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

Abstract

The present application relates to the field of composite ceramics, and aims to provide a new B4C-cBN composite ceramic with good mechanical properties and thermal conductivity, and a preparation method thereof, which comprises the following steps: S100, mixing boron carbide 5-9 parts, cubic boron nitride 1-5 parts and sintering additives to obtain a mixture; S200, obtaining the B4C-cBN composite ceramic through a sintering process; the sintering temperature is 1400-1550 DEG C; the sintering pressure is 4.9-5.5 GPa; and the sintering time is 1-3 h. The reason why aluminum and cobalt are selected as sintering additives is that the melting point of aluminum is low, and a liquid phase formed in the sintering process can effectively fill the pores through capillary action, thereby promoting the sintering process. Aluminum reacts with cubic boron nitride to form aluminum nitride, which can inhibit the conversion of cubic boron nitride into hexagonal boron nitride. Cobalt has oxygen affinity and can play a purifying role of "deoxidation and degassing", thereby reducing pores, improving density and improving the sintering performance of the composite ceramic. The optimal raw material ratio and sintering parameters are determined, so that the mechanical properties and thermal conductivity of the composite ceramic are both excellent.

Description

Technical Field

[0001] This invention relates to the field of composite ceramics, and more specifically, to a method for preparing B4C-cBN composite ceramics and the B4C-cBN composite ceramics themselves. Background Technology

[0002] Composite ceramics are a new type of material composed of two or more different types of ceramic materials (such as oxides, nitrides, carbides, etc.) through physical or chemical methods. The aim is to combine the advantages of different materials and overcome the shortcomings of single ceramic materials, thereby achieving superior performance. Composite ceramics are widely used in aerospace, biomedicine, electronic devices, cutting tools, and protective equipment.

[0003] Currently, in the preparation process of ceramics using boron carbide and cubic boron nitride as raw materials, boron carbide is mostly used as a binder, without making reasonable use of the physical properties of boron carbide. There is an urgent need for a ceramic and its preparation method that uses both boron carbide and cubic boron nitride as the main materials, 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 purpose of this invention is to provide a method for preparing B4C-cBN composite ceramics and the B4C-cBN composite ceramics. When boron carbide and cubic boron nitride are used together as the main materials, the ceramics have 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, by weight, 5-9 parts of boron carbide, 1-5 parts of cubic boron nitride and sintering aid are mixed to obtain a mixture;

[0008] S200, the mixture is sintered to obtain B4C-cBN composite ceramic; the sintering temperature is 1400-1550℃; the sintering pressure is 4.9-5.5GPa; and the sintering time is 1-3h.

[0009] Preferably, the sintering aid comprises: 6wt%-10wt% aluminum and 1wt%-3wt% 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℃, and the holding time is 5 minutes.

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

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

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

[0016] A B4C-cBN composite ceramic prepared by the aforementioned 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 reason for choosing aluminum and cobalt as sintering aids in this invention is that aluminum has a low melting point, and during the sintering process, it forms a liquid phase that can effectively fill pores through capillary action, promoting the sintering process. Furthermore, the reaction of aluminum with cubic boron nitride to form aluminum nitride can inhibit the conversion of cubic boron nitride to hexagonal boron nitride. Cobalt, with its oxidizing properties, can play a "deoxidizing and degassing" role, reducing porosity, increasing density, and improving the sintering performance of the composite ceramic. Multiple controlled experiments were conducted using the controlled variable method to thoroughly study the effects of pressure, temperature, raw material ratio, and synthesis time on the properties of the composite ceramic. The hardness, fracture toughness, density, phase composition, microstructure, wear ratio, and thermal stability of the composite ceramic were measured using a Vickers hardness tester, XRD diffractometer, SEM scanning electron microscope, Archimedes' water displacement method, wear ratio measuring instrument, and thermogravimetric analyzer. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

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

[0022] Figure 2 The hardness trend of boron carbide-cubic boron nitride composite ceramics synthesized under different pressures is shown in the figure.

[0023] Figure 3 The graph shows the variation trend of density and relative density of boron carbide-cubic boron nitride composite ceramics under different pressures.

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

[0025] Figure 5 Thermogravimetric curves of boron carbide-cubic boron nitride composite ceramics;

[0026] Figure 6 Thermogravimetric curves of boron carbide-cubic boron nitride composite ceramics at a synthesis pressure of 5.5 GPa are shown.

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

[0028] Figure 8 The graph shows the trend of hardness of boron carbide-cubic boron nitride composite ceramic as a function of synthesis temperature.

[0029] Figure 9 The graph shows the trend of wear ratio variation in boron carbide-cubic boron nitride composite ceramics at different synthesis temperatures.

[0030] Figure 10 The graph shows the trend of density and relative density 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 with different proportions;

[0032] Figure 12 SEM images of the fracture surfaces of boron carbide-cubic boron nitride composite ceramics with different proportions.

[0033] Figure 13 The graph shows the hardness variation trend of boron carbide-cubic boron nitride composite ceramics with different proportions.

[0034] Figure 14 The graph shows the trend of density and relative density variation of boron carbide-cubic boron nitride composite ceramics with different proportions.

[0035] Figure 15The trend of fracture toughness variation of boron carbide-cubic boron nitride composite ceramics with different cubic boron nitride contents is shown in the figure.

[0036] Figure 16 SEM images of the fracture surfaces of boron carbide-cubic boron nitride composite ceramics under different heat preservation times;

[0037] Figure 17 The graph shows the trend of hardness variation of boron carbide-cubic boron nitride composite ceramics under different heat preservation times.

[0038] Figure 18 The graph shows the trend of wear ratio of boron carbide-cubic boron nitride composite ceramics under different heat preservation times.

[0039] Figure 19 The trends of density and relative density variation of boron carbide-cubic boron nitride composite ceramics under different heat preservation times are shown. Detailed Implementation

[0040] To make the objectives, methods, and advantages of the embodiments of the present invention clearer, the methods in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

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

[0042] S100. 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 includes 6 wt% aluminum and 1 wt% 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 mixture is sintered to obtain B4C-cBN composite ceramic; the sintering temperature is 1400℃; the sintering pressure is 4.9GPa; the sintering time is 1h. The holding time during the sintering process is 1min.

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

[0045] The sintering process includes: the sintering pressure is 0.5 GPa in the initial stage, when the temperature is raised to 600°C, the pressure is increased to 2.6 GPa, 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. By weight, 9 parts of boron carbide, 5 parts of cubic boron nitride, and sintering aid are mixed to obtain a mixture; the sintering aid includes 10 wt% aluminum and 3 wt% 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 mixture is sintered to obtain B4C-cBN composite ceramic; the sintering temperature is 1550℃; the sintering pressure is 5.5GPa; the sintering time is 3h. The holding time during the sintering process is 10min.

[0049] The sintering process includes: first heating to 800°C at a rate of 50°C / min, holding at that temperature for 40 min, 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. 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 includes 8 wt% aluminum and 2 wt% 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 mixture is sintered to obtain B4C-cBN composite ceramic; the sintering temperature is 1450℃; the sintering pressure is 5.5GPa; the sintering time is 2.5h. The holding time during the sintering process is 5min.

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

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

[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 composite ceramics. Using appropriate pressure can effectively reduce the synthesis temperature, improve the sintering process, and enhance the mechanical properties of the composite ceramics. Based on previous research and our own work, this invention utilizes the controlled variable method to prepare boron carbide-cubic boron nitride composite ceramics at a temperature of 1450℃, a time of 5 min, a raw material ratio of 7:3, and sintering aids of 8 wt.% Al and 2 wt.% Co, using synthesis pressures of 4.9, 5.1, 5.3, 5.5, and 5.7 GPa. The effects of synthesis pressure on the mechanical properties and thermal stability of the composite ceramics were analyzed using XRD diffraction, SEM scanning electron microscopy, Vickers hardness tester, wear ratio meter, and thermogravimetric analysis.

[0058] Figure 1 SEM fracture images of boron carbide-cubic boron nitride composite ceramics under different pressures of 4.9, 5.1, 5.3, and 5.5 GPa are shown. Figure (a) shows a synthesis pressure of 4.9 GPa. As can be seen from the figure, the boron carbide composite ceramic synthesized under this pressure has many gaps and pores, and the components cannot effectively interpenetrate and grow together. This results in poor sample compactness, easy delamination, and low hardness, poor density, and low wear of the composite ceramic. Figure (b) shows the boron carbide-cubic boron nitride composite ceramic synthesized at a pressure increased to 5.1 GPa. Compared to Figure (a), with the increase of pressure, the gaps and pores on the fracture surface decrease, and the microstructure of the composite ceramic is improved. The grains interconnect and grow together to form larger grains, increasing the material density and improving the mechanical properties of the synthesized sample. In Figure (c), as the pressure continues to increase to 5.3 GPa, the grains engulf each other, and the gaps and pores between grains are significantly reduced, further improving the compactness of the sample. The components are well integrated, and the fracture surface begins to become smoother. As shown in Figure (d), when the pressure increases to 5.5 GPa, the fracture surface of the boron carbide-cubic boron nitride composite ceramic becomes smoother, the pores and gaps are further reduced, the compactness is improved, and the microstructure morphology of the composite ceramic is effectively improved. Therefore, the boron carbide-cubic boron nitride composite ceramic synthesized under this pressure has good mechanical properties.

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

[0060] Depend on Figure 2It can be seen that the hardness of the boron carbide-cubic boron nitride composite ceramic first increases and then decreases with increasing synthesis pressure. Furthermore, the upward trend in hardness slows down when the pressure reaches 5.5 GPa. At this point, the hardness of the composite ceramic reaches its maximum of 41.6 GPa. At lower synthesis pressures, there are more gaps and pores between grains, resulting in lower density, weak grain bonding, and a tendency for delamination. This leads to poor mechanical properties of the composite ceramic. When the pressure increases, B4C and cBN particles can effectively expel gas during particle rearrangement, reducing gaps and pores between grains. Under the influence of pressure and capillary force, the liquid phases Al and Co can effectively fill the pores and gaps within the composite ceramic, improving the material's density. When the synthesis pressure reaches 5.5 GPa, the hardness of the boron carbide-cubic boron nitride composite ceramic tends to plateau, reaching a maximum value of 41.6 GPa. Further increasing the pressure has little effect on improving the mechanical properties of the composite ceramic. Therefore, this invention selects 5.5 GPa as the pressure condition for synthesizing boron carbide-cubic boron nitride composite ceramics. Under higher pressure (5.5 GPa), the boron carbide and cubic boron nitride particles will break and undergo plastic deformation, which helps to improve the mechanical properties of the composite ceramics.

[0061] Experiment 3: Effect of synthesis pressure on the 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 the boron carbide-cubic boron nitride composite ceramic are both low at lower pressures, indicating poor compactness. With increasing pressure, both density and relative density show an upward trend, slowing down at 5.5 GPa, where the density and relative density of the composite ceramic reach their maximum values ​​of 2.88 g / cm³ and 98.59%, respectively. At lower pressures, the composite ceramic has more pores and gaps, and the liquid phase Al and Co cannot effectively encapsulate the boron carbide and cubic boron nitride particles, resulting in insufficient grain arrangement and poor performance of the sintered composite material. As the pressure increases, the gas inside the composite ceramic is expelled under pressure, significantly reducing the pores and gaps between grains. The liquid phase Al and Co can effectively fill these gaps under pressure and capillary action, improving the material's compactness and optimizing the mechanical properties of the composite ceramic.

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

[0064] Wear ratio is an important parameter for measuring the wear resistance of materials; the higher the value, the better the wear resistance. This invention investigates the effect of different synthesis pressures on the wear resistance of boron carbide-cubic boron nitride composite ceramics synthesized under different pressures by measuring the wear ratio. Figure 4The boron carbide-cubic boron nitride composite ceramic was synthesized under pressures of 4.9, 5.1, 5.3, and 5.5 GPa, and the trend of its wear ratio is shown in the figure. With increasing synthesis pressure, the wear ratio of the composite ceramic initially increases sharply and then levels off. At a pressure of 5.5 GPa, the wear ratio of the composite ceramic reaches its maximum of 2.5. At lower synthesis pressures, the components cannot effectively melt and grow together, resulting in a composite ceramic with poor density and low wear ratio. As the pressure increases, the liquid phases Al and Co, under the influence of pressure and capillary action, can effectively fill gaps and pores, connecting boron carbide and cubic boron nitride particles, improving the density of the composite material and enhancing its wear resistance.

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

[0066] Figure 5 The thermogravimetric curves (TGA) of boron carbide-cubic boron nitride composite ceramics synthesized at pressures of 4.9 GPa and 5.5 GPa are shown in Figure (a) at a synthesis pressure of 4.9 GPa and Figure (b) at 5.5 GPa. The TGA curve in Figure (a) clearly shows that the mass of the composite ceramic initially increases and then decreases with increasing temperature. Before 400℃, the mass change is small. At 600℃, the mass increases slowly, then increases sharply above 600℃, reaching its maximum at 1100℃, an increase of 1.004%. Further increasing the temperature, the mass decreases sharply again, reaching its lowest point at 1390℃. Boron carbide begins to oxidize to form boron oxide around 600℃, and the oxidation reaction becomes significant at 800℃. Therefore, the mass of the boron carbide composite ceramic increases sharply between 600℃ and 1100℃. Cubic boron nitride also undergoes an oxidation reaction to form boron oxide with increasing temperature, further increasing the mass of the composite ceramic. With continued temperature increase, more boron oxide volatilizes, and the mass of the composite ceramic rapidly decreases. The DTG curve shows that the rate of mass change of the composite ceramic increases slowly with increasing temperature, then drops sharply around 1000℃, due to the volatilization of boron oxide. In Figure (b), the TGA curve shows almost no change before 500℃, then rises sharply above 500℃, reaching its peak at 1090℃, with a mass increase of 0.61%. With further temperature increases, the mass of the composite ceramic decreases rapidly again. The DTG curve hovers around 0 before 400℃, then rises slowly with increasing temperature, reaching its peak at 1090℃ before dropping sharply. The rate of decline slows down when the temperature reaches around 1200℃. Combining Figures (a) and (b), it can be seen that increasing pressure improves the thermal stability of the composite ceramic.

[0067] Figure 6The thermogravimetric analysis (TGA) curves of the boron carbide-cubic boron nitride composite ceramic at a synthesis pressure of 5.7 GPa are shown. From the TGA and DTG curves, it can be seen that the mass change and rate of change of the composite ceramic initially increase and then decrease with increasing temperature.

[0068] Experiment 6: Effect of synthesis temperature on the microstructure of boron carbide-cubic boron nitride composite ceramics

[0069] Figure 7 SEM images of the fracture surfaces of boron carbide-cubic boron nitride composite ceramics at different synthesis temperatures are shown. Figures (a), (b), (c), and (d) correspond to synthesis temperatures of 1400℃, 1450℃, 1500℃, and 1550℃, respectively. Figure (a) shows that at lower temperatures, the fracture surface of the sample has more gaps, grains connect and fuse to form various shapes, grain boundaries begin to fuse, resulting in poor compactness and poor mechanical properties. In Figure (b), when the temperature rises to 1450℃, the temperature gradient provides sufficient driving force for grain boundary movement. The gaps on the fracture surface shrink into pores that are expelled from the sample, grain boundaries disappear, the fracture surface becomes smoother, the sample becomes more compact, and its mechanical properties improve to their optimal level. As shown in Figure (c), when the temperature is further increased to 1500℃, the excessively high temperature causes secondary grain growth, increasing pores and gaps, decreasing compactness, and reducing the mechanical properties of the sample. Figure (d) shows that when the temperature rises to 1550℃, the fracture surface of the sample becomes relatively smoother compared to Figure (c), with fewer pores and gaps, and improved density. However, the mechanical properties of the sample are not effectively improved. Combining Figures (a), (b), (c), and (d) with the corresponding mechanical property test results, the optimal synthesis temperature for boron carbide-cubic boron nitride composite ceramics is determined to be 1450℃. Under this temperature condition, the microstructure of the boron carbide-cubic boron nitride composite ceramics is most dense, and the mechanical properties are optimal.

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

[0071] from Figure 8This study demonstrates the trend of hardness change in boron carbide-cubic boron nitride composite ceramics with increasing synthesis temperature. The hardness of the composite ceramic initially increases with temperature, reaching a maximum of 41.6 GPa at 1450℃. Further temperature increases lead to a decrease in hardness. SEM fracture images show that the composite ceramic exhibits the densest microstructure and highest hardness at 1450℃. Further temperature increases result in abnormal grain growth, increasing internal pores and gaps, leading to decreased density and negatively impacting hardness. Therefore, selecting an appropriate synthesis temperature is crucial for synthesizing high-performance boron carbide-cubic boron nitride composite ceramics. Too low a synthesis temperature affects the sintering process, preventing proper cohesive growth of the components and resulting in more internal pores and gaps, thus affecting the composite's hardness. Too high a synthesis temperature promotes secondary grain growth, reducing density and lowering the material's hardness.

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

[0073] Figure 9 This graph shows the trend of the wear ratio of boron carbide-cubic boron nitride composite ceramics as temperature increases. It clearly shows that the wear ratio of the composite ceramic first increases and then decreases with rising temperature. The wear ratio reaches its maximum of 2.5 at 1450℃. This phenomenon occurs because when the synthesis temperature is too low, grain growth is affected, preventing them from connecting and growing together effectively. This results in more pores and gaps, lower density, and thus lower wear ratio. When the temperature rises to a suitable value, the temperature gradient provides sufficient driving force for grain growth, allowing the grains to grow and bond together better, greatly improving the material's density and wear resistance. However, excessively high temperatures can cause secondary grain growth, increasing the pores and gaps within the composite ceramic, decreasing density, and negatively impacting its wear resistance.

[0074] Experiment 10: Effect of synthesis temperature on the 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 shown. The figure reveals that both the density and relative density of the composite ceramic initially increase and then decrease with increasing temperature. The density and relative density reach their maximum at 1450℃. This phenomenon is attributed to the influence of temperature on the sintering process of the composite ceramic. When the synthesis temperature is too low, it hinders grain growth, preventing the grains from effectively connecting and growing together. This results in more pores and gaps within the material, leading to lower density and relative density. Further increasing the temperature to suitable conditions provides sufficient driving force for grain growth, effectively improving the microstructure of the composite ceramic and significantly enhancing its compactness, thus increasing its density and relative density. However, excessively high temperatures can cause overgrowth of grains, increasing pores and gaps within the composite material, reducing density, and consequently decreasing both density and relative density.

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

[0077] Figure 11 The XRD diffraction patterns of boron carbide-cubic boron nitride composite ceramics with different cubic boron nitride contents are shown in the figure at 5.5 GPa, 1450 °C, and a holding time of 5 minutes. The figure shows that the main phase composition of the boron carbide-cubic boron nitride composite ceramics includes boron carbide, cubic boron nitride, alumina, aluminum nitride, cobalt boride, cobalt carbide, and cobalt diboride. No boron nitride diffraction peaks are observed, indicating that cubic boron nitride did not transform to hexagonal boron nitride during the sintering process. The diffraction peak intensity of boron carbide decreases with increasing cubic boron nitride content, while the diffraction peak intensity of cubic boron nitride 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 cobalt boride peak gradually decreases with increasing cubic boron nitride content, while the peaks of cobalt diboride, aluminum nitride, and cobalt carbide gradually 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 proportions on the microstructure of boron carbide-cubic boron nitride composite ceramics

[0079] Figure 12SEM cross-sectional images of boron carbide-cubic boron nitride composite ceramics with different ratios at 5.5 GPa, 1450℃, and a holding time of 5 minutes are shown. 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 images represent boron carbide, and the gray-white grains represent cubic boron nitride. Figure (a) shows that the grain size on the fracture surface varies, and the grain shape is mostly a regular rhombic structure. Some grains connect and grow to form irregular polyhedral shapes, resulting in uneven grain distribution, numerous gaps, and poor sample compactness. Many irregularly shaped small grains are interspersed between large grains. The material filling the gaps is metallic aluminum, but the metallic aluminum does not effectively fill the gaps, leading to low sample density, low hardness, and poor fracture toughness and wear resistance. In Figure (b), the crystals become rounded and smooth, transforming into elliptical shapes. The gaps between grains are significantly reduced, improving the sample's density, hardness, fracture toughness, and wear resistance. In Figure (c), the fracture surface is relatively smooth, with large grains connecting and growing to form a smooth plane. Gap shrinkage reduces, forming pores, and the sample's density, hardness, fracture toughness, and wear resistance reach their highest values. Figure (d) shows that the boron carbide grains in the fracture surface are regular quadrilaterals, with large crystals engulfing smaller crystals. The grains connect to form a network-like structure, with small grains adhering to their surfaces. The porosity and gaps between grains increase, leading to a decrease in density. This indicates that excessively high cubic boron nitride content affects the boron carbide growth process. In Figure (e), the grain size decreases with increasing cubic boron nitride content, and the distribution of small grains is relatively uniform. However, the gaps and pores on the fracture surface increase, and the sample's density, hardness, fracture toughness, and wear resistance all decrease.

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

[0081] Figure 13 The graph shows the hardness variation trend of boron carbide-cubic boron nitride composite ceramics at 5.5 GPa, 1450℃, and a holding time of 5 minutes under different ratios. As can be seen from the graph, the hardness of the boron carbide-cubic boron nitride composite ceramics initially increases and then decreases with increasing cubic boron nitride content. The hardness reaches its maximum of 41.6 GPa when the cubic boron nitride content reaches 27 wt.%. This phenomenon is because cubic boron nitride is harder than boron carbide, and the introduction of cubic boron nitride helps to improve the hardness of the composite ceramic. However, when the cubic boron nitride content continues to increase, the hardness of the composite ceramic decreases sharply. This is because excessive cubic boron nitride content adversely affects the growth of boron carbide, reducing the hardness of the composite ceramic. This is also consistent with the intensity variation trend of the diffraction peaks of boron carbide and cubic boron nitride in the XRD diffraction pattern. Therefore, the optimal ratio of boron carbide-cubic boron nitride composite ceramics is 7:3.

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

[0083] Figure 14 This is a graph showing the density and relative density trends of boron carbide-cubic boron nitride composite ceramics with different proportions at 5.5 GPa, 1450℃, and a holding time of 5 minutes. From... Figure 4-14 It can be seen that the density of boron carbide-cubic boron nitride composite ceramics increases with the increase of cubic boron nitride content. This phenomenon occurs because the density of cubic boron nitride (3.48 g / cm³) is greater than that of boron carbide, and the density of the boron carbide-cubic boron nitride composite ceramic increases with the increase of cubic boron nitride content. However, the relative density of the composite ceramic does not follow the same trend as the density, showing an initial increase followed by a decrease. When the cubic boron nitride content is 27 wt.%, the relative density of the composite ceramic reaches its highest value of 98.59%. Further increasing the cubic boron nitride content leads to a decrease in the relative density of the composite ceramic. This is because excessive cubic boron nitride easily leads to agglomeration, preventing metallic aluminum from effectively filling the intergranular gaps and inhibiting the growth of boron carbide grains, thus hindering the drainage of pores within the composite ceramic. This results in a decrease in the relative density, hardness, and compactness of the composite ceramic.

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

[0085] Figure 15 The study revealed the trend of fracture toughness variation in boron carbide-cubic boron nitride composite ceramics under different cubic boron nitride contents. The fracture toughness of the composite ceramics initially increased and then decreased with increasing cubic boron nitride content, reaching a maximum of 4.2 MPa·m1 / 2 when the cubic boron nitride content was 27 wt.%. Adding a small amount of cubic boron nitride helps improve the fracture toughness of the composite ceramics. However, excessively high cubic boron nitride content easily leads to agglomeration, affecting the compactness of the sample and causing a decrease in fracture toughness. 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 microstructure of boron carbide-cubic boron nitride composite ceramics

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

[0088] In Figures (a), (b), (c), (d), and (e), the holding times are 1, 3, 5, and 10 min, respectively. Figures (a), (b), and (c) show that as the holding time increases, the gaps gradually shrink into pores, which are then expelled with the movement of grain boundaries. Large grains engulf small grains, and the grains connect and grow together, resulting in a smoother fracture surface and improved density of the composite ceramic. Figures (d) and (e), however, show that with further increases in holding time, grains are prone to secondary nucleation and overgrowth, leading to more pores and a decrease in the density of the composite ceramic, negatively impacting its mechanical properties. Therefore, if the holding time is too short, the grain growth of the composite ceramic is insufficient, resulting in more pores and gaps and poorer mechanical properties. Conversely, if the holding time is too long, excessive grain growth is likely, leading to decreased density and affecting the material's performance. An appropriate holding time should be selected to prevent insufficient growth of the composite ceramic during sintering due to insufficient holding time, while avoiding adverse effects on the mechanical properties of the sample due to excessive holding time. According to... Figure 4-16 SEM fracture images and corresponding performance tests of boron carbide-cubic boron nitride composite ceramics with different holding times were obtained, and it was found that the suitable 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 graph shows the hardness trend of boron carbide-cubic boron nitride composite ceramics after holding times of 1 min, 3 min, 5 min, 10 min, and 15 min. It is clear from the graph that the hardness of the composite ceramic initially increases with increasing holding time, reaching a maximum of 30.6 GPa at 5 min. Further extending the holding time leads to a decrease in hardness. The hardness increases slowly from 1 min to 3 min, but increases sharply from 3 min to 5 min. This indicates that grain growth is slow within the first 3 min, as the temperature has not yet fully covered the entire sample. After 3 min, the temperature effectively provides sufficient driving force for grain growth. When the holding time is extended to 10 min, the hardness of the composite ceramic begins to decrease. Further extending the holding time to 15 min results in a significant drop in hardness. This suggests that between 5 min and 10 min, some grains overgrow, affecting the hardness of the ceramic. Between 10 min and 15 min, the number of overgrown grains increases, severely impacting the mechanical properties of the material. Therefore, the suitable 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 varies under different holding times. The wear ratio initially increases and then decreases with increasing holding time. The wear ratio reaches its maximum of 2.3 when the holding time is 5 minutes. Shorter holding times result in incomplete sintering, poor material density, and lower wear ratio. Conversely, excessively long holding times lead to excessive grain growth, reducing the density of the composite ceramic and further decreasing the wear ratio. Therefore, selecting an appropriate holding time is crucial for 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 figure shows the density and relative density trends of boron carbide-cubic boron nitride composite ceramics under different holding times. As shown in the figure, both the density and relative density of the composite ceramics show a trend of first increasing and then decreasing. When the holding time is 5 min, the density and relative density of the ceramics reach their maximum, at 2.729 g / cm3 and 99.3%, respectively. Before 3 min, the density and relative density of the composite ceramics increase relatively slowly, but increase sharply to the maximum at 5 min. Combined with the SEM fracture image, this indicates that the sintering time of the composite ceramics is relatively short before 3 min, and gaps and pores are not completely eliminated, resulting in poor material compactness and low density and relative density. By 5 min, the gaps and pores inside the composite ceramics have almost been eliminated, the material compactness is improved, and the density and relative density rise rapidly. When the holding time is extended to 10 min, the density and relative density begin to show a slower decreasing trend, and further increasing the holding time to 15 min shows a severe downward trend. This indicates that excessive heat treatment time leads to excessive grain growth, increased porosity, decreased compactness of the composite ceramic, and a significant decrease in density and relative density. This is consistent with the patterns observed in the SEM fracture images and hardness trend diagrams of the composite ceramic. The microstructure morphology of boron carbide-cubic boron nitride composite ceramics closely affects its 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 ceramic tend to plateau, and its thermal stability reaches its peak. Further increasing the pressure has little effect on the mechanical properties of the composite ceramic, but the thermal stability deteriorates. Therefore, 5.5 GPa was chosen as the final synthesis pressure.

[0096] Under a pressure of 5.5 GPa, the variation of the mechanical properties of the composite ceramics in the range of 1400-1550℃ was investigated by changing the synthesis temperature. With increasing temperature, the microstructure of the boron carbide-cubic boron nitride composite ceramics became denser, and the density and relative density increased, leading to improved mechanical properties. The composite ceramics exhibited the best mechanical properties at a temperature of 1450℃.

[0097] The mechanical properties of boron carbide-cubic boron nitride composite ceramics initially increase and then decrease with increasing cubic boron nitride content. The composite ceramic exhibits optimal mechanical properties when the cubic boron nitride content is 27 wt.%.

[0098] Keeping other factors constant and only varying the holding time, the mechanical properties of the composite ceramics were systematically studied in the range of 1-15 minutes. With increasing holding time, the mechanical properties of the composite material first increased and then decreased. The composite ceramics exhibited optimal mechanical properties at a holding time of 5 minutes.

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

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

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

[0102] Experiment 20: B4C-cBN composite ceramics prepared according to the preparation methods provided in Examples 1-3 and Comparative Examples 1-3 were subjected to thermal conductivity tests at a 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] As can be seen from the experimental results of Examples 1-3, the B4C-cBN composite ceramic prepared by the preparation method provided by the present invention has high thermal conductivity.

[0106] A comparison of the experimental results of Comparative Examples 1-3 and Example 3 shows that the control of temperature and pressure increase and the particle size distribution of raw materials in the sintering process all affect the thermal conductivity of the product.

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

[0108] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should 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, by weight, 5-9 parts of boron carbide, 1-5 parts of cubic boron nitride and sintering aid are mixed to obtain a mixture; S200, The mixture is sintered to obtain B4C-cBN composite ceramic; The sintering temperature is 1400-1550℃; The sintering pressure is 4.9-5.5 GPa; the sintering time is 1-3 hours. 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; The sintering process includes: first heating to 600-800℃ at a rate of 30-50℃ / min, holding at that temperature for 20-40min, and then heating to the sintering temperature at a rate of 100-200℃ / min. The sintering process includes: the sintering pressure is 0.5-1 GPa in the initial stage, and when the temperature is raised to 600-800℃, the pressure is increased to 2.6-3.5 GPa, and when the temperature is raised to 1200-1400℃, the pressure is increased to the sintering pressure.

2. The production method according to claim 1, characterized by, The sintering aids include: 6wt%-10wt% aluminum and 1wt%-3wt% cobalt.

3. The production method according to claim 1, characterized by, The holding time during the sintering process is 1-10 minutes.

4. The production method according to claim 3, characterized by, The sintering pressure was 5.5 GPa, the sintering temperature was 1450℃, and the holding time was 5 min.

5. The method of any one of claims 1-4, wherein, The mass ratio of boron carbide to cubic boron nitride is 7:

3.

6. A B4C-cBN composite ceramic prepared by the preparation method according to any one of claims 1-5.

7. The B4C-cBN composite ceramic according to claim 6, characterized in that, Its Vickers hardness is 32.7-41.6 GPa, and its density is 2.21-2.88 g / cm³. 3 Relative density 97.16%-98.59%, fracture toughness 3.9-4.2 MPa·m 1 / 2 The wear ratio is 2.0-2.5.

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