Preparation method of high-thermal-conductivity B4C-cBN composite ceramic and B4C-cBN composite ceramic

Abstract

The invention relates to the field of composite ceramics, and aims to provide a novel B4C-cBN composite ceramic with good mechanical properties and thermal conductivity, a preparation method of the novel B4C-cBN composite ceramic comprises the following steps: S100, mixing 5-9 parts by weight of boron carbide, 1-5 parts by weight of cubic boron nitride and a sintering aid to obtain a mixture; s200, the mixture is subjected to a sintering process, and B4C-cBN composite ceramic is obtained; the sintering temperature is 1400 to 1550 DEG C; the sintering pressure is 4.9 to 5.5 GPa; the sintering time is 1-3 hours. The reason that aluminum and cobalt are selected as sintering aids is that the melting point of aluminum is low, a liquid phase is formed in the sintering process, pores can be effectively filled through the capillary action, and the sintering process is promoted. Aluminum and cubic boron nitride react to generate aluminum nitride, and conversion of cubic boron nitride to hexagonal boron nitride can be inhibited. Cobalt has oxyphilicity and can play a purifying role of deoxidizing and degassing, pores are reduced, the density is improved, and the sintering performance of the composite ceramic is improved. The optimal raw material ratio and sintering parameters are determined, so that the mechanical property and the heat-conducting property of the composite ceramic are relatively excellent.

Description

Technical Field

[0001] The present invention relates to the field of composite ceramics, and in particular to a preparation method of a B4C-cBN composite ceramic and the B4C-cBN composite ceramic. Background Art

[0002] Composite ceramics are a new type of material formed by combining two or more different ceramic materials (such as oxides, nitrides, and carbides) through physical or chemical methods. Their purpose is to combine the advantages of different materials, overcoming the shortcomings of a single ceramic material and achieving superior performance. Composite ceramics are widely used in aerospace, biomedicine, electronics, cutting tools, and protective equipment.

[0003] Currently, in the preparation process of boron carbide and cubic boron nitride as ceramic raw materials, boron carbide is mostly used as a binder, and the physical properties of boron carbide are not reasonably utilized. Now there is an urgent need for a ceramic with boron carbide and cubic boron nitride as the main materials and a preparation method thereof, which can not only ensure that it has good mechanical properties and thermal conductivity, but also effectively reduce the synthesis temperature. Summary of the Invention

[0004] The object of the present invention is to provide a preparation method of a B4C-cBN composite ceramic and a B4C-cBN composite ceramic. After boron carbide and cubic boron nitride are used as main materials, the ceramic has good mechanical properties and high thermal conductivity.

[0005] The embodiments of the present invention are achieved through the following technical solutions:

[0006] A method for preparing a high thermal conductivity B4C-cBN composite ceramic, comprising:

[0007] S100, mixing 5-9 parts by weight of boron carbide, 1-5 parts by weight of cubic boron nitride, and a sintering aid to obtain a mixture;

[0008] S200, the mixed material is sintered to obtain B4C-cBN composite ceramics; the sintering temperature is 1400-1550° C.; the sintering pressure is 4.9-5.5 GPa; and the sintering time is 1-3 hours.

[0009] Preferably, the sintering aid comprises: 6wt%-10wt% of aluminum and 1wt%-3wt% of cobalt.

[0010] Preferably, the average particle size of the boron carbide is 1-5 μm, and the particle size of the cubic boron nitride is 3-10 μm.

[0011] Preferably, the holding time during the sintering process is 1-10 minutes.

[0012] Preferably, the holding time is 5 minutes, the sintering pressure is 5.5 GPa, the sintering temperature is 1450° C., and the holding time is 5 minutes.

[0013] Preferably, the mass ratio of the boron carbide to cubic boron nitride is 7:3.

[0014] Preferably, the sintering process includes: first heating to 600-800°C at a rate of 30-50°C / min, keeping the temperature for 20-40 minutes, and then heating to the sintering temperature at a rate of 100-200°C / min.

[0015] Preferably, the sintering process includes: the sintering pressure is 0.5-1 GPa in the initial stage, when the temperature is raised to 600-800°C, the pressure is increased to 2.6-3.5 GPa, when the temperature is raised to 1200-1400°C, the pressure is increased to the sintering pressure.

[0016] A B4C-cBN composite ceramic prepared by the preparation method.

[0017] Preferably, the B4C-cBN composite ceramic has a Vickers hardness of 32.7-41.6 GPa, a density of 2.21-2.88 g / cm3, a relative density of 97.16%-98.59%, a fracture toughness of 3.9-4.2 MPa·m1 / 2, and a wear ratio of 2.0-2.5.

[0018] The present invention has at least the following beneficial effects:

[0019] The present invention selects aluminum and cobalt as sintering aids because aluminum has a low melting point and forms a liquid phase during the sintering process that can effectively fill pores through capillary action, thereby promoting the sintering process. In addition, aluminum reacts with cubic boron nitride to form aluminum nitride, which can inhibit the conversion of cubic boron nitride to hexagonal boron nitride. Cobalt has an affinity for oxygen and can play a purifying role of "deoxidation and degassing", thereby reducing pores, increasing density, and improving the sintering performance of the composite ceramic. By conducting multiple groups of control experiments using the controlled variable method, the effects of pressure, temperature, raw material ratio, and synthesis time on the performance of the composite ceramic were deeply studied. The hardness, fracture toughness, density, phase composition, micromorphology, wear ratio, and thermal stability of the composite ceramic were measured using a Vickers hardness tester, an XRD diffractometer, a SEM scanning electron microscope, an Archimedes drainage method, an abrasion ratio measuring instrument, and a thermogravimetric analyzer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 SEM fracture images of boron carbide-cubic boron nitride composite ceramics under different pressures;

[0022] Figure 2 The hardness trend diagram of boron carbide-cubic boron nitride composite ceramics synthesized under different pressures;

[0023] Figure 3 This is a graph showing the changing trend of density and relative density of boron carbide-cubic boron nitride composite ceramics under different pressures;

[0024] Figure 4 The change trend of wear resistance of boron carbide-cubic boron nitride composite ceramics under different pressures;

[0025] Figure 5 This is the thermogravimetric curve of boron carbide-cubic boron nitride composite ceramics;

[0026] Figure 6 This is the thermogravimetric curve of boron carbide-cubic boron nitride composite ceramics at a synthesis pressure of 5.5 GPa;

[0027] Figure 7 The fracture SEM images of boron carbide-cubic boron nitride composite ceramics prepared at different synthesis temperatures;

[0028] Figure 8 This is a graph showing the change trend of the hardness of boron carbide-cubic boron nitride composite ceramics as the synthesis temperature changes;

[0029] Figure 9 The wear ratio variation trend diagram of boron carbide-cubic boron nitride composite ceramics at different synthesis temperatures;

[0030] Figure 10 The density and relative density change trend diagram of boron carbide-cubic boron nitride composite ceramics at different synthesis temperatures;

[0031] Figure 11 XRD diffraction patterns of boron carbide-cubic boron nitride composite ceramics at different ratios;

[0032] Figure 12 The fracture SEM images of boron carbide-cubic boron nitride composite ceramics with different proportions;

[0033] Figure 13 The hardness variation trend diagram of boron carbide-cubic boron nitride composite ceramics with different proportions;

[0034] Figure 14 The density and relative density change trend diagram of boron carbide-cubic boron nitride composite ceramics with different proportions;

[0035] Figure 15This is a graph showing the fracture toughness variation of boron carbide-cubic boron nitride composite ceramics at different cubic boron nitride contents;

[0036] Figure 16 The fracture SEM images of boron carbide-cubic boron nitride composite ceramics at different holding times;

[0037] Figure 17 The hardness variation trend of boron carbide-cubic boron nitride composite ceramics under different holding times;

[0038] Figure 18 This is a graph showing the trend of wear ratio changes of boron carbide-cubic boron nitride composite ceramics under different holding times;

[0039] Figure 19 The density and relative density change trends of boron carbide-cubic boron nitride composite ceramics under different holding times. DETAILED DESCRIPTION

[0040] In order to make the purpose, method scheme and advantages of the embodiments of the present invention clearer, the method scheme in the embodiments of the present invention is clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0041] Example 1: A method for preparing a high thermal conductivity B4C-cBN composite ceramic, comprising:

[0042] S100. In parts by weight, 5 parts of boron carbide, 1 part of cubic boron nitride, and a sintering aid are mixed to obtain a mixture; the sintering aid comprises: 6 wt% of aluminum and 1 wt% of cobalt; the average particle size of the boron carbide is 1 μm, and the particle size of the cubic boron nitride is 3 μm.

[0043] S200, the mixed material is sintered to obtain a B4C-cBN composite ceramic; the sintering temperature is 1400° C., the sintering pressure is 4.9 GPa, the sintering time is 1 hour, and the holding time during the sintering process is 1 minute.

[0044] The sintering process includes: firstly heating to 600°C at a rate of 30°C / min, keeping the temperature for 20 minutes, and then heating to the sintering temperature at a rate of 100°C / min.

[0045] The sintering process includes: the sintering pressure is 0.5GPa in the initial stage, when the temperature is raised to 600°C, the pressure is increased to 2.6GPa, and when the temperature is raised to 1200°C, the pressure is increased to the sintering pressure.

[0046] Example 2: A method for preparing a high thermal conductivity B4C-cBN composite ceramic, comprising:

[0047] S100. In parts by weight, 9 parts of boron carbide, 5 parts of cubic boron nitride, and a sintering aid are mixed to obtain a mixture; the sintering aid comprises: 10 wt% of aluminum and 3 wt% of cobalt; the average particle size of the boron carbide is 5 μm, and the particle size of the cubic boron nitride is 10 μm.

[0048] S200, the mixed material is sintered to obtain a B4C-cBN composite ceramic; the sintering temperature is 1550° C., the sintering pressure is 5.5 GPa, the sintering time is 3 hours, and the holding time during the sintering process is 10 minutes.

[0049] The sintering process includes: firstly heating to 800°C at a rate of 50°C / min, keeping the temperature for 40 minutes, and then heating to the sintering temperature at a rate of 200°C / min.

[0050] The sintering process includes: the sintering pressure is 1 GPa in the initial stage, when the temperature is raised to 800° C., the pressure is increased to 3.5 GPa, and when the temperature is raised to 1400° C., the pressure is increased to the sintering pressure.

[0051] Example 3: A method for preparing a high thermal conductivity B4C-cBN composite ceramic, comprising:

[0052] S100. In parts by weight, 7 parts of boron carbide, 3 parts of cubic boron nitride, and a sintering aid are mixed to obtain a mixture; the sintering aid comprises: 8 wt% of aluminum and 2 wt% of cobalt; the average particle size of the boron carbide is 3 μm, and the particle size of the cubic boron nitride is 7 μm.

[0053] S200, the mixed material is sintered to obtain a B4C-cBN composite ceramic; the sintering temperature is 1450° C., the sintering pressure is 5.5 GPa, the sintering time is 2.5 hours, and the holding time during the sintering process is 5 minutes.

[0054] The sintering process includes: firstly heating to 700°C at a rate of 40°C / min, keeping the temperature for 30 minutes, and then heating to the sintering temperature at a rate of 160°C / min.

[0055] The sintering process includes: the sintering pressure is 0.8GPa in the initial stage, when the temperature is raised to 700°C, the pressure is increased to 3GPa, and when the temperature is raised to 1300°C, the pressure is increased to the sintering pressure.

[0056] Experiment 1: Effect of Synthesis Pressure on the Properties of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0057] Different synthesis pressures have a significant impact on the mechanical properties of the prepared boron carbide-cubic boron nitride. Using an appropriate pressure can effectively reduce the synthesis temperature, improve the sintering process, and enhance the mechanical properties of the composite ceramic. Based on the research results of predecessors and combined with the present invention's own work, the present invention utilizes a controlled variable method to prepare boron carbide-cubic boron nitride composite ceramics at a temperature of 1450°C, a time of 5 minutes, a raw material ratio of 7:3, and a sintering aid of 8 wt.% Al and 2 wt.% Co, using synthesis pressures of 4.9, 5.1, 5.3, 5.5, and 5.7 GPa, respectively. The effects of synthesis pressure on the mechanical properties and thermal stability of the composite ceramics were analyzed using XRD diffractometers, SEM scanning electron microscopes, Vickers hardness testers, wear ratio measuring instruments, and thermogravimetric analyzers.

[0058] Figure 1 Figure 2 shows SEM fracture images of boron carbide-cubic boron nitride (CBN) at different pressures of 4.9, 5.1, 5.3, and 5.5 GPa. The synthesis pressure in Figure (a) was 4.9 GPa. As can be seen, the boron carbide composite ceramic synthesized at this pressure has many gaps and pores, preventing the components from infiltrating and growing together properly. This results in poor sample density and prone to faulting, resulting in low hardness, poor density, and low wear resistance. Figure (b) shows the synthesis pressure of the boron carbide-cubic boron nitride composite ceramic increased to 5.1 GPa. Compared to Figure (a), as the pressure increases, the gaps and pores on the fracture surface decrease, and the microstructure of the composite ceramic improves. The grains connect and grow together to form larger grains, increasing the density of the material and improving the mechanical properties of the synthesized sample. As the pressure in Figure (c) continues to rise to 5.3 GPa, the grains merge with each other, significantly reducing the gaps and pores between the grains. This further improves the density of the sample, allowing the components to be effectively infiltrated together, and the resulting fracture surface begins to become smooth. As shown in Figure (d), when the pressure is increased to 5.5 GPa, the fracture surface of the boron carbide-cubic boron nitride composite ceramic becomes smoother, with further reductions in pores and gaps, increasing density, and effectively improving the microstructure of the composite ceramic. Therefore, the boron carbide-cubic boron nitride composite ceramic synthesized at this pressure exhibits excellent mechanical properties.

[0059] Experiment 2: Effect of Synthesis Pressure on the Hardness of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0060] Depend on Figure 2It can be seen that with increasing synthesis pressure, the hardness of the boron carbide-cubic boron nitride composite ceramic first increases and then decreases. The upward trend in hardness slows when the pressure reaches 5.5 GPa. At this point, the composite ceramic reaches a maximum hardness of 41.6 GPa. At lower synthesis pressures, intergranular gaps and pores are more numerous, resulting in lower density, loose grain bonding, and prone to delamination. This results in poor mechanical properties of the composite ceramic. As pressure increases, B4C and cBN particles effectively expel gas during particle rearrangement, reducing intergranular gaps and pores. Under the influence of pressure and capillary forces, liquid Al and Co effectively fill the pores and gaps within the composite ceramic, improving the material's densification. When the synthesis pressure reaches 5.5 GPa, the hardness of the boron carbide-cubic boron nitride composite ceramic levels off, reaching a maximum of 41.6 GPa. Further increases in pressure have little effect on improving the mechanical properties of the composite ceramic. Therefore, the present invention selects 5.5 GPa as the pressure condition for synthesizing boron carbide-cubic boron nitride composite ceramics. Under higher pressure (5.5 GPa), boron carbide and cubic boron nitride particles will break and plastically deform, which helps to improve the mechanical properties of the composite ceramics.

[0061] Experiment 3: Effect of Synthesis Pressure on Density and Relative Density of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0062] from Figure 3 It can be seen that the density and relative density of boron carbide-cubic boron nitride composite ceramics are low at low pressures, and their compactness is poor. As the pressure increases, their density and relative density both show an upward trend. When the pressure rises to 5.5GPa, the growth slows down, and the density and relative density of the composite ceramic reach their maximum values ​​of 2.88g / cm3 and 98.59%, respectively. When the pressure is low, there are many pores and gaps inside the composite ceramic, and the liquid phase Al and Co cannot effectively wrap the boron carbide and cubic boron nitride particles, resulting in insufficient alignment between the grains and poor performance of the sintered composite material. When the pressure increases, the gas inside the composite ceramic is expelled from the body under the action of pressure, and the pores and gaps between the grains are significantly reduced. The liquid phase Al and Co can effectively fill them under the action of pressure and capillaries, improving the density of the material and optimizing the mechanical properties of the composite ceramic.

[0063] Experiment 4: Effect of Synthesis Pressure on the Wear Ratio of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0064] The wear ratio is an important parameter for measuring a material's wear resistance; a larger value indicates better wear resistance. This study investigated the effect of different synthesis pressures on the wear resistance of boron carbide-cubic boron nitride composite ceramics by measuring the wear ratios of these ceramics at different synthesis pressures. Figure 4Boron carbide-cubic boron nitride composite ceramics were synthesized under conditions of 4.9, 5.1, 5.3, and 5.5 GPa, and the trend of the wear ratio is shown in the figure. With the increase of the synthesis pressure, the wear ratio of the composite ceramics shows a trend of first rising sharply and then tending to be gentle. When the pressure is 5.5 GPa, the wear ratio of the composite ceramics reaches a maximum of 2.5. When the synthesis pressure is low, the components cannot be well infiltrated and grown together, and the synthesized composite ceramics have poor density and low wear. When the pressure increases, the liquid phase Al and Co can effectively fill the gaps and holes under the action of pressure and capillaries, connect the boron carbide and cubic boron nitride particles, increase the density of the composite material, and improve the wear resistance of the composite ceramics.

[0065] Experiment 5: Effect of Synthesis Pressure on Thermal Stability of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0066] Figure 5 Thermogravimetric curves of boron carbide-cubic boron nitride composite ceramics synthesized at pressures of 4.9 GPa and 5.5 GPa at temperatures between 20 and 1400°C are shown. The synthesis pressure in Figure (a) is 4.9 GPa, while that in Figure (b) is 5.5 GPa. The thermogravimetric curve in Figure (a) clearly shows that the mass of the composite ceramic increases first and then decreases with increasing temperature. Before 400°C, the mass of the composite ceramic changes little. As the temperature rises to 600°C, the mass slowly increases, then rapidly increases to a maximum at 1100°C, increasing by 1.004%. With further increases in temperature, the mass of the composite ceramic decreases sharply again, reaching a minimum at 1390°C. Boron carbide begins to oxidize to form boron oxide at around 600°C, and the oxidation reaction is already very pronounced by the time the temperature reaches 800°C. Therefore, the mass of the boron carbide composite ceramic increases sharply between 600°C and 1100°C. As the temperature increases, cubic boron nitride also undergoes oxidation to form boron oxide, further increasing the mass of the composite ceramic. As the temperature continues to rise, more boron oxide volatilizes, causing the composite ceramic's mass to rapidly decrease. The DTG curve shows that the rate of mass change of the composite ceramic fluctuates and slowly increases with increasing temperature, before beginning to decline sharply around 1000°C, due to the volatilization of boron oxide. The TGA curve in Figure (b) shows little change before 500°C, then rises sharply to a maximum at 1090°C, with a mass increase of 0.61%. Further increases in temperature cause the composite ceramic's mass to rapidly decrease again. The DTG curve, on the other hand, hovers around 0 before 400°C, then slowly rises with increasing temperature, reaching a maximum at 1090°C before dropping sharply. The rate of decline slows down around 1200°C. Combined, Figures (a) and (b) demonstrate that increasing pressure improves the thermal stability of the composite ceramic.

[0067] Figure 6The thermogravimetric curves of boron carbide-cubic boron nitride composite ceramics at a synthesis pressure of 5.7 GPa are shown in the TGA and DTG curves. The mass change and rate of change of the composite ceramics increase first and then decrease with increasing temperature.

[0068] Experiment 6: Effect of Synthesis Temperature on the Micromorphology of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0069] Figure 7 Figures 2 and 3 show SEM images of the fracture surfaces of boron carbide-cubic boron nitride composite ceramics at different synthesis temperatures. Figures (a), (b), (c), and (d) correspond to synthesis temperatures of 1400°C, 1450°C, 1500°C, and 1550°C, respectively. Figure (a) shows that at lower temperatures, the sample exhibits numerous gaps on the fracture surface, the grains fuse together to form various shapes, and the grain boundaries begin to fuse, resulting in poor density and poor mechanical properties. In Figure (b), when the temperature rises to 1450°C, the temperature gradient provides sufficient driving force for grain boundary movement. The gaps on the fracture surface shrink to pores, which are then expelled from the sample. The grain boundaries disappear, the fracture surface becomes smoother, the sample becomes denser, and its mechanical properties improve to their optimal levels. As shown in Figure (c), when the temperature is further increased to 1500°C, the excessively high temperature causes secondary grain growth, increasing the number of pores and gaps, weakening the density, and degrading the sample's mechanical properties. Figure (d) shows that when the temperature rises to 1550°C, the fracture surface of the sample becomes relatively smoother than that of Figure (c), with fewer pores and gaps, and improved density. However, the mechanical properties of the sample do not significantly improve. Combining Figures (a), (b), (c), and (d) and the test results of their corresponding mechanical properties, it is concluded that the optimal synthesis temperature for boron carbide-cubic boron nitride composite ceramics is 1450°C. The boron carbide-cubic boron nitride composite ceramics synthesized under this temperature condition have the most dense microstructure and achieve the best mechanical properties.

[0070] Experiment 7: Effect of Synthesis Temperature on the Hardness of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0071] from Figure 8The hardness of boron carbide-cubic boron nitride composite ceramics changes with increasing synthesis temperature. The hardness of the composite ceramics initially increases with increasing temperature, reaching a maximum hardness of 41.6 GPa at 1450°C. With further temperature increases, the hardness decreases. Combined with SEM fracture images, the micromorphology of the composite ceramics reaches its highest density and hardness at 1450°C. Further increases in temperature lead to abnormal grain growth, increased pores and gaps within the ceramics, and decreased density of the composite, negatively impacting the hardness. Selecting an appropriate synthesis temperature is crucial for synthesizing high-performance boron carbide-cubic boron nitride composite ceramics. Excessively low synthesis temperatures impair the sintering process of the composite ceramics, preventing the components from growing together effectively. This results in a high number of pores and gaps within the composite, which compromises the hardness of the composite. Excessively high synthesis temperatures can lead to secondary grain growth, reducing the density of the ceramic and lowering its hardness.

[0072] Experiment 9: Effect of Synthesis Temperature on the Wear Ratio of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0073] Figure 9 The following graph shows the trend of the wear ratio of boron carbide-cubic boron nitride composite ceramics as the temperature increases. It can be clearly seen from the figure that the wear ratio of the composite ceramics first increases and then decreases with increasing temperature. At 1450°C, the wear ratio of the composite ceramics reaches a maximum of 2.5. This phenomenon occurs because when the synthesis temperature is too low, the growth of the grains is affected, and they cannot grow together well. There are many holes and gaps between them, and the density is low, resulting in relatively low wear of the composite ceramics. When the temperature rises to an appropriate value, the temperature gradient provides sufficient driving force for grain growth, and the individual grains can grow together well, greatly improving the density of the material and the wear resistance of the composite ceramics. However, when the temperature is too high, the grains will grow again, the holes and gaps within the composite ceramics will increase, the density will decrease, and its wear resistance will be adversely affected.

[0074] Experiment 10: Effect of Synthesis Temperature on Density and Relative Density of Boron Carbide-Cubic Boron Nitride Composite Ceramics

[0075] Figure 10The effect of synthesis temperature on the density and relative density of boron carbide-cubic boron nitride composite ceramics is demonstrated. The figure shows that the density and relative density of the composite ceramics first increase and then decrease with increasing temperature. The density and relative density of the composite ceramics reach their maximum at 1450°C. This phenomenon is attributed to the influence of temperature on the sintering process of the composite ceramics. When the synthesis temperature is too low, grain growth is impaired, preventing the grains from properly connecting and growing together. This results in a high number of pores and gaps within the material, a low density, and a low density and relative density of the ceramic. Further increasing the temperature to an optimal level provides sufficient driving force for grain growth, effectively improving the microstructure of the composite ceramics, significantly enhancing the material's compactness, and increasing the density and relative density of the composite ceramics. However, when the temperature is too high, excessive grain growth occurs, increasing the number of pores and gaps within the composite material, reducing the density, and consequently, the density and relative density of the composite ceramics.

[0076] Experiment 11: Effect of different ratios on the phase of boron carbide-cubic boron nitride composite ceramics

[0077] Figure 11 The XRD diffraction patterns of boron carbide-cubic boron nitride composite ceramics with varying cubic boron nitride contents were obtained at 5.5 GPa, 1450°C, and a 5-minute hold. The figure shows that the main phases of the boron carbide-cubic boron nitride composite ceramics include boron carbide, cubic boron nitride, aluminum oxide, aluminum nitride, cobalt boride, cobalt carbide, and dicobalt boride. The absence of a boron nitride diffraction peak indicates that cubic boron nitride did not transform into hexagonal boron nitride during the sintering process. The intensity of the boron carbide diffraction peak decreases with increasing cubic boron nitride content. However, the intensity of the cubic boron nitride diffraction peak increases with increasing cubic boron nitride content. When the cubic boron nitride content reaches 27 wt.%, the diffraction peak intensities of boron carbide and cubic boron nitride are closest. The peak of cobalt boride decreases with increasing cubic boron nitride content, while the peaks of cobalt diboride, aluminum nitride, and cobalt carbide increase. Boron carbide crystals mainly grow along the (021), (104), (012), and (003) planes, while cubic boron nitride mainly grows along the (111) and (220) planes.

[0078] Experiment 12: Effect of different ratios on the micromorphology of boron carbide-cubic boron nitride composite ceramics

[0079] Figure 12SEM cross-sectional images of boron carbide-cubic boron nitride composite ceramics at different ratios, held at 5.5 GPa and 1450°C for 5 minutes. The ratios of boron carbide to cubic boron nitride in Figures (a), (b), (c), (d), and (e) are 9:1, 8:2, 7:3, 6:4, and 5:5, respectively. The gray-black grains in the figure are boron carbide, and the gray-white grains are cubic boron nitride. Figure (a) shows that the grains on the fracture surface vary in size, with most exhibiting a regular rhombus structure. Some grains interconnect and grow to form irregular polyhedrons. The grain distribution is uneven, with numerous interstices, resulting in poor sample density. Numerous small, irregularly rounded grains are interspersed between the large grains. The interstices are filled with aluminum, which does not effectively fill the gaps, resulting in low density, low hardness, and poor fracture toughness and wear resistance. In Figure (b), the crystal corners become rounded and smooth, transitioning to an elliptical shape. The intergranular gaps are significantly reduced, significantly improving the sample's density, hardness, fracture toughness, and wear resistance. In Figure (c), the fracture surface is relatively smooth, with large grains interconnected and growing to form a relatively smooth surface. The gaps shrink, forming pores. The sample's density, hardness, fracture toughness, and wear resistance also reach their highest values. Figure (d) shows that the boron carbide grains in the fracture surface are regular quadrilaterals. Large crystals engulf smaller ones, interconnecting to form a network of chains. Small grains adhere to their surfaces, increasing the number of pores and gaps between the grains and reducing the density. This indicates that excessive cubic boron nitride content affects the growth process of boron carbide. In Figure (e), the grain size decreases with increasing cubic boron nitride content, and the small grains are more evenly distributed. However, the gaps and holes on the fracture surface increase, reducing the sample's density, hardness, fracture toughness, and wear resistance.

[0080] Experiment 13: Effect of different ratios on the hardness of boron carbide-cubic boron nitride composite ceramics

[0081] Figure 13 The following graph shows the hardness trends of boron carbide-cubic boron nitride composite ceramics at different ratios at 5.5 GPa, 1450°C, and a 5-minute hold. The graph shows that the hardness of the boron carbide-cubic boron nitride composite ceramics first increases and then decreases with increasing CBN content. When the CBN content reaches 27 wt.%, the composite ceramic reaches a maximum hardness of 41.6 GPa. This phenomenon is due to the higher hardness of CBN than that of BC, and the addition of CBN helps to increase the hardness of the composite ceramic. However, as the CBN content continues to increase, the hardness of the composite ceramic decreases sharply. This is because the excessive CBN content adversely affects the growth of BC, reducing the hardness of the composite ceramic. This trend is consistent with the intensity trends of the diffraction peaks of BC and CBN in the XRD diffraction pattern. Therefore, the optimal ratio of BC to CBN composite ceramics is 7:3.

[0082] Experiment 14: Effect of different ratios on the density and relative density of boron carbide-cubic boron nitride composite ceramics

[0083] Figure 14 The density and relative density change trend of boron carbide-cubic boron nitride composite ceramics with different proportions at 5.5GPa, 1450℃, and 5 minutes of heat preservation. Figure 4-14 It can be seen that the density of the boron carbide-cubic boron nitride composite ceramic shows an upward trend with the increase of cubic boron nitride content. The reason for this phenomenon is that the density of cubic boron nitride is 3.48g / cm3, which is greater than the density of boron carbide. As the cubic boron nitride content increases, the density of the boron carbide-cubic boron nitride composite ceramic increases. However, the relative density of the composite ceramic is inconsistent with the change trend of the density, showing a trend of first increasing and then decreasing. When the cubic boron nitride content is 27wt.%, the relative density of the composite ceramic reaches a maximum of 98.59%. As the cubic boron nitride content continues to increase, the relative density of the composite ceramic decreases. This is because excessive cubic boron nitride is prone to agglomeration, metallic aluminum cannot effectively fill the grain gaps, and inhibits the growth of boron carbide grains, making it difficult to drain the pores inside the composite ceramic. This leads to a decrease in the relative density of the composite ceramic, a decrease in hardness, and a decrease in density.

[0084] Experiment 15: Effect of different ratios on the fracture toughness of boron carbide-cubic boron nitride composite ceramics

[0085] Figure 15 The fracture toughness of boron carbide-cubic boron nitride composite ceramics at different CBN contents was revealed. The fracture toughness of the composite ceramics first increases and then decreases with increasing CBN content, reaching a maximum of 4.2 MPa·m1 / 2 at a CBN content of 27 wt.%. Adding a small amount of CBN helps improve the fracture toughness of the composite ceramics. However, when the CBN content is too high, agglomeration tends to occur, affecting the density of the sample and reducing the fracture toughness of the composite ceramic. Therefore, the optimal mixing ratio of boron carbide-cubic boron nitride composite ceramics is 7:3.

[0086] Experiment 16: Effect of holding time on the micromorphology of boron carbide-cubic boron nitride composite ceramics

[0087] Figure 16 These are SEM fracture images of boron carbide-cubic boron nitride composite ceramics at different holding times.

[0088] The holding times for Figures (a), (b), (c), (d), and (e) are 1, 3, 5, and 10 minutes, respectively. Figures (a), (b), and (c) show that as the holding time increases, the gaps gradually shrink into pores and are expelled by the movement of grain boundaries. Large grains engulf smaller grains, interconnecting and growing together. The fracture surface of the composite ceramic gradually becomes smoother, and the density is improved. Figures (d) and (e) show that as the holding time continues to increase, the grains are prone to secondary nucleation, excessive growth, and increased pores, resulting in a decrease in the density of the composite ceramic and a negative impact on the mechanical properties of the composite ceramic. This indicates that if the holding time is too short, the grain growth of the composite ceramic is insufficient, resulting in a large number of pores and gaps, and the resulting mechanical properties are poor. If the holding time is too long, the grains tend to grow excessively, which reduces the density of the composite ceramic and affects the material's performance. The appropriate holding time should be selected to prevent insufficient growth of the composite ceramics during sintering due to a too short holding time, and to avoid adverse effects on the mechanical properties of the sample due to a too long holding time. Figure 4-16 The SEM fracture images of boron carbide-cubic boron nitride composite ceramics with different holding times and the corresponding performance tests show that the appropriate holding time for the composite ceramics is 5 minutes.

[0089] Experiment 17: Effect of holding time on the hardness of boron carbide-cubic boron nitride composite ceramics

[0090] Figure 17 The following graph shows the hardness trend of a boron carbide-cubic boron nitride composite ceramic for holding times of 1 minute, 3 minutes, 5 minutes, 10 minutes, and 15 minutes, respectively. The graph clearly shows that the hardness of the composite ceramic initially increases with increasing holding time, reaching a maximum hardness of 30.6 GPa at 5 minutes. As the holding time increases, the hardness of the composite ceramic decreases. The hardness of the composite ceramic increases slowly from 1 to 3 minutes, but increases sharply from 3 to 5 minutes. This indicates that grain growth in the composite ceramic is slow within 3 minutes, and the temperature has not yet fully spread throughout the sample. After 3 minutes, the temperature effectively provides sufficient driving force for grain growth. However, when the holding time is extended to 10 minutes, the hardness of the composite ceramic begins to decrease. Further extending the holding time to 15 minutes shows a significant drop in hardness. This indicates that between 5 and 10 minutes, some grains overgrow, affecting the hardness of the ceramic. Between 10 and 15 minutes, the number of overgrown grains increases, severely impacting the mechanical properties of the material. Therefore, the appropriate holding time for boron carbide-cubic boron nitride composite ceramics is 5 minutes.

[0091] Experiment 18: Effect of holding time on the wear ratio of boron carbide-cubic boron nitride composite ceramics

[0092] Figure 18 The wear ratio of boron carbide-cubic boron nitride composite ceramics changes under different holding times. The wear ratio of the composite ceramics increases first and then decreases with the change of holding time. When the holding time is 5 minutes, the wear ratio of the composite ceramic reaches a maximum of 2.3. When the holding time is short, the composite ceramics are not fully sintered, the density of the material is poor, and the wear is relatively low. When the holding time is too long, it will lead to excessive grain growth, reduce the density of the composite ceramics, and reduce the wear ratio of the material. Therefore, choosing a suitable holding time is of great significance to improving the mechanical properties of composite ceramics.

[0093] Experiment 19: Effect of holding time on density and relative density of boron carbide-cubic boron nitride composite ceramics

[0094] Figure 19 The following graph shows the density and relative density trends of boron carbide-cubic boron nitride composite ceramics at different holding times. As shown, the density and relative density of the composite ceramics first increase and then decrease. At a holding time of 5 minutes, the density and relative density of the ceramics reach their maximum values ​​of 2.729 g / cm³ and 99.3%, respectively. The density and relative density of the composite ceramics increase slowly before 3 minutes, but then increase dramatically to their maximum values ​​at 5 minutes. Combined with SEM fracture images, this indicates that the sintering time before 3 minutes is too short, and gaps and pores within the composite ceramics have not been completely eliminated, resulting in poor material compactness and low density and relative density. By 5 minutes, the gaps and pores within the composite ceramics have been nearly completely eliminated, improving the material's compactness and causing a rapid increase in density and relative density. When the holding time is extended to 10 minutes, the density and relative density begin to decline slowly, and further increasing the holding time to 15 minutes leads to a significant decline. This indicates that excessive holding time can lead to excessive grain growth, increased pores and gaps, poor compactness, and a significant decrease in density and relative density of the composite ceramic. This is consistent with the patterns shown in the SEM fracture images and hardness trend graphs of the composite ceramic. The microstructure of boron carbide-cubic boron nitride composite ceramics closely influences their properties.

[0095] Pressure is a crucial factor in the sintering process of boron carbide-cubic boron nitride composite ceramics. At a pressure of 5.5 GPa, the density and relative density of the composite ceramics level off, and thermal stability reaches its highest level. Further increases in pressure have little effect on the mechanical properties of the composite ceramics, but deteriorate their thermal stability. Therefore, 5.5 GPa was selected as the final synthesis pressure.

[0096] By varying the synthesis temperature under a pressure of 5.5 GPa, the researchers investigated the changes in the mechanical properties of the composite ceramics within the range of 1400-1550°C. As the temperature increased, the microstructure of the boron carbide-cubic boron nitride composite ceramics became denser, the density and relative density increased, and the mechanical properties improved. The composite ceramics achieved optimal mechanical properties at 1450°C.

[0097] The mechanical properties of the boron carbide-cubic boron nitride composite ceramics show a trend of increasing first and then decreasing with the increase of cubic boron nitride content. When the cubic boron nitride content is 27wt.%, the mechanical properties of the composite ceramics are the best.

[0098] By keeping all other factors constant and varying only the holding time, we systematically studied how the mechanical properties of the composite ceramics change over holding times ranging from 1 to 15 minutes. As holding time increases, the mechanical properties of the composite material first increase and then decrease. The optimal mechanical properties are achieved when the holding time is 5 minutes.

[0099] Comparative Example 1: The difference from Example 3 is that the sintering pressure in the initial stage is the sintering pressure.

[0100] Comparative Example 2: The difference from Example 3 is that in the sintering process, the temperature is directly increased to the sintering temperature at a rate of 200° C. / min.

[0101] Comparative Example 3: The difference from Example 3 is that the average particle size of boron carbide and cubic boron nitride is 3 μm.

[0102] Experiment 20: B4C-cBN composite ceramics were prepared according to the preparation methods provided in Examples 1-3 and Comparative Examples 1-3, and thermal conductivity tests were performed at a test temperature of 25°C. The test results are shown in Table 1.

[0103] Table 1

[0104] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Thermal conductivity 87.6 91.2 95.1 69.2 73.3 65.7

[0105] It can be seen from the test results of Examples 1-3 that the B4C-cBN composite ceramics prepared by the preparation method provided by the present invention have high thermal conductivity.

[0106] From the comparison of the test results of Comparative Examples 1-3 and Example 3, it can be seen that the regulation of temperature and pressure increase in the sintering process and the particle size distribution of the raw materials all affect the thermal conductivity of the product.

[0107] In summary, when boron carbide and cubic boron nitride are mixed in a ratio of 7:3, under the conditions of pressure 5.5GPa, temperature 1450℃ and time 5min, the obtained composite ceramic has good comprehensive properties, with Vickers hardness of 41.6GPa, density of 2.88g / cm3, relative density of 98.59%, fracture toughness of 4.2MPa·m1 / 2, wear ratio of 2.5, and thermal conductivity of 95.1W / (m·K).

[0108] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations are readily apparent to those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing a high thermal conductivity B4C-cBN composite ceramic, characterized in that: include: S100, mixing 5-9 parts by weight of boron carbide, 1-5 parts by weight of cubic boron nitride, and a sintering aid to obtain a mixture; S200, the mixed material is sintered to obtain B4C-cBN composite ceramics; the sintering temperature is 1400-1550° C.; the sintering pressure is 4.9-5.5 GPa; and the sintering time is 1-3 hours.

2. The preparation method according to claim 1, characterized in that The sintering aid comprises: 6wt%-10wt% of aluminum and 1wt%-3wt% of cobalt.

3. The preparation method according to claim 1, characterized in that The average particle size of the boron carbide is 1-5 μm, and the particle size of the cubic boron nitride is 3-10 μm.

4. The preparation method according to claim 1, characterized in that The holding time during the sintering process is 1-10 minutes.

5. The preparation method according to claim 4, characterized in that The holding time is 5 minutes, the sintering pressure is 5.5 GPa, the sintering temperature is 1450° C., and the holding time is 5 minutes.

6. The preparation method according to any one of claims 1 to 5, characterized in that The mass ratio of the boron carbide to cubic boron nitride is 7:

3.

7. The preparation method according to claim 6, characterized in that The sintering process includes: firstly heating to 600-800°C at a rate of 30-50°C / min, keeping the temperature for 20-40 minutes, and then heating to the sintering temperature at a rate of 100-200°C / min.

8. The preparation method according to claim 7, characterized in that The sintering process includes: the sintering pressure is 0.5-1GPa in the initial stage, when the temperature is raised to 600-800°C, the pressure is increased to 2.6-3.5GPa, and when the temperature is raised to 1200-1400°C, the pressure is increased to the sintering pressure.

9. A B4C-cBN composite ceramic prepared according to the preparation method according to any one of claims 1 to 8.

10. The B4C-cBN composite ceramic according to claim 9, characterized in that The Vickers hardness is 32.7-41.6GPa, the density is 2.21-2.88g / cm3, the relative density is 97.16%-98.59%, the fracture toughness is 3.9-4.2MPa·m1 / 2, and the wear ratio is 2.0-2.5.

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