CN115611642B - SiC whisker with BN coating on surface and molten salt method preparation method thereof - Google Patents

Abstract

The invention discloses a SiC whisker with a BN coating on the surface and a preparation method thereof by a molten salt method. powder; then heat-up and sinter the uniformly mixed powder to make it fully react to obtain SiC whiskers coated with BN coating mixed with molten salt, and SiC whiskers coated with BN coated with mixed molten salt After washing with water, the molten salt attached to the surface is washed off to obtain SiC whiskers with a BN coating on the surface; finally, the powder obtained by suction filtration is dried to obtain SiC whiskers with a BN coating on the surface. The method of the invention is suitable for the preparation of BN coatings on the surface of SiC whiskers and chopped fibers, and the obtained SiC whiskers covered with scaly BN can be used for strengthening and toughening SiC-based composite materials.

Description

A kind of SiC whisker with BN coating on the surface and preparation method thereof by molten salt method

technical field

The invention generally relates to the field of composite materials, in particular to a SiC whisker with a BN coating on the surface and a preparation method thereof by a molten salt method.

Background technique

With the development of science and technology, scientific research workers have also emerged in an endless stream of research on composite materials.

Ceramic matrix composites are widely used in the aerospace field because of their excellent high temperature resistance, corrosion resistance and wear resistance. Among them, SiC whiskers have high strength, high modulus and high temperature resistance, and are widely used as reinforcement materials for ceramic matrix composites. However, during high-temperature sintering, chopped fibers or whiskers are easy to form interfacial bonds with the ceramic matrix, so that the composite material will exhibit poor mechanical properties. In order to adjust the interfacial bonding strength between the reinforcement and the matrix so that the matrix cracks are deflected at the interface and the interface can transfer the load from the matrix to the reinforcement, it is necessary to prepare an interface layer.

As a coating that can improve the performance of SiC whiskers, BN coating has the following advantages:

1. Good oxidation resistance. Compared with PyC, BN interface coating has better oxidation resistance, maintains chemical stability in the middle temperature region (800 °C), and BN interface coating has self-lubricating properties at high temperatures.

2. The toughening effect is better. As a reinforcement, BN will be oxidized to B ₂ O ₃ under high temperature conditions, and the volume will expand slightly from BN to B ₂ O ₃ , which can fill cracks on the one hand; on the other hand, B ₂ O ₃ will seal cracks and prevent cracks from further developing. extension, which can enhance the fracture toughness of the composite.

3. Strong chemical corrosion resistance.

At present, there are mainly two methods for preparing BN coatings, namely CVD method and dipping cracking method. However, the BN coating deposition process in the prior art still has the following deficiencies: The coating process prepared by CVD method is mostly used for deposition on SiC fibers, and less applied to SiC whiskers. The coating prepared by the CVD method has high requirements on the ratio of the mixed gas, deposition time and pressure, and the deposition speed will affect the uniform formation of the coating. Although the preparation process of the precursor dipping and pyrolysis method is simple, the resulting coating has many defects, and the dipping and pyrolysis preparation process also has a certain degree of damage to the original reinforcement. Compared with the first two methods, the preparation of BN coating on SiC whiskers by the molten salt method provided by the present invention has the advantages of low cost and high efficiency. The difficulty lies in the flake structure of BN coating, so in controlling the coating The degree is somewhat difficult, but the coating coating can be controlled by controlling the mass ratio of molten salt, reactants and SiC whiskers and the holding time. At the same time, the change of temperature will affect the crystallinity of BN. As the temperature rises, the crystallinity of BN Increased, the generated BN will be better attached to the SiC whiskers.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art, and provide a SiC whisker with a BN coating on the surface and a preparation method thereof by a molten salt method. In the SiC whisker with a BN coating on the obtained surface, the BN coating The layer forms a uniform interface on the whisker surface, and the preparation method of the invention has high efficiency and low cost.

The technical solution of the present invention is that the present invention firstly provides a SiC whisker with a BN coating on the surface, the BN coating is formed on the surface of the SiC whisker by a molten salt method, and the BN coating is formed on the surface of the SiC whisker. The scales are evenly distributed, the BN of the BN coating is a hexagonal boron nitride structure (h-BN), and the thickness of the BN coating is in the range of 10-50nm.

The present invention also provides a molten salt method for preparing the SiC whiskers with a BN coating on the surface, comprising the following steps:

1. Powder mixing: ball mill and mix molten salt, BN-forming reactants, and SiC whiskers in a certain proportion to obtain uniformly mixed powder;

2. Sintering treatment: heat up and sinter the homogeneously mixed powder obtained in step 1 to allow it to fully react to obtain SiC whiskers coated with BN coating of mixed molten salt;

3. Suction filtration treatment: wash the SiC whiskers coated with BN coating of the mixed molten salt obtained in step 2, wash off the molten salt attached to the surface, and obtain SiC whiskers coated with BN coating on the surface ;

4. Drying treatment: after drying the SiC whiskers with a BN coating on the surface obtained in step 3, SiC whiskers with a BN coating on the surface are obtained.

Further, the molten salt in the above step 1 is sodium chloride and potassium chloride, and the reactants for generating BN are melamine and borax.

Further, in the above step 1: the molar ratio of nitrogen in melamine to boron in borax is (1:1)~(4:1); the mass ratio of sodium chloride to potassium chloride is 11:14; BN is generated The mass ratio of reactants to molten salt is (1:1)~(1:2); the mass ratio of the total mass of reactants and molten salt to SiC whiskers is (5:1)~(10:1).

Further, the heating and sintering operation method in the above step 2 is as follows: under a nitrogen atmosphere, at a heating rate of 3-5°C/min, heat the uniformly mixed powder to 900-1200°C and keep it warm for 1-4 hours, Then cool to room temperature under a nitrogen atmosphere, and the equipment used in the temperature-raising sintering treatment is a vacuum cracking furnace.

Further, the operation method of the suction filtration treatment in the above step 3 is: put the product obtained in the step 2 into deionized water and mix evenly, then use a suction filtration device to filter out the liquid, and repeat 4-5 times.

Further, the drying temperature in the above step 4 is 80-90°C.

Compared with the prior art, the present invention has the following advantages:

1. The present invention provides silicon carbide whiskers with a surface interface layer of BN. The evenly distributed BN layer can improve the physical and chemical properties of silicon carbide whiskers and expand its application field.

2. The BN coating on the surface of silicon carbide whiskers of the present invention presents a scaly shape microscopically, which is different from the uniformly spread BN coating prepared on SiC fibers in the prior art. The hexagonal BN presents a scaly shape without BN coating treatment The whiskers will be combined with the powder during the sintering process to lose its original strengthening and toughening effect. After the BN coating is prepared on the surface of the whiskers, when the composite material is subjected to external force, cracks will occur when it encounters the coating. Therefore, the SiC whiskers attached to the scaly BN coating can enrich the application of SiC whiskers in the field of reinforcement and toughening of ceramic matrix composites.

3. In the preparation method of the present invention, the crystallinity and grain size of the generated h-BN are controlled by adjusting and controlling the rate and temperature range of heating and sintering, and the temperature range of the present application can make the crystal grains of BN grow to a certain extent Larger, while ensuring that the crystallinity is optimally controlled, so that the more scaly BN is generated, the sintering below 900°C will not be conducive to the crystallization of the BN coating, and the sintering above 1200°C will cause the grain growth of the generated BN coating If it is too large, the BN layer loses the burr feeling of scales.

4. The present invention adopts the molten salt method to prepare the BN coating, and the salt in the product can be washed with deionized water, and the conversion rate of the product is higher.

Description of drawings

These and/or other aspects and advantages of the present invention will become clearer and easier to understand from the following detailed description of the embodiments of the present invention in conjunction with the accompanying drawings, wherein:

Fig. 1 is the schematic flow chart of preparation method of the present invention;

Fig. 2 is the SEM picture of the SiC whisker covered with BN coating prepared in Example 1 of the present invention;

Fig. 3 is the SiC whisker SEM picture that is covered with BN coating prepared in the embodiment 2 of the present invention;

Fig. 4 is the XRD contrast figure of the SiC whisker that is covered with BN coating that the embodiment of the present invention 1 and 2 prepare;

Fig. 5 is the SiC whisker SEM picture that is covered with BN coating prepared in the embodiment 3 of the present invention;

Fig. 6 is the SiC whisker SEM picture that is covered with BN coating prepared in the embodiment 4 of the present invention;

Fig. 7 is a comparison chart of XRD of SiC whiskers coated with BN coating prepared in Examples 4 and 5 of the present invention.

Detailed ways

In order to enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example 1:

In this embodiment, the method for preparing a BN coating on the surface of SiC whiskers is shown in Figure 1, and is carried out according to the following steps:

1. Powder mixing: Take 4g of β-SiC whiskers with a length of about 20-40μm and a diameter of 50-200nm; a total of 10g of molten salt (sodium chloride 4.4g, potassium chloride 5.6g); a total of 10g of reactants for forming BN (The ratio of the amount of B to N is 1:1), mix, stir, and grind the above-mentioned substances to obtain a uniform powder.

2. Sintering treatment: Sinter the uniformly mixed powder in step 1. The sintering equipment is a vacuum cracking furnace. The sintering process conditions are: under nitrogen atmosphere, the temperature is raised to 900°C at a rate of 5°C/min, and the temperature is kept for 3 hours. The nitrogen atmosphere is cooled with the furnace to obtain molten salt coated SiC whiskers with a BN coating on the surface.

3. Suction filtration treatment: the SiC whiskers coated with BN coating mixed with molten salt obtained in step 2 were washed with deionized water and suction filtered 4 times, and the salt attached to the surface was washed off, and the BN coating coated with the surface was obtained. layer of SiC whiskers;

4. Drying treatment: Dry the powder obtained in step 3 in an oven at 85° C. to obtain the final SiC whiskers with BN coating on the surface.

Example 2:

In this embodiment, the method for preparing a BN coating on the surface of SiC whiskers is carried out according to the following steps:

1. Powder mixing:

Take 4g of β-SiC whiskers with a length of about 20-40μm and a diameter of 50-200nm; a total of 10g of molten salt (sodium chloride 4.4g, potassium chloride 5.6g); a total of 10g of reactants to form BN (substances of B and N The ratio of the amount is 1:1), mix the above materials, stir and grind to mix evenly.

2. Sintering treatment:

Sinter the uniformly mixed powder in step 1. The sintering equipment is a vacuum cracking furnace. The sintering process conditions are: under nitrogen atmosphere, heat up to 1200°C at a rate of 5°C/min, keep the temperature for 3 hours, and keep the nitrogen atmosphere to cool with the furnace , to obtain molten salt-coated SiC whiskers with BN coating on the surface.

3. Suction filtration treatment: the SiC whiskers coated with BN coating mixed with molten salt obtained in step 2 were washed with deionized water and suction filtered 4 times, and the salt attached to the surface was washed off, and the BN coating coated with the surface was obtained. layer of SiC whiskers;

4. Drying treatment: Dry the powder obtained in step 3 in an oven at 85° C. to obtain the final SiC whiskers with BN coating on the surface.

The SEM images of Example 1 and Example 2 are shown in Figures 1 and 2. It can be seen that there is a BN coating on the surface of SiC whiskers at 900°C and 1200°C. Compared with 900°C, the flaky BN coating produced at 1200°C is more uniform and has a heavier burr feeling, indicating that increasing the temperature is conducive to the formation of BN coatings. According to the XRD images of the two in Figure 3, it can be seen that the generated BN is h-BN, and the higher the temperature, the better the crystallinity of BN, and the more obvious the diffraction peak of BN.

Example 3:

In this embodiment, the method for preparing a BN coating on the surface of SiC whiskers is carried out according to the following steps:

1. Powder mixing: Take 3.33g of β-SiC whiskers with a length of about 20-40μm and a diameter of 50-200nm, and a total of 10g of molten salt (4.4g of sodium chloride and 5.6g of potassium chloride); 10g (the ratio of the amount of B to N is 1:1), mix the above substances, stir and grind them evenly.

2. Sintering treatment: sinter the uniformly mixed powder in step 1. The sintering equipment is a vacuum cracking furnace. The sintering process conditions are: under nitrogen atmosphere, heat up to 1200°C at a rate of 5°C/min, keep warm for 1 hour, and keep The nitrogen atmosphere is cooled with the furnace to obtain molten salt coated SiC whiskers with a BN coating on the surface.

3. Suction filtration treatment: the SiC whiskers coated with BN coating mixed with molten salt obtained in step 2 were washed with deionized water and suction filtered 4 times, and the salt attached to the surface was washed off, and the BN coating coated with the surface was obtained. layer of SiC whiskers;

4. Drying treatment: Dry the powder obtained in step 3 in an oven at 85° C. to obtain the final SiC whiskers with BN coating on the surface.

Example 4:

In this embodiment, the method for preparing a BN coating on the surface of SiC whiskers is carried out according to the following steps:

Powder mixing: Take 3.33g of β-SiC whiskers with a length of about 20-40μm and a diameter of 50-200nm, a total of 10g of molten salt (sodium chloride 4.4g, potassium chloride 5.6g); a total of 10g of reactants (B and N The ratio of the amount of the substances is 1:1), the above substances are mixed, stirred and ground to mix evenly.

Sintering treatment: Sinter the uniformly mixed powder in step 1. The sintering equipment is a vacuum cracking furnace. The sintering process conditions are: under the condition of nitrogen atmosphere, the temperature is raised to 1200°C at a rate of 5°C/min, kept for 4 hours, and the nitrogen atmosphere is maintained. After cooling in the furnace, SiC whiskers coated with molten salt and having a BN coating on the surface are obtained.

3. Suction filtration treatment: the SiC whiskers coated with BN coating mixed with molten salt obtained in step 2 were washed with deionized water and suction filtered 4 times, and the salt attached to the surface was washed off, and the BN coating coated with the surface was obtained. layer of SiC whiskers;

4. Drying treatment: Dry the powder obtained in step 3 in an oven at 85° C. to obtain the final SiC whiskers with BN coating on the surface.

The SEM images of Example 3 and Example 4 are shown in Figures 5 and 6. It can be seen that the holding time mainly affects the growth of the crystal grains. When the heat preservation time is 1 hour, the BN coating grains attached to the surface are smaller, showing It is in the form of small flakes, and the overall surface of the SiC whiskers has obvious burrs; when the temperature is kept for 4 hours, the grains of BN grow significantly, and because the grains grow and cover the entire SiC whiskers, there is no burrs on the overall surface of the SiC whiskers feel. According to the XRD patterns of the two in Figure 7, it can be seen that both of them have h-BN formation, and the BN diffraction peak after 4 hours of incubation is more obvious.

Having described various embodiments of the present invention, the foregoing description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and alterations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (2)

1. a method for preparing SiC whiskers with a BN coating on the surface, characterized in that, the BN coating is formed on the SiC whisker surface by molten salt method, and the BN coating is scaly on the SiC whisker surface Uniform distribution, the BN of the BN coating is a hexagonal boron nitride structure, and the thickness of the BN coating is in the range of 10~50nm; Include the following steps: 1. Powder mixing: ball mill and mix molten salt, BN-forming reactants, and SiC whiskers in a certain proportion to obtain uniformly mixed powder; 2. Sintering treatment: heat up and sinter the homogeneously mixed powder obtained in step 1 to allow it to fully react to obtain SiC whiskers coated with BN coating of mixed molten salt; 3. Suction filtration treatment: wash the SiC whiskers coated with BN coating of the mixed molten salt obtained in step 2, wash off the molten salt attached to the surface, and obtain SiC whiskers coated with BN coating on the surface ; 4. Drying treatment: after drying the SiC whiskers with a BN coating on the surface obtained in step 3, obtain SiC whiskers with a BN coating on the surface; Molten salt is sodium chloride and potassium chloride in described step 1, and the reactant that generates BN is melamine and borax; The SiC whiskers have a length of 20-40 μm and a diameter of 50-200 nm; The molar ratio of nitrogen in melamine to boron in borax is (1:1)~(4:1); The mass ratio of sodium chloride to potassium chloride is 11:14; The mass ratio of the reactant to the molten salt to generate BN is (1:1)~(1:2); The mass ratio of the total mass of reactants and molten salt to SiC whiskers is (5:1)~(10:1); The heating and sintering operation method in the step 2 is as follows: in a nitrogen atmosphere, according to a heating rate of 3-5°C/min, the temperature of the uniformly mixed powder is raised to 900-1200°C, kept for 1-4 hours, and then Cool to room temperature under a nitrogen atmosphere, and the equipment used in the temperature-raising sintering treatment is a vacuum cracking furnace. 2. the molten salt method preparation method that the surface has the SiC whisker of BN coating as claimed in claim 1, is characterized in that, The operation method of the suction filtration treatment in the step 3 is: put the product obtained in the step 2 into deionized water and mix evenly, then use a suction filtration device to filter out the liquid, and repeat 4-5 times.

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