A large-size silicon nitride ceramic ball valve core and its preparation method and application

Abstract

The present invention relates to the field of ceramic materials, and discloses a large-size silicon nitride ceramic ball valve core and a preparation method and application thereof, wherein a mixture of silicon nitride powder, a sintering aid and a beta silicon nitride seed crystal is used as a raw material, pressed into a green body, and then machined into a valve core blank; the valve core blank is then sintered to prepare a silicon nitride ceramic ball valve core. Through a special pressing method and optimizing the sintering curve at the same time, the product has a bimodal distribution microstructure, which improves the uniformity, density, fracture toughness and Weibull modulus of the product, thereby reducing the occurrence of cracks during the preparation process and use process, and improving the product's qualified rate and wear resistance.

Description

Technical Field

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

[0002] With the explosive growth of application demands in photovoltaics, semiconductor power devices and other fields, the requirements for purity and metal contamination control during material transportation are extremely strict. Even trace amounts of impurities may have a serious impact on the performance and reliability of silicon devices. Therefore, the related industry chain has an increasing demand for large-size, highly wear-resistant and low-pollution ball valves.

[0003] When metal ball valves are in contact with materials, they will wear out over a long period of time, which will lead to the generation of wear debris, which will seriously affect the quality of the final product. Silicon nitride (Si3N4) ceramic ball valves have the characteristics of high hardness, good wear resistance, high temperature resistance, corrosion resistance, high thermal conductivity, and high insulation, making them an ideal substitute. More importantly, compared with other ceramic materials, silicon nitride has the characteristics of high strength and low density, which is especially important for large material transfer equipment that requires lightweight design.

[0004] However, the green strength of large-sized silicon nitride ceramic ball valves is low and the manufacturing is difficult. During the sintering process of large-sized silicon nitride ceramic ball valves, due to the influence of factors such as temperature and thermal expansion coefficient differences, cracks, deformation, poor uniformity and other problems are prone to occur, resulting in the size of large-sized silicon nitride ceramic ball valves in batches being below φ200mm, and it is difficult to obtain larger-sized silicon nitride ceramic ball valves with both high pass rate and wear resistance. Summary of the invention

[0005] In order to solve the problem that large-sized silicon nitride ceramics are prone to cracks during the sintering process, the present invention provides a method for preparing a large-sized silicon nitride ceramic ball valve core. By adopting a specific pressing and sintering process, a larger-sized silicon nitride ceramic ball valve core is obtained, and the product's qualification rate and wear resistance are improved.

[0006] In a first aspect, the present invention provides a method for preparing a large-sized silicon nitride ceramic ball valve core.

[0007] In an optional embodiment, a method for preparing a large-size silicon nitride ceramic ball valve core comprises the following steps:

[0008] S1, using a mixture of silicon nitride powder, a sintering aid and β-silicon nitride seed crystals as raw materials, sequentially pressing and forming the mixture into spherical particles, and pressing the mixture into a green body for a second time;

[0009] S2, turning the blank obtained in S1 into a valve core blank;

[0010] S3, sintering the valve core blank obtained in S2 to obtain a silicon nitride ceramic ball valve core;

[0011] The sintering process includes the following steps:

[0012] S31. Under a vacuum degree of 80-110Pa, heat the room temperature to 1150-1200°C and keep it warm for 30-60min;

[0013] S32, heat to 1480-1520℃, keep warm for 30-100min, and fill with nitrogen until the pressure is 0.5-1MPa;

[0014] S33, heat to 1690-1740℃ and keep warm for 60-120min; at 1690-1740℃, the nitrogen pressure is 1.5-2.5MPa;

[0015] S34, heat to 1820-1860℃ and keep warm for 100-200min, nitrogen pressure is 3-4MPa;

[0016] S35, cool down to 1400-1450℃ and keep warm for 300-400min; then cool down naturally.

[0017] In an optional embodiment, in S1, the α-phase content in the silicon nitride powder is ≥ 93wt%;

[0018] and / or, the content of the β-silicon nitride seed crystal is 1wt%-2wt% of the total weight of the raw material;

[0019] And / or, the median particle size D50 of the silicon nitride powder is 0.3 μm-0.6 μm.

[0020] In an optional embodiment, the weight ratio of silicon nitride powder, sintering aid, and β-silicon nitride seed crystals is (90-96): (3-9): (1-2);

[0021] And / or, the sintering aid includes one or more of cadmium oxide, aluminum oxide, magnesium oxide, lanthanum oxide, nickel oxide, and lutetium oxide;

[0022] and / or, the average length of the β-silicon nitride seed crystal is 2.5 μm-3.5 μm, and the average aspect ratio is 9-11;

[0023] And / or, the metal content of the silicon nitride powder is ≤300ppm.

[0024] In an optional embodiment, in S1, silicon nitride powder, sintering aid, and β-silicon nitride seed crystals are dispersed in an organic solvent and ball-milled to form a mixture, the rotation speed of the ball mill is 250 r / min-300 r / min, and the organic solvent is anhydrous ethanol;

[0025] And / or, in S3, the valve core blank is placed on a boron nitride plate, and then the boron nitride plate with the valve core blank is placed in a silicon nitride sagger for sintering to obtain a silicon nitride ceramic ball valve core.

[0026] In an optional embodiment, in S1, the first pressing is performed by cold isostatic pressing, with a pressure of 80-100 MPa and a holding time of 50-100 s;

[0027] And / or, the second pressing is performed by cold isostatic pressing at a pressure of 160-200 MPa and a holding time of 100-200 s.

[0028] In an optional embodiment, the heating rate in S31 is 5-9°C / min;

[0029] and / or, the heating rate in S32 is 2-3°C / min;

[0030] and / or, the heating rate in S33 is 1-2°C / min;

[0031] and / or, the heating rate in S34 is 1-2°C / min;

[0032] And / or, the cooling rate in S35 is 3-5°C / min.

[0033] In a second aspect, the present invention provides a silicon nitride ceramic ball valve core, which is manufactured using the method for preparing a large-size silicon nitride ceramic ball valve core.

[0034] In an optional embodiment, the silicon nitride ceramic ball valve core is a spherical body with a through hole at the center;

[0035] The outer diameter of the silicon nitride ceramic ball valve core is greater than 260 mm;

[0036] And / or, the diameter of the through hole is greater than 175 mm.

[0037] In a third aspect, the present invention provides a ball valve, which adopts the silicon nitride ceramic ball valve core.

[0038] In an optional embodiment, the ball valve comprises a silicon nitride ceramic ball valve core and a valve body, wherein the silicon nitride ceramic ball valve core is a spherical body with a through hole at the center, 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, a valve stem groove is provided on one side of the outer wall of the spherical body, and a metal valve stem seat is installed at the groove of the valve stem groove by using strong glue;

[0039] The connection between the metal valve stem seat and the valve stem is sprayed with an anti-corrosion coating material, wherein the anti-corrosion coating material includes one or more of PFA, PTFE, and PEEK;

[0040] The spherical body is connected to the valve stem through the valve stem groove and the metal valve stem seat.

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

[0042] The method for preparing a silicon nitride ceramic ball valve core provided by the present invention uses a mixture of silicon nitride powder, a sintering aid and a beta silicon nitride crystal seed as raw materials, presses the green body into a blank, and then processes the green body into a valve core blank; then the valve core blank is sintered to prepare a silicon nitride ceramic ball valve core. Through a special pressing method and optimizing the sintering curve at the same time, the product has a bimodal distribution microstructure, which improves the uniformity, density, fracture toughness and Weibull modulus of the product, thereby reducing the occurrence of cracks during the preparation process and use process, and improving the qualified rate and wear resistance of the product.

[0043] 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

[0044] 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.

[0045] Figure 1 Schematic diagram of the structure of the silicon nitride ball valve prepared in Example 1 of the present invention;

[0046] Figure 2 This is a physical picture of the silicon nitride ball valve prepared in Example 1 of the present invention;

[0047] Figure 3 1 is a SEM image and a distribution diagram of the grain aspect ratio of the silicon nitride ball valve prepared in Example 1 of the present invention;

[0048] Figure 4 1 is a SEM image and a distribution diagram of the grain aspect ratio of the silicon nitride ball valve prepared in Example 2 of the present invention;

[0049] Figure 5 1 is a SEM image and a distribution diagram of the grain aspect ratio of the silicon nitride ball valve prepared in Example 3 of the present invention;

[0050] Figure 6 is a SEM image of the silicon nitride ball valve prepared in Comparative Example 3 of the present invention;

[0051] Figure 7 is a metallographic diagram of the silicon nitride ball valve prepared in Example 1 of the present invention;

[0052] Figure 8 is a metallographic diagram of the silicon nitride ball valve prepared in Comparative Example 2 of the present invention;

[0053] Fig. 9 is an EPMA image of the silicon nitride ball valve prepared in Example 1 of the present invention;

[0054] Fig.10 is an EPMA image of the silicon nitride ball valve prepared in Example 4 of the present invention;

[0055] Fig.11 It is the SEM image and the grain aspect ratio distribution diagram of the silicon nitride ball valve prepared in Comparative Example 1 of the present invention.

[0056] Description of reference numerals:

[0057] 1. Silicon nitride ceramic ball valve core; 11. Through hole; 12. Valve stem groove. DETAILED DESCRIPTION

[0058] 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.

[0059] 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.

[0060] In the description of the embodiments of the present invention, “plurality” means more than two, unless otherwise clearly and specifically defined.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] In the present invention, wt% refers to weight percentage, and vol% refers to volume percentage.

[0065] When using silicon nitride powder to prepare ball valve cores, the most difficult problem to solve during the production process is that cracks are prone to occur during the sintering process, resulting in a low product qualification rate. There are many reasons for the occurrence of cracks, such as high impurity content in the raw materials or poor powder particle grading, which can easily lead to the generation of pores or cracks, uneven pressing of ceramic ball valve blanks, resulting in uneven internal structure of ceramics, and cracks after sintering; unstable temperature changes or too high or too low temperatures can easily lead to stress concentration and cause pores, and insufficient sintering time affects the bonding of particles in the ceramic, which can easily lead to the generation of pores or cracks, resulting in low fracture toughness of silicon nitride ceramics. For large-sized silicon nitride ceramic ball valve cores, cracks are more likely to occur, resulting in a low product qualification rate.

[0066] The related prior art proposes a method for preparing silicon nitride ceramic materials, which is to heat to 1300-1500℃ for 3-6h at a nitrogen pressure of 0.1-0.3MPa; then heat to 1600-1800℃ for 6-12h at a nitrogen pressure of 0.1-0.3MPa; finally heat to 1800-2000℃ for 6-12h at a nitrogen pressure of 1.5-2MPa. The results show that the method does not carry out the low-temperature insulation process of silicon nitride, but directly heats to 1300-1500℃ in one step. The overall temperature of the valve core blank is inconsistent, resulting in inconsistent shrinkage of the large-sized ceramic ball valve valve core, and defects such as cracks in the ceramic ball valve valve core.

[0067] In order to solve the problems existing in the above-mentioned related art, according to a first aspect of the present invention, a method for preparing a large-sized silicon nitride ceramic ball valve core is provided, comprising the following steps:

[0068] S1, using a mixture of silicon nitride powder, a sintering aid and β-silicon nitride seed crystals as raw materials, sequentially pressing and forming the mixture into spherical particles, and pressing the mixture into a green body for a second time;

[0069] S2, turning the blank obtained in S1 into a valve core blank;

[0070] S3, sintering the valve core blank obtained in S2 to obtain a silicon nitride ceramic ball valve core;

[0071] The sintering process includes the following steps:

[0072] S31. Under a vacuum degree of 80-110Pa, heat from room temperature to 1150-1200°C and keep warm for 30-60min;

[0073] S32, heat to 1480-1520℃, keep warm for 30-100min, and fill with nitrogen until the pressure is 0.5-1MPa;

[0074] S33, heat to 1690-1740℃ and keep warm for 60-120min; at 1690-1740℃, the nitrogen pressure is 1.5-2.5MPa;

[0075] S34, heat to 1820-1860℃ and keep warm for 100-200min, nitrogen pressure is 3-4MPa;

[0076] S35, cool down to 1400-1450℃ and keep warm for 300-400min; then cool down naturally.

[0077] The present invention uses a special pressing and sintering process to form a bimodal distribution microstructure in the product, thereby improving the uniformity, density, fracture toughness and Weibull modulus of the product, thereby reducing the occurrence of cracks during the preparation process and use process, and improving the qualification rate and wear resistance of large-sized silicon nitride ceramic ball valve core products.

[0078] The present invention makes the mixture into spherical powder through the first pressing and crushing, thereby improving the fluidity and uniformity of the ceramic powder. By pressing the spherical particles for the second time, the spherical particles of the green body are arranged evenly and compactly, which promotes the formation of a bimodal distribution microstructure after sintering. This structure contains fine matrix grains and larger rod-shaped grains without abnormally grown grains. In the process of crack propagation, it will induce toughening mechanisms such as crack deflection and bridging, effectively consume fracture energy, and improve the fracture toughness and wear resistance of silicon nitride ceramic materials.

[0079] By optimizing the heating curve of the sintering process, the valve core blank is first heated from room temperature to 1150-1200℃, and before the α-phase silicon nitride begins to transform into β-phase silicon nitride, it is kept warm for 30-60 minutes to make the overall temperature of the valve core blank basically consistent. In the subsequent sintering process, uneven heating can be avoided to cause deformation of the ceramic ball valve during the sintering process, affecting its dimensional accuracy and shape. S32 and S33 are heated and kept warm in stages, and the α-phase silicon nitride is uniformly transformed into β-phase silicon nitride, which promotes the formation of a self-reinforced microstructure with a bimodal distribution and reduces the generation of cracks.

[0080] In addition, the S31 stage is carried out under vacuum. The ceramic ball valve core is prone to residual gas on the surface during the manufacturing process or the residual material on the surface of the ball valve reacts with silicon nitride during the sintering process to form gas. Therefore, vacuum sintering is used in the low-temperature stage to reduce the gas content of the ball valve and avoid the formation of open pores on the surface of the ball valve.

[0081] During the sintering process, the temperature is controlled below 1860°C. When the sintering temperature is too high, the silicon nitride particles will decompose, resulting in a decrease in the density and mechanical strength of the silicon nitride ceramic products.

[0082] As an example, the starting temperature of S31 is 10°C-35°C, for example, it can be 10°C, 20°C, 25°C, 30°C, 35°C, etc., or within the range formed by any two of the above values, and the end temperature of S31 can be 1150°C, 1185°C, 1190°C, 1195°C, 1200°C, etc., or within the range formed by any two of the above values;

[0083] As an example, the starting temperature of S32 is 1150-1200°C, for example, it can be 1150°C, 1185°C, 1190°C, 1195°C, 1200°C, etc., or within the range formed by any two of the above values, and the end temperature of S32 can be 1480°C, 1500°C, 1510°C, 1515°C, 1520°C, etc., or within the range formed by any two of the above values;

[0084] As an example, the starting temperature of S33 is 1480-1520°C, for example, it can be 1480°C, 1500°C, 1510°C, 1515°C, 1520°C, etc., or within the range formed by any two of the above values, and the end temperature of S33 can be 1690°C, 1700°C, 1710°C, 1730°C, 1740°C, etc., or within the range formed by any two of the above values;

[0085] As an example, the starting temperature of S34 is 1690-1740°C, for example, it can be 1690°C, 1700°C, 1710°C, 1730°C, 1740°C, etc., or within the range formed by any two of the above values, and the end temperature of S34 can be 1820°C, 1830°C, 1840°C, 1850°C, 1860°C, etc., or within the range formed by any two of the above values;

[0086] As an example, the starting temperature of S35 is 1820-1860°C, for example, it can be 1820°C, 1830°C, 1840°C, 1850°C, 1860°C, etc., or it is within the range formed by any two of the above values, and then the temperature is reduced to 1400-1450°C, which can be 1450°C, 1440°C, 1430°C, 1420°C, 1410°C, etc., or it is within the range formed by any two of the above values; and then the temperature is naturally reduced to room temperature 10°C-35°C, which can be 10°C, 20°C, 25°C, 30°C, 35°C, etc., or it is within the range formed by any two of the above values.

[0087] In some embodiments, the outer diameter of the large-sized silicon nitride ceramic ball valve core is greater than 200 mm; optionally, the outer diameter of the large-sized silicon nitride ceramic ball valve core is greater than 260 mm; the outer diameter of the large-sized silicon nitride ceramic ball valve core is 260-300 mm.

[0088] In some embodiments, the content of α-phase silicon nitride powder in the silicon nitride powder is ≥ 93wt%;

[0089] and / or, the content of the β-silicon nitride seed crystal is 1wt%-2wt% of the total weight of the raw material;

[0090] The present invention adds a small amount of β-silicon nitride seed crystals, which effectively reduces the activation energy of the β phase precipitation and continued growth on the surface of the seed crystals, thereby promoting the phase change process and improving the sintering densification degree of the product. When the amount of β-silicon nitride seed crystals added is too much, excessive coarse columnar grains are easily formed during sintering, and many defects such as pores, dislocations, and primary cracks will be generated. These defects will become the source of material fracture, resulting in reduced mechanical properties of large-sized silicon nitride ceramic ball valves.

[0091] In some embodiments, the median particle size D50 of the silicon nitride powder is 0.3 μm-0.6 μm;

[0092] For the raw material powder of large-sized silicon nitride ceramic ball valves, the finer the better, and ultrafine powder is conducive to reducing the firing temperature. However, for molding, the finer the powder particle size, the worse the fluidity, the more likely it is to produce voids, and the density is not high. The impact is more obvious when molding large-sized silicon nitride products.

[0093] In some embodiments, in S1, the weight ratio of silicon nitride powder, sintering aid, and β-silicon nitride seed crystal is (90-96): (3-9): (1-2), preferably, (92-96): (3-6): (1-2);

[0094] And / or, the sintering aid includes one or more of cadmium oxide, aluminum oxide, magnesium oxide, lanthanum oxide, nickel oxide, and lutetium oxide;

[0095] Because the self-diffusion coefficient of silicon nitride is very small, the volume diffusion and grain boundary diffusion rates are also very small, and the ratio of its grain boundary energy to powder surface energy is much larger than that of other ionic compounds and metals, resulting in a very small sintering driving force. In addition, silicon nitride is very easy to decompose and oxidize at high temperatures. Therefore, a small amount of sintering aids needs to be added to the sintering of silicon nitride ceramics to produce a liquid phase at high temperatures, inhibit the decomposition of silicon nitride, and accelerate the reaction rate of silicon nitride. However, more sintering aids may lead to the formation of more glass phases at the grain boundaries. The presence of this glass phase will reduce the hardness and wear resistance of the material, and cause pollution to the products in material transportation. In particular, the lower the sintering aid content applied to large-sized silicon nitride ceramic ball valve cores, the better. Therefore, the content of sintering aids in the raw materials is controlled to be 3wt%-6wt%, optimizing the size and distribution of the grains, while reducing pollution to material transportation.

[0096] And / or, the average length of the β-silicon nitride seed crystal is 2.5 μm-3.5 μm, and the average aspect ratio is 9-11; preferably, the average length of the β-silicon nitride seed crystal is 3 μm, and the average aspect ratio is 10;

[0097] Experiments have found that adding β-silicon nitride seeds with an average length of 2.5μm-3.5μm and an average aspect ratio of 9-11 can better obtain a bimodal distribution microstructure.

[0098] And / or, the metal content in the silicon nitride powder is ≤300ppm.

[0099] In some embodiments, in S1, silicon nitride powder, sintering aid, and β-silicon nitride seed crystals are dispersed in an organic solvent and ball-milled to form a mixture; the rotation speed of the ball mill is 250 r / min-300 r / min. The organic solvent is preferably anhydrous ethanol.

[0100] Optionally, the ball milling time is 15 hours to 25 hours. As an example, the ball milling speed can be 250 r / min, 260 r / min, 270 r / min, 290 r / min, 300 r / min, etc., or within the range formed by any two of the above values; the ball milling time can be 15 hours, 18 hours, 20 hours, 22 hours, 24 hours, 25 hours, etc., or within the range formed by any two of the above values.

[0101] Mixing by ball milling promotes the formation of spherical particles and makes the mixture more uniform. Adding a small amount of β-silicon nitride seeds and specific pressing and sintering processes improves the uniformity of the ceramic blank, and produces more coarse columnar β-Si3N4 grains during sintering, which improves the fracture toughness and Weibull modulus of silicon nitride ceramics.

[0102] In some embodiments, in S3, the valve core blank is placed on a boron nitride plate, and then the boron nitride plate with the valve core blank is placed in a silicon nitride sagger for sintering to obtain a large-sized silicon nitride ceramic ball valve core.

[0103] Large-sized valve core blanks will shrink to a certain extent during the sintering process. The shrinkage of graphite and the valve core blank is inconsistent, and the friction generated is too large, causing defects such as deformation of the ceramic ball valve. The present invention adopts a boron nitride plate to support the valve core blank, and utilizes the lubricity of the boron nitride plate to effectively reduce the friction when the valve core blank shrinks, avoiding defects such as cracks or fractures caused by large friction caused by inconsistent shrinkage, thereby improving the sintering quality and yield rate of large-sized ball valves.

[0104] In the prior art, graphite saggers are generally used for sintering silicon nitride. However, the thermal expansion coefficients of graphite and silicon nitride are inconsistent. Large-sized valve core blanks are prone to cracks or deformation during the sintering process. The present invention uses high thermal conductivity silicon nitride saggers, which have the following advantages: 1. The thermal expansion coefficients of silicon nitride saggers and silicon nitride ceramic products are consistent, avoiding internal stress caused by mismatched thermal expansion coefficients, thereby reducing the risk of cracks and deformation; 2. Avoiding the influence of impurities brought by graphite saggers (carburization), thereby affecting product stability; 3. More uniform heat transfer is conducive to better rearrangement and densification of grains inside silicon nitride ceramics, reducing the risk of cracking of ceramic ball valves.

[0105] In some embodiments, in S1, the first pressing is performed by cold isostatic pressing, with a pressure of 80-100 MPa and a holding time of 50-100 s;

[0106] And / or, the second pressing is performed by cold isostatic pressing at a pressure of 160-200 MPa and a holding time of 100-200 s.

[0107] In some embodiments, the heating rate in S31 is 5-9° C. / min;

[0108] and / or, the heating rate in S32 is 2-3°C / min;

[0109] and / or, the heating rate in S33 is 1-2°C / min;

[0110] and / or, the heating rate in S34 is 1-2°C / min;

[0111] And / or, the cooling rate in S35 is 3-5°C / min.

[0112] As an example, the heating rate in S31 is, for example, 5°C / h, 5.5°C / h, 7°C / h, 8.5°C / h, 9°C / h, etc., or is within the range of any two of the above values; the heating rate in S32 is, for example, 2°C / h, 2.2°C / h, 2.5°C / h, 2.6°C / h, 3°C / h, etc., or is within the range of any two of the above values; the heating rate in S33 is, for example, 1°C / h, 1.2°C / h, 1.5°C / h, etc. / h, 1.6℃ / h, 2℃ / h, etc. or are within the range formed by any two of the above values; the heating rate in S34 is, for example, 1℃ / h, 1.2℃ / h, 1.5℃ / h, 1.6℃ / h, 2℃ / h, etc. or are within the range formed by any two of the above values; the heating rate in S35 is, for example, 3℃ / h, 3.5℃ / h, 4℃ / h, 4.8℃ / h, 5℃ / h, etc. or are within the range formed by any two of the above values.

[0113] The S31 stage heats up faster, reaching the specified temperature as soon as possible, compressing the heating time as much as possible, and saving costs; this temperature is lower than the conversion temperature of α silicon nitride, and after reaching the set temperature, it is kept warm so that the overall temperature of the valve core blank reaches a consistent level;

[0114] In the S32 stage, the temperature is slowly increased to convert the particles with lower melting points in the valve core blank into liquid phase. The silicon nitride particles move under the action of the surface tension of the liquid phase, and the rapid diffusion of the grain boundaries causes the sharp corners or diamond-shaped parts on the silicon nitride particles to become smaller or even disappear, making the silicon nitride particles gradually smooth, which is more conducive to the rearrangement of the silicon nitride particles, so that the gaps inside the valve core blank will migrate, merge, disappear, etc., the number of pores will be significantly reduced, and the valve core blank will be more dense. Therefore, in this stage, the heating rate is reduced, the nitrogen pressure is increased, and the insulation time is set, so that the capillary force formed by the liquid phase between the silicon nitride particles and the viscous flow of the liquid phase itself cause the silicon nitride particles to rearrange to achieve the most compact distribution; at the same time, α silicon nitride is transformed into β silicon nitride;

[0115] In the S33 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. The pre-added β silicon nitride grains as crystal nuclei promote the continued growth of new grains on them and promote the transformation process of α→β phase, thereby improving the densification degree of the valve core blank. Therefore, in this stage, the heating rate is reduced again, the nitrogen pressure is increased, and the insulation time is extended, so that the β silicon nitride grains in the valve core blank grow fully, while the small-particle silicon nitride powder tends to decrease, and the particle surface tends to be flat and smooth. The microstructure is manifested as an increase in the average grain size, the development of the enhanced bimodal structure, and the densification degree of the silicon nitride valve core blank is further improved, thereby increasing the bending strength and toughness of silicon nitride ceramics.

[0116] In the S34 stage, the silicon nitride particles are close to each other, contacting and bonding to form a continuous skeleton. In the process of dissolution and reprecipitation, there must be a region with less or no liquid phase. The silicon nitride particles are in direct contact through the sintering neck and form interconnected voids. In order to ensure the mechanical strength and qualified rate of the valve core blank, the heating rate is reduced again in this stage, the nitrogen pressure is significantly increased, and the insulation time is extended, so that the interconnected voids are gradually closed and transformed into isolated pores. Subsequently, with the extension of the insulation time, the stable shrinkage of the isolated pores gradually disappears, so that the bending strength and toughness of the silicon nitride ceramics reach the best.

[0117] In the S35 stage, when silicon nitride ceramics are cooling down, if the cooling speed is too fast, the internal stress may exceed the bearing capacity of the material, which may easily lead to stress concentration, resulting in cracks or fractures. Therefore, the cooling rate is controlled at 3-5℃ / min in this stage to improve the qualified rate in the production process.

[0118] In a second aspect, the present invention provides a silicon nitride ceramic ball valve core, which is manufactured using the method for preparing a large-size silicon nitride ceramic ball valve core.

[0119] In some embodiments, the silicon nitride ceramic ball valve core is a spherical body with a through hole in the center;

[0120] The outer diameter of the silicon nitride ceramic ball valve core is greater than 260 mm;

[0121] And / or, the diameter of the through hole is greater than 175 mm.

[0122] In a third aspect, the present invention provides a ball valve, which adopts the silicon nitride ceramic ball valve core.

[0123] In some embodiments, a silicon nitride ceramic ball valve core and a valve body are included. The silicon nitride ceramic ball valve core is a spherical body with a through hole in the center. 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. A valve stem groove is arranged on the outer diameter of one side of the spherical body. A metal valve stem seat is installed at the groove of the valve stem groove by using strong glue.

[0124] The connection between the metal valve stem seat and the valve stem is sprayed with an anti-corrosion coating material, wherein the anti-corrosion coating material includes one or more of PFA, PTFE, and PEEK;

[0125] The spherical body is connected to the valve stem through the valve stem groove and the metal valve stem seat.

[0126] 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.

[0127] Example 1

[0128] This embodiment provides a large-sized silicon nitride ceramic ball valve core, and the preparation method thereof comprises the following steps:

[0129] S1. Preparation of silicon block

[0130] S11, according to the weight percentage of ceramic powder, 92% of high-purity silicon nitride powder, 4% of aluminum oxide, 2% of lanthanum oxide, and 2% of β-silicon nitride seeds are dispersed in anhydrous ethanol and put into a ball mill for thorough mixing, the ball mill speed is 250r / min, and the mixing time is 20 hours to form ceramic powder; wherein the high-purity silicon nitride powder has a D50 particle size of ≤0.6μm, an α phase content of ≥93wt%, and a metal content (Fe, Al, Ca) of ≤300ppm; the average length of the β-silicon nitride seeds is 3μm, and the average aspect ratio is 10;

[0131] S12, putting the ceramic powder obtained in S11 into a mold, pressing and molding it through cold isostatic pressing, crushing it through a crusher, passing it through a 50-mesh sieve, and taking the sieve-under material as a spherical powder with a particle size D50 of about 250 μm to form a mixed granulation powder; the cold isostatic pressing pressure is 85 MPa, and the holding time is 60 s;

[0132] S13, placing the mixed granulated powder obtained in S12 in a mold, and obtaining a cylindrical body after cold isostatic pressing; the cold isostatic pressing pressure is 180 MPa, and the holding time is 150 s;

[0133] S2, turning the cylindrical blank obtained in S13 into a valve core blank (a spherical body with a through hole in the center and a valve stem groove on the outer diameter of one side of the spherical body);

[0134] S3. Preparation of silicon nitride ceramic ball valve core blank

[0135] The valve core blank made of S2 is placed on a boron nitride plate, and then the boron nitride plate with the valve core blank is placed in a silicon nitride sagger and placed in a sintering furnace for gas pressure sintering under a nitrogen atmosphere to prepare a silicon nitride ceramic ball valve core with an outer diameter of φ260 mm and a center through hole diameter of 175 mm.

[0136] Gas pressure sintering includes four heating stages and two cooling stages, specifically:

[0137] S31. Under the condition of vacuum degree of 100Pa, the first stage of sintering process is heated from room temperature to 1170℃, the heating rate is 7.5℃ / min, and the temperature is kept for 45min; the first stage is heated faster, reaching 1170℃ as soon as possible, and the heating time is compressed as much as possible to save costs; this temperature is lower than the conversion temperature of α silicon nitride, and after reaching the predetermined temperature, the temperature is kept to make the overall temperature of the valve core blank consistent;

[0138] S32, the second sintering process is heated from 1170℃ to 1520℃, with a heating rate of 2.3℃ / min; keep warm at 1520℃ for 45min, fill with nitrogen to a pressure of 0.6MPa and keep the pressure; in this stage, the slow heating makes the particles with lower melting point of the valve core blank transform into liquid phase, the silicon nitride particles move under the action of the surface tension of the liquid phase, and the rapid diffusion of the grain boundaries causes the sharp corners or diamond parts on the silicon nitride particles to become smaller or even disappear, making the silicon nitride particles gradually become smooth, which is more conducive to the rearrangement of the silicon nitride particles, so that the gaps inside the valve core blank migrate, merge, disappear, etc., the number of pores will be significantly reduced, and the valve core blank will be more dense. Therefore, in this stage, the heating rate is reduced, the nitrogen pressure is increased, and the insulation time is set, so that the capillary force formed by the liquid phase between the silicon nitride particles and the viscous flow of the liquid phase itself cause the silicon nitride particles to rearrange to achieve the most compact distribution; at the same time, α silicon nitride is transformed into β silicon nitride.

[0139] S33, the third sintering process is from 1520℃ to 1720℃, the heating rate is 1.4℃ / min; keep at 1720℃ for 80min, the nitrogen pressure is 2.2MPa; as the sintering temperature further increases, 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. The pre-added β silicon nitride grains as crystal nuclei promote the continued growth of new grains on them, and promote the transformation process of α phase silicon nitride to β phase, thereby improving the densification degree of the valve core blank. Therefore, in this stage, the heating rate is reduced again, the nitrogen pressure is increased, and the holding time is extended, so that the β silicon nitride grains in the valve core blank grow fully, and the small particles of silicon nitride powder tend to decrease, and the particle surface tends to be flat and smooth. The microstructure shows an increase in the average grain size, which enhances the development of the bimodal structure and further improves the densification of the silicon nitride valve core blank, thereby increasing the bending strength and toughness of the silicon nitride ceramic.

[0140] S34, the fourth sintering process is from 1720℃ to 1830℃, the heating rate is 1.2℃ / min; keep warm at 1830℃ for 130min, and the nitrogen pressure is 4MPa. In this stage, the silicon nitride particles are close to each other, contact, and bond to form a continuous skeleton. In the process of dissolution and reprecipitation, there must be a liquid phase area with less or no liquid phase. The silicon nitride particles are in direct contact through the sintering neck and form interconnected gaps. In order to ensure the mechanical strength and qualified rate of the valve core blank, the heating rate is reduced again in this stage, the nitrogen pressure is significantly increased, and the insulation time is extended, so that the interconnected gaps are gradually closed and transformed into isolated pores. Subsequently, the insulation time is extended, and the isolated pores shrink steadily until they gradually disappear, so that the bending strength and toughness of silicon nitride ceramics reach the best.

[0141] S35, cooling includes two stages:

[0142] The first stage is at a rate of 3.5℃ / min, cooling to 1450℃ and keeping warm for 360min. In this stage, if the cooling speed of silicon nitride ceramic is too fast, the internal stress may exceed the bearing capacity of the material, which may easily lead to stress concentration and cracks or fractures. Therefore, the cooling rate is controlled in this stage to improve the qualified rate in the production process.

[0143] The second stage is free cooling.

[0144] The structural diagram of the sintered silicon nitride ceramic ball valve core is as follows Figure 1The valve core 1 of the silicon nitride ceramic ball valve is a spherical body with a through hole 11 in the center, and a valve stem groove 12 for installing a valve stem seat is provided on the outer wall of one side of the spherical body; the actual picture of the valve core of the silicon nitride ceramic ball valve is shown in Figure 2 Its microstructure can be seen Figure 3 .

[0145] S4. A ball valve is prepared by using the sintered silicon nitride ceramic ball valve core in S35. The silicon nitride ceramic ball valve core is arranged in the valve body. The central through hole of the silicon nitride ceramic ball valve core is connected to the channel of the valve body. A stainless steel valve stem seat is installed at the groove of the valve stem groove with strong glue to reduce the torque force borne by the ball valve during opening and closing. The outer surface of the stainless steel valve stem seat is sprayed with PFA (polyperfluoroethylene propylene) to improve the corrosion resistance of the ceramic valve. The spherical body is connected to the valve stem through the valve stem groove and the metal valve stem seat.

[0146] The average density of the large-sized silicon nitride ball valve core prepared in this embodiment is 99.52% measured by the drainage method, the average bending strength is 1189MPa measured by the three-point bending method, and the average Weibull modulus is 26.5 measured by GB / T 40005-2021. The Weibull modulus is an important parameter to describe the brittle fracture characteristics of the material, which is used to reflect the discreteness of the material strength and is particularly important for ceramic materials; improving the Weibull modulus and fracture toughness of the large-sized silicon nitride ball valve core can reduce the probability of cracks in the large-sized silicon nitride ball valve core during the preparation process and use.

[0147] Example 2

[0148] This embodiment provides a large-sized silicon nitride ceramic ball valve core, and the preparation method thereof comprises the following steps:

[0149] S1. Preparation of silicon block

[0150] S11, according to the weight percentage of ceramic powder, 94% of high-purity silicon nitride powder, 3% of aluminum oxide, 2% of nickel oxide, and 1% of β-silicon nitride seeds are dispersed in anhydrous ethanol and put into a ball mill for thorough mixing, the ball mill speed is 280r / min, and the mixing time is 22 hours to form ceramic powder; wherein the median particle size of the high-purity silicon nitride powder is ≤0.5μm, the α-phase content is ≥94wt%, and the metal content (Fe, Al, Ca) is ≤300ppm; the average length of the β-silicon nitride seeds is 3.5μm, and the average aspect ratio is 11;

[0151] S12, putting the ceramic powder obtained in S11 into a mold, pressing and molding it through cold isostatic pressing, crushing it through a crusher, passing it through a 50-mesh sieve, and taking the sieve-under material as a spherical powder with a particle size D50 of about 250 μm to form a mixed granulation powder; the cold isostatic pressing pressure is 80 MPa, and the holding time is 50 s;

[0152] S13, placing the mixed granulated powder obtained in S12 in a mold, and obtaining a cylindrical body after cold isostatic pressing; the cold isostatic pressing pressure is 160 MPa, and the holding time is 200 s;

[0153] S2, turning the cylindrical blank obtained in S13 into a valve core blank (a spherical body with a through hole in the center and a valve stem groove on the outer diameter of one side of the spherical body);

[0154] S3. Preparation of silicon nitride ceramic ball valve core

[0155] The valve core blank made of S2 is placed on a boron nitride plate, and then the boron nitride plate with the valve core blank is placed in a silicon nitride sagger and placed in an atmosphere pressure sintering furnace for gas pressure sintering under a nitrogen atmosphere to prepare a silicon nitride ceramic ball valve core with an outer diameter of φ260mm and a center through hole diameter of 175mm.

[0156] Gas pressure sintering includes four heating stages and two cooling stages, specifically:

[0157] Under the condition of vacuum degree of 80Pa, the first stage of sintering process was heated from room temperature to 1200℃, the heating rate was 5℃ / min, and the holding time was 60min;

[0158] The second sintering process was from 1200℃ to 1480℃, with a heating rate of 2.3℃ / min; the temperature was kept at 1480℃ for 30min, and the nitrogen atmosphere pressure was 0.5MPa;

[0159] The third sintering process was from 1480°C to 1690°C, with a heating rate of 1.4°C / min; the temperature was kept at 1690°C for 60 minutes, and the nitrogen atmosphere pressure was 1.5MPa.

[0160] The fourth sintering process was from 1690°C to 1840°C, with a heating rate of 1.2°C / min; the temperature was kept at 1840°C for 100 minutes, and the nitrogen atmosphere pressure was 4 MPa.

[0161] Cooling consists of two stages:

[0162] The rate of the first stage was 5°C / min, the temperature was reduced to 1400°C and kept at this temperature for 300 min.

[0163] The second stage is free cooling.

[0164] The microstructure of the sintered silicon nitride ceramic ball valve core is shown in the figure. Figure 4 .

[0165] S4. In S3, a metal valve stem seat is installed at the groove of the ball valve core using strong glue to reduce the torque force borne by the ball valve during opening and closing. In addition, the stainless steel valve seat is coated with PFA (polyperfluoroethylene propylene) to improve the corrosion resistance of the ceramic valve.

[0166] The large-sized silicon nitride ball valve prepared in this embodiment has a density of 99.71%, a bending strength of 1260 MPa, and a Weibull modulus of 30.

[0167] Example 3

[0168] This embodiment provides a large-sized silicon nitride ceramic ball valve core, and the preparation method thereof comprises the following steps:

[0169] S1. Preparation of silicon block

[0170] S11, according to the weight percentage of ceramic powder, 96% of high-purity silicon nitride powder, 2% of aluminum oxide, 1% of cadmium oxide, and 1% of β-silicon nitride seeds are dispersed in anhydrous ethanol and put into a ball mill for thorough mixing, the ball mill speed is 300r / min, and the mixing time is 24 hours to form ceramic powder; wherein the median particle size of the high-purity silicon nitride powder is ≤0.3μm, the α-phase content is ≥95wt%, and the metal content (Fe, Al, Ca) is ≤300ppm; the average length of the β-silicon nitride seeds is 2.5μm, and the average aspect ratio is 9;

[0171] S12, the ceramic powder obtained in S11 is placed in a vacuum bag, pressed by cold isostatic pressing, crushed by a crusher, passed through a 50-mesh sieve, and the sieve-through material is a spherical powder with a particle size D50 of about 250 μm, forming a mixed granulation powder; the cold isostatic pressing pressure is 100 MPa, and the holding time is 100 s;

[0172] S13, placing the mixed granulated powder obtained in S12 in a mold, and obtaining a cylindrical body after cold isostatic pressing; the cold isostatic pressing pressure is 200 MPa, and the holding time is 100 s;

[0173] S2, turning the cylindrical blank obtained in S13 into a valve core blank (a spherical body with a through hole in the center and a valve stem groove on the outer diameter of one side of the spherical body);

[0174] S3. Preparation of silicon nitride ceramic ball valve core

[0175] The valve core blank made of S2 is placed on a boron nitride plate, and then the boron nitride plate with the valve core blank is placed in a silicon nitride sagger and placed in an atmosphere pressure sintering furnace for gas pressure sintering under a nitrogen atmosphere to prepare a silicon nitride ceramic ball valve core with an outer diameter of φ260mm and a center through hole diameter of 175mm.

[0176] Gas pressure sintering includes four heating stages and two cooling stages, specifically:

[0177] Under the condition of vacuum degree of 110Pa, the first stage of sintering process was from room temperature to 1150℃, the heating rate was 9℃ / min, and the holding time was 30min;

[0178] The second sintering process was from 1150℃ to 1520℃, with a heating rate of 2℃ / min; the temperature was kept at 1520℃ for 100min, and the nitrogen atmosphere pressure was 1MPa;

[0179] The third sintering process was from 1520°C to 1740°C, with a heating rate of 1.2°C / min; the temperature was kept at 1740°C for 120 minutes, and the nitrogen atmosphere pressure was 2.5MPa.

[0180] The fourth sintering process was from 1740°C to 1860°C, with a heating rate of 1°C / min; the temperature was kept at 1860°C for 200 minutes, and the nitrogen atmosphere pressure was 4 MPa.

[0181] Cooling consists of two stages:

[0182] The rate of the first stage was 3°C / min, the temperature was reduced to 1400°C and kept at this temperature for 400min.

[0183] The second stage is natural cooling to room temperature.

[0184] The microstructure of the sintered silicon nitride ceramic ball valve core is shown in the figure. Figure 5 .

[0185] S4. In S3, a metal valve stem seat is installed at the groove of the ball valve core using strong glue to reduce the torque force borne by the ball valve during opening and closing. In addition, the stainless steel valve seat is coated with PFA (polyperfluoroethylene propylene) to improve the corrosion resistance of the ceramic valve.

[0186] The large-sized silicon nitride ceramic ball valve core prepared in this embodiment has a density of 99.8%, a bending strength of 1293 MPa, and a Weibull modulus of 35.

[0187] Example 4

[0188] The present embodiment provides a large-sized silicon nitride ceramic ball valve core, and its preparation method is basically the same as that of Example 1, except that the sintering aid content is 9%, and the ceramic powder is calculated by weight percentage, specifically 90% high-purity silicon nitride powder, 5% aluminum oxide, 4% lanthanum oxide, and 1% β-silicon nitride seeds.

[0189] The sintered silicon nitride ceramic ball valve core was analyzed by an electron probe microanalyzer, and the analysis results were compared with the silicon nitride ceramic ball valve core of Example 1. The EPMA image of the silicon nitride ceramic ball valve core of Example 1 is as follows: Fig. 9 The EPMA image of the silicon nitride ceramic ball valve core of Example 4 is as follows: Fig.10 It can be seen that the grain distribution of the silicon nitride ceramic ball valve core of Example 1 is more uniform, and there is less glass phase inside, so that the wear resistance of the ceramic ball valve is enhanced.

[0190] The density of the silicon nitride ceramic ball valve core of Example 4 is 98.3%, the bending strength is 894 MPa, and the Weibull modulus is 20.

[0191] Comparative Example 1

[0192] This comparative example provides a large-sized silicon nitride ceramic ball valve core, and its preparation method is basically the same as that of Example 1, except that the sintering process is as follows:

[0193] The green billet was heated to 1300°C under a nitrogen pressure of 0.2 MPa and kept at this temperature for 2 hours.

[0194] Maintain the nitrogen pressure at 0.3 MPa, raise the temperature to 1700°C, and keep it for 10 hours.

[0195] Increase the nitrogen pressure to 2 MPa, increase the temperature to 2000°C, and keep warm for 12 hours.

[0196] The density of the sintered silicon nitride ceramic ball valve core is 97.12%, the bending strength is 740MPa, the Weibull modulus is 15, and its microstructure is shown in Fig.11 .

[0197] Comparative Example 2

[0198] This comparative example provides a large-sized silicon nitride ceramic ball valve core, and its preparation method is basically the same as that of Example 1, except that the pressing process does not use the method of cold isostatic pressing to granulate into spherical powder, but directly uses cold isostatic pressing for pressing, and S12 and S13 are combined into:

[0199] S12, putting the ceramic powder obtained in S11 into a mold, and pressing it into a cylindrical body by cold isostatic pressing; the cold isostatic pressing pressure is 180 MPa, and the holding time is 150 s.

[0200] The sintered silicon nitride ceramic ball valve core is compared with the silicon nitride ceramic ball valve core of Example 1. The metallographic image of the silicon nitride ceramic ball valve core of Example 1 is as follows: Figure 7 The metallographic image of the silicon nitride ceramic ball valve core of comparative example 2 is as follows: Figure 8It can be seen that the silicon nitride ceramic ball valve core of Example 1 has uniform grain distribution and almost no pores.

[0201] The density of the silicon nitride ceramic ball valve core of Comparative Example 2 is 98.0%, the bending strength is 800 MPa, and the Weibull modulus is 18.

[0202] Comparative Example 3

[0203] This embodiment provides a large-sized silicon nitride ceramic ball valve core, and its preparation method is basically the same as that of embodiment 1, except that no β-silicon nitride seed crystal is added, and step S11 is specifically as follows:

[0204] Weigh high-purity silicon nitride powder, aluminum oxide, and lanthanum oxide of the same weight as in Example 1, disperse them in anhydrous ethanol, and place them in a ball mill for thorough mixing. The ball mill speed is 250 r / min, and the mixing time is 20 hours to form a ceramic powder; wherein the high-purity silicon nitride powder has a D50 particle size of ≤0.6 μm, an α phase content of ≥93wt%, and a metal content (Fe, Al, Ca) of ≤300 ppm;

[0205] Comparative Example 3 Sintered silicon nitride ceramic ball valve core, its microstructure Figure 6 The density of silicon nitride ceramic ball valve core is 98.5%, the bending strength is 925MPa, and the Weibull modulus is 23.

[0206] Mass production test

[0207] According to the preparation method of Example 1, 6 large-sized silicon nitride ceramic ball valve cores were prepared at the same time, marked as ABCDEF, and these large-sized silicon nitride ceramic ball valve cores were tested. The results are shown in Table 1:

[0208] Table 1 Mass production test results

[0209]

[0210] The results show that the average density of the large-size silicon nitride ceramic ball valve core prepared by the preparation method of Example 1 is 99.52%, the average bending strength is 1189 MPa, and the average Weibull modulus is 26.5, indicating that this method is suitable for batch production. The density range of the product is between 99.4% and 99.6%, the bending strength range is between 1120 MPa and 1220 MPa, and the Weibull modulus range is between 25 and 28, indicating that the ceramic ball valve prepared by this method has good stability and a high product qualification rate.

[0211] Test Case

[0212] According to the theoretical analysis of the microstructure diagram, the density of the silicon nitride ceramic ball valve core prepared in Examples 1 to 3 is 99.52-99.8%, the bending strength is 1189-1293MPa, and the Weibull modulus is 26.5-35. The mechanical properties of silicon nitride ceramics depend on their microstructure, especially the morphology and size of β-Si3N4 grains. According to the crack bridging and deflection toughening model, when the number of coarse grains with high aspect ratio and wide diameter in the silicon nitride microstructure increases, its mechanical properties will be improved. The SEM photos of Examples 1 to 3 after polishing and corrosion are shown in Figures 3 and 4, respectively. Figure 4 , Figure 5 As shown in the figure, it can be seen that the microstructure of each sintered body is composed of well-developed long columnar β-Si3N4 grains of different particle sizes stacked together. The addition of β-Si3N4 seeds makes the grain development tend to be perfect, and makes the peak type of the aspect ratio of some columnar grains develop from narrow and high to wide and low (grain coarsening phenomenon), indicating that the distribution range of the grain aspect ratio is gradually expanded, the geometric size difference between the grains is improved, and the double-peak mode microstructure is strengthened, thereby improving the mechanical properties of silicon nitride ceramics.

[0213] Compared with Example 1, Comparative Example 3 did not add β-silicon nitride seed crystals for mixing. The SEM photo of Comparative Example 3 after polishing and etching is as follows: Figure 6 As shown in the figure, only the dissolution and re-sintering process of silicon nitride itself is relied on to realize the transformation process of α→β phase and form long columnar grains. The grain size is relatively small compared with that of the sample with added β-Si3N4, which makes the grain aspect ratio distribution diagram narrow and high, thereby weakening the double-peak microstructure. Secondly, the fine grains can absorb less energy than the coarse long columnar grains in the process of resisting crack propagation, so the mechanical properties of the ceramic ball valve are reduced.

[0214] Figure 3 , Figure 4 , Figure 5 , Figure 6 The left picture in the figure is the SEM picture of each sample, and the right picture is the statistical result of measuring the aspect ratio of columnar crystals by selecting 100 representative grains from each photo. It can be seen that the aspect ratio of columnar crystals developed from narrow and high to wide and low after adding β-silicon nitride seeds compared with that without adding β-silicon nitride seeds, indicating that the distribution range of silicon nitride ball valve grains has been expanded, that is, the bimodal mode microstructure has been enhanced.

[0215] Compared with Example 1, Comparative Example 2 did not use cold isostatic pressing to prepare the ball valve, which may lead to uneven particle distribution, affect the uniformity of the microstructure of the ceramic, increase the porosity in the local area, and reduce the mechanical properties of the silicon nitride ceramic ball valve. Figure 7 and Figure 8 As shown in the figure, Example 1 contains a small amount of tiny pores with a pore diameter of only 1-2 μm, while Comparative Example 2 does not undergo a granulation process, resulting in a large number of pores in a local area with a pore diameter of about 5.86-48.34 μm, thereby reducing the wear resistance of the silicon nitride ceramic ball valve.

[0216] Compared with Example 1, Example 4 increases the sintering aid content in the large-size ball valve to 9%. The polished and uncorroded photos of Example 1 and Example 4 are as follows: Fig. 9 , Fig.10 As shown. The sintering aid forms a liquid phase during the sintering process, which promotes the densification of silicon nitride ceramics and improves its microstructure, and finally forms a glass phase (white area in the figure) and silicon nitride grains that are integrated. From the comparison of the two figures, it can be concluded that the glass phase content of Example 4 is greater than that of Example 1 (the white area is the glass phase), and the glass phase is usually softer than the silicon nitride matrix. Under the working conditions in the wear-resistant field, the glass phase will become a prone area of ​​wear, thereby reducing the overall wear resistance of the ball valve.

[0217] Test method: Test the weight of a batch of silicon nitride valve cores and then put them in a ball mill, add water to completely cover the silicon nitride valve core, the ball mill speed is 300rpm, and after ball milling for 12 hours, the silicon nitride valve core is dried, the weight is measured, and the wear result is calculated. The calculation formula is wear = (weight before ball milling - weight after ball milling) / weight before ball milling. Table 2 is the wear test results of silicon nitride balls prepared by the same process of Example 1, Example 4 and Comparative Examples 1-3.

[0218] Table 2 Wear test results

[0219]

[0220] Compared with Example 1, the maximum sintering temperature in the sintering process of Comparative Example 1 is significantly increased, which accelerates the grain growth rate, resulting in abnormal coarsening of the grains, making the grain size difference in the local area large, weakening the bimodal microstructure and possibly causing cracks and dislocations, ultimately affecting the mechanical properties of the ceramic ball valve. The SEM photos of the two after polishing and corrosion are shown in FIG. Figure 3 and Fig.11 It can be seen that the aspect ratio of the columnar grains in Example 1 develops from narrow and high to wide and low, indicating that the distribution range of the silicon nitride ball valve grains is expanded, that is, the bimodal mode microstructure is enhanced, while the opposite is true for Comparative Example 1. In addition, with the increase of sintering temperature (≥1900℃), the grains overgrow on the β-Si3N4 seed crystals, which may produce more microscopic pores, reducing the bending strength of the ceramic ball valve.

[0221] 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 method for preparing a large-size silicon nitride ceramic ball valve core, characterized in that: The steps include: S1, using a mixture of silicon nitride powder, a sintering aid and β-silicon nitride seed crystals as raw materials, sequentially pressing and forming the mixture into spherical particles, and pressing the mixture into a green body for a second time; S2, turning the blank obtained in S1 into a valve core blank; S3, placing the valve core blank obtained in S2 on a boron nitride plate, and then placing the boron nitride plate with the valve core blank into a silicon nitride sagger for sintering to obtain a silicon nitride ceramic ball valve core; The outer diameter of the silicon nitride ceramic ball valve core is greater than 200 mm; The sintering process includes the following steps: S31. Under a vacuum degree of 80-110Pa, heat from room temperature to 1150-1200°C and keep warm for 30-60min; S32, heat to 1480-1520℃, keep warm for 30-100min, and fill with nitrogen until the pressure is 0.5-1MPa; S33, heat to 1690-1740℃ and keep warm for 60-120min; at 1690-1740℃, the nitrogen pressure is 1.5-2.5MPa; S34, heat to 1820-1860℃ and keep warm for 100-200min, nitrogen pressure is 3-4MPa; S35, cool down to 1400-1450℃ and keep warm for 300-400min; then cool down naturally; In S1, the first pressing is performed by cold isostatic pressing, with a pressure of 80-100 MPa and a holding time of 50-100 s; The second pressing is performed by cold isostatic pressing, with a pressure of 160-200 MPa and a holding time of 100-200 s.

2. The preparation method according to claim 1, characterized in that: In S1, the content of α-phase silicon nitride powder in the silicon nitride powder is ≥ 93wt%; and / or, the content of the β-silicon nitride seed crystal is 1wt%-2wt% of the total weight of the raw material; And / or, the median particle size D50 of the silicon nitride powder is 0.3 μm-0.6 μm.

3. The preparation method according to claim 2, characterized in that: The weight ratio of silicon nitride powder, sintering aid and beta silicon nitride seed is (90-96):(3-9):(1-2); And / or, the sintering aid includes one or more of cadmium oxide, aluminum oxide, magnesium oxide, lanthanum oxide, nickel oxide, and lutetium oxide; and / or, the average length of the β-silicon nitride seed crystal is 2.5 μm-3.5 μm, and the average aspect ratio is 9-11; And / or, the metal content of the silicon nitride powder is ≤300ppm.

4. The preparation method according to any one of claims 1 to 3, characterized in that: In S1, silicon nitride powder, sintering aid, and β-silicon nitride seed crystals are dispersed in an organic solvent and ball-milled to form a mixture. The rotation speed of the ball mill is 250 r / min-300 r / min, and the organic solvent is anhydrous ethanol.

5. The preparation method according to any one of claims 1 to 3, characterized in that: The heating rate in S31 is 5-9°C / min; and / or, the heating rate in S32 is 2-3°C / min; and / or, the heating rate in S33 is 1-2°C / min; and / or, the heating rate in S34 is 1-2°C / min; And / or, the cooling rate in S35 is 3-5°C / min.

6. A silicon nitride ceramic ball valve core, made by the method for preparing a large-size silicon nitride ceramic ball valve core according to any one of claims 1 to 5.

7. The silicon nitride ceramic ball valve core according to claim 6, characterized in that: The silicon nitride ceramic ball valve core is a spherical body with a through hole at the center; The outer diameter of the silicon nitride ceramic ball valve core is greater than 260 mm; And / or, the diameter of the through hole is greater than 175 mm.

8. A ball valve, characterized in that: The ball valve adopts the silicon nitride ceramic ball valve core described in claim 6 or 7.

9. The ball valve according to claim 8, characterized in that: It includes a silicon nitride ceramic ball valve core and a valve body. The silicon nitride ceramic ball valve core is a spherical body with a through hole in the center. 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. A valve stem groove is arranged on the outer wall of one side of the spherical body. A metal valve stem seat is installed at the groove of the valve stem groove by using strong glue. The connection between the metal valve stem seat and the valve stem is sprayed with an anti-corrosion coating material, wherein the anti-corrosion coating material includes one or more of PFA, PTFE, and PEEK; The spherical body is connected to the valve stem through the valve stem groove and the metal valve stem seat.

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