CN115259836B - B with fracture toughness and hardness 6 O-diamond composite material and preparation method thereof - Google Patents

Abstract

The invention discloses a B with fracture toughness and hardness ₆ O-diamond composite material and preparation method thereof, B ₆ O and carbon black nano powder are used as raw materials, and B with good sintering is synthesized under high pressure and high temperature ₆ O-diamond composite material. Carbon black is converted into diamond nano particles under the conditions of high pressure and high temperature, and high strength B is formed simultaneously ₆ O-diamond coherent interface. Superfine B ₆ O and diamond nanoparticles and high strength B ₆ The O-diamond coherent interface cooperates to construct excellent mechanical properties of the composite material. B (B) ₆ The hardness (43 GPa) of the O-diamond composite material is equivalent to that of polycrystalline diamond PCD (40-60 GPa), and the fracture toughness (7.6 MPa.m) ^1/2 ) Than previously synthesized B ₆ O ceramic (1.7-3.1 MPa.m) ^1/2 ) And B ₆ O-based composite material (3-4 MPa.m) ^1/2 ) The improvement is multiple times. Analysis of the breaking behaviour shows that B ₆ The toughening mechanism of the O-diamond composite material mainly comprises nano twin crystal toughening, crack deflection and crack bridging.

Description

B with fracture toughness and hardness 6 O-diamond composite material and preparation method thereof

Technical Field

The invention relates to the technical field of composite materials, in particular to a B with fracture toughness and hardness ₆ An O-diamond composite material and a preparation method thereof.

Background

The synthesis of novel ceramic materials with excellent mechanical properties has been one of the leading topics of high-pressure scientific research, and the materials are usually composed of light elements such as boron, carbon, nitrogen, oxygen and the like, and the light elements have incomparable resistance to external shearing due to long and short interatomic bonds and strong covalent bonds. The most typical examples are diamond and cubic boron nitride (cBN). The excellent mechanical properties have led to a long-standing lead in the application of hard material tools. In addition, scientists have developed a range of other ceramic materials of excellent hardness, such as boron carbide (B) ₄ C) Carbon nitride (C) ₃ N ₄ ) B-C-N ternary Compound (BC) ₂ N), boron suboxide (B) ₆ O), and the like. Wherein B is ₆ O is undoubtedly an attractive branch because of its more pronounced mechanical properties, hardness similar to cBN and fracture toughness comparable to diamond.

In the past, peoplePair B of ₆ Densification and sintering of O have been a series of efforts. Unfortunately, due to B ₆ The diffusion coefficient of O is low, the vapor pressure of boron oxide at the sintering temperature is high, and B can be realized only by pressure-assisted sintering ₆ Complete densification of O. The concrete method is that B ₆ O powder or B and B ₂ O ₃ The mixture of powders is sintered at a high temperature of 1700 to 2200 ℃ under pressure, but even at such high temperature, the sinterability is poor, leading to B ₆ The fracture toughness of O ceramic is poor (1.7-3.1 MPa.m) ^1/2 ). The addition of additives is a straightforward way to improve their sinterability. Past experiments have demonstrated that in B ₆ Alkaline earth, rare earth, alkaline oxide, transition metal and oxide thereof are added as additives in the O sintering process, so that the sintering temperature can be effectively reduced, and densification is facilitated. For example, alkaline earth, rare earth and basic oxides can form stable liquid oxides with boron oxides under sintering conditions, while transition metals and their oxides can form stable liquid oxides with B or B ₆ The O reaction forms a stable boride. However, these additives inevitably have an adverse effect on the properties of the sintered composite block, which results from byproducts (mainly amorphous or crystalline borates and boron oxides) generated during the sintering reaction. B to which these materials are added ₆ The hardness of the O-based composite material is generally in the range of 28-36 GPa, and the fracture toughness is 3-4 MPa m ^1/2 . In addition, other hard materials such as boron carbide, cubic boron nitride and diamond are also possible B ₆ And an O additive. The composite material prepared from the hard material has relatively high hardness but relatively poor fracture toughness, and generally not more than 1.8 MPa.m ^1/2 It is difficult to have a wide application value. It was found that whatever additive was added was insufficient to increase the fracture toughness of the composite to B ₆ O single crystal (4.5 MPa.m) ^1/2 ) Is not sufficiently excellent. The previous research results show that B ₆ O-based composite material and polycrystal B ₆ The main reason for poor O fracture toughness is that the fracture resistance mode is single and the main reason is that the crystal-through fracture is adopted. Typical toughening mechanisms for ceramic materials, such as crack deflection, crack bridging, internal stresses, etc., are rarely developed in these materialsNow, the process is performed. Thus, in holding B ₆ The hardness of the O-based composite material and the improvement of the fracture toughness are B at present ₆ One of the major difficulties in the study of O-based composites.

Disclosure of Invention

In order to solve the problems existing in the prior art, the invention provides a B with fracture toughness and hardness ₆ O-diamond composite material and preparation method thereof, and B is maintained ₆ The hardness of the O-based composite material is improved, and meanwhile, the fracture toughness of the O-based composite material is improved, so that the problems in the background art are solved.

In order to achieve the above purpose, the present invention provides the following technical solutions: b with fracture toughness and hardness ₆ O-diamond composite material, B ₆ O-diamond composite material B ₆ The O powder and the carbon black nano powder are used as raw materials and sintered and synthesized at high pressure and high temperature; the B is ₆ The Vickers hardness of the O-diamond composite material can reach 43GPa, and the fracture toughness of the O-diamond composite material can reach 7.6MPa m ^{1 /2} 。

B with fracture toughness and hardness ₆ The preparation method of the O-diamond composite material comprises the following steps:

s1, amorphous boron (B) and amorphous boron oxide (B) ₂ O ₃ ) B is synthesized by solid-liquid reaction as raw material ₆ O powder;

s2, B is ₆ Mixing O powder and carbon black powder by a mechanical alloying method to obtain precursor powder;

s3, preprocessing the precursor powder to remove impurities on the surface of the powder;

s4, performing high-temperature and high-pressure experiments by adopting a 10MN double-stage large-volume multi-anvil system, and sintering the precursor powder obtained in the step S3 at high temperature and high pressure to obtain B ₆ O-diamond composite material.

Further, the B is ₆ The molar ratio of O powder to carbon black powder was 1:3.

Further, in the precursor powder, the particle diameter of the carbon black powder is 20-100 nm; the B is ₆ The particle size of the O powder is 200-700 nm.

Further, the method comprises the steps of,the pretreatment in step S3 is specifically 3.0X10 ^-5 And (3) treating for 30min under vacuum condition of Pa and 900 ℃.

Further, the sintering in the step S4 is performed at high temperature and high pressure, specifically, the sintering is performed for 30min under the conditions of 25GPa and 1600 ℃.

The beneficial effects of the invention are as follows: the invention prepares B through high-temperature high-pressure sintering ₆ Ultrafine grain B of O and carbon black nano powder ₆ O-diamond composite material, B in sintering process ₆ High-density deformation twin crystals are formed inside the O nano crystal grains, and carbon black is converted into nano crystal diamond, B ₆ And high-strength coherent interfaces are formed between the O crystal grains and the diamond crystal grains. Superfine B ₆ Synergistic action of O and diamond nano-crystal grains, B ₆ High density nano twin substructure in O nano-grains and high strength B ₆ The O-diamond coherent interface provides excellent mechanical properties for the composite material. B synthesized under the conditions of 25GPa and 1600 DEG C ₆ The O-diamond composite material has both fracture toughness and hardness, the Vickers hardness is 43GPa, and the fracture toughness is 7.6 MPa.m ^1/2 。

Drawings

FIG. 1 shows carbon black and B in the examples ₆ Structural characterization and particle size distribution diagram of O powder, (a) TEM image of carbon black nano powder, (B) HRTEM image of carbon black nano particle, showing microstructure of long-range disorder but short-range order, (c) particle size diagram of carbon black nano powder obtained by transmission electron microscope measurement, (d) B ₆ SEM image of O nano powder, grain size of submicron order, (e) B ₆ High-magnification SEM image of O nano powder, (f) is B obtained by SEM measurement ₆ An O nano powder particle size chart;

FIG. 2 shows precursor powder and B prepared ₆ X-ray diffraction XRD pattern of O-diamond composite material, (a) is precursor powder, (B) is B ₆ An O-diamond composite;

FIG. 3 is a B synthesized at 25GPa and 1600 DEG C ₆ A microstructure of the O-diamond composite, (a) a large-scale BF-STEM image of the composite, (B) a SAED image of a framed region in (a), and (c) a framed B ₆ HRTE of O particlesM image, (d) is B ₆ HRTEM image of O inter-grain interface, (e) is B ₆ HRTEM images of interfaces between O grains and diamond grains;

FIG. 4 is B ₆ The mechanical property diagram of the O-diamond composite material is shown in the specification, wherein (a) is the function relation between nano hardness and external load, (B) is the function relation between Vickers hardness and external load, (c) is the function relation between fracture toughness and synthesis temperature, and (d) is B ₆ O-diamond composites versus typical boride, oxide and carbide ceramics hardness versus fracture toughness;

FIG. 5 is B ₆ Schematic diagram of analysis of fracture behavior of O-diamond composite material, (a) B ₆ SEM image of cracks during indentation of O-diamond composite, (B) B ₆ STEM-BF images of cracks in the indentation process of the O-diamond composite material;

FIG. 6 is B ₆ Morphology of O-C mixed nano powder precursor, (a) is SEM image, (b) is high-power SEM image;

FIG. 7 is an XRD contrast plot of the precursor feedstock to composite material conversion, (a) to reduce the carbon phase of the precursor, (B) to 25GPa and B at 1600℃ ₆ An O-diamond composite;

FIG. 8 is B ₆ The method comprises the steps of (a) carrying out SEM (scanning electron microscope) on the surface of a sample of the O-diamond composite material and a corresponding EDS image of the O-diamond composite material, (a) carrying out typical SEM image on the surface of the sample, (b) carrying out EDS image on the surface of the sample, (c) carrying out EDS image on a bright area of the surface of the sample, and (d) carrying out EDS image on a dark area of the surface of the sample;

FIG. 9 is B ₆ B in O-diamond composite material ₆ Particle size distribution diagram of O and diamond, (a) is B ₆ O, (b) is diamond.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

In the present embodiment, B ₆ O and carbon black powder are used as raw materials to synthesize superfine crystal B under high pressure and high temperature HPHT conditions ₆ O-diamond composite material. The high-temperature and high-pressure condition induces the phase transition from carbon black to diamond, thereby greatly promoting the sintering densification of the composite material. Synthetic B ₆ The hardness value of the O-diamond composite material is equivalent to that of the PCD material produced in industry, and the fracture toughness is B ₆ O is more than 1.5 times of that of the previously synthesized polycrystal B ₆ O and B ₆ An O-based composite material. Because of their good combination of hardness and toughness, these ingredients show unlimited potential as attractive alternatives to PCD in industrial processes.

The preparation method comprises the following steps: with amorphous boron and amorphous boron oxide (B) ₂ O ₃ ) As raw material, B is synthesized by solid-liquid reaction ₆ O powder. The mechanical alloying method is adopted to prepare the B ₆ The O powder was mixed with carbon black in a 1:3 molar ratio. The precursor is mixed at 3.0X10 ^-5 And (3) treating for 30min under vacuum conditions of Pa and 900 ℃ to remove impurities on the surface of the powder, and then carrying out high-temperature and high-pressure experiments. And a 10MN double-stage large-volume multi-anvil system is adopted to carry out high-temperature and high-pressure experiments. The mixed powder was compressed to 25GPa and then heated to the specified temperature for 30 minutes. Obtaining B ₆ The O-diamond composite material has a Vickers hardness of 43GPa and a fracture toughness of 7.6MPa.m ^1/2 。

Structural characterization

The phase structure of the sample was analyzed by Cu ka radiation X-ray diffractometer XRD, and the morphology and chemical composition of the sample were characterized by scanning electron microscope SEM and energy spectrometer EDS. The microstructure of the sample with the acceleration voltage of 200kV is characterized by adopting a transmission electron microscope TEM. And preparing a transmission electron microscope sample by adopting a Ga Focused Ion Beam (FIB) milling method. The pre-sheet was cut first at an accelerating voltage of 30kV and a current of 27nA and then reworked at currents of 15, 7, 5, 1-0.5 nA until the thickness was less than 100nm. Low energy Ar milling is further employed to minimize knockout damage to the slices.

As shown in FIG. 1, FIG. 1 is carbon black nano-powder, B ₆ O nano powderMorphology and statistical particle size map thereof. As can be seen in high resolution transmission electron microscopy HRTEM images, the diameter of the carbon black particles varies from 20 to 100nm, and the particles are formed by interweaving short-range ordered graphite flakes. In contrast, B ₆ The particle size of the O particles is larger and is between 200 and 700nm. It is further noted that almost every single crystal of boron oxide exhibits typical twinning characteristics. Through mechanical grinding and mixing, the two precursors are well mixed, the carbon black powder is uniformly distributed and diffused in the boron oxide powder, and the carbon black powder has no C and B ₆ The O-rich zone is shown in fig. 6.

As shown in FIG. 2, FIG. 2 shows precursor powder and B prepared ₆ X-ray diffraction XRD data for O-diamond composites. The clear diffraction peak found corresponds to B ₆ The combination of O and diamond indicates that a phase change from carbon black to diamond has been completely converted at 25GPa and 1200-1600 ℃ and that diamond and B at 25GPa@1800℃ ₆ Chemical reactions between O occur, resulting in a product no longer being B ₆ O-diamond composite material. For boron suboxide, although the nominal composition is B ₆ O, however, is widely regarded as non-stoichiometric due to its inherent hypoxia. Using the Rietveld method, it was investigated whether the stoichiometric ratio of boron oxide was changed during the conversion from raw material to composite material at 25GPa and 1600 c using a fine XRD pattern, as shown in fig. 7. The finishing result shows that: before and after HPHT sintering B ₆ The stoichiometric ratio of oxygen in O is 0.86 and 0.82, respectively; description B ₆ Oxygen content in O has little change in sintering process, and super-pressure can well inhibit B ₆ Decomposition and structural change of O nano powder. This further illustrates that B, at high pressures of 25GPa to 1600 DEG C ₆ Neither a solid solution nor a new compound is formed between O and diamond.

As shown in FIG. 3, from the STEM image, diamond regions of several hundred nm size are uniformly distributed in B ₆ This is also seen in the O matrix, as shown in FIG. 8, which shows an SEM and its corresponding EDS image, where (a) in FIG. 8 is a typical SEM image of the sample surface, the surface roughness is high, and the bright areas represent protrusions, indicating that the areas are hardThe degree is higher; the dark areas represent depressions, indicating that the hardness of the areas is relatively low. FIG. 8 (B-d) shows an EDS corresponding to a, and it can be seen that the bright region is B-C phase and the dark region is B-O phase, illustrating B in the precursor ₆ O and C react chemically under these conditions. As shown in the STEM-BF image of FIG. 9, the diamond and B were counted ₆ The grain sizes of O are about 9nm and 300nm respectively, and match the size of carbon black and B respectively ₆ O precursor, indicating inhibition of current B under high pressure conditions ₆ The grains of the O-diamond composite are grown. Further observation of B with HRTEM ₆ Microstructure of the O-diamond composite. FIGS. 3B and 3c are block diagram areas B of FIG. 3a, respectively ₆ Regional electron diffraction (SAED) and HRTEM images of O grains. Notably, in the SAED image (FIG. 3B), the (101) plane diffraction spot was severely elongated, indicating that B ₆ The O-grain (101) face has a large number of planar defects, which may be stacking faults or twins. This is confirmed by the HRTEM image (FIG. 3 c), which shows B ₆ Stacking faults and nano twin substructure that are prevalent in the O grains. In fact, this is no exception, since almost all B ₆ A large number of stacking faults and nano twinning substructures can be observed for the O grains, as shown in fig. 3 a. This is in accordance with initial B ₆ The structure of the O nano powder is closely related, and in the initial stage B ₆ A large number of twin crystal substructures can be found in the O nano powder, and further deformation twin crystals can be caused by the polyhedron particle morphology in the sintering densification process. FIGS. 3d and 3e show B respectively ₆ inter-O-grains and B ₆ Grain boundaries between O and diamond grains. Under the action of high temperature and high pressure, B is in the sintering process ₆ Between the O grains and B converted from carbon black ₆ And a coherent grain boundary is formed between the O crystal grains and the diamond crystal grains, and no pore and amorphous layer are formed at the grain boundary. B (B) ₆ Both the high density nano twin substructure within the O grains and the coherent structure of the grain boundaries indicate synthesized B ₆ The O-diamond composite material has good mechanical properties.

Performance testing

All test specimens were mirror polished with a diamond paste using a polisher before hardness testing. Nan (nanometer)The rice indentation test uses a nanoindentation tester with Berkovich diamond stones. Nano hardness H was calculated using the Au Li Fufa Oliver and Pharr method, respectively _N And an elastic modulus E. Vickers hardness H was performed on a microhardness tester _V The loading and residence times employed were 30s and 15s, respectively. H _V From H _V ＝1854.4F/L ² Where F is the applied load and L is the arithmetic average of the two diagonals of the indentation. Fracture toughness of the composite was estimated from vickers indentation cracks using the Evans and An Sidi s Anstins equation: k (K) _Ic-Evans ＝0.16×H _V ×a ² /c ^1.5 Where a is half of the indentation diagonal and c is the radial crack length, K _Ic-Anstis ＝0.016×(E/H _V ) ^1/2 ×F/c ^1.5 E is Young's modulus and c is radial crack length.

The hardness test was performed on the composite material synthesized using the nanoindentation method and the vickers indentation method, in which the nanohardness and the elastic modulus of all prepared samples were increased with an increase in sintering temperature from 1200 to 1600 ℃ and time t, and then significantly decreased in a temperature range from 1600 to 1800 ℃ as shown in fig. 4 a. As the sintering temperature increased from 1200 ℃ to 1800 ℃, the change in vickers hardness of the composite was well matched with the change in nano-hardness, and the composite synthesized at 1600 ℃ still maintained the optimal hardness value (fig. 4 b). This is mainly due to the stronger grain boundaries of the carbon black and more thorough phase transitions with increasing temperature, the graphite-like product gradually disappearing between 1200 ℃ and 1600 ℃, and at 1800 ℃, B ₆ O and diamond react chemically to reduce diamond content, hardness and strength B ₆ The destruction of the O diamond interface reduces the toughness of the composite (fig. 4 c). Notably, these hardness values (37-43 GPa) are comparable to high quality commercial superhard materials such as PCBN and PCD. Meanwhile, B synthesized under the conditions of 25GPa and 1600 DEG C ₆ The fracture toughness of the O-diamond composite material also reaches 7.6MPa.m ^1/2 Far exceeds the previously reported B ₆ Fracture toughness (1.8 MPa.m) of O-diamond composite material ^1/2 ) Even withB ₆ O-based metal carbide and metal boride composites (3-4 MPa.m ^1/2 ) In comparison with the prior art. Typically, commercial WC-Co ceramics known to us require more than 30% of the hardness to achieve 100% toughening. FIG. 4d summarizes the hardness and toughness of typical engineering ceramics, particularly B, compared to typical boride, oxide and carbide ceramics ₆ Ashby plot of hardness versus fracture toughness for O-diamond composites. Error bars shown in the figure represent standard deviation of the data, showing B for the present work composition ₆ The perfect combination of high hardness and high toughness of the O-diamond composite material.

To systematically clarify B ₆ The toughening mechanism of the O-diamond composite material is used for deeply characterizing radial cracks generated by the Vickers indentation, as shown in figure 5. Fig. 5a shows a representative SEM image of radial cracks of a polished surface of a composite material. In the figure, the white area and the black area correspond to the diamond area and the B area respectively ₆ O region as shown by SEM EDS analysis in fig. 8. The radial cracks generated from the indentations exhibit a nano-scale zigzagged propagation path. Notably, when a crack encounters B ₆ At the time of O grains, a distinct through-grain fracture characteristic was found. Characterization of B in composite material by TEM ₆ The O crystal grains have a nano twin crystal substructure with high density. Nano twin crystals have great influence on the hardness and toughness of superhard materials, so that B is known ₆ The high density nano twin substructure in the O crystal grain is one of the reasons for the high toughness of the composite material. In addition, another characteristic of crack propagation in the composite material is crack propagation to B ₆ When O is in interface with diamond, crack bridging phenomenon often exists. This illustrates a large number of B ₆ The O diamond interface can prevent crack growth, reduce stress concentration and greatly dissipate energy, so that the composite material has excellent fracture toughness

Based on more detailed observations of STEM-BF images (FIG. 5B), those found in B ₆ Cracks without discontinuities at the O-diamond interface tend to undergo large angular deflections, while at B ₆ There is a significant amount of crack bridging in both the O-region and the diamond region. Composite materials synthesized at lower temperatures have poorer toughness becauseThere is a phase transition mesophase residue at the weaker diamond grain boundaries. This fatal defect also exists in previously synthesized B ₆ In O-diamond composites, this is due to the poor sinterability of diamond in the precursor. B synthesized in this study ₆ The O-diamond composite material effectively disperses the energy in the material through the high-density nano twin crystal substructure, crack bridging and crack deflection, thereby greatly improving the fracture toughness. This is in accordance with B ₆ O and carbon black are used as raw materials, and the grain boundary which is well sintered and obtained by phase transition of the carbon black under the conditions of high temperature and high pressure is not separated.

In conclusion, the invention prepares B by high-temperature high-pressure sintering ₆ An ultra-fine grain diamond composite material of O and carbon black nano powder. During HPHT sintering process, B ₆ High-density deformation twin crystals are formed inside the O nano crystal grains, and carbon black is converted into nano crystal diamond, B ₆ And high-strength coherent interfaces are formed between the O crystal grains and the diamond crystal grains. Superfine B ₆ Synergistic action of O and diamond nano-crystal grains, B ₆ High density nano twin substructure in O nano-grains and high strength B ₆ The O-diamond coherent interface provides excellent mechanical properties for the composite material. B synthesized under the conditions of 25GPa and 1600 DEG C ₆ The O-diamond composite material has a Vickers hardness of 43GPa, which is equivalent to that of polycrystalline diamond PCD (40-60 GPa), and a fracture toughness of 7.6 MPa.m ^1/2 Compared with the previously synthesized polycrystal B ₆ O ceramic (1.7-3.1 MPa.m) ^1/2 ) And B ₆ O-based composite material (3-4 MPa.m) ^1/2 ) The improvement is several times, which is far superior to the prior reported composite materials. The detailed characterization of microcracks shows that various toughening mechanisms such as nano twin crystal toughening, crack deflection, crack bridging and the like are B ₆ The main reason for the enhanced fracture toughness of the O-diamond composite material. The current research method is expected to be high-performance B ₆ O-based composite ceramics offer new research strategies and synthetic solutions, especially with covalent materials with phase transitions under high temperature and pressure conditions as additives.

Although the present invention has been described with reference to the foregoing embodiments, it will be apparent to those skilled in the art that modifications may be made to the embodiments described, or equivalents may be substituted for elements thereof, and any modifications, equivalents, improvements and changes may be made without departing from the spirit and principles of the present invention.

Claims (1)

1. B with fracture toughness and hardness ₆ An O-diamond composite material characterized in that the B ₆ O-diamond composite material B ₆ O powder and carbon black nano powder are used as raw materials, and are sintered and synthesized at high pressure and high temperature, and B is obtained in the sintering process ₆ High-density deformation twin crystals are formed inside the O nano crystal grains, and carbon black is converted into nano crystal diamond, B ₆ Forming high-strength coherent interfaces between the O crystal grains and the diamond crystal grains; the B is ₆ The Vickers hardness of the O-diamond composite material can reach 43GPa, and the fracture toughness of the O-diamond composite material can reach 7.6MPa m ^1/2 The method comprises the steps of carrying out a first treatment on the surface of the B having both fracture toughness and hardness ₆ The preparation method of the O-diamond composite material comprises the following steps:

s1, synthesizing B by taking amorphous boron and amorphous boron oxide as raw materials through solid-liquid reaction ₆ O powder;

s2, B is ₆ Mixing O powder and carbon black powder by a mechanical alloying method to obtain precursor powder;

s3, preprocessing the precursor powder to remove impurities on the surface of the powder;

s4, sintering the precursor powder obtained in the step S3 at high temperature and high pressure to obtain B ₆ An O-diamond composite;

the B is ₆ The molar ratio of the O powder to the carbon black powder is 1:3; in the precursor powder, the particle size of the carbon black powder is 20-100 nm; the B is ₆ The particle size of the O powder is 200-700 nm;

the pretreatment in step S3 is specifically 3.0X10 ^-5 Treating for 30min under vacuum condition of Pa and 900 ℃; the sintering in the step S4 is carried out at high temperature and high pressure, specifically, the treatment is carried out for 30min under the conditions of 25GPa and 1600 ℃.