A mold, silicon nitride ball valve core, and preparation method and application thereof

Abstract

The present invention relates to the field of ceramic materials, and discloses a mold, a silicon nitride ball valve core, and a preparation method and application thereof. A mold having the shape of a ball valve core is formed by enclosing a base, a top seat, a side mold, and a core mold to achieve a better molding effect. Silicon nitride powder, a sintering aid, tungsten carbide, and a binder are mixed, put into a mold for molding, debinding, and sintering to obtain a silicon nitride ceramic ball valve core. The uniformity of silicon nitride grains is improved by adding tungsten carbide, and the adhesive helps to evenly arrange the ceramic slurry particles, prevent particle sedimentation and aggregation, optimize the particle grading of the spherical mixed granulation powder, and use a mold to avoid manual errors. In summary, the preparation method of the present invention not only effectively avoids the problem of valve core eccentricity, but also has significant improvements in production efficiency, cost control, and product quality stability, thereby improving the product qualification rate and being suitable for large-scale production.

Description

Technical Field

[0001] The invention relates to the field of ceramic materials, and in particular to a mold, a silicon nitride ball valve core, and a preparation method and application thereof. Background Art

[0002] Ball valves are important and indispensable fluid control components in modern industries such as metallurgy and petrochemicals. They are mainly used to control the on / off, flow direction, flow rate, pressure and temperature of fluids in pipelines. Silicon nitride ceramics are an advanced structural ceramic material with unique high strength, high hardness, wear resistance, corrosion resistance, high temperature resistance and reliability. They have been widely used in metallurgy, coal chemical industry, petrochemical industry, transportation of positive and negative electrode materials for lithium batteries and other fields.

[0003] However, silicon nitride ceramic ball valves are prone to eccentricity during the manufacturing process, resulting in uneven contact between the ball and the valve seat, which causes uneven wear of the ball and the valve seat and affects the sealing effect, thereby reducing the service life of the ceramic ball valve. Secondly, the eccentricity of the ball valve may increase the operating torque required to open and close the valve, which not only imposes an additional burden on the valve drive device but also deteriorates the valve's adjustment performance, making it difficult to achieve precise control, thereby affecting the stability and reliability of the entire valve. Summary of the invention

[0004] In order to solve the problem that silicon nitride ceramics are easily eccentric during the preparation process, resulting in uneven contact between the ball and the valve seat of the ball valve, affecting the sealing effect and service life of the ceramic ball valve, the present invention provides a mold, a silicon nitride ball valve core and a preparation method and application thereof, which adopts mold molding and adds tungsten carbide balls by mechanical ball milling to avoid the problem of eccentricity of the ball and improve the qualified rate of silicon nitride ball valves.

[0005] In a first aspect, the present invention provides a mold, comprising a base, a top seat, a side mold and a core mold; the base and the top seat are both provided with an annular groove, the upper and lower ends of the side mold are respectively embedded in the annular grooves of the base and the top seat, and the base, the top seat and the side mold are enclosed to form a cavity;

[0006] The core mold is arranged in the chamber, the top end and the upper outer wall of the core mold are in contact with the top seat groove of the top seat, and the top seat groove is a groove formed by the lower bottom surface of the top seat and the outer wall of the annular groove; the bottom end and the lower outer wall of the core mold are in contact with the base groove of the base, and the base groove is a groove formed by the upper top surface of the base and the outer wall of the annular groove;

[0007] The inner wall of the side mold is a curved surface, and the core mold is inserted into the chamber to form an injection cavity in the shape of a ball valve core; the ball valve core is a spherical body with a through hole in the center.

[0008] In an optional embodiment, the upper outer wall of the side mold is provided with a first limiting protrusion, the inner wall of the annular groove of the top seat is provided with a first groove, and the first limiting protrusion is matched with the first groove; the lower outer wall of the side mold is provided with a second limiting protrusion, the inner wall of the annular groove of the base is provided with a second groove, and the second limiting protrusion is matched with the second groove;

[0009] And / or, the side mold is formed by plugging and fixing a left half side mold and a right half side mold, and the left half side mold is symmetrical with the center of the right half side mold; wherein, plug blocks are provided on both sides of the left half side mold, and sockets are provided on both sides of the right half side mold, and the plug blocks are plugged into the sockets to plug and fix the left half side mold and the right half side mold;

[0010] And / or, the core mold is a cylinder.

[0011] In a second aspect, the present invention provides a method for preparing a silicon nitride ball valve core.

[0012] A method for preparing a silicon nitride ball valve core comprises the following steps:

[0013] Tungsten carbide balls are added to a mixture of silicon nitride powder and a sintering aid for ball milling to form an intermediate; a binder is then added for spheroidization to form a mixed granulated powder; the particle size of the tungsten carbide balls is 5-25 mm;

[0014] Putting the mixed granulated powder into the mold for pressing and molding to obtain a valve core blank;

[0015] The prepared valve core blank is debinded and then sintered in a nitrogen atmosphere to obtain a silicon nitride ceramic ball valve.

[0016] In an optional embodiment, the sintering aid includes one or more of yttrium oxide, aluminum oxide, magnesium oxide, lanthanum oxide, gadolinium oxide, and lutetium oxide;

[0017] And / or, the binder is at least one or more of PEG, PVB, PEG and PVB;

[0018] and / or, the particle size D50 of the silicon nitride powder is ≤1 μm, wherein the metal content is <0.05wt%, and the α-phase content is ≥92wt%;

[0019] And / or, the particle size D50 of the mixed granulated powder is 100-200 μm.

[0020] In an optional embodiment, the mass ratio of silicon nitride powder, sintering aid, and tungsten carbide ball is (86-93): (5-8): (2-6);

[0021] and / or, the mass ratio of the binder to the intermediate is (3-5): (95-97);

[0022] And / or, the intermediate is mixed with a binder and then spray granulated to form the mixed granulated powder, wherein the inlet temperature of the spray granulation is 250°C-300°C and the outlet temperature is 100-150°C.

[0023] In an optional embodiment, tungsten carbide balls and an organic solvent are added to a mixed powder of silicon nitride powder and a sintering aid, and the mixture is ball-milled and dried to form an intermediate;

[0024] The tungsten carbide balls include large balls, medium balls and small balls; the particle size of the large balls is 20-25 mm, the particle size of the medium balls is 12-18 mm, and the particle size of the small balls is 5-10 mm; the weight ratio of the large balls, medium balls and small balls is 1:2:2;

[0025] And / or, the ball milling speed is 200-300 r / min, and the ball milling time is 15-30 h;

[0026] and / or, during the ball milling process, the weight ratio of the mixed powder to the organic solvent is 1:(1.2-1.8);

[0027] And / or, the drying temperature is 80-100°C.

[0028] In an optional embodiment, the press molding includes cold isostatic pressing with a pressure of 160-200 MPa and holding the pressure for 100-200 seconds;

[0029] And / or, during the debinding process, carbon dioxide is introduced, and the material is heated to 1000-1100° C. and kept warm for 60-180 min, with a heating rate of 5-10° C. / min.

[0030] In an optional embodiment, the sintering process adopts the following process:

[0031] S1. Evacuate to below 50Pa, and heat from room temperature to 1100-1200℃ at a heating rate of 6-10℃ / min. During this sintering process, keep evacuating to below 50Pa;

[0032] S2. Raise the temperature to 1350-1450°C at a heating rate of 2-5°C / min and keep warm for 50-100 min, and introduce nitrogen until the pressure reaches 0.3-1MPa;

[0033] S3, heating up to 1600-1700°C at a heating rate of 1-2°C / min and keeping at that temperature for 50-100min, with the nitrogen atmosphere pressure being 1.5-2.5MPa;

[0034] S4. Heat to 1800-1830°C at a heating rate of 1-2°C / min and keep warm for 60-150 min. The nitrogen atmosphere pressure is 3-4 MPa.

[0035] In a third aspect, the present invention provides a silicon nitride ball valve core, which is manufactured using the method for manufacturing a silicon nitride ball valve core;

[0036] The outer diameter of the valve core of the silicon nitride ball valve is less than 150 mm.

[0037] In a fourth aspect, the present invention provides a ball valve, wherein the ball valve adopts the silicon nitride ball valve core.

[0038] In an optional embodiment, the ball valve core is a spherical body with a through hole in the center, and a switch groove is provided on the outer diameter of one side of the spherical body; the spherical body is arranged in the valve body, the central through hole of the spherical body is connected to the channel of the valve body, and the spherical body is connected to the handle through the switch groove and the valve stem;

[0039] The groove of the ball valve is provided with a metal valve stem seat by using strong glue;

[0040] And / or, an anti-corrosion coating material is sprayed on the connection between the metal valve stem seat and the valve stem, and the anti-corrosion coating material includes one or more of PFA, PTFE, and PEEK.

[0041] The technical solution of the present invention has the following advantages:

[0042] The present invention provides a mold, including a base, a top seat, a side mold and a core mold; the base and the top seat are both provided with an annular groove, the upper and lower ends of the side mold are respectively embedded in the annular grooves of the base and the top seat, and the base, the top seat and the side mold are enclosed to form a chamber; the core mold is inserted into the chamber to form an injection cavity having the shape of the ball valve core. According to the shape of the ball valve core, by using various components in combination, a better molding effect can be achieved, and the eccentricity problem of the ball valve core can be effectively avoided.

[0043] The preparation method of the silicon nitride ball valve core provided by the present invention comprises the following steps: adding tungsten carbide balls to a mixture of silicon nitride powder and a sintering aid for ball milling to form an intermediate; then adding a binder for spheroidization to form a mixed granulated powder; the particle size of the tungsten carbide balls is 5-25 mm, and the balls are put into a mold for compression molding, debinding, and then sintering in a nitrogen atmosphere to obtain a silicon nitride ceramic ball valve core. The tungsten carbide balls are used as ball milling media and also as raw materials, which improves the uniformity of silicon nitride grains; the binder helps to uniformly arrange the ceramic slurry particles, prevents the particles from settling and agglomerating, and makes the size and shape of the mixed granulated powder particles treated by spheroidization basically consistent; the use of a mold avoids manual errors, and the molding effect is better. In summary, the preparation method of the present invention not only effectively avoids the problem of valve core eccentricity, but also has significant improvements in production efficiency, cost control, and product quality stability, improves the qualified rate of the product, and is suitable for large-scale production.

[0044] Additional aspects and advantages of embodiments of the present invention will be described and shown in part in the following description, or will be explained through the implementation of embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0046] Figure 1 is a schematic structural diagram of a mold in an embodiment of the present invention;

[0047] Figure 2 is a cross-sectional view of a mold in an embodiment of the present invention;

[0048] Figure 3 The mold in the embodiment of the present invention is not equipped with a top seat;

[0049] Figure 4 It is a schematic structural diagram of the cylindrical mold in Comparative Example 2 of the present invention;

[0050] Figure 5 It is a cross-sectional view of the cylindrical mold in Comparative Example 2 of the present invention.

[0051] Description of reference numerals:

[0052] 1. Base; 2. Top seat; 3. Side mold; 4. Core mold; 5. Mixed granulation powder; 31. Left half side mold; 32. Right half side mold; 311. First limiting protrusion; 312. Second limiting protrusion; 33. Insert block. DETAILED DESCRIPTION

[0053] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0054] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which the present invention belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention; the terms "including" and "having" in the description and claims of the present invention and any variations thereof are intended to cover non-exclusive inclusions.

[0055] In the description of the embodiments of the present invention, “plurality” means more than two, unless otherwise clearly and specifically defined.

[0056] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present invention. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0057] In the present invention, the term "and / or" is merely a term used to describe the association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B may represent three situations: A exists alone, A and B exist at the same time, and B exists alone.

[0058] In the present invention, when it comes to percentage content, unless otherwise specified, for solid-liquid mixing and solid-solid mixing, it refers to mass percentage, and for liquid-liquid mixing, it refers to volume percentage.

[0059] In the present invention, wt% refers to weight percentage, and vol% refers to volume percentage.

[0060] In the prior art, silicon nitride powder is used to prepare ball valve cores. During the preparation process, a large amount of machining is required after the core is pressed into a cylindrical shape. Due to the low precision of existing machine tools or improper manual operation, the ceramic ball valve may easily become eccentric.

[0061] In order to solve the problems existing in the above-mentioned related technologies, according to a first aspect of the present invention, the present invention provides a mold, the mold comprising a detachable base, a top seat, a side mold and a core mold; the base and the top seat are both provided with an annular groove, the upper and lower ends of the side mold are respectively embedded in the annular grooves of the base and the top seat, and the base, the top seat and the side mold are enclosed to form a cavity;

[0062] The core mold is arranged in the chamber, the top end and the upper outer wall of the core mold are in contact with the top seat groove of the top seat, and the top seat groove is a groove formed by the lower bottom surface of the top seat and the outer wall of the annular groove; the bottom end and the lower outer wall of the core mold are in contact with the base groove of the base, and the base groove is a groove formed by the upper top surface of the base and the outer wall of the annular groove;

[0063] The inner wall of the side mold is a curved surface. The core mold is inserted into the chamber to form an injection cavity having the shape of the ball valve core. The ball valve core is a spherical body with a through hole at the center.

[0064] A spherical space with a through hole in the center is formed by enclosing the base, top seat, side mold and core mold. According to the shape of the ball valve core, the various components are used in coordination to achieve a better molding effect, effectively avoid the eccentricity problem of the ball valve core, and facilitate demoulding.

[0065] The core mold is made of steel material to avoid the eccentricity of the ball valve to the greatest extent; the side mold, base, and top seat are all made of rubber material with a Shore hardness of 60-70 degrees Shore D, preferably 65 degrees Shore D, to avoid the deformation of the ball valve to the greatest extent; the inner wall of the side mold is a curved surface that matches the bottom arc of the top seat to form the shape of the ball valve core, which is better pressed and not easy to be eccentric. Due to the large-sized and complex cold isostatic pressing molds, the mold cost and the difficulty of degumming large-sized thick-walled ball valves must be considered in the design and manufacture. The mold provided by the present invention is suitable for medium to small sizes, and the outer diameter of the ball valve core is less than 150mm.

[0066] When in use, the base, side mold and core mold are installed, the mixed granulation powder is injected from the middle hollow part of the side mold and the core mold, and then the top seat is installed, and cold isostatic pressing is performed. After the mold is opened, it is taken out to obtain the valve core blank of the silicon nitride ball valve core.

[0067] During the cold isostatic pressing process, the outer periphery of the side mold is fixed with a clamp, which is more conducive to maintaining the shape of the ceramic ball valve blank and avoiding problems such as eccentricity.

[0068] In some embodiments, the upper outer wall of the side mold is provided with a first limiting protrusion, the inner wall of the annular groove of the top seat is provided with a first groove, and the first limiting protrusion is matched with the first groove; the lower outer wall of the side mold is provided with a second limiting protrusion, the inner wall of the annular groove of the base is provided with a second groove, and the second limiting protrusion is matched with the second groove;

[0069] And / or, the side mold is formed by plugging and fixing a left half side mold and a right half side mold, and the left half side mold is symmetrical with the center of the right half side mold; wherein the left half side mold is provided with an insert block, and the right half side mold is provided with a socket, and the insert block is inserted into the socket to plug and fix the left half side mold and the right half side mold;

[0070] And / or, the core mold is a cylinder.

[0071] The second groove of the base matches the first limiting protrusion of the side mold, which is convenient for assembly and plays a role in positioning and fixing; the side mold is divided into two parts, which is convenient for taking out the ball valve blank, and the two parts are for the convenience of taking out the ball valve blank. At the same time, a plug block and a socket are provided at the connection part of the two parts of the side mold to prevent liquid from penetrating into the ball valve blank during the cold isostatic pressing process.

[0072] There are many factors that affect the eccentricity of the valve core of silicon nitride ball valves. In addition to the precision of machine tools and the influence of manual operation mentioned above, there are also the uniformity of raw materials, the molding process of valve core blanks, the sintering process, etc.; the uniformity of ceramic raw materials is one of the key factors affecting the quality of ceramic products. The uniformity of raw materials refers to the uniformity of the distribution of each component in the raw materials of the valve core blank, including particle size, shape, chemical composition, etc. The factors affecting the uniformity of ceramic raw materials mainly include the following aspects: 1. Pretreatment of raw materials: Appropriate pretreatment of raw materials (such as drying, crushing, screening, etc.) can improve their particle size and distribution, thereby improving mixing uniformity. 2. Mixing process of raw materials: Mixing is a key step to ensure the uniformity of ceramic raw materials. The selection of mixing equipment, the length of mixing time, the speed of mixing, and the mixing method (such as dry mixing or wet mixing) will affect the uniformity of the final raw materials. 3. The use of additives: The type, amount and method of adding additives need to be precisely controlled. Additives added to ceramic raw materials (such as binders, dispersants, etc.) will also affect mixing uniformity. If there is uneven stress or density during the molding or sintering process, it may also lead to inconsistent shrinkage in all directions. Or, in the subsequent processing or use process, the contact surface between the valve core and the valve seat may change due to uneven wear of the valve core, causing eccentricity.

[0073] In a second aspect, the present invention provides a method for preparing a silicon nitride ball valve core, comprising the following steps:

[0074] Tungsten carbide balls are added to a mixture of silicon nitride powder and a sintering aid for ball milling to form an intermediate; a binder is then added for spheroidization to form a mixed granulated powder; the particle size of the tungsten carbide balls is 5-25 mm;

[0075] Putting the mixed granulated powder into the mold and pressing it into shape to obtain a valve core blank;

[0076] The prepared valve core blank is debinded and then sintered in a nitrogen atmosphere to obtain a silicon nitride ceramic ball valve.

[0077] Before adding the tungsten carbide balls, the silicon nitride powder is first mixed with the sintering aid. The tungsten carbide balls can be used as grinding balls. The amount of tungsten carbide balls added is reduced by the collision and friction between the tungsten carbide grinding balls and the mixed powder and the ball mill, thereby increasing the amount of tungsten carbide particles introduced into the mixed powder. As the ball milling time continues to increase, the amount of tungsten carbide introduced also continues to increase. The present invention adds a small amount of tungsten carbide balls and uses the tungsten carbide balls as ball milling media and raw materials to produce a pinning effect on the silicon nitride matrix at a microscopic level, thereby hindering the abnormal growth of silicon nitride grains. In addition, the ball milling of the tungsten carbide balls makes the raw material mixing more uniform and delicate, thereby improving the uniformity of the silicon nitride grains. The tungsten carbide balls can also effectively promote the transformation of silicon nitride from the α phase to the β phase, thereby improving the densification and mechanical properties of silicon nitride ceramics.

[0078] The binder helps to evenly arrange the ceramic slurry particles, prevents the particles from settling and agglomerating, optimizes the particle size distribution of the mixed granulation powder after spheroidization, and is beneficial to the uniformity of green billet forming.

[0079] The present invention adopts mold pressing and forming without turning, which solves the problem of valve core eccentricity caused by low precision of existing machine tools or improper manual operation. At the same time, a special mold is adopted to achieve good pressing and forming effect and a smooth valve core surface. The various components of the mold are properly distributed, and the stress of the blank is uniform during the forming process, the raw materials are uniformly distributed, and the density is uniform, which effectively avoids the problem of spherical eccentricity and improves the qualified rate of the product.

[0080] The residual carbon after thermal decomposition of the binder will inhibit the densification process of silicon nitride, causing the pore size to become larger and more numerous, reducing the strength of the silicon nitride ball valve. Therefore, the present invention needs to debind the valve core blank before sintering to reduce the impact of residual carbon.

[0081] It can be seen that in the preparation method of the present invention, tungsten carbide balls improve the uniformity of silicon nitride grains, the adhesive helps to evenly arrange the ceramic slurry particles, prevents particle sedimentation and aggregation, optimizes the particle grading of the mixed granulation powder after spheroidization treatment, and the use of molds avoids human errors. In summary, the preparation method of the present invention not only effectively avoids the problem of valve core eccentricity, but also has obvious improvements in production efficiency, cost control and product quality stability, thereby improving the product qualification rate and being suitable for large-scale production.

[0082] In some embodiments, the mass ratio of silicon nitride powder, sintering aid, and tungsten carbide ball is (86-93): (5-8): (2-6);

[0083] and / or, the mass ratio of the binder to the intermediate is (3-5): (95-97);

[0084] and / or, the intermediate is mixed with a binder and then spray granulated to form the mixed granulated powder, wherein the inlet temperature of the spray granulation is 250° C.-300° C. and the outlet temperature is 100-150° C.;

[0085] And / or, the particle size D50 of the mixed granulated powder is 100-200 μm.

[0086] The spheroidization treatment by spray granulation can avoid the reagglomeration or sedimentation of the components in the ceramic slurry, so that the slurry can be kept uniform; at the same time, the slurry is evenly atomized to obtain spherical particles with uniform particle size distribution and good fluidity. Such particles have excellent formability and sintering properties, which is beneficial to improve the density and mechanical properties of silicon nitride ceramic ball valves.

[0087] The addition of binders to the raw materials helps stabilize the ceramic particles in the suspension during the spray granulation process, prevents the particles from settling and agglomerating, and thus maintains the uniformity of the slurry. The amount of binder used can adjust the slurry viscosity, spheroidize the granules through the spray granulation equipment, and optimize the particle size distribution of the mixed granulation powder, which is beneficial to the uniformity of the ceramic green body.

[0088] In some embodiments, the particle size D50 of the silicon nitride powder is ≤1 μm, wherein the metal content is <0.05wt%, and the α-phase content is ≥92wt%;

[0089] And / or, the sintering aid includes one or more of yttrium oxide, aluminum oxide, magnesium oxide, lanthanum oxide, gadolinium oxide, and lutetium oxide;

[0090] And / or, the binder is at least one or more of PEG, PVB, PEG and PVB; the binder is a mixture of PEG and PVB, or PVB and PVB, or PEG and PVB.

[0091] In some embodiments, tungsten carbide balls and an organic solvent are added to a mixed powder of silicon nitride powder and a sintering aid, and the mixture is ball-milled and dried to form an intermediate;

[0092] And / or, the tungsten carbide balls include large balls, medium balls and small balls; the particle size of the large balls is 20-25 mm, the particle size of the medium balls is 12-18 mm, and the particle size of the small balls is 5-10 mm; the weight ratio of the large balls, the medium balls and the small balls is 1:2:2;

[0093] And / or, the ball milling speed is 200-300 r / min, and the ball milling time is 15-30 h;

[0094] and / or, during the ball milling process, the weight ratio of the mixed powder to the organic solvent is 1:(1.2-1.8);

[0095] During ball milling, an organic solvent is added to the mixed powder of silicon nitride powder and sintering aid, and anhydrous ethanol is preferably used as the organic solvent to achieve a more uniform mixing of high-purity silicon nitride powder, sintering aid and tungsten carbide balls.

[0096] And / or, the drying temperature is 80-100°C.

[0097] In some embodiments, the press molding comprises cold isostatic pressing at a pressure of 160-200 MPa and holding the pressure for 100-200 seconds;

[0098] Conventional ball valve molding processes include dry pressing, cold isostatic pressing, machining and other methods. Cold isostatic pressing has significant advantages in processing products with complex shapes, high density and high uniformity requirements. The incompressible nature of liquids and the property of uniform pressure transmission are used to uniformly pressurize the ball valve from all directions to avoid problems such as low strength, stratification, and poor uniformity of the ball valve green body.

[0099] And / or, during the debinding process, carbon dioxide is introduced, and the mixture is heated to 1000-1100°C and kept warm for 60-180 minutes, with a heating rate of 5-10°C / min, and a mixed gas of carbon dioxide and nitrogen is introduced to a pressure of 0.1-0.3Mpa, such as 0.1MPa, 0.2MPa, 0.3MPa, etc., or within the range of any two of the above values. In some embodiments, the flow ratio of carbon dioxide to nitrogen is (0.8-1.2):1, and the flow rate of carbon dioxide is 0.8L / min-1.2L / min.

[0100] The debinding process can concentrate the volatilization of the residual carbon of the binder in the ceramic ball valve. During the debinding process, carbon dioxide gas is introduced to react with the residual carbon in the green body, and the generated gaseous reaction products are discharged, so as to prevent the residual carbon from affecting the close contact between the silicon nitride particles and the material transfer during the sintering process, affecting the density of the silicon nitride ceramic, thereby improving the batch stability of the product. As an example, during the debinding process, the starting temperature is 10°C-35°C, for example, it can be 10°C, 20°C, 250°C, 30°C, 35°C, etc., or within the range of any two of the above values, and the end temperature can be 1000°C, 1020°C, 1040°C, 1060°C, 1080°C, 1100°C, etc., or within the range of any two of the above values;

[0101] In some embodiments, the sintering process uses the following process:

[0102] S1. Raise the temperature from room temperature to 1100-1200℃ at a heating rate of 6-10℃ / min. During this sintering process, evacuate to below 50Pa and maintain the vacuum. Under the evacuated state, reach the conversion temperature of α silicon nitride as soon as possible, and α silicon nitride begins to transform into β silicon nitride. Try to compress the heating time to save costs. After reaching the predetermined temperature, keep the temperature to make the overall temperature of the valve core blank consistent.

[0103] S2. Heat to 1350-1450°C at a heating rate of 2-5°C / min and keep warm for 50-100min. The nitrogen atmosphere pressure is 0.3-1MPa.

[0104] The beneficial effect of the technical solution of the above steps is that the ceramic grains are rearranged, dissolved, and precipitated, and at the same time, the pores are eliminated and the grain boundaries begin to migrate; reducing the heating rate is conducive to the grain boundary migration rate of the ceramic grains being lower than the pore migration rate, which is conducive to reducing the number of closed pores between the grains in the ceramic ball valve;

[0105] S3, heating to 1600-1700°C at a heating rate of 1-2°C / min and keeping at that temperature for 50-100min, with the nitrogen atmosphere pressure being 1.5-2.5MPa;

[0106] In this stage, a small amount of silicon nitride particles will dissolve in the liquid phase, and the solubility will gradually increase with the increase of temperature. Silicon nitride powder with smaller particle size and the parts with edges or protrusions on the surface generally dissolve first, and when the concentration of silicon nitride particles in the liquid phase exceeds the saturation concentration, the supersaturated atoms in the liquid phase will first re-precipitate on the surface of large particles. At this time, the introduction of micron-sized tungsten carbide after ball milling helps to promote the α→β phase transformation of silicon nitride ceramics. Therefore, in this stage, the heating rate is reduced, the nitrogen pressure is increased, and the insulation time is set;

[0107] S4. Heat up to 1800-1830℃ at a heating rate of 1-2℃ / min and keep warm for 60-150min, with the nitrogen atmosphere pressure of 3-4MPa. The heating rate is reduced again to avoid the migration rate of grain boundaries being higher than the migration rate of pores, thereby avoiding the phenomenon of pores being wrapped in grains, which is beneficial to improving the density and strength of silicon nitride ceramic ball valves.

[0108] As an example, the end temperature of S1 can be 1100°C, 1120°C, 1140°C, 1180°C, 1200°C, etc., or within the range of any two of the above values, the heating rate is, for example, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, etc., or within the range of any two of the above values, and the pressure is evacuated to 50Pa, 45Pa or 40Pa, etc., or within the range of any two of the above values;

[0109] As an example, the starting temperature of S2 may be 1100°C, 1120°C, 1140°C, 1180°C, 1200°C, etc., or within the range of any two of the above values, the end temperature may be 1350°C, 1360°C, 1380°C, 1400°C, 1430°C, 1450°C, etc., or within the range of any two of the above values, the heating rate may be, for example, 2°C / min, 3°C / min, 4°C / min, 5°C / min, etc., or within the range of any two of the above values, and the nitrogen may be introduced to a pressure of 0.3MPa, 0.5MPa, 0.7Mpa, 1Mpa, etc., or within the range of any two of the above values;

[0110] As an example, the starting temperature of S3 may be 1350°C, 1360°C, 1380°C, 1400°C, 1430°C, 1450°C, etc., or within the range formed by any two of the above values; the end temperature may be 1600°C, 1630°C, 1650°C, 1670°C, 1690°C, 1700°C, etc., or within the range formed by any two of the above values; the heating rate may be, for example, 1°C / min, 1.2°C / min, 1.5°C / min, 2.0°C / min, etc., or within the range formed by any two of the above values; the nitrogen gas may be introduced to a pressure of 1.5MPa, 1.6MPa, 1.9MPa, 2.2MPa, 2.5MPa, etc., or within the range formed by any two of the above values;

[0111] As an example, the starting temperature of S4 may be 1600°C, 1630°C, 1650°C, 1670°C, 1690°C, 1700°C, etc., or within the range formed by any two of the above values; the end temperature may be 1800°C, 1810°C, 1820°C, 1825°C, 1830°C, etc., or within the range formed by any two of the above values; the heating rate may be, for example, 1°C / min, 1.2°C / min, 1.5°C / min, 2.0°C / min, etc., or within the range formed by any two of the above values; the nitrogen gas may be introduced to a pressure of 3MPa, 3.2MPa, 3.4MPa, 3.8MPa, 4.0MPa, etc., or within the range formed by any two of the above values;

[0112] In a third aspect, the present invention provides a silicon nitride ball valve core, which is manufactured using the method for manufacturing a silicon nitride ball valve core;

[0113] The outer diameter of the valve core of the silicon nitride ball valve is less than 150 mm.

[0114] The preparation method also includes fine processing, opening a switch groove on the outer diameter of one side of the ball valve core; using strong glue to add a metal valve stem seat at the groove of the ball valve core to reduce the torque force that the ball valve bears during the opening and closing process. In addition, PTFE (polytetrafluoroethylene) is sprayed at the connection between the stainless steel valve seat and the valve stem to improve the corrosion resistance of the stainless steel connection of the ceramic ball valve.

[0115] In a fourth aspect, the present invention provides a ball valve, wherein the ball valve adopts the silicon nitride ball valve core.

[0116] In some embodiments, the ball valve core is a spherical body with a through hole in the center, and a switch slot is provided on the outer diameter of one side of the spherical body; the spherical body is arranged in the valve body, the central through hole of the spherical body is connected to the channel of the valve body, the spherical body is connected to the handle through the switch slot and the valve stem, and the diameter of the through hole is less than 95 mm;

[0117] The groove of the ball valve is provided with a metal valve stem seat by using strong glue;

[0118] And / or, an anti-corrosion coating material is sprayed on the connection between the metal valve stem seat and the valve stem, and the anti-corrosion coating material includes one or more of PFA, PTFE, and PEEK.

[0119] The present invention is further described in detail below in conjunction with specific examples, which should not be construed as limiting the scope of the invention. In all examples and comparative examples of the present invention, the unit % represents mass percentage.

[0120] In the following embodiments, the mixed granulated powder is pressed into a spherical silicon nitride ball valve core with a through hole in the center by using a mold. Figure 1-3 , Figure 1 Schematic diagram of the structure of the mold; Figure 2 is a cross-sectional view of the mold; Figure 3 It is a schematic diagram of the structure of the mold without the top seat installed;

[0121] The mold includes a base 1, a top seat 2, a side mold 3 and a core mold 4; the base 1 and the top seat 2 are both provided with an annular groove, and the upper and lower ends of the side mold 3 are respectively embedded in the annular grooves of the base 1 and the top seat 2, and the base 1, the top seat 2, and the side mold 3 enclose a cavity;

[0122] The core mold 4 is arranged in the chamber, and the top and upper outer wall of the core mold 4 abut against the top seat groove of the top seat 2, and the top seat groove is a groove formed by the lower bottom surface of the top seat 2 and the outer wall of the annular groove;

[0123] The bottom end and the lower outer wall of the core mold 4 abut against the base groove of the base 1, and the base groove is a groove formed by the upper top surface of the base 1 and the outer wall of the annular groove;

[0124] The inner wall of the side mold 3 is a curved surface, and the core mold 4 is inserted into the chamber to form an injection cavity having the shape of the silicon nitride ball valve core; the ball valve core is a spherical body with a through hole in the center.

[0125] The upper outer wall of the side mold 3 is provided with a first limiting protrusion 311, and the top seat 2 is provided with a first groove, the first limiting protrusion is matched with the first groove, and the first groove is arranged on the inner wall of the annular groove; the lower outer wall of the side mold 3 is provided with a second limiting protrusion 312, and the base 1 is provided with a second groove, the second limiting protrusion is matched with the second groove, and the second groove is arranged on the inner wall of the annular groove;

[0126] The side mold 3 is formed by plugging and fixing a left half side mold 31 and a right half side mold 32, and the left half side mold 31 is symmetrical with the center of the right half side mold 32; wherein the left half side mold 31 is provided with an insert block 33, and the right half side mold 32 is provided with a socket, and the insert block 33 is inserted into the socket to plug and fix the left half side mold 31 and the right half side mold 32;

[0127] The core mold 4 is a cylinder.

[0128] When in use, the left half side mold 31 and the right half side mold 32 are plugged and fixed to form the side mold 3, and then the upper end of the side mold 3 is inserted into the annular groove of the base 1, and then the core mold 4 is installed, and the mixed granulation powder 5 is injected from the middle hollow part of the side mold 3 and the core mold 4. Figure 3 As shown, then install the top seat 2, as shown Figure 1 As shown, cold isostatic pressing is performed, and the valve core blank of the silicon nitride ball valve core is obtained after the mold is opened and taken out.

[0129] During the cold isostatic pressing process, the outer periphery of the side mold is fixed with a clamp, which is more conducive to maintaining the shape of the ceramic ball valve blank and avoiding problems such as eccentricity.

[0130] The Chinese name of PEG involved in the following embodiments is polyethylene glycol, and the molecular weight is 6000±10%; the Chinese name of PVB is polyvinyl butyral ester, and the mass fraction of butyraldehyde group is 43%-50%.

[0131] Example 1

[0132] This embodiment provides a silicon nitride ball valve core, and the preparation method thereof comprises the following steps:

[0133] 1. Weigh silicon nitride powder, aluminum oxide, yttrium oxide, and tungsten carbide balls in a mass ratio of 86:4:4:2; put high-purity silicon nitride powder, aluminum oxide, and yttrium oxide into a ball mill, add ethanol, and the amount of ethanol added is 1.2 times the total weight of high-purity silicon nitride powder, aluminum oxide, and yttrium oxide. Add tungsten carbide balls while ball milling, mix thoroughly, and dry at 100°C to form an intermediate. The ball mill speed is 200r / min, and the ball milling time is 30 hours. The tungsten carbide balls include large balls, medium balls and small balls; the particle size of the large balls is 20-25mm, the particle size of the medium balls is 12-18mm, and the particle size of the small balls is 5-10mm; the weight ratio of the large balls, medium balls and small balls is 1:2:2; the particle size of the tungsten carbide balls is 5-25mm; the median particle size of the high-purity silicon nitride powder is ≤1μm, the α phase content is ≥94wt%, and the metal content (Fe, Al, Ca) is <0.05wt%;

[0134] 2. The binder PEG and the intermediate are mixed in a mass ratio of 5:95 to form a mixed powder, and the mixed powder is granulated (spheroidized) in a centrifugal spray granulator. The inlet temperature is controlled at 265°C and the outlet temperature is controlled at 130°C to obtain a mixed granulated powder with a median particle size D50 of 100-200 μm;

[0135] 3. Mix the granulated powder and put it into a specific ball valve mold (such as Figure 1-3 ), the mold base, top seat and side mold are all made of rubber material, the core mold is made of steel, the base is fixed to the core mold to avoid the ball valve from forming an eccentric structure to the greatest extent, the side mold and top seat are detachably connected to the core mold respectively, so that the ball valve blank can be smoothly demolded; after cold isostatic pressing, the silicon nitride ball valve blank is obtained; the cold isostatic pressing pressure is 160 MPa, and the cold isostatic pressing time is 100s;

[0136] 4. Place the silicon nitride ball valve blank in a debinding furnace and introduce carbon dioxide to debind it, then place it in a large pressure furnace for sintering;

[0137] The starting temperature of the debinding process is heated from room temperature to 1000°C, with a heating rate of 5°C / min, and kept at 1000°C for 120 minutes. During the debinding process, a mixed gas of carbon dioxide and nitrogen is introduced to a pressure of 0.1Mpa; the flow ratio of carbon dioxide to nitrogen is 0.8:1, and the flow rate of carbon dioxide is 0.8L / min.

[0138] The first stage of sintering process is to heat up to 1100℃ after debinding, with a heating rate of 6℃ / min. During this stage of sintering, the vacuum is evacuated to below 50Pa;

[0139] The second sintering process was from 1100 to 1350 °C, with a heating rate of 2.8 °C / min; the temperature was kept at 1350 °C for 50 min, and the nitrogen atmosphere pressure was 0.3 MPa;

[0140] The third sintering process was from 1350℃ to 1600℃, with a heating rate of 1.9℃ / min; the temperature was kept at 1600℃ for 70min, and the nitrogen atmosphere pressure was 2MPa;

[0141] The fourth sintering process was from 1600°C to 1800°C, with a heating rate of 1.6°C / min; the temperature was kept at 1800°C for 110 min, and the nitrogen atmosphere pressure was 3.5 MPa;

[0142] 5. After the sintered silicon nitride ball valve core is naturally cooled to room temperature, the outer diameter of the silicon nitride ball valve core is 145 mm, and the diameter of the through hole is 90 mm, and fine processing is performed;

[0143] 6. Use strong glue to install a metal valve stem seat at the groove of the ball valve core to reduce the torque force that the ball valve bears during the opening and closing process. In addition, spray PTFE (polytetrafluoroethylene) at the connection between the stainless steel valve seat and the valve stem to improve the corrosion resistance of the stainless steel connection of the ceramic ball valve.

[0144] Example 2

[0145] This embodiment provides a silicon nitride ball valve core, and the preparation method thereof comprises the following steps:

[0146] 1. Weigh silicon nitride powder, aluminum oxide, lanthanum oxide and tungsten carbide balls in a mass ratio of 93:3:2:4; put high-purity silicon nitride powder, aluminum oxide and lanthanum oxide into a ball mill, add ethanol, the amount of ethanol added is 1.8 times the total weight of high-purity silicon nitride powder, aluminum oxide and yttrium oxide, add tungsten carbide balls while ball milling, mix thoroughly, dry at 80°C to form an intermediate, the ball mill speed is 280r / min, the ball milling time is 15 hours, the median particle size of the high-purity silicon nitride powder is ≤0.8μm, the α phase content is ≥93wt%, and the metal content (Fe, Al, Ca) is <0.05wt%; tungsten carbide balls include large balls, medium balls and small balls; the particle size of the large balls is 20-25mm, the particle size of the medium balls is 12-18mm, and the particle size of the small balls is 5-10mm; the weight ratio of the large balls, medium balls and small balls is 1:2:2;

[0147] 2. The binder PEG and the intermediate are mixed in a mass ratio of 4:96 to form a mixed powder, and the mixed powder is granulated in a centrifugal spray granulator, the inlet temperature is controlled at 250°C, and the outlet temperature is controlled at 150°C, and the median particle size of the mixed granulated powder is about 100-200 μm;

[0148] 3. The granulated powder is placed in a ball valve mold of a specific size, and a silicon nitride ball valve blank is obtained after cold isostatic pressing; the cold isostatic pressing pressure is 170 MPa, and the cold isostatic pressing time is 160 seconds;

[0149] 4. Place the silicon nitride ball valve blank in a debinding furnace and introduce carbon dioxide to debind it, then place it in a large gas pressure furnace for sintering;

[0150] The starting temperature of the debinding process is heated from room temperature to 1040°C, with a heating rate of 5°C / min, and kept at 1040°C for 100min. During the debinding process, a mixed gas of carbon dioxide and nitrogen is introduced to a pressure of 0.3Mpa; the flow ratio of carbon dioxide to nitrogen is 1.2:1, and the flow rate of carbon dioxide is 1.2L / min.

[0151] The first stage of sintering process is to heat up to 1160℃ after debinding, with a heating rate of 7℃ / min. During this stage of sintering, the vacuum is evacuated to below 50Pa;

[0152] In the second stage of sintering, the temperature was raised from 1160 to 1420 °C at a rate of 2.5 °C / min; the temperature was kept at 1420 °C for 65 min, and the nitrogen atmosphere pressure was 0.4 MPa;

[0153] The third sintering process was from 1420℃ to 1660℃, with a heating rate of 1.7℃ / min; the temperature was kept at 1660℃ for 70min, and the nitrogen atmosphere pressure was 2.1MPa;

[0154] The fourth sintering process was from 1660°C to 1815°C, with a heating rate of 1.4°C / min; the temperature was kept at 1815°C for 120 min, and the nitrogen atmosphere pressure was 3.8 MPa;

[0155] 5. After the sintered silicon nitride ball valve core is naturally cooled to room temperature, the outer diameter of the silicon nitride ball valve core is 140mm, and the diameter of the through hole is 90mm, and fine processing is performed;

[0156] 6. Use strong glue to install a metal valve stem seat at the groove of the ball valve core to reduce the torque force that the ball valve bears during the opening and closing process. In addition, spray PTFE (polytetrafluoroethylene) at the connection between the stainless steel valve seat and the valve stem to improve the corrosion resistance of the stainless steel connection of the ceramic ball valve.

[0157] Example 3

[0158] This embodiment provides a silicon nitride ball valve core, and the preparation method thereof comprises the following steps:

[0159] 1. Weigh silicon nitride powder, alumina, lutetium oxide, and tungsten carbide balls in a mass ratio of 89:3:2:6; put high-purity silicon nitride powder, alumina, and lutetium oxide into a ball mill, add ethanol, and the amount of ethanol added is 1.6 times the total weight of high-purity silicon nitride powder, alumina, and yttrium oxide. Add tungsten carbide balls while ball milling, mix thoroughly, and dry at 80°C to form an intermediate. The ball mill speed is 300r / min, and the ball milling time is 2 4 hours; the median particle size of high-purity silicon nitride powder is D50≤0.6μm, the α phase content is ≥94wt%, and the metal content (Fe, Al, Ca) is <0.05wt%; the tungsten carbide balls include large balls, medium balls and small balls; the particle size of the large balls is 20-25mm, the particle size of the medium balls is 12-18mm, and the particle size of the small balls is 5-10mm; the weight ratio of the large balls, medium balls and small balls is 1:2:2;

[0160] 2. The binder PVB and the intermediate are mixed in a mass ratio of 3:97 to form a mixed powder, and the mixed powder is granulated in a centrifugal spray granulator. The inlet temperature is controlled at 300°C and the outlet temperature is controlled at 100°C. The median particle size of the mixed granulated powder is about 100-200 μm;

[0161] 3. The granulated powder is placed into a specific ball valve mold and subjected to cold isostatic pressing to obtain a silicon nitride ball valve blank; the cold isostatic pressing pressure is 200 MPa, and the cold isostatic pressing time is 200 seconds;

[0162] 4. Place the silicon nitride ball valve blank in a debinding furnace and introduce carbon dioxide to debind it, then place it in a large pressure furnace for sintering;

[0163] The starting temperature of the debinding process is heated from room temperature to 1100°C, with a heating rate of 7°C / min, and kept at 1100°C for 180 minutes. During the debinding process, a mixed gas of carbon dioxide and nitrogen is introduced to a pressure of 0.2Mpa; the flow ratio of carbon dioxide to nitrogen is 1.0:1, and the flow rate of carbon dioxide is 1.0L / min;

[0164] The first stage of sintering process is to heat up to 1200℃ after debinding, with a heating rate of 8℃ / min. During this stage of sintering, the vacuum is evacuated to below 50Pa;

[0165] The second sintering process was heated from 1200 to 1450 °C at a rate of 2.3 °C / min; the temperature was kept at 1450 °C for 80 min, and the nitrogen atmosphere pressure was 1 MPa;

[0166] The third sintering process was from 1450℃ to 1700℃, with a heating rate of 1.4℃ / min; the temperature was kept at 1700℃ for 100min, and the nitrogen atmosphere pressure was 2.5MPa;

[0167] The fourth sintering process was from 1700°C to 1830°C, with a heating rate of 1°C / min; the temperature was kept at 1830°C for 150min, and the nitrogen atmosphere pressure was 4MPa;

[0168] 5. After the sintered silicon nitride ball valve core is naturally cooled to room temperature, the outer diameter of the silicon nitride ball valve core is 148mm, and the diameter of the through hole is 90mm, and fine processing is performed;

[0169] 6. Use strong glue to install a metal valve stem seat at the groove of the ball valve core to reduce the torque force that the ball valve bears during the opening and closing process. In addition, spray PTFE (polytetrafluoroethylene) at the connection between the stainless steel valve seat and the valve stem to improve the corrosion resistance of the stainless steel connection of the ceramic ball valve.

[0170] Six silicon nitride ball valve cores from the same batch were prepared according to the method of Example 3, and the six silicon nitride ball valve cores from the same batch were measured. The results are shown in Table 1 below.

[0171] Table 1 Silicon nitride ball valve core performance test

[0172]

[0173] It can be seen from the test results of 6 ball valves that the preparation method of the present invention has high stability, and the prepared products have high density, hardness and fracture toughness, are free of eccentricity, have high qualification, and are suitable for large-scale production.

[0174] Example 4

[0175] This embodiment provides a silicon nitride ball valve core, and its preparation method is basically the same as that of Embodiment 1, except that in step (4), no carbon dioxide gas is introduced during the debinding process, and only nitrogen gas is introduced, and the gas pressure is the same during the debinding process.

[0176] Comparative Example 1

[0177] This comparative example provides a silicon nitride ball valve core, and its preparation method is basically the same as that of Example 1, except that tungsten carbide powder is directly used for mixing in step 1.

[0178] Step 1 is as follows:

[0179] High-purity silicon nitride powder, aluminum oxide, yttrium oxide, and tungsten carbide powder are fully mixed in a weight ratio of 86:4:4:2, and dried at 100°C to form an intermediate. The median particle size of the high-purity silicon nitride powder is ≤0.6μm, the α phase content is ≥94wt%, and the metal content (Fe, Al, Ca) is <0.05wt%; the D50 of the tungsten carbide powder is 5μm.

[0180] Comparative Example 2

[0181] This comparative example provides a silicon nitride ball valve core, and its preparation method is basically the same as that of Example 1, except that in step 3, the mold of the present invention is not used, but the ball valve is mainly manufactured by lathe processing; step 3 is as follows:

[0182] The granulated powder is placed in a cylindrical mold and subjected to cold isostatic pressing to obtain a cylindrical silicon nitride ball valve blank; Figure 4 and Figure 5 Then the cylindrical silicon nitride ball valve blank is turned to form a spherical silicon nitride ball valve blank. The outer diameter and the diameter of the through hole of the silicon nitride ball valve core finally obtained are the same as those in Example 1.

[0183] Comparative Example 3

[0184] This comparative example provides a silicon nitride ball valve core, and its preparation method is basically the same as that of Example 1, except that the spray granulation process is not performed in step 2, and in step 3, the dried intermediate is directly placed in a specific ball valve mold for cold isostatic pressing to form a ceramic ball valve, and the subsequent processes remain the same. The preparation method of the intermediate is the same as that of Example 1.

[0185] Test Case

[0186] The silicon nitride ball valve cores of Examples 1-5 and Comparative Examples 1-2 were sampled and tested for ball valve density, HV10 hardness, fracture toughness, and eccentricity.

[0187] Ball valve density determination method: Archimedes drainage method is used, and the weighing is carried out using a digital electronic balance with an accuracy of 0.001g. The calculation formula is as follows:

[0188]

[0189] Where: d—volume density of the sample (g / cm 3 ); —Density of liquid (pure water) in air (g / cm 3 ); m0—the mass of the sample in a dry environment (g); m1—the mass of the sample in water (g).

[0190] HV10 hardness determination method: The Vickers hardness of silicon nitride ceramic samples is calculated by calculating the length of the indentation diagonal. The indenter load is 10kg, the loading time is 10s, and the formula for Vickers hardness is as follows:

[0191]

[0192] Where: HV = Vickers hardness of ceramic material (GPa); P = pressure load (kg); d = average length of two indentation diagonals (um).

[0193] Fracture toughness determination method: Fracture toughness is calculated by indentation method. The length of the indentation diagonal a and the crack extension length l are measured under an optical microscope. The fracture toughness of silicon nitride material is calculated by Niihara formula, where H is the hardness of the sample being tested; E is the elastic modulus of the material (310 for silicon nitride ceramics); P is the indentation load (10kg); l is the indentation crack length; a is the half length of the indentation diagonal. When c / a>2.5, the fracture toughness value of the sample is calculated by formula 1; when c / a<2.5, the fracture toughness value of the sample is calculated by formula 2.

[0194]

[0195] Eccentricity determination method:

[0196]

[0197] Where e=eccentricity; c=semi-focal length; a=semi-major axis / semi-real axis.

[0198] The results of Examples 1-4 and Comparative Examples 1-3 are shown in Table 2.

[0199] Table 2 Test results

[0200]

[0201] It can be seen from Table 1 that the density of the silicon nitride ball valve core prepared in Examples 1 to 3 is 3.232-3.256 g / cm 3 , fracture toughness 10.1-10.5MPa·m 1 / 2 , HV10 hardness is 1719-1780.

[0202] Compared with Example 1, Example 4 does not introduce carbon dioxide gas to remove residual carbon in the green body, thereby inhibiting the normal growth of grains, resulting in a decrease in the density of the sintered body, and slightly reducing the hardness and fracture toughness of the material.

[0203] Compared with Example 1, the density, Vickers hardness and fracture toughness of the product of Comparative Example 1 are all reduced by adding tungsten carbide powder. The reason is that the tungsten carbide particles introduced by ball milling are smaller in size (the particle size of tungsten carbide balls after grinding is D50≤1μm). The tungsten carbide balls introduced by ball milling will not undergo component segregation in the liquid phase during the sintering process, but will be dispersed in the ceramic matrix, resulting in a particle dispersion enhancement mechanism, which improves the mechanical properties of the product. However, when tungsten carbide powder particles are directly added, the raw materials are uneven due to the large difference in particle size between tungsten carbide and silicon nitride, which affects the uniformity of the ceramic slurry process, resulting in inconsistent shrinkage and densification during the sintering process, and ultimately causing eccentricity of the ball valve and reduced mechanical properties.

[0204] Compared with Example 1, Comparative Example 2 does not use the special mold of the present invention to prepare the silicon nitride ball valve core, which slightly reduces the hardness and fracture toughness of the prepared ball valve core. During the lathe processing, more stress concentration and defects may be generated due to the cutting force and precision, resulting in a decrease in mechanical properties and an increase in eccentricity.

[0205] Compared with Example 1, in Comparative Example 3, the spray granulation process was not performed, and the dried powder was directly cold isostatically pressed into a ceramic ball valve, resulting in poor powder fluidity. Problems such as voids and poor uniformity may occur during the cold isostatic pressing process. It is difficult to achieve uniform densification during the sintering process and the shrinkage consistency in all directions is poor, resulting in decreased mechanical properties and increased eccentricity.

[0206] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A mold, characterized in that: The mold comprises a base, a top seat, a side mold and a core mold; the base and the top seat are both provided with an annular groove, the upper and lower ends of the side mold are respectively embedded in the annular grooves of the base and the top seat, and the base, the top seat and the side mold are enclosed to form a cavity; The core mold is arranged in the chamber, the top end and the upper outer wall of the core mold are in contact with the top seat groove of the top seat, and the top seat groove is a groove formed by the lower bottom surface of the top seat and the outer wall of the annular groove; the bottom end and the lower outer wall of the core mold are in contact with the base groove of the base, and the base groove is a groove formed by the upper top surface of the base and the outer wall of the annular groove; The inner wall of the side mold is a curved surface, and the core mold is inserted into the chamber to form an injection cavity in the shape of a ball valve core; the ball valve core is a spherical body with a through hole in the center.

2. The mold according to claim 1, characterized in that: The upper outer wall of the side mold is provided with a first limiting protrusion, the inner wall of the annular groove of the top seat is provided with a first groove, and the first limiting protrusion is matched with the first groove; the lower outer wall of the side mold is provided with a second limiting protrusion, the inner wall of the annular groove of the base is provided with a second groove, and the second limiting protrusion is matched with the second groove; And / or, the side mold is formed by plugging and fixing a left half side mold and a right half side mold, and the left half side mold is symmetrical with the center of the right half side mold; wherein, plug blocks are provided on both sides of the left half side mold, and sockets are provided on both sides of the right half side mold, and the plug blocks are plugged into the sockets to plug and fix the left half side mold and the right half side mold; And / or, the core mold is a cylinder.

3. A method for preparing a silicon nitride ball valve core, characterized in that: The steps include: Tungsten carbide balls are added to a mixture of silicon nitride powder and a sintering aid for ball milling to form an intermediate; a binder is then added for spheroidization to form a mixed granulated powder; the particle size of the tungsten carbide balls is 5-25 mm; Putting the mixed granulated powder into the mold of claim 1 or 2 for pressing and molding to obtain a valve core blank; The prepared valve core blank is debinded and then sintered in a nitrogen atmosphere to obtain a silicon nitride ceramic ball valve.

4. The method for preparing a silicon nitride ball valve core according to claim 3, characterized in that: The sintering aid includes one or more of yttrium oxide, aluminum oxide, magnesium oxide, lanthanum oxide, gadolinium oxide, and lutetium oxide; And / or, the binder is at least one or more of PEG and PVB; and / or, the particle size D50 of the silicon nitride powder is ≤1 μm, wherein the metal content is <0.05wt%, and the α-phase content is ≥92wt%; And / or, the particle size D50 of the mixed granulated powder is 100-200 μm.

5. The method for preparing a silicon nitride ball valve core according to claim 3, characterized in that: The mass ratio of silicon nitride powder, sintering aid and tungsten carbide ball is (86-93): (5-8): (2-6); and / or, the mass ratio of the binder to the intermediate is (3-5): (95-97); And / or, the intermediate is mixed with a binder and then spray granulated to form the mixed granulated powder, wherein the inlet temperature of the spray granulation is 250°C-300°C and the outlet temperature is 100-150°C.

6. The method for preparing a silicon nitride ball valve core according to claim 5, characterized in that: Adding tungsten carbide balls and organic solvents to a mixture of silicon nitride powder and a sintering aid, ball milling and drying to form an intermediate; And / or, the tungsten carbide balls include large balls, medium balls and small balls; the particle size of the large balls is 20-25 mm, the particle size of the medium balls is 12-18 mm, and the particle size of the small balls is 5-10 mm; And / or, the ball milling speed is 200-300 r / min, and the ball milling time is 15-30 h; and / or, during the ball milling process, the weight ratio of the mixed powder to the organic solvent is 1:(1.2-1.8); And / or, the drying temperature is 80-100°C.

7. The method for preparing a silicon nitride ball valve core according to claim 3, characterized in that: The pressing and forming comprises cold isostatic pressing with a pressure of 160-200 MPa and holding the pressure for 100-200 seconds; And / or, during the debinding process, carbon dioxide is introduced, and the temperature is heated to 1000-1100°C and kept for 60-180 minutes, with a heating rate of 5-10°C / min; And / or, during the debinding process, a mixed gas of carbon dioxide and nitrogen is introduced to a pressure of 0.1-0.3 MPa; the flow ratio of carbon dioxide to nitrogen is (0.8-1.2):1, and the flow rate of carbon dioxide is 0.8L / min-1.2L / min.

8. The method for preparing a silicon nitride ball valve core according to claim 3, characterized in that: The following processes are used during the sintering process: S1. Evacuate to below 50Pa and heat to 1100-1200℃ at a heating rate of 6-10℃ / min; S2. Raise the temperature to 1350-1450°C at a heating rate of 2-5°C / min and keep warm for 50-100 min, and introduce nitrogen until the pressure reaches 0.3-1MPa; S3, heating to 1600-1700°C at a heating rate of 1-2°C / min and keeping at that temperature for 50-100min, with the nitrogen atmosphere pressure being 1.5-2.5MPa; S4. Heat to 1800-1830°C at a heating rate of 1-2°C / min and keep warm for 60-150 min. The nitrogen atmosphere pressure is 3-4 MPa.

9. A silicon nitride ball valve core, made by the method for making a silicon nitride ball valve core according to any one of claims 3 to 8; The outer diameter of the valve core of the silicon nitride ball valve is less than 150 mm.

10. A ball valve, characterized in that: The ball valve adopts the silicon nitride ball valve core described in claim 9.

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