simple process, industrialization feasibility, and significant enhancement effect has important theoretical significance and engineering application value. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for preparing carbon nanotube toughened silicon carbide ceramic composites. The present invention uses carbon nanotubes with a short aspect ratio and a high degree of defects as the second phase and an uniaxial solid-state sintering method. While ensuring the improvement of the strength and toughness of the material through toughening mechanisms such as the pull-out, bridging, and fracture of carbon nanotubes, a method for preparing silicon carbide ceramic composites with high efficiency, simplicity, and economy is realized.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing carbon nanotube toughened silicon carbide ceramic composites, comprising the following steps:

[0010] 1) Dispersion of carbon nanotubes: Disperse the carbon nanotubes in an ethanol solution and perform ultrasonic dispersion to obtain a carbon nanotube slurry. The average aspect ratio of the carbon nanotubes is 30 - 54, and the I D / I G value is 0.8 - 1.2;

[0011] 2) Mixing and dispersion: Mix the carbon nanotube slurry obtained in step 1) with silicon carbide raw powder, boron carbide, and phenolic resin, and then perform ball milling to obtain a uniformly mixed slurry;

[0012] 3) Powder preparation: Dry, grind, and screen the slurry obtained in step 2) to obtain a powder;

[0013] 4) Compression molding: Prepare the powder obtained in step 3) into a preformed green body, and then perform cold isostatic pressing on the preformed green body to obtain a dense green body;

[0014] 5) Pyrolysis debinding: Heat the dense green body obtained in step 4) under argon protection and hold for a certain time, and then cool to obtain a green body after pyrolysis debinding;

[0015] 6) Uniaxial sintering: Perform uniaxial sintering treatment on the green body after pyrolysis debinding in step 5) under argon protection to obtain carbon nanotube toughened silicon carbide ceramic composites.

[0016] Further, the time for ultrasonic dispersion in step 1) is 0.5 - 5h.

[0017] Further, the mass ratio of silicon carbide raw powder : boron carbide : phenolic resin : carbon nanotubes in step 2) is 1 : (0.0025 - 0.015) : (0.03 - 0.16) : (0.005 - 0.04).

[0018] Further, the ball milling in step 2) uses silicon carbide grinding balls, the ball milling time is 2 - 8h, and the ball - to - material ratio is (3 - 8) : 1.

[0019] Further, the drying time in step 3) is 15 - 30 h, the drying temperature is 60 - 75 °C, and the sieve mesh for sieving is 40 - 60 meshes.

[0020] Further, in step 4), the pressure for preparing the preformed body is 20 - 40 MPa, the pressure holding time is 40 - 80 s, the pressure of cold isostatic pressing is 150 - 300 MPa, and the pressure holding time is 150 - 250 s.

[0021] Further, in step 5), the heating rate is 2 - 10 °C / min, the heat preservation temperature is 800 - 900 °C, and the heat preservation time is 1.5 - 3 h.

[0022] Further, in step 6), the temperature of pressureless sintering is 2000 - 2200 °C, the heating rate is 2 - 20 °C / min, and the heat preservation time is 1.5 - 3 h.

[0023] Through the systematic design of the aspect ratio and defect degree (I D / I G value) of carbon nanotubes, the present invention optimizes the dispersion and interfacial bonding behavior of carbon nanotubes in the silicon carbide matrix. Specifically, the short aspect ratio helps to inhibit the agglomeration of carbon nanotubes during the slurry preparation and sintering processes, improves the dispersion uniformity in the silicon carbide matrix, and thus enhances the load transfer efficiency. The high defect degree enables carbon nanotubes to easily react with the silicon carbide matrix during the high-temperature pressureless sintering process, promotes the formation of stable chemical bonds at the interface, and enhances the interfacial strength. The parameter selection of the aspect ratio and I D / I G value in the present invention is based on the synergistic optimization of the overall material ratio, ultrasonic dispersion, ball milling process, pyrolysis debinding, and pressureless sintering temperature system, enabling the strengthening and toughening mechanisms of carbon nanotubes (including bridging, pulling out, and fracture energy consumption) to be fully exerted under the condition of no external pressure, thereby significantly improving the flexural strength and fracture toughness of the silicon carbide ceramic composite. In the traditional process, the influence of the aspect ratio and defect degree of carbon nanotubes on the strength and toughness of the silicon carbide ceramic composite was not clearly considered, and there was a lack of a complete idea of synergistically regulating the two to adapt to the pressureless sintering system and maximize the interfacial bonding strength and material toughness. By the matching regulation of the structural characteristics of carbon nanotubes and the sintering process conditions, the present invention overcomes the problems of serious agglomeration of carbon nanotubes, weak interfacial bonding, and limited strengthening effect in the prior art, and significantly improves the comprehensive performance of the silicon carbide ceramic composite.

[0024] The present invention also provides a carbon nanotube toughened silicon carbide ceramic composite, which is prepared by using the method for preparing a carbon nanotube toughened silicon carbide ceramic composite as described above.

[0025] Advantages and beneficial effects of the present invention:

[0026] 1. The present invention prepares a ceramic composite material by a pressureless solid-phase sintering method. This method is simple, low-cost, and universal, and is easy to scale up production.

[0027] 2. Carbon nanotubes with a short aspect ratio can effectively inhibit their agglomeration tendency in the ceramic matrix and have better dispersibility in the silicon carbide matrix, which helps to improve the strengthening and toughening effects of carbon nanotubes on the composite material.

[0028] 3. Carbon nanotubes with a high degree of defects have a higher interfacial bonding strength with the ceramic matrix, ensuring the effective transfer of load between the carbon nanotubes and the ceramic matrix. As a result, the crack formation ability of the ceramic composite material is improved and the crack propagation speed is reduced, thereby enhancing the strength and fracture toughness of the material. The flexural strength and fracture toughness can reach 529.05 MPa and 4.74 MPa·m respectively. 1 / 2 。 Brief Description of the Drawings

[0029] Figure 1 is a schematic diagram of the preparation process of the carbon nanotube toughened silicon carbide ceramic composite material described in the present invention;

[0030] Figure 2 is the Raman spectrum of carbon nanotubes with different aspect ratios and degrees of defects;

[0031] Figure 3 is the scanning morphology diagram of carbon nanotubes with different aspect ratios and degrees of defects;

[0032] Figure 4 is the flexural strength, fracture toughness, and bulk density of sintered specimens with different types of carbon nanotubes added;

[0033] Figure 5 is the XRD pattern of sintered specimens with different carbon nanotube addition amounts;

[0034] Figure 6 is the fracture surface morphology of sintered specimens with different carbon nanotube addition amounts;

[0035] Figure 7 is the schematic diagram of crack propagation of a sintered specimen with 1 wt.% carbon nanotube addition;

[0036] Figure 8 is the bulk density, apparent porosity, and relative density of sintered specimens with different carbon nanotube addition amounts;

[0037] Figure 9 is the flexural strength, hardness, and fracture toughness of sintered specimens with different carbon nanotube addition amounts. Detailed Embodiments

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. In addition, it is worth noting that the raw materials involved in the present invention are all ordinary commercially available products without special instructions.

[0039] Example 1

[0040] A method for preparing carbon nanotube toughened silicon carbide ceramic composite material, and its preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0041] 1) Dispersion of carbon nanotubes: Carbon nanotubes with an average aspect ratio of 40 and I D / I G = 0.98 are dispersed in an ethanol solution and ultrasonically dispersed for 2 h to obtain a carbon nanotube slurry;

[0042] 2) Mixed dispersion: The carbon nanotube slurry obtained in step 1) is added to a slurry containing silicon carbide raw powder, boron carbide, and phenolic resin, and ball-milled for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder: boron carbide: phenolic resin: carbon nanotubes = 1: 0.005: 0.1: 0.01;

[0043] 3) Powder preparation: The uniformly mixed slurry obtained in step 2) is placed in an oven and dried for 24 h, then fully ground and sieved through a 50-mesh sieve to obtain a powder;

[0044] 4) Compression molding: The powder obtained in step 3) is pressed into a preformed green body with a casting steel mold under a molding pressure of 30 MPa and a pressure holding time of 60 s, and then the preformed green body is cold isostatically pressed at 200 MPa and a pressure holding time of 180 s to obtain a dense green body;

[0045] 5) Pyrolysis debinding: The dense green body obtained in step 4) is placed in an atmosphere box furnace, heated to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon, and held for 2 h, and then cooled with the furnace to obtain a green body after pyrolysis debinding;

[0046] 6) Pressureless sintering: The green body after pyrolysis debinding in step 5) is placed in a pressureless sintering furnace, evacuated and filled with argon as a protective gas, and sintered at a sintering temperature of 2200 °C for 2 h under pressureless sintering treatment, and then cooled with the furnace to obtain Sample SC2.

[0047] Figure 2 a and Figure 3 a has a short aspect ratio (average aspect ratio of 40) and a high degree of defect (I D / IG The Raman spectrum of carbon nanotubes with an average aspect ratio of 40 and I D / I G = 0.98 and the scanning morphology after 30 minutes of ultrasonic treatment show that the carbon nanotubes with a short aspect ratio have good dispersibility.

[0048] Example 2

[0049] A method for preparing carbon nanotube toughened silicon carbide ceramic composite, and its preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0050] 1) Dispersion of carbon nanotubes: Disperse carbon nanotubes with an average aspect ratio of 40 and I D / I G = 0.98 in an ethanol solution and ultrasonically disperse for 2 h to obtain a carbon nanotube slurry;

[0051] 2) Mixed dispersion: Add the carbon nanotube slurry obtained in step 1) to a slurry containing silicon carbide raw powder, boron carbide and phenolic resin, and ball mill for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder: boron carbide: phenolic resin: carbon nanotubes = 1: 0.005: 0.1: 0.005;

[0052] 3) Powder preparation: Put the uniformly mixed slurry obtained in step 2) into an oven and dry for 24 h, then grind thoroughly and pass through a 50-mesh sieve to obtain a powder;

[0053] 4) Compression molding: Press the powder obtained in step 3) with a cast steel mold at a molding pressure of 30 MPa and hold the pressure for 60 s to obtain a preformed green body, and then subject the preformed green body to cold isostatic pressing at 200 MPa and hold the pressure for 180 s to obtain a dense green body;

[0054] 5) Pyrolysis and debinding: Put the dense green body obtained in step 4) into an atmosphere box furnace, heat it up to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon and hold for 2 h, and then cool it with the furnace to obtain a green body after pyrolysis and debinding;

[0055] 6) Pressureless sintering: Put the green body after pyrolysis and debinding in step 5) into a pressureless sintering furnace, evacuate and fill with argon as the protective gas, sinter at a sintering temperature of 2200 °C for 2 h under pressureless sintering treatment, and then cool it with the furnace to obtain sample SC1.

[0056] Example 3

[0057] A method for preparing carbon nanotube toughened silicon carbide ceramic composite, and its preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0058] 1) Dispersion of carbon nanotubes: Disperse carbon nanotubes with an average aspect ratio of 40 and I D / IG Carbon nanotubes with an aspect ratio of 0.98 were dispersed in an ethanol solution and ultrasonically dispersed for 2 h to obtain a carbon nanotube slurry.

[0059] 2) Mixing and dispersion: The carbon nanotube slurry obtained in step 1) was added to a slurry containing silicon carbide raw powder, boron carbide, and phenolic resin, and ball-milled for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder:boron carbide:phenolic resin:carbon nanotubes == 1:0.005:0.1:0.02.

[0060] 3) Powder preparation: The uniformly mixed slurry obtained in step 2) was placed in an oven and dried for 24 h, then thoroughly ground and passed through a 50-mesh sieve to obtain a powder.

[0061] 4) Compression molding: The powder obtained in step 3) was pressed into a preformed green body using a cast steel mold at a molding pressure of 30 MPa and held for 60 s. Then, the preformed green body was cold isostatically pressed at 200 MPa and held for 180 s to obtain a dense green body.

[0062] 5) Pyrolysis debinding: The dense green body obtained in step 4) was placed in an atmosphere box furnace and heated to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon and held for 2 h, and then cooled with the furnace to obtain a pyrolysis debound green body.

[0063] 6) Pressureless sintering: The pyrolysis debound green body obtained in step 5) was placed in a pressureless sintering furnace, evacuated and filled with argon as the protective gas, and sintered at a sintering temperature of 2200 °C for 2 h under pressureless sintering treatment, and then cooled with the furnace to obtain Sample SC3.

[0064] Example 4

[0065] A method for preparing carbon nanotube toughened silicon carbide ceramic composite materials, the preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0066] 1) Dispersion of carbon nanotubes: Carbon nanotubes with an average aspect ratio of 40 and I D / I G = 0.98 were dispersed in an ethanol solution and ultrasonically dispersed for 2 h to obtain a carbon nanotube slurry.

[0067] 2) Mixing and dispersion: The carbon nanotube slurry obtained in step 1) was added to a slurry containing silicon carbide raw powder, boron carbide, and phenolic resin, and ball-milled for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder:boron carbide:phenolic resin:carbon nanotubes == 1:0.005:0.1:0.04.

[0068] 3) Powder preparation: Put the uniform slurry obtained in step 2) into an oven for drying for 24 h, then grind it thoroughly and sieve it through a 50-mesh sieve to obtain the powder;

[0069] 4) Compression molding: Use the powder obtained in step 3) with a cast steel mold under a molding pressure of 30 MPa, hold the pressure for 60 s to form a preformed green body, and then subject the preformed green body to cold isostatic pressing at 200 MPa for 180 s to obtain a dense green body;

[0070] 5) Pyrolysis debinding: Put the dense green body obtained in step 4) into an atmosphere box furnace, heat it up to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon, hold the temperature for 2 h, and cool it with the furnace to obtain the green body after pyrolysis debinding;

[0071] 6) Pressureless sintering: Put the green body after pyrolysis debinding in step 5) into a pressureless sintering furnace, evacuate and fill it with argon as the protective gas, carry out pressureless sintering treatment at a sintering temperature of 2200 °C for 2 h, and cool it with the furnace to obtain the SC4 sample.

[0072] Comparative Example 1

[0073] A preparation method for preparing silicon carbide ceramic materials includes the following steps:

[0074] 1) Mixing and dispersion: Weigh silicon carbide raw powder, boron carbide and phenolic resin respectively according to the mass ratio of 1:0.005:0.1, add them into a ball milling tank, and carry out ball milling for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry;

[0075] 2) Powder preparation: Put the uniform slurry obtained in step 1) into an oven for drying for 24 h, then grind it thoroughly and sieve it through a 50-mesh sieve to obtain the powder;

[0076] 3) Compression molding: Use the powder obtained in step 2) with a cast steel mold under a molding pressure of 30 MPa, hold the pressure for 60 s to form a preformed green body, and then subject the preformed green body to cold isostatic pressing at 200 MPa for 180 s to obtain a dense green body;

[0077] 4) Pyrolysis debinding: Put the dense green body obtained in step 3) into an atmosphere box furnace, heat it up to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon, hold the temperature for 2 h, and cool it with the furnace to obtain the green body after pyrolysis debinding;

[0078] 5) Pressureless sintering: Put the green body after pyrolysis debinding in step 4) into a pressureless sintering furnace, evacuate and fill it with argon as the protective gas, carry out pressureless sintering treatment at a sintering temperature of 2200 °C for 2 h, and cool it with the furnace to obtain the SC0 sample.

[0079] Comparative Example 2

[0080] A method for preparing carbon nanotube toughened silicon carbide ceramic composite material, the preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0081] 1) Dispersion of carbon nanotubes: Carbon nanotubes with an average aspect ratio of 40 and I D / I G = 0.17 are dispersed in an ethanol solution and ultrasonically dispersed for 2 h to obtain a carbon nanotube slurry;

[0082] 2) Mixing and dispersion: The carbon nanotube slurry obtained in step 1) is added to a slurry containing silicon carbide raw powder, boron carbide, and phenolic resin, and ball-milled for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder: boron carbide: phenolic resin: carbon nanotubes = 1: 0.005: 0.1: 0.01;

[0083] 3) Powder preparation: The uniformly mixed slurry obtained in step 2) is placed in an oven and dried for 24 h, then sufficiently ground and passed through a 50-mesh sieve to obtain a powder;

[0084] 4) Compression molding: The powder obtained in step 3) is pressed into a preformed green body with a steel casting mold under a molding pressure of 30 MPa and a holding pressure of 60 s, and then the preformed green body is cold isostatically pressed at 200 MPa and held for 180 s to obtain a dense green body;

[0085] 5) Pyrolysis debinding: The dense green body obtained in step 4) is placed in an atmosphere box furnace, heated to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon and held for 2 h, and then cooled with the furnace to obtain a green body after pyrolysis debinding;

[0086] 6) Pressureless sintering: The green body after pyrolysis debinding in step 5) is placed in a pressureless sintering furnace, evacuated and filled with argon as the protective gas, and sintered at a sintering temperature of 2200 °C for 2 h under pressureless sintering treatment, and then cooled with the furnace to obtain the SCA number sample.

[0087] Figure 2 b and Figure 3 b is the Raman spectrum and the scanning morphology map after 30 minutes of ultrasonic treatment of carbon nanotubes with the same short aspect ratio (average aspect ratio of 40) and lower defect degree (I D / I G = 0.17). It can be seen that the dispersibility of carbon nanotubes with the same short aspect ratio is close.

[0088] Comparative example 3

[0089] A method for preparing carbon nanotube toughened silicon carbide ceramic composite material, the preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0090] 1) Dispersion of carbon nanotubes: Carbon nanotubes with an average aspect ratio of 950 and I D / I G = 1.20 were dispersed in an ethanol solution and ultrasonically dispersed for 2 h to obtain a carbon nanotube slurry;

[0091] 2) Mixed dispersion: The carbon nanotube slurry obtained in step 1) was added to a slurry containing silicon carbide raw powder, boron carbide, and phenolic resin, and ball-milled for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder:boron carbide:phenolic resin:carbon nanotubes = 1:0.005:0.1:0.01;

[0092] 3) Powder preparation: The uniformly mixed slurry obtained in step 2) was placed in an oven and dried for 24 h, then sufficiently ground and passed through a 50-mesh sieve to obtain a powder;

[0093] 4) Compression molding: The powder obtained in step 3) was pressed into a preformed green body using a cast steel mold at a molding pressure of 30 MPa and held for 60 s, and then the preformed green body was cold isostatically pressed at 200 MPa and held for 180 s to obtain a dense green body;

[0094] 5) Pyrolysis debinding: The dense green body obtained in step 4) was placed in an atmosphere box furnace, heated to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon and held for 2 h, and then cooled with the furnace to obtain a pyrolysis debound green body;

[0095] 6) Pressureless sintering: The pyrolysis debound green body obtained in step 5) was placed in a pressureless sintering furnace, evacuated and filled with argon as a protective gas, and sintered at a sintering temperature of 2200 °C for 2 h under pressureless sintering treatment, and then cooled with the furnace to obtain an SCB sample.

[0096] Figure 2 c and Figure 3 c is the Raman spectrum of carbon nanotubes with a large aspect ratio (average aspect ratio of 950) and a high degree of defects (I D / I G = 1.20) and the scanning morphology map after 30 minutes of ultrasonic treatment. It can be seen that the dispersion effect of carbon nanotubes with a large aspect ratio is poor and there is still agglomeration.

[0097] Comparative Example 4

[0098] A method for preparing carbon nanotube toughened silicon carbide ceramic composite materials, and its preparation process is shown in Figure 1 , and the specific preparation method includes the following steps:

[0099] 1) Dispersion of carbon nanotubes: Carbon nanotubes with an average aspect ratio of 20 and I D / I GCarbon nanotubes with an aspect ratio of 0.96 were dispersed in an ethanol solution and ultrasonically dispersed for 2 h to obtain a carbon nanotube slurry;

[0100] 2) Mixing and dispersion: The carbon nanotube slurry obtained in step 1) was added to a slurry containing silicon carbide raw powder, boron carbide, and phenolic resin, and ball-milled for 5 h with silicon carbide grinding balls to obtain a uniformly mixed slurry, where the mass ratio of silicon carbide raw powder: boron carbide: phenolic resin: carbon nanotubes = 1: 0.005: 0.1: 0.01;

[0101] 3) Powder preparation: The uniformly mixed slurry obtained in step 2) was placed in an oven and dried for 24 h, then thoroughly ground and passed through a 50-mesh sieve to obtain a powder;

[0102] 4) Compression molding: The powder obtained in step 3) was pressed into a preformed green body using a cast steel mold at a molding pressure of 30 MPa and held for 60 s, and then the preformed green body was cold isostatically pressed at 200 MPa and held for 180 s to obtain a dense green body;

[0103] 5) Pyrolysis debinding: The dense green body obtained in step 4) was placed in an atmosphere box furnace, heated to 900 °C at a heating rate of 5 °C / min under the protection of flowing argon and held for 2 h, and then cooled with the furnace to obtain a pyrolysis debound green body;

[0104] 6) Pressureless sintering: The pyrolysis debound green body obtained in step 5) was placed in a pressureless sintering furnace, evacuated and filled with argon as the protective gas, sintered at a sintering temperature of 2200 °C for 2 h under pressureless sintering treatment, and then cooled with the furnace to obtain the SCC number sample.

[0105] Figure 2 d and Figure 3 d is the Raman spectrum of carbon nanotubes with an ultra-short aspect ratio (average aspect ratio of 20) and also a relatively high degree of defect (I D / I G = 0.96) and the scanning morphology map after 30 minutes of ultrasonic treatment. It can be seen that the carbon nanotubes with an ultra-short aspect ratio also have a better dispersion effect.

[0106] Performance test of carbon nanotube toughened silicon carbide ceramic materials:

[0107] Performance tests were carried out on the samples obtained in Examples SC1 to SC4 and Comparative Examples SC0, SCA, SCB, and SCC, and their bulk density, open pores, relative density, and mechanical properties were measured, and XRD pattern characterization and fracture morphology analysis of the specimens were carried out.

[0108] Figure 4The flexural strength, fracture toughness, and bulk density of sintered specimens (SCO, SCA, SC2, SCB, SCC) with a carbon nanotube content of 1 wt% and different aspect ratios and defect degrees. From Figure 4 It can be seen that compared with the SC0 specimen without carbon nanotubes, the flexural strength and fracture toughness of the sintered specimens SC2, SCB, and SCC with carbon nanotubes of higher defect degrees have been improved to a certain extent. However, both too large and too small aspect ratios will affect the strengthening and toughening effects of carbon nanotubes. Moreover, the flexural strength and fracture toughness of the sintered specimen SCA with carbon nanotubes of low defect degrees not only do not increase, but instead show a certain downward trend. This is because carbon nanotubes with high defect degrees have better interfacial bonding with the silicon carbide matrix, while carbon nanotubes with low defect degrees have weak interfacial bonding with the silicon carbide matrix and their stable structure has a large resistance to the diffusion of silicon carbide particles during sintering, resulting in a decrease in the densification degree and affecting the mechanical properties of the specimens. It can also be seen from the figure that the bulk density of all sintered specimens with carbon nanotubes decreases. On the one hand, it is because the density of the added carbon nanotubes is much lower than that of silicon carbide, and on the other hand, due to the agglomeration of carbon nanotubes, the porosity of the specimens increases. In short, it can be seen from the figure that the specimen SC2 exhibits the best mechanical properties and densification effect, with the flexural strength and fracture toughness reaching 529.05 MPa and 4.74 MPa·m 1 / 2 .

[0109] Figure 5 XRD patterns of sintered specimens with different carbon nanotube addition amounts. From Figure 5 It can be seen that obvious diffraction peaks of 4H-SiC appear in the SC0 specimen without carbon nanotubes, indicating that a 6H→4H polymorphic transformation occurred during its high-temperature sintering process. For the SC1 and SC4 specimens with added carbon nanotubes, only weak 4H peaks exist, indicating that the addition of carbon nanotubes can significantly inhibit the polymorphic transformation of 6H-SiC to 4H-SiC during high-temperature sintering. In addition, obvious carbon peaks exist in all specimens, especially obvious in the specimen SC4, which is due to the residual of the sintering aid carbon and the existence of carbon nanotubes.

[0110] Figure 6 a - e are fracture morphology diagrams of sintered specimens with different carbon nanotube addition amounts. Among them Figure 6 a corresponds to the specimen SC0 without carbon nanotubes, Figure 6 b corresponds to the specimen SC1 with a carbon nanotube addition amount of 0.5 wt.%, Figure 6 c corresponds to the specimen SC2 with a carbon nanotube addition amount of 1 wt.%, Figure 6 d corresponds to the specimen SC3 with a carbon nanotube addition amount of 2 wt.%, Figure 6Specimen SC4 with a carbon nanotube addition of 4 wt.%. As the carbon nanotube addition increased from 0 wt.% to 1 wt.%, the number of pores in the specimen was low, and no obvious aggregation of carbon nanotubes was seen inside the pores. Only when the addition exceeded 1 wt.% did the number of pores increase significantly, and the carbon nanotube aggregates increased with the increase in the addition, thus affecting the mechanical properties of the specimen.

[0111] Figure 7 a-c are the crack propagation diagrams of the sintered specimen SC2 with a carbon nanotube addition of 1 wt.%. In Figure 7 a-b, the pulling out and bridging phenomena of carbon nanotubes can be clearly seen. In Figure 7 c, the deflection and branching of cracks can also be seen. The action mechanisms of these carbon nanotubes can disperse the energy at the crack tip, slow down the crack propagation speed, change the crack propagation direction, and achieve the effect of strengthening and toughening. And in Figure 7 a-b, it can be observed that the surface of the carbon nanotubes is coated with coarse silicon carbide particles, indicating that the carbon nanotubes with a high degree of defects have chemical bonding with the silicon carbide matrix, thus having a high interfacial bonding strength.

[0112] Figure 8 It is the trend diagram of the bulk density, apparent porosity, and relative density of the sintered specimens with different carbon nanotube additions. As can be seen from the figure, the bulk density and relative density of the specimens showed a slow downward trend as the carbon nanotube addition increased from 0 wt.% to 1 wt.%, and the porosity increased slightly. However, when the carbon nanotube addition exceeded 1 wt.%, the bulk density and relative density of the specimens decreased rapidly, and the porosity increased significantly. An appropriate amount of carbon nanotube addition has little effect on the material density, but too much carbon nanotube addition results in more aggregates, thus causing pores to form in the matrix, leading to a significant decrease in the density of the specimen.

[0113] Figure 9 It is the schematic diagram of the flexural strength, hardness, and fracture toughness of the sintered specimens with different carbon nanotube additions. As can be seen from the figure, the flexural strength of the material first increased and then decreased, and the fracture toughness increased. When the carbon nanotube addition was 1 wt.%, the flexural strength reached 529.05 MPa, and the fracture toughness was 4.74 MPa·m 1 / 2 , which were increased by 62.6% and 32.4% respectively compared with the specimen SC0 without carbon nanotubes. When the carbon nanotube addition was in the range of 0 wt.% - 1 wt.%, the hardness of the specimen changed little. Only when the addition exceeded 1 wt.% did the hardness value decrease significantly. It can be seen that the addition of an appropriate amount of carbon nanotubes with a short aspect ratio and a high degree of defects can play a role in strengthening and toughening the silicon carbide ceramic, but too much carbon nanotube addition will cause an increase in the porosity of the silicon carbide ceramic, affecting the mechanical properties of the ceramic.

[0114] It should be noted that, for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent shall be subject to the appended claims.

Claims

1. A method for preparing carbon nanotube toughened silicon carbide ceramic composite material, comprising the following steps: 1) Dispersion of carbon nanotubes: The carbon nanotubes are dispersed in an ethanol solution and ultrasonically dispersed to obtain a carbon nanotube slurry. The average aspect ratio of the carbon nanotubes is 30 - 54, and the I D / I G value is 0.8 - 1.2; 2) Mixing and dispersion: Mix the carbon nanotube slurry obtained in step 1) with silicon carbide raw powder, boron carbide and phenolic resin, and then perform ball milling to obtain a uniformly mixed slurry; 3) Powder preparation: Dry, grind and screen the slurry obtained in step 2) to obtain powder; 4) Press molding: Prepare the powder obtained in step 3) into a preformed green body, and then perform cold isostatic pressing on the preformed green body to obtain a dense green body; 5) Pyrolysis and debinding: Heat up and keep the dense green body obtained in step 4) under argon protection for a certain period of time, and cool to obtain a green body after pyrolysis and debinding; 6) Pressureless sintering: Perform pressureless sintering treatment on the green body after pyrolysis and debinding in step 5) under argon protection to obtain a carbon nanotube toughened silicon carbide ceramic composite material.

2. The method for preparing the carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, wherein The time of ultrasonic dispersion in step 1) is 0.5 - 5 h.

3. The method for preparing the carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, wherein In step 2), the mass ratio of silicon carbide raw powder: boron carbide: phenolic resin: carbon nanotubes is 1:(0.0025 - 0.015):(0.03 - 0.16):(0.005 - 0.04).

4. The method for preparing a carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, wherein In step 2), silicon carbide grinding balls are used for ball milling, the ball milling time is 2 - 8 h, and the ball-to-material ratio is (3 - 8):

1.

5. The method for preparing the carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, wherein The drying time in step 3) is 15 - 30 h, the drying temperature is 60 - 75 °C, and the screen mesh for screening is 40 - 60 meshes.

6. The method for preparing a carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, wherein, In step 4), the pressure for preparing the preformed green body is 20 - 40 MPa, the pressure holding time is 40 - 80 s, the pressure of cold isostatic pressing is 150 - 300 MPa, and the pressure holding time is 150 - 250 s.

7. The method for preparing a carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, wherein In step 5), the heating rate is 2 - 10 °C / min, the holding temperature is 800 - 900 °C, and the holding time is 1.5 - 3 h.

8. The method for preparing a carbon nanotube toughened silicon carbide ceramic composite material according to claim 1, characterized in that, In step 6), the temperature of pressureless sintering is 2000 - 2200 °C, the heating rate is 2 - 20 °C / min, and the holding time is 1.5 - 3 h.

9. A carbon nanotube toughened silicon carbide ceramic composite material, characterized in that, Prepared by using the method for preparing a carbon nanotube toughened silicon carbide ceramic composite material according to any one of claims 1 - 8.

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