A method for preparing a b-c-n superhard composite

Abstract

The preparation method of the B-C-N superhard composite material takes diamond powder with an average particle size of 0.5 mu m and cubic boron nitride powder with an average particle size of 3 mu m as raw materials, the molar ratio of the diamond powder to the cubic boron nitride powder is one of 16:1, 12:1, 8:1, 4:1, 1:8 and 1:4, or the volume ratio of the diamond powder to the cubic boron nitride powder is 1:1, and the steps of the preparation method are as follows: (1) respectively acid-dissolving and removing impurities from the diamond powder and the cubic boron nitride powder; (2) uniformly mixing and measuring the impurity-removed diamond powder and the impurity-removed cubic boron nitride powder according to the molar ratio or the volume ratio to form mixed powder; (3) performing vacuum heat treatment on the mixed powder; (4) pre-pressing the mixed powder after the vacuum heat treatment to form a green body; and (5) performing high-temperature and high-pressure sintering on the obtained green body. The B-C-N superhard composite material prepared by the method has improved hardness, which can reach 107.7 GPa, and better thermal stability, which can reach 934 DEG C.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of superhard composite material preparation, and particularly relates to a preparation method of B-C-N superhard composite material. BACKGROUND

[0002] Superhard material is a general term for materials with Vickers hardness greater than 40 GPa. In general, superhard materials are composed of single light element atoms or multiple light element atoms, wherein the light elements are boron (B), carbon (C), nitrogen (N), oxygen (O), etc. For example, diamond and cubic boron nitride (cBN) are the two most widely used superhard materials, which are composed of carbon atoms and boron and nitrogen atoms, respectively. Although diamond and cBN are widely used, their shortcomings in the field of material processing cannot be ignored. Diamond, as the hardest known substance in nature, has a Vickers hardness of 60-120 GPa, but its thermal stability and chemical inertness are poor at room temperature and pressure. The oxidation temperature of diamond in air is about 600℃, and diamond is easily chemically reacted with iron group metals and their alloys and loses its effectiveness. In comparison, cBN has better thermal stability and chemical inertness than diamond. The oxidation temperature of cBN in air is about 1100℃ and cBN is not easily chemically reacted with iron group metals and their alloys, but the hardness of cBN is only about half of that of diamond. Therefore, more and more attention has been paid to the preparation of new superhard materials that combine the advantages of diamond and cBN by researchers in the field of superhard materials.

[0003] Based on the success of carbon, boron, and nitrogen in manufacturing superhard materials, people have tried to prepare B-C-N ternary composite superhard materials under high temperature and high pressure conditions, so that they have both high hardness and high wear resistance of diamond and high stability of cBN. The journal "Applied Physics Letters" discloses a diamond-cubic boron nitride alloy material (Wang P, He D, Wang L, et al. Diamond-cBN alloy: a universal cutting material [J]. Applied Physics Letters, 2015, 107(10): 101901.). The material uses diamond and cBN powders with a particle size of 2-4 μm as raw materials, the molar ratio of diamond to cBN powder is the same, and is sintered under high temperature and high pressure conditions of 11-20 GPa and 1300-2600 K (1026.85-2326.85℃). The B-C-N composite material obtained has a diameter of about 3 mm, a maximum hardness of 76 GPa, and a thermal stability of 796℃. Through turning experiments, it is shown that the B-C-N composite material successfully solves the problems of "diamond cannot turn iron group black metals" and "cBN has low hardness", but the hardness and thermal stability of the B-C-N composite material need to be improved. SUMMARY

[0004] The present application aims to overcome the deficiencies of the prior art, and provide a preparation method of B-C-N superhard composite material, so as to obtain B-C-N superhard composite material with higher hardness and better thermal stability.

[0005] The technical scheme of the present application is: taking diamond powder with a specific particle size and cubic boron nitride powder with a specific particle size as raw materials, and dosing the diamond powder and the cubic boron nitride powder according to a specific molar ratio or volume ratio, then pre-pressing the mixed powder of the diamond powder and the cubic boron nitride powder to form a “date cake model” structure in high-temperature and high-pressure sintering; the “date cake model” structure realizes high-strength diamond wrapping cBN and inhibits the growth of cBN under high temperature and high pressure, while reducing porosity and promoting the bonding of carbon in diamond and boron and nitrogen atoms in cBN to form new B-C bonds and C-N bonds. The “date cake model” structure formed in the sintering process can improve the entropy value of the overall material, thereby reducing the Gibbs free energy of the system, so that the obtained B-C-N superhard composite material is more stable and has better comprehensive performance. To form the “date cake model” structure, the particle size and the ratio of the raw materials are key factors.

[0006] The preparation method of the B-C-N superhard composite material according to the present application takes diamond powder with an average particle size of 0.5 μm and cubic boron nitride powder with an average particle size of 3 μm as raw materials, and the molar ratio of the diamond powder to the cubic boron nitride powder is one of 16:1, 12:1, 8:1, 4:1, 1:8 and 1:4, or the volume ratio of the diamond powder to the cubic boron nitride powder is 1:1. The steps of the preparation method are: (1) separately acid-dissolving and removing impurities from the diamond powder and the cubic boron nitride powder; (2) measuring and uniformly mixing the diamond powder and the cubic boron nitride powder after impurity removal according to the molar ratio or the volume ratio to form a mixed powder; (3) removing adsorbed gas and organic impurities in the mixed powder by vacuum heat treatment; (4) pre-pressing the mixed powder after vacuum heat treatment to form a green body by wrapping with a tantalum foil, a rhenium foil or a molybdenum foil; (5) high-temperature and high-pressure sintering the obtained green body, and then taking out the wrapped body after high-temperature and high-pressure sintering after pressure reduction and temperature reduction to normal pressure and room temperature, and then cleaning to remove the metal wrapping and impurities attached to the surface of the B-C-N superhard composite material.

[0007] In step (1) of the above method, the operation of acid-dissolving and removing impurities from the diamond powder and the cubic boron nitride powder is as follows:

[0008] (11) the diamond powder and the cubic boron nitride powder are respectively added into hydrofluoric acid with a mass concentration of 20-30% and a temperature of 50-60°C and stirred uniformly, then soaked, the supernatant is removed after the powder is settled, and the diamond powder and the cubic boron nitride powder are respectively washed with deionized water until neutral (i.e. the washing water is neutral), and the weight ratio of the diamond powder and the cubic boron nitride powder to the hydrofluoric acid is 1:2.5-3.5;

[0009] (12) the diamond powder and the cubic boron nitride powder washed with hydrofluoric acid are respectively added into hydrochloric acid with a mass concentration of 20-25% and a temperature of 50-60°C and stirred uniformly, then soaked, the supernatant is removed after the powder is settled, and the diamond powder and the cubic boron nitride powder are respectively washed with deionized water until neutral (i.e. the washing water is neutral), and the weight ratio of the diamond powder and the cubic boron nitride powder to the hydrochloric acid is 1:2.5-3.5;

[0010] The above operation is repeated 2-3 times, and then the obtained impurity-removed diamond powder and cubic boron nitride powder are dried for standby use.

[0011] In step (3) of the above method, the vacuum degree of vacuum heat treatment of the mixed powder is 1×10 -1 ~1×10 - 5 Pa, the temperature is 500-1500°C, the treatment time is 0.1-10h, the temperature rising rate from room temperature to the treatment temperature and the temperature falling rate from the treatment temperature to room temperature are 10-25°C / min.

[0012] In step (4) of the above method, the pressure intensity of the mixed powder in the mold during pre-pressing is 200-600MPa, and the pre-pressing time is determined according to the compactness of the green body formed by pre-pressing, which is greater than 30%.

[0013] In step (5) of the above method, the pressure intensity of high-temperature and high-pressure sintering of the green body is 10-20GPa, the temperature is 1800-2500°C, and the holding time is 1min-1.5h; the pressure rising rate from normal pressure to the sintering pressure intensity and the pressure falling rate from the sintering pressure intensity to normal pressure are 2-4.2GPa / h, and the temperature rising rate from room temperature to the sintering temperature and the temperature falling rate from the sintering temperature to room temperature are 50-200°C / min.

[0014] The present application has the following beneficial effects:

[0015] 1. Since the particle size and the ratio of the raw material diamond powder and the cubic boron nitride powder in the method can form a "date cake model" structure under high temperature and high pressure, the hardness of the prepared B-C-N superhard composite material is improved, which can reach 107.7GPa, and the thermal stability is better, which can reach 934°C.

[0016] 2. The raw materials of the method described in this invention are easy to obtain, the preparation method is simple, and the equipment used is conventional equipment, thus facilitating industrial production. Attached Figure Description

[0017] Figure 1 These are microscopic morphology and indentation diagrams of the BCN superhard composite material prepared by the molar ratio of diamond powder to cubic boron nitride powder of Example 1 of the present invention, wherein (a) is the microscopic morphology and (b) is the indentation.

[0018] Figure 2 The images show the polished surface and cross-sectional micromorphology of the BCN superhard composite material prepared in Example 2 of the present invention, wherein (a) is the polished surface and (b) is the cross-sectional micromorphology.

[0019] Figure 3 This is an optical photograph of the BCN superhard composite material prepared in Example 2 of the present invention;

[0020] Figure 4 This is a schematic diagram of the "sweet date cake model" structure of the BCN superhard composite material prepared in Example 2 of the present invention;

[0021] Figure 5 This is a comparison of the thermal stability test temperatures of the BCN superhard composite material prepared in Example 2 of the present invention and pure polycrystalline diamond. Detailed Implementation

[0022] The preparation method of the BCN superhard composite material of the present invention will be further described below through embodiments and in conjunction with the accompanying drawings. It should be noted that the following embodiments should not be construed as limiting the scope of protection of the present invention. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In the following examples, the raw materials, diamond powder and cubic boron nitride powder, were purchased commercially. Hydrofluoric acid, hydrochloric acid, and nitric acid were chemically pure. The high-temperature and high-pressure device used in the high-temperature and high-pressure sintering of Example 1 was a domestically produced hinged six-sided press of model 6×8MN; the high-temperature and high-pressure device used in the high-temperature and high-pressure sintering of Example 2 was a domestically produced hinged six-sided press of model 6×25MN.

[0024] Example 1

[0025] In this embodiment, diamond powder with an average particle size of 0.5 μm and cubic boron nitride powder with an average particle size of 3 μm were used as raw materials, and six BCN superhard composite material samples were prepared according to the molar ratio of diamond powder to cubic boron nitride powder of 16:1, 12:1, 8:1, 4:1, 1:8 and 1:4.

[0026] The process steps are as follows:

[0027] (1) Acid purification of diamond powder and cubic boron nitride powder

[0028] The operation of acid purification of diamond powder and cubic boron nitride powder is as follows:

[0029] (11) Diamond powder and cubic boron nitride powder are respectively added into two purification kettles at a weight ratio of 1:3 with 20% hydrofluoric acid, and stirred in a water bath to heat to 60°C, then soaked at 60°C, and after the powder settles, the supernatant is removed, and then washed with deionized water until the washing water is neutral;

[0030] (12) Diamond powder and cubic boron nitride powder washed with hydrofluoric acid are respectively added into two purification kettles at a weight ratio of 1:3 with 25% hydrochloric acid, and stirred in a water bath to heat to 60°C, then soaked at 60°C, and after the powder settles, the supernatant is removed, and then washed with deionized water until the washing water is neutral;

[0031] The above operation is repeated twice, and then the obtained purified diamond powder and cubic boron nitride powder are respectively placed in an oven and dried at 50°C for standby;

[0032] (2) Formation of mixed powder

[0033] The purified diamond powder and cubic boron nitride powder are weighed according to the molar ratio of 16:1, 12:1, 8:1, 4:1, 1:8, and 1:4, and then mixed uniformly with a three-dimensional mixer to form six kinds of mixed powder;

[0034] (3) Vacuum heat treatment of mixed powder

[0035] The operation of vacuum heat treatment is as follows:

[0036] (31) Six corundum crucibles are cleaned with alcohol and dried, and then the six kinds of mixed powder are respectively poured into the cleaned six corundum crucibles;

[0037] (32) The corundum crucible containing the mixed powder is placed in a vacuum furnace, and then the furnace door is closed to vacuumize and preheated to 400°C, and the vacuum degree is controlled at 5×10 -3 Pa, and then the temperature in the furnace is increased to 800°C at a rate of 20°C / min for 8h of heat preservation treatment;

[0038] (33) After the heat preservation treatment is completed, the temperature in the furnace is decreased to room temperature at a rate of 20°C / min;

[0039] (4) Pre-pressing

[0040] The six kinds of mixed powders after vacuum heat treatment were respectively put into 0.025mm thick tantalum sheet for wrapping and were respectively put into a mold, and then were respectively pre-pressed into shape by a jack. For the six kinds of mixed powders, the pre-pressing pressure of the jack was 30MPa, the pre-pressing pressure of the mixed powders in the mold was 400MPa, and the pre-pressing time was 20s. The compactness of the six green compacts was measured after pre-pressing, and is shown in the following table:

[0041]

[0042]

[0043] (5) High temperature and high pressure sintering

[0044] The six green compacts were respectively put into a high pressure assembly, and then the six high pressure assemblies were respectively sintered by a high temperature and high pressure device. The operation was as follows: after the pressure was increased to 16GPa at a pressure increasing rate of 3.6GPa / h, the pressure was kept unchanged, then the temperature was increased to 2200℃ at a temperature increasing rate of 100℃ / min and was kept for sintering for 5min. After the sintering time arrived, the temperature was first decreased to room temperature at a temperature decreasing rate of 100℃ / min, and then the pressure was decreased to normal pressure at a pressure decreasing rate of 3.6GPa / h. Then the wrapped body after high temperature and high pressure treatment was taken out, was put into a mixed acid (hydrofluoric acid and nitric acid with a volume ratio of 1:1) with a mass concentration of 30% of hydrofluoric acid and a mass concentration of 40% of nitric acid to remove the wrapping material tantalum, and then was sequentially cleaned with alcohol and deionized water by ultrasonic to remove external impurities, so as to obtain six B-C-N superhard composite material samples.

[0045] The six B-C-N superhard composite material samples prepared were respectively subjected to hardness test and microstructure analysis by scanning electron microscope. The results of hardness test are shown in the following table:

[0046] Sample The ratio of diamond powder to cubic boron nitride powder in the mixed powder Vickers hardness (GPa) First Molar ratio 16:1 97.2 Second Molar ratio 12:1 100.9 Third Molar ratio 8:1 94.1 Fourth Molar ratio 4:1 90.3 Fifth Molar ratio 1:8 81.7 Sixth Molar ratio 1:4 83.8

[0047] The microstructure and pressure pits of the third sample (the molar ratio of diamond powder to cubic boron nitride powder in the mixed powder was 8:1) are shown in Figure 1 . It can be seen from Figure 1 (a) that the diamond grains and the cBN grains are tightly combined, most of the cBN grains are wrapped by the diamond grains, and the size of the cBN grains after sintering is basically consistent with the size of the initial powder grains; and it can be seen from Figure 1 (b) that the pressure pits are regular and the crack propagation length of the pressure pits is short. The microstructure and pressure pits of the other five samples are similar to Figure 1 .

[0048] Example 2

[0049] This example uses diamond powder with an average particle size of 0.5 μm and cubic boron nitride powder with an average particle size of 3 μm as raw materials, and a B-C-N superhard composite sample is prepared with a volume ratio of diamond powder to cubic boron nitride powder of 1:1.

[0050] (1) Acid purification of diamond powder and cubic boron nitride powder

[0051] The operation of acid purification of diamond powder and cubic boron nitride powder is as follows:

[0052] (11) Diamond powder and cubic boron nitride powder are added separately into two purification kettles with hydrofluoric acid of 20% mass concentration at a weight ratio of 1:3, and heated to 60°C in a water bath while stirring, then kept at 60°C for a while, and soaked after settling. The supernatant is removed, and the powder is washed with deionized water until the washing water is neutral;

[0053] (12) Diamond powder and cubic boron nitride powder washed with hydrofluoric acid are added separately into two purification kettles with hydrochloric acid of 25% mass concentration at a weight ratio of 1:3, and heated to 60°C in a water bath while stirring, then kept at 60°C for a while, and soaked after settling. The supernatant is removed, and the powder is washed with deionized water until the washing water is neutral;

[0054] The above operation is repeated 3 times, and the obtained purified diamond powder and cubic boron nitride powder are dried in an oven at 50°C for standby;

[0055] (2) Formation of mixed powder

[0056] The purified diamond powder and cubic boron nitride powder are measured at a volume ratio of 1:1, and then mixed uniformly with a three-dimensional mixer to form a mixed powder;

[0057] (3) Vacuum heat treatment of mixed powder

[0058] The operation of vacuum heat treatment is as follows:

[0059] (31) The corundum crucible is cleaned with alcohol and dried, and then the mixed powder is poured into the cleaned corundum crucible;

[0060] (32) The corundum crucible with the mixed powder is placed into a vacuum furnace, and then the furnace door is closed to create vacuum and preheated to 400°C, with a vacuum degree of 5x10 -3 Pa, and the temperature in the furnace is raised to 800°C at a rate of 20°C / min for heat treatment for 8h;

[0061] (33) After the heat treatment, the temperature in the furnace is lowered to room temperature at a rate of 20°C / min;

[0062] (4) Pre-pressing

[0063] The mixed powder after vacuum heat treatment is put into a tantalum sheet with a thickness of 0.025 mm for wrapping and into a mold, and then pre-pressed into a shape by a jack, the pre-pressing pressure of the jack being 30 MPa, the pre-pressing pressure of the mixed powder in the mold being 400 MPa, and the pre-pressing time being 20 s, and the compactness of the green body after pre-pressing being 56%;

[0064] (5) High-temperature and high-pressure sintering

[0065] The obtained green body is put into a high-pressure assembly, and then the high-pressure assembly is sintered by a high-temperature and high-pressure device, the operation being as follows: the pressure is increased to 16 GPa at a rate of 3.6 GPa / h, the pressure is kept unchanged, then the temperature is increased to 2200 ℃ at a rate of 100 ℃ / min and sintered for 5 min, after the sintering time, the temperature is first decreased to room temperature at a rate of 100 ℃ / min, and then the pressure is decreased to normal pressure at a rate of 3.6 GPa / h, and then the wrapped body after high-temperature and high-pressure treatment is taken out, put into a mixed acid composed of hydrofluoric acid with a mass concentration of 30% and nitric acid with a mass concentration of 40% (the volume ratio of hydrofluoric acid to nitric acid being 1:1) to remove the wrapping material tantalum, and then sequentially cleaned with alcohol and deionized water by ultrasonic to remove external impurities, thereby obtaining a B-C-N superhard composite material sample.

[0066] The prepared B-C-N superhard composite material sample is photographed by a camera, the polished surface and the cross section thereof are analyzed by a scanning electron microscope, the hardness thereof is tested, and the thermal stability thereof is tested, the optical photograph of the prepared B-C-N superhard composite material sample is shown in Figure 3 , the microstructure of the polished surface and the cross section thereof is shown in Figure 2 , and the thermal stability thereof is shown in Figure 5 . It can be seen from Figure 3 that the B-C-N superhard composite material sample prepared in the embodiment is in a cylindrical shape, and the diameter of the cross section is 10 mm. It can be seen from Figure 2 (a) that the B-C-N superhard composite material sample prepared in the embodiment has no obvious exposed cBN grains, the diamond grains well wrap the cBN grains and inhibit the recrystallization of the cBN grains, the two kinds of grains are combined closely, and form the jujube cake model structure shown in Figure 4 . It can be seen from Figure 2 (b) that the grain boundaries of the sample cross section are blurred, and the grain boundaries between the grains cannot be distinguished, and at the same time, the grains of the cross section also show obvious transgranular fracture. The result of the hardness test is that the Vickers hardness is 107.7 GPa. It can be seen from Figure 5 that the thermal stability reaches 934 ℃, which is far higher than the decomposition temperature 630 ℃ of pure-phase polycrystalline diamond.

Claims

1. A method for preparing a BCN superhard composite material, characterized in that... Using diamond powder with an average particle size of 0.5 μm and cubic boron nitride powder with an average particle size of 3 μm as raw materials, the molar ratio of diamond powder to cubic boron nitride powder is one of 16:1, 12:1, 8:1, 4:1, 1:8, or 1:4, or the volume ratio of diamond powder to cubic boron nitride powder is 1:

1. The preparation method consists of the following steps: (1) acid dissolving and impurity removal of diamond powder and cubic boron nitride powder respectively; (2) metering and mixing the impurity-removed diamond powder and cubic boron nitride powder according to the molar ratio or volume ratio. (3) Vacuum heat treatment is performed on the mixed powder to remove the gas and organic impurities adsorbed in the mixed powder; (4) The mixed powder after vacuum heat treatment is wrapped with metal tantalum foil, rhenium foil or molybdenum foil and pre-pressed to form a blank; (5) The obtained blank is first pressurized to the sintering pressure and then heated to the sintering temperature for high temperature and high pressure sintering. After sintering, it is first cooled to room temperature and then pressured to normal pressure. Then the wrapped body after high temperature and high pressure sintering is taken out and cleaned to remove the metal wrapping and impurities attached to the surface of BCN superhard composite material. In step (5), the pressure for high-temperature and high-pressure sintering of the green blank is 10~20 GPa, the temperature is 1800~2500℃, and the holding time is 1min~1.5h; the pressure increase rate from atmospheric pressure to sintering pressure and the pressure decrease rate from sintering pressure to atmospheric pressure are 2~4.2 GPa / h, the temperature increase rate from room temperature to sintering temperature and the temperature decrease rate from sintering temperature to room temperature are 50~200 ℃ / min.

2. The method for preparing the BCN superhard composite material according to claim 1, characterized in that... The acid dissolution and impurity removal operations for diamond powder and cubic boron nitride powder in step (1) are as follows: (11) Add diamond powder and cubic boron nitride powder to hydrofluoric acid with a mass concentration of 20-30% and a temperature of 50-60℃ and stir evenly. Then let it stand and soak. After the powder settles, remove the supernatant and wash with deionized water until neutral. The weight ratio of diamond powder and cubic boron nitride powder to hydrofluoric acid is 1:2.5-3.

5. (12) Add the diamond powder and cubic boron nitride powder washed with hydrofluoric acid to hydrochloric acid with a mass concentration of 20-25% and a temperature of 50-60℃ and stir evenly. Then let it stand and soak. After the powder settles, remove the supernatant and wash with deionized water until neutral. The weight ratio of the diamond powder and cubic boron nitride powder to the hydrochloric acid is 1:2.5-3.

5. Repeat the above operation 2-3 times, and then dry the obtained purified diamond powder and cubic boron nitride powder separately for later use.

3. The method for preparing the BCN superhard composite material according to claim 1 or 2, characterized in that... In step (3), the vacuum degree of the mixed powder underwent vacuum heat treatment is 1×10⁻⁶. -1 ~1×10 -5 Pa, temperature is 500~1500℃, treatment time is 0.1~10h; the heating rate from room temperature to treatment temperature and the cooling rate from treatment temperature to room temperature are 10~25℃ / min.

4. The method for preparing the BCN superhard composite material according to claim 1 or 2, characterized in that... In step (4), the pressure on the mixed powder in the mold during pre-pressing is 200~600 MPa, and the pre-pressing time is determined by the density of the preform formed by pre-pressing being greater than 30%.

5. The method for preparing the BCN superhard composite material according to claim 3, characterized in that... In step (4), the pressure on the mixed powder in the mold during pre-pressing is 200~600 MPa, and the pre-pressing time is determined by the density of the preform formed by pre-pressing being greater than 30%.

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