Preparation method of boron nitride toughened high thermal conductivity silicon nitride ceramic

Abstract

The present invention discloses a method for preparing a high-thermal-conductivity silicon nitride ceramic toughened with boron nitride, belonging to the technical field of silicon nitride ceramics. The preparation method comprises the following steps: pretreatment of boron nitride fibers, vapor deposition, raw material mixing, compression molding, and gas pressure sintering. The vapor deposition method comprises the following steps: adopting an unbalanced magnetron sputtering method, using a boron carbide target and a magnesium oxide target as target materials to perform vapor deposition on the surface of activated boron nitride fibers. The high-thermal-conductivity silicon nitride ceramic prepared by the present invention can improve its toughness while maintaining its high thermal conductivity. The thermal conductivity of the prepared silicon nitride ceramic is 79.2-81.0 W / (m•K), the flexural strength is 738-745 MPa, and the fracture toughness is 11.3-11.6 MPa•m 1 / 2 , elastic modulus is 263~270GPa, density is 3.09~3.15g / cm 3 .

Description

Technical Field

[0001] The invention belongs to the technical field of silicon nitride ceramics, and particularly relates to a method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics. Background Art

[0002] Silicon nitride (Si3N4) ceramics, due to their excellent mechanical properties, dielectric properties, and thermal shock resistance, have become a popular material in structural ceramics, attracting widespread attention and in-depth research. Due to their excellent overall performance, they are widely used in fields such as metallurgy, aerospace, energy, machinery, military technology, optics, and the glass industry.

[0003] Si3N4 is a strong covalent bond compound with high atomic bonding strength and excellent comprehensive performance. In addition, due to the directionality and saturation of covalent bonds, there are very few slip systems in Si3N4 ceramics composed of covalent bonds, and they usually break before slip occurs, resulting in Si3N4 ceramics showing obvious brittle characteristics. The fracture toughness of Si3N4 ceramics is low, and they are very sensitive to local cracks inside the material. This has become a fatal flaw of Si3N4 ceramics, seriously affecting their service life and reliability, and greatly limiting their scope of application. In order to improve the fracture toughness of Si3N4 ceramics, the existing technology mainly focuses on material composite design, microstructure regulation and preparation process optimization. Among them, the better effect is to improve the toughness by adding fibers or whiskers, including second phase materials such as silicon carbide whiskers, silicon carbide fibers or boron nitride fibers, and using crack deflection, whisker pullout and other mechanisms to absorb fracture energy.

[0004] Boron nitride fibers / whiskers have excellent high-temperature stability, low dielectric loss, and a low thermal expansion coefficient close to that of silicon nitride, making them the preferred toughening material for silicon nitride ceramics. However, many problems arise when boron nitride fiber / whiskers are composited into a ceramic matrix, such as a strong interfacial reaction between the fiber and the matrix, and a significant decrease in the density of the composite material after the fiber is added. The strong interfacial reaction between the fiber and the matrix will weaken the toughening effect. Specifically, in terms of the properties of silicon nitride itself, the higher the sintering temperature (>1700°C), the better the thermal conductivity and mechanical properties. However, when composited with boron nitride fiber, in the area with low sintering temperature (1400-1600°C), the boron nitride fiber and the silicon nitride matrix form an interface through physical bonding and weak chemical bonds. This interface has moderate bonding strength, which can not only transmit loads but also allow crack deflection or fiber pullout, thereby improving fracture toughness through energy dissipation mechanism. When the sintering temperature is higher than 1700°C, boron nitride and silicon nitride generate a hard and brittle Si-BN ternary phase, forming a rigid interface, which hinders crack deflection and causes brittle fracture when external force is applied, and the fracture toughness cannot be effectively improved.

[0005] As a new generation of high-temperature structural materials, high thermal conductivity silicon nitride ceramics have always been restricted in their engineering applications by the contradiction between their excellent thermal conductivity and inherent brittleness. Although the introduction of boron nitride fibers or whiskers for toughening can significantly improve the fracture toughness of the material, there is an inherent conflict between the toughening mechanism and thermal conductivity, making it difficult to balance toughening and high thermal conductivity. In order to promote the application of high thermal conductivity silicon nitride ceramics in more fields, it is urgent to resolve the contradiction between its thermal conductivity and toughness. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the present invention provides a method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics. The prepared silicon nitride ceramics can maintain the high thermal conductivity of the silicon nitride ceramics while improving its toughness.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows: a method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics, comprising the following steps: boron nitride fiber pretreatment, vapor deposition, raw material mixing, pressing and forming, and gas pressure sintering;

[0008] The boron nitride fiber pretreatment method comprises: ultrasonically cleaning the boron nitride fiber with ethanol for 18 to 22 minutes, drying it, placing it in a nitric acid solution, treating it at 60 to 70° C. for 1 to 1.5 hours, taking it out, cleaning it, and drying it to obtain activated boron nitride fiber;

[0009] The boron nitride fiber has a diameter of 5 to 8 μm and an aspect ratio of 50,000 to 80,000:1;

[0010] The concentration of the nitric acid solution is 65-68 wt %.

[0011] The vapor deposition method is to use an unbalanced magnetron sputtering method with a boron carbide target and a magnesium oxide target as target materials. The vapor deposition method is to place the activated boron nitride fiber on a sample holder of a magnetron sputtering device and evacuate to 5.0×10 -3 Pa, introduce 16~18sccm of argon, apply a bias of -300~-400V on the substrate, ionize argon ions to sputter and clean the sample surface for 15~20min to remove oxides and other contaminants on the sample surface, then adjust the substrate bias to -70~-100V, set the argon flow rate to 25~28sccm, control the boron carbide target current to 1.5~2.0A, and deposit for 3.5~4h, then adjust the substrate bias to -180~-200V, control the magnesium oxide target current to 1.5~2.0A, keep the argon flow rate unchanged, and deposit the magnesium oxide surface for 15~20min. After the deposition is completed, take out the fiber and then chop it to obtain a composite coated boron nitride fiber;

[0012] The purity of the boron carbide target is ≥99.95%;

[0013] The purity of the magnesium oxide target is ≥99.95%.

[0014] The raw material mixing method comprises adding α-silicon nitride powder, composite-coated boron nitride fiber, sintering aid, β-silicon nitride seed crystals and ethanol into a ball mill, ball milling for 4-6 hours to obtain a silicon nitride suspension, filtering the suspension and drying it to obtain a mixed raw material;

[0015] The length of the composite coated boron nitride fiber is 10-15 mm;

[0016] The sintering aid is composed of Y2O3 and MgF2, and the mass ratio of Y2O3 to MgF2 is 3-4:2-3;

[0017] The aspect ratio of the β-silicon nitride seed crystal is 5 to 8:1;

[0018] The mass ratio of the α-silicon nitride powder, composite coated boron nitride fiber, sintering aid, β-silicon nitride seed crystal and ethanol is 100:10-15:5-7:3-5:70-90;

[0019] The balls and jars used in the ball milling are silicon nitride balls and nylon ball mill jars respectively;

[0020] The ball-to-material ratio is 2~4:1.

[0021] The compression molding method comprises placing the mixed raw materials into a mold, performing mechanical compression molding at a pressure of 60-80 MPa, and then performing cold isostatic pressing at a pressure of 120-140 MPa for 3-5 minutes to obtain a blank.

[0022] The gas pressure sintering method comprises placing the blank in a gas pressure sintering device, using nitrogen as a protective gas, a pressure of 1.2-2.0 MPa, maintaining the temperature at 1700-1800° C. for 2-3 hours, and then cooling and releasing the pressure to obtain silicon nitride ceramics.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The silicon nitride ceramics prepared by the method of the present invention can improve their toughness while maintaining their high thermal conductivity. The thermal conductivity is 79.2-81.0 W / (m·K), the bending strength is 738-745 MPa, and the fracture toughness is 11.3-11.6 MPa·m 1 / 2 , elastic modulus is 263~270GPa, density is 3.09~3.15g / cm 3 . DETAILED DESCRIPTION

[0025] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific embodiments of the present invention are now described.

[0026] Example 1

[0027] A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics is as follows:

[0028] 1. Boron nitride fiber pretreatment

[0029] Boron nitride fiber with a diameter of 5 μm and an aspect ratio of 50,000:1 was ultrasonically cleaned with ethanol for 18 minutes, dried, and then placed in a 65wt% nitric acid solution at 60°C for 1.5 hours. After being taken out, cleaned, and dried, the activated boron nitride fiber was obtained.

[0030] 2. Vapor deposition

[0031] The unbalanced magnetron sputtering method was used, with a 99.95% pure boron carbide target and a 99.95% pure magnesium oxide target as the target material. The activated boron nitride fiber was placed on the sample holder of the magnetron sputtering equipment and vacuumed to 4.8×10 -3 Pa, 16 sccm of argon was introduced, a bias of -300 V was applied to the substrate, and the ionized argon ions were sputtered to clean the sample surface for 20 minutes to remove oxides and other contaminants on the sample surface. Then the substrate bias was adjusted to -70 V, the argon flow rate was set to 25 sccm, the boron carbide target current was controlled to 1.5 A, and the deposition was carried out for 4 hours. Then the substrate bias was adjusted to -180 V, the magnesium oxide target current was controlled to 1.5 A, the argon flow rate remained unchanged, and the magnesium oxide surface layer was deposited for 20 minutes. After the deposition was completed, the fiber was taken out and then chopped to obtain a composite coated boron nitride fiber.

[0032] 3. Raw material mixing

[0033] α-Silicon nitride powder, composite-coated boron nitride fiber, Y2O3, MgF2, β-Silicon nitride seed crystals, and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and nylon ball mill jars, respectively. After ball milling for 4 hours at a ball-to-material ratio of 2:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0034] The length of the composite coating boron nitride fiber is 10 mm;

[0035] The aspect ratio of the β-silicon nitride seed crystal is 5:1;

[0036] The mass ratio of the α-silicon nitride powder, the composite-coated boron nitride fiber, Y2O3, MgF2, the β-silicon nitride seed crystal and the ethanol is 100:10:3:2:3:70.

[0037] 4. Pressing

[0038] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 60 MPa, and then subjected to cold isostatic pressing at a pressure of 120 MPa for 5 minutes to obtain a blank.

[0039] 5. Gas pressure sintering

[0040] The blank was placed in a gas pressure sintering device with nitrogen as protective gas at a pressure of 1.2 MPa and kept at 1700°C for 3 hours. After cooling and releasing the pressure, silicon nitride ceramics were obtained.

[0041] Example 2

[0042] A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics is as follows:

[0043] 1. Boron nitride fiber pretreatment

[0044] Boron nitride fibers with a diameter of 6 μm and an aspect ratio of 60,000:1 were ultrasonically cleaned with ethanol for 20 minutes, dried, and then placed in a 66 wt% nitric acid solution at 65°C for 1.2 hours. After removal, the fibers were washed and dried to obtain activated boron nitride fibers.

[0045] 2. Vapor deposition

[0046] The unbalanced magnetron sputtering method was used, with a 99.98% pure boron carbide target and a 99.98% pure magnesium oxide target as target materials. The activated boron nitride fiber was placed on the sample holder of the magnetron sputtering equipment and vacuumed to 4×10 -3 Pa, 17sccm of argon was introduced, a bias of -350V was applied to the substrate, and the ionized argon ions were sputtered to clean the sample surface for 18 minutes to remove oxides and other contaminants on the sample surface. Then the substrate bias was adjusted to -80V, the argon flow rate was set to 26sccm, the boron carbide target current was controlled to 1.8A, and the deposition was carried out for 3.8 hours. Then the substrate bias was adjusted to -190V, the magnesium oxide target current was controlled to 1.8A, the argon flow rate remained unchanged, and the magnesium oxide surface layer was deposited for 18 minutes. After the deposition was completed, the fiber was taken out and then chopped to obtain a composite coated boron nitride fiber.

[0047] 3. Raw material mixing

[0048] α-Silicon nitride powder, composite-coated boron nitride fiber, Y2O3, MgF2, β-Silicon nitride seed crystals, and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and nylon ball mill jars, respectively. After ball milling for 5 hours at a ball-to-material ratio of 3:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0049] The composite coating boron nitride fiber has a length of 12 mm;

[0050] The aspect ratio of the β-silicon nitride seed crystal is 6:1;

[0051] The mass ratio of the α-silicon nitride powder, the composite-coated boron nitride fiber, Y2O3, MgF2, the β-silicon nitride seed crystal and the ethanol is 100:12:3.5:2.5:4:80.

[0052] 4. Pressing

[0053] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 70 MPa, and then subjected to cold isostatic pressing at a pressure of 130 MPa for 4 minutes to obtain a blank.

[0054] 5. Gas pressure sintering

[0055] The blank was placed in a gas pressure sintering device with nitrogen as the protective gas and a pressure of 1.6 MPa, kept at 1750°C for 2.5 hours, and then cooled and the pressure was released to obtain silicon nitride ceramics.

[0056] Example 3

[0057] 1. Boron nitride fiber pretreatment

[0058] Boron nitride fibers with a diameter of 8 μm and an aspect ratio of 80,000:1 were ultrasonically cleaned with ethanol for 22 minutes, dried, and then placed in a 68 wt% nitric acid solution at 70°C for 1 hour. After removal, the fibers were cleaned and dried to obtain activated boron nitride fibers.

[0059] 2. Vapor deposition

[0060] The unbalanced magnetron sputtering method was used, with a 99.99% pure boron carbide target and a 99.99% pure magnesium oxide target as target materials. The activated boron nitride fiber was placed on the sample holder of the magnetron sputtering equipment and vacuumed to 3.5×10 -3 Pa, 18 sccm of argon is introduced, a bias of -400 V is applied to the substrate, and the ionized argon ions are sputtered to clean the sample surface for 15 minutes to remove oxides and other contaminants on the sample surface. Then the substrate bias is adjusted to -100 V, the argon flow rate is set to 28 sccm, the boron carbide target current is controlled to 2.0 A, and the deposition is carried out for 3.5 hours. Then the substrate bias is adjusted to -200 V, the magnesium oxide target current is controlled to 2.0 A, the argon flow rate remains unchanged, and the magnesium oxide surface layer is deposited for 15 minutes. After the deposition is completed, the fiber is taken out and then chopped to obtain a composite coated boron nitride fiber.

[0061] 3. Raw material mixing

[0062] α-Silicon nitride powder, composite-coated boron nitride fiber, Y2O3, MgF2, β-Silicon nitride seed crystals, and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and a nylon ball mill jar, respectively. After ball milling for 6 hours at a ball-to-material ratio of 4:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0063] The length of the composite coating boron nitride fiber is 15 mm;

[0064] The aspect ratio of the β-silicon nitride seed crystal is 8:1;

[0065] The mass ratio of the α-silicon nitride powder, the composite-coated boron nitride fiber, Y2O3, MgF2, the β-silicon nitride seed crystal and the ethanol is 100:15:4:3:5:90.

[0066] 4. Pressing

[0067] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 80 MPa, and then subjected to cold isostatic pressing at a pressure of 140 MPa for 3 minutes to obtain a blank.

[0068] 5. Gas pressure sintering

[0069] The blank was placed in a gas pressure sintering device, with nitrogen as the protective gas, a pressure of 2.0 MPa, and kept at 1800°C for 2 hours. After cooling and releasing the pressure, silicon nitride ceramics were obtained.

[0070] Example 4

[0071] A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics is as follows:

[0072] 1. Boron nitride whisker pretreatment

[0073] Boron nitride whiskers with a diameter of 6 μm and an aspect ratio of 500:1 were ultrasonically cleaned with ethanol for 20 minutes, dried, and then placed in a 66 wt% nitric acid solution at 65°C for 1.2 hours. After removal, they were cleaned and dried to obtain pretreated boron nitride whiskers.

[0074] 2. Vapor deposition

[0075] The unbalanced magnetron sputtering method was used, with a 99.98% pure boron carbide target and a 99.98% pure magnesium oxide target as the target material. The pretreated boron nitride whiskers were placed on the sample holder of the magnetron sputtering equipment and vacuumed to 4×10 -3Pa, 17sccm of argon was introduced, a bias of -350V was applied to the substrate, and the ionized argon ions were sputtered to clean the sample surface for 18 minutes to remove oxides and other contaminants on the sample surface. Then the substrate bias was adjusted to -80V, the argon flow rate was set to 26sccm, the boron carbide target current was controlled to 1.8A, and the deposition was carried out for 3.8 hours. Then the substrate bias was adjusted to -190V, the magnesium oxide target current was controlled to 1.8A, the argon flow rate remained unchanged, and the magnesium oxide surface layer was deposited for 18 minutes to obtain a composite coating boron nitride whisker.

[0076] 3. Raw material mixing

[0077] α-Silicon nitride powder, composite-coated boron nitride whiskers, Y2O3, MgF2, β-Silicon nitride seeds, and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and a nylon ball mill jar, respectively. After ball milling for 5 hours at a ball-to-material ratio of 3:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0078] The aspect ratio of the β-silicon nitride seed crystal is 6:1;

[0079] The mass ratio of the α-silicon nitride powder, the composite coating boron nitride whisker, Y2O3, MgF2, the β-silicon nitride seed crystal and the ethanol is 100:12:3.5:2.5:4:80.

[0080] 4. Pressing

[0081] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 70 MPa, and then subjected to cold isostatic pressing at a pressure of 130 MPa for 4 minutes to obtain a blank.

[0082] 5. Gas pressure sintering

[0083] The blank was placed in a gas pressure sintering device with nitrogen as the protective gas and a pressure of 1.6 MPa, kept at 1750°C for 2.5 hours, and then cooled and the pressure was released to obtain silicon nitride ceramics.

[0084] Example 5

[0085] A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics is as follows:

[0086] 1. Boron nitride fiber pretreatment

[0087] Boron nitride fibers with a diameter of 6 μm and an aspect ratio of 60,000:1 were ultrasonically cleaned with ethanol for 20 minutes, dried, and then placed in a 66 wt% nitric acid solution at 65°C for 1.2 hours. After removal, the fibers were cleaned and dried to obtain activated boron nitride fibers.

[0088] 2. Vapor deposition

[0089] The unbalanced magnetron sputtering method was used, and a boron carbide target with a purity of 99.98% was used as the target material. The activated boron nitride fiber was placed on the sample holder of the magnetron sputtering equipment and vacuumed to 4×10 -3 Pa, 17sccm of argon was introduced, a bias of -350V was applied to the substrate, and the sample surface was sputtered and cleaned by ionized argon ions for 18 minutes to remove oxides and other contaminants on the sample surface. Then the substrate bias was adjusted to -80V, the argon flow rate was set to 26sccm, the boron carbide target current was controlled to 1.8A, and the deposition was carried out for 3.8 hours. After the deposition was completed, the fiber was taken out and then chopped to obtain a composite coated boron nitride fiber.

[0090] 3. Raw material mixing

[0091] α-Silicon nitride powder, composite-coated boron nitride fiber, Y2O3, MgF2, β-Silicon nitride seed crystals, and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and nylon ball mill jars, respectively. After ball milling for 5 hours at a ball-to-material ratio of 3:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0092] The composite coating boron nitride fiber has a length of 12 mm;

[0093] The aspect ratio of the β-silicon nitride seed crystal is 6:1;

[0094] The mass ratio of the α-silicon nitride powder, the composite-coated boron nitride fiber, Y2O3, MgF2, the β-silicon nitride seed crystal and the ethanol is 100:12:3.5:2.5:4:80.

[0095] 4. Pressing

[0096] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 70 MPa, and then subjected to cold isostatic pressing at a pressure of 130 MPa for 4 minutes to obtain a blank.

[0097] 5. Gas pressure sintering

[0098] The blank was placed in a gas pressure sintering device with nitrogen as the protective gas and a pressure of 1.6 MPa, kept at 1750°C for 2.5 hours, and then cooled and the pressure was released to obtain silicon nitride ceramics.

[0099] Example 6

[0100] A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics is as follows:

[0101] 1. Boron nitride fiber pretreatment

[0102] Boron nitride fibers with a diameter of 6 μm and a fiber length of 12 mm were ultrasonically cleaned with ethanol for 20 minutes, dried, and then placed in a 66 wt% nitric acid solution at 65°C for 1.2 hours. After being taken out, cleaned, and dried, the activated boron nitride fibers were obtained.

[0103] 2. Raw material mixing

[0104] α-Silicon nitride powder, activated boron nitride fiber, Y2O3, MgF2, β-Silicon nitride seed crystals and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and nylon ball mill jars, respectively. After ball milling for 5 hours at a ball-to-material ratio of 3:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0105] The aspect ratio of the β-silicon nitride seed crystal is 6:1;

[0106] The mass ratio of the α-silicon nitride powder, activated boron nitride fiber, Y2O3, MgF2, β-silicon nitride seed crystal and ethanol is 100:12:3.5:2.5:4:80.

[0107] 3. Pressing

[0108] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 70 MPa, and then subjected to cold isostatic pressing at a pressure of 130 MPa for 4 minutes to obtain a blank.

[0109] 4. Gas pressure sintering

[0110] The blank was placed in a gas pressure sintering device with nitrogen as the protective gas and a pressure of 1.6 MPa, kept at 1750°C for 2.5 hours, and then cooled and the pressure was released to obtain silicon nitride ceramics.

[0111] Example 7

[0112] A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics is as follows:

[0113] 1. Boron nitride fiber pretreatment

[0114] Boron nitride fibers with a diameter of 6 μm and a fiber length of 12 mm were ultrasonically cleaned with ethanol for 20 minutes, dried, and then placed in a 66 wt% nitric acid solution at 65°C for 1.2 hours. After being taken out, cleaned, and dried, the activated boron nitride fibers were obtained.

[0115] 2. Raw material mixing

[0116] α-Silicon nitride powder, activated boron nitride fiber, Y2O3, MgF2, β-Silicon nitride seed crystals and ethanol were added to a ball mill. The balls and jar used for ball milling were silicon nitride balls and nylon ball mill jars, respectively. After ball milling for 5 hours at a ball-to-material ratio of 3:1, a silicon nitride suspension was obtained. The suspension was filtered and dried to obtain a mixed raw material.

[0117] The aspect ratio of the β-silicon nitride seed crystal is 6:1;

[0118] The mass ratio of the α-silicon nitride powder, activated boron nitride fiber, Y2O3, MgF2, β-silicon nitride seed crystal and ethanol is 100:12:3.5:2.5:4:80.

[0119] 3. Pressing

[0120] The mixed raw materials were placed in a mold and mechanically pressed at a pressure of 70 MPa, and then subjected to cold isostatic pressing at a pressure of 130 MPa for 4 minutes to obtain a blank.

[0121] 4. Gas pressure sintering

[0122] The blank was placed in a gas pressure sintering device with nitrogen as protective gas at a pressure of 1.6 MPa and kept at 1500°C for 2.5 hours. After cooling and releasing the pressure, silicon nitride ceramics were obtained.

[0123] Test example

[0124] The thermal conductivity, flexural strength, fracture toughness, elastic modulus, and density of the silicon nitride ceramics prepared in Examples 1-7 were tested. The thermal conductivity was measured using the transient plane heat source method, the flexural strength was calculated using the three-point bending method, the fracture toughness was tested using the indentation method, the elastic modulus was measured using the three-point bending method, and the density was measured using the Archimedes drainage method. The test results are shown in Table 1.

[0125] Table 1 Thermal conductivity, flexural strength, fracture toughness, elastic modulus and density of silicon nitride ceramics

[0126]

[0127] The present invention uses boron nitride fiber to toughen silicon nitride ceramics. To address the poor toughening effect caused by the high interfacial forces when sintering boron nitride fibers at temperatures above 1700°C, a double coating of boron carbide and magnesium oxide is deposited on the boron nitride fibers. The deposition takes 15 to 20 minutes to form a thin magnesium oxide layer, which dissolves to form a weak interfacial layer, allowing for moderate fiber bonding and balancing pull-out efficiency with load transfer efficiency. Furthermore, magnesium oxide has high stability in a high-temperature nitrogen environment, inhibiting surface oxidation of the boron nitride fibers and maintaining fiber performance. Boron carbide has a thermal expansion coefficient between the radial values ​​of silicon nitride and boron nitride fibers. The boron carbide coating can alleviate interfacial thermal adaptation stress and avoid transverse fracture caused by boron nitride's high radial thermal expansion coefficient. Furthermore, the high hardness of boron carbide can guide crack propagation along the coating / substrate interface, promoting fiber pull-out rather than direct fracture, thereby improving the toughness of the resulting silicon nitride ceramic.

[0128] Example 4 uses boron nitride whiskers as the toughening material. Compared with boron nitride fiber toughening material, the obtained silicon nitride ceramic has slightly higher elastic modulus and fracture toughness, and slightly lower thermal conductivity and bending strength, because the fibers form a continuous heat conduction path to enhance heat transfer; long fibers effectively inhibit crack propagation by bridging cracks and bearing loads, and whiskers are short and randomly distributed, and their blocking effect on the main crack is weaker than that of continuous fibers. However, whiskers can increase fracture toughness through crack deflection, whisker breakage and small amount of pullout, and the effect of increasing fracture toughness is better than that of fibers.

[0129] In Example 5, only a layer of boron carbide coating is deposited on the outside of the boron nitride fiber. The values ​​of thermal conductivity, flexural strength, and fracture toughness are slightly lower than those in Example 2, indicating that a layer of magnesium oxide deposited on the outer surface of the boron carbide coating can improve the thermal conductivity and mechanical properties of the obtained silicon nitride ceramic.

[0130] In Example 6, boron nitride fibers without any treatment are directly used to toughen silicon nitride ceramics. The thermal conductivity, flexural strength, fracture toughness, elastic modulus and density of the obtained ceramics are lower than those in Example 2.

[0131] In Example 7, based on Example 6, the sintering temperature is lowered. At a sintering temperature of 1500° C., the fracture toughness of the silicon nitride ceramic obtained is improved, but the thermal conductivity and flexural strength are reduced.

[0132] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing boron nitride toughened high thermal conductivity silicon nitride ceramics, characterized in that: The preparation method comprises the following steps: pretreatment of boron nitride fiber, vapor deposition, raw material mixing, pressing and forming, and gas pressure sintering; The vapor deposition method is to use an unbalanced magnetron sputtering method, with a boron carbide target and a magnesium oxide target as target materials, place the activated boron nitride fiber on the sample holder of the magnetron sputtering equipment, and evacuate to 5.0×10 -3 Pa, introduce 16~18sccm of argon, apply a bias of -300~-400V on the substrate, ionize argon ions to sputter and clean the sample surface for 15~20min, then adjust the substrate bias to -70~-100V, set the argon flow rate to 25~28sccm, control the boron carbide target current to 1.5~2.0A, and deposit for 3.5~4h, then adjust the substrate bias to -180~-200V, control the magnesium oxide target current to 1.5~2.0A, keep the argon flow rate unchanged, and deposit the magnesium oxide surface for 15~20min. After the deposition is completed, take out the fiber and then cut it into pieces to obtain a composite coated boron nitride fiber; The raw material mixing method comprises the following steps: adding α-silicon nitride powder, composite-coated boron nitride fiber, sintering aid, β-silicon nitride seed crystals and ethanol into a ball mill, ball milling for 4-6 hours to obtain a silicon nitride suspension, filtering the suspension and drying the suspension to obtain a mixed raw material.

2. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: In the vapor deposition step, the purity of the boron carbide target is ≥99.95%, and the purity of the magnesium oxide target is ≥99.95%.

3. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: The boron nitride fiber pretreatment method comprises the following steps: ultrasonically cleaning the boron nitride fiber with ethanol for 18 to 22 minutes, drying the fiber, placing the fiber in a nitric acid solution, treating the fiber at 60 to 70° C. for 1 to 1.5 hours, and then removing the fiber from the solution, cleaning the fiber, and drying the fiber to obtain activated boron nitride fiber.

4. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 3, characterized in that: In the step of pretreating the boron nitride fiber, the diameter of the boron nitride fiber is 5-8 μm, the aspect ratio is 50,000-80,000:1; and the concentration of the nitric acid solution is 65-68 wt%.

5. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: In the step of mixing the raw materials, The length of the composite coated boron nitride fiber is 10-15mm; The sintering aid is composed of Y2O3 and MgF2, and the mass ratio of Y2O3 to MgF2 is 3~4:2~3; The aspect ratio of the β-silicon nitride seed crystal is 5 to 8:1; The mass ratio of α-silicon nitride powder, composite coated boron nitride fiber, sintering aid, β-silicon nitride seed crystal and ethanol is 100:10~15:5~7:3~5:70-90.

6. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: In the step of mixing the raw materials, the balls and jars used for ball milling are silicon nitride balls and nylon ball mill jars respectively; the ball-to-material ratio is 2-4:

1.

7. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: The compression molding method comprises placing the mixed raw materials into a mold, performing mechanical compression molding at a pressure of 60-80 MPa, and then performing cold isostatic pressing at a pressure of 120-140 MPa for 3-5 minutes to obtain a blank.

8. The method for preparing a boron nitride toughened high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: The gas pressure sintering method comprises placing the blank in a gas pressure sintering device, using nitrogen as a protective gas, a pressure of 1.2-2.0 MPa, maintaining the temperature at 1700-1800° C. for 2-3 hours, and then cooling and releasing the pressure to obtain silicon nitride ceramics.

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