A method for preventing surface deposition of corundum in normal pressure solid phase sintered silicon carbide ceramics

Abstract

A method for preventing the deposition of schillerization on the surface of normal pressure solid phase sintered silicon carbide ceramic; including at least one of the following steps: a step before sintering, a step during sintering or a step after sintering; the step before sintering includes covering the surface of the sintered sample with graphite paper, or spraying a boron carbide ceramic coating or a boron nitride ceramic coating on the surface of the sintered sample; the step during sintering includes reducing the maximum sintering temperature from 2200 DEG C to 2050-2100 DEG C, while extending the holding time by 0.5-1 hour, or increasing the gas flow of the protective gas by 5-15 m 3 / h; the step after sintering includes placing the silicon carbide ceramic product into a container containing molten NaOH for high temperature corrosion, effectively reducing the surface roughness of the sealing material, realizing near net size forming of the silicon carbide ceramic sealing element, and having great practical significance for reducing production cost.

Description

Technical Field

[0001] This application relates to the field of silicon carbide ceramic sintering application technology, and more specifically, to a method for preventing the deposition of scintillation crystals on the surface of silicon carbide ceramics sintered under normal pressure. Background Technology

[0002] Silicon carbide (SiC) ceramics possess high hardness, excellent high-temperature resistance, corrosion resistance, and high thermal conductivity, enabling their application in harsh environments such as hot water, hot oil, radiation, environments containing solid particles, and corrosive media. Therefore, SiC has seen rapid development in mechanical seals and wear-resistant components in recent years. In the chemical industry, SiC ceramics are commonly used to prepare pump seals for corrosive media such as strong acids and alkalis, significantly increasing equipment reliability and stability. SiC is also frequently used for sealing rings in marine seawater pumps. Because seawater often contains a large amount of solid particles, soft materials, although having a low coefficient of friction, are easily scratched by these particles, leading to seal failure. SiC, with its high hardness, maintains good operating conditions in seawater environments containing solid particles, improving equipment reliability. Bearings made of SiC ceramics also have many outstanding advantages compared with other existing materials. For example, compared with tungsten carbide bearings, SiC has higher hardness and lower coefficient of friction; compared with graphite bearings, SiC has good wear resistance and longer service life; compared with metal bearings, SiC has excellent corrosion resistance and high temperature resistance, and can be used in various environments and media. In addition, due to its high hardness, SiC is also commonly used to make grinding wheels and other abrasives, pump shafts of magnetic pumps, thrust discs, bushings, and other components that require high wear resistance and corrosion resistance.

[0003] Atmospheric pressure solid-state sintered silicon carbide ceramics, due to their low sintering aid content during preparation and the absence of second-phase or grain boundary phases (except for a small amount of residual carbon), exhibit excellent high-temperature performance, remaining largely unchanged up to 1600℃. The absence of a second phase also fully demonstrates the chemical stability of SiC, making it resistant to corrosion from various strong acids and alkalis. Therefore, atmospheric pressure solid-state sintered silicon carbide is widely used in sealing materials. Furthermore, atmospheric pressure sintering allows for densification of SiC without external pressure, thus unrestricting the shape of the product and enabling near-net-shape fabrication without subsequent processing, significantly reducing production costs. However, the sintering temperature of currently prepared atmospheric pressure solid-state sintered silicon carbide ceramics is typically high, around 2200℃. At this temperature, the surface of the silicon carbide sealing material is prone to flash crystallization, leading to increased surface roughness (greater than 0.5 μm). This necessitates further processing to meet application requirements, increasing production costs.

[0004] Therefore, it is necessary for the inventors to design a method to prevent the deposition of scintillation crystals on the surface of silicon carbide ceramics sintered under normal pressure, so as to overcome the problem of large surface roughness caused by scintillation crystals during the solid-state sintering of silicon carbide ceramics under normal pressure. Summary of the Invention

[0005] The main objective of this application is to provide a method for preventing the deposition of scintillates on the surface of silicon carbide ceramics sintered under normal pressure, so as to overcome the problem of large surface roughness caused by scintillates during the solid-state sintering of silicon carbide ceramics under normal pressure.

[0006] To achieve the above objectives, this application provides a method for preventing the deposition of scintillation crystals on the surface of silicon carbide ceramics sintered under normal pressure, comprising at least one of a pre-sintering step, a sintering step, or a post-sintering step.

[0007] The pre-sintering steps include covering the surface of the sintered sample with graphite paper, or spraying a boron carbide ceramic coating or a boron nitride ceramic coating onto the surface of the sintered sample.

[0008] The sintering process includes reducing the maximum sintering temperature from 2200℃ to 2050-2100℃, while extending the holding time by 0.5-1 hour, or increasing the flow rate of the protective gas by 5-15 m³ / h when introducing the protective gas at 1200-1700℃. 3 / h;

[0009] The post-sintering steps include placing the silicon carbide ceramic product with a crystalline surface into a container filled with molten NaOH for high-temperature etching, removing it, cleaning it with dilute hydrochloric acid, rinsing it with deionized water, and then drying it.

[0010] The main reason for the formation of scintillation crystals on the surface of silicon carbide ceramics during atmospheric pressure solid-state sintering is that, during the densification stage of ceramic sintering, silicon carbide sublimates from solid to SiC, SiC2, and Si2C gas at high temperature. These gases are transported to the surface of silicon carbide ceramic grains along with the protective gas Ar. Then, the silicon carbide gas is deposited and grows into SiC grains on the surface of silicon carbide grains in the ceramic, which leads to an increase in the surface roughness of the material, with Ra greater than 0.5 μm.

[0011] Therefore, in order to reduce the growth of these excess grains and maintain the roughness of the prepared silicon carbide ceramic sealing surface at the post-forming state (Ra below 0.2 μm), the main methods adopted are as follows:

[0012] (1) Reduce the maximum sintering temperature to 2050-2100℃ (usually sinter at 2200℃ for 1 hour), and the heating rate is 1-5℃ / min. This can prevent silicon carbide ceramics from sublimating into SiC-like gases. When introducing Ar protective gas at 1200-1700℃, control the Ar gas flow rate to 10-25m³. 3 / h; at the same time, in order to promote densification, the heat preservation time is 1.5-3 hours.

[0013] (2) The sintering temperature is 2200℃, the holding time is 1-2 hours, and when Ar protective gas is introduced at 1200-1700℃, the Ar gas flow rate is increased to control the Ar gas flow rate to 25-30m³. 3 / h, by utilizing the increased Ar gas flow rate, SiC-type gases can be transported to the exhaust valve for discharge, thus preventing SiC vapor from depositing on the surface of silicon carbide ceramics.

[0014] (3) Cover the sintered sample surface with 0.1-0.3 mm of graphite paper. The graphite paper can be made into a cover shape to cover the entire sintered sample. On the one hand, it can block the deposition of SiC vapor. On the other hand, it can serve as a heat insulation layer between the heating element and the sample, reducing the direct radiation of the heating element's thermal field. This can lower the sintering temperature by 1-5℃ and make the temperature more uniform, thus preventing SiC vapor from depositing locally at low temperatures.

[0015] (4) Spray boron carbide or boron nitride ceramic slurry that does not react with silicon carbide directly onto the surface of the sintered silicon carbide sample. The solid content of the slurry is 30-45wt%. Control the pressure and spray the slurry coating evenly onto the sample surface through the atomizing nozzle. The thickness is 10-30um. After sintering, the surface of the silicon carbide ceramic can be cleaned directly by purging gas.

[0016] (5) The sintered silicon carbide ceramic product with a crystalline surface can be placed in a crucible or pool filled with molten NaOH. The temperature should be controlled between 500℃ and 700℃, and the etching time in air should be between 10s and 600s. After removal, it should be cleaned with dilute hydrochloric acid and then with deionized water before drying. The principle is: SiC + 2NaOH + 2O2 = Na2SiO3 + CO2 + H2O.

[0017] A further improvement is that the graphite paper thickness is 0.1-0.3 mm, the maximum sintering temperature is 2200℃, the holding time is 1-2 hours, and an inert protective gas is introduced at 1200-1700℃, with the protective gas flow rate controlled at 10-25 m³ / h. 3 / h.

[0018] A further improvement is that the solid content of the ceramic slurry for the boron carbide or boron nitride ceramic coating is 30-45 wt%, the thickness of the ceramic coating is 10-30 μm, and the surface of the sintered silicon carbide ceramic is cleaned by purging gas after sintering. The maximum sintering temperature is 2200℃, the holding time is 1-2 hours, and an inert protective gas is introduced at 1200-1700℃, with the flow rate of the protective gas controlled at 10-25 m³ / h. 3 / h.

[0019] A further improvement is that the heating rate is 1-5℃ / min.

[0020] A further improvement is that, in the post-sintering step, the temperature during high-temperature corrosion is controlled at 500℃~700℃, and the corrosion time is 10s~600s.

[0021] A further improvement is that the protective gas is Ar.

[0022] A further improvement is that, in the pre-sintering step, the post-sintering step, and the sintering step, the maximum sintering temperature is reduced, and the protective gas flow rate is 10 m³ / s. 3 / h.

[0023] A further improvement is that the flow rate of the protective gas is increased in the pre-sintering step, the post-sintering step, and the sintering step, and the holding time is 1 hour.

[0024] The present invention provides a method for preventing the deposition of scintillator crystals on the surface of silicon carbide ceramics sintered under normal pressure. Compared with the prior art, its beneficial effects are as follows: by adopting methods such as lowering the ceramic sintering temperature and increasing the holding time, increasing the flow rate of the protective gas during ceramic sintering, covering the surface of the sintered ceramic sample with graphite paper, spraying a protective and isolating ceramic coating on the surface of the ceramic sample, or corroding the ceramic surface with molten NaOH, the growth of excess SiC grains is reduced, which can prevent the deposition of scintillator crystals on the surface of silicon carbide ceramic sealing materials, effectively reduce the surface roughness of the sealing material, achieve near-net-shape forming of silicon carbide ceramic sealing parts, and reduce production costs. Detailed Implementation

[0025] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0026] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be used interchangeably where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0027] In addition, the term "multiple" should mean two or more.

[0028] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to specific embodiments.

[0029] In this application, a method for preventing the deposition of scintillation crystals on the surface of silicon carbide ceramics sintered under normal pressure includes at least one of the following steps: a pre-sintering step, a sintering step, or a post-sintering step.

[0030] The pre-sintering steps include covering the surface of the sintered sample with graphite paper, or spraying a boron carbide ceramic coating or a boron nitride ceramic coating onto the surface of the sintered sample.

[0031] The sintering process includes reducing the maximum sintering temperature from 2200℃ to 2050-2100℃, while extending the holding time by 0.5-1 hour, or increasing the flow rate of the protective gas by 5-15 m³ / h when introducing the protective gas at 1200-1700℃. 3 / h;

[0032] The post-sintering steps include placing the silicon carbide ceramic product with a crystalline surface into a container filled with molten NaOH for high-temperature etching, removing it, cleaning it with dilute hydrochloric acid, rinsing it with deionized water, and then drying it.

[0033] Example 1

[0034] During the high-temperature sintering stage, the sintering temperature was controlled at 2100℃, the heating rate was 1℃ / min, and the holding time was 1.5h. When Ar protective gas was introduced at 1700℃, the Ar gas flow rate was controlled at 10m³ / min. 3 / h, no flash crystallization was observed on the sample surface after sintering, the surface roughness was 0.15 μm, and the density was 3.11 g / cm³. -3 Its Vickers hardness is 24.84 GPa, its flexural strength is 423 MPa, and its thermal conductivity at room temperature is 142 W / m². -1 K -1 The coefficient of thermal expansion is 4.47*10. -6 (25-1000℃).

[0035] Example 2

[0036] During the high-temperature sintering and holding process at 2200℃ for 1 hour, Ar protective gas was introduced at 1700℃, and the Ar gas flow rate was controlled at 30 m³ / h. 3 / h, the surface roughness of the sintered sample is 0.2um, no flash crystals appear on the surface, and the density is 3.13gcm³. -3Its Vickers hardness is 24.5 GPa, its flexural strength is 436 MPa, and its thermal conductivity at room temperature is 157 W / m. -1 K -1 The coefficient of thermal expansion is 4.63*10. -6 (25-1000℃).

[0037] Example 3

[0038] During the high-temperature sintering and holding process at 2200℃ for 1 hour, when Ar protective gas was introduced at 1700℃, the Ar gas flow rate was controlled to be 10 m³ / h. 3 / h, 0.1mm graphite paper was placed on the sample surface. After sintering, the sample surface roughness was 0.18um, no scintillator appeared on the surface, and the density was 3.11gcm³. -3 Its Vickers hardness is 25.45 GPa, its flexural strength is 418 MPa, and its thermal conductivity at room temperature is 163 W / m². -1 K -1 The coefficient of thermal expansion is 4.77*10. -6 (25-1000℃).

[0039] Example 4

[0040] During the high-temperature sintering and holding process at 2200℃ for 1 hour, Ar protective gas was introduced at 1700℃, with the Ar gas flow rate controlled at 10 m³ / h. 3 During sintering, a boron nitride ceramic coating is directly sprayed onto the surface of the silicon carbide ceramic sample. The boron nitride ceramic slurry has a solid content of 30 wt%. The pressure is controlled to ensure the boron nitride ceramic slurry is evenly sprayed onto the silicon carbide ceramic sample surface through an atomizing nozzle, with a thickness of 20 μm. After sintering, the silicon carbide ceramic surface can be directly cleaned with purging air. The sample surface roughness is 0.2 μm, no flash crystals appear on the surface, and the density is 3.11 g / cm³. -3 It has a Vickers hardness of 24.79 GPa, a flexural strength of 442 MPa, and a room temperature thermal conductivity of 88.7 W / m. -1 K -1 The coefficient of thermal expansion is 4.66*10. -6 (25-1000℃).

[0041] Example 5

[0042] During the high-temperature sintering and holding process at 2200℃ for 1 hour, when Ar protective gas was introduced at 1700℃, the Ar gas flow rate was controlled to be 10 m³ / h. 3After sintering, the surface roughness of the sample was 0.5 μm, and flash crystals appeared on the surface. The sintered silicon carbide ceramic product was placed in a pool filled with molten NaOH, and the temperature was controlled at 650℃ for 30 seconds. After removal, it was cleaned with dilute hydrochloric acid and deionized water, and then dried. The resulting silicon carbide ceramic had a surface roughness of 0.2 μm and a density of 3.14 g / cm³. -3 Its Vickers hardness is 24.74 GPa, its flexural strength is 423 MPa, and its thermal conductivity at room temperature is 156 W / m². -1 K -1 The coefficient of thermal expansion is 4.89*10. -6 (25-1000℃).

[0043] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preventing the deposition of scintillation crystals on the surface of silicon carbide ceramics sintered under normal pressure, characterized in that: Including steps prior to sintering; The pre-sintering steps include covering the surface of the sintering sample with graphite paper, the graphite paper being 0.1-0.3 mm thick; the maximum sintering temperature being 2200℃; the holding time being 1-2 hours; and the introduction of an inert protective gas at 1200-1700℃, with the protective gas flow rate controlled at 10-25 m³ / h. 3 / h.

2. The method for preventing the deposition of scintillation crystals on the surface of silicon carbide ceramics under normal pressure solid-phase sintering as described in claim 1, characterized in that: The heating rate is 1-5℃ / min.

3. The method for preventing the deposition of scintillation crystals on the surface of silicon carbide ceramics under normal pressure solid-phase sintering as described in claim 1, characterized in that: The protective gas is Ar.