High-entropy boride-based ceramic and method of making the same

Abstract

This invention discloses a high-entropy boride-based ceramic and its preparation method, belonging to the technical field of advanced ceramics. The preparation method involves mixing HfO2, ZrO2, TiO2, Ta2O5, and Nb2O5 in a non-equimolar ratio of Nb to obtain a mixed oxide powder. Boron carbide and a carbon source are added to the mixed oxide powder to obtain a reactant mixture. The mixture is then ball-milled, shaped, and sintered under an inert atmosphere or vacuum at a programmed temperature. This invention obtains a high-entropy boride-based ceramic by controlling the C / O and B / M ratios combined with pressureless sintering. This ceramic possesses a high-entropy boride main phase and an intergranular Nb-containing carbide second phase generated in situ. The preparation method avoids the residual carbon retention problem caused by excess carbon in traditional carbothermal reduction methods, resulting in a significantly improved ceramic material density and mechanical properties.

Description

Technical Field

[0001] This invention belongs to the technical field of advanced ceramics, specifically relating to a high-entropy boride-based ceramic and its preparation method. Background Technology

[0002] Ultra-high temperature ceramics (UHTCs) are widely used in key components of hypersonic vehicles, reusable launch vehicles, and next-generation aero engines. Furthermore, with technological advancements, increasingly stringent requirements are being placed on the comprehensive performance of UHTC materials. Transition metal borides (such as ZrB2, HfB2, and TiB2) have become ideal candidates for UHTC thermal protection materials due to their ultra-high melting point (>3000℃), high hardness (20-30 GPa), excellent thermal and electrical conductivity, and good oxidation resistance.

[0003] However, traditional binary boride ceramics have many shortcomings in practical applications: the performance adjustability of single-component borides is limited, making it difficult to meet various extreme service requirements; boride ceramics are inherently brittle and have low fracture toughness, making them prone to catastrophic fracture under thermal cycling and mechanical loads; traditional pressureless sintering is difficult to achieve densification, requiring expensive equipment such as hot pressing (HP) or spark plasma sintering (SPS).

[0004] High-entropy borides, by introducing four or more transition metal elements in equimolar ratios into a single crystal lattice, exhibit high-entropy effects, lattice distortion effects, hysteresis diffusion effects, and cocktail effects, thereby achieving superior overall performance compared to single-component borides. However, the preparation of high-entropy borides faces two key challenges:

[0005] First, carbothermal reduction is one of the most commonly used methods for preparing high-entropy borides. Existing literature and practical preparation often employ a strategy of excess carbon to ensure complete reduction. However, the residual carbon caused by excess carbon can severely hinder the densification process, leading to a sharp decline in mechanical properties. Currently, systematic research on the relationship between carbon content and the microstructure and mechanical properties of high-entropy borides is still incomplete.

[0006] Second, the hysteresis diffusion effect in high-entropy ceramics makes it difficult to achieve high density in pressureless sintering. Although hot pressing and spark plasma sintering can achieve high density, the equipment is expensive and difficult to promote industrially.

[0007] Therefore, achieving high density and excellent overall performance of high-entropy borides remains the core requirement for promoting their engineering applications. Summary of the Invention

[0008] This invention addresses the shortcomings of existing technologies by providing a high-entropy boride-based ceramic and its preparation method.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] A method for preparing high-entropy boride-based ceramics includes the following steps:

[0011] S1. Mix HfO2, ZrO2, TiO2, Ta2O5, and Nb2O5 in a molar ratio of Hf, Zr, Ti, Ta, and Nb of 1:1:1:1:x to obtain a mixed oxide powder; where x is 1.2-2.0.

[0012] S2. Add boron carbide (B4C) and a carbon source to the mixed oxide powder to obtain a reaction raw material mixture, wherein the ratio of the total molar amount of carbon provided by boron carbide and carbon source to the total molar amount of oxygen provided by the mixed oxide powder (C / O) is 0.45-0.75, and the ratio of the molar amount of boron provided by boron carbide to the total molar amount of metal elements provided by the mixed oxide powder (B / M) is 2.3-2.9.

[0013] S3. After ball milling and mixing the reactant mixture, it is shaped and placed under an inert atmosphere or vacuum condition for programmed temperature sintering, including...

[0014] Low-temperature degassing section: Heat to 1200-1500℃ at a rate of 5-10℃ / min, and hold for 0.5-1 hour;

[0015] High-temperature reaction sintering section: Heat to 1800-2100℃ at a rate of 3-5℃ / min, and hold for 1-3 hours;

[0016] S4. The furnace is cooled to room temperature to obtain a high-entropy boride-based ceramic, which has a high-entropy boride main phase and an intergranular in-situ generated Nb-containing carbide second phase.

[0017] Precise control of the carbon / oxygen molar ratio within the range of 0.45-0.75 avoids residual carbon retention due to excess carbon while ensuring complete reduction reaction. A suitable amount of carbon-vacant carbide second phase is generated in situ within the high-entropy boride matrix, utilizing the carbon vacancy effect to promote sintering densification. The in-situ generated NbC is distributed at the high-entropy boride grain boundaries, inhibiting grain growth through the Zener pinning effect, thus achieving grain refinement and increased density.

[0018] The selection of Nb as the non-equimolar excess control element is based on a specific thermodynamic competition mechanism: in a high-temperature carbothermic reduction environment, Nb has the weakest affinity for boron in the pentagonal system. Therefore, Nb is most easily extruded from the boride sequence, and then spontaneously and singly combines with carbon to form NbC in the later stage of reduction, without disturbing the high-entropy stability of the main phase borides. Simultaneously, the thermal expansion coefficients of NbC and the matrix are well matched, avoiding cooling microcracks.

[0019] More preferably, the ratio of the total molar amount of carbon provided by the boron carbide and carbon source to the total molar amount of oxygen provided by the mixed oxide powder (C / O) is 0.48-0.6.

[0020] More preferably, the ratio of the molar amount of boron provided by the boron carbide to the total molar amount of metal elements provided by the mixed oxide powder, B / M, is 2.5-2.7.

[0021] Optionally, the carbon source is either carbon black or graphite powder.

[0022] Optionally, step S2 further includes adding 0.5-3 wt% B2O3 to the reaction mixture. Introducing B2O3 as a transient liquid-phase sintering aid, B2O3 melts at 450°C to form a liquid phase, promoting particle rearrangement and mass transport through capillary forces; it completely volatilizes above 1860°C, leaving no residue in the final product, thus avoiding the problem of residual glass phase reducing high-temperature performance in traditional liquid-phase sintering.

[0023] More preferably, the amount of B2O3 added is 1.5-2.5 wt% of the reaction raw material mixture.

[0024] Optionally, in step S3, the ball mill rotation speed is 200-500 rpm, the ball-to-material mass ratio is (5-15):1, the ball milling medium is anhydrous ethanol, and the time is 4-12 hours.

[0025] Optionally, the molding method is cold isostatic pressing, and the molding pressure is 100-300 MPa.

[0026] A high-entropy boride-based ceramic is prepared by the above-described preparation method.

[0027] Optionally, the high-entropy boride main phase is a hexagonal AlB2 type structure (Hf,Zr,Ti,Ta,Nb)B2 high-entropy boride solid solution; the Nb-containing carbide second phase is distributed at the grain boundaries of the high-entropy boride, with a volume fraction of 5-20%.

[0028] The beneficial effects of this invention are as follows:

[0029] The preparation method of this invention avoids the problem of residual carbon retention caused by excessive carbon in the traditional carbothermal reduction method, significantly reducing the residual carbon content and resulting in a significant increase in the density and mechanical properties of the obtained ceramic material. During the carbothermal reduction process, a second NbC phase is generated in situ. This in-situ generated NbC directly nucleates at the grain boundaries during the reaction, resulting in a clean interface, high bonding strength, and more uniform distribution compared to externally added NbC powder. NbC inhibits grain growth through the Zener pinning effect, achieving grain refinement, while simultaneously filling grain boundary pores to promote densification. Through the synergistic effect of the process, a high-entropy boride-based multiphase ultra-high temperature ceramic with excellent comprehensive performance is obtained.

[0030] The process is simple and low-cost, employing a pressureless sintering process, making it suitable for practical production applications.

[0031] The prepared high-entropy boride-based ceramics have wide applications in ultra-high temperature structural components and neutron-absorbing materials for nuclear engineering. These high-entropy boride-based ceramics contain two neutron-absorbing elements, boron (B) and neutron (Hf). B-10 has a thermal neutron absorption cross-section of 3840 barns, while Hf has a thermal neutron absorption cross-section of 104 barns. This allows the material to function as both an ultra-high temperature structural material and a neutron absorber, expanding its applications to nuclear engineering scenarios such as reactor control rods and neutron shielding structures.

[0032] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0033] Figure 1 The XRD patterns of the examples and comparative examples are shown below, where (a) is the XRD pattern of the sample of Example 1, (b) is the XRD pattern of the sample of Example 2, (c) is the XRD pattern of the sample of Comparative Example 1, (d) is the XRD pattern of the sample of Comparative Example 2, and (e) is the XRD pattern of the sample of Comparative Example 3.

[0034] Figure 2 The images shown are SEM images of the polished surfaces of the examples and comparative examples, where (a) is the SEM image of the sample of Example 1, (b) is the SEM image of the sample of Example 2, and (c) is the SEM image of the sample of Comparative Example 1.

[0035] Figure 3 This is an EDS surface scan result of the sample from Example 2. Detailed Implementation

[0036] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0037] Example 1

[0038] Weigh out 4.21g (0.02mol) of HfO2, 2.46g (0.02mol) of ZrO2, 1.60g (0.02mol) of TiO2, and 4.42g (0.01mol) of Ta2O5 (containing 0.02mol of Ta) in equimolar ratios. Weigh out 3.99g (0.015mol, containing 0.03mol of Nb) of Nb2O5. Mix them to obtain a mixed oxide powder with a molar ratio of Hf, Zr, Ti, Ta, and Nb of 1:1:1:1:1:1.5. The total number of moles of metal elements is 0.11mol, and the total number of moles of oxygen is 0.245mol.

[0039] B4C and carbon black were added to the mixed oxide powder to obtain a reaction mixture. The amount of B4C added was 3.95 g, approximately 0.0715 mol, providing approximately 0.286 mol of B and 0.0715 mol of C; the amount of carbon black added was 0.60 g, approximately 0.050 mol of C. At this point, the total carbon content in the system (including carbon introduced by B4C and carbon in the carbon black) was approximately 0.1215 mol, and its molar ratio (C / O) to the total oxygen (0.245 mol) was approximately 0.50; the molar ratio (B / M) of B to the total metal elements was approximately 2.6.

[0040] The above-mentioned reactant mixture and tungsten carbide milling balls were placed in a ball mill jar at a ball-to-material ratio of 10:1. Anhydrous ethanol was added as the milling medium, and the mixture was milled at 300 rpm for 10 hours. After milling, the mixture was dried and sieved to obtain a uniformly mixed precursor powder.

[0041] The precursor powder was packed into a casing and pressed into shape using cold isostatic pressing at 200 MPa. The green body was then placed in a vacuum sintering furnace. Sintering was performed according to the following procedure: the temperature was increased to 1500℃ at a rate of 5℃ / min and held for 0.5 hours to allow adsorbed moisture and some volatile components to be expelled; the temperature was increased to 1900℃ at a rate of 4℃ / min and held for 2 hours to complete the carbothermic reduction reaction and form a high-entropy boride phase; the furnace was then cooled to room temperature. The sample was then removed.

[0042] like Figure 1 As shown in Figure (a), the sample in this embodiment was analyzed by XRD. The main phase of the product was a hexagonal high-entropy (Hf, Zr, Ti, Ta, Nb)B2 solid solution, which conforms to the AlB2 type structure (P6 / mmm space group). At the same time, characteristic diffraction peaks of NbC (rock salt type structure) were observed, and the volume fraction of NbC was about 8%-12%. The relative density was measured to be 90% by Archimedes method.

[0043] The NbC phase content was estimated using a semi-quantitative calculation method combining the integrated area of ​​X-ray diffraction peaks with reference intensity ratio (RIR) and density correction. Specifically, this included:

[0044] Summing the integral areas of the characteristic peaks of each phase, and calculating the equivalent integral intensity based on the standard relative intensity of the selected peaks:

[0045] ;

[0046] Where i represents the phase, ΣI(f) i The standard relative intensities of the selected diffraction peaks for this phase are: HEB phase 231.7 and NbC phase 210.6.

[0047] The relative quality factor is further calculated based on the reference strength ratio: ;

[0048] The apparent volume fraction was converted using the relative quality factor. .

[0049] The parameters used are as follows: HEB phase ( The density is 14.13. It is 11.843 g / cm³ 3 The standard relative intensity of the selected peak is 231.7; the NbC phase ( The value is 9.85, and the density is... It is 7.802 g / cm³ 3 The standard relative intensity of the selected peak is 210.6.

[0050] Table 1. Summary of Selected Peaks and Integral Areas:

[0051]

[0052] Table 2. Summary of Semi-Quantitative Calculation Results:

[0053]

[0054] Scanning electron microscopy (SEM) observation of the polished surface of the sample from Example 1 revealed that, as shown... Figure 2 As shown in (a), the sample surface is generally smooth and continuous, with only a few minor local defects, indicating that the sample achieved good sintering bonding and a certain degree of densification under pressureless sintering conditions at 1900℃. Combined with XRD analysis and mechanical property test results, it can be seen that the excessive design of non-equimolar Nb is beneficial to improving the sintering densification behavior of the material, enabling the sample to achieve a relative density of 90% and a Vickers hardness of 15.6 GPa.

[0055] In the system of this embodiment, Nb does not entirely enter the main phase lattice in the form of diborides, but can react with the carbon source in the system to form an Nb-containing metal carbide second phase. This second phase can manifest as NbC-type rock salt structure carbides, Nb-rich carbides, or Nb-containing multimetallic carbides. The above results demonstrate that in-situ NbC effectively suppresses grain growth (Zener pinning effect) and regulates grain boundary structure and pore evolution behavior, thereby promoting sample densification. It should be understood that since the additionally introduced Nb can partially enter the hexagonal AlB2-type high-entropy boride main phase and partially form an intergranular Nb-containing carbide second phase during the reaction, the actual occupancy ratio of each metal element in the final main phase may not strictly conform to the theoretical equimolar ratio. The above effects are mainly reflected in the regulation of the multiphase structure and will not substantially change the basic structural characteristics of the material, which is mainly composed of high-entropy borides and contains an in-situ generated intergranular Nb-containing carbide second phase, and therefore does not necessarily lead to performance degradation. Compared with composite methods involving added carbide particles, in-situ formation of the Nb-containing carbide second phase during reaction sintering typically exhibits better interfacial bonding and uniform distribution, which is beneficial for improving the microstructure stability and mechanical properties of multiphase ceramics.

[0056] Example 2

[0057] Weigh out 4.21g (0.02mol) of HfO2, 2.46g (0.02mol) of ZrO2, 1.60g (0.02mol) of TiO2, and 4.42g (0.01mol) of Ta2O5 (containing 0.02mol of Ta) in equimolar ratios. Weigh out 3.99g (0.015mol, containing 0.03mol of Nb) of Nb2O5. Mix them to obtain a mixed oxide powder with a molar ratio of Hf, Zr, Ti, Ta, and Nb of 1:1:1:1:1:1.5. The total number of moles of metal elements is 0.11mol, and the total number of moles of oxygen is 0.245mol.

[0058] B4C and carbon black were added to the mixed oxide powder to obtain a reaction mixture. The amount of B4C added was 3.95 g, approximately 0.0715 mol, providing approximately 0.286 mol of B and 0.0715 mol of C; the amount of carbon black added was 0.60 g, approximately 0.050 mol of C. At this point, the total carbon content in the system (including carbon introduced by B4C and carbon in the carbon black) was approximately 0.1215 mol, and its molar ratio (C / O) to the total oxygen (0.245 mol) was approximately 0.50; the molar ratio (B / M) of B to the total metal elements was approximately 2.6.

[0059] The above-mentioned reactant mixture and tungsten carbide milling balls were placed in a ball mill jar at a ball-to-material ratio of 10:1. Anhydrous ethanol was added as the milling medium, and the mixture was milled at 300 rpm for 10 hours. After milling, the mixture was dried, sieved, and 0.42 g (approximately 2 wt%) of B2O3 was added. The mixture was then ground and mixed in a mortar for 15 minutes to obtain the precursor powder.

[0060] After the precursor powder is cold isostatically pressed (200 MPa), it is placed in a vacuum sintering furnace and sintered according to the following procedure: the temperature is increased to 1500℃ at 5℃ / min and held for 0.5 hours. During this process, B2O3 melts to form a liquid phase, which promotes particle rearrangement. The temperature is increased to 2000℃ at 4℃ / min and held for 2 hours. Carbothermic reduction is completed, and the high-entropy boride main phase is formed. At the same time, the Nb component participates in the formation of the Nb-containing metal carbide second phase. B2O3 gradually volatilizes or is consumed by the reaction, and finally densification is completed. The furnace is then cooled to room temperature.

[0061] Remove the sample. XRD analysis is performed as follows. Figure 1 As shown in (b), the main phase of the product is high-entropy (Hf, Zr, Ti, Ta, Nb)B2, with NbC diffraction peaks and no B2O3 residue. The relative density is 95%, higher than that of Example 1. The Vickers hardness is 19.3 GPa.

[0062] The polished surface of the sample in Example 2 was observed using a scanning electron microscope as follows: Figure 2 As shown in (b), the polished surface is relatively flat and dense, with good surface continuity, and no obvious large-sized pores or loose defects are observed. Compared with Example 1, the surface of the sample in Example 2 is more uniform and dense, indicating that, based on the Nb excess design, the sintering bonding state and densification degree of the material can be further improved by introducing a small amount of B2O3 additive and increasing the sintering temperature.

[0063] B2O3, acting as a transient liquid-phase sintering aid, promotes particle rearrangement and initial densification in the low-temperature region (450-1500℃), creating closer particle contact conditions for subsequent high-temperature reactions. In-situ NbC fills grain boundary pores and pins grain boundaries in the high-temperature region (>1800℃), completing the final densification. This demonstrates good process compatibility and synergistic gain effects between carbon content control and Nb composition control.

[0064] The sample prepared in Example 2 was used as a representative sample for EDS surface scanning analysis. The results are as follows: Figure 3 As shown, Hf, Zr, Ti, Ta, and Nb elements are relatively uniformly distributed in the sample, and no obvious elemental segregation is observed, indicating that the prepared material has good compositional uniformity.

[0065] In this embodiment, the ratio (C / O) of the total molar carbon provided by B4C and carbon black to the total molar oxygen in the mixed oxides is the same as in Example 1. This C / O ratio ensures a certain match between the carbothermic deoxidation reaction and the oxygen content in the system, helping to reduce the adverse effects of insufficient reduction due to insufficient carbon source or residual carbon due to excessive carbon source on the subsequent sintering process. This ensures that no free carbon is left to hinder liquid phase penetration after deoxidation and venting at 1500°C; while in the high-temperature region of 1500-2000°C, in-situ grain boundary nucleation of NbC seamlessly takes over, filling grain boundary capillaries and pinning grain boundaries to inhibit grain refinement. The entire process spans different temperature zones from 450°C to 2000°C, with multiple mechanisms working in tandem to achieve high-density performance in pressureless high-temperature sintering.

[0066] Comparative Example 1

[0067] 4.21 g (0.02 mol) of HfO2, 2.46 g (0.02 mol) of ZrO2, 1.60 g (0.02 mol) of TiO2, 4.42 g (0.01 mol) of Ta2O5 (containing 0.02 mol Ta), and 2.66 g (0.01 mol) of Nb2O5 (containing 0.02 mol Nb) were weighed in equimolar ratios and mixed to obtain a mixed oxide powder. The total molar amount of metal element M was 0.1 mol. Calculate the total molar amount of oxygen in the mixed oxide to be 0.22 mol.

[0068] B4C and carbon black were added to the mixed oxide powder to obtain a reaction mixture. The amount of B4C added was 3.59 g (approximately 0.065 mol), providing approximately 0.26 mol of B and 0.065 mol of C; the amount of carbon black added was 0.57 g (approximately 0.0475 mol of C). At this point, the total carbon content in the system was approximately 0.1125 mol, and the molar ratio of carbon to total oxygen (C / O) was approximately 0.51; the molar ratio of B to total metal elements (B / M) was approximately 2.6.

[0069] The above-mentioned mixed powder and tungsten carbide milling balls were placed in a ball mill jar at a ball-to-powder ratio of 10:1. Anhydrous ethanol was added as the milling medium, and the mixture was milled at 300 rpm for 10 hours. After milling, the powder was dried and sieved to obtain a uniformly mixed precursor powder.

[0070] The precursor powder was packed into a casing and pressed into shape using cold isostatic pressing at 200 MPa, then placed in a vacuum sintering furnace. Sintering was performed according to the following procedure: the temperature was increased to 1500℃ at a rate of 5℃ / min and held for 0.5 hours to allow adsorbed moisture and some volatile components to be removed; the temperature was increased to 1900℃ at a rate of 4℃ / min and held for 2 hours to complete the carbothermic reduction reaction and form a high-entropy boride phase; the furnace was then cooled to room temperature.

[0071] The sample was removed, yielding a grayish-black blocky product. XRD analysis revealed... Figure 1 As shown in (c), the main phase of the product is a hexagonal high-entropy (Hf, Zr, Ti, Ta, Nb)B2 solid solution, conforming to the AlB2 type structure (P6 / mmm space group), with no obvious carbon peaks or residual oxide peaks. SEM scan images are shown below. Figure 2 As shown in (c), the polished surface of Comparative Example 1 sample has poor smoothness, with obvious undulations, holes, or loose areas in some places, and its surface continuity is not as good as that of Examples 1 and 2. This result indicates that under conventional equimolar metal feeding conditions, the sintering bonding degree and densification level of the samples are low.

[0072] Comparative Example 2

[0073] Weigh out 4.21g (0.02mol) of HfO2, 2.46g (0.02mol) of ZrO2, 1.60g (0.02mol) of TiO2, and 4.42g (0.01mol) of Ta2O5 (containing 0.02mol of Ta) in equimolar ratios. Weigh out 3.99g (0.015mol, containing 0.03mol of Nb) of Nb2O5. Mix them to obtain a mixed oxide powder with a molar ratio of Hf, Zr, Ti, Ta, and Nb of 1:1:1:1:1:1.5. The total number of moles of metal elements is 0.11mol, and the total number of moles of oxygen is 0.245mol.

[0074] B4C and carbon black were added to the mixed oxide powder to obtain a reaction mixture. The amount of B4C added was 3.95 g (approximately 0.0715 mol), providing approximately 0.286 mol of B and 0.0715 mol of C; the amount of carbon black added was 2.00 g (approximately 0.167 mol of C). At this point, the total carbon content in the system was approximately 0.238 mol, and the C / O ratio was approximately 0.97, creating a high carbon potential reaction environment; the molar ratio of B to total metal elements, B / M, was approximately 2.6.

[0075] The above-mentioned mixed powder and tungsten carbide grinding balls were placed in a ball mill jar at a ball-to-powder ratio of 10:1, and anhydrous ethanol was added as the grinding medium. The mixture was ball-milled at 300 rpm for 10 h. After ball milling, the powder was dried and sieved to obtain a uniformly mixed precursor powder. Subsequently, 0.42 g of B2O3 powder was added to the precursor powder, and the mixture was manually mixed in a mortar for 15 min. The resulting powder was then packed into a casing and pressed into shape in a cold isostatic press at a pressure of 200 MPa.

[0076] The obtained green body was placed in a graphite crucible and then placed in a vacuum sintering furnace. Sintering was carried out according to the following procedure: the temperature was increased to 1500 ℃ at a rate of 5 ℃ / min and held for 0.5 h to allow adsorbed moisture and some volatile components to be removed; then the temperature was increased to 2000 ℃ at a rate of 4 ℃ / min and held for 2 h to allow the carbothermic reduction reaction and the formation of the high-entropy boride phase to proceed fully; after the reaction was completed, the furnace was cooled to room temperature and the sample was removed.

[0077] The obtained sample retains its overall blocky morphology, but a small amount of black residue is visible on the surface. For example... Figure 1 As shown in (d), XRD results indicate that the main phase of Comparative Example 2 is still a hexagonal high-entropy (Hf, Zr, Ti, Ta, Nb)B2 solid solution. Significant NbC characteristic peaks are detected near approximately 35° and 40°. In the main peak region near approximately 26°, the diffraction peak intensity is higher than in Example 2, suggesting possible peak overlap, indicating the possible presence of a small amount of carbon-containing phase or residual carbon in the system. Combined with surface observations, it can be inferred that under conditions of high carbon content, in addition to the formation of a second Nb carbide phase, a small amount of unreacted carbon-containing components may remain in the system. Compared to Example 2, the higher carbon content in Comparative Example 2 leads to an increased amount of NbC phase formation and causes some residual carbon to remain in the microstructure, thus affecting the effective bonding between particles and the sintering densification process.

[0078] The relative density of the sample in Comparative Example 2 was 88%, and its Vickers hardness was 12.0 GPa. Both its density and hardness were lower than those of Example 2, indicating that while increasing the sintering temperature and using B2O3 additives and cold isostatic pressing are beneficial for material densification based on an excess Nb design, a high carbon source content will still weaken the uniformity of the microstructure and the densification effect due to the increase in carbide phases and the presence of a small amount of residual carbon. This is not conducive to obtaining high-entropy diboride-based multiphase ceramics with superior performance. This further illustrates that the carbon content needs to be controlled within a suitable range; simply increasing the amount of carbon source added cannot continuously improve the material properties.

[0079] Comparative Example 3

[0080] Comparative Example 3 was essentially the same as Example 1 in terms of raw material composition and feeding ratio. Green bodies were prepared using the same method as in Example 1, and the resulting green bodies were placed in a graphite crucible and then placed in a vacuum sintering furnace. Sintering was performed according to the following procedure: the temperature was increased to 1500℃ at a rate of 5℃ / min and held for 0.5h to allow adsorbed moisture and some volatile components to be removed; then the temperature was increased to 1700℃ at a rate of 4℃ / min and held for 2h to allow the carbothermic reduction reaction and the formation of the high-entropy boride phase to proceed fully; after the reaction, the furnace was cooled to room temperature, and the samples were removed. Figure 1As shown in (e), the XRD results indicate that in addition to the target high-entropy diboride main phase, there are still relatively obvious residual peaks of simple diborides such as HfB2 in the Comparative Example 3 sample. Compared with Example 1, these residual peaks are more obvious in Comparative Example 3, indicating that a lower sintering temperature is not conducive to promoting the full reaction of each component in the system and the formation of a stable high-entropy diboride structure, resulting in a decrease in the degree of main phase formation and an increase in the residual secondary phase.

[0081] It should be noted that some comparative samples exhibited significant brittleness during the grinding and polishing process, easily experiencing surface particle detachment, localized edge chipping, and breakage. This made it difficult to form a smooth, continuous, and complete test surface that met the requirements for hardness testing, thus failing to obtain stable, effective, and representative hardness test data. These phenomena indicate that the comparative sample conditions are not conducive to obtaining structurally stable sintered materials suitable for subsequent processing and performance evaluation.

[0082] The above embodiments are only used to further illustrate a high-entropy boride-based ceramic and its preparation method according to the present invention. However, the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing high-entropy boride-based ceramics, characterized in that, Includes the following steps: S1. Mix HfO2, ZrO2, TiO2, Ta2O5, and Nb2O5 in a molar ratio of Hf, Zr, Ti, Ta, and Nb of 1:1:1:1:x to obtain a mixed oxide powder; where x is 1.2-2.

0. S2. Add boron carbide and a carbon source to the mixed oxide powder to obtain a reaction raw material mixture, wherein the ratio of the total molar amount of carbon provided by boron carbide and carbon source to the total molar amount of oxygen provided by the mixed oxide powder (C / O) is 0.45-0.75, and the ratio of the molar amount of boron provided by boron carbide to the total molar amount of metal elements provided by the mixed oxide powder (B / M) is 2.3-2.

9. S3. After ball milling and mixing the reaction raw materials, the mixture is shaped and then sintered under an inert atmosphere or vacuum at a programmed temperature, including: Low-temperature degassing section: Heat to 1200-1500℃ at a rate of 5-10℃ / min, and hold for 0.5-1 hour; High-temperature reaction sintering section: Heat to 1800-2100℃ at a rate of 3-5℃ / min, and hold for 1-3 hours; S4. After furnace cooling to room temperature, a high-entropy boride-based ceramic is obtained, which has a high-entropy boride main phase and an intergranular in-situ generated Nb-containing carbide second phase.

2. The method for preparing high-entropy boride-based ceramics according to claim 1, characterized in that: The ratio of the total molar amount of carbon provided by the boron carbide and carbon source to the total molar amount of oxygen provided by the mixed oxide powder (C / O) is 0.48-0.

6.

3. The method for preparing high-entropy boride-based ceramics according to claim 2, characterized in that: The ratio of the molar amount of boron provided by the boron carbide to the total molar amount of metal elements provided by the mixed oxide powder, B / M, is 2.5-2.

7.

4. The method for preparing high-entropy boride-based ceramics according to claim 1, characterized in that: The carbon source is either carbon black or graphite powder.

5. The method for preparing high-entropy boride-based ceramics according to claim 1, characterized in that: Step S2 also includes adding 0.5-3 wt% B2O3 to the reaction mixture.

6. The method for preparing high-entropy boride-based ceramics according to claim 5, characterized in that: The amount of B2O3 added is 1.5-2.5 wt%.

7. The method for preparing high-entropy boride-based ceramics according to claim 1, characterized in that: In step S3, the ball milling speed is 200-500 rpm, the ball-to-material mass ratio is (5-15):1, the ball milling medium is anhydrous ethanol, and the time is 4-12 hours.

8. The method for preparing high-entropy boride-based ceramics according to claim 1, characterized in that: The molding method is cold isostatic pressing, and the molding pressure is 100-300 MPa.

9. A high-entropy boride-based ceramic, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 8.

10. The high-entropy boride-based ceramic according to claim 9, characterized in that: The high-entropy boride main phase is a hexagonal AlB2 type structure (Hf, Zr, Ti, Ta, Nb)B2 high-entropy boride solid solution; the Nb-containing carbide second phase is distributed at the grain boundaries of the high-entropy boride, with a volume fraction of 5-20%.

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