Ceramic with ultra-fine grain structure as well as preparation method and application of ceramic

Abstract

The invention belongs to the field of ceramic material preparation, and discloses ultra-fine grain structure ceramic and a preparation method and application thereof. The grain size is controlled to be below 100 nm through an ultra-fast high-temperature sintering process, and the microstructure is uniform and compact. The preparation method comprises the following steps: uniformly mixing superfine precursor ceramic powder with a sintering aid, and sieving to obtain mixed powder; the mixed powder is placed in a sintering mold to be pre-pressed and formed, high-temperature sintering and cooling aftertreatment are carried out, and the ultra-fine grain structure ceramic is obtained; wherein the heating rate of the high-temperature sintering is 500 DEG C / min to 1500 DEG C / min, the temperature of the high-temperature sintering is 1500 DEG C to 1700 DEG C, the pressure of the high-temperature sintering is 5 MPa to 100 MPa, and the time of the high-temperature sintering is 10 seconds to 60 seconds; the initial particle size D50 of the ultrafine precursor ceramic powder is 100-300 nm, and the specific surface area is 8-16 m < 2 > / g. The method disclosed by the invention supports high-temperature sintering of ceramics with various structures, has the characteristics of short process period, good grain size controllability, excellent mechanical property and the like, and is suitable for the fields such as aerospace hot-end components, high-end bearings and the like which have strict requirements on material performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of ceramic materials, and particularly relates to an ultra-fine grain structure ceramic and a preparation method and application thereof, which is suitable for large-scale industrial preparation and is suitable for high-strength high-temperature-resistant scene applications. BACKGROUND

[0002] Silicon nitride structural ceramics are materials with excellent mechanical properties, thermal stability and chemical inertness, and are widely used in aerospace, precision machinery, biomedicine and other fields. With the continuous improvement of the performance requirements of materials, traditional micron-sized silicon nitride has been difficult to meet the extreme performance requirements such as high toughness, high wear resistance and high strength. Nano-crystalline silicon nitride, with its grain size reduced to tens to hundreds of nanometers, has higher fracture strength, better wear resistance and better mechanical properties, and is an important direction of research on structural ceramic materials.

[0003] The nano-crystalline structural ceramics in the prior art still have problems in the specific preparation process, such as difficulty in controlling the grain size, long high-temperature sintering time, and limited material performance. SUMMARY

[0004] The present application provides an ultra-fine grain structure ceramic and a preparation method and application thereof, aiming at the problems in the preparation technology of existing nano-crystalline structural ceramics. Through an innovative preparation process, the grain size of the nano-crystalline structural ceramic is accurately controlled, and the material performance is significantly improved.

[0005] The purpose of the present application can be achieved by the following technical solutions.

[0006] In the first aspect of the present application, a preparation method of an ultra-fine grain structure ceramic is provided, comprising the following steps: The ultra-fine precursor ceramic powder and the sintering aid are uniformly mixed and sieved to obtain a mixed powder; The mixed powder is pre-pressed into a sintering mold, and after high-temperature sintering and cooling treatment, the ultra-fine grain structure ceramic is obtained, and the grain size in the ultra-fine grain structure ceramic is ≤100 nm; wherein, The temperature rising rate of the high-temperature sintering is 500℃ / min-1500℃ / min, the temperature of the high-temperature sintering is 1500℃-1700℃, the pressure of the high-temperature sintering is 5 MPa-100 MPa, the temperature rising rate of the high-temperature sintering is 500℃ / min-1500℃ / min, and the time of the high-temperature sintering is 10s-60s; The initial particle size D 50 of the ultra-fine precursor ceramic powder is 100 nm-300 nm, the specific surface area is 8 m 2 / g-16 m 2 / g.

[0007] Preferably, the heating rate of the high-temperature sintering is 1000 ℃ / min-1500 ℃ / min, and the high-temperature sintering time is 30 s.

[0008] This application employs extremely high heating rates and extremely short holding times to suppress grain growth in structural ceramics; a suitable high-temperature sintering temperature ensures sufficient densification driving force, preventing nanostructure instability. This preparation method achieves grain size refinement through a high-temperature phase transformation-induced mechanism, resulting in a uniform and dense microstructure. During high-temperature sintering, the ultrafine-grained ceramic undergoes a phase transformation, which reverses upon rapid cooling, accompanied by lattice reconstruction, inducing grain fragmentation and refinement. This preparation method offers advantages such as strict control of grain growth, simple technical path, good stability, a clear nanostructure formation mechanism, short high-temperature sintering time, and low residual silica content, making it suitable for large-scale industrial preparation. The mechanical properties of the high-temperature sintered ultrafine-grained ceramic material prepared in this application are significantly improved.

[0009] Preferably, the uniform mixing method includes ball milling, and the sieving is through a 60-mesh sieve.

[0010] Preferably, the cooling rate of the post-cooling treatment is ≥500℃ / min, and the post-cooling treatment is coordinated with the heating rate of the high-temperature sintering to form an ultra-fast thermal cycling process. Ultra-fast cooling can trigger a reverse phase transformation between structural ceramics. During this process, the internal stress generated by the lattice reconstruction of each phase induces microcracks within the grains, which then break down into finer grains. This mechanism contrasts sharply with the traditional diffusion-controlled grain growth mechanism.

[0011] The selection of different sintering aids in this application determines the differences in the high-temperature sintering performance of the materials. Due to the different characteristics of various aids, they also exhibit their own advantages in terms of mechanical properties. Preferably, the sintering aids include at least one of oxides, rare earth oxides, composite sintering aids, and non-oxide aids; the initial particle size D of the sintering aids... 50 The range is 30nm-100nm.

[0012] Preferably, the uniform mixing method includes liquid phase mixing, wherein the liquid phase includes at least one of ethanol and isopropanol, and the mass ratio of the liquid phase to the mixed powder is (1-1.5):1.

[0013] Preferably, the mass ratio of the sintering aid to the mixed powder is (1-15):1. An appropriate amount of sintering aid can maintain a suitable amount of liquid phase generation to wet the grain boundaries of the ultrafine-grained ceramic structure; if the sintering aid exceeds this range, a continuous glassy phase network will form, reducing the mechanical properties of the structural ceramic. The preparation method of this application achieves a balance between high performance and low aid content, requiring only 1-15 wt% of sintering aid to achieve high density (≥98%), far lower than the aid usage in traditional processes, thus avoiding the formation of a glassy phase from excessive aid.

[0014] This application not only significantly improves process efficiency but also achieves superior overall performance using only a small amount of sintering aids. This groundbreaking high-temperature sintering process provides a completely new technical route for the industrial production of high-performance silicon nitride ceramics, possessing significant technological advantages and market competitiveness.

[0015] The raw materials selected in this application are highly versatile and applicable to a variety of high-temperature structural ceramics. Preferably, the ultrafine precursor ceramic powder includes one of silicon nitride, silicon carbide, boron carbide, tantalum carbide, and aluminum nitride.

[0016] Preferably, the high-temperature sintering method includes discharge plasma sintering, and the atmosphere for high-temperature sintering is high-purity nitrogen (purity ≥ 99.99%) or vacuum (vacuum degree ≤ 10). -2 The pulse current parameters during high-temperature sintering are: DC pulse frequency 500 Hz~1 kHz, single pulse width 5 ms-10 ms, and current density 200 A / cm². 2 ~500 A / cm 2 .

[0017] Preferably, the sintering mold is a high-strength graphite mold with a compressive strength ≥50 MPa and an ash content ≤0.1%. The high-temperature sintering process is carried out under conditions that prevent carbon pollution. The inner wall of the sintering mold is coated with an isolation layer, the material of which includes boron nitride. Boron nitride can isolate the phase reaction between the ceramic powder and the graphite mold. The carbon content in the ultrafine-grained ceramic is ≤0.5%.

[0018] In a second aspect, this application provides a method for preparing an ultrafine-grained ceramic, wherein the ultrafine-grained ceramic prepared by the aforementioned method has a grain size ≤100 nm, and the ultrafine-grained ceramic further satisfies at least one of the following characteristics: (1) Density ≥ 98%; (2) Bending strength ≥ 1000 MPa; (3) Fracture toughness ≥ 8 MPa·m 1 / 2 ; (4) Hardness ≥ 1900HV1.0; (5) The content of residual oxides is ≤3%.

[0019] In a third aspect, this application provides an application of ultrafine-grained ceramics prepared by the aforementioned method, or the aforementioned ultrafine-grained ceramics, in aerospace engine components, precision mechanical bearings, cutting tools, biomedical implants, or electronic packaging thermal management materials. Attached Figure Description

[0020] Figure 1 This is a morphological image of the material after high-temperature sintering; Figure 2 These are morphological images of the product after high-temperature sintering using conventional methods. Figure 3 This is a particle size distribution diagram after high-temperature sintering of this application. DETAILED DESCRIPTION The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the ultrafine-grained ceramics of this application, their preparation methods, and applications. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of essentially the same structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0022] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. The range defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range.

[0023] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0024] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0025] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0026] The key to preparing nanocrystalline silicon nitride lies in obtaining ultrafine precursor powder and suppressing abnormal grain growth during high-temperature sintering. Current mainstream preparation routes include: high-energy ball milling to refine silicon nitride powder, sol-gel synthesis of nitrogen-containing precursors followed by heat treatment to produce powder, carbothermal reduction nitridation to synthesize nanopowders, and the synthesis of nanostructure precursors using vapor deposition or plasma processing. Regarding high-temperature sintering, to achieve densification of the nanocrystalline structure, researchers widely employ low-temperature rapid high-temperature sintering techniques, such as spark plasma high-temperature sintering, microwave high-temperature sintering, and high-pressure high-temperature sintering. These methods can effectively suppress grain growth and achieve the preparation of high-density nanocrystalline silicon nitride. In recent years, with the deepening research in high-temperature sintering kinetics control, interface engineering, and precursor design, nanocrystalline silicon nitride has shown broad application prospects in wear-resistant components, microelectromechanical systems (MEMS), and thermal management materials, becoming a research hotspot in the field of structural ceramics.

[0027] Currently, the preparation of high-performance silicon nitride ceramics mainly employs spark plasma sintering (SPS) technology. While existing SPS processes, through rapid heating (100℃ / min-200℃ / min), can suppress grain growth to some extent, they still require long holding times (over 5 minutes), making it difficult to control grain size below 200 nm. More importantly, when using nanoscale powders as raw materials, the high specific surface area exacerbates oxidation during high-temperature sintering, severely impacting material properties. Existing methods for preparing high-hardness, high-toughness silicon nitride ceramics using spark plasma sintering employ a two-step high-temperature sintering process, controlling the phase transformation by combining high-temperature pre-high-temperature sintering and low-temperature high-temperature sintering. However, this method is not only complex but also has a long total high-temperature sintering time (over 10 minutes), making it impossible to control grain size below 200 nm. Existing technologies disclose a silicon nitride ceramic with high toughness and high hardness, its preparation method, and applications, using a composite sintering aid system. While this improves material properties, its dependence on special additives is detrimental to improving mechanical properties.

[0028] This application addresses the shortcomings of existing technologies by innovatively proposing an ultra-fast high-temperature sintering process for ultrafine-grained ceramics. The preparation method includes the following steps: The ultrafine precursor ceramic powder is uniformly mixed with sintering aid and then sieved to obtain a mixed powder. The mixed powder is pre-pressed in a sintering mold, and then subjected to high-temperature sintering and cooling post-treatment to obtain the ultrafine-grained ceramic, wherein the grain size of the ultrafine-grained ceramic is ≤100 nm; wherein, The heating rate of the high-temperature sintering is 500℃ / min-1500℃ / min, the temperature of the high-temperature sintering is 1500℃-1700℃, the pressure is 5 MPa-100 MPa, and the time of the high-temperature sintering is 10s-60s. The initial particle size D of the ultrafine precursor ceramic powder 50 The wavelength range is 100 nm to 300 nm, and the specific surface area is 8 m². 2 / g-16m 2 / g.

[0029] This preparation method, through extremely high heating rates and extremely short holding times, effectively suppresses grain growth while ensuring sufficient densification of the material. The characteristics of this ultra-fast high-temperature sintering are: firstly, the extremely rapid heating rate significantly shortens the residence time of the powder in the high-temperature zone, greatly reducing the risk of oxidation; secondly, the ultra-short holding time effectively controls the grain boundary diffusion process, enabling the grain size of the final product to be stably controlled below 100 nm.

[0030] The following specific embodiments illustrate the solution of this application. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0031] Example 1: The mixed powder in this embodiment is made from the following raw materials: silicon nitride ultrafine precursor ceramic powder and sintering aid; The preparation method of the nanocrystalline silicon nitride has very high requirements for raw materials. When the size of silicon nitride powder is reduced to tens of nanometers, the high specific surface area promotes a large amount of oxidation of the silicon nitride powder, resulting in more than half of the silicon oxide remaining in the product after high-temperature sintering, which leads to a decrease in its performance. Therefore, α-Si3N4 silicon nitride powder was selected, with an initial particle size D 50 It is around 200nm and has a specific surface area greater than 8m². 2 / g and less than 16 m 2 / g, purity ≥99.9%; The sintering aids include Y2O3 and Al2O3, and the initial particle size of the powder is D. 50 The nanometer size is approximately 50 nm, and the purity is ≥99.9%; the composite sintering aid has a mass ratio of 1:1 and a total content of 10 wt.%. The preparation steps of the ultrafine-grained ceramic in this embodiment are as follows: S1. The sintering aid and silicon nitride ultrafine precursor ceramic powder are mixed evenly by ball milling. The ball milling process includes: milling all powders with alcohol at a mass ratio of 1:1 for 12 hours; the milling media is silicon nitride balls; S2. After drying the uniformly mixed powder, grind it and pass it through a 60-mesh sieve to obtain powder with uniform size. S3. The obtained powder is loaded into a graphite mold and pre-pressed into shape; S4. Place the pre-pressed sample in an SPS and heat it to 1550℃ at a heating rate of 1000 ℃ / min under a normal pressure nitrogen atmosphere. The pressure is 30 MPa. After holding at the temperature for 30 s, the temperature is rapidly reduced to obtain an ultrafine-grained ceramic with a grain size of less than 100 nm. S5. The obtained samples are processed to be suitable for various application scenarios; The composite additive (Y2O3-Al2O3) reacts with SiO2 on the surface of Si3N4 at 1550℃ to generate a Y-Al-Si-ON liquid phase (viscosity approximately 10). 3 The liquid phase wets the grain boundaries and fills the pores, promoting particle rearrangement through capillary forces; at the same time, rapid cooling amorphizes the liquid phase, forming a nanoscale grain boundary phase that pins the grain boundaries and inhibits grain growth. In this embodiment, appropriate sintering aids maintain a suitable amount of liquid phase generation to wet the silicon nitride grain boundaries; a sufficiently fast heating rate and a sufficiently short holding time suppress grain growth; and the selection of a suitable high-temperature sintering temperature ensures sufficient densification driving force to prevent nanostructure instability.

[0032] The ultrafine-grained ceramic prepared in this embodiment has a uniform and dense microstructure, with a grain size of less than 100 nm and a residual silicon oxide content of less than 3%. In this embodiment, during the high-temperature sintering process, α-Si3N4 is partially transformed into β-Si3N4, which is reversed during rapid cooling and accompanied by lattice reconstruction, inducing grain cleavage and refinement.

[0033] The high-temperature sintering equipment in this embodiment is a discharge plasma high-temperature sintering system. The high-temperature sintering atmosphere is high-purity nitrogen (purity ≥ 99.99%) or vacuum (vacuum degree ≤ 10). -2 Pa); preferably, high-temperature sintering is selected under a nitrogen atmosphere to suppress the oxidation of silicon nitride; the other ceramic materials are sintered in the most preferred high-temperature atmosphere according to the characteristics of the materials themselves.

[0034] In this embodiment, the pulse current parameters during the high-temperature sintering process are: DC pulse frequency 800 Hz, single pulse width 7 ms, and current density 400 A / cm². 2 The pulse current parameters ensure a heating rate of 1000℃ / min.

[0035] The sintering mold selected in this embodiment is a high-strength graphite mold (compressive strength ≥50 MPa, ash content ≤0.1%), with the inner wall coated with a boron nitride isolation layer to prevent carbon contamination.

[0036] The silicon nitride ceramic prepared by the method in this embodiment has a grain size of less than 100 nm, a density of 98.9%, a flexural strength of 1304 MPa, and a fracture toughness of 9.23 MPa·m. 1 / 2 Its hardness is 2218.7 HV1.0.

[0037] The method described in this embodiment is applicable to the preparation of various high-temperature structural ceramics, including but not limited to silicon nitride, silicon carbide, boron carbide, tantalum carbide, and aluminum nitride.

[0038] Example 2: The sintering aid used in the preparation of the ultrafine-grained ceramic in Example 2 consisted of yttrium oxide:magnesium oxide with a ratio of 3:1 and a total content of 12 wt.%. Other preparation methods were the same as in Example 1.

[0039] Example 3: The sintering aid used in the preparation of the ultrafine-grained ceramic in Example 3 was lanthanum oxide:ytterbium oxide, and the other preparation methods were the same as in Example 1.

[0040] Example 4: The sintering aid used in the preparation of the ultrafine-grained ceramic in Example 4 was aluminum nitride:alumina, with an aluminum nitride:alumina ratio of 2:1 and a total content of 18 wt.%. Other preparation methods were the same as in Example 1.

[0041] Example 5: The sintering aid used in the preparation of the ultrafine-grained ceramic in Example 5 was strontium oxide:zirconia, with a ratio of 1:2 and a total content of 9 wt.%. Other preparation methods were the same as in Example 1.

[0042] Example 6: The sintering aid used in the preparation of the ultrafine-grained ceramic in Example 6 was cerium oxide, with a total content of 1.5 wt.%, and the other preparation methods were the same as in Example 1.

[0043] After surface polishing treatment, the ultrafine-grained ceramics prepared in Examples 2-6 were subjected to mechanical property tests such as hardness and fracture toughness, as well as microstructure and structural characterization. It was found that the ultrafine-grained ceramics maintained high density, with grain size below 100 nm, and high flexural strength, fracture toughness and hardness. The high-temperature sintering mechanisms produced by the different sintering aids are different.

[0044] In Example 1, Y2O3 and Al2O3 react with the Si3N4 surface oxide layer (SiO2) at high temperature to form a low-melting-point Y-Al-Si-ON liquid phase, which wets the grain boundaries and fills the pores, significantly improving the density (98.9%). An appropriate amount of liquid phase (10 wt.%) forms an amorphous phase during rapid cooling, which pins the grain boundaries and restricts the grain size.

[0045] In Example 2, the addition of MgO lowers the liquid phase formation temperature (approximately 1400°C) and accelerates densification (density 98.9%); Mg 2+ It combines with impurity oxygen to reduce the glassy phase at grain boundaries; MgO refines the long columnar crystals of β-Si3N4, thus optimizing mechanical properties.

[0046] In Example 3, rare earth oxides form a high-viscosity liquid phase, inhibiting grain boundary migration; La 3+ and Yb 3+ The difference in ionic radius introduces lattice distortion, which improves high-temperature stability.

[0047] In Example 4, AlN inhibits the β-Si3N4 phase transformation, promotes the formation of equiaxed crystals, and significantly improves hardness; AlN reacts with SiO2 to generate the SiAlON phase, reducing residual silicon oxide (<3%).

[0048] In Example 5, ZrO2 undergoes a martensitic phase transformation upon cooling, absorbing crack energy and providing fracture toughness; SrO purifies grain boundaries, reduces amorphous phases, and provides thermal conductivity to silicon nitride ceramics.

[0049] In Example 6, CeO2 achieves a density of 98.1% by promoting diffusion through oxygen vacancies; Ce 4+ / Ce 3+ The redox cycle consumes free oxygen, reducing high-temperature sintering oxidation (residual SiO2 < 2%).

[0050] Table 1 Different Sintering Aids

[0051] Comparative Example 1: The high-temperature sintering process of this comparative example differs from that of Example 1: the pre-pressed sample is placed in an SPS and heated to 1550 ℃ at a heating rate of 100 ℃ / min under normal pressure nitrogen atmosphere, with a pressure of 30 MPa. After holding at this temperature for 10 min, the temperature is lowered. Other conditions are the same as in Example 1, resulting in silicon nitride ceramic.

[0052] The heating rate of 100 ℃ / min prolongs the residence time of the powder in the α→β phase transition temperature range (1400-1600℃), and the β-Si3N4 grains grow into micron-sized columnar crystals through the dissolution-precipitation mechanism, resulting in the deterioration of mechanical properties.

[0053] The silicon nitride ceramic prepared by this comparative method has a grain size of 1 μm, a density of 98.5%, a flexural strength of 936 MPa, and a fracture toughness of 5.43 MPa·m. 1 / 2 Its hardness is 1894.2 HV1.0.

[0054] The slow heating rate and long reaction time of the high-temperature sintering process promote grain growth, which in turn reduces the mechanical properties of silicon nitride ceramics.

[0055] Comparative Example 2: In this comparative example, the sintering aid content was increased to 25 wt.%, and other process conditions were the same as in Example 1. Silicon nitride ceramics were obtained after high-temperature sintering.

[0056] The silicon nitride ceramic prepared by this comparative method has a grain size of approximately 150 nm, a density of 93.6%, a flexural strength of 845 MPa, and a fracture toughness of 5.83 MPa·m. 1 / 2 Its hardness is 1798.5 HV1.0.

[0057] The method generates an excessive liquid phase that promotes grain growth, produces a continuous glassy phase that weakens grain boundaries, and thus reduces the mechanical properties and density of silicon nitride ceramics.

[0058] Comparative Example 3: In this comparative example, the reaction temperature was lowered to 1300℃, and other process conditions were the same as in Example 1. Silicon nitride ceramics were obtained after high-temperature sintering.

[0059] The silicon nitride ceramic prepared by this comparative method has a grain size similar to that of the powder before high-temperature sintering, and a density of 82.9%.

[0060] The method did not reach the high-temperature sintering temperature, and the high-temperature liquid-phase sintering was not achieved during the reaction process, resulting in low density of the obtained silicon nitride ceramic and failure of growth and fracture of individual grains at the microscopic level.

[0061] Comparative Example 4: In this comparative example, the reaction time was increased to 5 min, and other process conditions were the same as in Example 1. After high-temperature sintering, silicon nitride ceramics were obtained.

[0062] The silicon nitride ceramic prepared by this comparative method has a grain size of approximately 400 nm, a density of 97.3%, a flexural strength of 1025 MPa, and a fracture toughness of 6.87 MPa·m. 1 / 2 Its hardness is 1969.3 HV1.0.

[0063] The long reaction time of the method results in a large increase in grain size, and the rapid cooling process cannot induce grain fragmentation and refinement to below 100nm.

[0064] Comparative Example 5: This comparative example does not add any sintering aids and directly performs SPS rapid high-temperature sintering. Its high-temperature sintering process is the same as that in Example 1. After high-temperature sintering, silicon nitride ceramics are obtained.

[0065] The silicon nitride ceramic prepared by this comparative method has a grain size similar to that of the powder before high-temperature sintering, and a density of 78.6%.

[0066] The method, due to the absence of sintering aids, failed to reach the high-temperature sintering temperature and did not achieve high-temperature liquid-phase sintering during the reaction process, resulting in low density of the obtained silicon nitride ceramic and failure of grain growth and fracture at the microscopic level.

[0067] Table 2 Performance of each embodiment and comparative example

[0068] The embodiments herein do not exhaustively cover the points not covered by the technical scope claimed in this application, and new technical solutions formed by equivalent substitutions of one or more technical features in the technical solutions of the embodiments are also within the scope of protection claimed in this application. At the same time, in all the listed or unlisted embodiments of the solution in this application, each parameter in the same embodiment merely represents an instance of its technical solution (i.e., a feasible solution), and there is no strict matching or limiting relationship between the parameters. The parameters can be substituted for each other without violating axioms and the claims of this application, unless otherwise stated.

[0069] The technical means disclosed in this application are not limited to those described above, but also include technical solutions composed of any combination of the above technical features. The above descriptions are specific embodiments of this application. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered within the scope of protection of this application.

[0070] The specific embodiments described herein are merely illustrative examples of the spirit of this application. Those skilled in the art to which this application pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of this application or exceeding the scope defined by the appended claims.

Claims

1. A method for preparing ultrafine-grained ceramics, characterized in that, Includes the following steps: The ultrafine precursor ceramic powder is uniformly mixed with sintering aid and then sieved to obtain a mixed powder. The mixed powder is pre-pressed in a sintering mold, and then subjected to high-temperature sintering and cooling post-treatment to obtain the ultrafine-grained ceramic, wherein the grain size of the ultrafine-grained ceramic is ≤100 nm; wherein, The heating rate of the high-temperature sintering is 500℃ / min-1500℃ / min, the temperature of the high-temperature sintering is 1500℃-1700℃, the pressure of the high-temperature sintering is 5 MPa-100 MPa, and the time of the high-temperature sintering is 10s-60s. The initial particle size D of the ultrafine precursor ceramic powder 50 The wavelength range is 100 nm to 300 nm, and the specific surface area is 8 m². 2 / g-16m 2 / g.

2. The method for preparing an ultrafine-grained ceramic according to claim 1, characterized in that: The cooling rate of the post-cooling treatment is ≥500℃ / min.

3. The method for preparing an ultrafine-grained ceramic according to claim 1, characterized in that: The sintering aid includes at least one of oxides, rare earth oxides, composite sintering aids, and non-oxide aids; the initial particle size D of the sintering aid... 50 The range is 30 nm to 100 nm.

4. The method for preparing an ultrafine-grained ceramic according to claim 1, characterized in that: The uniform mixing method includes liquid phase mixing, wherein the liquid phase includes at least one of ethanol and isopropanol, and the mass ratio of the liquid phase to the mixed powder is (1-1.5):

1.

5. The method for preparing an ultrafine-grained ceramic according to claim 3, characterized in that: The mass ratio of the sintering aid to the mixed powder is (1-15):

1.

6. The method for preparing an ultrafine-grained ceramic according to claim 2, characterized in that: The ultrafine precursor ceramic powder includes one of silicon nitride, silicon carbide, boron carbide, tantalum carbide, and aluminum nitride.

7. The method for preparing an ultrafine-grained ceramic according to claim 1, characterized in that: The high-temperature sintering method includes discharge plasma sintering, and the atmosphere for high-temperature sintering is high-purity nitrogen or vacuum; the pulse current parameters in the high-temperature sintering are: DC pulse frequency 500 Hz~1 kHz, single pulse width 5 ms-10 ms, and current density 200 A / cm². 2 ~500 A / cm 2 .

8. The method for preparing an ultrafine-grained ceramic according to claim 1, characterized in that: The sintering mold is a high-strength graphite mold with a compressive strength ≥50 MPa and an ash content ≤0.1%. The inner wall of the sintering mold is coated with an isolation layer, the material of which includes boron nitride. The carbon content in the ultrafine crystalline ceramic is ≤0.5%.

9. A ceramic with an ultrafine-grained structure, characterized in that, The ultrafine-grained ceramic obtained by the preparation method according to any one of claims 1-8 has a grain size ≤100 nm, and the ultrafine-grained ceramic also satisfies at least one of the following characteristics: (1) Density ≥ 98%; (2) Bending strength ≥ 1000 MPa; (3) Fracture toughness ≥ 8 MPa·m 1 / 2 ; (4) Hardness ≥ 1900HV1.0; (5) The content of residual oxides is ≤3%.

10. The application of an ultrafine-grained ceramic obtained by any one of claims 1-8 or the ultrafine-grained ceramic as described in claim 9 in aerospace engine components, precision mechanical bearings, cutting tools, biomedical implants, or electronic packaging thermal management materials.

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