CN113956049A

CN113956049A - Method for preparing high-density ceramic by pressureless sintering of beta-SiC powder synthesized by self-propagating combustion

Info

Publication number: CN113956049A
Application number: CN202111322817.7A
Authority: CN
Inventors: 陆有军; 林立群; 李茂辉; 张明君; 刘乡; 杨璐同
Original assignee: North Minzu University
Current assignee: North Minzu University
Priority date: 2021-11-09
Filing date: 2021-11-09
Publication date: 2022-01-21

Abstract

The invention provides a method for preparing high-density ceramic by pressureless sintering of beta-SiC powder synthesized by self-propagating combustion, which comprises the following steps: (1) mechanical activation; (2) acid washing and purifying; (3) ball milling and pulping; (3) dry pressing and forming; (4) and (4) pressureless sintering. The invention adopts the beta-SiC powder synthesized by self-propagating combustion, improves the surface activation energy and the purity through mechanical activation and acid cleaning purification, and prepares the high-performance submicron powder suitable for sintering. And then by controlling the sintering aid C, B and the sintering process, the beta-SiC powder synthesized by self-propagating combustion is used as a raw material to prepare the compact beta-SiC ceramic material with excellent mechanical properties. When the addition amount of B is 0.7 wt% and the addition amount of C is 4 wt%, sintering is performedThe density and mechanical property of the prepared SiC ceramic are optimal under the conditions that the temperature is 2100 ℃ and the heat preservation time is 90min, the loss on ignition is 7.9 percent, the shrinkage rate is 15.6 percent, and the density is 3.14g/m³Hardness of 25.62 +/-0.92 GPa and fracture toughness of 4.84 +/-0.84 MPa.m^1/2Three-point bending strength 401.74 +/-8.66 GPa.

Description

Method for preparing high-density ceramic by pressureless sintering of beta-SiC powder synthesized by self-propagating combustion

Technical Field

The invention relates to the technical field of preparation of ceramics by sintering beta-SiC, in particular to a method for preparing high-density ceramics by pressureless sintering of beta-SiC powder synthesized by self-propagating combustion.

Background

Silicon carbide (SiC) ceramic material is an important structural ceramic material, and can be widely applied to aerospace, engine parts, nuclear reactors and grinding tools due to high temperature resistance, abrasion resistance, small thermal expansion coefficient, high thermal conductivity and excellent oxidation resistance and corrosion resistance. For example, in petrochemical industries, for use in various corrosion resistant pipelines; as a seal ring component in the automotive industry; in the mechanical industry, various wear-resistant, high-temperature-resistant and corrosion-resistant devices are manufactured. And silicon carbide is considered by scientists as an ideal cladding material for nuclear fuel due to its low reactivity under neutron radiation.

SiC is a strongly covalent compound second only to diamond, with silicon-carbon bond covalency: the ionic property is 7.3: 1. the SiC molecule configuration is Si-C tetrahedron, the Si atom is positioned at the center, and four C atoms surround the Si atom respectively. The basic structural unit of SiC is covalently bonded [ SiC ]₄]And [ CSi₄]Coordinative tetrahedrons of these [ SiC ]₄]And [ CSi₄]The bases of (a) are stacked in parallel or antiparallel association with each other, the tetrahedra being coterminous to form a planar layer and connected at vertices to the next stack of tetrahedra to form a three-dimensional structure. The SiC has beta and alpha crystal structures, exists in the form of beta-SiC at the temperature of below 1600 ℃, has a face-centered cubic system sphalerite structure and has a density of 3.215g/cm³. When the temperature is higher than 1600 ℃, the beta-SiC begins to convert to the alpha-SiC, and when the temperature is higher than 2100 ℃, the beta-SiC is completely converted into the alpha-SiC, and the alpha-SiC has polytypes of 2H, 4H, 6H, 15R and the like, the polytype density of the alpha-SiC is about 3.217g/cm,among them, the 6H polytype is widely used in industry.

The preparation method of the beta-SiC can be divided into three methods, namely a solid phase method, a liquid phase method and a gas phase method, wherein the solid phase method comprises a carbothermic method (namely an Acheson method), a silicon-carbon direct reaction method, a mechanical alloying method and a self-propagating combustion synthesis method; liquid phase methods include sol-gel methods and polymer decomposition methods; the vapor phase method includes a chemical vapor deposition method (CVD method), a laser induced chemical vapor deposition method (LICVD method), and a plasma method. Among them, the liquid phase method and the gas phase method are not applicable to industrial mass production due to the limitation of experimental conditions and yield, while the carbothermic method in the solid phase method is high in energy consumption and pollution, impurities are introduced in the mechanical alloying method, the silicon-carbon direct reaction method also requires high temperature and long time for reaction synthesis of beta-SiC, and self-propagating combustion synthesis (SHS) is performed. The high temperature combustion wavefront propagates in the medium, thereby converting the precursor to the desired product. Some features of the SHS method include: 1) the reaction time is short; 2) the energy efficiency is high; 3) simple technical equipment; 4) high reaction rate and relatively fast cooling rate. Due to the extremely high combustion temperature, most impurities are burnt, and the self-purification effect is generated. Therefore, the technology for synthesizing the beta-SiC powder by self-propagating combustion is a low-cost, energy-saving and environment-friendly production technology which can be applied to large-scale industrial production of high-quality and ultrafine beta-SiC powder.

At present, the beta-SiC powder produced by the Ahsceon process is based on the theory of solid phase sintering SiC proposed by Prochazka, a small amount of B, C is added as a sintering aid, the density of pressureless sintering SiC ceramic can reach more than 97%, but the density of SiC ceramic sintered by the beta-SiC powder synthesized by self-propagating combustion only reaches 86%. The reason for this is that the sintering driving force of the SiC ceramic comes from the surface energy of the powder, the average particle size of the beta-SiC powder synthesized by self-propagating combustion is large, the specific surface area is small, and the densification of the SiC ceramic is hindered, so that further mechanical activation is required to reduce the particle size of the beta-SiC powder synthesized by self-propagating combustion and increase the surface energy thereof; secondly, the initial purity of the beta-SiC powder synthesized by self-propagating combustion is only about 90 percent, and can only be improved to about 94 percent even through acid cleaning purification, so the acid cleaning purification process aiming at the beta-SiC powder synthesized by self-propagating combustion needs to be researched, and the purity of the beta-SiC powder synthesized by self-propagating combustion can reach more than 98 percent; thirdly, the beta-SiC powder is used as a raw material, and the beta phase → alpha phase crystal form transformation of the SiC powder occurs at high temperature (>1950 (C)), and the abnormal growth of crystal grains preferentially oriented along the one-dimensional direction is accompanied, so that the densification of the sample is hindered, and the density is reduced, therefore, the sintering temperature and the heat preservation time are strictly controlled, and the excessive growth of the crystal grains is inhibited, so that the densification is realized.

In conclusion, compared with the traditional alpha-SiC, the beta-SiC micropowder synthesized by self-propagating combustion has the advantages of short reaction time, high purity and the like. However, the beta-SiC micropowder prepared by the self-propagating combustion synthesis method has large particle size, small specific surface area and low surface activity, does not meet the requirement of pressureless solid phase sintering, and is difficult to prepare high-quality ceramic, so the beta-SiC micropowder needs to be subjected to treatment such as activation and the like to improve the surface activation energy, and meanwhile, the sintering process is researched to expand the application of the beta-SiC micropowder synthesized by the self-propagating combustion in the field of silicon carbide ceramic materials.

Disclosure of Invention

The invention aims to solve the technical problem of providing a method for preparing high-density ceramic through pressureless sintering of beta-SiC powder synthesized by self-propagating combustion, which improves the surface activation energy and the purity through mechanical activation and acid cleaning purification, and prepares and obtains compact SiC ceramic material with excellent mechanical property by taking the beta-SiC powder synthesized by the self-propagating combustion as a raw material through controlling a sintering aid C, B and a sintering process.

The invention provides a method for preparing high-density ceramic by pressureless sintering of beta-SiC powder synthesized by self-propagating combustion, which comprises the following steps:

(1) mechanical activation: putting beta-SiC raw powder into a stirring mill, adding deionized water, adding tetramethylammonium hydroxide as a dispersing aid, adding zirconia balls as a grinding medium, and grinding for 24-72 hours for mechanical activation to obtain active beta-SiC submicron powder;

(2) acid washing and purifying: placing the mechanically activated beta-SiC submicron powder into a container, wetting the beta-SiC submicron powder by using boiling distilled water, carrying out water bath at the temperature of 80 +/-5 ℃ to constant temperature, adding acid liquor one or two times, uniformly stirring, reacting for 30-90 min each time, adding the acid liquor which is mixed acid consisting of one or more of concentrated sulfuric acid, hydrofluoric acid or concentrated hydrochloric acid, cooling and discharging acid after the reaction is finished, and drying to obtain beta-SiC micropowder with the purity of more than 98%;

(3) ball milling and pulping: weighing solid raw material active beta-SiC micro powder according to a certain weight proportion, adding B source B₄C and C source phenolic resin, and ZrO is added₂Ball milling media are added into the ball mill together for ball milling and mixing for 10-20 min, then 5-15 wt% of dispersing agent is added, and ball milling and mixing are continued for 30-60 min to obtain slurry;

(4) dry pressing and forming: drying and sieving the slurry, placing the sieved mixed powder into a hydraulic forming machine, performing dry pressing forming under the pressure of 50-150 MPa to form a primary blank, and then placing the primary blank into a cold isostatic press to press the primary blank under the pressure of 150-250 MPa to form a blank body;

(5) pressureless sintering: placing the blank in a sintering furnace, vacuumizing, filling protective gas, heating to 700-900 ℃ at the speed of 6 ℃/min, and preserving heat for presintering for 30 min; then heating to 1950-2130 ℃ at the speed of 8 ℃/min, carrying out heat preservation sintering for 30-120 min, and then naturally cooling to room temperature.

Preferably, the primary average particle diameter of the beta-SiC raw powder in the step (1) is 5.471 μm, and the specific surface area is 6.34m²Per g, the purity is 90.4 percent, and the oxygen content is 2 weight percent.

Preferably, the solid weight ratio after adding water in the step (1) is 60%.

Preferably, the particle size of the zirconia ball milling medium in the step (1) is 0.4-2 mm.

Preferably, the ball milling time in the step (1) is 72h, firstly adding a zirconia ball with the diameter of 2mm, grinding for 36h, then replacing the zirconia ball with the diameter of 0.4mm, and continuing to grind for 36 h.

Preferably, the mass ratio of the concentrated sulfuric acid solution to the hydrofluoric acid solution in the mixed acid in the step (2) is 1: 1, wherein the mass concentration of concentrated sulfuric acid is not less than 90%, and the concentration of hydrofluoric acid solution is not less than 40%.

Preferably, the acid discharging step in the step (2) is as follows: adding tap water into the mixture, standing for layering, discharging supernatant, repeating the operation until the pH is 5, adding deionized water, naturally standing for layering, discharging supernatant, repeating the operation until the pH is 7, and discharging all the supernatants.

Preferably, the adding mass of the boiling distilled water in the step (2) is 1-3 times of that of the beta-SiC powder.

Preferably, in the step (3), the addition amount of B is 0.7 wt%, the addition amount of C is 4 wt%, and the addition amount of a dispersant is 10 wt%; the dispersing agent comprises the following components in parts by weight: 6-8 parts of ethanol, 0.3-2 parts of oleic acid, 0.4-1 part of polyvinyl alcohol and 0.1-1 part of tetramethyl ammonium hydroxide.

Preferably, the sintering temperature in the step (5) is 2100 ℃, and the heat preservation sintering time is 90 min.

The invention has the beneficial effects that: the invention adopts the beta-SiC powder synthesized by self-propagating combustion, improves the surface activation energy and the purity through mechanical activation and acid cleaning purification, and prepares the high-performance submicron powder suitable for sintering. And then by controlling the sintering aid C, B and controlling the sintering process, beta-SiC powder synthesized by self-propagating combustion is used as a raw material to prepare the compact beta-SiC ceramic material with excellent mechanical properties. Specifically, the method comprises the following steps:

(1) the primary average particle diameter was 5.471 μm, and the specific surface area was 6.34m²Per g, purity 90.4%, oxygen content 2 wt% of self-propagating combustion synthesized beta-SiC powder, using stirring mill grinding to make mechanical activation to make it reach sinterable submicron grade, preparing D₅₀The best mechanical activation mode is that 2mm zirconia balls are added firstly and ground for 36h, then 0.4mm zirconia balls are replaced, and the grinding is continued for 36 h; the size and the granularity of the particles of the required product are achieved by means of impacting, colliding, shearing, dispersing, grinding and the like of the material through mechanical activation, and meanwhile, when energy generated by the action of mechanical force is applied to the surfaces of the particles, the particles can be subjected to mechanical activationThe particles generate lattice defects, amorphization, lattice defects and the like, and simultaneously are accompanied with surface free radical formation, and the activation efficiency is improved through scientific mechanical activation treatment.

(2) The beta-SiC powder after mechanical activation contains ZrO₂The impurities are equal, and the beta-SiC micro powder is subjected to concentrated H₂SO₄: HF ═ 1: 1, acid washing and purifying the mixed acid to obtain beta-SiC micropowder which is suitable for pressureless sintering and has high purity, the purity of the beta-SiC micropowder is more than 98 percent, the oxygen content is 0.89 percent by weight, and the specific surface area is 18.46m²Per g, better sintering activity, benefit to sintering, concentrated H under the comprehensive consideration of economic cost and time efficiency₂SO₄The combination with HF is optimal compared to the other three groups.

(3) Selecting beta-SiC micropowder synthesized by self-propagating combustion with good sintering activity, adding B₄C and phenolic resin as sintering aid, with average particle diameter D₅₀0.22 μm B₄C is taken as a boron source, phenolic resin is taken as an organic carbon source, the organic carbon source is organic matter, and the molecular formula is (C)₈H₈O₂) n, also known as bakelite, in which phenolic resin acts as a binder, increasing the strength of the biscuit, which phenolic resin converts H and O into H under an inert atmosphere at high temperature₂The proportion of O is removed, leaving C. This is because C and SiO pyrolyzed during the process of sintering SiC ceramics₂SiC and CO are generated in a combined manner, so that the surface energy (Eb) of the powder is increased, the grain boundary energy (Es) is reduced, and the sintering activity of the beta-SiC powder is increased.

(4) The density and the mechanical property of the beta-SiC synthesized by the pressureless sintering self-propagating combustion are improved by controlling the addition amount of B, C, the sintering temperature and the heat preservation time. When B (B)₄C conversion) of 0.7 wt% and C (phenolic resin conversion) of 4 wt%, the SiC ceramic prepared under the conditions of the sintering temperature of 2100 ℃ and the holding time of 90min has the best compactness and mechanical property, and the loss on ignition is 7.9%, the shrinkage is 15.6%, and the density is 3.14g/m³Hardness of 25.62 +/-0.92 GPa and fracture toughness of 4.84 +/-0.84 MPa.m^1/2The three-point bending strength is 401.74 +/-8.66 GPa.

Drawings

FIG. 1 is a particle size distribution diagram before and after mechanical activation of a beta-SiC powder, wherein: (a) the particle size distribution of the beta-SiC micropowder before mechanical activation; (b) example 1; (c) example 2; (d) example 3; (e) example 4;

FIG. 2 is a graph of the random mechanical activation time variation of D10, D50 and D90 during the mechanical activation of beta-SiC powder, wherein: (a) example 1; (b) example 2; (c) example 3; (d) example 4;

FIG. 3 is a SEM comparison of beta-SiC powder before and after mechanical activation, wherein: (a) - (d) is the main morphology of the beta-SiC raw powder; (e) - (h) is D obtained using example 4₅₀The main morphology of 0.418 μm β -SiC powder;

FIG. 4 is an XRD pattern of the raw powder, the pulverized and refined beta-SiC powder after purification;

FIG. 5 is EDS elemental surface sweep of beta-SiC powder, where: (a) beta-SiC raw powder; (b) example 4 mechanically activated beta-SiC powder; (c) example 4 acid washing of purified beta-SiC powder;

FIG. 6 is SEM analysis of ceramic surfaces at different addition levels, wherein: (a) example 1; (b) example 2; (c) example 3; (d) example 4;

FIG. 7 shows fracture morphology of ceramics at different addition levels, where: (a) example 2; (b) example 3; (c) example 4.

FIG. 8 is a schematic diagram of three-point bending for bending strength testing, wherein, 1-test sample bar, 2-roller bar, 3-loading pressure head, sample bar size is a length a more than or equal to 35mm, width b is 4 + -0.2 mm, and height c is 3 + -0.2 mm; FIG. 9 is a schematic illustration of indentation for hardness and fracture toughness testing, wherein: 1-indentation, 2-cracking.

Detailed Description

In order to make the technical scheme of the invention easier to understand, the technical scheme of the invention is clearly and completely described by adopting a mode of specific embodiments with reference to the attached drawings.

Detailed description of the preferred embodiments

Example 1:

the method for preparing high-density ceramic through pressureless sintering of beta-SiC powder synthesized by self-propagating combustion comprises the following steps:

(1) machine for workingMechanical activation: putting beta-SiC raw powder into a stirring mill, adding deionized water, adding tetramethylammonium hydroxide as a dispersing aid, adding 2mm zirconia balls as a grinding medium, and grinding for 72 hours for mechanical activation to obtain the active beta-SiC submicron powder. Wherein: the initial average particle diameter of the beta-SiC raw powder is 5.471 mu m, and the specific surface area is 6.34m²Per g, the purity is 90.4%, the oxygen content is 2 wt% of beta-SiC powder synthesized by self-propagating combustion; the solid weight ratio after adding water is 60%;

(2) acid washing and purifying: placing the mechanically activated beta-SiC submicron powder into a container, wetting the beta-SiC submicron powder by using boiling distilled water, carrying out water bath at the temperature of 80 +/-5 ℃ to constant temperature, then adding concentrated sulfuric acid for reaction for 0.5h, pouring out the concentrated sulfuric acid, adding hydrofluoric acid for reaction for 0.5h, cooling, discharging acid, treating, and drying to obtain the beta-SiC micropowder with the purity of more than 98%. Wherein: the mass concentration of concentrated sulfuric acid is not less than 90%, and the concentration of hydrofluoric acid solution is not less than 40%; the acid discharge step is as follows: adding tap water into the mixed material, standing for layering, discharging supernatant, repeating the operation until the pH is 5, adding deionized water, naturally standing for layering, discharging supernatant, repeating the operation until the pH is 7, and discharging all supernatant; the adding mass of the boiling distilled water is 1-3 times of that of the beta-SiC powder.

(3) Ball milling and pulping: weighing solid raw material active beta-SiC micro powder according to a certain weight proportion, adding B source B₄C and C source phenolic resin, and ZrO is added₂Ball milling media are added into the ball mill together for ball milling and mixing for 10-20 min, then 10 wt% of dispersing agent is added, and ball milling and mixing are continued for 30-60 min to obtain slurry; the addition amount of B is 0.7 wt%, and the addition amount of C is 4 wt%; the dispersing agent comprises the following components in parts by weight: 7 parts of ethanol, 1.5 parts of oleic acid, 0.6 part of polyvinyl alcohol and 0.3 part of tetramethyl ammonium hydroxide.

(4) Dry pressing and forming: and drying and sieving the slurry, placing the sieved mixed powder into a hydraulic forming machine, performing dry pressing forming under the pressure of 60-120 MPa to form a primary blank, and then placing the primary blank into a cold isostatic press to press the primary blank under the pressure of 180-240 MPa to form a blank body.

(5) Pressureless sintering: placing the blank in a sintering furnace, vacuumizing, filling protective gas, heating to 700 ℃ at the speed of 6 ℃/min, and preserving heat for presintering for 30 min; heating to 1950 deg.C at 8 deg.C/min, sintering for 30min, and naturally cooling to room temperature.

Example 2:

(1) mechanical activation: putting beta-SiC raw powder into a stirring mill, adding deionized water, adding tetramethylammonium hydroxide as a dispersing aid, adding a zirconia ball with the thickness of 0.4mm as a grinding medium, grinding for 72 hours, and performing mechanical activation to obtain the active beta-SiC submicron powder. Wherein: the primary average particle diameter of the beta-SiC raw powder is 5.471 mu m, and the specific surface area is 6.34m²Per g, the purity is 90.4%, the oxygen content is 2 wt% of beta-SiC powder synthesized by self-propagating combustion; the weight ratio of the solid after adding water is 60 percent.

(2) Acid washing and purifying: placing the mechanically activated beta-SiC submicron powder into a container, wetting the beta-SiC submicron powder by using boiling distilled water, carrying out water bath at the temperature of 80 +/-5 ℃ to constant temperature, then adding concentrated sulfuric acid for reaction for 0.5h, pouring out the concentrated sulfuric acid, adding a mixed solution of concentrated hydrochloric acid and hydrofluoric acid for reaction for 0.5h, cooling and discharging acid for treatment, and then drying to obtain the beta-SiC micropowder with the purity of more than 98%. Wherein: the mass concentration of the concentrated sulfuric acid is not lower than 90 percent, and the concentration of the hydrofluoric acid solution is not lower than 40 percent; the acid discharge step is as follows: adding tap water into the mixture, standing for layering, discharging supernatant, repeating the operation until the pH is 5, adding deionized water, naturally standing for layering, discharging supernatant, repeating the operation until the pH is 7, and discharging all the supernatants; the adding mass of the boiling distilled water is 1-3 times of that of the beta-SiC powder.

(3) Ball milling and pulping: weighing solid raw material active beta-SiC micro powder according to a certain weight proportion, adding B source B₄C and C source phenolic resin, and ZrO is added₂Ball milling media are added into the ball mill together for ball milling and mixing for 10-20 min, then 5-15 wt% of dispersing agent is added, and ball milling and mixing are continued for 30-60 min to obtain slurry; the addition amount of B is 0.5-1.0 wt%,the addition amount of C is 3-5 wt%; the dispersing agent comprises the following components in parts by weight: 6 parts of ethanol, 2 parts of oleic acid, 0.4 part of polyvinyl alcohol and 1 part of tetramethyl ammonium hydroxide.

(4) Dry pressing and forming: and drying and sieving the slurry, placing the sieved mixed powder into a hydraulic forming machine, performing dry pressing forming under the pressure of 50-100 MPa to form a primary blank, and then placing the primary blank into a cold isostatic press to press the primary blank under the pressure of 150-200 MPa to form a blank body.

(5) Pressureless sintering: placing the blank in a sintering furnace, vacuumizing, filling protective gas, heating to 800 ℃ at the speed of 6 ℃/min, and preserving heat for presintering for 30 min; then heating to 2000 ℃ at the speed of 8 ℃/min, preserving heat, sintering for 60min, and naturally cooling to room temperature.

Example 3:

(1) mechanical activation: putting beta-SiC raw powder into a stirring mill, adding deionized water, adding tetramethylammonium hydroxide as a dispersing aid, and mixing according to the weight ratio of 1: adding zirconia balls with the diameter of 0.4mm and 2mm as grinding media according to the proportion of 1, grinding for 72 hours, and carrying out mechanical activation to obtain the active beta-SiC submicron powder. Wherein: the primary average particle diameter of the beta-SiC raw powder was 5.471 μm, and the specific surface area was 6.34m²Per g, the purity is 90.4%, the oxygen content is 2 wt% of beta-SiC powder synthesized by self-propagating combustion; the solid weight ratio after adding water is 60%.

(2) Acid washing and purifying: placing the mechanically activated beta-SiC submicron powder into a container, wetting the beta-SiC submicron powder by using boiling distilled water, carrying out water bath at the temperature of 80 +/-5 ℃ to constant temperature, then adding a mixed solution of concentrated sulfuric acid and hydrofluoric acid for reaction for 0.5h, pouring out mixed acid, adding concentrated hydrochloric acid for reaction for 0.5h, cooling and discharging acid, and then drying to obtain the beta-SiC micropowder with the purity of more than 98%. Wherein: the mass concentration of the concentrated sulfuric acid is not lower than 90%, and the concentration of the hydrofluoric acid solution is not lower than 40%; the acid discharge step is as follows: adding tap water into the mixture, standing for layering, discharging supernatant, repeating the operation until the pH is 5, adding deionized water, naturally standing for layering, discharging supernatant, repeating the operation until the pH is 7, and discharging all the supernatants; the adding mass of the boiling distilled water is 1-3 times of that of the beta-SiC powder.

(3) Ball milling and pulping: weighing solid raw material active beta-SiC micro powder according to a certain weight proportion, adding B source B₄C and C source phenolic resin, and ZrO is added₂Ball milling media are added into the ball mill together for ball milling and mixing for 10-20 min, then 5-15 wt% of dispersing agent is added, and ball milling and mixing are continued for 30-60 min to obtain slurry; the addition amount of B is 0.5-1.0 wt%, and the addition amount of C is 3-5 wt%; the dispersing agent comprises the following components in parts by weight: 8 parts of ethanol, 0.3 part of oleic acid, 1 part of polyvinyl alcohol and 0.1 part of tetramethyl ammonium hydroxide.

(4) Dry pressing and forming: and drying and sieving the slurry, placing the sieved mixed powder into a hydraulic forming machine, performing dry pressing forming under the pressure of 100-150 MPa to form a primary blank, and then placing the primary blank into a cold isostatic press to press the primary blank under the pressure of 200-250 MPa to form a blank body.

(5) Pressureless sintering: placing the blank in a sintering furnace, vacuumizing, filling protective gas, heating to 900 ℃ at the speed of 6 ℃/min, and preserving heat for presintering for 30 min; heating to 2130 deg.C at 8 deg.C/min, sintering for 120min, and naturally cooling to room temperature.

Example 4:

(1) mechanical activation: putting beta-SiC raw powder into a stirring mill, adding deionized water, adding tetramethylammonium hydroxide as a dispersing aid, and adding 2mm ZrO₂Grinding the ball milling medium for 36h, and then replacing the ball milling medium with ZrO of 0.4mm₂Grinding the ball milling medium for 36h, and carrying out mechanical activation to obtain the active beta-SiC submicron powder. Wherein: the primary average particle diameter of the beta-SiC raw powder was 5.471 μm, and the specific surface area was 6.34m²Per g, the purity is 90.4%, the oxygen content is 2 wt% of beta-SiC powder synthesized by self-propagating combustion; the weight ratio of the solid after adding water is 60 percent.

(2) Acid washing and purifying: placing the mechanically activated beta-SiC submicron powder into a container, wetting the beta-SiC submicron powder by using boiling distilled water, carrying out water bath at the temperature of 80 +/-5 ℃ to constant temperature, and then adding a concentrated sulfuric acid solution and a hydrofluoric acid solution according to the mass ratio of 1: 1 for 1 hour, cooling, carrying out acid treatment, and drying to obtain the beta-SiC micropowder with the purity of more than 98%. Wherein: in the mixed acid, the mass concentration of concentrated sulfuric acid is not less than 90%, and the concentration of hydrofluoric acid solution is not less than 40%; the acid discharge step is as follows: adding tap water into the mixture, standing for layering, discharging supernatant, repeating the operation until the pH is 5, adding deionized water, naturally standing for layering, discharging supernatant, repeating the operation until the pH is 7, and discharging all the supernatants; the adding mass of the boiling distilled water is 1-3 times of that of the beta-SiC powder.

(3) Ball milling and pulping: weighing solid raw material active beta-SiC micro powder according to a certain weight proportion, adding B source B₄C and C source phenolic resin, and ZrO is added₂Ball milling media are added into the ball mill together for ball milling and mixing for 10-20 min, then 5-15 wt% of dispersing agent is added, and ball milling and mixing are continued for 30-60 min to obtain slurry; the addition amount of B is 0.5-1.0 wt%, and the addition amount of C is 3-5 wt%; the dispersing agent comprises the following components in parts by weight: 7 parts of ethanol, 1 part of oleic acid, 0.8 part of polyvinyl alcohol and 0.7 part of tetramethyl ammonium hydroxide.

(4) Dry pressing and forming: and drying and sieving the slurry, placing the sieved mixed powder into a hydraulic forming machine, performing dry pressing forming under the pressure of 80-120 MPa to form a primary blank, and then placing the primary blank into a cold isostatic press to press the primary blank under the pressure of 180-230 MPa to form a blank body.

(5) Pressureless sintering: placing the blank in a sintering furnace, vacuumizing, filling protective gas, heating to 800 ℃ at the speed of 6 ℃/min, and preserving heat for presintering for 30 min; then heating to 2100 ℃ at the speed of 8 ℃/min, preserving heat, sintering for 90min, and naturally cooling to room temperature.

Second, testing the performance of the material

1. Compactness test

The SiC ceramic density is measured by Archimedes drainage method, and the dry weight (m) of the sample is firstly weighed₁) Boiling the sample with boiling water for 0.5-1h to reach saturation, weighing the sample to be wetHeavy (m)₂) And the floating weight (m)₃). The measured density is calculated according to the formula (2-1):

ρ＝[m₁/(m₃-m₂)]×100％ (2-1)

in the formula: m is₁-dry weight of SiC;

m₂-wet weight of SiC;

m₃-SiC float weight.

2. Bending strength test

The SiC ceramic is processed into a sample strip with the thickness of 40mm multiplied by 4mm multiplied by 3mm, three-point bending resistance test is carried out according to GB/T6569 plus 2006 Fine ceramic bending strength test method, the loading speed of a pressure head is 0.5 mm/min, and the span is 30 mm.

Figure 2.3Schematic diagram of three-point bending

The bending strength of the three-point bending is calculated according to the formula (2-2):

in the formula:

σ f-three point bending strength (MPa);

f-maximum load (N);

l-lower span of the clamp (mm);

b-width of the specimen (mm);

d-specimen height (thickness) parallel to the loading direction (mm).

3. Hardness and fracture toughness testing

The hardness test adopts a Vickers hardness tester, the load is 5kgf, the pressure maintaining time is 10s, and the average value of 7 points is taken to calculate the hardness and calculate the standard deviation.

The fracture toughness adopts an indentation method, a conical diamond pressure head of a Vickers hardness tester is used for generating indentation, the diagonal line of the indentation and the length of the crack are measured as shown in figure 2.4, each sample is measured for 10 times, the average value and the standard deviation are calculated, and the calculation formula is shown as formula (2-3):

in the formula:

a-half length of the diagonal of the indentation;

e-elastic modulus (GPa);

h-hardness (GPa);

c-radial crack length (measured from the center of the indentation).

4. Phase and microstructure (SEM) analysis

The beta-SiC powder and the sintered SiC ceramic are subjected to phase analysis by XRD-6000 of Shimadzu Japan, and an EDS (electron dispersive Spectroscopy) is adopted to characterize the phase distribution of the beta-SiC powder.

The grain size of the beta-SiC powder and the planar appearance, fracture appearance, grain size and the like of the SiC sintered product are observed by adopting an SEM electron microscope.

5. Analysis of powder particle size and oxygen content

The particle size distribution of the beta-SiC powder is tested by a laser particle sizer by using (CH)₃)₄NOH is used as a dispersing agent, a small amount of powder is added into water deionized water to prepare water-based suspension, and the particle size distribution of the suspension is measured after the suspension is dispersed for 5min by ultrasonic waves.

Testing the oxygen content of the beta-SiC powder by using an oxygen content analyzer, filling Ar atmosphere as protective gas, wrapping a beta-SiC sample by using a tin sac, and reacting O in the beta-SiC with graphite at the high temperature of 3000 ℃ of 2000-₂Further determines the content of O in SiC.

Third, the technical scheme and data of the embodiment

Table 1: mechanical activation technical scheme comparison table

Table 2: comparison table of acid cleaning and purifying technical scheme

Table 3: non-pressure sintering scheme comparison table

Fourthly, result characterization and analysis:

1. particle size analysis and microstructure analysis before and after mechanical activation

1-1 particle size analysis

The ultrasonically dispersed beta-SiC powder was analyzed for particle size as shown in FIGS. 1(a) - (e). Wherein, FIG. 1(a) shows the particle size distribution of beta-SiC fine powder before mechanical activation; FIG. 1(b) is a particle size distribution of a fine β -SiC powder of example 1; FIG. 1(c) shows the particle size distribution of the fine β -SiC powder of example 2; FIG. 1(d) is a particle size distribution of a fine β -SiC powder of example 3; FIG. 1(e) shows the particle size distribution of the fine β -SiC powder of example 4.

FIGS. 2(a) to (D) show beta-SiC powders D of examples 1, 2, 3 and 4₁₀、D₅₀And D₉₀Graph of time dependence of mechanical activation. As can be seen from FIGS. 2(a) - (d), the best mechanical activation effect is shown in example 4, i.e. 2mm ZrO was used first₂Mechanically activated for 36h and then with 0.4mm ZrO₂After the ball has been mechanically activated for the same time, D₅₀To 0.418 μm, D₉₀The particle size of the beta-SiC fine powder reaching 0.733 mu m, namely 90 percent of the beta-SiC fine powder reaches submicron level, and the particle size distribution range is narrow, as shown in figure 1 (e). And as can be seen from fig. 1 and 2, grinding media (ZrO)₂Spheres) has a great influence on the mechanical activation efficiency of beta-SiC when ZrO is present₂The smaller the spheres, the smaller the volume, ZrO₂The larger the contact surface with β -SiC, the higher the mechanical activation efficiency.

Random mechanical activation change of beta-SiC particle size for example 1FIG. 2(a), the final particle size distribution of which is shown in FIG. 1(b), ZrO 2mm in diameter₂D of beta-SiC powder after grinding of balls for 12h₅₀Reaching 0.741 mu m, and continuously grinding to find that the grain diameter of the SiC powder is reduced to a small extent, and after 72 hours, the beta-SiC powder D₅₀Reaches 0.523 mu m, D₉₀Reaches 0.867 μm; the particle diameter of the beta-SiC powder of example 2 was changed by random mechanical activation, as shown in FIG. 2(b), when ZrO having a diameter of 0.4mm was used₂SiC powder D after ball grinding for 12h₅₀Reduced to 0.891 μm, D₉₀The grain size of the beta-SiC powder is reduced to 1.892 mu m, the mechanical activation efficiency is not as high as that of example 1 within 12h initially, and the grain size of the beta-SiC powder is reduced to 72h after the beta-SiC powder D is obtained by prolonging the mechanical activation time₅₀Reaches 0.554 mu m, D₉₀Reaches 0.748 mu m; the variation of the particle size of the beta-SiC particles in accordance with the mechanical activation in example 3 is shown in FIG. 2(c) when ZrO 2mm and 0.4mm are present₂After the balls are mixed for use, the grain diameter of the beta-SiC powder is mechanically activated for 12h, and D is₅₀Reaches 0.834 mu m, D₉₀Reaches 1.494 mu m, and beta-SiC powder D is obtained after 72 hours₅₀To 0.564 μm, D₉₀The particle size reaches 0.82 mu m, and the mechanical activation effect is the worst compared with other schemes; the change of the beta-SiC particle size with mechanical activation of example 4 is shown in FIG. 2(d), starting with 2mm ZrO₂After ball machinery is activated for 12 hours, beta-SiC powder D₅₀Reaches 0.793 mu m, SiC powder D after 36h₅₀Reaches 0.548 mu m, and is replaced by ZrO of 0.4mm₂Ball, mechanical activation for 36h, then beta-SiC powder D₅₀Reaches 0.418 mu m, D₉₀Reaches 0.733 mu m, and the mechanical activation efficiency is highest. Four schemes for beta-SiC powder D₁₀The mechanical activation efficiency of the beta-SiC is almost the same, which shows that the mechanical activation is mainly aimed at the larger particles of the beta-SiC, and the mechanical activation of the smaller particles is not obvious.

ZrO from comparison of the four variants of examples 1 to 4₂The smaller the diameter of the ball is, the smaller the grain size of beta-SiC obtained by ball milling is, and the higher the mechanical activation efficiency is. In a stirred mill, the beta-SiC particles are subjected to ZrO during the initial period of mechanical activation₂The impact force of the ball is large, the particle size of the particles is reduced obviously, and the particle size is reduced relatively quickly, so that the ball-milling medium with larger diameter is selected in the initial activation stageThe mechanical activation of the SiC particles can be more fully performed. The particle size decreases with increasing mechanical activation time to the limit size at which ZrO of large size (2mm) is obtained₂The ball can not continuously reduce the grain diameter of the beta-SiC grains, the mechanical extrusion and impact energy acting on the surfaces of the grains are reduced, the ball milling effect is weakened, the mechanical activation mode of the beta-SiC grains is mainly friction crushing, and at the moment, ZrO with small size (0.4mm) is replaced₂The ball increases the contact surface with the beta-SiC, and the mechanical activation efficiency is improved.

In example 1, only ZrO of large size (2mm) was selected₂The balls are superior in the initial stage of grinding, but are weak in the later stage, so that beta-SiC particles cannot be ground more effectively, and the final result cannot reach the required 0.500 mu m; in example 2, only ZrO of small size (0.4mm) was selected₂Balls, perform less well at the early stage of milling than large size (2mm) balls, as for D in the later stage of milling₅₀The particle size of the particles did not reach 0.4 μm, probably because the initial particle size was not sufficiently reduced, and the frictional pulverization could not be smoothly started; in example 3, ZrO of large size (2mm)₂Ball-mixing of small size (0.4mm) ZrO₂As is clear from FIG. 2(c), the initial effect of the balls was not obtained by selecting only 2mm ZrO when the particle size of the beta-SiC grains reached 0.834. mu.m at the initial stage of grinding₂The ball grinding effect is good because the contact area between the grinding medium and the material is reduced and the activation efficiency is less obvious because the gap formed between the two balls is too large; in example 4, large-size (2mm) ZrO was used first₂Grinding beta-SiC particles by balls to quickly reduce the particle size of the beta-SiC large particles, and replacing ZrO with small size (0.4mm)₂And (4) continuously ball-grinding the beta-SiC particles to the limit particle size by using the balls. Due to the reduction of ZrO₂The diameter of the ball can increase the contact surface between the grinding medium and the material particles, which is more beneficial to improving the grinding efficiency, so that the beta-SiC particles are more refined and the surface activity is higher.

1-2 microstructure analysis

FIG. 3 shows the synthesis of beta-SiC raw powder (D) by self-propagating combustion₅₀5.471 μm) and example 4 mechanically activated beta-SiC powder (D)₅₀0.418 μm), wherein fig. 3(a) - (d) are β -SiCThe four main morphologies of the raw powder, fig. 3(a) is a large particle formed by aggregating a plurality of small particles, fig. 3(b) is a long-strip tree root shape, fig. 3(c) is a cylindrical super-hard aggregate with a width of about 10 μm, fig. 3 (d) is a filament state, the majority of the agglomerated particle size is more than 5 μm, and the particle morphology is complex, because the driving force for sintering the SiC ceramic comes from the surface energy of the SiC powder, the finer the powder, the better the sphericity, the larger the surface energy of the powder particle, and the higher the sintering activity. FIGS. 3(e) - (h) are D obtained in example 4₅₀As is evident from fig. 3(e) - (h), after grinding and activation, the particle size reaches submicron level, the particle sphericity increases, the particles are equiaxed irregular polygons, and the particle size distribution is uniform, which lays a good foundation for pressureless sintering densification of β -SiC ceramics.

2. Analysis of phase composition, oxygen content and microstructure of beta-SiC powder before and after acid cleaning and purification

2-1 phase composition analysis

Fig. 4 shows XRD patterns of the β -SiC raw powder and the β -SiC powder after mechanical activation and acid cleaning purification, and it is understood from fig. 4 that the β -SiC powder is 3C-SiC and the X-ray diffraction pattern of the powder shows an additional peak at 2 θ of 33.7. The reason for this is due to the presence of stacking faults in the powder, not other polytypes (α -SiC). It can be seen from the figure that the diffraction peak intensity of the milled β -SiC powder becomes lower and the peak width becomes wider, because the milled β -SiC grains become finer, as can be seen from Scherrer equation (1).

The Scherrer formula can calculate the average grain size D, and for the same substances with different grain sizes, when k, gamma and theta are not changed, the average grain size D is inversely proportional to the half-height width B of the diffraction peak of the sample, i.e. the larger B, the smaller D, as can be seen from fig. 4, the half-height width of the diffraction peak of the ground beta-SiC is larger than that of the original powder, thus proving that the grain size after mechanical activation is smaller than that of the original powder.

According to beta-SiCThe purity of the initial powder was 90.4% in comparison with the purity before and after grinding, while the purity of the milled and refined beta-SiC powder was reduced to 78% due to ZrO₂The hardness of the ceramic is lower than that of SiC ceramic, and ZrO in the grinding process₂The introduction of impurities resulted in a decrease in the purity of the raw material, but ZrO was not found in the phase analysis after grinding₂Diffraction peaks due to ZrO₂The balls have high hardness without beta-SiC particles, and when the mechanical activation time is longer, ZrO is lost₂Balls of ZrO on their surfaces₂Grinding the mixture into beta-SiC slurry, and grinding, rubbing, shearing and the like to possibly cause that beta-SiC particles are applied to ZrO₂Destruction of the crystal structure of the particles, reduction of the diffraction power, and ZrO₂The impurity content of (2) is small, and thus the diffraction peak disappears.

FIGS. 5(a) - (C) are elemental surface scans of self-propagating combustion synthesized beta-SiC raw powder, submicron beta-SiC powder, and acid-washed purified beta-SiC powder, and it can be seen from FIG. 5(a) that the self-propagating raw powder exists with three elements of Si, C, and O, i.e., contains beta-SiC and partially oxidized SiO₂FIG. 5(b) beta-SiC powder after mechanical activation of example 4 in which four elements of Si, C, Zr and O are present, except for beta-SiC and SiO₂In the presence of (A), ZrO has also been found₂And Zr is uniformly distributed in the SiC powder, further proving that longer-time grinding destroys ZrO₂FIG. 5(c) shows that the SiC powder of example 4, which had been purified by acid washing to a purity of 98% or more, had only beta-SiC and SiO₂The pickling effect is very obvious, Zr element does not exist in an EDS energy spectrum, and ZrO does not exist₂The beta-SiC powder is removed by mixed acid, thus successfully obtaining the pressureless sintering beta-SiC powder with submicron grade purity of more than 98 percent.

2-2. analysis of oxygen content and specific surface area

The oxygen content and specific surface area of the beta-SiC raw powder synthesized by self-propagating combustion and the beta-SiC powder purified by grinding and acid washing are tested by using an oxygen content analyzer and a specific surface area analyzer, the oxygen content of the raw powder is 2.0 wt%, and the specific surface area is 6.34m²(ii)/g; as can be seen from Table 2, the oxygen content of the beta-SiC fine powder acid-washed and purified in example 4 was 0.89% by weight, and the specific surface areaThe product is 18.46m²(ii) in terms of/g. The results show that acid washing removes mechanically activated incorporated ZrO₂Impurities and SiO on the surface of beta-SiC₂And an oxide layer improves the sintering activity of the beta-SiC powder.

3. Effect analysis of pressureless sintering scheme

3-1, analysis of influence of sintering aid and process on density and mechanical property of SiC ceramic

Using activated and purified beta-SiC micropowder as raw material, adding B₄C and phenolic resin are used as sintering aids, and the influences of the addition amount of B, C, the sintering temperature and the heat preservation time on the densification and the mechanical properties of beta-SiC synthesized by pressureless sintering self-propagating combustion are researched. The effects of the addition amounts of B and C on the density of the SiC ceramics synthesized by self-propagating combustion were analyzed by comparing examples 1 to 4, and the results are shown in Table 3.

(1) C addition amount: the addition amount of C is within the range of 1-5 wt%, the density of the SiC ceramic is increased along with the increase of the addition amount of C, and when the addition amount of C reaches 4 wt%, the SiC density reaches the maximum value of 3.14g/cm³(ii) a As C continues to increase, the density decreases instead, since C and SiO mix during the sintering of the SiC ceramic₂SiC and CO are generated in a combined mode, so that the surface energy of the powder is increased, the grain boundary energy is reduced, the sintering activity of the beta-SiC powder is increased, the SiC can generate decomposition-sublimation reaction at the temperature higher than 1880 ℃, the sublimation of the SiC can be hindered by the existence of C, and the carbon is introduced to react with an oxide layer of the SiC. When the carbon addition is above 4 wt%, the oxide layer is no longer eliminated and carbon inclusions are formed in the sintered sample.

The hardness of the SiC ceramic tends to increase firstly and then decrease along with the increase of the addition amount of C, and the hardness of the SiC ceramic is 25.62 +/-0.92 GPa when the addition amount of C is 4 wt%; at the addition of 5 wt% of C, there is a decrease in hardness, which is similar to the tendency of the C addition content and the density of SiC ceramics, probably because C at the time of sintering SiC binds SiO of the SiC surface₂The residual C left in the SiC ceramic after reduction results in a reduction in hardness.

The fracture toughness of the SiC ceramic tends to increase first and then decrease along with the increase of the addition amount of C, and when the addition amount of C is 4 percent, the fracture toughness value of the SiC ceramic reaches a maximum of 4.84 +/-0.89MPa·m^1/2However, actually, the fluctuation of the fracture toughness value is small, and the average fracture toughness value is 4 to 5 MPa.m^1/2Compared with the alpha-SiC sintered in a solid phase, the fracture toughness of the alpha-SiC sintered in a solid phase is 3-4.5 MPa.m^1/2In other words, the value is slightly larger, which may be because the beta-SiC grains exhibit a long columnar structure, and have better toughness than the equiaxed grains of the alpha-SiC, and when cracks propagate, the long columnar grains of the beta-SiC are subjected to crack extension, deflection, bridging and the like, so that the fracture toughness of the SiC is improved.

The three-point bending strength of the SiC ceramic shows a tendency of increasing and then decreasing with the increase of the C content, the three-point bending strength of the SiC ceramic reaches a maximum value of 401.79 + -8.66 MPa when the C addition amount is 4 wt%, but the three-point bending strength of the SiC thereof decreases with the continued increase of C, which is probably due to the decrease of the SiC strength caused by the residual enrichment of C in SiC.

(2) B addition amount: the addition amount of B is in the range of 0.2-1.0 wt%, the density of the SiC ceramic is increased along with the increase of the content of B, and when the addition amount of B is 0.7 wt%, the density of the SiC ceramic reaches a maximum value of 3.14g/cm³(ii) a As the B content continues to increase, the density of the SiC ceramic decreases instead. B plays a role in reducing the grain boundary energy in the sintered SiC, namely under the conditions that the covalent property of the SiC is strong and the SiC is difficult to diffuse, a small amount of B can change the phenomenon, and when the content of B reaches 1 wt%, the density of the SiC ceramic is reduced on the contrary because the excessive addition of B causes the crystal grains of the SiC ceramic to be too long, so that the bonding among the crystal grains is not tight, and the density of the SiC ceramic is reduced.

The hardness of the SiC ceramic increases along with the addition of B until the hardness of the SiC ceramic reaches a maximum of 25.62 +/-0.92 GPa when the content of B is 0.7 wt%, and when B is continuously added to 1 wt%. The hardness is rather decreased, which is probably caused by abnormal growth of crystal grains of the SiC ceramic due to excessive introduction of B to cause non-tight bonding between the crystal grains, which is approximately the same tendency as the B addition amount and the SiC ceramic density.

The fracture toughness of the SiC ceramic increases with the addition of B, and the fracture toughness of the SiC ceramic is increased at 0.7 wt%The toughness value reaches the maximum and is 4.84 +/-0.84 MPa.m^1/2When B is continuously added to 1 wt%, the fracture toughness value of the SiC ceramic is rather lowered due to abnormal growth of crystal grains.

The three-point bending strength of the SiC ceramic increases with the addition of B, the maximum three-point bending strength is 401.79 +/-8.66 MPa when the addition of B is 0.7 wt%, but the three-point bending strength of SiC is greatly reduced with the continuous increase of B, which is probably because the excessive B is introduced to cause the abnormal growth of SiC crystal grains, so that the three-point bending strength of SiC is greatly reduced due to the untight combination of the SiC crystal grains.

(3) Sintering temperature: as can be seen from Table 3, when the sintering temperature is 2000 ℃, the density of the SiC ceramic is lower, and the number of pores is more, which results in lower hardness of the SiC ceramic; when the sintering temperature is 2100 ℃, the hardness of the SiC ceramic is improved to 25.62 +/-0.95 GPa; when the sintering temperature is continuously increased to 2130 ℃, the inter-grain bonding is not tight enough, the density is reduced, and the hardness is reduced to 21.43 +/-0.7 GPa. When the sintering temperature is 2100 ℃, the hardness of the SiC ceramic has the same trend with the change of density, shrinkage and ignition loss along with the time.

Firstly, since the density at 2000 ℃ is too low, a large amount of the particles exists in the particles, and the fracture toughness is measured by an indentation method, so that cracks are not obvious, the fracture toughness value at 2000 ℃ is abandoned, the sintering temperature is 2050 ℃, 2100 ℃ and 2130 ℃ shows a tendency of increasing firstly and then decreasing, the reason is also related to whether the grain size and the intercrystalline bonding are tight, and the fracture toughness reaches 4.84 +/-0.89 MPa.m at 2100 DEG^1/2It is noted here that the fracture toughness of ordinary α -SiC is generally 3 to 4MPa · m^1/2Because the crystal grains of the beta-SiC are rod-shaped, when cracks are expanded, the long columnar crystal grains of the SiC can have the functions of crack extension, deflection, bridging and the like, so that the fracture toughness is improved;

(4) sintering time: as can be seen from Table 3, when the holding time is 30min, the crystal grains on the surface of the ceramic are fine but not tightly combined, so that the hardness is only 21.16 +/-0.66 GPa; when the heat preservation time is prolonged to 60min, SiC ceramic grains grow gradually, the combination is tight, the pores are reduced, the compactness is improved, and the hardness is increased, until the hardness reaches the maximum value at 90min, and the hardness is 25.62 +/-0.62 GPa; when the heat preservation time is continuously prolonged to 120min, SiC crystal grains grow abnormally, so that the structures among the crystal grains are loose, and the hardness is reduced.

When the holding time is 90min, the SiC fracture toughness reaches the maximum value of 4.84 +/-0.89 MPa.m^1/2. This is because the bonding between the SiC grains is not tight enough at 60min, and the SiC grains are coarse and the bonding between the grains is not tight at 120min, resulting in low fracture toughness.

3-2. analysis of influence of sintering aid and process on SiC ceramic microstructure

(1) Fig. 6(a) - (d) SEM analysis of ceramic surfaces at different addition levels:

as shown in FIG. 6(a), when the amount of the sintering aid C added was 1 wt% and the B content was 0.2 wt%, the SiC ceramic structure was extremely loose. Since the amount of C added is insufficient, the silicon oxide layer cannot be completely removed, diffusion of Si and C to the neck portion is hindered by the presence of oxygen, and the SiC is sublimated due to the insufficient amount of C added, so that the internal structure of the SiC is loosened. The addition of a small amount of B may cause densification of SiC ceramics because a small amount of B is dissolved into SiC grains, lowering the grain boundary energy of SiC.

As shown in fig. 6(B), when the C addition amount is 3 wt% and when the B addition amount is 0.5 wt%, the SiC ceramic is substantially dense, the compactness degree reaches 97.2%, and only 1 wt.% of the C difference exists, because the C uniformly dispersed in the SiC hinders the sublimation of the SiC to stabilize it, and the inter-grain bonding is tight, but some grains grow abnormally, which may be because when phenolic resin is used as an organic carbon source, the carbon is distributed inside the SiC grains, the pinning effect at the grain boundary is not strong, and some SiC grains grow excessively.

As shown in FIG. 6(C), when the addition amount of C was increased to 4 wt% and when the addition amount of B was 1.0 wt%, the density of the SiC ceramic reached 3.14g/cm³The appropriate C content does not increase the density of the SiC ceramic much but the grain growth is more uniform, and the excessive growth of the SiC ceramic has fewer grains, which is also the reason for achieving the highest compactness.

As shown in FIG. 6(d), when the C content was increased to 5 wt% and when B was added to 0.7 wt%, large grains appeared on the surface of SiC, which had a SiC density of 3.12g/cm³There is a slight decrease because of the density decrease due to the excess enrichment of C. Further, when B is increased to 1 wt% and the density of silicon carbide is rather decreased, part of SiC is plate-like crystal grains having a small aspect ratio, because excessive B causes abnormal growth of SiC crystal grains.

(2) And (3) analyzing the microstructure of the SiC ceramic fracture under different addition amounts:

referring to fig. 7(a) - (c) showing the microstructures of the SiC ceramic sections of examples 2 to 4, it can be seen that the red circles show a river-like pattern, and thus the fracture mode is transgranular fracture, but it can also be seen that the SiC grains are long columnar or rod-like grains with a relatively complete and smooth surface, and thus the fracture mode is intergranular fracture, and thus the fracture mode is a mixed fracture mode of transgranular fracture and intergranular fracture. As can be seen from the comparison of the three-point bending strength data and the fracture surface structure, the SiC ceramic has the highest density and the highest strength because the addition amount of C is 4 wt/%, and the bonding between crystal grains is tight. Although the SiC ceramic density is close to each other at 3 wt% and 5 wt% C addition, the reason why the strength is far different is probably that the amount of carbon-rich in the SiC ceramic increases when the C addition is increased to 5 wt%, because the density and strength of C are lower than those of SiC, resulting in a decrease in the density and strength of the SiC ceramic.

Further, when the amount of B added is 1 wt%, the mixed fracture mode of transgranular fracture and intergranular fracture is obtained, and the fracture surface has abnormally large crystal grains and plate-like crystal grains having a small aspect ratio are formed, so that the bending strength is the lowest when the amount of B added is 1 wt%.

The above results show that: as in example 4, when B (B)₄C conversion) is 0.7 wt%, and when the C (phenolic resin conversion) is 4 wt%, under the conditions that the sintering temperature is 2100 ℃ and the heat preservation time is 90min, the prepared SiC ceramic has the best compactness and mechanical property, the loss on ignition is 7.9%, the shrinkage is 15.6%, and the density is 3.14g/m³(the compactness is 97.8 percent), the hardness is 25.62 +/-0.92 GPa, and the fracture toughness is 4.84 +/-0.84 MPa·m^1/2The three-point bending strength is 401.74 +/-8.66 GPa. Excessive addition of boron and carbon causes abnormal growth of SiC grains, and leads to a decrease in density and mechanical properties.

It should be noted that the embodiments described herein are only some embodiments of the present invention, and not all implementations of the present invention, and the embodiments are only examples, which are only used to provide a more intuitive and clear understanding of the present invention, and not to limit the technical solutions described in the present invention. All other embodiments, as well as other simple substitutions and various changes to the technical solutions of the present invention, which can be made by those skilled in the art without inventive work, are within the scope of the present invention without departing from the spirit of the present invention.

Claims

1. A method for preparing high-density ceramic by pressureless sintering of beta-SiC powder synthesized by self-propagating combustion is characterized by comprising the following steps:

(2) acid washing and purifying: placing the mechanically activated beta-SiC submicron powder into a container, wetting the beta-SiC submicron powder by using boiling distilled water, carrying out water bath at the temperature of 80 +/-5 ℃ to constant temperature, adding acid liquor once or twice, uniformly stirring, reacting for 30-90 min each time, adding the acid liquor which is mixed acid consisting of one or more of concentrated sulfuric acid, hydrofluoric acid or concentrated hydrochloric acid, cooling and discharging acid after the reaction is finished, and drying to obtain beta-SiC micropowder with the purity of more than 98%;

(4) dry pressing and forming: drying and sieving the slurry, placing the sieved mixed powder into a hydraulic forming machine, carrying out dry pressing forming under the pressure of 50-150 MPa to form a primary blank, and then placing the primary blank into a cold isostatic press to press the primary blank under the pressure of 150-250 MPa to form a blank body;

2. The pressureless sintering process for preparing high-density ceramic by synthesizing beta-SiC powder through self-propagating combustion according to claim 1, wherein the beta-SiC raw powder in the step (1) has an initial average particle size of 5.471 μm and a specific surface area of 6.34m²Per g, the purity is 90.4 percent, and the oxygen content is 2 weight percent.

3. The pressureless sintering process for preparing high-density ceramic by synthesizing beta-SiC powder through self-propagating combustion according to claim 1, wherein the solid weight ratio after adding water in the step (1) is 60%.

4. The method for preparing high-density ceramic through pressureless sintering of beta-SiC powder synthesized through self-propagating combustion according to claim 1, wherein the grain size of the zirconia ball-milling medium in the step (1) is 0.4-2 mm.

5. The method for preparing high-density ceramic through pressureless sintering of self-propagating combustion synthesis beta-SiC powder according to claim 1, wherein in the step (1), the ball milling time is 72 hours, 2mm zirconia balls are added and milled for 36 hours, then 0.4mm zirconia balls are replaced, and the milling is continued for 36 hours.

6. The pressureless sintering method for preparing high-density ceramic by synthesizing beta-SiC powder through self-propagating combustion according to claim 1, wherein the mass ratio of the concentrated sulfuric acid solution to the hydrofluoric acid solution in the mixed acid in the step (2) is 1: 1, wherein the mass concentration of concentrated sulfuric acid is not less than 90%, and the concentration of hydrofluoric acid solution is not less than 40%.

7. The pressureless sintering method for preparing high-density ceramic by synthesizing beta-SiC powder through self-propagating combustion according to claim 1, wherein the acid discharge step in the step (2) is as follows: adding tap water into the mixture, standing for layering, discharging supernatant, repeating the operation until the pH is 5, adding deionized water, naturally standing for layering, discharging supernatant, repeating the operation until the pH is 7, and discharging all the supernatants.

8. The method for preparing high-density ceramic through pressureless sintering of beta-SiC powder synthesized through self-propagating combustion according to claim 1, wherein the adding mass of the boiling distilled water in the step (2) is 1-3 times that of the beta-SiC powder.

9. The pressureless sintering self-propagating combustion synthesis method of beta-SiC powder as claimed in claim 1, wherein the addition amount of B in the step (3) is 0.7 wt%, the addition amount of C is 4 wt%, and the addition amount of dispersant is 10 wt%; the dispersing agent comprises the following components in parts by weight: 6-8 parts of ethanol, 0.3-2 parts of oleic acid, 0.4-1 part of polyvinyl alcohol and 0.1-1 part of tetramethyl ammonium hydroxide.

10. The pressureless sintering self-propagating combustion synthesis method of beta-SiC powder as claimed in claim 1, wherein the sintering temperature in the step (5) is 2100 ℃, and the heat preservation sintering time is 90 min.