CN114315361B - Nanocrystalline silicon carbide superhard bulk material and preparation method thereof - Google Patents
Nanocrystalline silicon carbide superhard bulk material and preparation method thereof Download PDFInfo
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- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
- C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
- C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
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Abstract
The application relates to a nanocrystalline silicon carbide superhard bulk material and a preparation method thereof. The average grain size of the nanocrystalline silicon carbide superhard bulk material is less than 100nm, and the Vickers hardness is equal to or higher than 40GPa. The preparation method comprises the following steps: a) Pretreatment: weighing silicon carbide nano powder with certain mass, carrying out acid washing treatment, diluting with water to be nearly neutral, taking out and drying; b) And (5) pre-pressing and forming: prepressing the pretreated raw material of the A) to obtain a prepressing blank; c) Sintering: sintering the pre-pressed blank body obtained in the step B) at high temperature and high pressure; d) Discharging: and C), cooling and releasing pressure of the high-temperature high-pressure sintering equipment in the step C), taking out the sintered silicon carbide block, and performing optional post-treatment to obtain the nanocrystalline silicon carbide superhard block.
Description
Technical Field
The application belongs to the field of inorganic nonmetallic materials, and particularly relates to a nanocrystalline silicon carbide superhard bulk material and a preparation method thereof.
Background
Materials with vickers hardness exceeding 40GPa are called superhard materials. Superhard materials are widely used in the industrial fields of cutting, turning, grinding and the like. Diamond and cubic boron nitride have been successfully synthesized in the last 50 th century, however, superhard materials have been limited to these two families to date. Researchers have made diligent efforts to obtain superhard materials other than diamond and cubic boron nitride, with little return. Meanwhile, the method of structural nanocrystallization is widely applied to increase the hardness of diamond and cubic boron nitride materials. For example, by reducing the grain size of cubic boron nitride to 14nm, the hardness can be raised to 85GPa, twice that of single crystal cubic boron nitride. However, conventional hard materials have not achieved superhard properties due to the difficulty in sintering a block of hard material to be dense and the limited range of grain size reduction at the same time.
The cubic silicon carbide is used as a common grinding and cutting material, and has good advantages in the aspects of mechanical property, chemical and thermal stability, preparation process and the like. However, as a conventional hard material, the hardness of cubic silicon carbide is around 26GPa, which limits its wide application in industry. In experiments, the nanocrystalline bulk material prepared by spark plasma sintering and hot-pressed sintering has smaller grain size, but has poor compactness and different degrees of air holes. The presence of air voids can adversely affect the mechanical properties of the material. In addition, researchers have also sintered silicon carbide nanopowders at 4GPa and 1500-1900 ℃ by a high temperature and high pressure method, and although dense nanocrystalline silicon carbide blocks are obtained, the Vickers hardness of the blocks is not more than 26GPa due to the large grain size of the blocks.
In addition, chinese patent application CN104086179a discloses a method for obtaining silicon carbide bulk ceramic by ball milling silicon powder and graphite powder, followed by sintering. The bulk density reported in this patent is 93.0 to 97.0% and the hardness is 31.4 to 42.4GPa, but the hardness test method is not disclosed. However according to other authoritative documents (e.g., "Monolithic Nanocrystalline SiC Ceramics", branko
Journal of the European Ceramic Society, volume 36,Issue 12,September 2016,Pages 3005-3010) is reported to be only about 23GPa, and it is therefore presumed that the hardness values disclosed in CN104086179a are not vickers or asymptotic, since the true measure of vickers hardness (the measure of hardness in the region where the hardness value remains unchanged with load, i.e., the asymptotic hardness, in the hardness-load curve obtained at the time of hardness measurement) of a silicon carbide block of similar density is only about 23 GPa. The silicon carbide bulk ceramic obtained in CN104086179a is amorphous or amorphous containing a very small amount of nanocrystalline precipitated phase. Compared withCrystalline materials, amorphous materials have limited use ranges; because amorphous phase transformation from amorphous to crystalline phase occurs at high temperature, thereby impairing its own excellent characteristics. In practical engineering applications, when silicon carbide is used as an isolation layer or abrasive, it is often necessary to be exposed to high temperature environments. Therefore, in these cases, the use of amorphous materials is very disadvantageous.
Disclosure of Invention
In view of the above-described shortcomings of the prior art silicon carbide ceramic materials, there is a great need in the industry to develop new types of silicon carbide crystal materials having ultra-high hardness.
The ultra-hard nanocrystalline silicon carbide block is obtained by sintering various pretreated silicon carbide raw materials at high temperature and high pressure.
Accordingly, the first aspect of the present application provides a nanocrystalline silicon carbide bulk material having an average grain size of less than 100nm and a vickers hardness of 40GPa or higher.
The technical scheme of the second aspect of the application is a preparation method of a nanocrystalline silicon carbide block, which comprises the following steps:
a) Pretreatment: weighing silicon carbide nano powder with certain mass, carrying out acid washing treatment, diluting with water to be close to neutral, taking out, and drying;
b) And (5) pre-pressing and forming: and prepressing the dried raw materials to obtain a prepressing blank.
C) Sintering: and C) performing high-temperature and high-pressure sintering on the pre-pressed blank obtained in the step B), wherein the sintering pressure is 4-28 GPa, the sintering temperature is 1000-2500 ℃, and the heat preservation time is 1-60 min.
D) Discharging: and C), cooling and releasing pressure of the high-temperature high-pressure sintering equipment in the step C), taking out the sintered silicon carbide block, and performing optional post-treatment.
Compared with the prior art, the invention has the following advantages:
1. compared with the sintering technology without pretreatment, the pretreatment technology provided by the invention can effectively remove the silicon dioxide layer on the surface of the silicon carbide nano-particles, thereby improving the activity of the silicon carbide and reducing the sintering conditions (pressure, temperature and the like) required by compact sintering.
2. Different from various pressureless hot pressing and spark plasma sintering technologies, the sintering temperature required by the high-temperature and high-pressure technology is lower, no sintering aid is needed, the grain boundary sintering of the silicon carbide block is good, and the sample is nearly transparent.
3. Different from the superhard silicon carbide film obtained by chemical vapor deposition, the silicon carbide obtained by high-temperature high-pressure sintering is a block material, has thicker thickness and is more suitable for machining. In addition, the size of the sample can be further increased, and the practical application range is wider.
4. In high-temperature and high-pressure sintering, high temperature can promote combination among grains and generation of a strong interface, high pressure can restrict atomic diffusion and limit coarsening of grains, so that the density and hardness of a sintered sample are improved. The average grain size of the obtained silicon carbide sample is small and can reach below 10nm at minimum. And the nanocrystalline superhard silicon carbide blocks with various grain sizes can be obtained by adjusting the raw material precursors and the sintering process.
5. The obtained silicon carbide sample has the actual measured value of the Vickers hardness equal to or higher than 40GPa, is a novel superhard material and enriches the superhard material family. The material can be made into cutters and has great application potential in the field of precision machining. Therefore, the research result has important significance for expanding the application range of the silicon carbide and guiding the synthesis of new superhard materials.
The nanocrystalline silicon carbide bulk material and the preparation method thereof are described in further detail below with reference to the accompanying drawings and the detailed description.
Drawings
Fig. 1 (a), (B), (C), (D), and (E) show optical photographs of samples obtained in examples 1, 2, 3, 4, and 5 of the present application, respectively;
FIG. 2 shows an X-ray diffraction pattern of nanocrystalline SiC superhard bulk material obtained in example 4 of the present application;
FIG. 3 shows a fracture scanning electron microscope image of nanocrystalline silicon carbide superhard bulk material obtained in example 4 of the present application;
fig. 4 shows a transmission electron microscope image of a nanocrystalline silicon carbide superhard bulk material obtained in example 4 of the present application, (a) is a low-power transmission electron microscope image of a sample, (B) is a high-angle annular dark field image of a certain grain in the sample, and (C) is a selected area electron diffraction pattern of the sample;
fig. 5 shows the hardness-load curve for nanocrystalline silicon carbide superhard bulk material obtained in example 4 of the present application.
Detailed Description
The technical solution of the first aspect of the present application is a nanocrystalline silicon carbide bulk material, whose average grain size is less than 100nm, and whose vickers hardness is equal to or higher than 40GPa.
In this application, unless otherwise indicated or defined, all terms are to be construed to have meanings as commonly understood by those skilled in the art.
In the context of the present application, "nanocrystalline silicon carbide" refers to a material that is composed, from a chemical composition perspective, of substantially pure silicon carbide, and from a microstructure perspective, of numerous silicon carbide grains of a size on the order of nanometers.
In the context of this application, "vickers hardness" refers to the hardness of a material measured by indentation, i.e., the vickers hardness of a sample obtained by applying a load such that a indenter (typically a diamond indenter) is pressed into the sample under test, leaving a trace of plastic deformation on the surface of the sample after the load is removed, and then dividing the applied load by the area of the indentation. When the Vickers hardness is measured, a diamond regular pyramid pressure head with an included angle of 136 degrees between opposite surfaces is used for vertically pressing into the surface of a material under the action of a certain pressure, and the material is kept for a certain time, so that diamond indentations generated by plastic deformation are left on the surface of a sample; the area of the indentation was calculated from the length of the diagonal of the indentation left after unloading. Specifically, the calculation formula of the vickers hardness is: h v ＝1854.4F/L 2 Wherein F is the load size of the load, and the unit is N; l is the average diagonal length of the indentations in μm. Notably, since the hardness value of the material varies within a certain pressure load range, particularly for superhard materials, the stiffness of the material is high, and the elastic strain of the indentation is large under a small load, resulting in a bias in the hardness measurementHigh, the material hardness tends to a constant value, i.e. the progressive wire hardness value, only when the load is greater than a certain limit value. For the nanocrystalline silicon carbide block, all vickers hardness is the real hardness value of the material obtained in a variable load measurement mode, namely, the hardness of the material is measured in a variable load mode, and the hardness value of a constant-load area (i.e. progressive line hardness value) of the hardness value in a hardness-load curve is taken as a final measured value.
Those skilled in the art will appreciate that there are a variety of techniques by which microstructure information of a material can be obtained; for example, the analysis may be performed by direct observation using an instrument such as a scanning electron microscope or a transmission electron microscope, or by an X-ray spectrogram. For accurate characterization, it is preferred to analyze the sample inside the material; in addition, to avoid interference from small amounts of contaminants that may be present in the sample, multiple (e.g., 3 or more or 5 or more) sample spots may be randomly taken for analysis.
It will be appreciated by those skilled in the art that the nanocrystalline silicon carbide bulk of the present application consists essentially of pure silicon carbide, but does not exclude the presence of unavoidable small amounts of impurities in the feedstock itself or that are inadvertently introduced during processing and manufacturing. For the purposes of this application, the weight content of impurities in the nanocrystalline silicon carbide material of this application is typically at most 5%, preferably less than 4%, more preferably less than 3%, for example less than 2%, less than 1%, less than 0.5%.
The grain size of the silicon carbide grains in the nanocrystalline silicon carbide bulk material is generally between 1nm and 100nm, for example, the grain size can be greater than 2nm, greater than 3nm, greater than 4nm, greater than 5nm, greater than 8nm, greater than 10nm, greater than 20nm, greater than 30nm, greater than 40nm and greater than 50nm, and the grain size can be less than 100nm, less than 90nm, less than 80nm, less than 70nm, less than 60nm and less than 50nm. The grain size of silicon carbide can be calculated by one skilled in the art from XRD data in combination with the scherrer formula.
The nanocrystalline silicon carbide of the present application is bulk, i.e., used in the form of bulk materials. "bulk" is understood in the present application as a single mass of material which exists independently, without a great difference between the dimensions of its three dimensions, and thus different fromFibrous, sheet or film-like materials, and powder or granular materials. The nanocrystalline silicon carbide of the present application is bulk, typically having a volume of at least 0.05mm 3 For example at least 0.1mm 3 At least 0.5mm 3 At least 1mm 3 At least 5mm 3 At least 10mm 3 At least 20mm 3 At least 50mm 3 At least 100mm 3 The upper limit of the volume of the bulk material is not particularly limited, and may be selected as appropriate according to actual needs and equipment processing capacity, for example, up to 1000cm 3 。
The shape of the nanocrystalline silicon carbide block is not particularly limited, and according to practical needs, the nanocrystalline silicon carbide block can be directly manufactured by adopting a corresponding die or can be processed into blocks with various shapes by cutting and grinding, such as a cylinder shape, a prismatic shape, a sphere shape and the like. For example, the nanocrystalline silicon carbide bulk material may be cylindrical with a diameter of 0.5 to 80mm and a height of 0.5 to 50 mm.
The nanocrystalline silicon carbide blocks of the present application have a high hardness, typically having a vickers hardness of greater than or equal to 40GPa, such as greater than 40.5GPa, greater than 41.5GPa, greater than 42GPa, greater than 42.5GPa, greater than 43GPa, greater than 44GPa, or even greater than 45GPa.
A second aspect of the present application provides a method of preparing the nanocrystalline silicon carbide bulk of the first aspect, the method comprising the steps of:
a) Pretreatment: weighing silicon carbide nano powder with certain mass, carrying out acid washing treatment, diluting with water to be close to neutral, taking out, and drying;
b) And (5) pre-pressing and forming: prepressing the dried raw materials to obtain a prepressing blank;
c) Sintering: carrying out high-temperature high-pressure sintering on the pre-pressed blank body obtained in the step B), wherein the sintering pressure is 4-28 GPa, the sintering temperature is 1000-2500 ℃, and the heat preservation time is 1-60 min;
d) Discharging: and C), cooling and releasing pressure of the high-temperature high-pressure sintering equipment in the step C), taking out the sintered silicon carbide block, and performing optional post-treatment.
The carbon raw material used in step A) of the method is silicon carbide nano-powder, i.e. silicon carbide powder with a nano-scale grain size. Preferably, the silicon carbide nanopowder used has a grain size of 5 to 100nm (e.g., 6nm, 8nm, 10nm, 12nm, 15nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm or any grain size within any numerical range ending with these sizes), preferably 5 to 50nm. The method can use silicon carbide nano powder with single grain size or a mixture of silicon carbide nano powder with several different grain sizes. The grain size of the powder can be estimated from XRD data by the well-known scherrer equation in the art.
The various starting materials used in step a) of the process are preferably of high purity, for example preferably of a purity of more than 97%, more preferably of more than 98%, more than 99% or more than 99.5%.
The acid used for the pickling in step A) of the process is preferably hydrofluoric acid, preferably in a concentration of 10% to 40%, and the pickling time is generally 15 to 90 minutes.
The water used in step a) of the process is preferably deionized water. The pickling solution may be diluted with water to a near neutral (about pH 7), i.e. about pH 6-8, more preferably pH 6.5-7.5.
The acid washing in step A) of the process is preferably carried out in a polytetrafluoroethylene vessel under the action of ultrasound. For example, silicon carbide nanopowder and acid may be placed in a polytetrafluoroethylene bottle, and the polytetrafluoroethylene bottle placed in an ultrasonic cleaner. The ultrasonic treatment process can be carried out for example by taking 10min of working and 5min of standing as one cycle (15 min of one cycle duration is recorded), a plurality of cycles can be carried out, and the total duration of ultrasonic treatment can be the same as or different from the pickling time. The ultrasonic treatment can ensure that the raw materials are fully dispersed in the acid.
In some preferred embodiments, a centrifuge and an ultrasonic cleaner are used simultaneously in the pickling process in step a) of the process. The centrifugal treatment accelerates the sedimentation of the raw materials, so that the raw materials are separated from the acid liquor, for example, the rotating speed is set to be 4000-11000 r/min, and the centrifugal time is set to be 10-20 min; and the ultrasonic treatment ensures that the settled raw materials are uniformly mixed with newly added deionized water, and the ultrasonic treatment time is 5-10 min. Repeated several times until the pH was close to 7.
Step A of the methodThe drying of (c) may be carried out in various conventional drying apparatuses. Preferably, the drying operation in step A) is performed using a vacuum oven with a vacuum setting above 1X 10 -1 Pa, the drying temperature is 40-80 ℃, and the drying time is 12-24 h.
The pretreatment in the step A) of the method can effectively remove the silicon dioxide layer on the surface of the silicon carbide nano particles, thereby improving the activity of the silicon carbide, reducing the sintering conditions (pressure, temperature and the like) required by compact sintering of the silicon carbide, and being more beneficial to improving the hardness and the compactness of a sintered sample.
And B) pre-pressing the dried raw materials in the step B) of the method to obtain a pre-pressed blank. The blank may be of any shape as desired, such as cylinders, blocks, spheres, and the like. The pre-compaction may be carried out on a conventional briquetting machine or hydraulic press, usually operating at room temperature, and may be carried out in air or in an inert atmosphere. The pre-pressing pressure in the second step of the method of the present application is not particularly limited as long as the compact body is ensured, but in order to achieve a better effect, the pre-pressing pressure should be 5MPa or more, for example, 8MPa, 10MPa, 20MPa, etc., and the pre-pressing pressure is preferably 5 to 10MPa. The pre-pressing holding time is preferably 0.5min or more and 10min or less, more preferably 1 to 5min.
In a preferred embodiment of the present application, the pre-compression mode is bi-directional compression. Therefore, powder in the forming die can be stressed bidirectionally and uniformly, the vertical pressure gradient of the pre-pressed blank is reduced, the difference between the vertical density of the pre-pressed blank is avoided to be larger, and the overall density of the sintered block is further influenced, so that the density of the blank is higher, and the performance of a synthesized sample is improved.
In step C) of the method, the pre-pressed blank obtained in step B) is sintered at high temperature and high pressure. In a preferred embodiment of the present application, sintering is performed at a sintering pressure of 4GPa to 28GPa and a temperature of 1000 ℃ to 2500 ℃ for a holding time of 1 to 60 minutes to obtain a silicon carbide ceramic having high hardness. The sintering pressure is, for example, 4GPa, 5GPa, 6GPa, 7GPa, 8GPa, 9GPa, 10GPa, 11GPa, 12GPa, 13GPa, 14GPa, 15Pa, 16GPa, 17GPa, 18GPa, 19GPa, 20GPa, 21GPa, 22GPa, 23GPa, 24GPa, 25GPa, 26GPa, 27GPa, 28GPa, or the like, or any pressure within these pressure ranges. In general, higher sintering pressure is advantageous for increasing the hardness of the sintered sample, and thus the sintering pressure is preferably 10 to 28GPa. The sintering temperature may be, for example, 1000 ℃, 1100 ℃, 1200 ℃, 1300 ℃, 1400 ℃,1500 ℃, 1600 ℃, 1700 ℃, 1800 ℃, 1900 ℃, 2000 ℃, 2100 ℃, 2200 ℃, 2300 ℃, 2400 ℃, 2500 ℃ or any temperature within these temperature ranges. In step C), the incubation time is preferably from 1 to 60 minutes, for example from 5 to 45 minutes or from 5 to 30 minutes. The temperature rising rate in the step C) can be controlled between 50 and 150 ℃/min.
The sintering process may be performed using high temperature and high pressure sintering equipment well known to those skilled in the art, such as a T25 press manufactured by America Rockland Research or a hexahedral hydraulic press CS-1B manufactured by Guilin metallurgical machinery general factories. The sintering process may be carried out in an inert atmosphere or in air, as desired.
For high temperature and high pressure sintering, the pre-pressed green body obtained in step B) is usually put into a crucible (e.g. a boron nitride crucible) and then put into an assembly block for high temperature and high pressure sintering. The assembly block comprises a heating body, a heat-insulating layer, a pressure-transmitting layer and the like, wherein the heating body is a rhenium sheet and/or graphite, the heat-insulating layer is zirconia and/or alumina, and the pressure-transmitting layer is magnesia and/or lanthanum chromate.
After sintering, cooling the sintering sample in the step D), cooling the sintering sample and the cavity of sintering equipment to room temperature, regulating the internal pressure of the equipment to the ambient pressure (namely releasing pressure), taking out the sintering sample, and performing optional processing and post-treatment (such as grinding and polishing) to obtain the nanocrystalline silicon carbide superhard bulk material. The cooling in step D) may be natural cooling or controlled cooling, for example at a cooling rate of 50-150 ℃/min.
According to the invention, the nanocrystalline silicon carbide superhard bulk material is obtained by optimizing high-temperature high-pressure sintering conditions and taking the silicon carbide nano powder as a raw material. Densification of bulk material and refinement of grains are necessary preconditions for the production of nanocrystalline silicon carbide with higher hardness. According to the invention, by adjusting the size of the raw materials, improving the pretreatment process and adjusting sintering parameters (pressure, temperature, heat preservation time and the like), the nanocrystalline silicon carbide with good compactness and finer grains is obtained, and the superhard silicon carbide ceramic block is obtained. The sintering pressure of high-temperature high-pressure sintering is higher than that of spark plasma sintering and hot-pressing sintering, and compact nanocrystalline bulk is easier to obtain.
The nanocrystalline silicon carbide bulk and the method of preparation of the present application are further described below with reference to examples.
Examples
The embodiments described below are some, but not all, of the embodiments of the present application. The detailed description of the embodiments of the present application is not intended to limit the scope of the application, as claimed, but is merely representative of particular embodiments of the application. Based on the embodiments herein, all other embodiments that may be made by those of ordinary skill in the art without departing from the principles of the present application and without making any inventive faculty are intended to be within the scope of the present application.
For simplicity, some materials, equipment, and method steps conventionally employed in the art are not noted one by one in the examples. All processes and analytical testing procedures (and related parameters) not specifically noted in the examples were performed as commonly employed by those skilled in the art; materials and equipment not specifically identified are laboratory conventional materials and equipment.
Apparatus and material analysis and detection method
The high-temperature and high-pressure sintering device used in each embodiment is a T25 press manufactured by Rockland Research company in the United states or a hexahedral hydraulic press CS-1B manufactured by Guilin metallurgical machinery general factories; the silicon carbide powder feedstock used in the examples was supplied by Alfa Aesar.
The main analytical detection methods and the instruments used in the examples are as follows:
x-ray diffraction: XRD pattern determination was performed using Rigaku DMAX-2500/P. The scanning speed is 0.1 degree per minute and ranges from 20 degrees to 90 degrees.
Grain size of powder raw material and block: the grain size of the feedstock and the mass was estimated from XRD data in combination with the scherrer formula.
SEM: sample fracture analysis was performed using the Semer fly Scios DualBeam. The voltage was 5KV and the current was 0.1nA.
TEM and HAADF: TEM samples (5X 10X 0.08 μm) were prepared using a Focused Ion Beam (FIB), and tested using a Siemens Feeden F200X scanning transmission electron microscope (Siemens Feisha technology Co., USA) with an acceleration voltage of 200kV.
Vickers hardness detection method: and (3) carrying out microhardness test on the sample by using a German KB-30-S microhardness tester, wherein the test parameters are 10S for loading and 15S for pressure maintaining. Namely, a diamond regular pyramid pressure head with an included angle of 136 degrees between opposite surfaces is vertically pressed into the surface of a material under the action of a certain pressure, the material is kept for a certain time (loading is carried out for 10 seconds and pressure maintaining is carried out for 15 seconds), diamond indentations generated by plastic deformation are reserved on the surface of a sample, and then the applied load is divided by the area of the indentations, so that the hardness is measured by an indentation method. The calculation formula of the Vickers hardness is: h v ＝1854.4F/L 2 Wherein F is the load size of the load, and the unit is N; l is the average diagonal length of the indentations in μm. The hardness measurement was performed in a variable load manner, and the measured value of the sample hardness value in the constant load region (i.e., the progressive line hardness value) was taken as the final hardness measured value, as shown in fig. 5.
Example 1
0.3g of silicon carbide raw material (average grain size 5 nm) was treated with hydrofluoric acid at a concentration of 10%, and the pickling time was 60 minutes. Deionized water is used for diluting the acid liquor, and a centrifugal machine and an ultrasonic cleaner are comprehensively used in the process. Centrifuging to separate acid liquor from raw materials, and mixing the raw materials with newly added deionized water uniformly by ultrasonic treatment. Repeated several times until the PH approaches 7. The rotational speed of the centrifugal machine is 4000r/min, the centrifugal time is 15min, and the ultrasonic time is 5min. And drying the raw materials obtained by the last centrifugation at 60 ℃ for 18 hours.
And (3) adding 10MPa pressure to the dried powder, prefabricating a cylindrical blank with the diameter of 3.5mm and the height of 3mm, putting the cylindrical blank into a hexagonal boron nitride crucible, putting the assembled block into a domestic hexahedral top synthesizer, and sintering under the conditions of the pressure of 5GPa and the temperature of 1900 ℃ for 20min. The heating rate is 100 ℃/min, and the cooling rate is 100 ℃/min. The synthesized silicon carbide block is shown in fig. 1 (a). The hardness of the samples is shown in Table one.
Example 2
0.3g of silicon carbide raw material (average grain size 30 nm) was treated with hydrofluoric acid at a concentration of 10%, and the pickling time was 30min. Deionized water is used for diluting the acid liquor, and a centrifugal machine and an ultrasonic cleaner are comprehensively used in the process. Centrifuging to separate acid liquor from raw materials, and mixing the raw materials with newly added deionized water uniformly by ultrasonic treatment. Repeated several times until the PH approaches 7. The rotational speed of the centrifugal machine is 11000r/min, and the centrifugal time is 20min. The ultrasonic time was 10min. And drying the raw materials obtained by the last centrifugation. The drying temperature is 40 ℃ and the drying time is 24 hours.
And (3) adding 10MPa pressure to the dried powder, prefabricating a cylindrical blank with the diameter of 1.2mm and the height of 2.3mm, putting the cylindrical blank into a hexagonal boron nitride crucible, putting the assembled block into a T25 press, sintering under the conditions that the pressure is 15GPa and the temperature is 1500 ℃, and keeping the temperature for 5min. The temperature rising rate is 150 ℃/min, and the temperature reducing rate is 150 ℃/min. As shown in fig. 1 (B), the synthesized silicon carbide block is composed of silicon carbide as the most central block, and silicon carbide, boron nitride crucible, and rhenium plate in this order from the inside to the outside. The hardness of the samples is shown in Table one.
Example 3
0.15g of a 5nm average grain size silicon carbide feedstock and 0.15g of a 70nm average grain size silicon carbide feedstock were mixed. The mixed raw material was treated with hydrofluoric acid having a concentration of 10%. The pickling time is 45min. Deionized water is used for diluting the acid liquor, and a centrifugal machine and an ultrasonic cleaner are comprehensively used in the process. Centrifuging to separate acid liquor from raw materials, and mixing the raw materials with newly added deionized water uniformly by ultrasonic treatment. Repeated several times until the PH approaches 7. The rotational speed of the centrifugal machine is 11000r/min, and the centrifugal time is 15min. The ultrasonic time was 5min. And drying the raw materials obtained by the last centrifugation. The drying temperature is 40 ℃ and the drying time is 24 hours.
And (3) adding 10MPa pressure to the dried powder, prefabricating a cylindrical blank with the diameter of 1.2mm and the height of 2.3mm, putting the cylindrical blank into a hexagonal boron nitride crucible, putting the assembled block into a T25 press, sintering under the conditions that the pressure is 20GPa and the temperature is 1500 ℃, and keeping the temperature for 5min. The heating rate is 100 ℃/min, and the cooling rate is 100 ℃/min. The synthesized silicon carbide block is shown in fig. 1 (C). The most central block is silicon carbide, and the silicon carbide, the boron nitride crucible and the rhenium sheet are sequentially arranged from inside to outside. The hardness of the samples is shown in Table one.
Example 4
0.3g of silicon carbide raw material (average grain size 8 nm) was treated with 20% strength hydrofluoric acid for 15min. Deionized water is used for diluting the acid liquor, and a centrifugal machine and an ultrasonic cleaner are comprehensively used in the process. Centrifuging to separate acid liquor from raw materials, and mixing the raw materials with newly added deionized water uniformly by ultrasonic treatment. Repeated several times until the PH approaches 7. The rotational speed of the centrifugal machine is 11000r/min, and the centrifugal time is 15min. The ultrasonic time was 5min. And drying the raw materials obtained by the last centrifugation. And drying by using a vacuum drying oven, wherein the drying temperature is 60 ℃ and the drying time is 18 hours.
And (3) adding 10MPa pressure to the dried powder, prefabricating a cylindrical blank with the diameter of 1.2mm and the height of 2.3mm, putting the cylindrical blank into a hexagonal boron nitride crucible, putting the assembled block into a T25 synthesis device, sintering under the conditions that the pressure is 25GPa and the temperature is 1400 ℃, and keeping the temperature for 5min. The temperature rising rate is 50 ℃/min, and the temperature reducing rate is 50 ℃/min. As shown in fig. 1 (D), the synthesized silicon carbide block is composed of silicon carbide as the most central block, and silicon carbide, a boron nitride crucible and rhenium pieces in this order from the inside to the outside. The hardness of the samples is shown in Table one.
An X-ray diffraction pattern of the sample of example 4 is shown in fig. 2. The pictures of the fracture scanning electron microscope and the pictures of the transmission electron microscope (including high resolution) are shown in fig. 3 and fig. 4, respectively. The hardness-load curve is shown in figure 5. Fig. 3 demonstrates that it has been sintered very densely, while fig. 4 reflects that the grain size of the sample is small and that the grain interior has a certain amount of twinned substructure. As is apparent from fig. 4 (a), the grain composition is evident, and the grain boundary between grains is quite evident; in fig. 4 (C), the diffraction ring is quite clear, illustrating that the sample is a polycrystalline sample; the periodic arrangement of atoms in the grains in fig. 4 (B) exhibits long range order, illustrating that the sample is a polycrystalline sample. Figure 5 reflects that the actual hardness measurement of the sample exceeded 40GPa, reaching the superhard standard.
Example 5
0.3g of silicon carbide raw material (average grain size 10 nm) was treated with 30% strength hydrofluoric acid for 30min. Deionized water is used for diluting the acid liquor, and a centrifugal machine and an ultrasonic cleaner are comprehensively used in the process. Centrifuging to separate acid liquor from raw materials, and mixing the raw materials with newly added deionized water uniformly by ultrasonic treatment. Repeated several times until the PH approaches 7. The rotational speed of the centrifugal machine is 4000r/min, and the centrifugal time is 10min. The ultrasonic time was 10min. And drying the raw materials obtained by the last centrifugation. And drying by using a vacuum drying box, wherein the drying temperature is 80 ℃ and the drying time is 24 hours.
And (3) adding 5MPa pressure to the dried powder, prefabricating a cylindrical blank with the diameter of 1.2mm and the height of 2.3mm, putting the cylindrical blank into a hexagonal boron nitride crucible, putting the assembled block into a T25 synthesis device, sintering under the conditions that the pressure is 25GPa and the temperature is 1700 ℃, and keeping the temperature for 30min. The temperature rising rate is 50 ℃/min, and the temperature reducing rate is 50 ℃/min. The synthesized silicon carbide block is shown in fig. 1 (E), the most central block is silicon carbide, and silicon carbide, a boron nitride crucible and rhenium sheets are sequentially arranged from inside to outside. The hardness of the samples is shown in Table one.
List one
The present invention is not limited to the above-mentioned embodiments, and any changes or substitutions that can be easily understood by those skilled in the art within the technical scope of the present invention are intended to be included in the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.
Alternative materials for the various components are listed in the description of the invention, but those skilled in the art will appreciate: the list of component materials mentioned above is not limiting, and is not exhaustive, and each component may be replaced by other equivalent materials not mentioned in the description of the present invention, while still achieving the objects of the present invention. The particular embodiments mentioned in the specification are also for purposes of illustration only and are not intended to limit the scope of the invention.
In addition, the range of the amount of each component of the present invention includes any combination of any lower limit and any upper limit mentioned in the specification, and also includes any range in which the specific content of the component in each specific embodiment is constituted as the upper limit or the combination of the lower limits: all such ranges are intended to be encompassed within the present invention as they are for brevity and for the sake of brevity, the ranges which are not explicitly recited in the specification. Each feature of the invention recited in the specification may be combined with any other feature of the invention, and such combination is also within the scope of the disclosure: for the sake of brevity, these combinations are not explicitly recited in the specification.
Claims (9)
1. The average grain size of the nanocrystalline silicon carbide block is less than 100nm, the progressive line hardness value of the Vickers hardness is higher than 40.5GPa, and the volume is greater than 0.05mm 3 The nanocrystalline silicon carbide bulk is prepared by a method comprising the following steps:
a) Pretreatment: weighing silicon carbide nano powder with certain mass, carrying out acid washing treatment, diluting with water to be close to neutral, taking out, and drying, wherein the grain size of the silicon carbide nano powder is 5-100 nm, and the silicon carbide nano powder is single-size grains or a mixture of grains with a plurality of sizes;
b) And (5) pre-pressing and forming: prepressing the dried raw materials to obtain a prepressing blank;
c) Sintering: carrying out high-temperature high-pressure sintering on the pre-pressed blank obtained in the step B), wherein the sintering pressure is 15-28 GPa, the sintering temperature is 1400-1900 ℃, and the heat preservation time is 5-20 min;
d) Discharging: and C), cooling and releasing pressure of the high-temperature high-pressure sintering equipment in the step C), taking out the sintered silicon carbide block, and performing optional post-treatment.
2. A preparation method of a nanocrystalline silicon carbide block, wherein the average grain size of the nanocrystalline silicon carbide block is less than 100nm, the progressive line hardness value of Vickers hardness is higher than 40.5GPa, and the volume is larger than 0.05mm 3 The method comprises the following steps:
a) Pretreatment: weighing silicon carbide nano powder with certain mass, carrying out acid washing treatment, diluting with water to be close to neutral, taking out, and drying, wherein the grain size of the silicon carbide nano powder is 5-100 nm, and the silicon carbide nano powder is single-size grains or a mixture of grains with a plurality of sizes;
b) And (5) pre-pressing and forming: prepressing the dried raw materials to obtain a prepressing blank;
c) Sintering: carrying out high-temperature high-pressure sintering on the pre-pressed blank obtained in the step B), wherein the sintering pressure is 15-28 GPa, the sintering temperature is 1400-1900 ℃, and the heat preservation time is 5-20 min;
d) Discharging: and C), cooling and releasing pressure of the high-temperature high-pressure sintering equipment in the step C), taking out the sintered silicon carbide block, and performing optional post-treatment.
3. The method for preparing a nanocrystalline silicon carbide bulk according to claim 2, characterized in that: the acid used in the step A) is hydrofluoric acid, the concentration of the acid is 10% -40%, and the acid washing time is 15-90 min.
4. The method for preparing a nanocrystalline silicon carbide bulk according to claim 2, characterized in that: the acid washing in step A) is carried out in a polytetrafluoroethylene container under the action of ultrasonic waves.
5. The method for preparing the nanocrystalline silicon carbide bulk according to claim 4, wherein: and C), treating the acid washing process in the step A) by using a centrifugal machine.
6. A bulk nanocrystalline silicon carbide according to any of claims 2 to 5The preparation method of (2) is characterized in that: the drying operation in step A) is performed using a vacuum drying oven with a vacuum degree higher than 1×10 -1 Pa, the drying temperature is 40-80 ℃, and the drying time is 12-24 hours.
7. The method for producing a bulk nanocrystalline silicon carbide according to any one of claims 2 to 5, characterized in that: the pre-pressing force applied in the step B) is 5-10 MPa, and the pre-pressing mode is bidirectional pressing.
8. The method for producing a bulk nanocrystalline silicon carbide according to any one of claims 2 to 5, characterized in that: in the step C), the pre-pressed blank obtained in the step B) is firstly put into a boron nitride crucible, and then is put into an assembly block for high-temperature and high-pressure sintering, wherein the assembly block comprises a heating body, a heat preservation layer and a pressure transmission layer, the heating body is rhenium sheet and/or graphite, the heat preservation layer is zirconia and/or alumina, and the pressure transmission layer is magnesia and/or lanthanum chromate.
9. The method for preparing a nanocrystalline silicon carbide bulk according to claim 8, wherein: in the step C), the temperature rising rate is 50-150 ℃/min.
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